Recovery of tars and heavy oils by gas extraction
US 3126961 A
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March 1964 F. F. CRAIG, JR.. ETAL 3,126,961
RECOVERY OF TABS AND HEAVY OILS BY GAS EXTRACTION Filed Nov. 23, 1959 FORREST F. CRAIG, JR. HOWARD GREKEL KAROL L. HUJSAK INVENTORS ATTORNEY United States Patent OfiFice 3,126,961 RECOVERY OF TARS AND HEAVY OILS BY GAS EXTRACTION Forrest F. Craig, Jr., Howard Grekel, and Karol L.
Hujsak, Tulsa, 01:121., assignors to Pan American Petroleum Corporation, Tulsa, Okla, a corporation of Delaware Filed Nov. 23, 1959, Ser. No. 854,665 13 Claims. (Cl. 166-40) The present invention relates to an improved method for recovering valuable products, such as petroleum, from underground reservoirs by means of injecting a hot gas into such reservoirs. More particularly, it is concerned with a novel method for the recovery of hydrocarbons such as, for example, heavy oils or tars by contacting them with a hot gas or vapor condensable under the conditions prevailing in the reservoir.
Some of the largest known petroleum deposits in the world are the Athabasca tar sands located in northern Alberta. It has been estimated that this area alone contains approximately three hundred billion barrels of oil.
"Other huge deposits of a similar nature are to be found in areas of the United States and in Venezuela. Owing, however, to the highly viscous nature of these deposits, their production has presented an extremely difiicult problem. Numerous proposals have been made in an effort to recover such material including, for example, processes involving mining the tar sand and thereafter centrifuging it in the presence of certain solvents and surface active agents to separate the tar from the sand with which it is associated. Also, attempts have been made to recover oil from the tar sand by subjecting the latter to treatment with hot water and separating the resulting upper oil layer. These and other methods which have been used, however, all require large labor and capital expenditures rendering such procedures economically unattractive. Also, recovery of tars and heavy oils in place by means of steam extraction has been proposed. However, the large heat requirements and the resulting high condensate-to-tar ratios obtained under the conditions used rendered such methods impractical.
Accordingly, it is an object of our invention to provide a practical method for recovering heavy oil or tars in place by contacting such materials with a hot condensable gas, thus reducing the viscosity of said oil and tars, and allowing them to flow to the production well. It is a further object of our invention to provide a means for recovering these hydrocarbons in place under conditions such that maximum utilization of the heat in said gas is realized. It is another object to assist production of the heavy oil or melted tar by a pressurized gas drive in addition to the normal drainage of hydrocarbons into the well due to gravity forces. Another object of our invention is to obtain this gas drive by maintaining a packer or equivalent seal in the Well opposite the oil or tar-bearing zone and at or below the liquid level in said zone, thereby causing said gas to force the heated hydrocarbon of reduced viscosity into a zone of lower pressure below the packer.
Briefly, the process of our invention involves setting a packer opposite the heavy oil-bearing zone in a cased or uncased hole and thereafter injecting high pressure steam or equivalent gas into the well, causing this injected gas to enter the oil-bearing zone at a level above the packer. Preferably, before the packer is set, the formation is warmed up to a temperature of from about 250 to about 450 F. by the injection of a hot gas, ordinarily preferably steam or mixtures of light hydrocarbons and steam. In the event it is difiicult to set a packer in the open hole, a suitable perforated casing should be used. With or without such casing, the heavy oi1bearing formation is 3,126,961 Patented Mar. 31, 1964 heated to a temperature such that the oil in or near the formation face has its viscosity reduced sufficiently and drains into the Well. When the oil in the formation adjacent the well is draining freely, which in some cases might require a week or more, a packer is set preferably on tubing a few feet, for example 5 or 6 feet, below the top of the pay zone. The pressure below the packer is controlled so that it is less than that above it, thus accelerating the production rate of oil into the well. In our work, We have found this rate to be approximately 3 times that secured through the use of steam aided only by gravity drainage. The introduction of the hot gas is continued and the packer is left in the same position as long as the producing condensate-to-oil ratio is less than 3 barrels per barrel of tar. The elapsed time before the packer needs to be reset in a typical case where tar sands are involved, is ordinarily about six months. This is particularly true with respect to the first re-setting. As the formation becomes more and more depleted with respect to tar, more condensate tends to form over a given period of time. The period required for resetting the packer will also, of course, be dependent upon the thickness of the tar section above the packer.
When the produced liquid indicates a ratio of producing condensate-to-oil of about 3, the packer should be lowered another several feet, typically 5 or 6 feet and reset, after which injection of hot condensable gas is resumed. This operation is continued until the producing condensate-oil volumetric ratio again reaches a value of about 3. Such procedure should be employed in order to keep the packer at a location where only heavy oil or tar and/or condensate can flow into the well below the packer. When steam or gas channels past the packer, it should be lowered as indicated above. This condition, i.e., the bypassing of steam around the packer, is indicated when the condensate-oil ratio reaches a value of about 3. Moreover, if the packer is not lowered further down the well after a ratio of 3 is reached, the condensatetar ratio increases rapidly.
The packer is repeatedly lowered until it reaches a level of about two-thirds to four-fifths of the way through the producing zone. When the process has progressed to this extent, production should be placed on an intermittent basis. Thus, the well is shut in for a short period of time, e.g. 20 days, to allow draining oil to resaturate the formation up to the packer level, after which hot gas injection can be resumed until substantial bypassing around the packer occurs. At that time, the well is again shut in and the cycle repeated. As production under these conditions continues, the periods for which the well is shut in are preferably increased to avoid unnecessarily short producing periods.
In carrying out the process of our invention, it is important to provide conditions such that the driving gas is forced to pass through a relatively thick section of the pay zone before it re-enters the Well below the packer as condensate. For a packer of given length, the tendency of the driving gas to bypass it will, of course, be greater when the pressure drop across the packer is high. This tendency of the gas to bypass can be dealt with generally by varying the length of the packer directly with the amount of pressure drop across it. Thus, where this pressure drop is only of the order of about 25 psi, the packer may be no more than about one foot in length. However, for each 25 psi increase in pressure drop, the packer length should be increased about one foot up to a maximum of from about six to eight feet for pressure drops of the order of 200 psi. and above. When operating in a cased hole, the perforations made opposite the tar or oil deposit and below the packer at a given setting should be sufficiently large or numerous to give an over-all open area that will in no way tend to restrict the flow of liquid hydrocarbons into the Well. Ordinarily, the perforated area through which the hot gas is injected into' the deposit need not be as great as the perforated area below the packer so as to promote a relatively unrestricted flow of oil into the well. In place of perforated casing opposite the producing formation, it may be desirable in certain instances to employ sections of highly porous materials. As examples of such materials there may be mentioned formed bodies made from ceramic or sintered particles.
One of the outstanding features of our invention is the fact that we employ conditions which permit production ofoil by means of a combination of gravity drainage and hot gas drive. This is accomplished by maintaining a pressure differential across the packer of from about to about 200 p.s.i., and preferably from about 50 to about l60'p.s.i. This pressure differential may be controlled in a number of ways, however, it is most generally practical to regulate it via the extent to which the production outlet at the well head is opened.
As long as a liquid seal is maintained in contact with the packer, it is impossible for the gas to bypass into the zone of lower pressure below the packer. Accordingly, maximum utilization of the heat is realized by condensation of the gas within the formation. A further advantage afforded by our invention, although it is incidental to the main oil recovery process, is the fact that when the resulting hot condensate reaches the low pressure zone of the well bore below the packer, it flashes, and in so doing assists the flow of oil out of the well. In this connection, it is to be pointed out that the pressure in the portion of the well bore below the packer should not be too low or excessive flashing of condensate may occur causing an undesirable cooling elfect on the oil, increasing its viscosity and making it difiicult to flow readily.
Although the composition of the gas or vaporous mixture used in the process of our invention to heat the oil and force it into the well may vary widely and, hence, render possible the use of a large number of substances as the driving gas, such materials should all have the common property of being not only condensable under the reservoir conditions encountered, but should, in addition, be capable of forming a liquid condensate that is substantially immiscible with the oil. While this, of course, rules out pure or substantially pure hydrocarbons and other solvents for these heavy oils or tars, we may use mixtures of the lower molecular weight hydrocarbons such as butane, hexane, kerosene and the like, with steam in addition to the use of steam by itself. Generally, a good source of suitable hydrocarbons for this purpose is the light ends produced when the tar product is subjected to visbreaking operations. In the case of heavy oils, such light ends may be obtained in a similar manner or by milder heat treatment thereof. Also, we may add to the steam a solvent (other than a hydrocarbon) for the tar or oil to give a satisfactory driving medium. Likewise, mixtures of such solvents, together with hydrocarbons of the type specifically mentioned immediately above, may be admixed with steam to form a suitable driving medium. The amount of hydrocarbon added, however, should not be sufiiciently great to give a mixture having essentially the same extraction characteristics as the pure hydrocarbon. These hydrocarbons or non-hydrocarbonaceous solvents may be added to the steam. From our observations, incorporation of these materials into the steam in amounts substantially exceeding 2 or 3 weight percent has no added beneficial effect.
Our invention will be further illustrated by reference to the accompanying drawing wherein a casing 2 is set through the overburden 4 and extends for a short distance into tar sand formation 6. Casing 2 is held rigidly in place by cement 3. Steam, at about 400 F., is injected into the well through pipe 10. The flow of steam, controlled by valve 11, proceeds down the well and along the face of the tar sand 6 situated above packer 12, mounted on tubing 14 forcing the steam into that portion of the sand above the level of the packer. The region 16 represents a portion of the formation from which tar has drained, as the flowing tar is shown diagrammatically in a zone or layer 18. When the steam enters and penetrates the formation, it gives up its heat to the tar and the resulting water of condensation flows downwardly, and mixes with the melted tar designated in layer 13 to form a nonhomogeneous composition resting on an uncontacted portion of the sand 21. It should be pointed out here that a sharp separation between hydrocarbon and water layers usually does not occur, since in some instances, particularly in the case of tars, the specific gravity of the hydrocarbon frequently is about equal to or slightly greater that that of water. For example, the specific gravity of Athabasca tar is typically about 1.03. Also, in some instances heavy oil-water emulsions may be formed.
As steam injection continues, the liquid of layer 18 and 21 flows into the well below packer 12. The flow of both hydrocarbon and condensate liquids into the Wellbelow packer 12 is aided by the fact that the pressure in that portion of the well is approximately to about 200 p.s.i. less than the pressure above the packer. This pressure difference is controiled by manipulation of valve 22 in pipe 24, which may be open to the atmosphere. The mixture of liquid tar and hot condensate flows into the well and into tubing 14 within which is a ball valve 26. Above the valve and connected to the bottom of the tubing is a conventional type pump 27, operated from the ground surface by means of sucker rod 28. The power source at the surface for operating the pump has been omitted since the pump mechanism, including the tubing, pump, valve and power source at the surface are of conventional design.
As liquid tar flows into tubing 14 via valve 26, its upward travel is assisted by the flashing of liquid condensate as it flows into the reduced pressure region within the well and is converted into steam. The tar is easily main tained in a readily flowable condition owing to the fact that tubing 14 is heated along substantially its entire length above the packer by steam entering the well through pipe 10. In some cases, particularly in shallower wells, the lifting force of the steam, coupled with flow of liquids from the formation caused by the initial steam drive, is sufficient to make the well flow without use of a pump.
When the top of layer 18 falls to a level approaching or just below the bottom side of packer 12, bypassing of high pressure steam around the packer occurs, causing an increase in the condensate-oil ratio in the product obtained through pipe 24. Packer 12 is then lowered to a level below the bottom of zone 16, but above the bottom of layer 18, and the above cycle repeated. Proper location of packer 12 is determined by the decrease in the condensate-oil ratio appearing in the product to a value less than 3 and preferably from about 1.5 to 2.
The eifectiveness of the process of our invention over ordinary steam, or equivalent, extraction methods as applied to tar sands, is shown in the examples that follow.
Example 1 Into a cased well approximately 1000 feet deep and having a tar sand section 60 feet thick extending down to the Well bottom, steam at 234 p.s.i. (400 F) is introduced for a period of about one week in order to heat up the formation and reduce the viscosity of the tar near the face of the tar-bearing zone. Thereafter, the casing is perforated at a level corresponding to about the top of the tar sand section. Other perforations of the casing are made at about 10 or 12 foot intervals down to the base of the sand. A packer is next set on production tubing approximately 6 feet below the top of the tar sand and between the first and second set of perforations. Af the Steam has had an opportunity to melt the tar and force it through the perforations below the packer and into the well, liquid tar and condensate flow up the tubing (without pumping) and out of the well at the rate of about 64 barrels per day. While producing at this rate, the pressure drop across the packer is about 60 p.s.i. Initially, the condensate-to-tar ratio is about 1. When the ratio reaches a value of 2.9, the packer is lowered to about the midway point between the second and third set of perforations and the above cycle repeated. This procedure is continued until the bottom of the sand is reached. Throughout the operation, the well produces at the rate of from about 60 to 65 barrels of oil per day.
Example II Under conditions identical to those in Example I, except for the fact that no packer is used to create two zones of different pressure, tar is produced at a rate of 24.2 barrels per day with a condensate-tar ratio of about 3.2. The well bore extending into the tar sand is uncased.
Example 111 Another run is made under conditions identical with those used in Example H except that instead of pure steam, a mixture of steam and hexane (10.6 weight percent) is used. The production rate observed is about 28 barrels of tar per day, with a condensate-tar ratio of about 2.8.
Example IV In another well, the injection gas was altered by incorporating into the steam approximately 2 weight percent of hexane. Otherwise, the conditions used are identical to those set out in Example I. The average daily production over about a two-year period is of the order of 71.3 barrels per day. The average condensate-tar ratio is about 1.5. During this period, no resetting of the packer is required.
When the producing condensate-tar ratio exceeds a value of about 3.0 the packer should be reset. As previously mentioned, we have found no additional benefit to result from using concentrations of hydrocarbons in steam greater than about 2 or 3 weight percent. For instance, in Example III a hexane concentration of 10.6 weight percent was mixed with the steam to get a daily recovery of 28 barrels, whereas in another run, only 1.8 weight percent hexane was mixed with steam under otherwise identical conditions to produce liquid hydrocarbons at the rate of 30 barrels per day.
The temperature employed in carrying out the process of our invention will vary rather widely depending in part at least on the nature of the hydrocarbon being recovered. If the product involved is Athabasca tar or similar tarry material, the gas temperature should ordinarily not be allowed to fall appreciably below 300 F.; otherwise the viscosity of the tar becomes too high to produce readily. On the other hand, with some low gravity crudes, gas temperatures of the order of about 250 F. may be used without appreciable drop in production rate. In any event, the pressure drop maintained across the packer is equally important, since in order to achieve the desired flow from the formation into the well bore, this pressure difference should be present. As previously mentioned, this pressure drop should not be so great as to cause cooling of the hydrocarbons to the point where the increase in viscosity interferes with practical production rates.
As used in the present description and claims, the expression heavy hydrocarbon oil is intended to mean and include bituminous tars as well as heavy viscous oils and low-gravity crudes. Also, it should be understood that the term gas as used herein is intended to refer only to a substance that can be condensed under the reservoir conditions employed in the process of our invention, and which forms a condensate that is substantially immiscible with the produced hydrocarbons.
1. In a method of recovering a heavy hydrocarbon oil from an underground deposit thereof penetrated by a Well by injecting a hot gas into said well and then into said deposit, said gas being capabale of forming a liquid condensate under reservoir conditions that is substantially immiscible with said oil, the improvement which comprises forming upper and lower zones in said well by placing across the bore of said well at an intermediate level of said deposit a barrier forming a substantially fluidtight seal with said well, introducing said hot gas into said upper zone and then into said deposit whereby the viscosity of said oil is substantially reduced and to produce a liquid condensate of said gas in said deposit so as to form a substantially gas-tight liquid seal at the level of said barrier, maintaining a pressure differential across said barrier of from about 15 to about 200 p.s.i. so as to direct the flow of oil into said lower zone, periodically adjusting the level of said barrier so that the latter coincides with the level of said liquid condensate, the level of said seal moving downwardly with time as fluids are produced into said well from said lower zone, and producing from said well the fluids entering from said lower zone.
2. In a method for recovering a heavy hydrocarbon oil from an underground deposit thereof penetrated by a well by injecting a hot condensable gas into said well and then into said deposit, said gas being capable of forming a liquid condensate under reservoir conditions that is substantially immiscible with said oil, the improvement which comprises forming upper and lower zones in said well by placing across the bore of said well at an intermediate level of said deposit a barrier forming a substantially fluid-tight seal with said well, introducing said hot gas into said upper zone and then into said deposit whereby the viscosity of said oil is substantially reduced and to produce a liquid condensate of said gas in said deposit so as to form a substantially gas-tight liquid seal at the level of said barrier, maintaining a pressure differential across said barrier of from about 15 to about 200 p.s.i. so as to direct the flow of oil and the resultant condensate into said lower zone and producing from said well the fluids entering from said lower zone until the ratio of condensate to oil is not more than about 3, thereafter resetting said barrier at a lower level opposite said deposit and coinciding with the level of said liquid condensate to form new upper and lower zones and to again form a substantially gas-tight liquid seal at the level of said barrier, and repeating the above cycle, said lower level being such that on subsequent injection of said hot gas and producing said well, the condesate-oil ratio is substantially less than 3.
3. In a method for recovering a heavy hydrocarbon oil from an underground deposit thereof penetrated by a well by injecting a hot gas into said well and then into said deposit, said gas being capable of forming a liquid condensate under reservoir conditions that is substantially immiscible with said oil, the improvement which comprises running a production tubing into said well to a depth below the top of said deposit, sealing off upper and lower zones in said well at a level opposite an intermediate point in said deposit and at an intermediate point on said tubing, introducing said hot gas into said upper zone and then into said deposit whereby the viscosity of said oil is substantially reduced and to produce a liquid condensate of said gas in said deposit so as to form a substantially gas-tight liquid seal in said deposit at said level, maintaining a pressure differential across said upper and lower zones of from about 15 to about 200 p.s.i. so as to direct the flow of oil into said lower zone, producing from said well the fluids entering said lower Zone, and periodically adjusting the level of the seal between said upper and lower zones so that the level of said seal coincides with the level of said liquid condensate, the level of said seal moving downwardly with time as fluids are produced into said well from said lower zone.
4. In a method for recovering a heavy hydrocarbon oil from an underground deposit thereof penetrated by a well by injecting a hot gas into said well and then into said deposit, said gas being capable of forming a liquid condensate under reservoir conditions that is substantially immiscible with said oil, the improvement which comprises running a production tubing carrying a packer into said well to a depth below the top of said deposit, setting said packer opposite an intermediate level in said deposit to form a substantially fluid-tight seal in said well, introducing said hot gas into said upper zone and then into said deposit whereby the viscosity of said 011 is substantially reduced, and to produce a liquid condensate of said gas in said deposit so as to form a substantially gas-tight liquid seal in said deposit at the level of said packer, maintaining said lower zone at a lower pressure than said upper zone so as to direct the flow of oil into said lower zone,tperiodically adjusting the level of said packer so that the latter coincides with the level of said liquid condensate, the level of said seal moving downwardly with time as fluids are produced into said well from said lower zone, and producing from said well the fluids entering from said lower zone.
5. The process of claim 1 in which the operation is carried out in a cased hole and wherein communication between said upper and lower zones is established through perforations in the casing above and below said barrier.
6. The process of claim 1 in which the operation is carried out in an uncased hole.
7; The process of claim 1 in which the hot gas is steam.
8. The process of claim 7 in which the steam employed is at least about 250 F.
9. The process of claim 1 in which the deposit is tar and the hot gas is at a temperature of at least about 350 F.
10; The process of claim 2 in which the hot gas employed consists essentially of steam.
11. The process of claim 2 in which said hot gas consists essentially of a steam-hydrocarbon mixture employed at a temperature of at least about 250 F.
12. The process of claim 4 in which the hot gas employed consists essentially of steam.
13. The process of claim 4 in which the hot gas employed consists essentially of a steam-hydrocarbon mixture employed at a temperature of at least about 250 F.
References Cited in the file of this patent UNITED STATES PATENTS 1,816,260 Lee July 28, 1931 2,412,765 Buddrus et al Dec. 17, 1946 2,765,850 Allen Oct. 9, 1956 2,821,255 Spearow Jan. 28, 1958 2,862,558 Dixon Dec. 2, 1958 2,881,838 Morse et al Apr. 14, 1959