Water and Gas Cyclic Pulsing Method for Improved Oil Recovery
Authors: Raza S.H. 
Published: December 2007

In recovering additional oil from extensively fractured reservoirs, cyclic water pulsing has been found helpful. Cyclic gas pulsing can be even more effective. This study indicates that by judiciously combining the two methods some of the limitations of each could be overcome and additional oil recovery amounting to as much as 25 percent of the pore space contacted could be achieved.

Introduction
Low-permeability reservoirs that are dissected extensively by a network of interconnected fractures, solution channels, and vugs are generally not suited for waterflooding or gas injection. The injected fluid tends to channel through the high-conductivity network and bypass the low-permeability, oil-bearing matrix. Methods for improving sweep, such as treatment with silica gel, foam, and finely ground solids, generally are not applicable to such reservoirs. A production method that is applicable to extensively fractured reservoirs is waterflood imbibition displacement. This process is based on the concept that water imbibed into an oil-bearing, water-wet rock causes the countercurrent expulsion of oil. The high-conductivity fracture network simultaneously serves as a source of water for imbibition and as a sink for produced fluids. While this method has proved successful in recovering oil both in the laboratory and in the field, the rate of oil production that is imbibition controlled is too slow to be economical. Some field tests in which water was injected at rates higher than the imbibition rate showed a decline rather than an increase in oil production rate. It was hypothesized that the injection production rate. It was hypothesized that the injection of water at rates higher than imbibition rate interfered with the countercurrent flow of oil. An outgrowth of the waterflood imbibition displacement process, and one that is also applicable to highly fractured process, and one that is also applicable to highly fractured reservoirs, is waterflood pressure pulsing. This method consists of alternately pressuring and depressuring the reservoir. During the pressuring phase, the injected water is forced under high pressure from the fracture network into the low-permeability, oil-bearing matrix. During the depressuring phase, the fluids are produced out of the low-permeability matrix into the fracture network, which transmits them to the producing wells. The high-conductivity network alternately serves as a source of water for injection and a sink for the produced fluids. The steps of alternate pressuring and depressuring -- a pressure cycle -- are pressuring and depressuring -- a pressure cycle -- are repeated as often as economically feasible. Owens and Archer showed that, in the laboratory, cyclic water pressure pulsing is an effective technique for recovering oil in the amount of 5 to 10 percent of pore space contacted from water-wet and simulated oil-wet sandstone and limestone cores containing oils of 1.2- to 360-cp viscosity. Felsenthal and Ferrell, confirmed these results in tests in water-wet Berea sandstone models and further showed that the oil yield of a gas-water pressure pulsing cycle, in which a gas is injected ahead of water. is greater than that of a water pressure pulsing cycle. They attributed this to the beneficial effect of replenishing the expulsive energy with gas. Moreover, they reported that no oil was recovered in tests in which gas alone was used for pressure pulsing.
JPT
P. 1467