Da26 well is drilled under-balanced, using polysulfonate drilling fluid.No problem appeared until continual sloughing occurred after under balanced drilling.

• 大26井采用聚磺钻井液进行欠平衡钻井,前期钻井施工顺利,但在欠平衡施工完毕之后,出现了井壁持续掉块现象,不得不通井划眼,电测难以到底。