An Improved Pseudo­Time for Gas Reservoirs With Significant Transient Flow

The use of semi­analytic methods for correcting flow equations to accommodate changing gas properties with pressure, has become increasingly common. It is a mainstay of modern production decline analysis as well as gas deliverability forecasting. The use of pseudo­time is one method which enables a time­based correction of gas properties, honoring the gas material balance within the time­based flow equation. By using pseudo­time, the analytical well / reservoir models, derived for the liquid case (slightly compressible fluid) can be modified for gas by re­evaluating the gas properties as the reservoir pressure depletes. These gas correction procedures are well documented in the literature. Also well documented is the iterative nature of the gas properties correction methods, as original gas­in­place is a required input into the equations. The pseudo­time correction is based on the average reservoir pressure and works very well for boundary dominated flow. However, when transient flow prevails, the pseudo­time concept is not valid and its use can create anomalous responses. This will occur in low permeability systems or in reservoirs with