页岩气储层井壁坍塌压力研究

页岩气储层各向异性强，易发生井壁失稳。利用四川盆地所取页岩气储层岩芯测试钻井液对页岩基体及层理面强度的影响规律，结合单一弱面准则，建立页岩气井井壁稳定预测模型，分析层理面倾角、钻井方位、钻井时间及钻井液类型等因素对页岩储层水平井坍塌压力的影响规律。研究结果表明：层理面倾角小于45°时，井壁岩石发生层理面破坏，倾角大于45°时，在某些井眼方位井壁会发生本体破坏；沿最小水平地应力方位附近钻进水平井最容易发生井壁坍塌；随钻井时间的增加，坍塌压力逐渐升高，井壁破坏形式可能会由初始的页岩基体破坏变为层理面破坏；使用油基钻井液比使用水基钻井液更容易保持井壁的长期稳定。研究结果可以为页岩气井钻井设计提供参考依据。

RESEARCH ON COLLAPSING PRESSURE OF GAS SHALE

Shale gas reservoir has strong anisotropy and wellbore instability is prone to occur. Wellbore stability model in gas shale was created based on transverse isotropic theory and single plane of weakness theory. Core samples of gas shale were tested to research the influence of drilling fluid on shale matrix strength and bedding plane strength. The influences of dip of bedding，drilling azimuth，drilling time and drilling fluid，etc. on collapsing pressure of horizontal wells were analyzed. The results show that when the dip of bedding is smaller than 45°，failure along the bedding plane will take place on the wellbore wall；when the dip of bedding is higher than 45°，failure in shale matrix will take place in some drilling azimuth；drilling horizontal wells near the direction of minimum horizontal stress may easily lead to wellbore collapsing；with the hole open time increasing，the collapsing pressure will raise gradually；and the failure mode of wellbore wall may change from the initial shale matrix failure into bedding plane failure；oil based mud is more positive for long-term wellbore stability than water based mud. The results can provide a reference for drilling in gas shale.