页岩气藏水平井井壁稳定性研究

页岩气藏开发采用的井型一般为水平井，在层理发育的页岩地层中钻井一般都存在严重的井壁失稳问题。为此，根据有效应力理论改进了层理地层水平地应力计算模型，并结合川南地区所取岩心进行室内强度实验，选取适当的破坏准则，对比2个地层水平地应力计算模型和水平井眼井周应力状态，得出了页岩气藏水平井坍塌压力随层理面产状的变化规律。研究结果发现：对于页岩地层而言，采用常规的均质地层水平地应力预测模型研究井壁稳定性，低估了水平井的地层坍塌压力；采用改进的水平地应力预测模型计算得出的坍塌压力与实际情况吻合较好；沿最小水平地应力方向钻进的水平井，地层坍塌压力在某一地层倾角处存在极大值，更高的倾角反而有利于井壁稳定。研究结论可以为页岩气井钻井设计提供参考。

Wellbore stability of horizontal wells in shale gas reservoirs

Serious wellbore instability frequently occurs in horizontal wells commonly which are used for the development of shale gas reservoirs.In view of this,the model for predicting the horizontal crustal stress is improved based on the principle of effective stress.And in combination with the core samples from shale gas reservoirs in the southern Sichuan Basin,a laboratory experiment on the strength of cores is conducted to select appropriate failure criteria,the previous and improved models are compared,and the stress state around the horizontal boreholes are analyzed in order to discover patterns that will illustrate how the collapse pressure of a horizontal well in a shale gas reservoir is getting changed along with the variation of bedding planes.The following research findings are presented.When the previous prediction model is adopted,the collapse pressure of a horizontal well in a shale formation will be underestimated;but when the improved model is used,the calculated pressure agrees well with the actual result.In addition,if the horizontal wellbore is drilled along the direction of the minimum in-situ stress,the collapse pressure will come to the largest at a certain dip angle,except for which the higher the dip angle,the more stable the wellbore will become.This study provides reference for drilling design of shale gas wells.