CA1148854A

CA1148854A - Method and apparatus for recovering high viscosity oils

Info

Publication number: CA1148854A
Application number: CA000365436A
Authority: CA
Inventors: Joseph C. Allen
Original assignee: Barber Heavy Oil Process Inc
Current assignee: Barber Heavy Oil Process Inc
Priority date: 1979-12-31
Filing date: 1980-11-25
Publication date: 1983-06-28
Also published as: GB2066333B; DE3048179A1; GB2066333A

Abstract

METHOD AND APPARATUS FOR RECOVERING HIGH VISCOSITY OILS
ABSTRACT OF THE DISCLOSURE
Improved methods and apparatus are provided for recover-ing high viscosity oils from sub-surface earth formations. IN
particular, a large diameter shaft hole is employed and a plur-ality of substantially horizontal drill holes extend radially from the shaft hole into the formation. It is a feature of this invention to provide vertically spaced drill holes oriented to improve the recover of the oil. The methods are provided for further enhancing oil recovery by coordinating the injected fluid with the location of injection within the formation.

Description

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METHOD AND APPARATUS FOR RECOV~RIN~ HIGH VISCOSITY OILS

RELATED PATENT APPLICATIONS

This application is related in ~art to Canadian Patent Application No. 334,159, filed ~ugust 21, 1979.
(Sec. M-9 - C1. 166- Sub. Cl. 34) BACKGROUND OF THE INVENTION
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This invention relates to methods and apparatus for recovering high viscosity oil from subsurface earth formations, and more particularly relates to improved methods and apparatus for recovering such oils by employing a large diameter shaft hole and a plurality of drill holes extending radially from the shaft hole.
Early disclosures relating to the recovery of petroleum substances by utilizing a large diameter shaft hole and a plurality of drill holes are provided in U.S.
Patent Nos. 1,520,737 and 1,634,235, and a paper published by Ranney in the Petroleum Engineer in 1939 entitled "The World's First Horizontal Hole." These publIcations propose the drilling of a large diameter shaft into an oil-bearing formation and then drilling radial drill holes into the formation. More recently, U.S. Patent Nos. 4,020,901, 4,099,570, 4,099,783 and 4,160,431 provide improved systems for recovering petroleum substances employing large diameter shaft holes and radials. The above processes, however, suffer limitations relating to restrictions Oil the rate of introducing the injection fluid, and the overall effi-ciency of the oil recovery process according to the above techniques may be so poor that recovery of the oil is not economically feasible.

-2-The publication and patents referred to above teach that steam or other heated fluids can be injected into horizontal drill holes or laterals extending from a large diameter shaft hole. The steam or other heated injection fluid is introduced via laterals into the forma-tion in order to provide heat to the formation and thus reduce viscosity of the oil. The heated oil thereafter flows to recovery laterals, being assisted by gravity.
The above procedure is generally referred to as a "soak" operation, which should be distinguished from a "drive" operation. In a "soak" operation, the injected fluid is intended to lower the viscosity of the oil and thereby enable the oil to more freely flow toward recovery lines by gravitational forces. A soak operation may be enhanced by raising the pressure in the formation and assisting the recovery of oil by increasing the differen-tial between the formation pressure and the pressure at the point of recovery.
It is also common practice to inject fluid into a conventional vertical well for the purpose of driving oil within a formation horizontally to be recovered hy a distant vertical well. In this "drive" operation, injected fluid is intended to act as a vertical bank or wall in the formation and push or drive the oil horizontally toward the recovery well. Although the drive operation may be enhanced by in jecting either a heated fluid or a solvent to additionally lower the viscosity of the oil, the injected fluid is ideally acting as a piston head to move through the formation and drive the oil toward the point of recovery. ~orizontal drive often results in viscous fingering and/or gravity override which are very detrimental to a horizontal drive operation.

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Horizontal drive operations are therefore generally con-cerned with the "critical velocity" of the injected fluid, so that the driving force does not break down as a result of viscous fingering or gravity override. Since the fluid injected in horizontal drive operations is preferably intro-duced at rates to keep the driving operation below the critical velocity, the recovery rate, and the corresponding economics of this operation, are often unacceptable. Both phenomena commonly referred to as viscous fingering and gravity override, as well as the differences between "soak"
and "drive" operations, are more fully discussed in my co-pending application, Serial No. 334,159.
It is also well-known to inject water via conven-tional vertical bore holes into the bottom of an oil-bearing formation, while producing oil from conventional vertical bore holes near the top of the formation. In this procedure, often referred to as conventional vertical flooding, the oil is displaced vertically upward by the injected water.
By using conventional vertical bore holes terminating near the top of the formation and conventional vertical bore holes terminating near the bottom of the formation, it is also known that low viscosity oils can be collected by injecting a light hydrocarbon in the top of the formation while recovering the light oil from the vertical bore hole terminating at the bottom of the formation. In the latter procedure, the light oil is pushed downward by the lighter hydrocarbon. The recovery of oil utilizing conventional vertical wells and drive techniques are often poor, however, because a limited number of points of recovery are provided by the vertical wells.

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These conventional techniques have several in-herent problems. In order to properly "blanket" an area prior to establishing active horizontal driving displacement of the oil, the displacing fluid must be injected at a low rate. Also, if effective blanketing is to be achieved, the formation must have high permeability and -the difference in density between the oil and the injected fluid should be high. If a good blanket of injecting fluid is not formed, or if a good blanket is formed but the displacing fluid is injected at too high a rate, viscous fingering can result.
When this condition occurs, the injecting fluid no longer effectively displaces the oil, and the volumetric sweep efficiency of the operation falls off drastically.
In order to minimize the likelihood of viscous fingering when utilizing the prior art techinques, the injecting fluid is often introduced into the formation at low injection rates, e.g., 400 barrels of fluid per day per well. Optimum steam injection rates with vertical wells are more fully discussed in an article by Messrs.
Bursell and Pittman, appearing in the Journal of Petroleum Technology, August, 1975, beginning on page 997. Low in-jection rates, of course, generally result in low production rates, which significantly increases the cost of production.
The problems and disadvantages of the prior art are overcome with the present invention. Novel methods in apparatus are hereinafter provided for recovering high vis-cosity oils from sub-surface formations, wherein a greater percentage of the oil can be recovered from the formation and can be recovered in a more efficient manner.

SUMMARY OF THE INVENTION
In one embodiment of this invention, a vertical mine shaft of sufficient diameter to accommodate personnel and equipment is bored to and through a formation containing a high viscosity oil. The vertical shaft is enlarged near the top of the formation and near the bottom of the forma-tion forming chambers to provide increased space for men and equipment. Substantially horizontal drill holes or laterals are thereafter bored radially into the formation.
The numbering and spacing of these laterals will be dis-cussed in detail below. For the present, it is significant that one group of laterals is provided near the top of the formation, and another group of laterals is provided near the bottom of the formation. Each group or set of boreholes or laterals effectively blankets a particular portion of the formation, and the configuration of each blanket may be rectangular. Thereafter, drill holes are bored into the face of the formation and radially about the chamber, through which a suitable fluid is thereafter injected into the formation by way of a conduit leading to the surface.
Using the above-described apparatus with one set of laterals near the top of a formation and another set at the bottom of a formation, oil may be recovered by either (a) introducing an injected fluid into the bottom set of laterals while recovering oil from the top set of laterals, or (b) introducing an injected fluid into the top set of laterals while recovering oil from the bottom set of laterals.
The instant invention preferably utilizes a plu-rality of substantially horizontal laterals which are drilledin the formation for recovery of high viscosity oil through B~

"drive" techniques, while the prior art attempts to improve oil recovery by using boreholes or laterals within the formation and various "soak" techniques. In the present invention, the driving force of the injected fluid is sub-stantially vertical: vertical upward drive toward the top set of laterals, or vertical downward drive toward the bottom set of laterals. The advantage of driving vertically rather than horizontally, as taught by conventional oil recovery techniques, is that the injected fluid may be introduced at a much higher rate because the likelihood of viscous fingering is substantially reduced and detrimental gravity override is eliminated. A fluid may therefore be injected according to the present techniques at rates equivalent to the maximum output of most commercial genera-tors, without regard to the concept of critical velocity, fingering, or gravity override.
In conventional oil wells, it is often quite difficult to establish a blanket of injected fluid, although the fluid is injected at various points in the plane of the blanket. According to the present invention, the blanket of injected fluid may be more easily established since the fluid is not injected at points in the blanket, but rather is injected along entire lines lying within the blanket.
These lines effectively provide a multitudinous number of points of injection so that the blanket for the drive operation is more easily established.
As previously mentioned, the apparatus described may be utilized for vertical upward drive or vertical down-ward drive. This offers a distinct advantage over the prior art since the injected fluid can be selected for either upward or downward drive depending on the density of the oil in the formation, the desirability of using a mis-cible or an immiscible injected fluid, the availability of the injected fluid, and the particular characteristics of the formation. For instance, if the oil in the formation is a heavy crude oil, e.g., 14 API, and a relatively heavy injected fluid is available, e.g., carbon disulfide, ver-tical upward drive of the oil is feasible. Thus, carbon disulfide, may be injected in the bottom set of laterals, and, since the injected fluid is heavier than the oil, it would not tend to rise naturally in the formation. A
blanket of injected fluid may be thereby easily established in the bottom of the formation. As the injection of carbon disulfide continues, the injected fluid drives oil upward in the formation toward the top set of laterals. Fluid may continue to be injected in the bottom set of laterals while oil is recovered from the top set of laterals, until the injected fluid is effectively driven through the entire formation.
If, on the other hand, the formation pressure is high and heated water is readily available as an injected fluid, downward vertical drive may be desirable. Heated water at 500 F. is less dense than 14 API oil. Thus, heated water may be injected in the top set of laterals serving to drive the oil downward for recovery by the bottom set of laterals. In this manner, a blanket of injected fluid is established in the upper portion of the formation, and the oil is driven downward by the continued injection of fluid.
The particular spacing and arrangement of drill holes will, of course, depend upon the size and lithology of the formation of interest, but it is a feature of the 8~

invention to provide approximately eight different radially e~tending drill holes for each shaft hole, and to further extend such drill holes to a location ad~acent the ends of similar radials extending from an adjacent vertical shaft hole. As will hereinafter be explained in detail, each group of radial drill holes will then define a rectangular pattern within the field, and thus the field may be effec-tively "covered" with a blanket of such rectangular patterns.
The bore holes or radials may extend from the large diameter shaft hole in a generally horizontal direc-tion. Some of the radials, however, may be positioned at a slight upward angle, e~g., 3 - 7, so that gravity may assist the flow of oil within the lateral. Also, some for-mations have generally horizontal barrier zones which are impermeable to fluid flow. Relatively thin shale deposits are one example of these impermeable barrier zones. When such zones exist, the bore holes or laterals may be drilled at a greater upward angle, e.g., 10 - 18 upward from the horizontal, so that the bore holes would pass through one or more of these barrier zones. Thus, the recovery of oil and the efficiency of the operation will be increased since oil can be recovered by soak or drive techniques which would have previously remained within the formation because of the presence of the impermeable barrier zones.
It is within the concept of the present invention to locate the radials adjacent the lower limit of the formation, whereby the fluid injected therefrom will also tend to rise as well as travel laterally through the forma-tion, and also to provide additional pluralities of such radial drill holes at other higher locations within thicker formations. Furthermore, it is within the concept of this _g_ invention to inject fluid through only a portion of the radials, while also recovering oil from one or more of the other radials extending from the same shaft hole.
The invention is suitable for recovering oil from subsurface formations, and is particularly suitable for recovering high viscosity oils. High viscosity oils are generally inclusive of both medium gravity oils having an API range of 20to 25, and heavy crude oils having an API
range of 20 or less.
A particular feature of this invention is to pro-vide methods and apparatus that result in increased produc-tion rates for recovering high viscosity oil from subsurface earth formations.
It is another feature of this invention to provide methods and apparatus for driving oil within a subsurface earth formation vertically toward recovery laterals extend-ing radially from a large diameter shaft hole.
It is a further feature of this invention to provide methods and apparatus that will increase the over-all efficiency of recovering high viscosity oil from asubsurface earth formation by utilizing vertical drive techniques.
It is an additional feature of this invention to provide apparatus for recovering oil from a subsurface earth formationp comprising a shaft hole extending from the surface to said subsurface earth formation, a first plurality of boreholes extending radially from said shaft hole into said formation at a first elevation, a second plurality of boreholes extending into said formation at a second elevation, and injection means introducing a fluid through said second plurality of said boreholes and into ..~ .

said formation for driving said oil vertically for recovery by said first plurality of boreholes.
It is a further feature of this invention to pro-vide a method of recovering oil from a subsurface earth formation, comprising establishing a shaft hole extending from the surface of -the earth to said subsurface earth formation, drilling a first plurality of boreholes radially from said shaft hole into said formation at a first eleva-tion, drilling a second plurality of boreholes into said formation at a second elevation, and injecting a driving fluid into said second plurality of boreholes and into said formation for driving said oil vertically for recovery by said first plurality of boreholes.
The method of the present invention comprises establishing a shaft hole extending from the subsurface of the earth to said subsurface formation, drilling a plurality of boreholes substantially laterally from the shaft hole into said subsurface earth formation, drilling a second plurality of boreholes, all terminated substantially at another second level, injecting into said formation through said second plurality of boreholes a displacing fluid to establish a substantially horizontal stratum of said dis-placing fluid, injecting into said formation through said second plurality of boreholes a driving fluid under pressure to vertically drive said stratum of said displacing fluid through said formation, and withdrawing oil from said shaft hole through said first plurality of boreholes while simul-taneously injecting said driving fluid into said formation.
Also, the present invention provides or an apparatus that includes a shaft extending from the surface of the earth to said subsurface earth formation, a first plurality of boreholes extending radially from said shaft and lying within a substantially horizontal first plane within said formation at a first elevation, a second plura-lity of boreholes all terminated substantially at another second level and comprising fluid output means for defining a layer of fluid, injection means introducing a displacing fluid through said second plurality of boreholes to provide a layer of displacing fluid, pressurizing means introducing a driving fluid through second plurality of boreholes for driving the layer of displacing fluid vertically -toward said irst plurality of boreholes, and recovery means for with--drawing oil from said first plurality of boreholes.
These and other features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.
IN THE DRAWINGS
Figure 1 is a simplified pictorial representation, partly in cross-section, of an exemplary installation for recovering oil from a subsurface earth formation according to the present invention.
Figure 2 is a cross-sectional representation of a portion of the apparatus depicted in Figure 1~
Figure 3 is a simplified pictorial representation, partly in cross-section, of a portion of the apparatus depicted in Figure 1.
Figure 4 is a graph illustrative of the variations in specific gravity for fluids, and is referred to in the description to assist in illustrating the concepts of the present invention.
Figure 5 is a simplified pictorial representation, partly in cross-section, of another form of an exemplary installation according to the present invention.
Figure 6 is a cross~sectional representation of a portion of the apparatus depicted in Figure 5.
DETAILED DESCRIPTION

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In Figure 1, there may be seen a pictorial repre-sentation of an oil recovery system embodying the concepts of the present invention. In particular, the apparatus depicted in Figure 1 may be utilized for recovering high viscosity oil from subsurface earth formations. A substan-tially vertical mine shaft 2 is drilled or bored from the surface 4 to the oil-bearing formation 6. The oil-bearing formation 6 may typically be hundreds of feet below the surface 4, and is shown to be bounded by an upper layer 5 and a lower layer 7 of shale deposits, which are generally impregnable to fluid flow. ~s seen in Figure 1, the mine shaft 2 is drilled through the oil-bearing formation 6, and terminates at a sump hole 8~
The shaft 2 is expanded near the top portion of the formation to form an upper work chamber 10, and is expanded at the bottom portion of the formation to form a lower work chamber 12. A plurality of upper laterals 14 and 16 may be drilled into the formation 6 from the upper work chamber; a plurality of lower laterals 18 and 20 may likewise be drilled into the formation from the lower work chamber 12.
A steam generator 22 at the surface floor provides injection fluid to the upper laterals 14 and 16 by a steam line 24. If desired, a plurality of steam lines 24 may be provided, so that the injected fluid to each upper lateral may be separately controlled at the surface by conventional . ~ .

valving techniques. The lower laterals 18 and 20 may serve as recovery lines for oil, which is collected in recovery tank 26. Each of the lo~er laterals 18 and 20 may be con-nected to individual recovery lines 28 and 30 which may terminate at the recovery tank 26, or the recovery lines 28 and 30 may join at a subsurface manifold 32 so that oil from the various lower laterals 18 and 20 flows to the recovery tank 26 by a central recovery line 34.
Referring again to Figure 1, it may be seen that a portion of the radial or lateral 14 extending from the wall of the upper work chamber 10 may be suitably provided with a casing 36, with a perforated portion of the lateral 14 thereafter extending from the casing 36. The walls of the shaft 2 may be conveniently sealed with sections of bolted or welded steel plates to form a casing 38, or may be lined with an appropriate material such as gunite, to prevent caving or other collapse of the walls of the shaft 2. The diameter of the shaft 2 is preferably of a size sufficient to accommodate the passage of men and equipment from the surface 4 to the interior of the work chambers 10 and 12. The mine shaft 2 and each of the work chambers 10 and 12 may be constructed in the manner further described in U.S. Patent 4,160,481.
~ lt'neembodiment of this invention illustrated in Figure 1, fluid is injected into the formation via radial drill holes or laterals 14 and 16 which lie entirely within the formation 6. If the injected fluid is heated water or steam, heat loss by way of the steel casing 38 therein is not significant since the heat merely transfers to the formation. On the other hand, it is desirable for the injected fluid to enter the formation at a distance from ,~

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the shaft 2 or work chamber 10, so that the fluid will tend to move outwardly therefrom instead of bypassing back into the chamber 10. Furthermore, it may be preferable to insert pre-perforated pipe or casing into the radial drill holes, rather than to perforate the casing in a conventional manner after it has been inserted. The upper laterals may contain the casing 36, but the casing is not necessary according to the present invention. Thus, although the casing 36 is only shown for the upper lateral 14, it is within the con-cept of my invention that the casing be employed with all,some, or none of the injection laterals.
Referring now to Figure 2, there is illustrated a cross-sectional view of the laterals shown in Figure 1.
In Figure 2, four upper laterals radiate from the shaft 2 at 90 intervals, and are denoted as 14, 15, 16 and 17.
Four lower laterals are also spaced at 90 intervals, and the lower laterals 18, 19 r 20 and 21 are shown connected to their respective vertical recovery lines 28, 29, 30 and 31.
Both the upper set of laterals and the lower set of laterals depicted in Figures 1 and 2 lie in substantially horizontal planes. As explained in further detail below, the upper set of laterals, in combination with an injected fluid, form a blanket of injected fluid which act as the driving force for the recovery process. Typically, the laterals will radiate from the shaft 2 -to enable a substan-tially horizontal blanket to be established having an area of 25 acres. In Figure 2, the pattern of the blanket formed by the injection radials 14, 15, 16 and 17 is circular.
Other configurations are possible, and the particular con-figuration of the blanket formed by the injection laterals will depend, in part, on the characteristics of the for-mation, the viscosity of the oil in the formation, the injected fluid, and the possible use of injection laterals in combination with conventional wells. For instance, the configuration of the injection laterals may form a rectangular blanket, as shown in my co-pending patent application, which i5 Serial No. 334,159. The length of the laterals will depend on their relative position to one another, since the injection laterals preferably function to form a uniform substantially horizontal blanket in a portion of formation 6~ Further, the diameters of the laterals will depend primarily upon the type of matrix composing the formation 6, as well as the viscosity of the oil sought to be recovered therefrom.
Figure 3 illustrates a portion of the apparatus generally depicted in Figure 1. A tubular member 40 is typically inserted into any of the boreholes described, such as the laterals 14, 15, 16 and 17 in Figure 2. Like-wise, a tubular member 42 may be inserted into the laterals 18, l9, 20 and 21. The oil-bearing formation 44 is bounded on the top by a layer of shale or other rock 46, and on the bottom by a similar impregnable layer of shale or other rock 48. The layers 46 and 48 are typical of any number of materials which often exist on the upper and lower portions of an oil-bearing formation. Generally, these impregnable layers line a substantially horizontal plane, although perhaps not as uniform as the layers depicted~
As previously stated, it is preferable according to the present invention that the upper tubular member 40 be placed near the upper portion of the formation, and that the lower tubular member 42 be near the bottom portion of the formation. The exact spacing of the upper and lower .

laterals will possibly depend on the configuration of the layers 46 and ~8. If these layers are uniform and substan-tially horizontal within the area oE the recovery operation, the laterals may be drilled very close to the layers. If the layers are irregular in configuration, the upper laterals may be drilled in the upper fifth of the formation, and the lower laterals in the lower fifth of the formation. For example, if the formation 44 is approximately 60 feet thick and substantially uniform in configurationr the upper tubular member 40 is preferably 1 to 5 feet from the bottom of the top layer 46, and the lower tubular member ~2 is preferably 1 to 5 feet above the top of the bottom layer 48.
A simplified method of operation according to the present invention will now be described by reference to Figures 1, 2 and 3. For the present, it will be assumed that steam has been selected as the injection fluid, and that both steam and water at the temperature and pressure within the formation are less dense than the oil in the formation.
Steam from the generator 22 may be injected into upper laterals 14, 15, 16 and 17 by a plurality of steam lines 24. The steam may enter the formation 44 through a series of perforations in the upper tubular members, such as the perforations 50 in the tubular member 40. The per-forations 50 may be formed along the entire length of the tubular member 40, and are preferably formed along at least a substantial portion of the length of the tubular members.
Heat from the injected steam will decrease the viscosity of the oil in the formation adjacent the tubular member 40. As the formation 44 is heated, condensate may be formed which will be more dense than the steam, but less dense than the oil in the formation. The steam from the tubular member 40 will, in combination with the steam simul-taneously injected in the other upper laterals, produce a blanket of steam generally in the plane of the upper laterals.
Continued in~ection of steam into the upper lat-erals will cause a pressure increase in the top of the formation 44. This pressure increase will drive the oil in the formation downward toward the lower tubular member 42. Also, the condensate from the injected steam may serve as a face for the driving Eorce. In other words, the increase in pressure above the condensate layer will force the condensate layer downward, thus driving the oil downward from the formation.
As the steam is injected in the upper tubular member 40, oil will be simultaneously recovered through perforations 54 along a substantial portion of the length of the lower tubular member 52. Referring to Figure 2, steam injected into the upper laterals 14, 15, 16 and 17 will drive the oil downward for recovery by the lower laterals 18, 19, 20 and 21. In this manner, the oil from the formation 44 may be efficiently recovered by the down-ward drive of the injection fluid.
As stated above, the present invention may be effectively employed by utili2ing a driving fluid and driving the oil vertically toward recovery laterals. In a vertical driving technique, the pressure gradient within the formation may be altered by the injected fluid to force the oil toward the recovery lines. Because of the weight of the oil in the formation, the pressure near the upper section of the formation will generally be less than the pressure in the lower portion of the formation. By way of illustration and referring to Figure 1, if the formation 6 were 60 feet thick and the pressure at the top of the for-mation just below the layer 5 was 15 psia, the pressure near the bottom of the formation above the layer 7 may typically be 40 psia because of the pressure gradient of the oil in the formation 6.
According to the vertical steam drive techniques discussed herein, drawing the oil from the recovery lines and injecting a fluid into the injection lines will produce a pressure differential sufficient to force or drive the oil vertically. Preferably, a pressure differential of 100 psia or more is achieved during the driving process between the pressure at the place of injection and the pressure at the place of recovery within the formation. The preferred pressure differential will vary depending on the specific characteristics of the oil and the formation, and typically a pressure differential of 200 psia to 400 psia will be desired. The maximum pressure at the place of injection is generally limited for safety reasons to 1 psig per foot of overburden. For instance, if steam is injected into the laterals 14, 15, 16 and 17 which are ~00 feet below the surface 4, it may be desired to limit the pressure provided in the upper portion of the formation 6 to 400 psia.
As previously mentioned, vertical drive may be properly utilized without concern for gravity override, even with injection rates far exceeding the injection rate employed in conventional horizontal drive. In the embodi-ment described above, the density contrast between the oil and the injected fluid is deliberately utilized during the driving operation to increase the efficiency of the recovery --lg--~8~3S~
processes, while the same density contrast ma~v result in gravity override in horizontal drive operations which is detrimental to the efficiency of horizontal drive recovery processes. Also, s-ince the o;l is being driven vertically downward t the likelihood of viscous fingering during the driving operation ls substantially eliminated since (a) a uniform blanket of drlving fluid is initially formed, (b) the condensate layer 52 acts as a face for driving the oil from the formation~ and Cc) the oil is being driven vertically downward, and the injected fluid will not tend to pierce through the formation 44 because the injected fluid is less dense than the oil below the injected fluid. Further, as explained below, vIscous fingering prior to the driving opera-tion increases the efficiency of th.e recovery process accord-ing to the present invention, rather than being detrimental to the recovery eff.iciency as in horizontal drive techni~ues.
Steam may be continually injected into the upper tubular member 4Q until condensate or steam has been recovered from the lo~er tubular member 42. At this point, further injection of steam may not be economical, since little if any further oil ~ill be recovered from the lower tubular member 42.
If the vertical recovery lines 23 and 3a are used to connect each lateral to the recovery tank 26, it is possible to monitor the fluid being recovered at the surface 4 from each lower .`
lateral. In this manner, it may be desirable to discontinue the injection of steam into the upper laterals which are ad-jacent the lower laterals in which steam or condensate is being recovered, whi.le continuing to inject steam into the other upper lateral.sas long as oil is being produced from their æspective ~Ja-- 20 ~

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cent lower laterals. If the recovered oil is ei~her being ~orwarded to a subsurface manifold 32 or is being taken from a common sump hole 8, the monitoring of the recovered fluid from the individual lower laterals may be accomplished before that fluid is intermingled with fluid from the other lower laterals. Use of the common sump hole 8 for recover-ing laterals is more fully described in my co-pending appli-cation Serial No. 334,159.
Referring again to Figure 2, the following is an example of the monitoring procedure described above. Steam may be initially injected into the four la-terals 1~, 15, 16 and 17, forming a substantially horizontal blanket of steam in the plane of the upper laterals. Continued injection of the steam will drive the oil in the formation to the lower laterals. Once steam or condensate has been recovered from one of the lower laterals, e.g., lateral 21, steam may be continued to be injected into the upper laterals 15 and 16, while steam is not injected into the upper laterals 14 and 17. Thus, oil recovery would continue from the lower laterals 18, 19 and 20, but oil recovery from the lower lateral 21 would be effectively discontinued.
An alternative procedure that may be used when steam or condensate is recovered in one of the plurality of lower laterals is to inject cold water into the lower lateral while continuing steam injection into the upper laterals. Referring to the example described immediately above, if water or condensate is being recovered from the lower lateral 21, cold water may be injected into lower lateral 21 while continuing to inject steam in either upper laterals 15 and 16 or all the upper laterals 14, 15, 16 and 17. The introduction of the cold water into the lower lateral 21 effectively terminates the recovery of any fluid from the lateral 21, thus the steam injected into the for-mation would be effectively used to produce oil from the laterals 18, 19 and 20.
One of the advantages of the plurality of upper laterals 14, 15, 16 and 17 is that a blanket can be easily and rapidly established in the formation which will enhance the driving efficiency of the in,ected fluid. The rapid establishment of a blanket in the formation will increase the thermal efficiency of the recovery process since less heat and injection fluid will be lost to adjacent beds or escape from the formation entirely. Conventional vertical injection wells do not result in the rapid and efficient establishment of a blanket, since the fluid is injected at points within the plane of the blanket. According to the present invention, the laterals lie within the plane of the blanket to be established, and thus lines of injection are provided rather than mere points of injection. The parti-cular number of laterals used will depend on a variety of factors. It is within the concept of the present invention that a pluralilty of laterals radiate from the mine shaft at at least two vertical elevations. The embodiment illus-trated in Figure 2 shows four upper laterals and four lower laterals, but any number of upper and lower laterals may be used. For instance, eight equally spaced upper laterals could be used in conjunction with eight equally spaced lower laterals. The efficiency of the recovery process would be generally enhanced by the increased number of laterals, but the increased number of laterals also results in increased drilling and equipment costs.
It is preferable that equal numbers of upper ~18~35~
laterals and lower l~terals be provided, and that the individual upper laterals and individual lower laterals b.e equally spaced from each other. Thus, as~ shown in Fîgure 2, each of the upper laterals is ~0 apart from adjacent upper laterals, and the lower laterals are 90 apart from ad~acent lower laterals.
In addition, i`t ;s- preferable that lower laterals be spaced between upper laterals:, as compared to having the lower laterals directly below the upper laterals. As shown in Figure 2, each o~ the lower laterals is 45 apart in the horizontal direction from its adjacent upper laterals; if eight upper laterals were used in conjunction with eight lower laterals, each lower lateral would preferably be offset fro~ its adjacent upper lateral by 22.5. One advantage in offsetting the lower later-als from th.e upper laterals is that a longer path is there~y provided between the injection laterals and the recovery later-als. Thus, the injection fluid tends to spread out evenly through the formation and increase the swe~p eff.iciency of the recovery process. If th.e upper laterals were to be placed directly above the lower laterals, there is less likelihood that the oil near the bottom of the formation and between the lower laterals would be recovered.
As previ.ously-described, the use of upper and lower laterals within a formation enables a blanket to be easily formed for the driving process. It is possible to use conven-tional vertical injection wells instead of the injection laterals, although the blanket of driving fluid may not be established as:readiL~ as if horizontal boreholes or laterals were used for the injectors. Referring to Figure 5, there is illustrated another form of my invention which employs the use of conventional vertical wells 60 ,~,. . .

and 62. It will be seen that these conventional vertical wells 60 and 62 are drilled from the surface 64 through the top rock layer 66 and terminate in the upper portion of the formation 68. Fluid may be injected into the vertical wells from a common generator, or separate generators 70 and 74 may be employed for each vertical well. Valves 72 and 73 may be used to control the flow of fluid from the generators to the formation.
The laterals 76 and 78 extend radially from the work chamber 80 in the lower portion of the mine shaft 82.
The lower set of laterals preferably "blanket" a portion of the formation, as previously described. Valves 83, 84 and 86 may be used to control the recovery of oil from the laterals to the recovery tank 88. In Figure 5, both the laterals 76 and 78, and the work chamber 80 are entirely within the lower portion of the formation 88, and lie above the rock layer 90.
It is also within the concept of my invention to utilize conventional vertical boreholes in combination with both upper and lower laterals, if desired, to assist the formation of a blanket and the driving process. Thus, in Figure 5 there is illustrated vertical injection wells 60 and 62 plus horizontal laterals 92 and 94. By controlling valves 96, 98 and 100, the injection of fluid from the generator 70 to each of the horizontal injection laterals 92 and 94 may be controlled.
Referring now to Figure 6, there is shown a cross-sectional view of a portion of the apparatus depicted in Figure 5. The vertical injection wells are designed as 102, 103, 104, 105, 106, 107, 108 and 109, with wells 103 and 108 corresponding to the wells 60 and 62 shown in Figure 5.

The horizontal recovery laterals form a generally rectangular blanket, and are designated as ~aterals 112, 113, 114, 115, ~ 24 -~ t~t~ ~
116, 117, 118 and 119, with latera:Ls 113 and 117 corresponding to laterals 76 and 78 in Figure 5.
It will be seen that the vertical wells may be spaced be-tween the horizontal laterals, and that the injection wells are spaced geographically to cover the area to be blanketed.
If desired, other injection wells may be spaced inwardly to the vertical mine shaft from the injection wells shown in Figure 6.
If horizontal injection laterals are utilized, it is anticipated that vertical injection wells would generally not be necessary. On the other hand, if vertical injection wells are utilized, it generally may not be necessary to drill horizontal injection laterals. However, it is pos-sible to use both vertical and horizontal injections, and this may be desirable if only a relatively few number of injection laterals are utilized.
To assist in establishing a horizontal blanket if vertical injectors are used in combination with recovery laterals, fluid may be initially injected into a portion of the vertical boreholes while oil is recovered from the remaining vertical boreholes. Thus, fluid may initially be inserted into the odd-numbered vertical wells, and the oil may be driven horizontally and recovered by the adja-cent even-numbered vertical boreholes until the injection fluid is recovered in the adjacent even-numbered vertical boreholes. At this point, a horizontal blanket of injec-tion fluid is established, and ~luid may be injected in all the vertical boreholes 102 through 109 to drive the oil downward for recovery by the lower laterals 112 through 119. In this embodiment, the vertical boreholes are preEer-ably spaced between the injection laterals and toward the periphery of the blanketed zone to increase the efficiency of the recovery operation. As previously described, it is .~ `?
~.

within the concept oE this inventicn to drive vertically upward or vertically downward, and the conventional vertical boreholes herein described may terminate near the top of the formation if downward drive is desired, or may terminate near the bottom of the ormation if upward drive is desired.
It is within the concept of this invention to employ a plurality of substantially horizontal laterals and inject fluid to drive the oil in the formation vertically toward recovery laterals. Oil may be driven vertically upward or downward, and thus the upper laterals and lower laterals may function either as injection laterals or recovery laterals. For instance, steam may be effectively used in many operations wherein the steam i5 injected into the upper laterals, since bo~h the steam and the condensate at the temperature and pressure within the formation may be less dense than the oil. However, some miscible fluids which are more dense than the oil in the formation can also be effectively used as a driving fluid. If the injected fluid chosen, e.g., carbon disulfide, is more dense than the oil, it may be advantageous to inject the fluid in the lower laterals 18, 19, 20 and 21 while recovering oil from the upper laterals 14, 15, 16 and 17. In this case, the heavier injected fluid would easily and rapidly form a blanket in the lower portion of a formation, and continued injection would drive the oil upward toward the recovery laterals. Also, neither gravity override or viscous finger-ing is a problem in this case during the driving operation, since the heavier injected fluid is below the oil to be displaced.
It may therefore be seen that the present inven-tion is applicable with both miscible and immiscible injec-tion fluids. Also, it is within the concept of my invention to inject a liquid lighter than the oil in a formation into the upper laterals, and thereafter to drive the oil downward toward the lower laterals by injecting a gas or a gas/liquid mixture in the upper laterals as the driving force. More particularly, a solvent less dense than the oil may be injected into the upper laterals to form a blanket of fluid to act as the face of the driving force. ~uccessively lighter slugs of fluid, each slug having a lower density than the previous slug, may thereafter be injected into the formation. Finally, a gas may be injected. The injection slugs of fluid and gas produce a driving force, and the solvent first injected acts as the face of the driving force. This technique, which may be referred to as graded miscible displacement, may be highly effective, permit much higher injection rates, and yield higher production rates than possible according to the prior art.
The density of the injection fluid may vary with temperature changes to a different extent than the oil in the formation. It is possible, therefore, to inject a par-ticular fluid in the top laterals as a driving force when that fluid is lighter than the oil, and to inject the same fluid in the lower laterals as a driving force when the fluid is heavier than the oil in the formation. Referring now to Figure 4, there is depicted a chart which illustrates the approximate density of the water, 10 API oil, 12 API
oil, and 14 API oil as a function of temperature. It will be noted that at 100 F., water is denser than 12 API oil, but that at 500 F., 12 API oil is denser than water.
The principle of different density changes for fluids can be fully utilized according to the present invention. For example, high temperature steam can be in jected into the upper laterals, and 12 API oil can be recovered by the lower laterals. At the pressure within the formation, the condensate from the high temperature steam may form a layer oE water at 500 F. In this case, both the steam and the condensate are lighter than the oil, and can be efficiently used to drive the oil downward.
After steam or condensate has been recovered from one of the lower laterals, the injection of steam may be continued in the upper laterals that are not adjacent the lower lat-eral from which the condensate was recovered, as previously described. tAlternatively, steam may be injected in all the upper laterals while cold water is injected in the lower lateral from which condensate or steam has been recovered). If the formation were at a relatively shallow depth and therefore correspondingly lower formation pres-sures existed, it is within the concept of my invention to inject superheated steam to achieve the high temperature desired to maximize the efficiency of the recovery process.
After the downward steam drive has been completed, the driving process may be reversed and cold water may be injected in the lower laterals while oil is recovered from the upper laterals. From the graph in Figure 4, it may be seen that at a temperature of 100 Fo~ water is more dense than 1~ API oil, and thus the injection of water will now drive some of the remaining oil in the formation upward toward the upper laterals. Thus, during both the vertically upward driving stage and the vertically downward driving stage, the density of the injected fluid enables an effective blan~et to be established, and the likelihood of viscous fingering is substantially eliminated.

~ 3~BB5~

Although driving vertically in one direction ac-cording to the present invention will effectively sweep the formation, it is understood that the oil recovery process will not be 100~ ef~ective. Driving in both directions, i.e., driving downward and thereafter driving upward, will increase the percent of oil recovered. This increased recovery is due, in part, to the fact that formations are not completely homogeneous, and thus driving in the reverse direction will cause the driving fluid to invade some areas that may have been sheltered from the forward driving fluid.
Also, the injection of cold water in the lower laterals, as described above, may cause some water to flash to steam because of the residual heat in the formation, and the steam may lower the viscosity of some remaining oil to enhance the effectiveness of the upward driving process.
In the methods described above, oil recovery is based on a vertical drive process, which may be simplicity described as injecting the fluid in one set of boreholes while recovering oil from another set of laterals. It is within th~ concept of my invention, however, to improve the efficiency of the vertical driving process by providing or a limited soak cycle for each of the sets of laterals.
For instance, if a vertical downward drive of the oil is to be achieved, it may be initially desirable to inject steam in the lower laterals to soak the formation directly adja-cent the lower laterals and thus improve the subsequent driving process. Also, the upper laterals may be thereafter injected with steam to soak the formation adjacent the upper laterals and therefore enhance the uniformity of the driving blanket. After the lower laterals have been opened and oil begins to flow in the lower laterals, steam may thereafter ~ 3~

be injected in the upper laterals to drive the oil toward the lower laterals.
Soaking the formation about the upper and lower laterals prior to establishing the driving process, as de-scribed above, may be beneficial in most applications. If a high viscosity oil is to be efficiently driven, it may be desirable, if not essential, to establish a flow path between the point of injection and the point of recovery prior to the driving cycle. The heavy oil in many forma-tions has such a high viscosity that it is very difficultto displace the oil by an injection fluid supplied from commercial generators.
If steam is to be used as the injection fluid in the application depicted in Figure 1, the efficiency of the downward driving process may be substantial]y increased by first injecting steam in the lower set of laterals 18, 19, 20 and 21. As the lower portion of the formation 6 is being soaked, viscous fingering and gravity override will readily occur since the injected fluid is lighter than the oil, and is being introduced in the lower portion of the formation. As viscous fingering and gravity over-ride occur, heated communication paths will be established between the lower set of laterals and the upper set of laterals. Thus, the formation may be subjected to repeated steam soak cycles from steam supplied through the lower set of laterals, wherein steam is injected into the lower laterals, the laterals are stopped off or shut in, and the laterals are opened for recovery of oil as a result of the soaking process.
When the repeated soaking from the lower laterals results in steam fingering to the top of the formation 6, ~30-the soaking process may be discontinued and steam thereafter injected into the upper set oE laterals 14, 15, 16 and 17 for driving the oil downward while recovering oil from the lower set of laterals. The steam soak cycle therefore results in communication paths between the injection loca-tions and the production locations which may have a greater area than that commonly associated with horizontal driving techin~ues. For instance, when conventional horizontal drive between vertical wells is utilized in a formation 60 feet thick, the area of face of the driving formation is typically approximately 12,500 square feet per acre. If vertical drive is practiced between horizontal laterals according to the present invention, the area or face of the driving front increases to approximately 43,000 square feet per acre. Thus, if the same injection rate per area of driving front is utilized, fluid is injected at approx-imately 3.5 times the rate as in conventional horizontal drive. Moreover, since the oil is being driven vertically rather than horizontally, the injection rate per area of the driving ~ront may be substantially increased since viscous fingering is substantially eliminated during the driving operation.
The larger driving face area, therefore, enables more driving fluid to be injected into the formation while maintaining a relatively low, stable driving velocity across the formation. Also, as previously mentioned, the driving velocity may be substantially increased when compared to horizontal drive since vertical driving minimizes the like-lihood of viscous fingering. Further, the fluid may be injected at higher pressures and at higher rates than realized in the prior art, which improves the efficiency of the recovery process. For instance, steam which may have been injected at 25~ quality in hori~ontal drive operations may effectively be injected at the higher rates and with greater steam quality, e.g., 80%, than in the prior art.
Also, as previously mentioned, superheated steam may be used as the injection fluid.
Although this invention has principally been de-scribed with steam or water as the injection fluid, both heated and unheated fluid may be used as a driving force within the concept of my invention. For example, water, solvents, gas, oil, distillate, LPG, and naptha, or a com-bination of liquids and gases may be utilized as the driving fluid according to the present inventionO Examples of gases that may be used in this invention are air, oxygen, hydrogen, carbon dioxide, inert gas, stack gas, steam, anhydrous ammonia, natural gas, ethane, propane and butane.
Also, although no additives must be combined with the fluid to be injected, the addition of additives may enhance the recovery process. Less heat is lost through condensation, and the average temperature of the formation is raised at a faster rate than in the prior art. Thus, the increased injection rates yield higher production rates, which improve the economics of the recovery operation.
It is also within the concept oE this invention to utilize in situ combustion to achieve or enhance vertical drive of the oil toward horizontal recovery laterals. For instance, if horizontal laterals are placed near the bottom of the formation, in situ combustion near the top of the formation will tend to drive the oil downward. Air or oxygen for the combustion process may be injected via either con-ventional vertical injection wells from the surface to the , upper portion of the formation or horizontal laterals from the mine shaft. Referring to Figures 5 and 6, if desired air or oxygen may be injected via the odd-numbered vertical wells, and in situ combustion used to recover oil from the even-numbered vertical wells as the combustion process drives the oil in the upper portion of the formation hori-zontally. After in situ combustion has progressed in a horizontal direction and the producing even-numbered ver-tical wells become heated, all the conventional vertical wells 102 through 109 may be used to inject air or oxygen.
Thus, in situ combustion may thereafter be utilized to drive the oil downward for recovery by the laterals or boreholes 112 through 119. If desired, this latter operation may be conducted in rotating segments, rather than exploiting the entire area at one time, which would reduce the capital investment required for the air or oxygen compressors.
Alternatively, in situ combustion may be conducted in the upper portion of the formation between conventional vertical wells, as previously described, and thereafter another fluid such as water, steam, inert gas or mixtures thereof injected into all the vertical wells for driving the oil downward for recovery by the horizontal laterals.
If desired, the lower horizontal laterals may be initially steam soaked or soaked at intervals during the driving pro-cess to maintain the desired recovery flow. The in situ combustion between the conventional vertical wells may be conducted until enough heat has been added to the formation for an efficient oil recovery by subsequent vertical drive techniques. If water is injected into the formation as the driving fluid from the vertical conventional wells, the residual heat from the "burned out" zones may generate f~'5~

steam in situ whîch may efficiently drive the remaining oil downward for recovery by the laterals.
The above operation is especially attractive from an economic standpoint since little or no sur~ace fuel is used to heat the formation. Temperatures in the combustion zone may be 800 F. to 1000 F. or more, and very good drive of heavy oil can be achieved if the oil in the formation is heated to 250 to 300 F. Also, creating a blanket of combustion-cleaned matrix establishes extremely high verti-cal downward transmissibility for the injected fluids usedin the subsequent driving cycle. In situ combustion further dehydrates and breaks down any shale layers or lenses with-in the combustion zone.
The recovery of oil when utilizing vertical drive, as described according to my invention, may be further enhanced by reducing the pressure at the recovery laterals to a sub-atmospheric value by connecting the recovery lat-erals to suction-type pumps. In this manner, oil recovery may be enhanced regardless of whether the vertical drive is propagated by in situ combustion, steam injection, solvent injection, gas injection, or injection of any number of fluids commonly used to enhance the driving operation.
The injection ]aterals in the embodiments de-scribed are preferably positioned in a substantially horizon-tal plane. The laterals may be inclined to conform to ~he inclination of the barrier layer, but the injection laterals with the injected fluid principally serve to establish a uniform, substantially horizontal blanket to be driven vertically through the formation. The recovery laterals are generally positioned in a similar substantially hori-zontal plane so that the formation may be efficiently swept t,9~

of oil. Some deviation of the laterals is expected, and in that sense the upper and lower laterals may not lie precisely in flat planes. Further, the laterals typically lie in relatively thin discs approximately five feet thick, and these discs or "planes" may be inclined slightly to conform to the barrier layers.
It may also be seen that the present invention may be profitably employed by installing a plurality of vertical mine shafts and laterals, as described herein.
Also, by operating such multiple installations in a simul-taneous manner, as more fully described in my co-pending patent application, Serial No. 334,159, an entire field may be drained in a systematic manner.
It may be that two or more oil bearing formations exist at different elevations. In such a case, it is within the concept of my invention that a vertical mine shaft may be employed, and upper and lower laterals may extend from the mine shaft into each of the oil bearing formations.
As hereinbefore described, the techniques of the present invention are principal]y directed to recovery of relatively heavy oils. However, it should be noted that these techniques are not limited to heavy oils only, but can be used with substantial affect in recovering hydro-carbons of various weights and gravities.
Other alternate forms of the present invention will suggest themselves from a consideration of the appara-tus and practices hereinbefore discussed. Accordingly, it should be clearly understood that the systems and techniques depicted in the accompanying drawings, and described in the foregoing explanation, are intended as exemplary embodiments of my invention, and not as limitations thereto.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of recovering oil and the like from a subsurface earth formation, comprising the steps of:
establishing a shaft hole extending from the sub-surface of the earth to said subsurface formation;

drilling a plurality of boreholes substantially laterally from the shaft hole into said subsurface earth formation;

drilling a second plurality of boreholes, all terminated substantially at another second level;

injecting into said formation through said second plurality of boreholes a displacing fluid to establish a sub-stantially horizontal stratum of said displacing fluid;

injecting into said formation through said second plurality of boreholes a driving fluid under pressure to vertically drive said stratum of said displacing fluid through said formation; and withdrawing oil from said shaft hole through said first plurality of boreholes while simultaneously injecting said driving fluid into said formation.

2. The method described in claim 1, wherein said step of drilling said second plurality of boreholes includes drilling a plurality of boreholes from the surface of the earth to the portion of the formation above said first plurality of boreholes and making outlet holes from said second plurality of boreholes at a level above said first plurality of boreholes,

3. The method described in claim 2, wherein said displacing fluid comprises substantially steam while said fluid under pressure comprised substantially a non-condensible gas.

4. The method described in claim 2, further including the steps of:

monitoring recovery of said driving fluid from each one of said first plurality of boreholes; and injecting a fluid comprised substantially of water through selected ones of said first plurality of boreholes in accordance with withdrawal of said driving fluid from each one of said first plurality of boreholes in order to prevent further withdrawal of said driving fluid.

5. The method described in claim 2 wherein another driving fluid is injected through said first plurality of boreholes into the formation in order to vertically drive the oil towards said second plurality of boreholes; and withdrawal of oil from said second plurality of boreholes while simultaneously injecting said second driving fluid into said formation.

6. The method described in claim 2 further including the steps of:

injecting the soaking fluid through said first plurality of boreholes into said formation until said fluid reaches at least one of the second plurality of boreholes;

thereafter injecting said driving fluid through said second plurality of boreholes into said formation;

establishing at least 100 psi (7 kg/cm2) pressure difference between said first and second pluralities of borehole by injecting said driving fluid; and maintaining such pressure difference during driving the oil towards said first plurality of boreholes.

7. The method described in claim 1 wherein the step of drilling the second plurality of boreholes includes drilling said second plurality of boreholes form the surface of the earth into the portion of said formation located below said first plurality of boreholes where said second plurality of boreholes are termin-ated at substantially the same level.

8. The method described in claim 7 further including:

monitoring the recovery of said driving fluid from each one of said first plurality of boreholes; and injecting a fluid comprising substantially water through selected ones of said first plurality of boreholes depending upon the withdrawal of said driving fluid from each one of said first plurality of boreholes in order to prevent further withdrawal of said driving fluid,

9. The method described in claim 7 further including the steps of:
thereafter injecting another driving fluid into said formation through said first plurality of boreholes in order to vertically drive the oil towards said second plur-ality of boreholes; and recovering oil from said second plurality of bore-holes while simultaneously injecting into said formation said second driving fluid.

10. The method described in claim 1, wherein the step of drilling said second plurality of boreholes includes drilling another plurality of boreholes radially from said shaft hole at another level, different from that of said first plurality of boreholes.

11. The method described in claim 10, wherein said dis-placing fluid is comprised substantially of steam whereas said driving fluid is comprising substantially a non-condensible gas.

12. The method described in claim 10, further including the steps of:

thereafter injecting another driving fluid into said formation through said second plurality of boreholes in order to vertically drive the oil towards said first plurality of boreholes; and recovering oil from said first plurality of bore-holes while simultaneously injecting said driving fluid into said formation.

13. The method described in claim 12, further including the steps of:

measuring oil production from each one of said second plurality of boreholes; and controlling the injection of said driving fluid through selected ones of said first plurality of boreholes depending upon production of oil form each one of said second plurality of boreholes.

14. The method described in claim 10, wherein the step of injecting said driving fluid also includes:

establishing at least 100 psi (7kg/cm2) pressure differerence between said first and second pluralities of boreholes by injecting said driving fluid; and vertical driving of oil towards said second plur-ality boreholes.

15. The method described in claim 10, further including the setps of:

injecting said soaking fluid into said formation through said second plurality of boreholes until said soaking fluid reaches at least one of said first plurality of bore-holes; and injecting said soaking fluid which precedes the injection of said driving fluid into said formation.

16. Apparatus for recovering oil from a subsurface earth formation, comprising:

a shaft extending from the surface of the earth to said subsurface earth formation;

a first plurality of boreholes extending radially from said shaft and lying within a substantially horizontal first plane within said formation at a first elevation;

a second plurality of boreholes all terminated sub-stantially at another second level and comprising fluid out-put means for defining a layer of fluid;

injection means introducing a displacing fluid through said second plurality of boreholes to provide a layer of displacing fluid;

pressurizing means introducing a driving fluid through said second plurality of boreholes for driving the layer of displacing fluid vertically toward said first plur-ality of boreholes; and recovery means for withdrawing oil from said first plurality of boreholes.

17. Apparatus as defined in claim 16, wherein said first plurality of boreholes lies in a substantially horizontal first plane in a lower portion of said formation, each of said second plurality of boreholes extends from said surface of the earth and terminates at said second elevation in an upper portion of said formation, and said injection means comprises means for injecting a displacing fluid less dense than said oil.

18. Apparatus as defined in claim 16, wherein said first plurality of boreholes lies in a substantially horizontal first plane in an upper portion of said formation, each of said second plurality of boreholes extends from said surface of the earth and terminates at said second elevation in a lower portion of said formation, and said injection means comprises means for injecting a displacing fluid more dense than said oil.

19. Apparatus as defined in claim 16, wherein each of said second plurality of boreholes extends radially from said shaft.