US3323594A

US3323594A - Method of fracturing subsurface formations

Info

Publication number: US3323594A
Application number: US421208A
Authority: US
Inventors: Jimmie L Huitt; Bruce B Mcglothlin
Original assignee: Gulf Research and Development Co
Current assignee: Gulf Research and Development Co
Priority date: 1964-12-28
Filing date: 1964-12-28
Publication date: 1967-06-06
Anticipated expiration: 1984-06-06
Also published as: GB1100110A

Description

June 6, 1967 J. L. HUITT ETAL METHOD OF FRACTURING SUBSURFACE FORMATIONS Filed Dec. 28, 1964 BRUCE 8. A4: 62 07/ll //V United States Patent 3,323,594 METHUD 6F FRAQTURHNG SUBSURFACE FORMATIONS Jimmie L. Huitt, Glenshaw, and Bruce B. McGlothlin, OHara Township, Allegheny County, Pa, assignors to Gulf Research 8; Development Company, Pittsburgh, Pan, a corporation of Delaware Filed Dec. 28, 1964, Ser. No. 421,208 9 Claims. (Cl. 166-4-2) This invention relates to a method of increasing producti on from fluid-bearing underground formations penetrated by a well, and more particularly relates to a method of propping fractures in relatively soft formations subjected to high overburden pressures causing excessive embedment of propping agent particles in the face of the fracture.

Hydraulic fracturing of underground formations has been widely used in recent years as a method of stimulating production of fluids from underground formations penetrated by wells. In the hydraulic fracturing process, a fracturing liquid is pumped down the well. to subject a portion of the formation exposed at the borehole of the well to a hydraulic pressure adequate to cause the formation rock to rupture and thereby result in a fracture extending from the well. Additional fracturing liquid is pumped down the well and into the fracture to extend it for the desired distance from the well. A slurry of propping agent particles in a carrying liquid is displaced into the fracture to deposit the particles in the fracture and prevent it from closing when the pressure on the carrying liquid is released.

It has been found that fractures of maximum flow capacity can be obtained in hard formations by the deposition of the propping agent in the fracture in a partial monolayer. The particles of propping agent are deposited in the fracture in a concentration adequate to hold the faces of the fracture apart while leaving substantial space between the particles for the flow of formation fluids.

If the underground formation is relatively soft, the particles of propping agent may become embedded in the face of the fracture and thereby fail to hold the fracture open. Deformable propping agents which compress to provide a larger bearing surface against the face of the fracture have been used to reduce embedment of the propping agent in the farcture; however, many relatively soft formations are subjected to high overburden pressures and even the deformable propping agents become completely embedded and are not effective in holding the faces of the fracture apart. Moreover, if the concentration of the particles of deformable propping agents in the fracture is high, the openings between'the particles are small and the deformation of the propping agent particles when subjected to the weight of the overburden tends to close the openings between the particles and thereby further reduce the flow capacity of the fracture.

This invention resides in a method of forming a multilayer pack of rigid, substantially spherical propping agent particles in a fracture in a relatively soft formation exposed to an overburden pressure approaching that sufficient to cause substantially complete embedment of a full monolayer of propping agent particles. In the process through the fracture Without substantial loss to the adjacent formation until the outer boundary of the sealed faces of the fracture is reached and then flows: into the formation to deposit the propping agent in the fracture. Loss of liquid into the formation beyond the sealed faces of the fracture results in screening out of the propping agent in the vicinity of the outer boundary of the sealed faces. On continued displacement of the suspension of propping agent into the fracture, the particles of propping agent build up against the screen-out to form. a multilayer pack of propping agent from substantially the outer boundary of the sealed faces of the fracture to the well. Thereafter pressure in the well is reduced to allow formation fluids to flow through the fracture into the well through which such fluids flow to the surface.

In the drawings:

FIGURE 1 is a diagrammatic fragmentary vertical sectional view of a well from which a fracture has been made to extend into the surrounding formation;

FIGURE 2 is a diagrammatic fragmentary vertical sectional view of the well of FIGURE 1 after a portion nearest the well of the faces of the fracture has been sealed; and

FIGURE 3 is a diagrammatic fragmentary sectional view, similar to FIGURES l and 2, of the well during the deposition of a multilayer of propping agent in the fracture.

Referring to FIGURE 1, the lower portionof casing 10 of a well is illustrated penetrating a fluid-bearing forrnvation 12 that is to be fractured by the method of this invention. Casing it is shown surrounded by a cement sheath 14 positioned by conventional cementing procedures. As illustrated in FIGURE 1, casing 10 is severed at 16 by any suitable means such as mechanical milling or a shaped charge to expose a portion of formation 12. Because this invention is of principal utility in the creation of fractures of high fluid-carrying capacity in soft formations, it will ordinarily be desirable to set and cement casing through the fluid-bearing formation to support the formation around the borehole: and to create the fracture through a notch cut through the casing and extending outwardly into the surrounding formation. This invention is not so limited; however, and may be used when the fracture is created through perforations in the casing, or may be used to fracture formations having adequate strength to allow completion with an open borehole.

A fracture 18 is initiated from the well by pumping a penetrating liquid down through casing 10 and increasing the pressure until breakdown of the formation occurs. Suitable penetrating liquids are water, heavier aqueous liquids such as brines, and hydrocarbon oils. Frequently, an initial slug of dilute hydrochloric acid is advantageous as a penetrating liquid. A penetrating liquid is used during the extension as Well as the initiation of the fracture to avoid sealing the faces of the outer extremities of the fracture. The penetrating liquid is pumped into the fracture at a high rate exceeding 30 barrels per minute, and may ex ceed barrels per minute if the capacity of the pumping equipment available is adequate, to extend the fracture a desired distance such as .50 to feet, preferably approximately 100 feet from the well. Fractures of larger radial extent can be used but usually are not economic because they involve excessive costs for pumping and wellhead equipment. The combination of the high pumping rate and a high density liquid is effective in scouring the fracture faces and carrying solids washed from the fracture faces to the outer extremities of the fracture.

The penetrating liquid is followed by a liquid containing a fluid-loss reducing additive adapted to form a seal 20 on the faces of the fracture adjacent the well. The seal 20, which is formed by filtration of the fluid-loss reducing additive from liquid flowing across the faces of the fracture into the formation 12, greatly retards the loss of a subsequent liquid across the faces of the fracture and causes subsequently injected liquid to flow radially out- Ward through the fracture beyond the outer boundary of the seal. The liquid containing the fiuidloss reducing additive, hereinafter referred to as the low fluid loss liquid, is pumped into the fracture at substantially the same rate as the penetrating liquid used to initiate and extend the fracture.

Any of the conventional fluid-loss reducing additives that will form a substantially impervious but easily removed seal on the faces of the fracture can be used. Examples of such materials are guar gum, karaya gum, blown asphalt, and additives of the type described in U.S. Patent No. 2,779,735. A preferred fluid-loss reducing additive is silica flour suspended in water gelled with guar gum. The concentration of the additive in the liquid will depend on the pumping rate, formation characteristics, and the particular additive used but usually will be in the range of .003 to .25 pound per gallon.

It is important to this invention that the amount of fluidloss reducing additive be such that the seal 20 is formed over only a portion of the faces of the fracture and leaves the faces 22 of fracture 18 at its outer extremity unplugged. A method for calculating the fracture area and the volume of liquid required to seal temporarily a given fracture area is described in The Petroleum Engineer, volume 31, Nos. 4 and 5, April and May of 1959. Another method is described in the booklet entitled Fracplan issued by Halliburton Company in December of 1960. It is preferred that the seal extend from the well for a distance of at least 25 feet to provide a propped fracture of substantial radial extent and not over about 75 percent of the radius of the fracture to insure rapid loss of carrying liquid in the unsealed part of the fracture. For example, if the fracture 18 extends radially for a distance of 100 feet from the well, it is desirable to form the seal 20 on the faces of the fracture from the Well outwardly a distance of only 25 to 75 feet. A seal of that extent can be formed by 1000 to 5000 gallons of water gelled with guar gum and containing 0.1 pound silica flour per gallon.

The low fluid loss liquid is followed by a penetrating carrying liquid devoid of fluid-loss reducing additive having particles of a rigid, substantially spherical propping agent suspended in it. In this invention it is desirable that the carrying liquid flow easily into the formation beyond the seal 20 and through the multilayer pack to the outer extremities of the fracture after the screen-out commences. The term devoid of fluid-loss reducing additive used in describing the carrying liquid means that the carrying liquid contains only incidental amounts of such additives such as may be inadvertently picked up by the carrying liquid. For most effective use in this invention, it is not desirable to incorporate any fluid-loss reducing additive in the carrying liquid. The carrying liquid is displaced into fracture 18 at a low rate, less than barrels per minute, to reduce washing of seal from the faces of the fracture. Because of seal 20, little of the carrying liquid is lost from the fracture 18 until it flows beyond the outer boundary of the seal; hence, the carrying liquid is effective in carrying the prop ping agent to the outer limits of the seal 20. Loss of liquid through the unsealed faces 22 of the fracture causes deposition of the propping agent in the fracture in a multilayer pack 24 at the outer boundary of the sealed portion of the fracture. Continued displacement of the carrying liquid with propping agent suspended in it results in build-up of multilayers of propping agent in the fracture from the outer boundary of the sealed portion 20 to the well. As the pack 24 builds up toward the Well, carrying liquid from which the propping agent has been screened by pack 24 filters through the pack and then into the formation through unsealed faces 22. A sharp increase on the pressure on the carrying liquid which occurs when the screenout fills the fracture all of the way to the well indicates completion of the filling of the fracture and deposition within the Well bore. Thereafter pressure in the well is reduced to a pressure below the formation pressure, and the resultant flow of formation fluids flushes the seal from the faces of the fracture and into the well where it is removed with the production of the formation fluids. If the formation pressure is not adequate to cause the formation fluids to flow to the Wellhead, suitable lifting apparatus is used for producing the fluids.

Because of the small spaces between the particles of propping agent in the multilayer pack, it is essential that the propping agent be rigid, substantially spherical, have a highly uniform size to provide a pack having a high permeability, and have a strength which will preclude crushing of the particles. The particles of propping agent may have a size spanning 5 numbers or less within the range of 4 to 40 mesh of the U.S. Sieve Series. Suitable propping agent particle sizes are 8 to 12, 12 to 20, and 20 to 40 mesh. To provide a pack of maximum flow capacity, it is highly preferred to use a very narrow range of sizes of particles such as a range in which percent or more of the particles pass through one screen in the U.S. Sieve Series and are retained on the next smaller screen in the series. The particles should have an average roundness and sphericity of at least 0.8. Roundness and sphericity are defined in Stratigraphy and Sedimentation by Krumbein and Sloss, pages 78 through 83, published by W. H. Freeman Company, 1951 edition. The propping agent is suspended in the carying liquid in a concentration of 1 to 10 lbs/gal. for transporting into the fnacture.

Propping agents which are particularly suitable for use in this invention are substantially spherical glass beads ob tained by the rapid quenching of molten glass particles from a temperature in excess of 1800 F. to a temperature lower than 900 F. in a gas or in a liquid having a viscosity higher than the viscosity of water. The temperature of the quenching medium should not exceed 400 F. The rapid quenching of molten glass particles results in substantially spherical glass particles having a roundness and sphericity higher than 0.8 and characterized by an L/D ratio, referred to as loading strength, exceeding 50,000 p.s.i. when tested between steel plates having a 35 Rockwell C hardness, where L is the maximum compressive load in pounds that a particle can carry and D is the diameter of the particle in inches. The rapid quenching of molten glass globules allows the production of rigid particles having L/D ratios as high as 250,000 p.s.i. by suitable adjustment of glass compositions and treating conditions; however, the rapid quenching is effective in producing spherical glass particles of the desired strength from soda-lime glasses as Well as other glasses such as borosilicate and lead borosilicate glasses. Because this invention is of principal utility in the propping of relatively soft formations, glass particles having an L/D ratio higher than 30,000 p.s.i. are satisfactory and can be used safely to prop fractures in many formations without danger of crushing.

A preferred propping agent for use in this invention is a low density, substantially spherical glassy particle. Such particles, having a specific gravity as low as about 1.3, can be prepared, for example, by the rapid quenching of slaglike materials. Metallurgical slags, such as blast furnace slag and silica manganese slag, are suitable for the preparation of the slag spheres. Suitable propping agents have been prepared from silica manganese containing 39.2 percent silica, 24.8 percent alumina, 13 percent calcium oxide, 12.9 percent manganese, 6.6 percent magnesium oxide, and 3.5 percent barium oxide. Alumina silicate slags can also be used for the preparation of the high strength, low density particles. The slags are heated to a temperature about 400 above their melting point and quenched in a liquid having a viscosity greater than water or in a solid material such as dry carbon flour to a temperature below 900 F. Such slag particles have a specific gravity in the range of 1.3 to 1.7 as compared with ordinary glass beads which have a specific gravity of approximately 2.6. By suspending a low density rigid, substantially spherical propping agent in a carrying liquid having a high density, preferably a specific gravity higher than 1.2, such as a solution of sodium chloride, calcium chloride, and zinc chloride, the tendency of the propping agent to settle from the carrying liquid is reduced and the carrying of the propping agent to the outer boundary of the sealed portion of the fracture is more surely accomplished.

The process of this invention is effective in providing fractures of high flow capacity in those formations which because of the strength of the formation and the high overburden pressure to which .it is subjected cannot be efl'ectively propped with a monolayer of propping agent. The rigidity of the propping agent particles used as well as their high roundness and uniform size provides a multilayer pack having a high permeability which is not decreased substantially by deformation of the particles when the pack is subjected to the compressive load of the overburden. The formation of a substantially impermeable but temporary seal on only the portion of the faces of the fracture nearest the well insures flow of the carrying fluid outwardly for a substantial distance in the fracture before it is lost to the formation, and thereby causes the initial screen-out to occur at a substantial distance from the well and build the multilayer pack from the outer boundary of the sealed portion to the borehole wall of the well.

We claim:

1. A method for increasing the productivity of a subterranean formation penetrated by a well comprising pumping a penetrating liquid down the well to the formation and increasing the pressure on the penetrating liquid to rupture the formation rock and thereby create a fracture extending from the well, pumping liquid containing a fluid-loss reducing additive in an amount adapted to form a seal on the faces of the fracture outwardly from the well only a portion of the radial extent of the fracture, and thereafter pumping into the well and displacing into the fracture a second penetrating liquid having suspended therein substantially spherical particles of a rigid propping agent, said second penetrating liquid having a concentration of particles of propping agent suspended therein and being pumped at a rate such that screen-out of the particles occurs in the vicinity of the outer boundary of the seal formed on the faces of the fracture, and continuing the pumping of the second penetrating liquid having propping agent suspended therein to fill the fracture from the screen-out to the borehole with a multilayer pack of propping agent particles.

2. A method of increasing the productivity of a subterranean formation penetrated by a well comprising pumping a penetrating liquid down the Well and into the formation, increasing the pressure on the penetrating liquid to rupture the formation rock and create a fracture extending outwardly into the formation from the well, displacing down the well and into the fracture a low fluid loss liquid containing a fluid-loss reducing additive in an amount adapted to seal the faces of the fracture outwardly from the well for a distance of at least 25 feet but not over 75 percent of the distance from the well to the outer boundary of the fracture, pumping a carrying liquid devoid of sealing material and having suspended therein substantially spherical particles of a rigid propping agent, the rate of pumping said carrying liquid being less than about barrels per minute to avoid removing the seal from the faces of the fracture, and continuing pumping the carrying liquid containing the propping agent to form a multilayer pack of propping agent in the fracture adjacent the well.

3. A method of increasing the productivity of a subterranean formation penetrated by a well comprising pumping a penetrating liquid down the well into contact with the formation and increasing the pressure thereon to create a fracture extending 50 to 125 feet from the well, thereafter pumping down the well and into the fracture a low fluid loss liquid containing a fluid loss reducing additive in an amount to seal the faces of the fracture from the well outward for a distance from a minimum of 25 feet to a maximum of 75 percent of the radius of the fracture, displacing down the well and into the fracture at a rate adapted to cause screen out of the propping agent in the fracture a carrying liquid substantially devoid of fluid loss additive having suspended therein glass spheres having a particle size spanning not more than 5 screens in the range of 4 to 40 mesh in the U.S. Sieve Series, an average roundness and sphericity of at least 0.8, and a loading strength of at least 30,000 p.s.i., and continuing the displacement of the carrying liquid down the well to build a multilayer pack of propping agent' in the fracture from the outer region of the sealed faces of the fracture to the well.

4. A method of increasing the productive capacity of a subterranean formation penetrated by the borehole of a well comprising pumping a penetrating liquid down the well and into contact with the subterranean formation, increasing the pressure on the penetrating liquid to rupture the formation rock, continuing pumping the penetrating liquid at a rate of at least 30 barrels per minute to extend the fracture for a distance of at least 50 feet from the well, pumping a low fluid loss liquid down the well and into the fracture at a rate of at least 30 barrels per minute, said low fluid loss liquid containing a fluid loss reducing additive in an amount adapted to seal the faces of the fracture adjacent the well for a distance from a minimum of 25 feet to a maximum of 75 percent of the radius of the fracture, thereafter pumping down the well and into the fracture at a rate less than 10 barrels per minute a carrying liquid having suspended therein glass beads to build a multilayer pack of the glass beads from the outer region of the sealed faces of the fracture to the well, said glass beads having a. loading strength of at least 30,000 p.s.i., an average roundness and sphericity of at least 0.8, and a particle size such that at least percent of the particles pass through a screen in the range of 4 to 35 mesh in the U.S. Sieve Series and are retained on. the next smaller screen in the series.

5. A method of increasing the productivity of a sub terranean formation penetrated by a well comprising pumping a penetrating liquid down the well and into cont-act with the subterranean formation, increasing the pressure on the penetrating liquid to rupture the formation rock and create a fracture extending therefrom, pumping down the well and into the fracture a low fluid loss liquid having a fluid loss reducing additive incorporated therein, the amount of low fluid loss liquid and fluid loss reducing additive incorporated therein being adapted to seal the faces of the fracture adjacent the well for a distance of at least 25 feet from the well and not exceeding 75 percent of the radius of the fracture, pumping down the well and into the fracture at a rate not exceeding 10 barrels per minute a suspension of glass beads in a brine devoid of fluid loss reducing additive whereby brine is lost from the fracture adjacent the outer boundary of the sealed portion of the fracture to cause a screen-out of glass beads in the fracture, and continuing the pumping of the brine having glass beads suspended therein down the well and into the fracture whereby the brine filters through the screen-out glass particles and builds a multilayer of glass particles in the fracture to the well bore, said glass beads having a loading strength of at least 50,000 p.s.i. and a particle size spanning not more than 5 screens in the U.S. Sieve Series between 4 and 40 mesh, and said brine having a specific gravity of at least 1.2.

6. A method as set forth in claim 1 in which the propping agent consists of glassy particles having a loading strength of at least 30,000 p.s.i., a specific gravity of 1.3 to 1.7, and a particle size spanning not more than 5 screens of the U.S. Sieve Series in the range of 4 to 40 mesh.

7. A method as set forth in claim 1 in which the well has casing set through the formation and a continuous, substantially horizontal notch is cut through the casing and into the surrounding formation prior to pumping the penetrating liquid down the well.

8. A method as set forth in claim 2 in which the rigid propping agent is composed of glass beads having a loading strength of at least 50,000 p.s.i. and having been prepared by quenching molten glass globules from a temperature of 1800 F. to a temperature below 900 F. in a quenching medium selected from the group consisting of liquids having a viscosity greater than the viscosity of Water and gases.

References Cited UNITED STATES PATENTS 3,239,006 3/1966 Fast 166-42 3,242,032 3/1966 Schott l6642 3,245,866 4/1966 Schott 166--42 CHARLES E. OCONNELL, Primary Examiner.

N. C. BYERS, Assistant Examiner.

Claims

5. A METHOD OF INCREASING THE PRODUCTIVITY OF A SUBTERRANEAN FORMATION PENETRATED BY A WELL COMPRISING PUMPING A PENETRATING LIQUID DOWN THE WELL AND INTO CONTACT WITH THE SUBTERRANEAN FORMATION, INCREASING THE PRESSURE ON THE PENETRATING LIQUID TO RUPTURE THE FORMATION ROCK AND CREATE A FRACTURE EXTENDING THEREFORM, PUMPING DOWN THE WELL AND INTO THE FRACTURE A LOW FLUID LOSS LIQUID HAVING A FLUID LOSS REDUCING ADDITIVE INCORPORATED THEREIN, THE AMOUNT OF LOW FLUID LOSS LIQUID AND FLUID LOSS REDUCING ADDITIVE INCORPORATED THEREIN BEING ADAPTED TO SEAL THE FACES OF THE FRACTURE ADJACENT THE WELL FOR A DISTANCE OF AT LEAST 25 FEET FROM THE WELL AND NOT EXCEEDING 75 PERCENT OF THE RADIUS OF THE FRACTURE, PUMPING DOWN THE WELL AND INTO THE FRACTURE AT A RATE NOT EXCEEDING 10 BARRELS PER MINUTE A SUSPENSION OF GLASS BEADS IN A BRINE DEVOID THE FLIUD LOSS REDUCING ADDITIVE WHEREBY BRINE IS LOST FROM THE FRACTURE ADJACENT THE OUTER BOUNDARY OF THE SEALED PORTION OF THE FRACTURE TO CAUSE A SCREEN-OUT OF GLASS BEADS IN THE FRACTURE, AND CONTINUING THE PUMPING OF THE BRINE HAVING GLASS BEADS SUSPENDED THEREIN DOWN THE WELL AND INTO THE FRACTURE WHEREBY THE BRINE FILTERS THROUGH THE SCREEN-OUT GLASS PARTICLES AND BUILDS A MULTILAYER OF GLASS PARTICLES IN THE FRACTURE TO THE WELL BORE, SAID GLASS BEADS HAVING A LOADING STRENGTH OF AT LEAST 50,000 P.S.I. AND A PARTICLE SIZE SPANNING NOT MORE THAN 5 SCREENS IN THE U.S. SIEVE SERIES BETWEEN 4 AND 40 MESH, AND SAID BRINE HAVING A SPECIFIC GRAVITY OF AT LEAST 1.2.