考虑煤层气稳定解吸的数值试井方法

Numerical Well Testing Method for Well by Considering CBM Desorption

建立了考虑煤层气解吸作用的煤层气稳定解吸均质试井模型,在控制方程中引入稳定源来描述煤层气解吸作用,利用有限元方法求得其数值解。通过分析解吸系数的影响,明确了煤层气解吸作用能缓解地层中压力下降、减缓压力波的传播。当解吸作用达到一定强度时,煤层气解吸量已经达到生产量,压力停止传播,这种情形类似于遇到了定压边界。另外,还考虑了临界解吸压力对试井理论曲线的影响,临界解吸压力和初始地层压力相差的越小,煤层气解吸出现的时间越早,对测试曲线影响也越大。

The distinguish character between CBM well test and normal gas well test is the desorption phenomena in coalbed.This paper developed a new well test model for CBM well by considering the desorption effect in homogeneous coalbed.The stable source method is introduced to describe the desorption effect in the governing equation.And the numerical solution is obtained by finite element method.By analyzing the desorption coefficient,it shows that the desoption effect made the pressure drop slowly and the pressure wave propagate slowly also.When the desorption coefficient reached a certain value,the desorption rate will be equal to the production rate and the pressure wave will stop propagate.It is similar to that there exists a constant pressure boundary in the coalbed.In addition,by considering the effect of critical desorption pressure,it shows that the smaller difference between the critical desorption pressure and the initial pressure,the earlier emerging of the desorption effect and the greater impact on well test type test curve.