川南茅口组碳酸盐岩井壁裂缝摩擦特性与结构失稳模拟研究OA

To reveal the mechanisms of borehole wall instability occurred in drilling the Maokou carbonate formation in southern Sichuan province,systematic testing and discrete element numerical simulation were conducted on the friction characteristics of the surfaces of the fractures in the formation.Field data analysis confirms that borehole wall instability mainly takes place in the broken rocks in the Maokou formation in which fractures are developed.When drilling into this formation,caving and sloughing of the formation rocks frequently take place and serious local wash-out leads to enlarged holes,posing a threat to safety of drilling operation. Experimental results show that in the limestones of the Maokou formation, the surfaces of the natural/artificial fractures have coefficients of friction (COFs) ranging in 0.691-0.743 and 0.501-0.588. After contacting with the drilling fluid, the mechanical strengths of the surfaces of the fractures are reduced and the surfaces are also lubricated by the fluid, the COFs of the surfaces of the fractures are reduced by 16.9％-31.8％ in average. In high stress condition, the micro convex spots on the surfaces of the fractures become softened and damaged, the COFs of some filled fractures can be reduced to 0.2-0.3. Simulation using discrete element method of the borehole wall instability in the Maokou formation which contains complex structural faces shows that when the angle between the orientation of a fracture and the maximum horizontal principal stress is between 45° and 60°, the fracture around the wellbore is easiest to be activated; a decrease in the friction coefficient of the surfaces of the fracture significantly increases the risk of shear slip of the rocks, thereby inducing wellbore instability such as caving and sloughing of the borehole wall and hole enlargement. Based on the above understanding, for a carbonate rock formation with high matrix strength and plenty of natural fractures, it is necessary to accurately assess the friction strength of the natural fractures, and use anti-collapse drilling fluids with excellent plugging capacity to effectively plug the fractures, suppress their lubrication effects, and enhance the friction strength of the surfaces of the fractures, thereby ensuring borehole wall stability and drilling safety.