Superpressured Gas Reservoirs: Case Studies and a Generalized Tank Model

Abstract

Abstract Pressure decline data from 21 superpressured gas reservoirs in the Gulf Coast were reviewed to determine the characteristic slopes of the P/Z (pressure divided by the gas non-ideality factor) vs. cumulative production plots. Twelve of the reservoirs were substantially depleted. The data were fit with one or two straight lines which would indicate a change in the reservoir pressure support mechanism. The slopes of the plots were reviewed in particular to determine whether the apparent compressibility at reservoir pressures above the hydrostatic level was different than the apparent compressibility below the hydrostatic level, as proposed by Hammerlindl and others. Only four of the plots could reasonably be fit by two straight lines which indicated that the rock compressibility correction factor proposed by Hammerlind1 and others, that changes at the hydrostatic pressure point is not general and the method cannot be used indiscriminately. To better identify the factors that cause downward curving P/Z plots the gas law equation for a gas/aquifer reservoir was rewritten in a generalized form with additional coefficients included the fluid and rock compressibility changes with pressure and gas exsolving from solution. The P/Z plot calculated by the generalized model departs from a straight line, and curves downwards depending on the compressibilities and aquifer size. The generalized equation, with appropriate expressions for the non-ideal factors, can be solved analytically or with numerical computer methods to fit both the shape and position of the decline data. An example fit, using both the two straight line method and the generalized tank model, to a reservoir with a curving P/Z plot is given.