US3380527A - Oil production by vertical steam drive - Google Patents

Description

April 30, 1968 E. M. CRAIGHEAD 3,380,527

OIL PRODUCTIQN BY VERTICAL STEAM DRIVE Filed Oct. 21, 1965 PRODUCTION FIG. 3

INVENTOR E. M. CRAIGHEAD F/G.

A T TORNEYS United States Patent Gfice 3,38%,527 Patented Apr. 30, 1968 ABSTRACT OF THE DKSCLOSURE Oil is produced from an oil stratum adjacent and open to a gas stratum, the strata being penetrated by an injection well and a production well, by injecting steam either directly into the gas stratum, only, or directly, into a section of the oil stratum remote from the gas stratum, only,

to form a condensate barrier in the gas cap and apply steam pressure downwardly on the oil in the oil stratum while opening the prcduction well to flow from the oil stratum only at a level remote from the gas stratum.

This invention relates to a process for producing oil from an oil stratum by vertical steam drive from an adjacent gas stratum.

The occurrence of a gas stratum, such as a gas cap, adjacent an oil stratum is rather prevalent. In conducting a steam drive in such an oil stratum it has heretofore been considered disadvantageous in that the loss of steam to the gas stratum has been a substantial problem. The instant invention makes use of the adjacent gas stratum in applying a vertical steam drive to the oil stratum utilizing a specific technique of operation.

Accordingly, a principal object of the invention is to provide a process for producing oil from an oil stratum lying adjacent a gas stratum, utilizing steam as a heating and driving agent principally from the gas stratum. Another object is to provide a steam drive process which is effective in an oil stratum lying adjacent a gas cap. Other objects of the invention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

In accordance with the invention, steam is injected into a gas stratum lying adjacent an oil stratum so as to build up a substantially upright bank of aqueous liquid from steam condensate across the gas stratum so as to force the steam vertically against the oil in the oil stratum, the oil stratum is open to flow in the production well(s) only at a level remote from the gas stratum so that production must be thru a substantial section of the stratum and steam cannot escape directly into the production well(s) from the gas stratum. Steam injection is continued so as to move the aqueous liquid bank toward the production well(s) and increase the area of the oil stratum to which the vertical steam drive is applied. The steam drive heats and displaces oil from the oil stratum and maintains a liquid bank of hot highly fluid oil thru the area of the oil stratum which is contacted by the steam. This hot oil layer is extended toward the production well as the process continues and the aqueous liquid bank is moved closer to the production well. Eventually, a liquid bank is driven beyond the production well and a section of the oil stratum adjacent and beyond the production well is subjected to vertical steam drive so as to heat, render more fluid, and displace oil into the production well at a level remote from the gas stratum. This technique of operation assumes that wells in the oil stratum beyond the production wells are not open to flow so that a pressure differential exists between the steam in the gas stratum and the pressure in the production well.

Generally the gas stratum is a gas cap lying above the oil stratum but it also occurs in some instances as a subjacent stratum. The invention is operable in either situation. The gas stratum is essentially saturated with gas but may also contain minor amounts of oil and water with no appreciable permeability barrier between the two strata.

A more complete understanding of the invention may be had by reference to the accompanying schematic drawing of which FIGURES 1, 2, and 3 are elevations thru a gas cap and an oil stratum penetrated by a central injection well and a ring of production wells with certain essential equipment for performing the process.

Referring to FIGURE 1, stratum 10 lies below a gas cap 12 and these strata are penetrated by an injection well 14 and production wells 16 which may represent wells in a ring or in parallel lines of production wells lying on either side of a line of injection Wells 14. Well 14 is provided with a casing 18 extending thru the strata and with a tubing string 20 extending into oil stratum 1t Casing 18 is perforated in a lower section of stratum 10 by perforations 22. A steam injection line 24, connected with a steam boiler or generator not shown, connects with tubing string 20 and contains a control valve 26.

Production wells 16 are provided with casings 28 and tubing strings 30, the casings extending thru the oil stratum and being perforated by perforations 32 at a lower level of the oil stratum remote from gas cap 12. Packers 34 may be set in the production Wells to seal off the annulus above the oil stratum or above the gas cap as shown. Production lines 36 connect with tubing strings 39 of the production wells and contain control valves 33.

In the operation of the process utilizing the arrangement illustrated in FIGURE 1, steam is injected thru line 24 from any suitable source and is forced thru tubing string 20 and perforations 22 into a lower section of the oil stratum. As the injection of steam continues, oil is heated and displaced from the oil stratum into perforations 32 in the production wells as illustrated by arrows 4t and a substantial layer 42 of hot more fluid oil is established. The steam injected into the stratum breaks thru into gas cap 12 as illustrated by arrows 44 and a bank of aqueous liquid 46 formed by steam condensate in the relatively cool gas cap is formed and extends substantially upright across the gas cap, forming a. seal or barrier to the flow of steam away from the injection well and toward the production wells. The resistance to the flow of steam provided by liquid bank 46 has the effect of forcing the steam to apply a vertical drive on the oil in the oil stratum as illustrated by arrows 48.

Steam does not break thru liquid bank 46 because the liquid bank is self-sealing in that steam penetrating this bank contacts relatively cool stratum in the gas cap and causes condensation to form more liquid seal. Thus, there is a generally annular liquid seal surrounding well 14 within gas cap 12 which encloses a steam bubble under pressure and a substantial portion of this pressure is applied to and is efiiective in moving hot oil layer 42 toward the production wells and, specifically, perforations 32- in the lower section of the oil stratum. The produced oil is recovered from wells 16 by pumping or gas lift, depending upon the pressure differential between the surrounding stratum and the production well. It is to be understood that oil is being produced in the production wells substantially from the time steam injection commences or shortly thereafter and does not depend upon the arrival of hot, more fluid oil from layer 42. This layer increases in volume and thickness as the process continues and substantially sweeps the stratum of oil under the impetus of the vertical steam drive.

Steam has several advantages as a pressuring and driving agent in that this medium at atmospheric pressure and above reverses the wettability of sand grains in an oil stratum that are originally oil wet, thus releasing the oil normally retained as a film on the individual sand grains in such a stratum. The method of the invention provides other advantages such as providing a liquid pressure seal, which is self-sealing in the gas cap. Application of vertical steam drive on the stratum oil, substantially heats the oil and reduces the viscosity thereof, and permits confining the steam drive operation to a part of the reservoir at any given time. Thus, in the early stages of the drive when aqueous liquid seal 46 is relatively close to injection well 14, the vertical steam drive is applied to only that section of the oil stratum extending out as far as the liquid seal. As the liquid seal is driven thru the gas cap, the extent of the oil stratum subjected to vertical steam drive increases.

Referring to FIGURE 2, an oil stratum lies subjacent a gas cap 12 and these strata are penetrated by injection well 14 and production well(s) 16. The arrangement shown is similar to that in FIGURE 1, corresponding parts being correspondingly numbered. Each of tubing strings and terminate in downhole pumps 5% and 52, respectively. The principal differences between the arrangement illustrated in FIGURE 1 and that of FIGURE 2 are in the positioning of packer 54 in injection well 14 adjacent the interface of strata 10 and 12 and the perforation of casing 18 of injection well 14 above the packer 54 as at 56. Steam injection line 24 leads into the casingtubing annulus of well 14 instead of into the tubing string so that injection of steam is thru the annulus and into the gas cap directly thru perforations 56.

In operation with the arrangement of FIGURE 2, injection of steam directly into the gas cap builds up a liquid bank of condensate 46 and applies vertical steam drive on the oil in stratum 10 adjacent the gas cap, forming a layer of hot oil 42 as in FIGURE 1. During the initial phases of the steam drive with the arrangement illustrated in FIGURE 2, wells 16 are open to production thru perforations 32 but well 14 is not open to production until aqueous liquid bank 46 has moved out into the stratum a substantial distance such as at least 1 4 of the distance from the injection well to Well 16. At this stage of the process well 14 is opened to production and the steam drive produces thru both the central well 14 and the ring wells 16. Injection well 14 is provided with a production line 60 for transporting produced oil forced into this line by pump 50 when the well is open to flow by pumping and opening of valve 62.

While steam is generally injected thru a tubing string into a stratum, it can be injected directly into the casing at the wellhead.

It is also feasible to operate with the arrangement illustrated in FIGURE 2 by opening well 14 to production intermittently, thereby controlling the proportions of oil produced in the injection well and in the ring wells. By opening up well 14- to production simultaneously with the initiation of steam injection thru perforations 56, the first production is thru well 14 in view of the closeness of the perforations 22 to the steam bubble compared with perforations 32 and the vertical drive on the oil in the oil stratum is supplemented by a substantial horizontal drive because of the opening up of the stratum immediately surrounding injection well 14 early in the process.

Referring to FIGURE 3, the arrangement illustrated is similar to that shown in FIGURE 1, corresponding parts being correspondingly numbered. In addition pumps 52 are shown on tubing strings 30 as in FIGURE 2 and sucker rods 63 are also shown for operating the pumps. FIG- URE 3 illustrates that stage of the injection and production process of FIGURE 1 after the water seal 46 is driven beyond production wells 16 and vertical steam pressure is being applied on the oil layer 42 beyond the production wells whereby oil is being produced from the underlying oil stratum thru perforations 32 in the production wells. Steam injection is continued beyond the stage illustrated in FIGURE 3 so as to drive more oil into the production wells from the stratum intermediate wells 16 and injec- 4 tion well 14. This continued steam injection also produces more oil from the oil stratum beyond the production wells and moves the water seal further into the gas cap. When steam breaks thru into the production wells, a soaking period for heat to further penetrate the unproduced oil zone is allowed and steam injection is again resumed to produce additional oil in the production wells. The unproduced stratum beyond the production wells can be produced by similar vertical steam drive, utilizing a series of wells in the oil stratum beyond the pattern illustrated.

In a reservoir in which the original gas pressure has been depleted, or which may be operating under a partial vacuum, vertical drive is a vital factor in producing the oil. In addition to the vertical drive, heat is transferred from the steam zone to the oil zone by conduction and some movement of hot condensate downward by gravity segregation occurs as the oil in the upper portion of the oil zone is heated. Heating the oil from formation temperature to steam temperatures causes tremendous decrease in the viscosity of the oil, particularly if the oil is of medium or low API gravity. The hot oil of low viscosity is driven along the upper portion of the oil zone and lower portion of the gas zone by the steam flow. This is the hot zone or layer in the drawing. When the hot oil is displaced from the upper portion of the oil zone it is immediately replaced with hot condensate and steam from the steam bubble. This keeps steam in contact with the top of the oil layer resulting in maximum transfer of heat from the steam bubble to the oil in the fluid (liquid) saturated porous medium. Since the top of the gas cap is sealed by a non-porous medium, heat can only be lost to the overburden by heat conduction.

The injection of natural gas or hot combustion gases could be used to increase the gas cap pressure and thus increase the vertical drive. However, their use would be completely devoid of the self-sealing liquid bank pressure seal in the gas cap, which allows the described steam method to work on a single pattern, or lease, without requiring temperature and pressure build-up in the entire oil field gas cap. This feature is extremely important from an economic standpoint. It permits use of smaller steam units by preventing enormous heat losses encountered when large areas are heated for long periods of time, as would be the case if it were necessary to operate on the entire field at once. The use of gases other than steam to drive oil thru the stratum to the production well after steam break-thru is advantageous.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

1 claim:

1. A process for producing oil from an oil stratum lying adjacent a gas stratum and penetrated by an injection well and at least one offset production well, which comprises the steps of:

(1) injecting steam thru said injection well only into a section of said oil stratum remote from said gas stratum so as to heat and displace oil toward said production well which is open to flow only thru a section of said oil stratum spaced substantially from said gas stratum, whereby steam passes into said gas stratum;

( 2) continuing the injection of steam as in step (1) so as to heat and displace additional oil and cause more steam to flow into said gas stratum and form a water seal of condensed steam extending from said oil stratum transversely across said gas stratum to block flow of steam thru said gas stratum to said production well;

(3) continuing the injection of steam as in step (1) so as to heat and displace additional oil toward said production well and cause more steam to flow into said gas stratum, thereby moving said water seal toward said production well and producing oil therein; and

(4) recovering displaced oil from said production well.

2. The process of claim 1 wherein said gas stratum is a gas cap'.

3. The process of claim 1 wherein said gas stratum underlies said oil stratum.

4. The process of claim wherein said injection well is surrounded by a ring of several production wells and said ring wells are closed to steam production except thru a section of said oil stratum spaced substantially from said gas stratum.

S. A process for producing oil from an oil stratum lying adjacent and open to a gas cap, said stratum being penetrated by an injection well and at least one offset production well open to flow only thru a lower section of said stratum, which comprises the steps of:

(1) injecting steam only into a lower section of said oil stratum thru said injection well so as to heat and displace oil toward said production well, whereby a substantial portion of the injected steam passes into said gas cap;

(2) injecting additional steam as in step (1) so as to heat and displace additional oil and force oil into said production well from sair lower section of said stratum, a substantial portion of said additional steam passing into said gas cap forming condensate therein, and building up a bank of aqueous liquid as a seal to steam penetration;

(3) injecting further quantities of steam as in the foregoing steps so as to continue the displacement of 'oil into said production well and push said seal toward said production well, thereby applying vertical steam pressure on the oil in said oil stratum and forming a zone of hot oil extending thru said oil stratum adjacent said gas cap';

(4) continuing the injection of steam as in the foregoing steps so as to increase the depth of heating in said oil stratum and producing additional oil in said production well; and

(5) recovering the produced oil from said production Well.

'6. The process of claim 5 wherein said injection well is surrounded by a ring of said production wells and steam injection is continued so as to drive said bank of aqueous liquid beyond said production wells and apply vertical steam pressure on the subjacent oil stratum beyond said wells, thereby producing additional oil therefrom.

7. A process for applying a vertical steam drive to an oil stratum penetrated by an injection well and at least one production well and lying adjacent a gas stratum, which comprises the "steps of:

(1) injecting steam thru said injection well only into said gas stratum so as to efiect steam condensation and form a generally upright aqueous liquid b-ank across said gas stratum, thereby applying generally vertical steam pressure on the oil in said oil stratum;

(2) opening said production well to production only thru a level remote from said gas stratum;

(3) continuing the injection of steam so as to heat and displace oil into said production well by vertical drive and drive said bank of liquid toward said production Well; and

(4) recovering produced oil from said production well.

8. The process of claim 7 wherein steam injection in step (3) is continued so as to drive said bank of liquid beyond said production well and apply vertical steam drive on the oil in said stratum beyond said production 'well.

9. The process of claim 8 wherein said production well is one of a number of ring wells surrounding said injection well.

References Cited UNITED STATES PATENTS 3,027,942 4/1962 Willman et al. 166-11 3,042,114 7/1'962 Willman 16'6-1'1 3,129,758 4/ 1964 Closma-nn 166l1 3,193,009 7/1965 Wallace et al. 166- 11 3,259,186 7/1966 Dietz 166-11 3,279,538 10/1966 Doscher 166 1'1 OTHER REFERENCES Secondary Recovery of Oil In the United States (Sec- 0nd edition), publishd by American Petroleum Institute, 50 W. 50th St., New York, NY. (1950) (pp. 222-227 relied on).

STEPHEN J. NOVOSAD, Primary Examiner.

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