水驱气藏早期直线外推动储量探讨

Dynamic reserves calculated by linear relationship in the early development of water-drive gas reservoir

根据已有文献,水驱气藏在生产初期具有与定容气藏相似的压降特征,可通过直线外推求取动储量,但并未对适用的采气程度范围和计算的动储量误差进行评估。为此,以已有的物质平衡方程式和非稳态水域模型为理论基础,采用迭代匹配法计算了不同水体倍数、水域渗透率情况下,直线外推动储量误差随采气程度的变化。结果表明,以动储量误差≤10%为准则,适合水驱气藏早期直线外推动储量的采气程度范围很窄,一般小于5%。以东海西湖凹陷开发井为例,对比传统的定期压力监测方式与实时压力监测方式下早期直线外推动储量的差异:前者因测压时间间隔长,易导致对早期直线段的误判和动储量评估结果的误差;后者可精确判断水侵影响明显的开始时间,从而有效实现早期直线外推计算水驱气藏的动储量。该研究结果对生产初期水驱气藏动储量的正确计算具有重要指导意义。

According to existing references,in the early development stage,water-drive gas reservoir has similar pressure drop characteristics with constant volume gas reservoir,therefore the dynamic reserves of gas reservoir can be obtained by linear relationship pressure drop extrapolation methods. However,the applicable range of gas recovery degree and the calculated dynamic reserve error have not been evaluated. Based on material balance equation and unsteady state aquifer model,the variation of reserve error with gas recovery degree under different aquifer multiplier and water permeability was calculated by iteration algorithm method. The results show that,taking dynamic reserve error less than 10% as criterion,the gas recovery range suitable for the early reserves calculated by linear relationship in water drive gas reservoirs is very narrow,generally less than 5%. Taking waterdrive gas reservoir in Xihu Sag of East China Sea as an example,the differences between the conventionally used method of pressure monitoring and the method of real-time pressure monitoring in obtaining early dynamicreserves by linear relationship were compared. Because of the long-time interval of pressure monitoring,the former method is liable to lead to misjudgment of the linear part of p/Z vs cumulative production in the initial production stage and the deviation in the evaluation of dynamic reserves. The latter method can accurately determine the starting time of the water influx so as to use straight-line extrapolation method to calculate dynamic reserves of water-drive gas reservoirs. The research result has significantly guidance for correct evaluation of dynamic reserves of water-drive gas reservoir in the early development stage.