Analysis of geopressured gas reservoirs often times is particularly difficult. The effects of pore volume compressibility as well as the pore volume compressibility as well as the uncertain presence of water influx should usually be included in the calculation procedures. The solution plot method is a straight forward procedure which negates the need for estimating procedure which negates the need for estimating these two troublesome reservoir parameters. Computer simulation studies have shown variable pore volume compressibility effects exhibited by under-stressed, abnormally pressured gas reservoirs are based on grain expansion as well as pore collapse values. Normally compacted geopressured, gas reservoirs will display uniform pore volume compressibility effects. pore volume compressibility effects. The solution plot method permits the simultaneous estimation of the original gas in place, pore volume compressibility values as well place, pore volume compressibility values as well as a qualitative estimate of the affect of water influx.
Material balance plots of p/z vs. gas-produced for most geopressured gas reservoirs generally exhibit a dual slope curve (see Fig. 1). Extrapolation of the early (and lesser gradient) slope to estimate ultimate recovery will lead to an optimistic reserves value. This dual slope plot for geopressured gas reservoirs was first plot for geopressured gas reservoirs was first noticed in the literature by Harville and Hawkins. The authors felt the initial and lesser slope was caused by the pore volume, rock crystal and water compressibility effects not being included in the pressure depletion, gas material balance equation. The effects of shale water influx, compaction or rock failure, solution gas liberated from the connate water and the aquifer have also been mentioned in the literature as possible additional influences on the shape of the possible additional influences on the shape of the plot. plot. Compressibility values of gas, water and the pore volume are reflected in the slopes of the pore volume are reflected in the slopes of the plot during pressure depletion situations. Gas plot during pressure depletion situations. Gas compressibility is on the order of 40 × 10(-6) psi(-1) at high pressures (greater than 8,000 psi) psi(-1) at high pressures (greater than 8,000 psi) while at more moderate pressures the gas compressibility is 250 × 10(-6) psi. Water compressibility is approximately 3 × 10(-6) psi(-1) and remains essentially the same over all pressure ranges. The pore volume compressibility is generally felt to vary from 4 to 30 × 10(-6) psi. At low to moderate pressures the large gas compressibilities values override the water and normal pore volume compressibility terms. Therefore, the gas material balance equation ignoring any other compressibility effects works quite well. The calculation procedure becomes muddied when one tries the same method for the geopressured gas reservoir situation. At early times these other compressibility sources affect the slope of the p/z vs. Gp plot. Various authors have proposed methods to include these multitude of compressibility effects in the calculation procedure for the abnormal pressured, gas reservoir situation. pressured, gas reservoir situation. Hammerlind proposed two methods to alter the normal p/z vs. Gp plot. A coefficient (C) is determined by relating the abnormal compressibility effects to a normal pressured situation. The author used a normal pressured (0.55 psi/ft) base line study to relate to the abnormal pressured case. The paper included a plot of formation compressibility vs. depth of the plot of formation compressibility vs. depth of the reservoir.