Shale Water As A Pressure Support Mechanism in Superpressure Reservoirs

Abstract

Abstract One problem confronting reservoir engineers today is an analysis of the reservoir mechanisms involved in the depletion of abnormally high pressure reservoirs. Two mechanisms that have been proposed are (1) high pressure reservoirs. Two mechanisms that have been proposed are (1) high rock compressibility and (2) 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 viscosity. The shale properties were used in a porosity, and water viscosity. 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 md and an initial bulk compressibility of 40 × 10 psi, shale water influx significantly affects the reservoir depletion. For shale permeabilities of the order of 10 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. Introduction The discovery of abnormally high pressures in the Gulf Coast region is increasingly common today as more wells penetrate the deeper horizons in search of new oil and gas reserves. In some cases, subsurface pressure gradients approaching the geostatic gradient of 1.0 psi/ft have been encountered. Most of the super-pressure reservoirs occur psi/ft have been encountered. Most of the super-pressure reservoirs occur in sand lenses below the base of the main sand development in or below a major shale series. These reservoirs usually contain gas, are often of limited areal extent, and frequently do not have a significant associated aquifer. However, these reservoirs often do not behave as volumetric gas reservoirs. Reservoir engineers have observed that the extrapolation of p/z data yields extremely optimistic values for the initial gas in place as p/z data yields extremely optimistic values for the initial gas in place as compared with initial gas values obtained by volumetric calculations. Three possible explanations for this anomalous pressure behavior that have been proposed are (1) variable rock compressibility (rock collapse) (2) water influx from limited aquifers and (3) shale water influx. Harville and Hawkins have previously investigated the possibility of rock collapse as the dominant source of pressure support for the North Casum Field in Louisiana. They calculated that a rock compressibility that decreased from 28 × 10 psi to 6 × 10 psi as the undercompacted reservoir was produced would account for the observed pressure behavior. The calculated rock compressibility changes were similar to compressibility measurements reported by Fatt for an undercompacted Sespe sandstone sample.