An iteration-free approach for determining the average reservoir pressure and original gas in place by production data analysis: Methodology and field cases

Current gas well decline analysis under boundary-dominated flow (BDF) is largely based on the Arps' empirical hyperbolic decline model and the analytical type curve tools associated with pseudo-functions. Due to the nonlinear flow behavior of natural gas, these analysis methods generally require iterative calculations. In this study, the dimensionless gas rate (q_g/q_gi) is introduced, and an explicit method to determine the average reservoir pressure and the original gas in place (OGIP) for a volumetric gas reservoir is proposed. We show that the dimensionless gas rate in the BDF is only the function of the gas PVT parameters and reservoir pressure. Step-by-step analysis procedures are presented that enable explicit and straightforward estimation of average reservoir pressure and OGIP by straight-line analysis. Compared with current techniques, this methodology avoids the iterative calculation of pseudo-time and pseudo-pressure functions, lowers the multiplicity of type curve analysis, and is applicable in different production situations (constant/variable gas flow rate, constant/variable bottom-hole pressure) with a broad range of applications and ease of use. Reservoir numerical simulation and field examples are thoroughly discussed to highlight the capabilities of the proposed approach.