Average reservoir pressure must be known when material balance equations are used to determine gas in place. Most pressure data available for material balance analysis comes from single point tests (static bottom hole pressure tests). Unless the reservoir has been shut-in long enough for the pressure in the reservoir to stabilize there is no direct means of determining the average reservoir pressure.
A method has been developed to estimate gas in place in volumetric gas reservoirs using transient pressure data. The method is similar to the isochronal flow test. Pressure data recorded at a specific shut-in interval is collected during a portion of the producing life of the reservoir. A p/z plot is constructed using this pressure data and the corresponding production data. The p/z curve will plot as a straight line with a slope similar to the stabilized p/z curve. However, this nonstabilized p/z curve will be shifted slightly downward from the stabilized p/z curve. Gas in place can be estimated by drawing a line parallel to the nonstabilized p/z curve through a point on the stabilized p/z curve, such as pi/zi. This new line is extrapolated to the point of intersection on the (Gp) x-axis.
This analysis procedure was tested with production data simulated for a single well reservoir model with and without wellbore storage distortion. The procedure was also tested with production data simulated for a hydraulically fractured well model. The results indicate this method can be used to estimate gas in place within a margin of error acceptable for most engineering purposes.