Rock Compressibility and Failure as Reservoir Mechanisms in Geopressured Gas Reservoirs
Authors: Harville David W. , Hawkins Murray F. 
Published: December 2007

A study of the North Ossun field, Louisiana, reveals that as reservoir pressure is depleted the increase in net overburden pressure initially pressure is depleted the increase in net overburden pressure initially causes rock failure and as the failure continues with decreasing pore pressure, rock compressibility decreases until eventually it reaches a pressure, rock compressibility decreases until eventually it reaches a normal value.

Introduction
Rock compressibility has long been recognized as an important factor in material balance calculations of oil in place for closed reservoirs producing above bubble-point pressure. For example, if the pore volume compressibility of the reservoir rock is half of the compressibility of the undersaturated oil, neglect of the rock compressibility term results in about a 50 percent overestimation of oil in place. In general, it percent overestimation of oil in place. In general, it may be stated that in material balance calculations on closed reservoirs, consideration of rock compressibility becomes increasingly important as the fluid compressibility decreases. For this reason the effect of rock compressibility is commonly neglected in studies on gas reservoirs where gas compressibility is usually great. Because gas compressibilities decrease with increasing pressures, the consideration of rock compressibility becomes increasingly important for deeper, high pressure gas reservoirs. For example, the compressibility of the gas in the reservoir to be discussed is 30 microsip** at an initial reservoir pressure of 8,921 psia. For a nominal pore volume rock compressibility of 6 microsip, neglect of rock compressibility in material balance calculations on a closed reservoir will result in a 20 percent overestimation of initial gas in place. If the rock compressibility is larger than 6 microsip, then a still larger over-estimation of gas in place results. In this study we propose that because of low net overburden pressures, rock compressibilities in geopressured reservoirs are considerably greater than for similar rocks in normally pressured reservoirs. We further suggest that as pressured reservoirs. We further suggest that as reservoir pressure is depleted, the increase in net overburden pressure initially causes inelastic rock compaction or rock failure. As failure continues with decreasing pore pressure, rock compressibility decreases and eventually reaches normal values in the range of 6 microsip.

North Ossun Field, Louisiana
The mechanisms proposed in the previous paragraph are believed to be illustrated by the performance of the NS2B reservoir of the North Ossun field, Lafayette Parish, La. This is a geopressured gas reservoir with an initial pore pressure of 8,921 psia at 12,500 ft subsea depth, or a gradient of 0.725 psi/ft. Table 1 gives pertinent data on this reservoir. Good geologic control is indicated by the structure map, Fig. 1. Although a gas-water contact exists, it is doubtful that the associated aquifer is very large because the reservoir appears to shale out on the west. In addition, considerable complex faulting in the area almost certainly closes the reservoir with a small associated aquifer. Good core and log data have been used to calculate an initial hydrocarbon pore volume of 583 million cu ft, and, with PVT data, to calculate an initial gas in place of 114 Bscf.
JPT
P. 1528