离散单元法评价煤层井壁稳定性

Evaluation of wellbore stability in coal seams by use of the universal discrete element code (UDEC) method

煤层属于裂缝发育的地层,钻井过程中井壁稳定性分析复杂,对煤层井壁稳定性进行评价十分必要。为此采用山西省沁水盆地樊庄3号煤层及取得的基本物理参数和节理参数,利用适合模拟节理煤层的离散元软件UDEC建立直井煤层模型,通过改变井底压力参数来分析井壁位移变化情况,得出了井眼模型宏观变化图像、微观位移矢量图以及8种不同状态下模型应力数据,并定量分析了8种状态下井眼附近离散单元失效情况。综合位移矢量图和定量分析结果,建立了井眼煤岩离散单元安全系数法,评价了煤层井壁在欠平衡条件下的井壁稳定性,评价结果认为:在该煤层参数下煤层井眼的最佳稳定井底压力区间值为4～7MPa。所建立的评价煤层井壁稳定性方法,可为煤层气气井钻井提供一种井壁分析分析手段。

Due to well-developed fractures in coal beds,it is quite complex to make the analysis of wellbore stability during the drilling of a CBM gas well,so the evaluation of which,however,is very necessary.In view of this,a case study was conducted in the 3 # coal seam of the Fanzhuang Block,Qinshui Basin.The universal discrete element code(UDEC) was adopted to build a simulation model for vertical wells in coal seams,from which we obtained the macrograph,micro displacement vector and stress data under 8 different states.On this basis,aquantitative analysis was made of the failure of UDEC near the well bore under 8different states,and the safety factor of UDEC was established.And in combination with the displacement vector,the wellbore stability in coal seams is evaluated in an under-balanced condition.The results show that the wellbore stability will be easily maintained under the ideal bottomhole pressure of 4-7MPa in this case study.