Selective displacement of water in pressure communication with a hydrocarbon reservoir

#The present invention relates generally to the selective displacement of water from an area within a porous and permeable geological formation. More particularly, the present invention relates to a method for selectively displacing a portion of an aqueous fluid zone with injected gas in a displacement zone that is in pressure communication with a hydrocarbon reservoir.

#A variety of processes are used to recover viscous hydrocarbons, such as heavy oils and bitumen, from underground deposits. There are extensive deposits of viscous hydrocarbons around the world, including large deposits in the Northern Alberta tar sands, that are not amenable to standard oil well production technologies. The primary problem associated with producing hydrocarbons from such deposits is that the hydrocarbons are too viscous to flow at commercially relevant rates at the temperatures and pressures present in the reservoir. In some cases, such deposits are mined using open-pit mining techniques to extract the hydrocarbon-bearing material for later processing to extract the hydrocarbons. Alternatively, thermal techniques may be used to heat the reservoir to produce the heated, mobilized hydrocarbons from wells. One such technique for utilizing a single horizontal well for injecting heated fluids and producing hydrocarbons is described in U.S. Pat. No. 4,116,275, which also describes some of the problems associated with the production of mobilized viscous hydrocarbons from horizontal wells.

#One thermal method of recovering viscous hydrocarbons using two vertically spaced horizontal wells is known as steam-assisted gravity drainage (SAGD). Various embodiments of the SAGD process are described in Canadian Patent No. 1,304,287 and corresponding U.S. Pat. No. 4,344,485. In the SAGD process, steam is pumped through an upper, horizontal, injection well into a viscous hydrocarbon reservoir while hydrocarbons are produced from a lower, parallel, horizontal, production well vertically spaced proximate to the injection well. The injection and production wells are typically located close to the bottom of the hydrocarbon deposit.

#Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

#FIG. 1 is a vertical cross-sectional view of a formation showing well configuration and formation composition prior to initiating displacement;

#FIG. 2 is a vertical cross-sectional view of a formation showing well configuration and formation composition during displacement treatment;

#Generally, the present invention provides methods and systems for selectively displacing a volume of water (or more accurately an aqueous fluid) from a permeable underground formation with an amount of pressurized gas. The methods of the invention are selective in the sense that water is displaced from a target zone of a hydraulically continuous aqueous fluid zone. A target region of the formation may for example be selected that is in pressure communication with a hydrocarbon reservoir, such as a target zone within a larger top or bottom water zone associated with an oil sands reservoir.

#Methods of the invention involve an initial stage of hydrodynamically adjusting aqueous fluid pressures, so as to provide a horizontal fluid pressure differential within the zone, to define a low pressure zone within a high pressure fluid zone. As gas is injected into the low pressure fluid zone, it displaces the fluid, which is produced, forming an injected gas zone in the formation above the receding top of a low pressure fluid zone. As more gas is injected, the low pressure fluid zone moves down, and the injected gas zone expands.

#Simulations have surprisingly shown that it is possible to confine the expanding zone of injected gas by arranging hydrodynamic conditions so that a circumferential zone of relatively high fluid pressure maintains a confining fluid pressure that is: i) greater than the maximum fluid pressure at the top of the low pressure aqueous fluid zone (so that the gas zone preferentially expands into the low pressure fluid zone); and, ii) greater than the maximum gas pressure in the injected gas zone (so that the injected gas zone is confined by the high pressure fluid zone).