Hard brittle shale in Shahejie Formation,Nanpu oilfeild,is the typical formation with wellbore instability.During drilling process,wellbore collapse and drilling leakage frequently occur.Currently,for wellbore instability in shale formation,most of researches focus on wellbore collapse with structural plane.However,in drilling process,due to rich structural planes,like bedding planes and fracture planes,drilling leakage in shale is serious,extremely restricting safe and effective drilling.Therefore,based on rock mechanical test,considering the characteristics of shale structural planes,combining mechanism of leakage from failure along matrix,failure along bedding plane or fracture and failure propagation,a novel model of loss of pressure in shale formation has been established,determining the distribution and influence factors of loss of pressure.Results indicate that bedding plane and natural fracture are typical weak plane,and are the major controlling factor of causing leakage in shale formation.Compared to homogeneity condition,loss pressure in shale with multiply structural planes is hugely smaller.Also,well trajectory with safe drilling is decreased,increasing the difficulty of drilling.Furthermore,under the influence of weak plane,it is unlikely to see the failure along rock matrix.Leakage firstly happens at natural fracture.Besides,natural fractures has larger influence on leakage compared to bedding plane.The research results can offer theoretical basis for drilling design in hard brittle shale formation,and also meaningful for safe and efficient drilling in shale formation.
维普
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