Gas-rate forecasting in boundary-dominated flow: Constant-bottomhole- pressure decline analysis by use of rescaled exponential models

Gas-well performance forecasting during boundary-dominated flow (BDF) is largely based on the application of pseudopressure, pseudotime, and material-balance-pseudotime concepts to rate, pressure, and time data. Recently, Ayala H. and Ye (2012; 2013) and Ye and Ayala H. (2013) demonstrated the convenience and importance of a rescaled exponential model that successfully forecasted gas-well decline in BDF by use of density-based dimensionless parameters in place of pseudovariables. In this study, the interdependability and interchangeability of these methodologies is formally demonstrated with a rigorous derivation for rescaled exponential models on the basis of fundamental physical principles applicable to BDF conditions. The rescaled exponential equation is demonstrated to be a rigorous rate/time equation modeling gas-rate decline in wells produced against a constant-bottomholepressure specification. The proposed BDF decline equation is shown to be able to be expressed in terms of a dimensionless fluid parameter (B) that quantifies the μ _gc_g dependency on density for the depletion process of interest, which has been directly tied to the hyperbolic decline coefficient experienced by a declining gas well. Case studies are presented to demonstrate the capabilities of the rescaled exponential model for gas-rate forecasting for wells producing at a constant bottomhole pressure, and its performance is compared with all other available models in the literature.