Investigation on influence of drilling unloading on wellbore stability in clay shale formation  

作　　者：Yi Ding Xiang-Jun Liu Ping-Ya Luo 

机构地区：[1]State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu 610500,Sichuan,People's Republic of China [2]Deep Earth Energy Lab,Department of Civil Engineering,Monash University,Melbourne 3800,Australia

出　　处：《Petroleum Science》2020年第3期781-796,共16页石油科学（英文版）

基　　金：This work was supported by the National Natural Science Foundation of China(No.41772151);the National Science and Technology Major Project of the Ministry of Science and Technology of China(No.2011ZX05020-007-06);the Application Basic Research Project of Sichuan Province(No.2014JY0092)。

摘　　要：Wellbore collapse frequently happens in the clay shale formation.To maintain wellbore stability,appropriate mud pressure is a vital factor.When clay formation is opened,drilling unloading occurs,modifying rock structure and strength at the wall of borehole,which affects the selection of mud pressure.Currently,mechanism of drilling unloading is still poorly understood which in return will bring a concern to wellbore stability.Therefore,in this study,a combination of triaxial compressive test and ultrasonic wave test has been used to simulate drilling unloading and analyze its mechanism.Results indicate that more void space is created inside the clay shale sample due to unloading.This structure change leads to a decline of strength and acoustic amplitude.Additionally,unloading influence is depended on varying drilling unloading parameters.Small unloading range and fast unloading rate are able to enhance stability.With various degrees of unloading impact,collapse pressure equivalent density has a clear modification,proving that unloading is a non-negligible influencing factor of wellbore stability.Besides,the unloading effect is much stronger in large confining pressure,implying that more attention should be given to unloading when drilling is in extreme deep or high geostress formation.Findings in this paper can offer theoretical guidance for drilling in the clay shale formation.

关 键 词：Drilling unloading Clay shale Wellbore stability Rock strength 

分 类 号：TE28[石油与天然气工程—油气井工程]