上部含水层对煤层气开发效果影响的数值模拟研究

Numerical simulation study on the effect of upper aquifer on coalbed methane development

煤层气气井通常需要通过水力压裂才能实现建产。沁水盆地M区块一些煤层气气井产水量明显高于周围其他气井,存在不产气或低产气的特殊情况。由于水力压裂缝在煤层纵向上的穿透距离具有一定的不确定性,压裂过程中易将顶板砂岩水层压窜,导致气井生产动态与周围正常状态下的煤层气井差异大。为了解决这个问题,在研究工区煤层地质特征和区块顶部砂岩含水层的地质特征基础上,建立了煤层+顶部含水层的三维地质模型,应用数值模拟技术量化分析了顶板含水层对煤层气井生产状况的影响程度。综合结果表明:1)M区块15#煤层顶板以上0~20 m距离发育的含水砂岩可能是一些煤层气井产水量异常大的主要来源。如果该砂层平均孔隙度大于5%,距离煤层10 m附近有较大产水异常风险。2)排采情况及数值模拟结果表明,15#煤层顶板含水层对煤层压裂改造及降压解吸影响较大,对于煤层顶板距离含水层较近的井,可考虑进行工艺改进,释放15#煤层气产能。3)考虑了顶板含水层影响的煤层气数值模拟模型,可模拟部分气井压窜上部含水层的异常生产情况,显著提高研究区块数值模拟的历史拟合精度,提高剩余气研究的可靠性,为后续方案优化奠定基础。

Coalbed methane wells usually need hydraulic fracturing to achieve production. The water production of some CBM wells in block M of Qinshui Basin is significantly higher than that of other surrounding gas wells, and there is a special situation of no gas production or low gas production. Due to the certain uncertainty of the penetration distance of hydraulic fracturing fracture in the longitudinal direction of the coal seam, it is easy to press the roof sandstone water layer in the fracturing process, resulting in a large difference between the production performance of the gas well and the surrounding CBM wells under normal conditions. In order to solve this problem, based on the study of the geological characteristics of coal seams in the work area and the geological characteristics of sandstone aquifer at the top of the block, athree-dimensional geological model of coal seams + top aquifer is established, and the influence of roof aquifer on the production of coalbed methane wells is quantitatively analyzed by using numerical simulation technology. The comprehensive results show that: 1)The water bearing sandstone developed 0~20 m above the roof of 15#coal seam in block M may be the main source of abnormally large water production of some coalbed gas wells. If the average porosity of the sand layer is greater than 5%, there is a great risk of abnormal water production near 10 m away from the coal seam;2)The drainage and production situation and numerical simulation results show that the aquifer of 15#coalbed roof has a great impact on the fracturing transformation and depressurization and desorption of coal seam. For wells with coal seam roof close to the aquifer, process improvement can be considered to release 15#coalbed methane productivity;3)The CBM numerical simulation model considering the influence of roof aquifer can simulate the abnormal production of some gas wells channeling the upper aquifer, significantly improve the historical fitting accuracy of numerical simulation in the research block and improve the reliability of residual gas research, so as to lay a foundation for subsequent scheme optimization.