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 non-stabilized 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 non-stabilized p/z curve through a point on the stabilized p/z curve, such as p /z. This new line is extrapolated to the point of intersection on the (G) 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.
The material balance method can be used to estimate gas in place when pressure-production data for two or more history points are available. The accuracy of this method depends on the accuracy of the pressure-production data and selection of the proper material balance equation. Regardless of the form of the material balance equation, average reservoir pressure must be known for proper determination of gas in place. Average reservoir pressure is obtained by shutting in the reservoir until pressure stabilizes. In low permeability reservoirs, shutting in the reservoir long enough for pressure to stabilize may not be practical. Average reservoir pressure can be estimated using transient pressure data if the pressure buildup trend has been recorded beyond wellbore storage. However, when only a single pressure reading is taken there is no direct means of determining average reservoir pressure unless the pressure is stabilized in the reservoir. The purpose of the following study was to develop a method to estimate gas in place using transient, single point pressure data. The proposed method is based on the same principle as the isochronal flow test.
The material balance equation for a gas reservoir where there is no water influx and rock and water expansion are negligible is as follows: