Method for recovery of stranded oil

Abstract

The present invention provides a method of recovering stranded oil wherein heated fluid is injected into a reservoir containing stranded oil in a region near the reservoir ceiling, or alternatively the heated fluid is generated downhole using a downhole combustor and then injected into the reservoir. The heated oil drains toward the reservoir floor and is recovered via a production well.

Description

CROSS-REFERENCE

This application is a Continuation-In-Part of U.S. patent application Ser. No. 11/698,556 filed Jan. 26, 2007 and which in turn claims the benefit of U.S. Provisional Application No. 60/773,410 filed Feb. 15, 2006.

FIELD OF THE INVENTION

This invention relates to a method for the recovery of stranded oil left behind in an oil reservoir after recovery of oil by conventional means.

BACKGROUND OF THE INVENTION

Oil fields typically are abandoned after oil can no longer be produced economically by known secondary or tertiary recovery methods. At this point, the remaining oil in place often represents as much as two-thirds of the amount originally in place. Of the remaining oil, about twenty percent is considered technically recoverable using advanced techniques such as, for example, carbon dioxide flooding. Unfortunately, for stranded oil in most U.S. fields, carbon dioxide is not available. Moreover, there is a need to recover far more stranded oil than can be recovered by known advanced methods such as carbon dioxide flooding. It is therefore an object of the present invention to make possible economic recovery of nearly all the remaining stranded oil in place.

SUMMARY OF THE INVENTION

The oil coating on the reservoir minerals can be flushed off much more effectively than by carbon dioxide flooding. By heating the oil to an elevated temperature along with reservoir pressurization, the oil can be flushed off of the sand, for example, in the reservoir. In the present invention, hot fluids are injected near the top of the reservoir such that fluid flow is downward toward the reservoir floor by gravity drainage aided by the downward fluid flow. Accordingly, the stranded oil left behind in the oil reservoir after recovery of oil by conventional means can now also be extracted. In addition, inert gases present in injected fluids provide reservoir repressurization. Preferably, fluid is injected through a horizontal well to best distribute the flow across the reservoir ceiling. Advantageously, the hot combustion gases are produced using a downhole combustor. If available, carbon dioxide is added to combustion products for sequestration.

The present invention comprises a method of recovering stranded oil wherein heated fluid is injected into an oil reservoir containing stranded oil left behind in the oil reservoir after recovery of oil by conventional means. The heated fluid is injected into the oil reservoir in a region near the reservoir ceiling at a temperature greater than about 800 degrees F. to induce cracking of the stranded oil. The cracked oil drains toward the reservoir floor and is recovered via a production well.

The heated fluid that is injected into the oil reservoir may be produced downhole using a downhole combustor or it may be provided from the surface and passed downhole. The heated fluid also may be injected into the oil reservoir via a horizontal well, and the horizontal well may comprise a plurality of branches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides an isometric view of one embodiment of the present invention including a horizontal injection well with branches protruding into the oil reservoir.

FIG. 2 provides an isometric view of an alternate embodiment of the present invention including a horizontal injection well and a combustor for producing the heated fluid in the oil reservoir.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention 10, as shown in FIG. 1, hot fluids such as steam or combustion gases may be produced at the surface and injected downhole via injection well 12. However, heat losses advantageously may be reduced by generating the heated fluids downhole as by a downhole combustor with fuel and air supplied from the surface. Combustion gases produced by downhole combustion typically are reduced in temperature by addition of an inert fluid such as nitrogen or carbon dioxide. Steam may be produced by spraying water into the hot combustion products.

Hot fluid is provided near the reservoir ceiling 14 using any conventional method known in the art such that the flow forms a gas blanket layer which flows downward toward the reservoir floor. Thus heated oil drains toward the reservoir floor 16. Typically the fluid is injected via a horizontal well 18. To distribute the fluid over a wider area, the horizontal well may have horizontal branches 20 as shown in FIG. 1. As shown in FIG. 2, hot fluids also may be generated by in-situ combustion by supplying combustor 22 with an appropriate combustor fee 24 such as fuel and air or oxygen supplied from the surface. The drained or cracked oil is subsequently recovered via production well 24.

The temperature of the injected fluid in most cases is below that which would result in significant cracking of the oil. However, if significant reduction in the oil viscosity is desired, the temperature of the injected fluid is controlled to a value which will result in cracking of the oil. Temperature within the reservoir high enough for cracking can be provided by use of in-situ combustion. With the present invention, temperatures within the reservoir high enough for cracking, typically above about 800 degrees F., can be achieved. Cracking of the oil is desirable where the viscosity of the stranded oil is higher than that originally produced in primary production. Oil draining to the reservoir floor may be recovered through an existing production well. However, it may be advantageous to provide a new horizontal production well located on or near the reservoir floor.

After the heated fluid has heated an upper portion of the reservoir flushing the oil downward, heat from oil depleted layers may be utilized to heat a cold fluid such as nitrogen or carbon dioxide thus providing heated fluid for recovery of oil from lower levels.

While the present invention has been described in considerable detail with reference to a preferred method for the recovery of stranded oil left behind in an oil reservoir after recovery of oil by conventional means as described herein, other methods exhibiting the characteristics taught herein are contemplated. Therefore, the spirit and scope of the invention should not be limited to the description of the preferred embodiment described herein.

Claims

1. A method of recovering stranded oil comprising:

a) injecting heated fluid at a temperature greater than about 800 degrees F. into an oil reservoir containing stranded oil left behind in the oil reservoir after recovery of oil by conventional means, wherein the heated fluid is injected into the oil reservoir in a region near the reservoir ceiling;

b) cracking the stranded oil thereby producing heated oil;

c) allowing the heated oil to drain toward the reservoir floor thereby forming an accumulation of drained oil; and

d) recovering the drained oil via a production well.

2. The method of claim 1 wherein the heated fluid is produced downhole using a downhole combustor.

3. The method of claim 1 wherein the heated fluid is provided from the surface and passed downhole.

4. The method of claim 1 wherein the heated fluid is injected via a horizontal well.

5. The method of claim 4 wherein the horizontal well comprises a plurality of branches.