CA1095831A - Fragmentation of subterranean formations - Google Patents

Abstract

A B S T R A C T
A subterranean formation containing, for example, oil shale is prepared for in situ retorting by initially excavating a pair of spaced apart voids, leaving an intervening zone of unfragmented formation between the voids. The inter-vening zone has substantially parallel free faces adjoining the void. A plurality of elongate blasting holes are formed in the intervening zone of unfragmented formation, the longitudinal axis of each blasting hole being substant-ially perpendicular to the parallel free faces of the intervening zone. At least two deck loads of explosives are placed in each. blasting hole with each load longit-udinally spaced apart from each adjacent charge by stemming.
The loads of explosive are then detonated in a single round of explosions with a time delay between adjacent loads for expanding formation in the intervening zone toward both voids. The fragmented mass of formation particles is then retorted to recover, for example, shale oil from the oil shale.

Description

~9~

This inventioll relates to the f`ragmentation of a subterranean formation to produce an in situ retort, for example the fragmentation of a subterranean oil shale formation to produce an in situ oil shale retort for the production of shale oil from oil shale.
The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a formation comprising marlstone deposit containing an organic material called "kerogen"
which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation contaiDing kerogen that is called "oil shale" herein, and the liquid product is called "shale oil".
The recovery of liquid and gaseous products from oil shale deposits has been described in several Pa-tents, one of which is U.S. Patent No. 3,66l,423. This Patent describes in situ recovery of liquid and gaseous car-bonaceous materia]s from a subterranean formation containing oil shale by mining out a portion of the subterranean forlnation. Then explosive chargcs dis~
persed through a portion of the remaining formation are detonated to fragmel1t and expand the portion of the remaining formation to form a stationary, fragmented, permeable mass of formation particles containing oil Z5 shale, referred to herein as an "in situ oil shale retort". Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot rr-~torting gasr-s used for cor~ert~ ero~en contained in the oil shale~ as descri~cd in U.S. Patent No. 3,66l,ll~3, includes est-ablishirlg a combustion zorle in the retr>rt and intro-ducing all ox-~getl supplyirlg combustion ~one fecd into the retort on the trailing side of thr combustioIl zorJr tr ad~ance t}le com~ucjlioIl zcr!e throuph ihe fra~ncrll-ed mass. :rn thc combojcJo}~ zone oxygeo in thr- gasrous fer?r3 m1~turr; .s drpleted ~y rcacti-"~ ~ith hr)t calbr>
~r 109~

aceous materials to producc heat and combustion gas.
By the continued introduction o.f~the oxygen supplying feed into the combustion zone, the combustion zone is advanced through the fragmented mass. The ef.fluent gas from the combustion zone passes through the retort on the advancing s.ide of the combustion zone to heat the oil shale in a retortin~ zone to a temperature sufficient to cause retorting during which kerogen undergoes decomposition to gaseous and liquid products and leaves a residue of solid carbonaceous material.
The resulting liquid and gaseous products pass to the bottom of the retort for collection.
It is desirable that the retort contain a reasonably uniformly fragmented, reasonably uniformly permeable mass of formation particles having a reasonably uniformly distributed void volume or void fraction so gases can flow uniformly through the retort and result in maximum conversion of kerogen to shale oil. A uni.formly dis-tributed void fraction in the direction perpendicular to the direct.ion of advancernen.t of the combustion zone is important to avoid channe~ling of gas flow in the retort. 1.`he creation of a mass of particles of uniform void volume distribution prevents the formation of over-sized voids or channe].s which hinder total recove-y of shale oil and also provi.des a uni.for~n pressure drop through the entire mass of particles. In preparation for the described retorting process, it is importan1;
that 1,he I'ormation be fragmented and displaced, rather than simply fractured, in order to creatc high perme-ability; otherwise, too much pressure dif'ferential isre~uired to pass gas through the retort. It is also i.mportant that the retort contain a subst;antia.lly uni~
formly fragrnented mass of particles so uniform conversion of kerogen to liquid and gaseous products occurs during retorting, A wide distributiol1 of particle si~e can adversely affect the efficiency of retortir1g becausc smal]. parl;:icles can be conlpletel~ rei,orted :Iong 1>efore ~9~31 complet:iol1 of retorting the core of large particles.
It has been proposed that o~l shale be prepared for in situ recovery by first undercutting a portion of the formation to remove rrom about 5% to about 25~o of the total volume of` the in situ retort being formed.
The overlyin~ formation is then expanded by detonating explosives placed in the formation to fill the void created by the undercut.
The general art of blasting rock formations is discussed in The Blasters~ Handbook, 15th Edit;ion, pub-lished by E.I. duPont de Nemours & Company, Wilmington, Delaware.
One method of explosive expansion is the so-called "V-cut" method, described at pp. 246-7 of The Blasters' ~andbook, in which explosive charges are arranged within the formation and detonated in sequence so the formation is expanded in concentric sequential steps moving rad-ially outwardly and upwardly within the formation geIler-ating a conica] free face which propagates upwardly 20 through the formation in accordance with the -t;ime delays between the explosive charges. A free face is the ;~
exposed surface of a mass of rock such as a sur~ace in the ~icinity of a shothole at which rock is free to move under the force of an explosion. h purpose of

2~ the V-cut method of expansion is to produce particles of relatively small size; but it has the disadvantage of tenc1ing to create a radially non-uniform void volume distribut;ion throughout the expanded mass.
Rather than using the V-cut method of expansion, it has been proposecl to use a plura:lity of concel1trated charges uniformly distributed throu~hout the formation to be expanded to produce a w1irorml~r fragmer3ted mass of formation part;icles. U.S. Patent ~o. 3,~34,757 teaches sequential detonation of a series of c-xplosi~c in oil shale to form a permeahle ~one ilJ the o i 1 shalc . IIo-~-e~rer, it is both time con-sun-ling and expensive -to place a large mlmber of explosi~e charge.s t]r~ughout thc l~g~831 5.

formation.
~ lother method for preparin~ subterranean forma-tions for in si.tu recovery is described in U.S. Patent No. 4,043,597. ~ccording to the method described in this Patent, two voids vertically spaced apart from each other are excavated in the subterranean formation, leaving an intervening zone of unfragmented formation between the voids. The intervening zone of unfrag-mented formation is then fragmented in two or more stages by placing explosive in a lower portion of the interveni.ng æone of unfra~nented formation and deton-ating the explosive so as to fragment the lower portion of the intervening zone. Further explosive i5 there-after placed in a f,urther portion of the unfragmented zone above the fragmented lower portion and detonated to cause fragmentation of the further portion. This procedure is continued as necessary until the inter-vening zone is entirely fragmented, using a total of at least two consecutive stages of expansion.
It has now been found that advantageous rcsults are obtained with such a method if such an intervening zone is fraglnented in a single stage, rather than two or more stages, by detonating explosive located in at least an u})per region and a lo~er region of the inter-vening zone in a si.ngle round with a time delay between deton~tion of loads of explosive in different regions of the i.ntervening Y,one.
According]y, in one aspect, the present invention provides a method of frag~nenting a subterrancan form-ation to produce an in situ retort, comprising excavatingan upper void and a lower void vertically spaced apart from eac~ other in the sub~erranean fornlation and sep-arate(l by an intervening zone of unfragmented formatiorl~
at least a portioïl of the upper void being substantially d;rectly above the lower void; placi33g a plurality of loads of e,~.plosive itl at least ~n upl)el regi,on and a lower region of` the intervenj.3l{, ~,03-e of unf'ragmellted ~9S831 formation; and explosivcly exparlding formation in the intcrvening zone towards both voids by detonat.ing the loads of explosive in a single round of explosions with a time delay be~t~een detonation of loads of explosive in different regions of the intervening zone.
The loads of explosive can conveniently be placed in the formation by forming, e.g. drilling, a plurality of substantially vertical blasting holes in the inter-vening zone of unfragmen-ted formation and placing at least two deck loads of explosive in each such blasti,ng hole. The loads in each blasting hole are vertically spaced apart from each adjacent load by a mass of stemmi]lg~ The loads of explosive are detonated in a single round of explosions with a time delay between adjacent loads to expand formation in the intervening zone toward both voids.
~ n one version of this invention, the loads are detonated sequentially toward the vertical centre of mass of the in.tervening zone for expanding the formation uniformly toward both of the voids. Th~s is effected by detonating each such load in a blasting hole no later than detonation of any of the loads in the same blasting hole between such load and the vertical centre of mass of the portion of the formation being fra~nented.
Sequen.tial deton.ation allows creation of a new, sub-stantially horizontally extending free face by detonation of loads, thereby providing a new free f`ace for explo-sive expansion of formation in the intervening zone by subsequent detonation of loads, ~n another version of this invention, each load is detonated no earlier th.an detonation of any of the loads between. such load and a free face of the forma-tion being Fragmented: this expands formation pre-ferentially toward that free t`ace, Preferably the time between (letoJla.tion of each load and an adjacellt load in the same 'blasti.ng hc~,e is more t]-lan the l;:ime re4uired f'or creatioll of a free face l~9S831 by explosive expa.nsion of formation by detonation of the first of the ~djacent loads ~o be detonated.
~ .so, preferably the time between detonation of the first load to be detonated and the last load to be detonated is less than the time required for expanding formation beyond a selected void fraction by detonation of the first load to be detonated. This permits for-mation of a retort containing a reasonably uniformly permeable ma.ss of particles.
To avoid excessive seismic e~fect f`rom detonation of the loads of explosive, preferably the time between detonation of loads detonated success:ively is sufficient for the seismic wave produced by detonation of the first load to be detonated to pa.ss the second load to be deton-ated.
In a further aspect, the present invention providesa subterranean formation in all intermediate stage of prepara';ion of an in situ retort, conlprising an upper void and a lower void vertically spaced apart from each other in the subterranean formation and separated by an intervenirlg zone of unfragmented formation, at least a ~ortioll of the upper void being substantially dir-ectly above the lower void; a plurality of loads of explosive located i.n at least an upper region and a lower region of the i.ntervening zone of wlfragmented formation; and means for detonating -t]-le loads of explo--sive in a single round of explosions with a ti~le delay hetween detonation of loads of explosive in d:ifferent regions o~ the intervening zone.
lhese and other features of the invention wil] be more flllly understGod by refcrence to the following detailed description and accompal~ing drawings, in which:
F~GU~E 1 is a schematic perspect:ive vicw showing a subterr.lnean formation containing oil shale in an intermcdiate stage of preparlt-,.on for i.n situ recovery in accordance with this invc~tionS

~gS~l FIGUR~ 2 is a schematic cross-sectional elevation vicw taken on line 2-2 of Figure~l;
F:CGURE 3 is a cross-sectional plan view taken on line 3-3 of ~igure 2;
FIGURE 4 shows on an enlarged scale a blasting hole extendin.g between a pair of vertically spaced apart voids and containing explosive for expanding formation toward both voids; and ~ IGURE 5 shows on a scale similar to that of Figure 4 an alternative arrangernent of blasting holes extending into formation between a pair of vertically spaced apart voids.
~. General Discussion Referring to the drawings, ~igures 1 to 3 illus-trate a subterranean formation 10, such as a subterraneanformation containing oil shale, which is in an inter-mediate stage of preparation for i.n situ recovery of carbonaceous values, such as shale oil and hydrocarbon gaseous products. Generally speaking, in situ recovery is carried out by initially excavating fGrmation from a port:ion of the subterranean formation and then explo-sively cxpanding a remainlng port:;on of the formation to produce a fragmented permeable mass of formation particles containing, for example, oil shale. The present invention is described in the context of a method for ultimately producing a subterranean oil shale retort comprising an approximately rectangularly prism<-tic retort cavity, or roorn, 12 (illustrated in phantom lines in ~igures 1 to 3) coIItai.niJlg a reasonably uniforlrly fragmented, reasonably uniformly permeable mass oI expanded formation particles h<lvi.ng a reasollab~y uniforlnly distributed void fraction for economical retorting operations. In the illustrated embodilllent, the .in situ retort being formed is square in horizontal

3~ cross-sect-.on having a v-ertical dimension or height whi.ch is greater than i-ts maximum lateral d:i~nension or widtll~ The heiP;ht of the retort carl. of course~ a.ller--1095~31 natively be less than or the s~me as the width of theretort.
Referring to ~igure 2, access to the portion of the subterranean forlnation containing oil shale to be expanded is established by forming a horizontal tunnel, drift or adil; 14 extending to the bottom of the volume to be expanded. From the drift 14, the formation is undercut and a volume of formation is removed to form a 30wer void 20 at the bottom of the subterranean re-tort 12 to be formed. The rnaterial excavated fromthe lower void is hauled away through the drift 14 for removal to the surface via a shaft or a~it (not shown).
The lower void 20 can be continuous across the width of the volume to be expanded, with formation over-lying the lower void being completely un.supported anddefiIling an upper hori~ontal free face 21 of the form-ation immediately above the lower void. The lower void ~0 similarly defines a lower horizontal free face 22 at the bottom of the retort site. If desired, however, and as il]ustrated in Figure 2, one or more p:illars 4~ of unfragmented formation can be left in the lower void 20 to assist in supporting overlying formation, as described in greater detail hereafter.
l`he floor plan or llorizontal cross section of the lower void 20 cQn be generally square, although the void, and also the in situ retort to be formed, can be of other hori~ontal cross-sect:ion, such as rectangular, without departing from the scope of the invention. lhe lowcr horizontal free face or floor 22 o~ the lower void 20 is inclined downward]y in the ~irection of the drift 14 to facilitate the flow of shale oil in the direction Or the drift 14 during subsequent retort:ing operations.
A hori~ontal access tunnel or drift 30 is e~cavated at an elev~tion above the elevatiorl of the lower void 3~ 20~ ~rom the hori~ontal access drift 30, forlnation is excavated from the volume 1o oe e}.panded to form an intermediate void 32 at an ele~l+io;lllhovc t~e eLevati<-~9~

10 .

of t}lC lower void 20. The floor plan or horizontalcross-section of the intermediate void 32 substantially matches the horl~ontal cross-section and area of the : lower void 20 and the in situ retort 12 to be formed.
Thus, the intermediate void can, for ex~mple, be square or rectangular in shape, and preferably is substantially directly above the lower void 20 so the outer edges of the two voids lie in common vertical planes. If des-ired, pillars 49 can be left in the intermediate void 32 to support the overlyi~g formation. Alternativel~-, thc i~Jtermediate void 32 can be continuous across the width of the room 1~ so that the overlying portion of the forrnation is completely unsupported. The formation adjacent the intermediate void de-fines a pair of verti-cally spaced apart bottom and top horizontal free faces34 alld 36, respectively, adjoining the intermedi.ate void 32. The two voids 20 and 32 thus also define a lower illterYenillg zone 37 of unfragmented formation containing oil shale left within the boundaries of the subterranean re1-ort 12 between the substanti.ally paral-lel hori~ontal iree faces 21 and 3ll After the intermediate void 32 is formed, or con-curren-tly therewith, a horizonta] tunnel or drift 42 is excavated at an elevation above tlle e]evcltion of the intermediate void 32. Forrnation :is removed from within the bollndaries of the retort 12 being formed through the drift 42 to form an upper void 1~11 at an eleva~ion above the e:Levatioll of the intermediate voicl 32. The floor plan or hc-rizontal cross-section of the upper void 44 is substantially similar to the cross-section of tl1e lower arld intermediate voids of the retort 12.
The upper void pre~erably is ali~led with tlle voids below it so tb.at the outer edges Or tl-le~ upper void lie in co~nmon vertical planes with the outer edges of the voids below. Th.e upper voi.d ~4 ~ of appr<)ximately the s~me h.ei~Jt as the in~termetlia~;t void 3~ a.ntl can.lJe conti~ ous across the width of retorl 12. T11us, tne 1~9S83~

portion of the formation above it can be completely unsupported, ,If desired, however, one or more pil]ars ll9 of unfragmellted formation can be left in th.e upper void to help sl3pport the overlying formation, as shown in Figures 2 and 3. (Pillars llg are not shown in Fi~lre 1 in -thc interestsof clarity). When pillars 49 are left in any of the voids~ preferably the pillars 49 are at least as wide as they are high to maximize the stability of the overlying formation. l`he optimwn proporl:ion of formation extracted from the void and proportion temporarily left in the form o.f pillars of unfragmented formation. in any particular case depends on man~T factors such as rock properties, depth of over-burden, height of the voi.d, time the voi.d must remain open and the like. The optimum size alld location of pillars can be readily determined by conventional tec~miques by one s~illed in mi.ning.
The uppcr void 4/~ dcfines a pair of verti.cally spaced apart bottom and top horizontal free faces 48 and 50, respectively, of the unfragrnellted formation adjoin-in~ ~he void. The two voids 32 and 44 t~lUs also def,'inc a zone 51 of unfragmented formation left between the free faces 36 and 48. Further~ an intact zone 52 oI' unfrag~ented formation withi,n the bounclarics of the retort being formel is left above the uppermost free ~ce 50.
In the arrangement illustrated in Fi~ures 1 to 3, there is one intcrrnediate void 32 betwecn tlle u~per void 44 ~nd th.e lower void 20. In othcr alternatjve arrangements 9 however, there can be no intcrveniJIg void, or therc ca,n be two or more i.nterlned:i~1;e void~
one above anot.her, The total m.ln1ber of voids used deperlcls upon the height of the forl-nati~n to i)e expc-3.nded:
the greater the heigrht o~ the f.`orn-lation to ~e expan(lcd, 3~ t'he Inore void.s required.
Tl-le use of' mul.tipl,e i,nt~rmcdi.~ oid.s can be advaIItagcou:-; wl-lere -the height of i'~.^ rcto3t bein f'ori!3ed 109~831 lZ.

is very much la.r~er than its width. One or two inter-mediate void.s can be excavated between ~ e top and bottom vo:ids so t}-~a1; the in situ retort can have a substantial height without need for expanding excessively thick zol1es of forlnation between ad~jacent voids.
Conventiollal underground mining techniques and equipment are used for e~cavating the voids and access drifts.
~ fter the spaced apart voids 20, 32 a,nd 44- have been excav~ted in the formation, the intervening zones 37 and 51 of unfrap,mented forma-tion and the intact zone 52 above the upper vo:id 41~ are prepared for explosive expansion arld subsequent retorting operations.
A plurali,ty of substantially vertical blasting holes 53 are drilled in the lower intervening zone 37 upwardly from the lower void 20 or downwardly from the intermediate void 32. Similarly, a plurality of substantially verti-cal blasting holes 54 are drilled in the upper interven-i.ng zone 51 from the intermediate void 32 or the upper void 44, and a pl11ra.1ity of substantially verti.cal b3,ast-ing holes 55 are drilled upwardly from t'he upper void 44 i.nto the zol~e 52 of unfra~mented forinati.on above the upper void. Tlle b:Lasti,ng hol~s 53, 5ll and 55 extend tllrou~h the formation and are substantially perpendicular to thc free faces of tile zonos of unfragment~d formation.
In the i.ntercsts of clarity~ only on~ of each such verti-cal biastiJIg holes 53, 54 and 55 is shown in Figure 2.
FurthGr, in order to show clear]y placement of explosive and stemming in t.hese b],asting holes, the horizontal dimel~sion of the holes is exaggerated in ~igure 2.
If` pilla:rs such. as the pillars 49 in thc u.pper void ll4 have been left withirl the voids 20, 32 and 4~, hori-zon1,~3,ly exte,;ding blast:ing holes a.re dri].led in tlhe pillars in ~reparatlon for explosive expalision therc~f.
~`he blas1;ing holes are t'h.e1:l loaded with. generall~-cy].indrica'!, coil-mn loads o-f c-xplosive and stell11ning. The loads or exp:l,osive are di.stri1~ntcd in the bl~sti]lg hol.es ~9S831 3 .

53 and 5LI in the intervening zones 37 and 51, respect-ively, preferably using a variation of deck loading, as descrihed in ndbool~ at pages 220 and 229. In the method of deck loading, two or more loads of explosive are placed in a blasting hole in spaced-apart relaticnslliE). Each load is completely separated frorn an adjacerlt load by a mass or segment ~f stcmming material such as sand, gravel or drill cuttings. Each load is separate]y primed, either electrically or with detonating cord.
Deck loading has been used to enable the explosive to be distributed according to tlle hardness of the rock and for distributing a charge of explosive through a blasting llole preferentially toward the bottom of the hole to provide m->re energy for breaking the burden near the bottom of the blasting hole than compared to the energy provided for breaking the burden nearer the free face.
In practiee of preferred methods of this inventioll, deck loading is used to expand formation cxplosively toward two free faces to form a substantially unifor~lly ~ragmented, substantially uniformly permeahle rnass o~
~ormation particles. This i 5 effected by detonating the deck Loads of explosive in a blastillg hole in a single round of explosions with a time delay between adjacent loads to stagger detonation of the loads.
With re~erence to Figure 2, the blasting holes 53 in thc lo~er intervening 70ne 37 of unfra~lented form-ation each cont-ain three cylindrical column ]oads of explosive, an upper or top load 101, a middle or inter-mediilte load 102, and a lower or bot;tom load 103. ~ach load is sepa-rated ~rom the adjacent load or ]oads by stemnlin~: that is, there is a segm(rll; or majs 104 of stemming between the upper lc,a~ 101 and the lntermeclia-te 3~ loi~d 02, and there i~ a s<-~ment lO~j of stenllling betweer the intermediate load 102 and ~he o(,tt;oin load 103. One purpose of thc segnlents of ste;nm,;n{r1-etween ~ljacent .. .

lQ9S~31 14.

loads is to allow time delay between detonat;ion of adjacent loads. If the sogments of stemming were not prescnl, dctolllti.on of one load col71clllnavoida~1y lead to detonation of an adjacent load. There is also a S segment 106 of stcmming above the upper load 101 and a seglllent 107 o~ stemming below tllC? lower l.oad 103 to confine these loads to maximi~e efficiency of blasting.
Similarly, each blasting hole 54 in the upper inter-vening ~one 51 contains three load.s of explosive, a top load 1]..~, a middle or intervening load 11.~, and a bottom load 113. Between the top and the intermediate load i.s a segment; 1].4 of stemming, and betwcell the intermed.iate and the bot1om load :is a segment 115 of stelmning. Tlhere is also a segtnell-t 116 of stemming above the top load 111 and a segment 117 Or stemming below the bottom load 113.
]3ecause the ~one 52 of unfraglllented forrnatj.on above the u~,per void 44 is explosively e~panded towards only one void, the upper void 44, deck loading is not requ:ired in the b]asting holes 55, Thus, in each blasting hole 55 thcre is only one Joa.d ]21 of e~.plosi~e Wit;}l a segmenl;
12~ of stemming below cach load 121. If desi.red, how-ever, deck loading also can be used :i.n the b]asting holes 55 in the ~one 52 above the ul,por voi~
It sllould be unc~erstood that in preferred embodi-ments Or -th.is invcntion a plura~.ity o:t vcrt:i.cal blasting holes 53, 54 and 55 are drill.ed in eacll of the ~ones cf 37, 51 and 52, respec1;ive1y, and ea(1l h]asli.n{, hol.- is loa.ded w:ith exp].osive and ~temrning s~ stan~ti.al]y as shown in ~igure ~. The si~e ancl tc,tal n~r.rl~or of ola.-t-ing lJoles usec~ aro chosen to prov:idc sufficjerl1; total explosive ene:rg~r to expan(l and fra~meZl~ th~ formatio2l beillg bJastcd. ~or e~amr?]cS ~i~ule 3 illuitxates OJle poss-ible clrrangelrlent which can he use~l for placemerlt- of the blasiillg holes 51l in l-he uppc-~ :irtrven.ing ~on-- 51 of unf:ra~mente~ ~orma-tion~ Md~ r:ia1;ions ~r( also useful~

l~9S831 15 .

As nlentioned above, there are preferably at least two deck.].oads of exp:losive in each blas-ting hole in the intcrvc~irlg ZOlle(S) of un.rra~nen.ted formation in order to obtain ex~lc,sive expansion of formatiol1 towards two space(l apart voids. In the embodiment of Figure 2, blasting holes 53 and 54 each contai.n three loads of explo~sive, as described above. As a further example, Fi~lre 4 shows a vertically extending blasting hole 130 in a zone 132 of unfragrmented format:ion between lwo substantially parallel vertically spaced~apart voids 20 and 32, with an alternative loading arrangement of explos:ive, 1;he ~l.asting hole 130 containing fivc loads of explosive numbered :in Figure 4 from top to bottom as 141, 1~2 7 1'l3, 14ll and 145~ ~ach loa.d is separated from an acljacent load by a segment 146 of stemmirlg. A
segment 147 of stemlning is provided above the top load 11l1 and a se~ment 148 of xtemlrling is provided below the bottom l.oad 145.
Sufli.cient ste~ :ing is provided between adjacent loads that detona~iorl of one load does not inter:fere with subscquent detonation of an adjacent load and does not c~llse prellllture detonat:ion ol an adJacent; load.
Use of dcck lolcling with staggered detonation of the loads in a blasting holc can yield fragmented for-matior having a particle si~.e approaching that achievedby using a plurality o~ independent, concentratcd, spherical charges but at a considerably :Lower cos-t because it is not necessary wi.th deck :I.oading to ,vrovide a sep-arate l~las-ting hole for eac~l as is the case Wherl llSiIlg independen1; .spherical charges. For example, i.n the arran~rement illustrated in Fi{~1re l~5 five indiv:idu-al dec]f loads arc provided in one blasti.n~ hole iha-t can be cl:lilled at sigl~ icantly le~s expensc tllan tl3c Iive blas-tin~ ~ioLes necessary for f:ive individl1al cllarges.
~ 35 ~lother advanta~l;e of staggerirJg 1.hc detonal;.io3l of tl-le~ decl~ 103-ls in 1 blas~.,ir~ lOle iS that mc,re cf`:fec-tiv-f7~ nen-i;.lt;o-rl is acllievecl compared ~.;:i.th rcsults aclli^vcd 1~9S83 16 .

when detc)nating all the loads simultaneously. The improved fragment.lti.on res1llts from a preconditioning ef`fect, wherf~?by det;ollatio]l of a first charge precondi-tions adjacent-, formation by creating smal.l cracks and fissures i.n the adjacent formation. Thus, wl-len a subsequent lGad is detonated in the adjacent formation, the fissured formation is more readily fragnented.
Control of t:ime delay between detonation of the deck loads :in t1~e blasti]lg holes is important for obtaini.ng a. retort containing a uniforrnly fra~mented mass of particles. There are three constraints on the time dc1.ay betweell detonation of the deck ].oads.
n. Constra:int-.s on Timc ~elay ~. Constraint 1 Accord.ing to the first constraint, to Gbtain effec-tive fragmentation, each. deck load should not be deton-ated un-til the charge i.s sufficien.tly close to a free face that interven:ing burden is free to move due to the force of explosion of the deck load. For example, in the arra.ngement illustrated in Fi~lre 2, intermediate loads 102 and 1]2 in blasting holes 53 and 54, respect-ivel.y-, are preferably not detoDated unt:il. the load.s are adjacent a Iree face. For thc intc~rmcdi2te load 102 of explo~ive to be adjacent a f'ree face, itt ic neces~,lry that either the upper ~eck loact lOl or t;he lower deck load 103 i.n the blast:ing ~ole 53 be detonated 1;o expand f`ori[lat:ion. explos:ivc~.y towards an ori.~:inal free I'ace 3l~
or 2~.~ respectivel~, to create a new free face extending substarltially parallel to the original free faces.
~hus, to obtain eff`ective expansion of fragmented form-a1,ion to~ard two voids, the tim~.? be-twf?eJI<letonati.o~. of eac]-l:in-t,ermed;ate 1.oad and an ad.jacent J.oad betwf?en such i.ntermediate load and a f~ee f'ace should be morc than the time requi.red for creation of a ne~i f`ree face by explo.si.ve expansi.on o~ formatio,:~ by de~ton,11-i.on of tl~e first; O:r the adjacent dec].. load~s to b(? detonate~. Only niD.im~l c~parl~siol:l i.s ncc-ded t.o c~f.at.f th~ ne~, f`ree :f.l.ce;

~t~9S831 the formation is not completely expanded at the time of creatioM of tlle new free face.
l:ll accordance with this constraint there is pre-ferably sufficient expansion that the primary compression wave resultin~ from detonation oP a load is at least partly reflected at the adjacent free face, If there is insufflcient expansion, elastic deformation of form-ation at the free face can bridge the ~ap resulting in transmission of the primary compression wave across th.e free face with l:ittle, if any, reflection, ~eflection of the ~riMary compression wa.ve is important ~ecause it sets up a -tension wave in thc format:ion which contri.but~s great;ly to fragrncn-tation of the formation, W:ith refercnce to the ~lasting ho'e 51l show~l in Figure 2 in the upper æon.e 51 of the unfragmented forma-tion, each of the following sequences f`or detonation of deck loads satisfies this first cons-traint:
111, 112, 113 111, 113, 112 1:l3, 112, 111 1;3, 111, ~12 ~ny seclucnce of detonation startin~ with the middle load 112 vio:l~tes this contrain1;, As rcgards the loa.ding arrangement illllstrated in ~ re 4, a.ny sequence of detonati.oJI startin.g with any of the i.ntcrmedia-te deck loads 142, 11l3 or 141~, violates the fi.rst collstra.i.n1. Exemplary Or sc(luenc~s which satisfy the first constrain1; are t]lC following~:
~ 5, 1~12, 1/~ 13 14], 142, :L4 ~ 2, ~ll3, lL~
11ll, 142, 143, 1~ , ]ll~
Exemplary of sequcnces whicll vi.olatc l}lc firs-~ constra:int are the following:
i~ , l4" 1~
145, 142, 1111 5 ]4!l l.43 145, 14~ 143, ~ 5 1~1-2 - 1~9S831 l& .

Expansion of formation is required to create a new free facc. The tim~ required for creation of a new free face l.y c xpansion of formation by detonati on of an explosive dcc k load is usually from al~out 4 to about 6 5 t:irr~es the transi t time of the primary compression wave f`ormed from clctonati.orl of the load relative to the :r~ear-est subst~n-tially lloriæontal free face. The term "transit timc II is used herein to refer to the round tr ip time of the pr; mary compression wave from the 10 .~oad to the nearest free face cand back to the load.
Thus, accordingr to this constraint, wit;h the arrangemen.t illustrated in Fi~re 2, if lo~d 103 is detonat;ed before load 102 i71 blastin~ hole 53, then the time between detonat:ion of load .103 and dctonatj.on of load ]02 should 15 be at least equal to from about 4 to about 6 times t~le round trip time of the prima.ry compression wave result-ing fronl deto~lation of :Load 103 to the upper fre~ face 21 of the ].ower void 20 and back to ].oad 102. A clelay of at least about 4 to about 6 transit tirnes allows 20 formation of a new free face by explosivc expansi.on of format:ion i n tlle xonc o:l format:ion in which the 1 oad ] 03 j B p.l.aced. The clel.~3.y can be ~rreater th -an 6 transit timc~s~ sul~Joct to the second constrain.t descri.bed below.
~le primary compre~sion wave is the fi.~s l; wave 2~ resul.ting from de tor;ation of a l.oad of exp].osive and i:;
the high.est magni tude compression wave prodllced by sucl~
deton~tion The trarlsit time of the primary compre~r3io wave depe]idci upon. the distance l)e-tween Ihe loacl and the closest free face as well as l;he spced of propagration 30 of the compressi.on wave throu~.r:h the forlm-lt:Lon. The speed of t:he compression WdVC c.-.n depcn~J on t;he type of format:ion bci.~ r :fragr~ nted. li`or e~ ~mpl e-, in a formation coll1;ain.i]lg oi l sll-dle having a Fi:jcher ~ssay of 3() gca 1 lons per ton ~12, l.itres per tolm.e), the primary comp:ression 35 wave from det;ollation o:f c-xplosi~-e i ~ave3 S througril the formatioll perpendicular to the ~e~d~li rl~. p ~ anc-~ at a velo-ci -ty of frorri abou-t ~3,000 to 11 ~Or~ re~ri (2,~ 0 1;o 3,350 1~95831 19.

metres) p;~]- secondO Veloc,ities of ahout 5,500 to about 7,500 feet (l,G75 to 2,285 metres) per second are rea].isecl w~len deton.lti.ng exp:l,osive i,n l`ormatioll contain-ing oil sllale having a Fischer Assay of 18 gal]ons per ton (75 li1;rcs per tonne).
2. Con.stra.i.nt II
The second con~straint on the time of detonation of the deck loads in a ~one of` unfragn~en-ted formation is that the t:i.me betwe6il detonation of the first load and the last load to 'be detonated is prefcrably less than the time reqllired for expand:ing format,:i.on beyond a selected vo:i.d fraction by de1.o1lation 03^ the f:i rst load to be detonated. Tlle purpose of this constraint is to have all tlle forma-tion e~}and:irlg befoxe any por-tion of -I,lle format:iol~ is over-expallded beyond the selected void f:raction for creation of a substantially uniform permea,ble rnass of f'o:rmation parti,les throughollt the retor-t being formed.
As a specific exarnple of this pri.nciple, w:ith the arrangement of Fi~lre 2, if the deck loads in tlle bla.st-;ng hole 53 are detol~ated in tlle sequence of 103, lOl, 102, t~cn the rniddle load lO2 should ~e detonated before forrnation ~xpanc3ed by detonation of load 103 has expand-~d beyond a .selected void fraction~ If forma.tion i.s perrllitted to over-expaIld beyond the se]ecte~ void fra.ctj.on, :it is impossib]e to reduce economically the vo:i.d fraction of the over-expanded forMati.on. Over~
e~pans;on of a port:;on of 1;he~ forrnat:io/l:is un~l~sirable because it can result in another por-l,ion of the fra~nen-t-ed mass of parti.cles having a void frac1,ion su1-,stantially belo~ tl~e desired void fracti.on. ~or ~xample, if a f3agmeIlted I'ormat;on is to llave, an avf-~xage void fracl;~io of l5C~o ancl about 80<~o oi' 1;he forrna1:ion e}:pan~s l;o a vo:id fraction o~` ahout 30/o~ thcl1 the remain:i.lig 20-^~g of the forlrl-ation has no xoom ava:i]ablt~ for e~pc.nsioJl.
~ ecause tl-Jere is a time dt~la~f het-~Jeerl detonation of a load arîd e.~pan~-ion of f0rm<-lt.~-,n a(-i~iacerl 1; that ].oac7, 20 .

this time lag is considered when staggering the deton-ation of load in a zone of unfragmented formation.
For cxamplc?~ if tl1c ma~in1um desired voicl fractio1l in a ret;ort :i.s a~out 30~, then the ]ast load to be detonated should be c1etonated before formation expal1ding due to detonation of the first load to be detonated has expand-ed beyond a void. fraction of about 25~. Thus, the "selected void fracti.on" is 25~. The purpose of the extra 5/' of ]eeway is to accommodate the lag between ].0 the detonation of the last load to be detonated and expansion of formation due to detonation of the last load, 3. _ Constra:Lnt III
As the third cons-traint, prefera~ly the time het~een detonation of deck loads in the same blasting hole is suffi,cient for the primary seismic wave produced by the detona-tion of a particular, e.g. the first, load to pass the nex-t subsequent, e.g. the second, load to be detonatecl. The primary seismi.c wave is the sei.smic ~0 wave of maximum dmplitude produced in formation due to detonatlon of explos,ive, This delay i.s provided to avo:if] damage to strucl-ures and e~JUipment ~hich can occur if the primary se.ismic wave of two loads superimpose to yield an overly large pri.mary ~e.ismi.c wave.
The time required for the primary sc-,i.srlic wave fron1 one ].oad to pass anoth(?r load depends ~lpon fac-l;ors such (lS the cl;.stance between tho two loa(]s, the de1;on-ation velocity of -the explosive, the len~rth of -the column of explosive being detona-ted, and the propagation veloci,ty of the wave through the formation. The time can he as little as one milljsecor1d.
C. Se~uence of_Detol1at;iorl ~ CCordiDg to this invention all of -th,e explc~sive in a zone of unf`ragmen1,ed formatior. het~:een ver1,ical:ly spacec1 apart voids is detonated -;.n a si1]~le ro1l7ld.
All loa(,is Or explosive at the stme eleva -!,ion in a z<)ne of u1~:l`ra~lnen1;e~ format:i.on can, :if desi~-c~c1, 1-e deto71atc~1.

1t~9S831 s.imu]taneollsly. Thus, all of the top cleck loads 111 in tl~e l~la.si~ g 130].es 5'l in the upper i~ltervenin~ zone 5:l. o:r unfr~tglllc~]lted lor~ tion c~n be cletol~ated silnu].tan-eously. Lik~.?wise, all of the bottom ]o~ds 113 can be detonated s:ir.lultaneolJc;]y and all of 1he intermediate ]oads 112 can be detonated simultaneous]y. Detonating a plurality 03` loads at the same elevation in a ~one of nnfragmentecl formation creates a new flee face extending .s~bstantial~.y p~ralle]. to tll~ origina] free ]C faces ~ or ex.~mple, with. tl~e loacling arr~ngemerl.t of F:i.gure 2, assllmin~ a sequence of deto]lation. of~ e loads in the upper intervcr~ g ~03]e 51 of lll, 113, 1.12, detonfltion of all of tlle top loa-ls 111 in an upper portion or zone .1_5 241 of the upper interveni.ng Y,O]le 51 expands the upper portion 2'~1 toward the upper void 44 ~n(l c3e~tes ~ first new rre? face, shown schematical:ly by d~.she~ line 242 in Fi~lre 2, wh:icJ~ is substanti.~].ly paral.le] to thc original frec fa.ce 'l8 and the remai.ni.ng rree face 36 of the upper intervening Yone ~1.. Likewise, cletonal:ion of the bottom lc~ad., 1]3 rcsul.ts in cYpansiorl Or a lower portion 2ll3 oI t]~e upper :intervel)in~ æone tow~.rd tl~e intermedi.a1e voicl 32 with. cr-eatiorl of a secon.d new free face sl~own by dafil-lecl line 2'1l~ in ~i~rlire 2, wh-i.cll.is s1lbslal]l:ial:Ly 2~ parall-.?:l to 1;1-le first llOW f~cee iace 242. T~.lc f:i-rst ne~
free .t~lce ~ 2 and the second new iree f-ace 244 are neia]
to t]le top ancl bottom, respect;.vel.y, Or t;lle In:iddle chal-ge.s ]12 Or explosi.ve. T}len, by ~le-toll~l.irlg t~le remainillg middle loads ]12 of exp.los:i.ve, tlle rema:i~ling centr3.1 port:i.on O:r t;]-le upper inte3vening ~one 5~. i.s e~plosive].y e~p~lndecl t.oward both thc l~7?)l- r vo:il ~ and t:he ~.oweJ vo-id 32.
l`o Ivo;-l e~cessi.ve seisrrlic ~shock:~nd cdaln~e to above ground ar,d.l~elow ground structl;re however, i.1 may l)(? des;.rat)le ~`or load~s of c}~j)l si~e ..t ~}1(? Sanl'?
ele'Vat,i(>!l il~. t}?e Iornla~iorl to b-~ .-;erl;l-rlti.~ r ]ctoJ-Il-le(l.
Tll~s, ~1 ~olds o f e~l).l.osiv-c~ a.-l; ;l~ ! f ' ~` 1 ~? .i ~i t; i . ( ) 13 -i:rc ~9S831 .
22.

not necessarily detonated simultaneously.
Wllen e~plosively expanding formation toward two subst~ i.l11y p.arl:llel voi.ds, the seqllence of detonation of the deck loads .in a blast;ing hole~ the detonation point ol` each load, the type of explosive used for each load~ and t}le relative amount of expl.osive used for the l.oads can all affcct the proportion of formation which is explrlded to~al-ds each of -the voids. The effect of each of ~hese factors OIl the distribution of formation will now be consi(lered.
W:ilh respec-t -to se(luencc of detonation~ to expand a.n intervening ~orle of unfra~nented formation hetween a~
upper void and a lower voi.d uni.forlnly toward both voids~
each :I.oad in the blasting holes is preferably detona.ted no later than detonating any of the loads bctween such load and t;he ver-tical centre of mass of the portion of the formation bei.Jlg fragmellted.
~ ol~ exampl.e, w:i.th tlle :Loading arrangement of Figure 2, to expalld the upper intervening ~one 51 uniformly toward th.c interll3edi.ate vo:id 32 and the u~per void 4'~, then the middl.e ].oad. 11~ is the las-t load to be detor.-ated. Simi.l.ar]y~ referrin~ to Fi~ure ll, to achieve uniform cllstribution of the intervening ~one 132 of ullf.`ra~n1enl;ed format:io]l towlrds 1he upper void 3~ and lower void 20, the middle load 143 is the last load to be de-tonated, the upper i~Lte-rmediate charge 11l2 is detonated after the top load 1l~1, and the lowcr inter-medil-ie ]oad .1~ is detonatecl after 1he bottom ]oad ].1l5.
Ii`ur1,]1er, to expal3d :F`orlrlation exp~.osi ve].y JJrel'er-entially l;owards 0-3e of the two par.illel voi.ds~ each load i.n a blasting hole is deto-lla1ed no earlj.er 1;]1an deton-at:k)n O:r any of tlle loads be-l;wee3l such load a~ld the vo:i.d toward wl~:ic13 a higher proporticn of -/l~e formation is to be expanded. For exalllple, referri)-lg~ to Figure 2, to expand tlle lo~^?er intervr-~ni]i~ ~orle 3~ of unfra~enteci forlnation l~rel:`e7-erltiall)~ to.al(3s tl~e lower vo-id 20, the l.oads i.n -the b~lsti.-g ll.c):.cs ,~ arc rlet)i-licrl :rrorn 1:l~e 1~9S~31 bot-lom -tc) tl~- to~, i.e. a t;he bottom loads 103 are deton-ated rirst, fol:Lo~ed by the middle load.s 3,02, and finally tlle to1~ loac1 .1,~ ver1 witll tllis sequence of detonation, ho~c~ve:r, SOlllf' expilnsi on of f'ornlation towclrfl both voids is 5 unavo i clahle.
D. Locus O:r Init.i iltion l`hf.? ] OCUs O:i'` initi.ation of de-l;onatiol1 of' a load affects the d:irectio11 of expan.sion of rorinati.oll adjacent the :Lo~d. W:l1e11 detonat;o11 of a cylindricc~. load is 10 ill:i.t:i<:~ted at, one of its ends, f`or~nation tends to be prerere11tial].y e~.pilnclfed-lowards the end at ~h-ich de~tol1-ation is i.ni1;:inte(1. 'rh,is re~iul-l,.s f'rorn 1;]~c~ 1;ilne reqllired for the detorla-tion wave -to trave3. t3)rou{~h a column o~
exl~losive.
Re~rerring 1,o ~i.gure 2, e~-plo.si,v-e initjal,io1l devi,ces, such as elec1;r;,c blastin{r caps uscd i`or detonating explo-~iVf~? .l f)a~s a'r'f' (~ac']l r~ep:rf~sc~ntecl l)y i~ X~ 0~ I~'o:r~
ex.3n1p1c-~ it nlay bf? desirod to expand t;he ul)per interven-in.g ~.c"),e 51 ur~ `ormly tow,ar(1s tl1e u3pper void I~ md 1,he lo~err vc):id 3~, al-ld to e~t-)~tnd t3le lowe:r in-tervenig ~,ol~e ~7 prin]a.rily towclrds the :lowc,~r vo;,d ,?0. l`hus, in t31e ble~ i31i~ ho:les ~l in -the uppc3r in-tc3:r-ve11i,r~ e, de-to~l-atio)] of, each top load lll i,s i,rli.t:ii-~,i;ed substal1tially at ,its top~ detonation c-~ eac}1 1,o-l;toin J.oad 1l3 is ir)i.-~5 tiate~l subs(,al1t;i.al1y at its botl;om, ,and ~le-tonc1t-icin of eac~l nli(3c31e locld 117 is :init.i atecl sul~.s1,.-~l2l,-i,~:li,y in tl1e m.iClf33 f' Or ,i. ts ver-t:ical :he:i i~;ht~ 'rO e:~pcl:rl(1 a h:i f~her prc)rlortion c,:r 1,13.e lower :i1lte-cvi~nil~ Y,{)L-IC ~7 10~rds L~-1e bottonl vo:id 20, detona-tic)rl of -t1~c 1o~e:~- 10,ad.j :L03 c-2nfl 1,31e Inidd:Le loads 102 :in th,e blas1;i~ )olesc~ 53 is :i,ni-tiated subs-l;a11,ti,.ll,ly at t31e:ir bot'.,om, ar2d deto11a1,jon o:f the top loads 101 is ini-ti~2teil subst-lnt:i,~]:ly at their top~ '3`31e .seq-~lence o~ clo-ionL~1,,ion u-,ed -i.s thc~ bo-f,-tom lo,~ds 303 i~i rs t; ~ the rnidd1e loacls 30,' ne~ t, a12d. t32e -io ]oads ]01 last~ Prefer)1-)1y -t1-)e l.)ol;tom lo.1c.s t.1.3 in t}1~ u~ r i.l)tc~ ,o~ 5] ~ r~ ) locld~
~he 3ow~ ; 7,0~, 37 a2-e r7~l.(,?~ cll~-t,-;, 1095~

24.

simul1;alleously so tha-t formatioll can be subst~ntially exp~n-1e~1 frc)m 1)oth the upper interverling ~.ones 51 and t~le :lower intcr~enin~r zone 37 towards the interrrledi.lte void 32.
E. Si~e o-f` Loads an<1 Ioading l~atio T]-1c? relati.ve size of the load and 1l~e relative load.:ing ratio of.the e~pLosive -(Ised for dec~ loads ar~ects the proportion of formation. e~pan~led to~ard each. of two voids Loadil1.g~ ra1;:io refers to the qu~n1;ity in weight or vo~l.ume, for e~ample tons, tomles~ c~1~ic yards or cubi.c mctre.s, of forrrla.1iol~1Jl.as1-.ed for a given weif~l~t, for exalr)ple per po~nd or per kg, of expl.osive used.
~ecause 1;1:1e midd]e portion of a ~one of ~lnfrafrn1~L?nted forlrlat:i.on i.s 110t adjaccnt to a void, it can be more 1.5 difricult to :fragment and expaDd the mid~le portion of the zone toward the vo.icts tllan i.t is 1:o expand the upper aod :Lo-ier port.ions O-r t~ t zone. To count~ract Ll1is effect, the il1termed-iate loadcj 1Ise~l for e~p~ndirlg a~ld fr~gm~nt:ing the interme~iate portion oI` the formation can be ].ar{?;er and/or usc explosive having 1 hig1~er loading ra1-i.o tha11 t;he top and ~ottorn char~;es.
F. P.illars [f pill.~rs /l9 of unI`r.1gme11te(' forlnat:ion are left; :in the voillsl tlle pi]llrs are prererab:iy l`ra~r~l~nted beforc detor)lti~l~r th~ exp~osivo in the l,lastirlg ~loles ;n the intervcni:ng ~ones of format:ion so the p:i.llars do not t;erfere W:it]-?. e}plosi.ve exp~ns:i.oI1 of t~1e :i.Jltervenil)~r ~ol~es of :fo:rmat:i.on. Th.lls, pro:t`-7.;ll)1.y exr~J-~sive :in t~le upper interveI1:ir1g ~one 5:1 is no1 d-torlale~ n.tjl aft(r creatioll of tlle free face .~t tl1e j.1l1ct.llr- of the pil:lars 49 in llle upper void 4ll arld tho 1:lpper .int(7venio~r ~one 5:L 1~y de1;onc)t;i.on of e~plo.s-ive -in ~ p~ rs 11 Af`ter the pill.ar.s 1l9 ar(~ e~l!l o :-i ve:i.-y ,~:,~ rn~-n~ i,e~i ~
cav:in~r of ~o~)n<ltion suppor-..ed h~T tl.c ~, :LI..:.rs carl occ11r.
Bec:aus s1lcll cav-:in~; can i-n~er.fe~e .;iih c~:pl.o~ ?~J)~n-~ii Oll cl:r -(:~ upp~ .tf~ v'~ ?
voicl~ , exlj:l.o;.\ie :;n 1~;0 u~jper i.:i-(J~cl. j J)~, /.0nC iS

~95831 preferab]y ~etonated before or at the same time as cav-ing occur.s O:r tl1e :t~ormation 52 previou.sly supported by the pillars ~9. 'rO tllis same effect, e.~plosive in the ?~lasting ]lolcs 5~l i.n the upper ir,ltervening zone is prefer~bly detonated before o. at t,lle same timc as explo-sive is detonated in the b3.asting holes 55 in the zone 52 of un.:t'ragmented formatioII above t]le upper voi.d 44.
To o1)tairl a u1].if`c-rm distribution of formation in a voi.d col,tai.nin~ pillars, preferably c.xplosive in an.
~,0 ~nfragrneI1tec1 zone be].ow and/or a~ove tllc void is not ~3e1;onated unti.1. a1`tcr pillar frcl{~ment.s from f`ra~rlnentin~
the pilla:rs i.n th.' voids are uIIif`ormly distributed.
- Thus, preferably the loa,ds of explosi.ve in the u~per :interven;.l1g zone 51 and the loads 55 of` explosive in the ZO]lf' 52 above the upper void are not detonat~d until after pi.llar fragments result:Ls3.g fror fra~merlti.]lg the l)i l..l.a.rs 11~ i.n t]lf' upper void are subs-tantially uniformly distri1,lJtec1 i.n 1;]le up~er void 1~l~. In tl-~is regard it can be noted that cavinz of formation previ.ously suppor-te.~3 1~y pi.1lars is timc dependcnt, and I,l1e timing of the start of caving ~cpcnds 071 ~actors inc:L1lding the prv pcrties of` the :formation, its depth and the unsupported span. Irl some case~ many sec,onds can elar)se 1-Jetween remova] of p:illars ancl cavi.ng of ovc~r:Ly:i.ll~ I'orma.ii.vl1.
G. ~o:i.d l~raction Tllc distribut;ed void frac-t:i.on or vo~urr)e of the permea~le ma~ss o~'IJartic~.es ill tlJe re~-tort, i.e., the raf;io of tlle ~vo],urne of tlle voids or spaces between part-i.cles to thc- total volume of tlle f:La~ c~ ed permca1>lf3 mass of part;i,cles in, the su1~terralleaJ)~ i-tu retort ~ is contrc)1led hy the volunle of 1,hc e~caval;ed void.s int;o wllich the f'orma1;-ion i.s expanded. ~re:fera1~ly, the total volunle o:L' tl~e excavated vvids :is suffi.c.iently small compared to -tlle total vo1,ume of tlle re~t;ort that tlle expa]~dec1 formati.on i,s Cclp~a~l e o1` I`i'l1inz lhe voi.i3s and t;he Sp.lCe Gccupied by--tl~e ~pa,l-ldf~ orrnal;i(izl prio]-to -l-he e:;pa~1Si.O)-I" :lln oi:he:r wor(-J~, t,lle ~-o:lunle o:r~ Ih~

~95831 26.

vo.ids is sufficiently small that the retort .is full of expanded form<ltioIl. In fillinf~ the ~Toids and the space occllp:iod hy t;1Je zones of lmfraf,rmented formation prior to fragmerltation, tlle particles of the expanded formation 5 bccome jalnmed and wedf,red together tightly so they do not shift or move after fraginentation has been completed.
In numerical terms, the total volumc of the voids is preferably ].ess than ai~out 30% of the tatal vvlume of t'tle retort be:ingr formed. Ill one embodi.ment of this invelltion, the volwne of tlle voids is preferably-Ilot greatcr than about 25% of the votume of the retort being formed, a.s this is founc3 to provide a void fractioll in the fragmented formation conta~ ing o:i] shale ade(luate for s~at,:isfact,ory retortinfJ operation. If the voi.d fraction is more 1,han about 25~o~ an undue amount of excavation occurs without noncomi.tant improvement in permcabi.lity. ~ernova~ of the material from the voids is costly~ and kerogen c,onta:ined therein is wasted or retorted by costly above ground method.s.
The tot:al vulume of the excavated voids is prefer-ably also .sufficiently large compared to the total volume of the retort l~)at subs-tantially all of t]~e expandccl forrnation wi.th:in the retort i.s capable of mov:ing enouf~h. during e~plosive c-~xpctnsi.on to fralrlnent and ~or thc rragrnents t,o be disp1aced and/or reori.e~lted.
Sucli movelnent provides perrneability in the fragmented m~ss to perln:i-t :rlow of gas witlluut cxcess:ive pres~.s1lre requ:iremerJts for nloving 1,he gas. Wi-lcIl t;hc-~ fragmented parl;icles containin~ oil .sllale are ret()rtec3, they increase in size. Part of th.i.s size incrcase is tcmp-vrar~r and result.s from thermal exparls:ic,i~, and part is permanent a~-Jd is bro1lf;ht abou-t dllrj.nfr 1;:he rel,ortJn{-, of kero~er.i.i.n l,~e sh.a3.e. ~he VUi(:i fract:i (>n Or tho I`raf-,r-lne~lt,ed permeable mass of' s'llale pa-rticles s1~ould also be lar~e enou~rJ-) I'or el'ficient iIl situ retor-,,inf, as this si.ze inc,re~se OCCl1:L5. I~l nll!rlerici~.1. tern.~-, 1,]le nlinimuirl volllln~ o:î the V-JiC~L5 i.ll Vi~ T o:l t]lC ~ll)OV~ f`O11',i.(3(`t ~lt,lUll'i 1~95831 is preferably above about loo,b of the total volllme of the retort. ]3e]ow tllis average percentage value, an unde-sJrab]e alno~ of power is required to drive tl-e gas blowers causin~7r retorting gas to flow tllrough the retort.
The abo~e percentage values assume that al.l of the format:iGn within the bow~daries of the retort is -to be fragme]lted; that is, there are no l1nfragrnented regions left in t]-~e retor-i. If there are unfragmented regions l.eft within the outer boundar.ies of tl~e retort, e.g~., for support p:i.llare 4~ or the l.ike, the percentages would be l~ss.
l~. Exarl1l)le7 In one ~xampl.c! of practice of t;his invent:Lon, the total hei.frilt of tl-!e in situ retort or room 12 i.s about 268 feet (82rn). Tlle interrnediate vo:id 32 and the lower void 20 each have a lle-ight (r~presented by the dimension a ill ~igure ~) of al:)ollt 30 I`eet (9m), arld the lle;glLt (rel~resfntd by the dim~ns:ion b) of the upper void is al~ollt 23 feet (7m). Eacll void contai.ns pillars compr-isi.ng about 30% of the volume of the void.. Eacn inter-VelliI1~ zone of unfragmented forrnat:i.on 37 a.nd 5~. and ~-he zone of u1l:rragmented f`orma-tion 52 above the top void i6 ahout ]8l1 feet (56m) square (represen-tcd by dimension c .in ~igu3-e 2) in h.ori.~ontal c,ross-sec i;:i 011~ W}li C`}l es~en-25 tially matclles the hori7(>n-tal cross-sect:ion of the void.s 20, 32 and ~, al~though these can he a foot (0.3m) or so w:ide:r tC? accommodate dril:l.:i.ngr e-luiprnen-t nea.r the edgres.
Tlle thickncs7 (repro-:7ented by the dirrlell.sion c]) of each intervenlngr ~onc i.s ab(>ut 76 feet (23rn) and the thicknc7 of the uppcr zonc _?~ above 1;Jle uppc--r voi-l is a7~.?oll-t 33 feet (lOrn).
~ xl):l.osi.ve i.s d-ispersed in a pl1lrcrli.ty of vert:ical bla..sting 1loles iI1 the upper an:l lower intervenillg ~ones of unfl-agmented forMat:i.on and in tne ~one of unfragnlerl-tecl.
fortrlc-lt-ion ab(-ve tl).e top void slibst~llt:lai.~ s 5l10~.7I1 in re 2~ Means f`or defonati.nc-17~ .s ~ f explc).sivo are provided and a~e- placed i?Z the :.?.J-.I Or e~r)1.os:ive ~9 2Y3.

.SUbStall ti.ally -~i'; shown in Fi.gure 2. Deck .I.oad 103 is d~tonaled f:irst, I`o].lowecl by load :L02 about 25 to about 50 In~ eco}lds 1 ~1 tel:-, Loads l.Ol, lll, 113 <lnd 12] are all detull~ted su1~stantillly i-;imult.-lneously about 25 to 50 mill:i.sc?eoJ1ds a:[ter detollatin-r l.oad 102. Load 112 is the11 detol~ated about 25 milliseeonds later.
results in formation of a retort àbout ]8l, f'eet (5Gm) square l-lavill~r a hei.g11.t o:E' a~-30ut 2G8 feet (82m) fi]led with a rea.so11a.l~ly unil`orm]y frag~ ented, lO reasonal~].y unif'orm].y permeab]e mass o,~ partie]es having an avcra,r.? voi~3 f'rae-ti.on o:r al)ollt 2:L . 7~/o~ ~s deseri.1~ed a'bove, th-is void rraetion is withi11 l]le dcsired preferred ra1~ge f`or maxim:i~i.ng reeovery of s11~-l]-c oil fron~ the volume bei.ng retorted and fo:r providi3lg for a minimdl 15 pressu3-e drop f`rom top to bottom of the verl;ieal retort.
In anot11er l~x.amplc, two vertieal]y spaced apart voids are e:x:eav~ted in a i`orl-ni~tioll coII-tainillg oil shale wi.th tl,c .l.c~ ?r void 11aving .a hei~ht of a1~out; 30 feet .
(3n1) arld be:in, al~cut 18~1 f`et3t (561n) sq~ re in cross~
20 SCCti.OJI, The upp(!r vo.id h~s t]-Le salrle eross-seetion and j S ~13o1lt ]5 f.`ee~t (l~.5m) in heig]~ n interve11ing zone c-f un:t'3 ~g1ne111ed formatio1l a.1Jol1t 9G fect (2C~m) th:iek: :;s le:L`1 bet~c?o1l tllc- lower void 1rld t]le upper void.
~?our e:l.or~, nted pill~r~, eac3~J 1~ fect (5m) 'oy .17~ feet 25 (52nl) .i.n ~1or:izo:nt;~t7 ero~ss-seeti on, are left in l;he upper -1nd lower vo:i.c'ts in the pattern s,llcjwn. i.n F`:i~1re 3.
Vertiea]. bl.asti.ng 11oles lO inehes (O.~,n1) in c1i.a.111eter on 20 x '() f`eet (-> x 6rm) cc~ lres are dri.:lLed dos~ dly into t11c :interven;r]g ~ol1e ar1cl ea.ch i..s ~Lo~lded witll th3~e 30 dec:k load.s of explos:ive. The uppc3 1nd lo~.!er lo~(ls a:re ]3.5 :fec~t (/~m) :in }1e-i~r~3-l ind cor~ it ~n exp]osi.vc~
~laV~ r 3 lo~c~ g r~tio of 0.55 cul-3ic yc3rdi o:t' formatio~
per pOUllCd (0.()3m3 per 1 ~r) Ot` eXpJo~ ?~ T}le m-iddle loads are 40 I'eet (l~m) i.n ]~ciglrl i~ncl co1rlprisc dn cxplo-35 si~e 11~'iIlg a loa(linc rat;o of` ).,7 oll~)-iC -~.~rds ot`
I`Orillal;iOn p--r poul-3~1 (O,()~m3 "er l~r;~ o:L` i -~;r,l.(, ~i~e.
~?'t'h~CC:~1 i? ~ C] l l1r~J3 (` L' i. O ~l Ci ~ C ~l(' J J l' i ~ O '~ ) C l, s~' C i', !l 9~331 29.

each llliddJe ]oad and each lower load are 1~ fe~et (l.2m) Or ~sand st;en~ ing. Below each bo-ttom load and above eac]-3 upper loac1 are 10.5 feet (3,~1n) of stemming.
Electrical clct;onators are provided for each load at about i-ts vertical ce]-tre of` mass. ~fter detonation of explt)sive ;m the pi]Lars, 1;]le upper loads are deton-ated, and thel~ 25 to 50 milliseco1~ds later the bo1,tom loads are dc~tt,n.l-ted. Then tlle middle loads are det-onated froln abollt 75 to about 100 milliseconds after ]0 deton.ltion of t1~e upper loclcls~ i.e. about 25 to about 75 mil]isc~collds ~I'tor deto7latio1L of t]le l~ottoln loads.
Thi.s resu L-ts in formation of a su'tJ1;erranearl room or c~av:il;y abollt llll feet (93m) hig]l ancl al)out l8l~ reet (561rl) square in .1 hori~o~1t~l cross-sec,tion. 'rhe room (oJltains a substantially llniformly fragmented, subs1;ar)l:ially uni-formly permec-~blQ mass of formatiol1 particles. The rn~s~ )-las ~n a-verage voicl voll1lnc or voicl l`r~ctior1 of abou(; ;':1.5~0.
l~`ol:Lowi~lg explosive e-.Ypansion of` t]]e format;io3l, at :least one ,r,cls access communicatirlgr ~it}1 an uppcr levc~] 03~ t]lC rctort 12 ~ ec;tab]ished by fc,rn]ing a hori~,ol1tal tunne:L 58 ar)d severa:l conl~lunica1,:in,g vertical condll:its 60 to t1~e top of tl~e fr.~rr1cnted perrrlea~]c nlass o:~ c~x~anded I`ormatiorl co~]t~illed in tht~ roo~
?5 I. c~t,-~vt~ry O-r llr :,c1llc-l;
-T}1e reco~ery of sllale oil and asco11s product~ fromthe oil shale in the rotort gencrally invoLvcs t~c move-menl, of a retor-tin,~ zone tl-1rt>llrrh the fragrrlcrLted p~rmoa`b]e mass 03~ 3~0rmation particles ir1 1,he retort. The re-to3rt-ir~g zone can 1~e estab:lished on 1,he aclv.ln<,ill" s:ide oi' a COrllb1iS t;iC)n ZO~l(` iIl thce retort or i-t cc~n ~c es-tal~listlc~
l~y pa~ ing hc`.1 L (?Cl gas throu~h t;l]e retc,3-~t. lt is gerler-ally p ef`erred to adva~-1ce thc ret,orii~ onc ~rom trle top -l,o the botto]n Or a vertical:!y or:ie1l-tecl retor-1,, i.,~
35 a retort 11C1Vin~ ve:rtica~ -,:;dC? ho~7n(lcll-i.es. Wi.-~ t~3]:i.S
O]'iC'~lt<lt:i.Ol), ttle sha] e o:i I ,~nd ~ 'OC3.11Ct g;;S('.-; p7`0C1.~1C`C'C3 ill t11e ~ei;o3~ti-~ on~r~ clo~ r~] l;!~ ,c~

~19~831 of the retort for collection and recovery aided by the force of gravity and gases introduced at an upper elevation.
A combustion zone can be established at or near the upper boundary of a retort by any of a number of methods. In one method an access conduit 58 is provided to the upper boundary of the retort and a combustible gaseous mixture is introduced therethrough and ignited in the retort. Off gas is withdrawn through an access means such as the drift 14 extending to the lower boundary of the retort, thereby bringing about a movement of gases from top to bottom of the retort through the fragmented permeable mass of formation particles containing oil shale. A combustible gaseous mixture of a fuel, such as propane, butane, natural gas, or retort off gas, and air is introduced through the access conduit 58 to the upper boundary and is ignited to initiate a combustion zone at or near the upper boundary of the retort. Combustible gaseous mixtures of oxygen and other fuels are also suitable. The supply of combustible gaseous mixture to the combustion zone is maintained for a period sufficient for the oil shale at the upper boundary of the retort to become heated, usually to a temperature of greater than about 900 F (480C), so combustion can be sustained by the introduction of air without fuel gas into the combustion zone. Such a period can be from about one day to about a week in duration.
The combustion zone is sustained and advanced through the retort towards the lower boundary by introducing an oxygen containing retort inlet mixture through the access conduit 58 to the upper boundary of the retort, and withdrawing gas from below the retorting zone. The inlet mixture, which can be a mixture of air and a diluent such as retort off gas or water vapour, can have an oxygen content of about 10% to 20% of its s 1~9S831 vol.ume, T.l-~c reto:r-t ;nlet m:ixture is introduced to the rel,()rt at .i :rate of al)out 0.5 to 2 standard cubic feet of ~rris pf.-~ itlul;e pc:r s(luare foot (15,' to 6],o litres per millu-(,e pcr s(luare nletrc) of c:ross-secl;:ional arèa of the :retort.
The i]lt.roductioll of gas at the top anc3. th~ ~.ri.th-dr..~.~al Or of-~' f.rase-~s :;rom the retort at a lower elevation serves to mai.:ntain ci do~ ward pressure di~ferential o~
~,ras to c,ar:ry ~Jot Co~ (st.ioll prodlict grases alld 11031 ox-i.di~ec' :inlct, gases (.such hg nitrogen, rOr eXamplf`) f1-Om tlle COITlbl1.5 tiOIl ~.c~lle downwarclly tJ].rollgh the ret,ort~
l`]l:is f`lo~ O:r llot gas cs-l;ablici]lc~s a retc):t1,:i]lf~ zone tJle advaIlc~ ; si.de of t.lle coml~u~:i1,ion ~Ollf' Where'i]l ~)c1rti-cu.:Latc fra~,irlented forn~ l,ion co~l-ta:inillg o:il shalc is ]5 heal;ed. 1-13 1,he retort,ing zone~ Icerogell in the oil.
s]~ale is retorted to ]i~luid and gaseous product~s. The ~ id,prodlicts, incllldiJI~ sllca~e oil~ nlove IJY gr~vi, ty toi~d 1;]le l~case of` t3~e .retort wJlere 1;hey arf~ co].lecctf~d :in a ~surnp 5] ~and pulllpe(l to the sur~ace hy a pump 62 tllr(,llf~ 1-i.c3l:licl prodllcl; tral]sI`er lillc ~ . The gaseous prodl:~cl,~s f`:roin t]le .retortir]g ~one rl~:ix Wi.tJl thf` gases ov:inf~r do~rl~vard~y t-13ro~ tl tlle i.ll situ retort and are -rcm(jvc!~l as re-t,ort oi'~ ~as f`rorn .l lcvel bcelow the retort~
i~Jgr i;one, 'rl-?e -re~ort o:L'~ gas i s l;]lf~ r~c;S :renlove~ ~lom ~5 sucli lo~-~er l-~~vel of l,1le retort and trans~erred to t;ho sur:t`ac,e ~ g?.S pro~uct tran.sre:r liJle ~6. T]le of'f`
g~.s inc~udes rc~tort, jlJle1; rnixture Wh:iCIl iocs :not ta1~e parl, :in the COI1)!15 t:iO11 proccss, conibusl,ioll ~ras gene:rated in t]lc con~ stion Y.onc, produc1; ~;as ~e]l(.~.ratcd i.n -t,}l~
r~l,c~rt-i.~g ~one, and ca-rboll d;.oxic~e frorn c'.ccornposit;ion of ca:rl~o~lat,es colita:ined il3. tlle f'Orrllal iOII~
J. ~r:i,en1;a1;iorl ~ 3ally f'o~ ations conta.iJi.i.ilr~ o:il sllhl-e }laVe i)eddill~, pl~lie dips of':Lcss -tha]l abou~ ~ , :i.31 ~'rlli,CJI case the ed~es o~ l,he verbi.ca.lly spaoed apait ~o~ s s1-lol~.d be i.n .1 :-` IJ ~ , a]' 1; i n ~ ,' V~ 1' t i (~ lc.~ d !,~3 ~ , 0:~' t llc,s ~,l]l~.jtcinl;-ia11-~- ~r*rl,i.cal s,icle !)~,lnicl.il:ics. Lf ~ e dip - 1~95831 32 .

of l;hc rornlat:i on containing oil sl]a] e is Inore than abou i. 'j , thf! vo;.ds can hc-~ve thc;.r edges offsc t and be 1,:i.3.tf'(~ .Sf3 tl-3Qt thc frc~e faccs Or the i~ tervening :~one of` w-l.l'ra~ne~ cl forma l i on are sul~.stanti ally para] lel 5 to tllc l>eddi~ plane o~` t;lle format ion. Tlle result wo~l.d l.)e a rf~lor1; tha-t; :i s re-orie~lted accordingly to con:.'o:~ln to the bedding p].ane so that the side bounda:ries of tll~ rctort; are perpc~ f~i cular to the beddi.ng plane.
Thi--; provi.des oi l shale llaving approxima-te].y thc same :1.0 k.ero~;erl content .lcro.js the rc Lorti Jl~r z~or f a1; any prlrti cu3.ar 1 i Ine ai-* t]lC! ret~3r1; i.rl~r~; 7.0nc a-3vr~cecj tl-lrou.~rl-l th.e reto:rl,~ A1.s~" f XpaI~fli.ng fOrlllC_ t :i O~ in a (l:i.r c~c tiOll sul~st.-l1)t.i.al.1.y perpenclicll]c~r -I,o t,he bedclin~rl~lane Inaxi-lni.Y,e.s fr~a~rl~?I)l-;lt:i.on of 1J-l~ i.`ormation.
:I 5T}).-.! above d.cscri7Jcd llse of the .i.nvf~3rl;iol~ for :re-coveri ~1{~; car7~ol~accol s vcl].ues includi.ng sllQle oil fro ~sul) l ~ r~ I~JJfe ln f`orlllc-a t ion co:l-)-t ain:in~r oi..l s]lal c~ i s f`or :i.:L:L-ustl ~-t;:ive purposes c)7l:ly~ ld is nol; cons.i~lered to bc .~ :L:inl:itat:i.on o:L' tlle ~sfop- o:L` t}lC' i~ vf~nti;~Il. For 20 Cx;-.~!llr.~].~ e i]--vcntion can 'l~e u3cd iJl a vrl:r:i~ty Or :i l]~ t.~U)(~C!~ hcl-f~ :i l :is de.s:i rrl't)le I c) ~:r :IJarc r ubterr~nca ore ~orln;lt;:i.(>]-l I'or in ji. t;l~ recovery ~jlJf?re tl-le~ r)art.iclc sl.ze .~-ld subcje~Jucnt vo:i-l vol~ le d iri tr:i7)1lti.oll ~,:r t:hf~
O:rC par licles c-l:rc -to l~e coll1;ro'l 1 e(l -t-) 111~3X:; 11;:; :~,e t'he 25 recc)vcry ~r con-ilit;uel~t.s :l fonl t}lC' f'-~:rlrl.lt:ion add-i-l :i orl, in.s l c;lcl o:r loa~ ; upper ;IJJd 1 c~l.rer exl3:10.ci:i ve .oad~s into l;h(- s~llne l):Lc-lst:i~lcr; ho].e, or3e l~ .st-:i n~r llol.e- c~n con-l;a:irl ar~ )}~:,e:r e :~13-l o~ ve ] OiJrl ~r~(l arlOtll- r 7~ l.aSt:; IJ{~; l1G1f~ C.~) COII ;,a:; 11 .1 3.0W~ X}).1 O S j.VC 1 0.;d; ~1lf!l~C~
~30 t1Ie Cel~tr~ O:r ;I~C~.S~S O:f.~ t))(~ )f)c~r ~x~ J~ivf~ d i~i ~t a hi.~e:r eLevation t~ 1 t~ cclit:re Or mCrS~j 0f tllC ~OWf r ex~los~ 7e ].oac~
I~`or e~.lnl~le, wj1;h rcIercrlcf- tc) l`-i,ru:rf~ 5~ tl)erfe j~*
shfu-~rll .1 :fi.rli t 'b:l ~s ti f~ X t rlCI j (;f~ -V~ t:; C;1~
3 5tl3rouf- ;L ~ot]-L .~ upcr ~( l3~ 26 -: r)d a :1 ~ f r ~OIIC~ o.'`
.ln :i~'!1;f!1'Ve~-L.i~l~r~ 0:rl( oi:' wl.'`,r.l~,,-.n~ rf);';;~,i.iOll l)e-~Jl lll~pf':L` ~oici ~r! ~ JIC1 i.J :! ;~(-- ~ VOJ fJi ",;J, 'I`bf 1 C i''i <

1~9`58~3 3 } ~

Sccol~l b] ;ls1~ , hol~? ''6'7 which i s ad jacel~t to the first b]i.lst:inl~r ]~OI.c 2~;0 a:ll(l w]lich ex-tellds vcrt:icaLly from the upl:)er voi ~l , () t.]-lrou{,l-~ i he uppcr ~.or.te ,'~)2 of ullfraglrlented :ro:rrrlation. 1:1le i`irs1. blastillg hole contains a ] ower 5 cy~ :i ndri ~ l explosiv-c .l oad ,'6(; in the lower ~one 264-.
Be:l ow -I]~e lc,wer exp.l osi ve ] oad is a shor-t; s~gmen 1; 26& of s-l elnrl~in~" arl-l above tlle lower explosive ]oad is a longer se~ rl~t 27() of stelr"ni 71C~ fi~ , -Z;he fir~t blastinf~/r holc up i o the upper void 20. Tlle secolld bl astinC~ l~ol.e i s ] 0 loaded wi1;]~l an llp]~e:r cy.l il~flri ca~ e~;p] os-ive load 272 in the uppcr ~OllC 2G2, a:nd a short set,~nerlt 274 of s-temming above i;]~e u))per :loiicl~ Tllc c~n1;r c o:L` In,-l~;s of t]le ~ per exp:l.os::ive l.o~t(1 272 :is at- fl h~ ]ter e 3.e~ :i oll thall tlle c ~,ltl r~ l,:r Ir,.~ c; O:r tl,c~ Jo~7r~r r:~-J-,lOsi.vc! lr)c~l 2G6.
15 ur)I)o~r and ] o~er exp:l.os ive ] oa~ls arc de-l onated at; sep-ara -te time~ci :;.ll a sill~] c round f.`or exp:l os.i ~ely cxI~allclil-~g f`or~llat:ion be lweeIl. 1;hc upper a~-ld ]o~er v~.i.ds toward t;hc ul pc~r .aJI~. lo~er voi ds .
A p:LIlra:L.i. ty of sllch blas t:illg hole.c.. 2t~() and 2~;7 caD
~0 be usec1 ci il~cr acl,jaccll 1 to eac]l ot;}lel-, a.s sllown -in ~ urc 5~ or sp~lcccl aI)clrt rrom cacll o-t]Ler, rn clddi ti.orl~ 1,.1 as-t :i.lll, 11o'1 e~s coll-l,fl:in:illg c)~ 1 y an upI~c:r c~]~):l os:i.ve :l o~lcl and bl.-lsi;i~i~,r ]lolc~s corltainill,r oIlly a lowc~r cxplos;ve load cfln bc llse(3 :i.:.l co~ lJIction ~i t]l blasti)--~ hol(~s o 0nl;ai 25 :i nl~; bo t,]-) upp(~:r alld l owe r exp~ o ~-;:i ve l ofl(ls .

3o

Claims (71)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of recovering shale oil from a subterranean formation con-taining oil shale, which comprises the steps of:
a) excavating an upper void and a lower void vertically spaced apart from each other, the upper void containing at least one support pillar and being substantially directly above the lower void, thereby leaving an interven-ing zone of unfragmented formation between the voids having an upper free face adjacent the upper void and a lower free face adjacent the lower void;
b) drilling a plurality of blasting holes in the unfragmented formation between the upper void and the lower void;
c) loading each blasting hole with explosive and stemming by i) placing a first mass of stemming at the bottom of each blast-ing hole, ii) placing a bottom load of explosive into each blasting hole, iii) placing a second mass of stemming in each blasting hole on top of the bottom load of explosive, iv) placing a middle load of explosive into each blasting hole, v) placing a third mass of stemming into each blasting hole on top of the middle load of explosive, vi) placing a top load of explosive into each blasting hole, and vii) placing a fourth mass of stemming on top of the top load of explosive in each blasting hole, d) detonating explosive in such a pillar to fragment and explosively expand such pillar;
e) expanding formation at each free face of the zone of unfragmented formation adjacent each void toward each void to form a subterranean room con-taining a fragmented permeable mass of formation particles by i) detonating the top load of explosive in each blasting hole after detonating explosive in the pillars, ii) detonating the bottom load of explosive in each blasting hole from about 25 to about 50 milliseconds after detonating the top load of explosive, and iii) detonating the middle load of explosive in each blasting hole from about 75 to about 100 milliseconds after detonating the top load of explo-sive;
f) supplying gas to the top of the fragmented permeable mass in the room for establishing a retorting zone in the fragmented permeable mass and a downward flow of hot gas through the retorting zone; and g) recovering shale oil produced in the retort.

2. The method of claim 1 in which the combined total volume of the voids is in the range of from about 10 to 25% of the total volume of the in situ retort being formed.

3. The method of claim 1 in which detonation of the top load of explosive in each blasting hole is initiated substantially at the top of each such load.

4. The method of claim 3 in which detonation of the bottom load of explosive in each blasting hole is initiated substantially at the bottom of each such load.

5. The method of claim 4 in which detonation of the middle load of explosive in each blasting hole is initiated substantially at the middle of the vertical height of each such load.

6. The method of claim 1 in which the bottom load and the top load have a higher loading ratio than the middle load.

7. The method of claim 6 in which the middle load contains more explosive than both the bottom load and the top load.

8. The method of claim 1 in which the middle load contains more explosive than both the bottom load and the top load.

9. A method for fragmenting a portion of subterranean formation having an upper substantially horizontal free face and a lower substantially horizontal free face spaced below the upper free face the method comprising the steps of:
forming at least one substantially vertical blasting hole in a portion of the formation between the upper free face and the lower free face;
placing explosive in the blasting hole in at least two vertically spaced apart loads, each load being separated from an adjacent load by stemming;
and explosively expanding formation toward both free faces to form a fragmented permeable mass of formation particles by detonating the loads of explosive in a single round of explosions, wherein the time between detonation of each load and an adjacent load is more than the time required for creation of a free face by explosive expansion of formation by detonation of the first of the adjacent loads to be detonated, and wherein the time between detonation of the first load to be detonated and the last load to be detonated is less than the time required for expanding formation beyond a selected void fraction by detonation of the first load to be detonated.

10. The method of claim 9 in which each load in such a blasting hole is detonated at a different time from the other loads in such blasting hole.

11. The method of claim 10 in which the time between detonation of loads detonated successively is sufficient for the wave produced by detonation of the first load to pass the second load to be detonated.

12. The method of claim 9 in which the time between detonation of each load and an adjacent load is greater than about four times the transit time of the primary compression wave from the first of the adjacent loads to be detonated relative to the nearest substantially horizontal free face.

13. The method of claim 9 in which the time between detonation of each load and an adjacent load is greater than about six times the transit time of the primary compression wave from the first of the adjacent loads to be detonated relative to the nearest substantially horizontal free face.

14. The method of claim 9 in which one load of explosive has a different loading ratio than another load of explosive.

15. The method of claim 9 in which one load of explosive contains more explosive than another load of explosive.

16. The method of claim 9 in which each such load is detonated no earlier than detonating any of the loads between such load and one of the free faces of the portion of the formation being fragmented.

17. A method for fragmenting a subterranean formation comprising the stepsof:
excavating an upper void and a lower void vertically spaced apart from each other in the subterranean formation, at least a portion of the upper void being substantially directly above the lower void, thereby leaving an interven-ing zone of unfragmented formation between the voids;
forming a plurality of substantially vertical blasting holes in the intervening zone of unfragmented formation;
placing at least two loads of explosive in such a blasting hole with each of said loads vertically spaced apart from each adjacent load by stemming;
and detonating the loads of explosive in a single round of explosions with a time delay between adjacent loads for expanding formation in the intervening zone toward both voids.

18. The method of claim 17 in which the step of detonating comprises detonating the uppermost loads in each blasting hole before detonating the lower-most loads in each blasting hole.

19. The method of claim 17 in which the step of placing loads of explosive in such a blasting hole comprises placing at least three loads of explosive vertically spaced apart by stemming in such a blasting hole.

20. The method of claim 19 in which the step of detonating comprises sequentially detonating the loads of explosive by detonating the uppermost load of explosive in each blasting hole, then detonating the lowermost load of explosive in each blasting hole, and thereafter detonating the explosive there-between in each blasting hole.

21. The method of claim 19 in which the step of detonating comprises sequentially detonating the loads of explosive by detonating the lowermost load of explosive in each blasting hole, then detonating the uppermost load of explosive in each blasting hole, thereafter detonating the explosive therebetween in each blasting hole.

22. The method of claim 19 in which the step of detonating comprises sequentially detonating the loads of explosive by detonating the uppermost load of explosive in each blasting hole first and detonating the lowermost load of explosive in each blasting hole last.

23. The method of claim 19 in which the step of detonating comprises sequentially detonating the loads of explosive by detonating the lowermost load of explosive in each blasting hole first and detonating the uppermost load of explosive in each blasting hole last.

24. The method of claim 17 in which at least one of the voids contains at least one pillar for supporting formation above the void and the method comprises the additional step of detonating explosive in such a pillar to fragment such pillar before detonating explosive in the blasting holes.

25. A subterranean formation in an intermediate stage of preparation for in situ recovery of constituents from the formation comprising:
a) an upper void and a lower void located at vertically spaced apart elevations within the formation, at least a portion of the upper void being sub-stantially directly above the lower void;
b) a zone of unfragmented formation between the voids;
c) a plurality of substantially vertical blasting holes in the zone of unfragmented formation, each of at least a portion of the blasting holes containing at least two loads of explosive with a segment of stemming above the uppermost load, a segment of stemming below the lowermost load, and a segment of stemming between all adjacent loads; and d) means for detonating the loads of explosive in a single round of explosions with a time delay between adjacent loads so that detonation of explosive will expand formation in the zone of unfragmented formation toward each void and form a subterranean cavity containing a fragmented permeable mass of formation particles.

26. The subterranean formation of claim 25 in which such a blasting hole contains at least an upper load, a middle load, and a lower load of explosive.

27. The subterranean formation of claim 26 wherein each of at least a portion of the blasting holes contains an upper load of explosive, a lower load of explosive, and a middle load of explosive, wherein the means for detonating comprises means for detonating the upper load of explosive in such blasting hole, means for detonating the lower load of explosive in such blasting hole after detonating the upper load of explosive in such blasting hole, and means for detonating the middle load of explosive in such blasting hole after detonat-ing the lower load of explosive in such blasting hole.

28. The subterranean formation of claim 27 in which the means for detonat-ing comprises means for detonating the lower load in such blasting hole from about 25 to about 50 milliseconds after detonating the upper load in such blast-ing hole.

29. The subterranean formation of claim 28 in which the means for detonat-ing comprises means for detonating the middle load in such blasting hole from about 25 to about 75 milliseconds after detonating the lower load in such blast-ing hole.

30. The subterranean formation of claim 27 in which the means for detonat-ing comprises means for detonating the middle load in such blasting hole from about 75 to about 100 milliseconds after detonating the upper load in such blast-ing hole.

31. The subterranean formation of claim 27 in which the middle load con-tains more explosive than each of the lower and upper loads.

32. The subterranean formation of claim 25 in which the combined volume of the voids is in the range of from about 10% to about 25% of the total volume of the subterranean cavity produced after expansion of the formation.

33. The subterranean formation of claim 25 comprising means for initiation of detonation of each load of explosive substantially in the middle of the vertical height of such load.

34. The subterranean formation of claim 25 comprising means for initiation of detonation of the upper load of explosive in such a blasting hole substantial-ly at the top of such load.

35. The subterranean formation of claim 25 comprising means for initiation of detonation of the lower load of explosive in such a blasting hole substantial-ly at the bottom of such load.

36. A method of forming an in situ oil shale retort in a subterranean formation containing oil shale, the method comprising the steps of:
excavating an upper void and a lower void vertically spaced apart from each other, at least a portion of the lower void being substantially directly below the upper void, thereby leaving an intervening zone of unfragment-ed formation between the voids;
forming a plurality of substantially vertical blasting holes in the intervening zone of unfragmented formation;
placing in each of at least a portion of said blasting holes a bottom load of explosive, a top load of explosive, and at least one intermediate load of explosive therebetween with stemming between adjacent loads of explosive; and explosively expanding formation from the intervening zone of unfrag-mented formation toward both voids by detonating all of the loads of explosive in each blasting hole in a single round of explosions with a time delay between adjacent loads in each blasting hole.

37. The method of claim 36 wherein each top load and each bottom load are detonated before at least one of the intermediate loads therebetween.

38. The method of claim 37 in which at least one of the intermediate loads between such a top load and such a bottom load is detonated substantially in the middle of the vertical height of such intermediate load.

39. The method of claim 36 in which detonation of each top load is initiated substantially at the top of such load.

40. The method of claim 39 in which detonation of each bottom load is initiated substantially at the bottom of such load.

41. The method of claim 36 in which detonation of each bottom load is initiated substantially at the bottom of such load.

42. A method for forming an in situ oil shale retort in a subterranean formation containing oil shale by fragmenting a selected portion of the forma-tion having a pair of original substantially horizontal free faces, the method comprising explosively expanding a zone of formation between the free faces toward both free faces by the steps of:
commencing explosive expansion of a first zone of formation adjacent one of said original free faces for creating a first new free face extending substantially parallel to the remaining original free face;
commencing explosive expansion of a second zone of formation adjacent the other one of said original free faces for creating a second new free face extending substantially parallel to the first new free face; and commencing explosive expansion of a third zone of formation adjacent at least one of said new free faces between the first and second zones, the time between commencing expansion of the third zone and commencing expansion of the first zone being less than the time required for completing expansion of the first zone.

43. A method of forming an in situ oil shale retort in a subterranean formation containing oil shale, said in situ retort having top, bottom and side boundaries and containing a fragmented permeable mass of formation particles containing oil shale, comprising the steps of:
excavating a lower void within the boundaries of the retort being formed;
excavating an upper void within the boundaries of the retort being formed and above the lower void, and leaving unfragmented formation between the upper and lower voids;
placing explosive in an upper zone of the unfragmented formation between the upper and lower voids;
placing explosive in a lower zone of the unfragmented formation between the upper and lower voids, the lower zone being below the upper zone;
and detonating explosives in the upper and lower zones in a single round with a time delay between detonation of explosive in the upper zone and detonation of explosive in the lower zone for explosively expanding formation between the upper and lower voids toward the upper and lower voids.

44. The method of claim 43 wherein explosive is placed and detonated by:
forming a plurality of vertically extending blasting holes in the upper and lower zones;
loading a lower explosive load in such a blasting hole in the lower zone;
loading an upper explosive load in such a blasting hole in the upper zone and separated from the lower explosive load by stemming; and detonating the upper and lower explosive loads at separate times in a single round.

45. The method of claim 44 in which detonation of such a lower explosive load is initiated substantially at the bottom of such load for explosively expanding formation in the lower zone primarily toward the lower void.

46. The method of claim 44 in which detonation of such an upper explosive load is initiated substantially at the top of such load for explosively expand-ing formation in the upper zone primarily toward the upper void.

47. The method of claim 43 wherein explosive is placed and detonated by:
forming a plurality of vertically extending blasting holes in the upper and lower zones;
loading a lower explosive load in such a blasting hole in the lower zone;
loading an upper explosive load in such a blasting hole in the upper zone, the center of mass of the upper explosive load being at a higher elevation than the center of mass of the lower explosive load; and detonating the upper and lower explosive loads at separate times in a single round.

48. The method of claim 43 wherein explosive is placed and detonated by:
forming a plurality of vertically extending blasting holes in the upper zone;
forming a plurality of vertically extending blasting holes in the lower zone;
loading a lower explosive load in such a blasting hole in the lower zone;
loading an upper explosive load in such a blasting hole in the upper zone, the center of mass of the upper explosive load being at a higher elevation than the center of mass of the lower explosive load; and detonating the upper and lower explosive loads at separate times in a single round.

49. The method of claim 48 wherein such a blasting hole in the lower zone containing a lower explosive load is formed adjacent to such a blasting hole in the upper zone containing an upper explosive load.

50. The method of claim 49 wherein such a blasting hole in the lower zone adjacent to a blasting hole in the upper zone is formed to have an upper portion extending into the upper zone.

51. The method of claim 50 including the step of loading stemming in the upper portion of such blasting hole in the lower zone adjacent to a blasting hole in the upper zone.

52. A method of forming an in situ oil shale retort in a subterranean formation containing oil shale, said in situ retort having top, bottom and side boundaries and containing a fragmented permeable mass of formation particles containing oil shale, comprising the steps of:
excavating a lower void within the boundaries of the retort being formed;
excavating an upper void within the boundaries of the retort being formed and above the lower void, and leaving unfragmented formation between the upper and lower voids, such unfragmented formation having an upper zone and a lower zone, the lower zone being below the upper zone;
forming a first set of vertically extending blasting holes, where the blasting holes of the first set extend only in the upper zone;
forming a second set of vertically extending blasting holes, where the blasting holes of the second set extend in both the upper and lower zones, and such a blasting hole of the second set is adjacent a blasting hole of the first set;
loading an upper explosive load in such a blasting hole of the first set adjacent a blasting hole of the second set and loading a lower explosive load in such adjacent blasting hole of the second set, where the center of mass of the upper explosive load is at a higher elevation than the center of mass of the lower explosive load;
loading stemming in such adjacent blasting hole of the second set above the lower explosive load; and detonating the upper and lower explosive loads at separate times in a single round for explosively expanding formation between the upper and lower voids toward the upper and lower voids.

53. A subterranean formation in an intermediate state of preparation for in situ recovery of constituents from the formation comprising:
a) an upper void and a lower void located at vertically spaced apart elevations within the formation, at least a portion of the upper void being substantially directly above the lower void;
b) a zone of unfragmented formation between the voids;
c) explosive in an upper zone of the unfragmented formation between the upper and lower voids;
d) explosive in a lower zone of the unfragmented formation between the upper and lower voids, the lower zone being below the upper zone; and e) means for detonating explosives in the upper and lower zones in a single round with a time delay between detonation of explosive in the upper zone and detonation of explosive in the lower zone for explosively expanding formation between the upper and lower voids toward the upper and lower voids.

54. The subterranean formation of claim 53 including a plurality of vertically extending blasting holes in the upper and lower zones, where there is a lower explosive load in such a blasting hole in the lower zone and an upper explosive load in such a blasting hole in the upper zone, the center of mass of the upper explosive load being at a higher elevation than the center of mass of the lower explosive load, wherein the detonating means comprises means for detonating the upper and lower explosive loads at separate times in a single round.

55. The subterranean formation of claim 53 including a plurality of vertically extending blasting holes in the upper zone and a plurality of vertically extending blasting holes in the lower zone, where there is a lower explosive load in such a blasting hole in the lower zone and an upper explosive load in such a blasting hole in the upper zone, the center of mass of the upper explosive load being at a higher elevation than the center of mass of the lower explosive load, wherein the detonating means comprises means for detonating the upper and lower explosive loads at separate times in a single round.

56. The subterranean formation of claim 55 wherein such a blasting hole in the lower zone containing a lower explosive load is adjacent to such a blast-ing hole in the upper zone containing an upper explosive load.

57. The subterranean formation of claim 56 wherein such a blasting hole in the lower zone adjacent to a blasting hole in the upper zone has an upper portion extending into the upper zone.

58. The subterranean formation of claim 57 including stemming in the upper portion of such blasting hole in the lower zone adjacent to a blasting hole in the upper zone.

59. A method for forming an in situ oil shale retort in a subterranean formation containing oil shale comprising the steps of:
excavating within the formation a pair of spaced apart voids and leav-ing an intervening zone of unfragmented formation between the voids, the inter-vening zone having substantially parallel free faces adjoining the voids;
forming a plurality of elongated blasting holes in the intervening zone of unfragmented formation, the longitudinal axis of each blasting hole being substantially perpendicular to the parallel free faces of the intervening zone;
placing at least two cylindrical loads of explosive in such a blasting hole with the loads longitudinally spaced apart from each adjacent load by stemming; and detonating the loads of explosive in a single round of explosions with a time delay between adjacent loads in such a blasting hole for expanding formation in the intervening zone toward both voids.

60. The method of claim 59 in which the combined total volume of the voids is in the range of from about 10 to 25% of the total volume of the in situ retort being formed.

61. The method of claim 59 in which the time between detonation of loads detonated successively is sufficient for the wave produced by detonation of the first load to pass the second load to be detonated.

62. The method of claim 59 in which the time between detonation of each load and an adjacent load is greater than about four times the transit time of the primary compression wave from the first of the adjacent loads to be detonated relative to the nearest substantially horizontal free face.

63. The method of claim 59 in which the time between detonation of each load and an adjacent load is greater than about six times the transit time of the primary compression wave from the first of the adjacent loads to be detonated relative to the nearest substantially horizontal free face.

64. A subterranean formation in an intermediate stage of preparation for in situ recovery of constituents from the formation comprising:
a pair of spaced apart voids with an intervening zone of unfragmented formation therebetween, the intervening zone having substantially parallel free faces adjoining the voids;
a plurality of elongated blasting holes in the intervening zone of unfragmented formation, the longitudinal axis of each blasting hole being sub-stantially perpendicular to the parallel free faces of the intervening zone;
at least two cylindrical loads of explosive in such a blasting hole with the loads longitudinally spaced apart from each adjacent load by stemming;
and means for detonating the loads of explosive in a single round of explosions with a time delay between adjacent loads in such a blasting hole for expanding formation in the intervening zone toward both voids and for forming a subterranean cavity containing a fragmented permeable mass of formation particles.

65. The subterranean formation of claim 64 comprising means for initiation of detonation of each load of explosive substantially in the middle of the vertical height of such load.

66. The subterranean formation of claim 64 wherein each of at least a portion of the blasting holes contains an upper load of explosive, a lower load of explosive and a middle load of explosive, wherein the means for detonating comprises means for detonating the upper load of explosive in such blasting hole, means for detonating the lower load of explosive in such blasting hole, after detonating the upper load of explosive in such blasting hole, and means for detonating the middle load of explosive in such blasting hole after detonating the lower load of explosive in such blasting hole.

67. The subterranean formation of claim 66 in which the means for detonat-ing comprises means for detonating the lower load in such blasting hole from about 25 to about 50 milliseconds after detonating the upper load in such blast-ing hole.

68. The subterranean formation of claim 67 in which the means for detonat-ing comprises means for detonating the middle load in such blasting hole from about 25 to about 75 milliseconds after detonating the lower load in such blast-ing hole.

69. The subterranean formation of claim 66 in which the means for detonat-ing comprises means for detonating the middle load in such blasting hole from about 75 to about 100 milliseconds after detonating the upper load in such blasting hole.

70. A method of fragmenting a subterranean formation to produce an in situ retort, comprising excavating an upper void and a lower void vertically spaced apart from each other in the subterranean formation and separated by an inter-vening zone of unfragmented formation, at least a portion of the upper void being substantially directly above the lower void; placing a plurality of loads of explosive in at least an upper region and a lower region of the intervening zone of unfragmented formation; and explosively expanding formation in the inter-vening zone towards both voids by detonating the loads of explosive in a single round of explosions with a time delay between detonation of loads of explosive in different regions of the intervening zone.

71. A subterranean formation in an intermediate stage of preparation of an in situ retort, comprising an upper void and a lower void vertically spaced apart from each other in the subterranean formation and separated by an intervening zone of unfragmented formation, at least a portion of the upper void being sub-stantially directly above the lower void; a plurality of loads of explosive located in at least an upper region and a lower region of the intervening zone of unfragmented formation; and means for detonating the loads of explosive in a single round of explosions with a time delay between detonation of loads of explosive in different regions of the intervening zone.