Nonstatic Pressure History Analyses for Gas Reservoirs

Abstract

Abstract A method has been developed for using nonstatic pressure measurements directly in gas reservoir material balances composed of various energy mechanisms. Applying this method leads to simultaneous determinations of the reservoir p¯ history, gas in place, and other parameters relevant to water influx and effective compressibility. Well-known methods1–4 of determining average static pressure, p¯, have at least two shortcomings: (1) an estimation of reservoir shape and (2) an often-neglected implicit relationship between p¯ and the viscosity-compressibility product. Errors resulting from these deficiencies are minimized by the proposed method through a simple coupling of the well-known pseudosteady-state flow and material-balance equations. The solution of this coupling is obtained through nonlinear regression, and it allows simultaneous evaluations of gas initially in place, static pressure history, and several other reservoir parameters. These parameters can include the initial reservoir pressure, a stabilized gas-deliverability constant, the effective compressibility, aquifer diffusivity, and aquifer volume plus water-influx constants. The results of applying the method to six published cases are presented to illustrate the utility of the method.