Shale Gas Reservoir Borehole Wall Stability Study

Shale gas is a new energy, whose efficient development is of strategic importance.Multiple shale gas horizontal wells drilled in the Sichuan Basin have the borehole instability problem, which seriously hampers the development process. Current research in the field is still at the exploratory stage, a scientific basis has not been got. In this paper,on the basis of reservoir lithology, rock mechanics parameters, stress profile and direction, three pressure formation profile, rock brittle, joints, bedding and hydration, we carried out experiments, theoretical analysis, and field-proven research to identify key factors affecting wellbore stability, providing technical support for large-scale and efficient development of domestic shale gas.The main work of the paper is as follows.1) For the actual drilling of the study block—Changning-weiyuan Block, I made analysis of the geology, shale lithology, and technical difficulties in actual drilling. Finally, I identified the key issues of borehole stability at the study block and the main factors.2) On the basis of triaxial rock mechanics experiments, a log interpretation model of rock mechanics parameters suitable for the study block was established, realizing computing of the whole well sector and getting the profile of the rock strength parameters, which provided basic data for wellbore stability evaluation.3) Analysis of the direction and bigness of vertical stress and horizontal principal stresses, rock mechanical parameters as well as other stress-related parameters was made. Based on drilling and completion information, the size and orientation of the overall regional stress and its impact on wellbore stability was determined.4) By means of log interpretation, with the correction of rock mechanics parameters,profile of the three pressure was established, which was analyzed with the engineering and geological data.And then,on the basis of it, analysis of the shale instability mechanism was made.5) Safe mud density window was studied. And in accordance with the actual situation, the borehole instability countermeasures were given.6) Through field-tracking analysis, the experimental research and theoretical research in this paper was validated and improved, which proved to be of well adaptability and practical value.