Mechanisms of sidewall stability loss in horizontal wells drilled for shale gas development in Jiaoshiba Block

With beddings well developed, shale gas reservoirs in Jiaoshiba Block are characterized by high strength anisotrophy, so loss of sidewall stability is frequent in these reservoirs. Therefore, direct shearing and triaxial tests were performed on core samples taken from the Longmaxi Formation to develop a new way to characterize shale strength anisotropy based on single-group weak-plane strength theory. By combining the shale strength anisotropy characterization method and model of stress distribution on sidewall, a calculation method of sidewall collapsing pressure in shale formations has been worked out. Finally, calculation and analysis have been done with Well JY1HF as an example. The calculation results show that, with large amounts of weak bedding, shale in the Longmaxi Formation shows apparent anisotropy, and shearing slippage along weak bedding is the main mechanism of sidewall stability loss. Strength of the shale soaked in drilling fluid turns weaker, but such strength reduction is not significant; collapsing pressure calcualted by using conventional models decreases with the increase of deviation angle, whereas collapsing pressure calculated by using the model proposed in this paper increases first and decreases later with the increase of deviation angle, and the calculation results match well with field observation results; shearing slippage of weak bedding in shale is the key of sidewall collapsing in building-up intervals, while reduction in rock strength is not the controlling factor. The research results may provide references and guidelines for drilling design, trajectory optimization of shale gas wells in Jiaoshiba Block and other regions.