Accuracy of material balance applied to two-layered commingled gas reservoirs

Abstract

Material balance equation or the p/Z plot method is the well-known application for original gas in place estimation. This single tank model can be applied to commingled two-layered gas reservoirs as well. To apply the single tank material balance model to estimate the OGIP of commingled two-layered gas reservoirs, we should understand the factors that affect the pressure and cumulative gas production in a commingled system. In this study, we use a reservoir simulator to simulate cases with two gas sands in order to demonstrate the application of the method. Different scenarios of reservoirs properties contrast on thickness, area, porosity, horizontal permeability, and gas specific gravity were run to determine their effect on p/Z plot. Results from this study show that OGIP for the total system can be accurately determined via material balance despite there is contrast in thickness, area, porosity, horizontal permeability, or specific gas gravity. When there is any level of contrast in thickness or gas specific gravity or small contrast in area or permeability, only a single straight line is observed on the p/Z plot. In these cases, only total OGIP of the system can be estimated. On the other hand, when there is any level of contrast in porosity or medium to high contrast in area or permeability, two straight lines appear on the p/Z plot. Thus, OGIP can be estimated for each layer. However, only estimates for cases that have medium to large contrast in porosity or large contrast in permeability have acceptable error for layer OGIP estimate.