Straightline analysis of flow rate vs. cumulative-production data for the explicit determination of gas reserves

Production forecasting and prediction of original fluids in place are important gas-well-performance evaluations which are routinely conducted using rate/time decline-curve analysis. Currently, rate/time decline-curve analysis of natural-gas reservoirs relies heavily on the use of empirical curve fitting of boundarydominated rate/time production data using type curves or the Arps hyperbolic decline model. In this study, we show that originalfluids-in-place prediction and gas-well-performance evaluations can be conducted simply by straightline analysis of boundarydominated data in flow-rate vs. cumulative-production plots. We first analytically demonstrate that the hyperbolic decline exponent describing the depletion of single-phase volumetric gas reservoirs is not subject to empirical determination from rate/time data. Decline behaviour of gas wells producing at less than full potential is also shown to exhibit a hybrid decline character: hyperbolic during early boundary-dominated flow and exponential at later times. In both cases, explicit calculations of hyperbolic decline coefficients are possible, thus enabling the explicit calculation of gas reserves using flow-rate vs. cumulative-production straightline plots. Numerical and field case studies are presented to demonstrate the applicability and generality of the proposed reservesdetermination methodology.