Non-uniqueness problem in estimating original gas in place

Abstract

This thesis demonstrates the problem of non-uniqueness in estimating Original Gas in Place of gas reservoirs using the material balance technique. The presence of this non-uniqueness problem can cause an overestimation of the Original Gas in Place and a negligence of the effect of a considerable encroaching aquifer that could be thought not to exist. This study shows several examples for synthetic gas reservoir/aquifer systems that are modeled mathematically, then programmed, to simulate the performance of these systems. Aquifer analytical models for linear and radial shapes were used to calculate for the effect of the water encroachment. It is shown that the performance plots for material balance techniques (p/z technique and Havlena-Odeh technique) show a misleading behavior that can be misinterpreted to be of a closed (volumetric) reservoir, while actually it is for a water-drive gas reservoir of much less Original Gas in Place. Two models for reservoir/aquifer systems were developed to illustrate the non-uniqueness performance. In the first model, which is the forward model, we input the rate schedule and we obtain the pressure performance throughout the production history. In the second model, which is the inverse model, we assume a fake overestimated Original Gas in Place (G'), which implicitly defines a specific cumulative production/ pressure history of a linear p/z plot. The output of the inverse model is a unique production rate schedule for a synthetic case of a water-drive gas reservoir. If we produce our synthetic case using this rate schedule, we will obtain an exact performance on material balance plots of what historically reservoir engineers believed to be a closed reservoir. The extrapolation of the plots in this latter case show a false overestimated Original Gas in Place (G'). In conclusion, we can say that under certain circumstances, the material balance techniques for determining the Original Gas in Place of gas reservoirs can exhibit a clear non-uniqueness problem.