Viscous oil recovery method
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to copending application Ser. No. 06/947,932 filed Dec. 30, 1986 for Viscous Oil Recovery Method. 
     FIELD OF THE INVENTION 
     The present invention is concerned with a process for stimulating the production of viscous oil or petroleum from a subterranean reservoir. More particularly, this invention is concerned with a preconditioning treatment to be applied to a viscous oil-containing formation prior to steam injection in order to increase the steam injectivity in formations containing high concentrations of highly viscous oil or petroleum. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for treating a subterranean oil formation containing very viscous petroleum. It is well known to persons skilled in the art of oil recovery that many subterranean deposits of petroleum cannot be produced by conventional primary means because the viscosity of the petroleum is so high that virtually no petroleum flow can be obtained without applying some treatment to decrease the viscosity of the petroleum prior to production. Steam flooding has been used successfully in many such reservoirs with varying degrees of success. Injection of steam into a formation raises the temperature of the formation petroleum contacted by the steam, thereby reducing its viscosity and increasing its ability to flow when a sufficient pressure differential exists within the formation to move heated petroleum toward a production well where it can be recovered to the surface of the earth. 
     Steam injection has been utilized for recovering viscous petroleum from subterranean deposits in a number of different processes. In one class of steam stimulation process, steam is injected into a well, the well is shut in for a period of time, and then production is taken from the same well as was used for steam injection. This method is commonly referred to as cyclic steam injection or huff-puff steam stimulation. In another general class of steam-stimulated viscous oil recovery methods, steam is injected into a formation via one or more injection wells to displace petroleum through the formation toward a remotely-located well where it is recovered from the formation and produced to the surface of the earth. This second type of steam stimulation is referred to as steam drive. 
     Both of the above-described steam stimulation techniques require that the formation&#39;s steam injectivity be sufficiently high to permit injection of a minimum quantity of steam into the petroleum formation in order to raise the temperature of the petroleum, thereby reducing its viscosity sufficiently that it will move through the formation under the pressure differential imposed by the steam injection. When steam is injected into a subterranean reservoir containing viscous petroleum, the petroleum viscosity is decreased to a point where it will begin to migrate and thereby form a oil bank in the formation. An oil bank is a zone within the formation having a higher oil saturation than the original oil saturation, moving in the general direction of petroleum production well. 
     Certain formations have been found in which steam stimulation is not effective because the formation has very low steam receptivity. These Formations are characterized by high oil viscosity, high oil saturation and are usually fully liquid filled. Even if some steam can be injected at first, the oil bank formed begins to cool at its leading edge as it migrates away from the injection well, thereby resulting in the formation of a high viscosity oil bank which becomes immobile within the formation a short distance from the injection well. Once this occurs, further steam injection is not possible because the high oil saturation in the oil bank reduces the permeability of that portion of the formation which greatly reduces steam injectivity. Once the cooled oil bank forms, it becomes impossible to decrease the viscosity of the immobilized viscous oil bank by contact with steam because no more steam can be injected into the formation. 
     The above described problem has been recognized by persons experienced in oil recovery procedures, and numerous techniques have been described for improving injectivity of steam into formations containing relatively high oil saturations of very high viscosity petroleum. One of the classical methods for increasing the ability of a formation to accept injected fluid is fracturing, but it has been determined that in the formations such as those described above, fracturing of a formation prior to injection of steam is not a satisfactory solution. While steam will move into the formation through the fractures, as it warms the high viscosity petroleum in the portions of the formations adjacent to the openings created by the fracture process, the viscosity of the petroleum is reduced sufficiently to allow the petroleum to flow into the fractures where it is displaced away from the injection well by the injected steam. As the fluid moves ahead of the steam, it cools and again becomes immobile, closing off the fracture flow path (so long as the injection pressure is less than the fracture pressure) thereby resulting in the same problem as was obtained prior to the fracturing of the formation. 
     It has also been disclosed in certain prior references that injection of a non-condensable fluid into the formation prior to or simultaneously with the steam injection will maintain flow channels open sufficiently to permit continuing injection of the steam into the formation for successful steam drive viscous oil recovery. The non-condensable gas does indeed open up certain flow channels which permits deeper penetration of the steam into the formation initially, but the heated oil moves into these flow channels much as was described above for results obtained when the formation is fractured, and the flow channels are soon plugged with the viscous petroleum. 
     In view of the above discussion, it can be appreciated that there is still a substantial, unfulfilled need for a method for treating a subterranean formation having very low steam injectivity because of high content of very viscous petroleum to permit the successful development of a steam drive or a cyclic steam injection oil recovery process within the formation which does not result in the formation of a flow-impeding barrier within the formation as the viscous petroleum cools and becomes immobile. 
     DESCRIPTION OF PRIOR ART 
     U.S. Pat. No. 4,121,661 issued Oct. 24, 1978 describes a method for recovering petroleum from a viscous formation comprising injecting steam and a non-condensable gas in combination with sequentially applied throttling and blowdown steps. 
     U.S. Pat. No. 4,099,568 issued July 11, 1978 describes a steam flooding process for viscous oil formations involving injection of steam and a non-condensable, non-oxidizing gas ahead of or in combination with the steam, in order to reduce the tendency for flow channels to become blocked with viscous petroleum. 
     U.S. Pat. No. 3,908,762 issued Sept. 30, 1975 described a method for establishing a communication path in a tar sand deposit or other very viscous petroleum containing formation using steam and non-condensable gas in a certain described sequence. 
     U.S. Pat. No. 4,607,699 issued Aug. 26, 1986 describes a method for conditioning a subterranean viscous oil-containing formation by fracturing the drainage area by injecting liquid carbon dioxide at pressures in excess of the fracture pressure of the formation prior to injecting steam. 
     U.S. Pat. No. 4,617,993 issued Oct. 21, 1986 describes a carbon dioxide stimulation process employing hydrocarbon to kill the well after carbon dioxide injection. 
     U.S. Pat. No. 4,418,753 issued Dec. 6, 1983 and U.S. Pat. No. 4,434,852 issued Mar. 6, 1984 described oil recovery processes employing nitrogen injection. 
     SUMMARY OF THE INVENTION 
     We have discovered that the problem associated with low steam injectivity in subterranean petroleum-containing formations caused by a high concentration of high viscosity oil may be alleviated by a pretreatment with a totally gaseous phase injection fluid which is comprised of a mixture of from 20-60% inert, nonoxidizing gas which is essentially insoluble in petroleum such as nitrogen and a gas which is soluble in the subterranean petroleum such as carbon dioxide. The fluid choice and injection parameters are critical to the successful application of this process. The preferred fluid for use in this process is a mixture of from 20% to 60% and preferably from 25% to 50% nitrogen and the balance of the gas mixture comprising carbon dioxide or a mixture of carbon dioxide and low molecular weight hydrocarbon, e.g. C 1  -C 4  hydrocarbons. The injected fluid is heated to a temperature above the temperature at which the material would condense at reservoir conditions, in order to ensure that only gas phase treating fluid is injected into the formation. In a preferred embodiment, a mixture of nitrogen and carbon dioxide is heated to a temperature above its critical temperature, and so the fluid entering the formation is super-critical fluid. The injection rate is carefully controlled to maintain it above the injection rate at which carbon dioxide absorption from the mixture by the viscous petroleum would cause the formation of a bank of petroleum mobilized by the absorption of carbon dioxide, and yet safely below the fracture pressure which would cause fracturing of the formation. By maintaining the injection rate in the desired range, it is possible to inject a predetermined quantity of the nitrogen-carbon dioxide mixture into the formation such that substantially all of the nitrogen and carbon dioxide .initially will pass through the water saturated flow channels of the formation, without any significant portion of carbon dioxide being absorbed initially by the petroleum, thereby achieving substantial penetration of the formation with the injected gaseous nitrogen-carbon dioxide mixture before significant absorption of carbon dioxide by the formation petroleum occurs, and yet avoiding fracturing the formation. After the gas mixture has been forced into the water-saturated flow channels of the formation, the well is shut in for a sufficient period of time to permit absorption of gaseous carbon dioxide from the mixture in these flow channels previously occupied by water, into the viscous petroleum. The nitrogen portion of the mixture maintains the flow channels open, thereby preventing loss of injectivity. Steam may thereafter be injected into the formation via the injection well at a rate substantially greater than the rate originally possible prior to the pretreatment process. The viscosity of the oil for a substantial distance away from the injection well will have been decreased as a result of carbon dioxide absorption, which avoids the rapid formation of an immobile zone of viscous petroleum which would make it impossible to continue injection of steam into the formation. As the petroleum is heated by contact with steam, carbon dioxide evolves from the heated petroleum. The evolved carbon dioxide then moves ahead of the steam-heated oil bank and is absorbed by previously untreated petroleum within the formation as the steam bank moves through the formation toward the production well. Nitrogen from the injected mixture of carbon dioxide and nitrogen remains in the flow channels to maintain steam injectivity. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Our invention is concerned with a preconditioning process for treating a subterranean formation adjacent an injecting well which is to be used for steam injection for the purpose of stimulating the production of viscous petroleum contained in the subterranean formation with which the well is in communication. Although carbon dioxide and, under certain conditions, nitrogen, are effective oil recovery agents in their own right, each is useful mainly in applications where the subterranean formation contains oil of lower viscosity than is contemplated in the present application. The procedure that constitutes the present invention is designed to improve the receptivity of the formation to steam, and is particularly aimed at treating formations where steam injectivity is low initially and/or drops quickly to a very low value soon after initiating steam injection because of the formation of an immobile oil bank within the formation. This is ordinarily experienced in formations which have very viscous petroleum, e.g., petroleum whose viscosity is in excess of 200,000 centipoise at formation temperature, and relatively high oil saturations, e.g,. oil saturations of 60% or greater, and low or essentially zero initial gas saturation, which means the flow channels of the formation are essentially 100% liquid filled. These three factors, the high viscosity oil, the high oil saturation, and the low gas saturation are the common features of formations which exhibit low injectivity or low receptivity to steam, and therefore make steam stimulation of such formations impossible. 
     There are three essential parameters to be controlled in order to achieve the results described herein by application of our process. (1) The fluid injected into the formation must be substantially all in the gaseous phase at the injection pressure, in order to ensure that it does not itself cause plugging of the limited flow channels existing in the formation at the time the stimulation procedures are applied thereto. (2) The gaseous fluid injected into the formation must be a mixture comprising one component which is an inert, non-condensable gas, which is insoluble in the formation petroleum, and one component which is soluble in the petroleum present in the formation. Inert gas such as nitrogen is the preferred non-soluble gas for use in our process, and carbon dioxide is the preferred oil soluble gas. (3) The maximum benefit of our process will be achieved if the mixture of oil-soluble gas and oil insoluble gas is injected into the formation at a rate within a narrow range which ensures that the injected fluid displaces water from flow channels of the formation, thereby achieving fairly deep penetration of the formation through these previously water-saturated flow channels before significant amounts of absorption of the gaseous component of the injected gaseous mixture by the formation petroleum occurs, without causing permanent fracturing of the formation. 
     The treating fluid should contain from 20 to 60 percent and preferably from 25 to 50 percent by weight of the oil insoluble component, preferably nitrogen, with the remainder being the oil soluble component, usually carbon dioxide or a mixture of carbon dioxide and C 1  -C 4  hydrocarbons. In some situations, the concentration of the oil insoluble component should be tapered or increased as the total amount of treating fluid is injected. For example, the first 10-20% of the treating fluid may be 20% nitrogen and 80% carbon dioxide, with the nitrogen content being increased in steps or steadily to a maximum value up to 60% in the last portion of treating fluid injected into the formation. 
     Because of the above-described requirement that the injected gaseous fluid displace water and achieve significant invasion of petroleum formation prior to the oil absorbing the injected fluid, the injection rate is critical. If nitrogen-carbon dioxide mixture is injected into a formation such as that described herein at a relatively low rate, absorption of carbon dioxide by the formation petroleum near the well will begin immediately, resulting in swelling of the formation petroleum and also in reducing the viscosity of the formation petroleum. This will lead to the formation of a mobile bank of petroleum containing the absorbed carbon dioxide, but the viscosity reduction is not sufficient to permit continued movement of this bank through the formation. At the interface of the undisturbed portion of the formation and the bank of petroleum having carbon dioxide dissolved therein, there will be a transition zone of decreasing carbon dioxide content moving in the direction from the injection well toward the unaffected portion of the formation, with the result that an immobile bank of petroleum will form which cannot subsequently be mobilized either by injection of additional carbon dioxide or by injection of steam because injected fluids will not penetrate sufficiently far into the formation to contact the immobile oil bank. 
     The fluid to be injected into the formation should be one which is entirely in the gaseous phase at the time it enters the formation and which will not condense at formation conditions. When the injected fluid is a mixture of nitrogen and carbon dioxide, it may be necessary to heat the injected fluid in order to ensure that it is above the point where it might condense at injection pressures in the flow channels of the formation adjacent to the injection well. Liquification of the injected fluid will greatly reduce the mobility thereof in a formation which already has low permeability to fluid movement, and will probably result in the formation of an immobile petroleum bank in the formation which would prevent subsequent injection of any fluid into the formation. 
     The quantity of the nitrogen-carbon dioxide mixture treating fluid injected into the formation should be sufficient to substantially fill all of the water filled pore space of the formation in a volume around the wellbore having a radius in the range of from 20 to 50 feet. Sometimes it is impossible to inject the desired volume of the nitrogen-carbon dioxide mixture because gas begins migrating from the previously water saturated flow channels of the formation into the oil saturated zone in the portion of the treated zone immediately adjacent to the injection well immediately after the first portion of the gaseous treating fluid is injected, and so the dissolution of injected gaseous material into the petroleum occurs simultaneously with the passage of gas through the flow channels into the formation. As the petroleum absorbs the carbon dioxide or other gaseous treating fluid, the petroleum swells and also experiences reduction in viscosity, and the reduced viscosity of petroleum causes it to migrate into the flow channels which blocks further injection of gas into the formation. The nitrogen or other oil insoluble gas serves to dilute the CO 2  and prevents absorption of too much carbon dioxide too quickly by the viscous petroleum. Excess early absorption would lead to loss of fluid transmissibility in the flow channels as the oil swells and fills the flow channels. 
     If experience in a particular field indicates that the above-described problem occurs to a severe degree, our process should be applied using a higher concentration of nitrogen (within the above described range) in the mixture, to ensure maintaining the flow channels open. 
     The rate at which the mixture of nitrogen and carbon dioxide is injected into the formation is critical if the desired results described herein are to be achieved. In order to avoid fracturing a portion of the formation adjacent to the well, the injection pressure should be maintained safely below the fracture pressure of the formation. The fracture pressure of the formation is usually known or it can be determined, and for the purpose of our process it is desired to maintain the injection pressure at a value below the actual fracture pressure of the formation. The objective of this process is to displace water away from the injection well for a substantial distance, and then have the carbon dioxide migrate from the mixture in the flow channels into the petroleum-saturated portions of the formation adjacent to the previously water-saturated flow channels and be absorbed by the viscous petroleum. The nitrogen component of the mixture remains predominantly in the flow channels. If the mixture of nitrogen and carbon dioxide is injected very slowly into the formation, a substantial portion of the injected carbon dioxide will be absorbed by the formation petroleum in the portions of the formation very close to the injection point, which will cause an immediate drop in the receptivity of the treating fluid by the formation because the previously water saturated flow channels of the formation have become filled with viscous petroleum. 
     The injection rate is interrelated with the formation porosity and permeability and the pressure at which the fluid is injected into the formation. In one preferred embodiment, the mixture of nitrogen and carbon dioxide is injected at whatever rate can be achieved while maintaining the injection pressure at a value equal to from 50 to 95 and preferably 60 to 90% of the known or predetermined fracture pressure of the formation. In most formations of the type to which this process will be applied, this will result to an injection rate which is equal to from 1.25 to 20 and preferably 5 to 10 thousand standard cubic feet (MSCF) of injected gas per foot of formation thickness per day. Accordingly, another preferred method of operating according to the process of our invention is to inject at a pressure safely below the fracture pressure while maintaining the injection rate in the above-described range of standard cubic feet of gas per foot of formation thickness per day. 
     The volume of the mixture of nitrogen and carbon dioxide injected into the formation should be sufficient to essentially fill all of the water-saturated pore space of the formation out to a radius of from 20 to 50 feet. Generally, this will require a volume of gas in the range of from 5,000 to 30,000 standard cubic feet per foot of formation thickness, with the preferred range being from 10,000 to 20,000 standard cubic feet of gas per foot of formation thickness. 
     When all of the predetermined desired volume of the nitrogen-carbon dioxide mixture has been injected into the formation, some procedure must be utilized to avoid immediate backflow of the gas mixture into the well. In order to kill the well, e.g., prevent backflow of injected gas from the formation into the well, the well should be substantially filled with a liquid which will provide sufficient hydrostatic pressure to ensure that the nitrogen-carbon dioxide mixture does not flow back into the well during the brief soak period prior to steam injection. Cold water should not be used for this procedure, since there is a high probability that it will cause plugging of the formation. In a particularly preferred embodiment, the wellbore is filled with a low viscosity liquid hydrocarbon diluent such as a light crude oil, diesel oil or some other hydrocarbon solvent. This serves the dual function of maintaining the desired hydrostatic pressure on the wellbore which prevents backflow of carbon dioxide, and also helps prevent the formation of blockage along the well face caused by deposition of high molecular weight components of the formation petroleum. 
     The soak time, e.g., the period of time which the fluid should be left in the formation prior to the initiation of steam injection, is in the range of from two hours to 30 days, and preferably 2 to 20 days. Preferably the fluid should be allowed to remain in the formation while monitoring the pressure on the well, with the soak time being terminated when the pressure has stabilized at a constant value. For example, if the process of our invention is applied to a formation having an initial reservoir pressure of 350 pounds per square inch, and the nitrogencarbon dioxide mixture is injected at a pressure of 900 pounds per square inch, that injection pressure should be maintained until all of the predetermined desired volume of gas is injected. The mixture should be allowed to soak in the formation 2-20 days until the pressure has stabilized at a value of about 650 pounds per square inch. This is a typical pattern, with the stable pressure after soak period being ordinarily several hundred pounds per square inch above the reservoir pressure prior to the injection of carbon dioxide into the formation. Accordingly, one preferred method of operating according to the process of our invention involves injecting the predetermined desired volume of the mixture of nitrogen and carbon dioxide, and allowing it to remain in the formation until the formation pressure adjacent the injection well has dropped to a value which is from 100 to 400 and preferably from 200 to 300 pounds above the formation pressure prior to injection of the nitrogen-carbon dioxide mixture. 
     The next step after the conclusion of the soak phase will be to initiate injecting steam into the formation, and no further treatment should be necessary to maintain steam injectivity. As steam enters the formation, it contacts petroleum adjacent to the wellbore, which causes several separate effects on the petroleum. Increasing the temperature causes carbon dioxide to evolve from the petroleum, which would cause the viscosity of petroleum to increase, however, the increased temperature maintains a low petroleum viscosity. The carbon dioxide which has broken out of solution moves away from the formation near the injection well because of the pressure differential during the steam injection phase, and the carbon dioxide is absorbed by petroleum in portions of the formation not previously contacted by carbon dioxide during the first injection phase. Evolution of carbon dioxide from petroleum leaves some gas saturation in the petroleum-saturated flow channels of the formation adjacent to the injection well which improves steam injectivity somewhat. The residual nitrogen in the flow channels maintains steam injectivity. 
     Another effect caused by contact between steam and petroleum is the viscosity reduction inherent in increasing the temperature of the petroleum, which more than offsets the adverse effect of causing carbon dioxide to be released from the petroleum. Heated petroleum then moves in the same direction as the steam is moving, and causes the creation of a bank of heated petroleum within the formation. In this instance, the heated oil bank does not become immobile as occurs when steam is injected without the previous treatment according to our process because carbon dioxide is moving ahead of the petroleum bank, maintaining injectivity by occupying some of the flow channels in the formation thereby keeping them open and also preconditioning the oil ahead of the heated oil bank by dissolution of carbon dioxide into the oil which causes a reduction in the viscosity of the oil ahead of the heated oil bank. Nitrogen also moves through the flow channels ahead of the steam, maintaining them open during the steam injectivity phase. 
     An early experiment was conducted to determine whether the injection of nitrogen under conditions such as those described above in a formation containing high saturation of viscous petroleum would produce similar results. Nitrogen was injected into the formation, and allowed to soak for a period of time. Pressure response observed during and subsequent to the nitrogen injection was similar to that which would be experienced in applying our process; however, when steam injection was attempted, the steam injectivity behaved about the same as had been experienced in this formation when there had been no preconditioning treatment at all. In other words, it was observed that injection of pure nitrogen, a non-condensable gas which is essentially insoluble in the formation petroleum under the conditions existing in the reservoir did not cause the improved steam receptivity that is accomplished when a mixture of nitrogen and carbon dioxide is utilized in accordance with our teachings. 
     PILOT FIELD EXAMPLE 
     For purpose of additional disclosure including best mode operation, the following constitutes a description of what we consider to be the best mode of operating in accordance with the teachings of our invention. 
     A subterranean formation containing oil whose viscosity at the formation temperature (83° F.) is 362,500 centipoise and the oil saturation is 65% with water saturation of 35% and zero gas saturation. The formation porosity is 35%. Steam injection in this well is impossible because past field experience indicates that an immobile viscous oil barrier forms only a short distance from the injection well after a few days of steam injection, no matter how the steam injection is applied. 
     It is decided to inject a mixture comprising 30% nitrogen and 70% carbon dioxide into the formation in order to saturate the petroleum in the formation with carbon dioxide and to occupy most or all of the water filled pore space to a radius of about 40 feet from the injection well. The formation thickness is approximately 90 feet. In order to displace water from the formation and saturate at least a substantial portion of the oil in the treated zone, it is determined that the total amount of the mixture of nitrogen and carbon dioxide required is 1.25 MM standard cubic feet (65 tons). 
     The temperature of the petroleum formation is 83° F. In order to ensure that the mixture of nitrogen and carbon dioxide enters the formation entirely in the gaseous phase and that no condensation occurs within the flow channels after gas injection, the gas mixture is heated to a temperature of 110° F. It is desired to inject this total volume of gaseous mixture into the formation over a time period of 6 hours, and so the injection rate is maintained at an average of 210 thousand standard cubic feet per hour. 
     The pressure in the formation prior to the injection of the gas mixture was determined to be 350 pounds per square inch and the fracture pressure of the formation was calculated to be 1,000 pounds per square inch, although field experience indicated that the actual fracture pressure was several hundred pounds higher. In order to inject the nitrogen-carbon dioxide mixture at a pressure which is safely below the actual fracture pressure, it is determined that the injection pressure will be maintained at 900 psi. 
     The mixture of nitrogen and carbon dioxide is injected into the formation and the injection pressure and rate is monitored. The injection rate remains very close to the target injection rate of 2,170 pounds per hour, and it is determined that all of the nitrogen-carbon dioxide mixture will be injected into the formation during a single cycle while maintaining the pressure at about 900 psi. After all of the gas is injected, the well is killed by filling the wellbore with 35° API crude oil, a light oil which will maintain the pressure within the wellbore. Additionally, this light oil is slowly circulated past the perforations by injecting oil down the tubing at a rate of 30 barrels per day in order to ensure that no plugging of the formation face occurs as a result of high molecular weight hydrocarbons such as asphaltenes forming thereon during the soak period. The mixture of nitrogen and carbon dioxide is maintained in the formation for a period of approximately 36 hours. The pressure in the formation is monitored, and when the pressure has declined to about 650 psi, it is determined that sufficient carbon dioxide has been absorbed by the petroleum in the formation from the nitrogen-carbon dioxide mixture to allow injection of steam into the well. Next, 44% quality steam is injected into the well, and it is determined that the formation accepts steam at a rate of about 800 barrels of steam per day at an injection pressure of ˜1,000 psi. Steam injection is continued for a long period of time without any loss of injectivity, indicating that the low steam injectivity has been corrected by our preconditioning process. 
     While our invention has been described in terms of a number of illustrative embodiments, it is clearly not so limited since many variations thereof will be apparent to persons skilled in the related art without departing from the true spirit and scope of our invention. In addition, theories have been advanced to explain the benefits observed when this procedure is applied to the formation, although it is not necessarily implied that these are the only mechanisms responsible for the observed benefits. It is our wish and desire that our invention be limited and restricted only by those limitations and restrictions appearing in the claims appended immediately hereinafter below.