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Summary Static reservoir pressures are required for all gas material balance calculations. There are no techniques currently available for using isochronal, transient pressure buildup tests to estimate original gas in place.
A method to extrapolate the slope of a line generated from two or more isochronal or short-term pressure buildup tests to initial conditions to estimate gas pressure buildup tests to initial conditions to estimate gas reserves has been developed. Simulation as well as theoretical studies prove the technique may be applied to gas reservoirs with a permeability of one md or greater. However, the extrapolations become skewed for gas reservoirs displaying a permeability of less than one md. The isochronal test method may not be used with a high degree of accuracy for these low permeability reservoirs. The divergence of the extrapolations was found to be caused by the c product term evaluated at a subsequent pressure differing significantly from the c product term pressure differing significantly from the c product term evaluated at static conditions. This pressure drawdown effect was found to only be significant in reservoirs displaying a permeability of less than 1 md.
Analysis of the fundamental concepts has formulated a relationship between the diffusivity equation and the material balance equation to account for changing fluid properties during pressure buildup. A method has been developed from these relationships to adjust the real time isochronal interval for the changing gas fluid properties. The adjustment technique permits the transient pressure, isochronal method to be applied to very low permeability reservoirs as long as the pressure is 1,000 psia or greater.
Introduction Material balance or reservoir simulation models are used to estimate reserves and predict future producing characteristics for gas reservoirs. The pressure-fluid properties-production history relationships established in properties-production history relationships established in the material balance equations are assumed to be a function of average reservoir pressure. These pressures must be measured at static or stabilized conditions. Attaining static conditions in very low permeability or extremely heterogeneous reservoirs is often a practical impossibility because shut-in times may extend for several months. Shut-in times for moderate to high permeability reservoirs are of a much lesser interval. In any case. revenue is lost when a gas well is closed in for measurement of a static bottomhole pressure. The potential loss of income resulting from shutting a well in potential loss of income resulting from shutting a well in often inhibits operators from obtaining static bottomhole pressures. pressures. Short term, pressure testing to determine the gas well deliverability is a current industry standard for reducing total test time to estimate well potential. Different shut-in times reflect different reservoir radii of investigation. The isochronal test method is discussed by Cullender. Previous simulation work by Sullivan and Poston has shown isochronal, transient pressure tests Poston has shown isochronal, transient pressure tests under constant rate conditions may be used to determine gas reserves with a good degree of accuracy. The method permits extending the constant line slopes generated from permits extending the constant line slopes generated from short-term buildup pressures measured over at least two different periods to the initial p /z value on the vertical axis of a p/z vs. Gp plot. The line then is extended to the intersection of the x axis where original gas in place is determined. An estimate of cumulative gas produced at any subsequent pressure may then be made from the plot. Fig. 1 shows the mechanics of the old and new p/z vs.G plot concepts. plot concepts. Additional theoretical and simulation work has shown the isochronal, transient pressure analysis method requires the well or reservoir to be in a constant producing or a constant bottomhole flowing pressure condition. A different type of plot must be generated for either producing situation. The p /z plot for the constant pressure producing situation. The p /z plot for the constant pressure case intersects the static pressure p/z plot at the point where p /z equals the static p/z value. Fig. 2 shows the mechanics of the constant pressure process. The present paper extends our knowledge of the isochronal, transient paper extends our knowledge of the isochronal, transient pressure concept and presents an analysis of the pressure concept and presents an analysis of the fundamentals underlying the reserves estimation method.