Waterflood Pressure Pulsing for Fractured Reservoirs

Abstract

Abstract Conventional waterflooding often is uneconomic in highly fractured reservoirs because of the gross by passing of the reservoir oil by injected water. Imbibition and pressure pulse flooding have been used in at least one fractured reservoir in an attempt to achieve better oil recovery performance. This paper presents the results of laboratory flow tests conducted on large cores to evaluate the possible applicability of these methods (particularly pressure pulse flooding) to different types of reservoir systems. Test data were obtained on both water-wet and oil-wet systems, and on systems having two widely different levels of compressibility and flow capacity. Results indicate that pressure pulse recoveries from fractured reservoirs will likely not exceed 5 to 10 per cent pore space with maximum response achieved during the first pulse cycle. Improved recovery by this method is possible from both oil-wet and water-wet reservoirs. Comparable saturation distributions during imbibition and pressure pulse production suggest that an initial pressure pulse cycle to speed production response would not interfere with subsequent imbibition flooding in water-wet reservoirs. Introduction There is a steady increase in the number of fluid injection projects being initiated each year to improve oil and gas recovery over that obtained by primary production mechanisms. Experience gained in different geographical areas and with different recovery methods is teaching that reservoir anatomy is one of the more important factors controlling the success or failure of such projects. Fractured formations appear to be the rule rather than the exception, especially in carbonate reservoirs. Conventional methods of fluid injection, whether it is waterflooding, gas injection or miscible flooding, have limited applicability to highly fractured reservoirs because of the severe bypassing of reservoir fluids. The result is early break-through of the injected fluid and rapidly increasing ratios of injected to in-place fluid with an undesirable effect on return on investment. Thus, innovations to conventional methods must be developed if recovery from fractured reservoirs is to be optimized. One new method proposed for application to highly fractured reservoirs is pressure pulse waterflooding. Although pressure pulsing with gas has been suggested and tested in at least one reservoir, the Sohio Oil Co. is generally credited with the development and testing of pressure Pulsing in conjunction with waterflooding. Publications of Sohio's waterflood operations in the Spraberry Trend indicate that the idea for pressure pulse hooding evolved from a critical analysis of the disappointing early performance of their flood initiated in a portion of the trend in April, 1964. This flood was planned to take advantage of imbibition and high pressure gradients across the reservoir matrix blocks (unfractured blocks of the reservoir rock which are visualized to be generally surrounded by the fracture system) evaluated in pilot tests by the Atlantic Richfield Co. and Humble Oil and Refining Co. However, the early recovery and pressure performance of this flood indicated that high pressure gradients induced between the fracture system and the centers of the matrix blocks were forcing water into the periphery of the blocks and temporarily interfering with the countercurrent flow of oil due to imbibition. Fill-up in the blocks was occurring with re-solution of the free gas phase as pressures climbed above the original reservoir bubble point. It appeared that cessation of water injection to permit the capillary forces to become dominant and that expansion of the rock and its contained fluids during pressure reduction might aid in expulsion of oil from the rock matrix into the fractures. Subsequent production performance, after cessation of injection, proved this hypothesis correct. Thus, pressure pulse waterflooding was born. Pressure pulse flooding appears to have several advantages over imbibition type flooding in highly fractured reservoirs. First, all wells may be used for water injection which should hasten fill-up and achieve a more rapid increase in reservoir pressure. Second, because of increased injection pressures and rates, flooding gradient will force water into the reservoir matrix more rapidly than would be achieved by imbibition alone. Third, during the pressure depletion cycle of the flood, the compressibility of the system, which has been increased by resolution of the free gas phase, provides energy for the displacement of oil at a rate greater than would correspond to countercurrent flow during imbibition. Fourth. during depletion all wells may be put on production, thus contributing to higher oil withdrawal rates on a reservoir-wide basis. One possible disadvantage of pressure pulse flooding as compared to imbibition flooding, however, is that the outer periphery of each matrix block is flooded to a residual or near residual oil saturation during the water injection stage. JPT