Experimental study on the influence of shale content in fault zone on fault friction coefficient based on circular shear test

Water injection in fault block oil and gas reservoirs can trigger fault activity, leading to leakage and potential earthquakes, which may cause significant natural and economic losses. The friction coefficient of the fault is a crucial factor in fault activity and stability. Analyzing this relationship is essential for evaluating fault stability. This study focuses on the Penglai fault zone in the Bohai Bay Basin, China, investigating the weakening mechanism of fault gouge on the friction coefficient and integrating key factors that control its heterogeneity. The friction strength of fault gouge was evaluated through circular shear experiments conducted under in situ geological conditions, establishing a method for characterizing the heterogeneity of friction strength. Based on these findings, a model for characterizing the heterogeneity of fault friction coefficients was developed. By integrating a 3D prediction model of clay content on fault surfaces, the spatial distribution of non-uniform friction coefficients across fault planes was determined. The research findings indicate a negative correlation between the steady-state friction coefficient and the maximum static friction coefficient with respect to fault shale content. Within a mud content range of 0% to 35%, the friction coefficient remains approximately constant at around 0.6 with minimal fluctuation. As the mud content increases from 35% to 40% to 65%, there is a gradual decrease in the friction coefficient, followed by a rapid decline when the mud content reaches 65% to 75%. This trend suggests an increased influence of mudstone on frictional sliding. As the mud content increases, leading to greater involvement of mudstone in frictional sliding, significant fluctuations in the friction coefficient are observed due to the combined effects of mudstone and quartz sandstone. Ultimately, when the mud content reaches 80% to 100%, the friction coefficient decreases further, with a reduced fluctuation range. This highlights the predominant role of shale content in influencing frictional sliding behavior. The method for characterizing fault-mud-content-related friction coefficients at various levels significantly enhances the accuracy of fault stability evaluations, thereby promoting the safe and efficient development of oil and gas reservoirs.