Abstract:

According to basic theories of the linear elastic lithomechanics and rock fracture mechanics,the present paper dealt with the plugging mechanism of drilling liquid by preventing induced fractures from extension and enhancing wellbore pressure through the increase of fracture reopening pressure.A fracture mechanics model was developed by means of analyzing the pressure distribution in factures after plugging,requirements for preventing induced fractures from extension by plugging were presented,and mechanic behaviors of the induced fracture reopening pressure enhanced by sealing were dealt with based on a mathematical model for the induced stress field of fractures in neighboring rocks of wellbore.Theoretical studies demonstrated that the best way of plugging induced fractures was to apply the lost circulation materials (LCM) at a certain distance near fracture mouth,the hydrodynamic pressure at the pointed end of fractures must be lower than the minimum horizontal major stress and the lower the pressure,the better the sealing effect. The increase of the fracture reopening pressure depended on the induced hoop stress,the lost circulation materials (LCM) must prop fracture walls to maintain the induced hoop stress.As the plugging mechanism of drilling fluid by enhancing wellbore pressure contained both sealing and propping fractures,the LCM should have a sealing property and certain mechanical strength.

Key words: drilling fluid, plugging, fracture, stress intensity factor, induced stress, fracture reopening pressure

摘要：

应用线弹性岩石力学和岩石断裂力学基本理论，揭示了钻井液堵漏阻止诱导裂缝延伸并提高裂缝重启压力从而提高地层承压能力的作用机理。通过分析承压堵漏后缝内压力分布，建立了封堵诱导裂缝的断裂力学模型，并给出承压堵漏阻止诱导裂缝延伸的必要条件；基于井周围岩裂缝诱导应力场的数学模型，提出了堵漏提高诱导裂缝重启压力的力学实质。理论研究表明，堵漏材料在裂缝入口后一定距离封堵为封堵诱导裂缝的最佳位置形式；裂缝尖端部分流体压力必须低于水平最小主应力，且该压力越低，越有利于裂缝的阻裂；诱导裂缝重启压力的提高依赖于周向诱导应力，堵漏材料必须支撑裂缝以维持周向诱导应力；承压堵漏作用机理包含密封和支撑裂缝两方面，堵漏材料应同时具有封堵性能和机械强度。

关键词: 钻井液, 堵漏, 裂缝, 应力强度因子, 诱导应力, 裂缝重启压力