A P/Z Plot For Estimating Original Gas in Place in a Geo-pressured Gas Reservoir By The Use of A Modified Material Balance Equation

Abstract

Geo-pressured gas reservoirs are reservoirs with high pressure gradient of about (0.5 – 1.0) psi/ft. The increase in pressures has been proposed by some authors to be caused by inability of excess fluids to leak-off after major tectonic events of compressional folding or rapid deposition of thick sediments in young sedimentary rocks. The limitation of the conventional method in estimating original gas in place (OGIP) in a geo-pressured gas reservoir is that the solution gas in connate water is habitually neglected in estimating the OGIP. As a result, its contribution of the total gas production is omitted in the MBE. This leads to an inaccurate estimation of OGIP and gas reserve. To eliminate this error, a new form of MBE is presented by this study. It includes the contribution of solution gas in connate water to the total gas production. A comprehensive compressibility term that includes the rock compressibility (Cf), water compressibility (Cw) and the gas solubility in water (Rsw) are introduced into the material balance equation and P/Z plots are formulated. The result of the formulation indicates a 5 – 12% increase in the estimation of the OGIP as compared to some existing methods. This tells the importance of the solubility of gas in brine in the estimation of the OGIP and the importance of its inclusion in the Material Balance Equation.