Shale water as a pressure support mechanism in gas reservoirs having abnormal formation pressure

Abstract

One problem often confronting reservoir engineers is an analysis of the reservoir mechanisms involved in the depletion of abnormally high pressure reservoirs. Two mechanisms that have been proposed are: (a) high rock compressibility; and (b) shale water influx. This paper is a study of the shale water influx theory. A literature search was conducted to establish what is known about shale permeability, compressibility, porosity, and water viscoity. The shale properties were used in a calculation of water influx for two actual superpressure reservoirs using a linear diffusivity equation in which permeability and compressibility were a function of pressure. Of the shale parameters, permeability and compressibility have the most influence on water influx. For an initial shale permeability of the order of 10 −5 mD and an initial bulk compressibility of 40 × 10 −6 psi −1, shale water influx significantly affects the reservoir depletion. For shale permeabilities of the order of 10 −7 mD, shale water influx is insignificant. It was also noted that the pressure distribution in the shale is very steep and only the first few feet of shale contribute materially to the water influx. An approximate method for extrapolating early p/ z behavior in superpressure gas reservoirs is also presented.