低渗非达西煤层气井渗流数学模型建立与应用

Establishment and application of mathematical model for non-Darcy flow in low permeability coalbed methane wells

低渗透煤层气藏中的气体渗流存在非达西效应,使用常规方法对其进行产量预测与计算时易出现误差。为有效解决该问题,在考虑启动压力梯度影响的条件下,建立了低渗煤层气藏气、水两相渗流数学模型。利用有限差分法进行了数值求解,编制煤层气直井开采数值模拟计算程序进行了产能预测,并分析了煤层参数对产量变化的影响。研究表明:①煤层气井生产时,存在气产量迅速升高、水产量迅速降低的阶段;②吸附时间越短,气产量越早到达高峰期,一定时间内产量也越高;启动压力梯度越大,高峰期后产量下降越快,最终产量也越小;③吸附气超饱和煤层气产量>饱和煤层气产量>欠饱和煤层气产量。

Gas flow in low permeability coalbed methane reservoir has non-Darcy effect.Using conventional method to predict and calculate production tends to lead to error.In order to effectively tackle this problem,a mathematical model of gas/water two-phase flow has been established for low permeability coalbed methane reservoir by taking account of threshold pressure gradient effect.The model is solved by using finite difference method.A computer program is developed for numerical simulation of vertical CBM well productivity prediction.The impact of coal seam parameters on production is analyzed.The results show that in the process of CBM well production,there exists a stage during which gas production increases quickly and water production decreases quickly;the shorter the adsorption time is,the earlier the peak production is seen;the greater the threshold pressure gradient is,the faster the production decline is after peak production,and the lower the ultimate production is;CBM production of supersaturated adsorbed gas > saturated gas > undersaturated gas.