Study on shear slip mechanism of shale gas fractured formation

The highly efficient development of shale gas is a research hotspot in the world oil and gas industry. When drilling long horizontal wells in shale gas formation with water-based drilling fluid, the problems of wellbore collapse always happened. In this article, the Longmaxi formation shale in Sichuan Province was taken as the research object. Through the friction experiment, it was found that the lubricity of the fracture wall would be enhanced after aqueous solution intrusion into the shale fractures. Compared to pH 7, the maximum static friction coefficient of shale decreased by 23.6% and the root mean square roughness decreased from 41.9 nm to 25 nm after soaking in aqueous solution with pH 13. With the addition of viscosity modifiers CMC-Na and PAC, the friction coefficient even reduced by 37.5% in our experiment and the CMC-Na is more suitable for the stability of shale than PAC. The aqueous solution with 4% bentonite slurry makes the friction coefficient reduced by 26%. The increase of the lubricity of the fracture wall will make the maximum static friction coefficient between shale fracture and the time of the static friction stage decreased, resulting in the decrease of the friction force on the fracture wall. The shear slip on the fracture wall surface is more likely to happen with the decrease of the friction force, ultimately lead to the collapse of wellbore, which is not conducive to the stability of fractured shale formation.