基于幂函数形式物质平衡方法的高压、超高压气藏储量评价

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

高压、超高压气藏的储层岩石有效压缩系数、含水层体积及水侵量难以确定,常规视储层压力与累计产气量曲线外推法或其改进方法计算的该类气藏储量的准确度较低。为了提高储量评估的准确性和可靠性,在Gonzalez方法的基础上,建立幂函数形式的高压、超高压气藏物质平衡方程,并结合20个国外已开发高压、超高压气藏实例,确定幂指数经验值,分析了视储层压力衰竭程度和采出程度对储量计算可靠性的影响,确定影响储量评价可靠性的关键参数(视储层压力衰竭程度)的临界值,并与两段式临界值进行了对比和实例计算。研究结果表明:①幂函数形式物质平衡方法的幂指数经验值为1.028 47,其上限值为1.115 67;②经典二段式拐点对应的视储层压力衰竭程度介于0.14~0.38,平均值为0.23,第二直线段外推点对应的视储层压力衰竭程度介于0.23~0.50,平均值为0.33,对应的采出程度介于33%~65%,平均值为45%;③采用上述方法计算了高压、超高压气藏的储量,当视储层压力衰竭程度大于0.33时,计算结果误差小于10%。结论认为,针对高压、超高压及裂缝性应力敏感气藏,所提出的幂函数形式物质平衡方法避开了储层岩石有效压缩系数、含水层体积及水侵量等不确定性参数,具有计算过程简单、实用性较好、误差较小的优点。

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 in order 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 1.028 47, with an upper limit of 1.115 67. 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.