Method of Improved Oil Recovery by Simultaneous Injection of Water With an in-Situ Combustion Process

1. Field of the Invention

This invention concerns a thermal oil recovery method utilizing in-situ combustion and water in a method which permits efficient recovery of heavy oil from permeable, heavy oil-bearing reservoirs.

2. Background and Prior Art

It has been proposed to recover oil, in the nature of heavy viscous oils, from a subterranean reservoir by a method which is commonly known as in-situ combustion. In this method, an oxygen-containing gas is injected into the reservoir through an injection well with ignition of oil within the adjacent reservoir initiated by suitable means for establishing a combustion front. The reservoir is usually provided with one or more production wells for the production of oil. As the flow of oxygen-containing gas to the reservoir is continued, the combustion front is moved from the injection well toward the production wells. The heat generated by burning reduces the viscosity of the oil which is displaced before the combustion front toward the production wells from which the oil is recovered. The combustion front in displacing the mobile oil before it in the reservoir uses the residual carbonaceous deposit as fuel.

In these processes, the gaseous combustion products and light hydrocarbons are considerably lighter than the oil and water present in the reservoir and thus, because of gravity segregation, tend to rise to the top of the reservoir when vertical communication exists. Consequently, these products channel through the top of the formation to the producing well overriding a major portion of the reservoir and contacting only a small fraction of the reservoir oil. This behavior results in inefficient oil recovery and low vertical sweep efficiency.

Furthermore, in such in-situ combustion processes, large quantities of heated rock are left behind in the reservoir. This heat is therefore lost, which greatly reduces the thermal efficiency of the process.

Our improved invention provides a substantial advance in recovering heavy oil from permeable, heavy oil-bearing reservoirs utilizing in-situ combustion.

According to the present invention, we have found an improved method for recovering oil, especially viscous or heavy oil, from a permeable, heavy oil-bearing reservoir wherein in-situ combustion is established in the lower portion of the reservoir, water is injected into the upper portion of the formation at controlled rates and oil is recovered at a production well. An oxidizing gas such as air or oxygen or mixtures thereof is injected into the lower portion of the reservoir to form a combustion front which advances through the reservoir toward a production well. Heat generated by the in-situ combustion reaction heats the viscous oil as it advances through the reservoir thereby reducing its viscosity. The water injected above the in-situ combustion front, with a higher density, tends to segregate to the bottom of the reservoir because of gravitational forces, whereas the low density products of combustion tend to segregate to the top. In addition to very effective contact and heat exchange between the water and the gaseous products of combustion, the water tends to fill gas-swept channels thus impeding the flow of gases and diverting them to previously unswept paths resulting in higher vertical sweep efficiency. The water passing through the combustion-heated formation scavenges heat and becomes a hot water drive displacing oil from lower regions, not subjected to combustion, which further improves recovery efficiency per BTU of heat injected. This type of vertical crossflow of fluids within a reservoir enables the reservoir to be more efficiently heated over the areal extent of the reservoir thereby greatly enhancing the recovery of oil. Mobility control agents such as thickeners and water-soluble polymers may be added to the injected water to improve its areal sweep efficiency.

The drawing is a view in cross-section of an injection well and a production well penetrating a subterranean, permeable, heavy oil-bearing reservoir illustrating the invention.

Thus, according to the drawing, a subterranean, permeable, heavy oil-bearing reservoir 10, is overlain by overburden 12 and underlain by basement formation 14. The reservoir 10 is penetrated by an injection well 16 and a production well 18.

The injection well 16 has perforations 20 providing fluid communication into the upper portion of reservoir 10 and perforations 22 providing fluid communication into the lower portion of the reservoir. A tubing string 24 extends from the earth's surface to the lower portion of reservoir 10 forming an annulus 26 between the tubing string and injection well casing 28.

A packer 30 seals the outer tubing surface from the inside of casing 28. The top of injection well 16 is provided with a means 31 for injecting fluid into the annular space 26 between tubing 24 and casing 28.

The production well 18 has perforations 32 providing fluid communication into the lower portion of reservoir 10 for the production of oil and gases through conduit 34 to the surface of the earth.

According to one presently preferred mode of operation, the lower portion of the oil-bearing reservoir 10 is ignited in the vicinity of perforations 22. After ignition, an oxygen-containing gas such as oxygen or air or mixtures thereof is injected through tubing string 24 into the lower portion of the reservoir through perforations 22. A combustion front is formed which progressively advances from the injection well to the production well. After the combustion front has advanced a sufficient distance, water is injected through injection means 31 into the annulus 26 formed between casing 28 and tubing 24 and outwardly through perforations 20 and into the upper portion of the reservoir 10.

The water, with a higher density, tends to segregate to the bottom of the formation because of gravitational forces, whereas the low density combustion gases tend to segregate to the top. The water tends to fill gas-swept channels thus impeding the flow of gas and diverting them to previously unswept paths resulting in higher vertical sweep efficiency. The water passing through the combustion-heated reservoir scavenges heat and becomes a hot water drive displacing oil from lower regions of the reservoir, not subjected to combustion, which further improves recovery efficiency.

The combustion front and water advance through the reservor 10 contacting the oil and reducing its viscosity and displacing the oil towards production well 18. Admixtures of oil and gases enter production well 18 through the lower perforations 32, pass up through conduit 34 and are recovered at the surface of the earth.

Once the combustion front is sufficiently near to the production well which can be ascertained by a rise in the temperature at the production well, further injection of the oxidizing gas is discontinued. Water injection may be continued until water breaks through at the production well 18.

Water may be injected with the oxygen-containing combustion supporting gas after the initiation of in-situ combustion so as to absorb heat from the combustion zone, which is a technique known in the art as wet combustion. The amount of water injected, in relation to the oxygen-containing combustion supporting gas, will vary depending upon the amount necessary to keep the reservoir below excessive temperature levels. It must not be so great, of course, as to extinguish combustion as would be evidenced by the composition of the gases produced from the reservoir.

The oxygen-containing gas used to support combustion can be air, substantially pure oxygen, or oxygen-enriched air.

The amount of water injected into the upper portion of the reservoir is much larger than that used in conventional wet combustion processes in order to scavenge sufficient heat from the burned out portion of the reservoir and also to restrict gas flow in the burned zone. However, the injection rate must be controlled or intermittently discontinued to prevent the water from overrriding the combustion front or extinguishing it.

In another embodiment of the invention, separate injection wells may be used for the injection of the oxidizing gas such as air or oxygen and water into the selected portions of the reservoir. For example, two or more closely spaced injection wells may be used with air or oxygen injected near the bottom of the reservoir through one well and water injected near the top of all, or selected, separate wells as dictated by preferred engineering practices.

In another embodiment of the process of this invention, the areal sweep efficiency of the water injected into the upper portion of the reservoir may be improved by the addition thereto of mobility control agents such as thickeners or water-soluble polymers. These agents increase the viscosity of the water which decrease its mobility with a resulting increase in displacement efficiency. Examples of useful thickeners and water-soluble polymers useful in connection with this process are disclosed in U.S. Pat. No. 3,500,918 to Holm and U.S. Pat. No. 3,710,861 to Steeg, the disclosures of which are hereby incorporated by reference.

The present invention may be carried out utilizing any suitable injection and production system. The injection and production systems may comprise one or more wells extending from the surface of the earth into the oil-bearing formation. Such injection and production wells may be located and spaced from one another in any desired pattern. For example, a line drive pattern may be utilized in which a plurality of injection wells are arranged in a more or less straight line toward a plurality of production wells in a more or less straight line parallel to a line intersecting the plurality of injection wells. In addition, a circular drive pattern may be used in which the injection system comprises a central injection well and the production system comprises a plurality of production wells about the injection well in a ring pattern such as a 5-spot or 7-spot well pattern.