A Model for Development and Analysis of Gas Reservoirs with Partial Water Drive

Abstract

Abstract In common reservoir analysis computations, the average gas reservoir pressures are erroneously used in the Van Everdingen-Hurst unsteady state equation (VEHE) to calculate water influx. The VEHE requires the pressure at the original gas/water contact (GWC). The incorrect use of the average oil reservoir pressure in the VEHE can lead to a significant error in the results. This study was performed to show the quantitative potential of the error, using a mathematical model. The mathematical model combines the material balance equation (MBE) for a partial water-drive gas reservoir and the VEHE. The model generates pressure-production data for a conceptual partial water-drive gas reservoir, and for the following two cases: Case A: using average reservoir pressures in the VEHE.Case B: using pressures at the original GWC elevation in the VEHE. The error is defined as the difference in the calculated pressure performances between cases A and B. The results of this study showed that the error increases as the aquifer size increases and as the permeability decreases. The maximum error was found to be 14% in the range of reservoir and aquifer conditions studied. This study also showed that gas recovery increases as the permeability decreases, a result which is also reported by Agarawal, Al-Hussainy, and Ramay.8 This paper presents the procedure used to correct average gas reservoir pressure to the original GWC elevation.