A new enhanced oil recovery scheme, pressure cycling, is proposed. Numerical simulations indicate that it may be possible to recover a substantial additional amount of oil after primary and secondary production by this method. The pressure cycling scheme involves the injection of produced gas and water into a reservoir in order to re-energize it and to resaturate the oil. Injection is done through existing vertical wells; however, it appears necessary to drill infill horizontal production wells to attain good rates of production, particularly in quite thin reservoirs (5 m or less). The optimal amount of gas to inject is believed to be the amount required to re-saturate the oil when the reservoir is repressured by water to approximately original reservoir pressure.
A substantial portion of Saskatchewan's petroleum resources is in the form of heavy oils, much of which occurs in rather thin reservoirs (4 to 6 m). Once primary production, and waterflooding where applicable, have been done in such reservoirs there is a major problem in finding an enhanced oil recovery (EOR) scheme that is practical. The thinness of the reservoirs effectively rules out thermal methods such as steam drive and SAGD because of high heat losses. It is conceivable that combustion techniques might be applied; however, the few field trials done to date have failed to ignite interest. The nonviability of thermal EOR methods for thin reservoirs compels the consideration of non-thermal methods.
Preferable and practical non-thermal methods for thin heavy oil reservoirs will almost certainly be associated with low cost. Some of the lowest cost materials that could be injected in quantity would be water and produced gas. It is therefore natural to carefully consider all techniques that use these materials for EOR of heavy oil.
The pressure cycling process had its origins in an examination by SRC researchers of a variation of the water alternating gas (WAG) process. A schematic of the arrangement of the wells is given in Figure 1. The area used for the new WAG study used four vertical wells and a segment of horizontal well between them For reasons of continuity with previous work the distance between vertical wells surrounding the horizontal well is 440 m rather than the more standard 400 m The simulation was based on the Senlac field, in that uniform average values from this reservoir were used in the simulation (4.8 m reservoir thickness, 1300 cp dead oil, 5600 kPa original pressure, no bottomwater). The WAG variation considered was one using a horizontal production well. This variation of WAG did not give attractive results. It was observed, however, that the horizontal production well, when viewed as an infill well, was considerably more productive after the reservoir had been repressured by water injection for the WAG scheme. The increase in production was not simply due to increased pressure. This was established by observing production after having pressured the reservoir up to a series of progressively higher pressures.