Method of Cyclic Solvent Flooding to Recover Viscous Oils

1. Field of the Invention

This invention is directed to a method of recovering oil from viscous oil-containing subsurface formations. More particularly, this invention is directed to a cyclic hydrocarbon solvent recovery method for recovering oil from subsurface formations that are penetrated by at least one injection well and one production well which extend downwardly from the surface of the earth into the bottom of the oil-containing formation and then extend horizontally through the formation.

2. Description of the Prior Art

In co-pending application filed July 21, 1981 Ser. No. 285,696, now U.S. Pat. No. 4,373,585, to Fitch et al., there is disclosed a method of recovering viscous oil from a viscous oil-containing formation wherein a selected solvent is injected into a fluid communication path in the power portion of the formation intermediate at least an injection well and a production well. A hydrocarbon solvent having a density less than oil contained in the formation and a viscosity not greater than 1/100 the viscosity of the oil contained in the formation under formation conditions is injected into the communication path and fluids including oil are recovered from the production well until the fluid recovered contains an unfavorable ratio of oil to solvent. The production well is shut-in and an additional quantity of the hydrocarbon solvent is injected into the fluid communication path, preferably until a total amount of between 0.05 to 0.30 pore volume has been injected. The injected well is then shut-in along with the production well to permit the formation to undergo a soak period for a variable time, preferably for a time of between 2 to 20 days per vertical thickness in feet of oil-containing formation. A driving fluid such as water is then injected into the formation via the injection well and the oil is produced until there is an unfavorable ratio of oil to driving fluid. During the fluid drive recovery phase, the injection well and production well may be completed to be in fluid communication with the entire portion of the formation to obtain a more uniform displacement of the upgraded formation oil by the driving fluid.

In co-pending application Ser. No. 46,275, now U.S. Pat. No. 4,293,035 filed June 7, 1979, to John L. Fitch, there is disclosed a method of recovering viscous oil from a viscous oil-bearing subsurface formation wherein a solvent is injected into a high mobility channel formed in the bottom of the formation intermediate an injection well and a production well. The solvent is injected until the ratio of produced oil to solvent becomes unfavorable and thereafter the injection of solvent is terminated and gas is injected into the high mobility channel to produce solvent and oil from the formation.

In U.S. Pat. No. 3,838,738 there is described a method for recovering viscous petroleum from petroleum-containing formations by first establishing a fluid communication path low in the formation. A heated fluid is then injected into the fluid communication path followed by injecting a volatile solvent such as carbon disulfide, benzene or toluene into the preheated flow path and continuing injecting the heated fluid and recovering fluids including petroleum from the production on well.

In U.S. Pat. No. 3,500,917 there is disclosed a method for recovering crude oil from an oil-bearing formation having a water-saturated zone underlying the oil-saturated zone. A mixture of an aqueous fluid which has a density greater than the density of the crude oil and a solvent having a density less than the density of the crude oil are injected into the water-saturated zone and oil is produced from the formation.

U.S. Pat. No. 4,026,358 discloses a method for recovering heavy oil from a subterranean hydrocarbon-bearing formation traversed by at least one injection well and one production well wherein a slug of hydrocarbon solvent in amounts of 0.1 to about 20 percent of the formation pore volume and having a gas dissolved therein is injected into the formation via the injection well. Thereafter, a thermal sink is created in the formation by in-situ combustion or by injecting steam. The wells are then shut-in for a predetermined time to permit the formation to undergo a soak period, after which production is continued. Optionally, after the production period, the formation may be water flooded to recover additional oil from the formation.

This invention is directed to a method of recovering viscous oil from a viscous oil-containing formation that has no significant vertical permeability barrier in the portion of the formation being produced involving at least one injection well and at least one spaced-apart production well that penetrate the formation and then extend horizontally through the bottom of the formation for cyclic injection of solvent into the formation and recovery of oil from the formation. The injection well and production well are drilled into the formation extending downwardly from the earth's surface to the bottom of the oil-containing formation and then a substantial distance through the formation in a generally horizontal direction. That portion of the injection well and production well that extends horizontally through the bottom of the formation is perforated or otherwise equipped to provide fluid communication between both wells and the formation. The production well is spaced-apart from the injection well but the horizontal portion of the production well is at the same level as, and parallel to, the horizontal portion of the injection well. A hydrocarbon solvent having a viscosity not greater than 1/100 the viscosity of the oil contained in the formation and a specific gravity less than the oil contained in the formation under formation conditions, is injected into the bottom of the formation via the injection well and fluids including oil are recovered from the production well until the produced fluid contains an unfavorable amount of solvent, preferably at least 90 percent. When this occurs, injection of solvent is terminated and both the injection well and production well are shut-in for a predetermined period of time to allow the formation to undergo a soak period. The soak period is preferably for 2 to 20 days per foot of vertical thickness of the oil-containing formation. During this soak period, the light hydrocarbon solvent will tend to rise and the heavy oil will move downward in a gravity-driven convection process forming a pattern of fingers. This fingering causes a more effective contact between the oil and solvent thereby allowing the two to mix more effectively, thus providing a greater volume of oil that is reduced in viscosity. After the soak period, the hydrocarbon solvent is injected into the bottom of the formation via the production well and fluids including oil and solvent are recovered from the formation via the injection well until the fluid being produced contains an unfavorable amount of solvent, preferably at least 90 percent. In another embodiment of the invention, following completion of the soak period, the fluid produced from the injection well which is primarily solvent is re-injected or recycled into the formation via the production well in place of pure solvent until the amount of solvent in the produced fluid is less than 90%. Thereafter, recycle of the produced fluid is terminated and pure hydrocarbon solvent is injected into the formation via the production well and production of fluids from the injection well continued until the fluid being produced contains an unfavorable amount of solvent, preferably at least 90 percent. After injection of solvent into the production well is terminated, both wells are shut-in to allow the formation to undergo a second soak period for a predetermined period of time, preferably for 2 to 20 days per foot of formation thickness. Thereafter, a driving fluid such as water is injected into the injection well to displace the mixture of oil and solvent toward the production well for recovery. Production is continued until the fluid including oil recovered from the production well contains an unfavorable ratio of oil to driving fluid.

FIG. 1 is a diagrammatic plan view of an area which overlies an oil-containing formation, and it indicates by dotted lines an injection well and a production well which extends downwardly from the surface to the bottom of the oil-containing formation.

FIG. 2 is a sectional view through the area shown in FIG. 1 and it is taken along the plane indicated by the lines 2--2 in FIG. 1 showing the manner in which the injection well and production well penetrate the formation.

FIG. 3 is an isometric view of a rectangular block of the earth showing the position of the injection well and the producton well within the formation .

The invention relates to a hydrocarbon solvent recovery method for recovering viscous oil from a viscous oil-containing subsurface formation that has no significant vertical permeability barrier in the portion of the formation to be treated utilizing injection and production wells which extend from the earth's surface downwardly into the oil-containing formation and then extend horizontally through the lower portion of the oil formation. Viscous oil is a term used to identify oil having relatively high viscosity and includes those oils referred to as tars. Such viscous oils are also referred to as heavy oils. In general, the term viscous oil is used to include those heavy oils and tars such as are commonly found in formations referred to as heavy oil or tar sands that have viscosities that are great enough to severely restrict the production of the oils from the formations in which they are found. The API gravity of such viscous oils is normally 20.degree. API or less.

Referring to FIG. 1, a field 10 overlying an oil-containing formation is provided with an injection well 12 and a production well 14. As shown in FIGS. 2 and 3, the injection well 12 and the production well 14 extend downwardly from the earth's surface 16, through an overburden 18, into the oil-containing formation 20 and then extend horizontally through the bottom of the formation. The production well 14 is spaced apart from the injection well 12 and the horizontal portion of the production well is at the same level and parallel to the horizontal portion of the injection well. The horizontally extending portion of the injection well 12 and the production well 14 have perforations 22 providing fluid communication between that portion of each well and the lower portion of the formation 20. Although only one injection well and one production well are shown for purposes of illustration, it is understood that a plurality of alternating spaced apart injection wells and production wells may be used in our process with the number depending upon the characteristics of the oil-containing formation. The wells may be formed by conventional directional drilling means with the horizontally extending portion of each well in the bottom of the formation being within plus or minus 20.degree. of a horizontal plane.

A hydrocarbon solvent having a specific gravity less than the specific gravity of the oil contained in the formation and having a viscosity not greater than 1/100 of the viscosity of the oil contained in the formation under formation conditions is injected via the injection well 12 and fluid including oil is recovered from the formation via production well 14. Solvent injection is continued until the fluid being produced from the formation by means of production well 14 comprises an unfavorable amount of solvent, preferably at least 90 percent.

Thereafter, injection of the solvent is discontinued and both the injection well 12 and production well 14 are shut-in to allow the formation 20 to undergo a soak period for a variable time, preferably for a period of time between 2 to 20 days per foot of the vertical thickness of the oil-containing formation. It will be recognized by those skilled in the art of oil recovery that during this shut-in or soak period minor amounts of fluid may be injected or produced, such as for the purpose of testing, without significant detrimental effects on the process. During the soak period, the liquid hydrocarbon solvent being lighter than the oil contained in the viscous oil-containing formation, that is, having a specific gravity less than the specific gravity of the oil contained in the formation and having a viscosity not greater than 1/100 of the viscosity of the oil contained in the formation under formation conditions, will tend to flow by gravity-driven convection upward into the oil-containing formation and the heavy oil will flow by gravity-driven convection downward forming a pattern of fingers. This fingering is important in that the distribution, size and extension of these fingers will provide intimate contact between the solvent and the heavy oil allowing the two to mix more effectively, thus forming a greater volume of oil that is reduced in viscosity and which can be produced more readily.

Once the soak period has been completed, hydrocarbon solvent is injected into the formation via the production well 14 and fluid including oil is recovered from the formation via the injection well 12. Solvent injection is continued until the fluid being recovered from the formation by means of the injection well 12 comprises an unfavorable amount of solvent, preferably at least 90 percent. Thereafter, production of fluids from the formation by means of injection well 12 is terminated and both the injection well 12 and producton well 14 are shut-in for a predetermined period of time to allow the formation to undergo a second soak period. The second soak period is preferably 2 to 20 days per foot of formation thickness being treated.

When the injection well is placed on production at the end of the first soak period, the produced fluid will consist primarily of solvent. Therefore, in another embodiment of our process to permit maximum solvent usage together with minimizing the quantity required, the fluid recovered from the injection well after the first soak period is re-injected or recycled into the formation via the production well 14 in place of pure solvent until the amount of solvent in the fluid produced from the injection well is less than 90%. Once the solvent content of the produced fluid drops to less than 90%, recycle of this fluid is terminated and pure solvent is injected into the production well. Injection of the pure solvent is then continued until the amount of solvent in the fluid being produced from the formation by means of injection well 12 contains an unfavorable amount of solvent, preferably at least 90 percent.

The above sequence of solvent injection, production and soak period followed by repeating these steps after convering the production well to an injection well and the injection well to a production well may be repeated for as many cycles as is desired.

Thereafter, a driving fluid is injected into the formation 20 via the injection well 12 which displaces the oil of reduced viscosity from the formation into the production well 14 and production is continued until the recovered fluid contains an unfavorable amount of driving fluid, preferably at least 90 percent. The driving fluid for use in our process may be gaseous or liquid and is preferably an aqueous fluid. Suitable gaseous driving fluids include light aliphatic hydrocarbons having from one to four carbon atoms, carbon dioxide or nitrogen. Suitable aqueous fluids include water, brine or thickened water.

The hydrocarbon solvents used in our process must have a specific gravity less than that of the oil and less than that of the brine injected or naturally present in the formation and a viscosity not greater than 1/100 the viscosity of the oil contained in the formation under formation conditions. It is also highly desirable that the hydrocarbon solvent remain liquid under the temperature and pressure conditions that exist in the subsurface viscous oil-containing formation and not cause solids such as asphaltenes to precipitate from the oil in amounts sufficient to seriously plug the pores of the formation. The preferred hydrocarbon solvent is a light crude oil. Other examples of suitable hydrocarbon solvents include light oil condensates having an API gravity greater than 30 API degrees and partially refined tar which is generally known as syncrude. It may be desirable to include in the hydrocarbon solvent up to about 10 percent of aromatic material such as aromatic refinery stock to make the solvent compatible with the oil contained in the formations and to prevent the deposition of solid or gelatinous materials such as asphaltenes therefrom.

The viscous oil-containing subsurface formation to be treated by the present hydrocarbon solvent recovery method must be one that has no significant vertical permeability barriers in that portion of the formation to be treated. This allows the solvent that is injected into the bottom of the formation between the injection well and production well to flow upward into the formation by convection forces thus solubilizing the oil and forming a solvent-oil mixture of reduced viscosity that can be more readily produced.

In a further embodiment, the injection well and the production well may extend downwardly from the earth's surface into the bottom of the oil-containing formation with the lower portion of each well extending through the formation at a downward angle not greater than 20.degree. relative to a horizontal plane and the lower portion of each well in fluid communication with the formation. The production well is spaced apart from the injection well and the lower portion is at the same level and parallel to the lower portion of the injection well.