Reserve evaluation of high pressure and ultra-high pressure reservoirs with power function material balance method

Abstract

It is difficult to determine the effective reservoir rock compressibility, aquifer volume and water influx of high pressure and ultra-high pressure gas reservoirs, so when the traditional apparent reservoir pressure and cumulative gas production curve extrapolation method and its modified version are used to calculate the initial gas in-place of such gas reservoirs, the calculation accuracy is lower. The material balance equation in the form of power function for such gas reservoirs was established based on Gonzalez method to improve the accuracy and reliability of reserve evaluation. Then, based on 20 high pressure and ultra-high pressure reservoirs that had been developed abroad, the empirical value of the power exponent was defined, and the influences of the depletion degree of apparent reservoir pressure and the degree of reserve recovery on the reliability of reserve calculation were analyzed. Besides, the critical value of the key parameter affecting the reliability of reserve evaluation (depletion degree of apparent reservoir pressure) was determined and compared with the critical value of the two-linear trends. Finally, an example calculation was carried out. And the following research results were obtained. First, the empirical value of the power exponent calculated using the material balance method in the form of power function is 1.02847, with an upper limit of 1.11567. Second, the depletion degree of apparent reservoir pressure corresponding to the inflection point of the classical two-linear trends ranges from 0.14 to 0.38 with an average of 0.23, while that corresponding to the extrapolation point of the second straight line is between 0.23 and 0.50 with an average of 0.33, and the corresponding degree of reserve recovery is in the range of 33%–65% with an average of 45%. Third, the reserves of such high pressure gas reservoirs calculated with this new method has an error rate less than 10% if the depletion degree of apparent reservoir pressure is greater than 0.33. In conclusion, for such high pressure gas reservoirs and the stress-sensitive fractured gas reservoirs, the material balance method in the form of power function proposed in this paper can avoid the uncertainty parameters (e.g. effective reservoir rock compressibility, aquifer volume and water influx) and it is advantageous with simple calculation process, better practicability and small error.