US 3141502 A
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July 21, 1964 DEW ETAL 3,141,502
METHOD OF CONDUCTING IN SITU COMBUSTION Filed Nov. 12, 1959 5 Sheets-Sheet 1 (Kl/*Tf) Average 2 Flux, Scf/Hr. Ft.
INVENTORS J. N. DEW M. L. MART/N ATTORNEY July 21, 1964 J. N. DEW ETAL 3,141,502
METHOD OF CONDUCTING IN SITU COMBUSTION Filed Nov. 12, 1959 5 Sheets-Sheet 2 O Q E L6 3 x c o a '0 4 5 m L4 0 LL a: .Q l p 1.2
U m (I '6 1 o l l o INVENTORS J /V. DEW By WLMART/N ATTORNEY July 21, 196 J. N. DEW ETAL METHOD OF CONDUCTING IN sn'u COMBUSTION Filed NOV. 12, 1959 5 Sheets-Sheet 3 m 6 I kmmu mmom jmz, 20mm wozEbE 4454a ow o ow om o on om o o T Sbd mmm O "mmm I n" I o EOE 29582 8 @2502 2253: 8. 6E3? 58 July 21, 1964 J. N. DEW ETAL 3,141,502
METHOD OF CONDUCTING IN SITU COMBUSTION Filed Nov. 12, 1959 5 Sheets-Sheet 4 A FORMATION CONTAINING LIGHT CRUDE 1 200' L 800' T I HEAVY B CRgJDE LIGHT CRUDE 20% HYDROCARBON PORE VOLUME INJECTED L 220 I 780' 41 I T T 90% cm 5A1.
(; l0% GAS sAT. LIGHT CRUDE HEAVY CRUDE AIR OR GAS DRIVE BEFORE IGNITION 2o' 1 L 2I5' 765' CLEANED SAND I I COMBUSTED zoNE- D 25%; LIGHT CRUDE COKE ZONE IGNITION STARTED 8I COMBUSTION FRONT ADVANCED 600' 50 350' #4 I T I COKED ZONE E CLEANED SAND LIGHT CRUDE COMBUSTION ZONE HEAVY CRUDE COMBUSTION ZONE UGHT F CLEANED SAND I CRUDE com-:0 ZONE INVENTORS F G. 4 .1. /v. 05w
BY WLMART/N TTORNEY July 21, 1964 J. N. DEW ETAL METHOD OF CONDUCTING IN SITU COMBUSTION Filed NOV. 12, 1959 5 Sheets-Sheet 5 mmDbqmmmEmE. ZOrZsEOu DISTANCE FROM WELL BORE FIG. 54
T MINIMIZED HEAT LOSS DISTANCE FROM WELL BORE FIG. 55
United States Patent 3,141,502 METHQD 0F CONDUCTING IN SITU COMBUSTEGN John N. Dew and William L. Martin, Ponca City, Okla.,
assignors to Continental Oil Company, Ponca Qity,
Okla, a corporation of Delaware Filed Nov. 12, 1959, Ser. No. 852,302 6 @iaims. (Cl. 16611) This invention relates to a process of starting and propagating underground combustion and more particularly to a process for the recovery of oil by in situ combustion from reservoirs containing high gravity crude which normally burns by direct air injection without leaving sulficient combustion supporting residue for a self sustained combustion process in the immediate vicinity of the ignition well bore.
Petroleum is usually found associated with sandstone or porous limestone deposits situated between impervious layers of shale, or rock and the like. In most instances, the oil contains lighter gaseous hydrocarbons, such as methane, ethane, propane, etc., which may exist as free gases in contact with the oil or dissolved in the oil itself. The pressure under which the oil and associated gases exist is usually proportional to the depth of the deposit below the surface. When such an oil bearing sand is reached by drilling, oil is produced by flowing to the surface under the expansive force of the gases at well pressures, whether the gases are free or dissolved in the liquid oil. Thus, the oil and gas are both forced into the region of low pressure around the well bottom. Depending upon the total pressure exerted, and the conditions at the mouth of the well, the upward movement of the oil and gas may create a flush production, for example, in the form of a gusher. During this stage of production, most of the gas associated with the oil escapes, and the motive power bringing the oil to the surface is dissipated.
Pumping is then initiated in order to continue to recover oil. During this stage, gas associated with the oil continues to escape from the casing head. Subsequently, the flow of oil becomes economically unprofitable with respect to further utilization of pumping means.
When the well attains this condition in which the heavier hydrocarbons obstruct the pores of the sand and no longer flow freely to the well bottom, the method of repressuring the well system is commonly practiced. This involves forcing back into selected central wells air or gas which penetrates the sands and finds exit from the adjacent wells communicating with the oil reservoir under treatment. The air or gas mechanically forces the crude oil to the venting well bottoms where it is removed by pumping, and the more volatile portions of the residual oil are entrained in the gas or air stream and thus removed from the well.
In time repressuring becomes no longer expedient, and final resort may be had to the flooding of the oil field with water to drive additional amounts of the residual oil contained in these sands into the wells. After flooding has been utilized to the point where it is no longer profitable, the flooded field becomes non-productive and must be abandoned.
It is well known that in the fields subjected to the foregoing treatment, nearly half of the oil known to be initially present is still left as residual oil in the sands. At the present time, the problem of recovering this vast amount of residual oil has become an urgent one, particularly due to the increasing demand for petroleum and petroleum products, and the rapidly diminishing number of discoveries of new oil fields.
It has heretofore been recognized that oil may be recovered by applying heat to oil-containing sands in their native position. By thus heating the oil-containing sands,
3,3415% Patented July 21, 1964 the heavier hydrocarbons clogging the pores of the sand are rendered less viscous, and the flow thereof through the sand is facilitated. In addition, the more volatile hydrocarbons are distilled from the sand to the venting well casing. Various methods have been proposed for effecting a heating of the oil sands. For example, direct combustion of a portion of the residual oil utilizing air under pressure, or air and a combustible gas to initiate and support combustion of the oil has been proposed. It has also been proposed to pass heated products of combustion in gaseous form through the oil sands in order to effect a heating thereof, whereby the viscosity of the residual oil is reduced and the oil becomes more mobile. However, these prior methods have certain inherent disadvantages which are obviated by the present invention.
In the subterranean combustion process, the oxidizing gas, which may be air, oxygen, oxygen-enriched air, air admived with inert gas to reduce the proportion of oxygen, oxygen admixed with inert gas, or any other suitable oxidizing gas or mixture which will support combustion within the subterranean reservoir, is passed, as by pumping, through an input well to the reservoir in which the combustion process is to be effected and combustion within the reservoir is initiated by suitable means. The flow of oxidizing gas to the reservoir is continued and the combustion gases, oil, and the distillation and viscosity breaking products migrate in front of the combustion zone to an output well or wells leading from the reservoir, from which output well or wells these fluids are removed and thereafter treated for recovery of the desired valuable constituents. The heated fluids migrating in front of the combustion zone strip the oil-bearing sand of the greater portion of the oil leaving behind within the sand a carbonaceous hydrocarbon deposit. The carbonaceous deposit essentially is the fuel consumed in the process and combustion of the carbonaceous deposit continues until the deposit has been substantially entirely consumed. The carbonaceous deposit preferentially burns to form carbon dioxide irrespective of restriction in the supply of oxidizing gas and thereby provides a maximum amount of energy per unit amount of carbonaceous deposit consumed. However, where the amount of carbonaceous deposit is in excess of about 2 percent by weight of the sand, combustion of the carbonaceous deposit provides more than sufficient thermal energy for achieving efficient recovery of petroleum oil by the combustion process,
and the additional amount of oxidizing gas consumed by the carbonaceous deposit constitutes an economic waste not only with respect to the excessive amounts of oxidizing gas which must be pumped to the reservoir but also with respect to the excess pump capacity required.
One such method comprises establishing a combustion zone around a production well by conventional methods so as to provide a combustion zone and a heat reservoir of sufficient extent and temperature to permit cutting oif the direct flow of air through the production well and injecting air into the formation through one or more spacedapart wells from the production well so as to cause the air to flow to the combustion zone at the production well and support combustion therein so that the combustion front is advanced countercurrently to the flow of air toward the injection well or wells. This technique is designated inverse air injection in situ combustion as opposed to direct air injection through the well or bore hole around which combustion is initiated.
Another recent development in recovery of oil by in situ combustion comprises continuing the injection of air through one or more injection wells after the combustion front has been advanced, by inverse air injection, to the injection well or Wells so as to reverse the movement of the combustion front and drive the same back through the formation to the production well around which combustion was originally initiated. In this technique, designated thermal echo, the returning combustion front feeds on the residual carbon deposited in the formation during the inverse air injection phase of the process.
It has been found that in many oil-bearing formations the crude is of such high API gravity (and low carbon residue) that the hot gases from a combustion front initiated around an injection Well drive the hydrocarbon materials substantially completely away from the area in front of the combustion front thereby leaving insufficient fuel to sustain combustion and the fire goes out. This renders it impossible to initiate combustion and build up a sufficient combustion zone and heat reservoir to permit reversing the direction of the flow of air to the combustion zone in order to establish inverse air injection to support and drive the combustion zone toward the surrounding injection wells and away from the well in which the combustion is originally initiated.
J. C. Trantham and H. 0. Dixon in US. Patent 2,889,881 disclose a back burn process for the recovery of oil which comprises depositing a heavy hydrocarbon oil, such as heavy crude oil, in the formation surrounding a production well or bore hole and initiating combustion of the deposited heavy oil so as to burn a sufficient amount of the oil to establish a combustion zone which contains enough heat to permit cutting off the direct injection of air or other combustion-supporting gas and feeding air to the combustion zone from one or more spaced-apart wells or bore holes through the formation whereby the combustion zone is advanced toward the injection well and combustion products and produced hydrocarbons are driven to the production well from which they are recovered by conventional means. The heavy oil is injected into the formation through a well or bore at a selected location so as to penetrate the formation for several feet (at least 3 or 4 and preferably to feet) surrounding the bore hole. The deposited oil is then ignited by any suitable means such as by the use of a squib, an electric heater, a gas heater, or any other heating device supplemented by injection of an oxygen-containing gas (preferably air) to support the combustion of the heavy oil. It is also feasible to pump hot air at combustion supporting temperatures down the well or bore hole until the temperature of the deposited oil and formation which it occupies are brought to a temperature sufiicient to initiate combustion which is in the range of approximately 500 to 700 F.
Burning of the oil in the formation surrounding the bore hole leaves a carbon residue which is sufficiently hot to support combustion when the direction of the air is reversed and fed thereto from surrounding injection wells. The quantity of deposited oil burned in the formation must be sufiicient to build up a reservoir of heat which holds the temperature sufficiently high to support combustion when the inverse air reaches the combustion area. It is essential to burn only a portion of the deposited heavy oil so that a reserve is left to prevent vaporization of all of the original oil in the formation directly outside of the deposited oil and to provide a continuous bed of fuel from the heat reservoir or combustion zone to the injection wells. The arrival of the inverse air from. the surrounding injection wells then revives the combustion zone. After inverse air injection and attendant combustion is established, the remaining heavy oil is burned and, as the inverse air injection is continued, the combustion front is propagated radially outwardly and laterally past the limit of the heavy oil saturation and the burning is continued in the light oil reservoir so as to advance the combustion front to the injection wells.
It is an object of our invention to provide a process for initiating in situ combustion and recovery of oil in an oil reservoir containing crude of high API gravity by means of a forward drive process. Another object of our invention is to provide a method of oil recovery by in situ combustion from an oil bearing formation which is incapable of supporting in situ combustion during ignition by direct air injection. Other objects and advantages of the invention will become apparent as the invention is hereinafter disclosed.
Other objects and advantages of the invention will be evident from the following detailed description when read in conjunction with the accompanying drawings which illustrate our invention wherein:
FIGURE 1 is a graphic representation of the relationship of the ratio of effective formation gas permeability in millidarcies to the product of centipoise gas viscosity and formation temperature expressed in Rankine to the ratio of the distance of a combustion front from an injection well to the distance between the injection well to a production well.
FIGURE 2 depicts the relationship between fuel requirements for supporting combustion and the rate of advance of a combustion front.
FIGURE 3 shows the relationship between fuel residue in a combusted area necessary to support combustion and the radial distance from an injection well.
FIGURE 4A is a schematic representation of a linear system of porous media containing a crude oil not capable of supporting self-sustained in situ combustion.
FIGURE 4-B is a schematic representation of the systerrli of 4-A after injection of a slug of bituminous materra FIGURE 4-C depicts the displacement of bitumen into the system of 4-B by air or gas injection.
FIGURE 4D depicts the movement of a combustion front in the system of FIGURE 4-C after ignition of the bituminous slug.
FIGURE 4-E shows the condition of the system shown in FIGURE 4-D after further movement of the bituminous slug and combustion front.
FIGURE 4-F shows the condition of the system of FIGURE 4-E when the combustion front nears the outlet face of the porous medium.
FIGURE 5A shows formation temperature distributron characteristics in a recovery process using well bore heaters.
FIGURE 5B shows formation temperature distributron. characteristics in a recovery process using a moving in situ combustion front.
The foregoing objects and advantages are attained by a process which may be described briefly as follows: A low gravity crude oil which is rich in high boiling fractions and carbon residue is injected into an underground formatron containing hydrocarbons not capable of supporting self sustained combustion. The injected crude oil is then dispersed into the formation by means of gaseous pressure after which the injected crude oil is ignited. The combustion front is propagated through the formation by the continued injection of a combustion supporting gas into the formation.
Secondary recovery of oil by in situ combustion is a widely discussed method that is finding increasing applicat on. This method is based fundamentally on propagating a combustion front through a permeable oil sand from air injection wells toward oil recovery wells. Heat generated by burning a portion of the oil in place provides a mechanism for recovery of the remaining oil. The front moving outwardly from the injection wells actually consists of several contiguous zones:
(1) A completely combusted Zone (2) The burning zone (3) A coke zone (4) A vaporization-condensation zone (5 A zone flowing three fluid phases (oil, gas, and water It is the coke zone that keeps the combustion process going. Coke is generated as heat from the burning zone is transferred to the oil bank in the vaporization zone ahead of the front. The amount of coke that the oil originally in place can deposit on the sand is a critical factor in propagating the combustion front through the sand.
We have found that a certain amount of coke must be continuously deposited if the combustion process is to be self-sustaining. The heat available by the combustion of this coke must be enough to raise the temperature of the associated rock solids to combustion levels and replace heat losses to the surroundings. This requirement decreases with an increase in original reservoir temperature; therefore, all other factors being equal, the coke laydownrequirements will decrease with the depth of the reservoir. If heat losses are high, coke requirements to maintain self-sustaining combustion temperatures will also be high. If the amount of coke deposited per cubic foot of sand swept drops below a critical value fixed by rock heat capacity and heat losses, combustion will eventually cease. Fortunately, the minimum amount of coke required to maintain combustion at low rates of frontal advance declines as the combustion front moves away from the well bore, due primarily to preheat of the formation ahead of the combustion front. For instance, for a reservoir of 15% porosity at rates of advance in the order of 0.2 foot per day, about 4 pounds of coke per cubic foot of rock may be required to maintain combustion at a distance of 5 feet from the well bore; but at a distance of 30 feet, only 2.5 pounds of coke per cubic foot of sand may be required. Furthermore, the coke requirement decreases as the rate of advance of the combustion front increases. These differences in coke requirements can be attributed to reduced heat losses from the combustion zone as stabilization of the temperature profile of the advancing combustion front is approached.
There are many reservoirs that contain crude oils which are incapable of supplying sufiicient coke to maintain in situ combustion near the injection well bores, while some oils are incapable of supplying sufl'icient coke to maintain combustion in any portion of the reservoir. Frequently, these types of hydrocarbons are found in low porosity and low permeability host rocks. These rock properties further jeopardize the prospects for a selfsustaining combustion front, because we have found that the minimum coke requirement increases as the porosity decreases. The low permeability prevents the use of high air fluxes with correspondingly high rates of advance and reduced coke requirements.
We have discovered that a combustion wave can be initiated and propagated through a given body of porous sandstone with or without liquid saturations by means of the following steps:
(1) Injecting an appropriate fraction of a pore volume .of a hydrocarbon oil, such as a low gravity crude oil, out back asphalt, or residual hydrocarbon material (preferably a bituminous material rich in high boiling fractions 7 and carbon residue) into the sand body.
(2) Further displacing the hydrocarbon into the sand by means of air pressure and air sweep.
(3) Raising the temperature of the air inlet sand face to the ignition point of the hydrocarbon and/ or its residuum or coke.
(4) Propagating the combustion front through the sand by continued air injection.
We have further discovered that the distance through which self sustained combustion can be maintained in porous media is dependent on, and can be controlled by, the amount of hydrocarbons injected.
The heat loss from a moving combustion front varies inversely with the velocity of its advance through the porous media. The rate of advance varies directly with air flux (air rate per unit area) at the front.
The rate at which air or gas will flow between wells in a permeable reservoir may be calculated by means of an appropriate equation based on Darcys law. Equation 1 describes the flow of gas from a central injection r 6 well to four production Wells drilled in an isolated S-spo pattern.
During an in situ combustion operation, the combustion front progresses outwardly from the central injector toward the production wells. The reservoir rock behind the combustion zone is completely devoid of the original fluids and is at an elevated temperature. Thus, the effective permeability of the reservoir to the flow of injected air or gas is higher behind the front than ahead of it, and the viscosity and volume of gas is also higher behind the front. The ratio of permeabilities may vary about 3.3 and 10.
An average value for the ratio (k /nT which appears in Equation 1 may be calculated by means of Equation 2.
Values of the ratio (k T are shown by curves 1, 2,, and 3 on FIG. 1 as a function of the location of the combustion front and the size of well bores for Kl/Kg equal to 10. As shown in FIG. 5 of the paper, Process Variables of In Situ Combustion, by Martin, et al., published in February 1958, Journal of Petroleum Technology, the rate of advance of a combustion front in an oil reservoir is directly related to the air flux at the front.
With high combustion efficiencies this may be expressed by the following equation:
M( ROA 3 where u=air flux-s.c.f./ hr. -ft. A air required/cu. ft. sand cleaned but t,(1000 t, s.c.f. 4)
Definition 0 Symbols The air flux may be estimated at any desired lOCatlOIl between wells by means of Equation 4 above. Let us assume the characteristics of one of the well-known, high-gravity crude oil reservoirs. Thus, for each foot of not pay in the Bradford Pool in Pennsylvania with K =l0 md. and K rl md., a maximum injection pressure of 1000 psi. and 2 /2 acre 5-spot spacing, the maximum attainable air flux according to Equation 4 would vary with location, as shown by curve 4, FIG. 1. The corresponding rates of advance calculated by Equation 3 with A ==420 s.c.f./cu. ft. are shown on curve 1, FIG. 2. As can be seen, the maximum attainable rate of advance decreases rapidly as the front moves away from the injection well if injection pressures and rates are limited, as they commonly are, by formation depth or compressor rating.
As stated previously, the heat loss from an advancing a front varies inversely with velocity. The fuel required to replace these heat losses may be calculated with equations based on the laws of unsteady-state heat transfer. A method for using an analogue computer to solve the equations for fuel requirements has been described in a paper by Vogel and Krueger in the AIME Journal of Petroleum Technology, vol. 7, pp. 208-209, December 1955, entitled An Analogue Computer for Studying Heat Transfer During a Thermal Recovery Process.
By methods described by Vogel and Krueger, it can be shown that the fuel required to replace heat losses varies with location of the combustion front and its velocity of motion or rate of advance. Typical data are shown on FIG. 3.
If the data of FIG. 3 are used in conjunction with the rate of advance and location data of FIG. 2, an estimate of the fuel required per cubic foot of Bradford sand may be deduced. These data are shown as curve 2 on FIG. 2. The dotted line on this curve indicates the effect of decreasing vertical sweep efficiency (channeling) which may occur as the front moves out from the injection well.
The following conclusions may be drawn, based on the data of FIG. 2:
(1) The maximum attainable rate of advance decreases rapidly with distance as the front moves away from an injection well bore.
(2) When low formation permeability and/ or reservoir depth limits injection rates and pressures, fuel requirements are maximized in the vicinity of the injection well bore.
Our invention consists of supplying the required amounts of fuel at the combustion front during its movement through this critical region. We do this by injecting an appropriate amount of a bituminous material rich in high boiling fractions and carbon residue. This material is then further displaced into the formation by air or gas injection and ignited at the sand face by methods known in the art. The combustion front is then propagated away from the well bore at a velocity selected so as to provide for stabilization of the advancing temperature profile of the combustion front at an appropriate location away from the injection well bore. At this location, the crude oil originally in place can supply the required amount of coke deposition. The minimum fuel requirements for some low porosity, low permeability reservoirs may be in excess of that provided by the crude oils contained therein. Thus an extension of our invention consists of injecting a selected sized slug of high boiling, carbonaceous bitumen, so as to provide sufficient fuel to propagate the combustion front over the entire distance between wells. This scheme may be particularly attractive in those areas where heavy, black crudes are a glut on the market (or where heavy crude is easily imported) and high gravity crudes bring premium prices. The data cited above indicate that such a scheme would be required to propagate a combustion front between wells in pools such as the Bradford sand in Pennsylvania and the Ponca sand in the field near Ponca City, Oklahoma. Our data for the latter crude oil show that it does not provide sufficient fuel for self-sustained combustion even in high porosity, unconsolidated sandpacks.
To explain further the application of our invention, reference may be made to FIG. 4. FIG. 4-A is a schematic representation of a linear system of porous media containing a crude oil not capable of supporting self-sustained in situ combustion. FIG. 4-B depicts the injection of a slug of bituminous material which has been sized to provide sufficient fuel to propagate a combustion front through the media. FIG. 4-C depicts the displacement of the bitumen into the media by air or gas injection. The
resulting low gas saturation provides gas permeability through the bitumen and causes an expansion of the size of the slug. FIG. 4-D depicts the movement of the front and the slug into the porous media after ignition by continued air injection, As the front progresses through the media, the bitumen is consumed for fuel and the size of the slug decreases. However, the movement of the front drives the viscous bitumen slug through the media, thus effectively displacing the less viscous and more valuable crude oil originally present in the pore spaces. FIG. 4-E shows how the size of the ,slug has continued to decrease as the front progresses, and FIG. 4-F shows that the slug is essentially consumed as the front nears the outlet face of the porous media.
In practice in the field, the slug size used must provide for the effects of the expanding radius of the burning front as it moves away from the injection well. This is best done on the basis of the volume of rock to be swept and the corresponding fuel requirement. Our data indicate that almost complete recovery of the lighter crude oil may be accomplished by only propagating the combustion front part of the distance between wells. The smallest slug is that required to provide a critical temperature increase which may be desirable to effect complete recovery of fluids from condensate reservoirs and crude oil reservoirs which can support combustion at distances away from the injection well.
The procedure depicted in FIG. 4 has advantages over processes dependent on well bore heating. FIG. 5 shows the formation temperature distribution characteristic of the two procedures. FIG. 5-A shows that processes using well bore heating methods, such as downhole burners, steam generators, heaters, etc., require that the entire formation swept, from injection well outwardly, be held at a high temperature. This results in high heat losses to overand under-burden. FIG. 5B, in contrast, illustrates the relatively small proportion of the total reservoir which is required to be at a high temperature for the process which uses a moving in situ combustion wave for oil recovery. Furthermore, a combustion front can be moved more rapidly through a reservoir than a heat wave can be moved by the injection of heated fluids, further reducing the heat losses to the surroundings. The decreased heat losses accomplished by our invention, i.e. injecting bituminous material into the porous media and buning in situ, is reflected in lower fuel requirements in terms of surface barrels of fuel required to provide the recovery desired from the reservoir. This method of operation produces 2 to 3 barrels of oil originally in place for each barrel of bitumen injected. Based on price differential, the cost of fuel would be in the order of 1 dollar for each 4 to 6 dollars returned.
As to the amount of injected oil required for practicing the invention that can be calculated as follows: The first step is to determine the amount of coke-laydown per cubic foot of porous rock that is necessary to maintain the combustion process. This can be done by referring to FIG- URE 2 where coke required is plotted on the left-hand ordinate versus the fraction of the distance between in the injection and recovery wells that the flame front has covered (r/d). Near the well bore the amount of coke necessary will be seen to change rapidly, but at a point farther from the well the amount necessary levels off to about 1.2 pounds of coke per cubic foot of rock.
After the amount of coke necessary has been determined the proper crude must be selected for the particular reservoir. The proper crude will be one that supplies the amount of coke necessary as determined in the preceding paragraph. A prospective crude should be tested in a laboratory in situ combustion apparatus. As a result of considerable experimental work we have found that crudes of the following API gravities provided the amounts of coke indicated.
Pounds coke per API: foot of rock The amount of oil to be injected into the reservoir can now be calculated as follows:
Volume of oil (in barrels)=(0.125)(V)(l-) where V=volume of the reservoir in ft. to be swept up to the point where combustion will become self sustaining. =the pore fraction of the reservoir.
While the instant invention comprises the several steps and the relation of one or more of such steps to each of the other enumerated steps, it is to be clearly understood that various changes in the method of procedure may be resorted to without departing from the spirit of the invention, and further that the theories set forth, although believed to be accurate, are not to be considered as the sole basis of the operativeness of this invention, but that this method does operate successfully and effectively Whether or not upon the principles described herein, this invention to be limited only by the appended claims. Minor changes to make a preferred adptation to any particular situation may be readily made by one skilled in the art, as trial may indicate.
1. In a process for recovering high API gravity hydrocarbons through a recovery well from an underground formation wherein said high API gravity hydrocarbons are characterized upon combustion by insufficient coke laydown to provide self-sustained combustion in the vicinity of an injection well, the method which comprises the steps of injecting into said formation through said injection well a quantity of hydrocarbon oil sufficient to support combustion to a point in the formation wherein the high API gravity hydrocarbons are capable of providing self-sustained combustion, injecting gas through said injection well into said formation to disperse said hydrocarbon oil into said formation toward said recovery Well, igniting said hydrocarbon oil, injecting a combustion supporting gas into said formation through said injection well to propagate the combustion front through said formation toward said recovery well and recovering hydrocarbon by way of said recovery well.
2. The process of claim 1 in which the hydrocarbon oil is a low gravity crude oil.
3. The process of claim 1 wherein said combustion supporting gas is air.
4. The process of claim 1 wherein the combustion supporting gas isagas containing free oxygen.
5. The process of claim 1 wherein the injected crude oil has an API gravity varying from 10.9 to 41.7.
6. The process defined in claim 1 wherein the quantity of hydrocarbon oil is sufficient to support combustion only to the point in the formation whereat the high gravity hydrocarbons are capable of providing self-sustained combustion.
References Cited in the file of this patent UNITED STATES PATENTS 2,889,881 Trantham et al June 9, 1959