Decline curve analysis using rate normalized pseudocumulative function in a boundary dominated gas reservoir

Abstract

Decline curve analysis is an important component in the set of tools used in any reservoir management plan. Decline curve analysis not only provides current information regarding the fluid content and the formation characteristics, but also permits a prediction of future performance. Decline curve analysis has historically been performed using theoretical models, whichinvolve production rate , time and cumulative production data from gas reservoirs and routinely analyzed using dimensionless type curve, which are plots of numerical, analytical or sometimes empirical solution to gas diffusivity equation. These solutions are developedtheoretically by coupling the boundary-dominated flow equation with the gas material balance equation. Due to mathematical difficulty of this process, past efforts have focused on approximate models based on simplified assumptions such as approximate linearization schemes for correlating the non-linear viscosity-compressibility terms and assume production at zero or a constant non-zero bottomhole pressure.These limitations allow for the development of several semi-analytical expressions for the analysis of production rate and cumulative production data. This research proposes a decline curve analysis method based on pseudocumulative function to gas production data in a boundary dominated gas reservoir. In thesinglephasegas flow, the use of a pseudocumulative functiongiven as pseudocumulative production divided by flow rate, allows toobtain a flow equation that has the form of a harmonic decline. This equationpermits the decline analysis of variable pressure/variable rate production data; even though early production data are absent for high depleted reservoir.This method incorporatespseudocumulativefunction that provides a convenient approach to handle variations in viscosity-compressibilityvalues more rigorously with pressure since viscosity-compressibility ratio is afunction of cumulative production.The rate normalized pseudocumulative function does not have any significant effect on transient behavior. First time, the rate normalized pseudocumulative approach has successfully been applied indecline curve analysis to both simulated and field data, and, good results wereobtained.However, this method is strictly valid for boundary-dominated gas reservoir. Finally, anew technique has been developed for analyzing gas production data through decline curve analysis involvinggas production rate, pseudopressure and pseudocumulativeproduction. This analysis method of gas production data is relatively easy and the results are compatible with conventional pressure transient analysis.Therefore, production data analysis should be performed in reservoir engineering to save time and money for it involves no cost inlost production.This method provide a level of certainty as regardthe accuracy and quicker estimation of initial-gas-in-place, reservoir properties, particularly for field data and should enhance the ability of the engineer to perform reliable analysis.