Improved gas resource calculation using modified material balance for overpressure gas reservoirs

The p/z plot of an overpressure gas reservoir with a closed boundary typically is a downward concave curve. Overestimation of the original gas in place (OGIP) caused by incorrect extrapolation of the early production data is often observed in reserve evaluation. To eliminate this error, a comprehensive compressibility term that includes pore volume compressibility, water compressibility, and gas solubility in water has been introduced into the p/z plot. To achieve the above objective, it is critical to obtain the right average reservoir pressure corresponding to the drained gas reserve at the time of interest. But for overpressure gas reservoirs, if we completely ignore the permeability changes as the reservoir pressure declines, the reservoir performance will not be representative. Another substantial deficiency of the conventional method is that the solution gas in connate water has been neglected in estimating the OGIP. As a result, the contribution of solution gas to the total gas production is omitted in the material balance equation (MBE). These missing terms lead to an inaccurate estimation of the OGIP and gas reserve. Considering that the permeability is not constant throughout the reservoir life, but a function of pressure, rock and fluid properties, production volume, and original pore volume, we present a new form of MBE which includes the effects of the permeability change due to pore volume change and the contribution of solution gas in connate water to the total gas production. With the proposed semi-analytical equations, the average reservoir pressure and reservoir deliverability can be more accurately estimated. Therefore, the evaluations of OGIP and recoverable gas are more reliable.