Simulation Research on Friction Characteristics of Borehole Wall Fracture Surfaces and Structural Instability in Maokou Carbonate Formation in Southern Sichuan
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摘要: 为揭示川南地区茅口组碳酸盐岩地层井壁失稳机制，针对该地层系统性地开展了裂缝面摩擦特性测试与离散元数值模拟研究。现场数据分析证实，井壁失稳主要发生在茅口组地层裂缝发育、岩体破碎地带，钻遇该层位后井壁掉块频发，局部扩径严重，威胁钻井安全。实验研究表明：茅口组灰岩中天然/人工裂缝面的摩擦系数分别为0.691～0.743和0.501～0.588；钻井液作用后，受到流体对裂缝面力学强度的弱化和润滑作用，裂缝面摩擦系数平均降幅16.9%～31.8%；高应力条件下由于裂缝面微凸体的软化与破坏，部分充填裂缝的摩擦系数可降至0.2～0.3；采用离散元方法进行含复杂结构面的茅口组井眼井壁失稳模拟发现，当裂缝走向与最大水平主应力夹角为45°～60°时，井周裂缝最易被激活；裂缝面摩擦系数降低显著增加岩体剪切滑移风险，诱发井壁掉块与扩径等失稳现象。基于以上认识，针对基质强度高、天然裂缝发育的碳酸盐岩地层，准确评价天然裂缝的摩擦强度，使用强封堵防塌钻井液体系，可有效封堵裂缝、抑制润滑效应，提升裂缝面摩擦强度，从而保证井壁稳定与钻井安全。Abstract: 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.
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表 1 茅口组碳酸盐岩井壁失稳模型宏-细观参数表
组别 细观参数 单位 取值 组别 细观参数 单位 取值 地层-井眼宏观参数 σH MPa 82.7 颗粒细观参数 弹性模量 GPa 35 σh MPa 76.3 粒径上限 mm 5.4～7.2 σv MPa 72.8 颗粒密度 g·cm−3 2.7 钻井液密度 g·cm−3 1.53 刚度比 无因次 3 井眼尺寸 mm 311.9 摩擦系数 无因次 0.5 平行粘结模型 法向刚度 GPa 24 000 平滑节理模型 法向刚度 GPa 11 100 切向刚度 GPa 8000 切向刚度 GPa 2400 弹性模量 GPa 35 摩擦系数 无因次 0.3～0.5 抗拉强度 MPa 8 抗拉强度 MPa 3 黏聚力 MPa 28 黏聚力 MPa 5 内摩擦角 35 内摩擦角 （°） 20 抗剪强度 MPa 15 抗剪强度 MPa 5 
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