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渗流力对裸眼井筒周围应力场及地层破裂压力影响机理研究

STUDY ON THE INFLUENCE MECHANISM OF SEEPAGE FORCE ON THE STRESS FIELD AROUND OPEN HOLE WELLBORE AND FORMATION BREAKDOWN PRESSURE

  • 摘要: 流体渗流进入储层岩石的孔隙喉道时会给岩石骨架施加渗流力作用, 该力会打破储层岩石原有的应力平衡状态, 影响岩石的变形与破坏. 尽管大量实验与数值模拟研究已经证实渗流力对岩土破坏有显著影响, 但在石油工程领域有关渗流力的研究鲜有报道. 渗流力对水力压裂裂缝起裂与扩展的影响机理仍不清楚. 基于此, 文章首先基于土力学渗流力定义式和Biot固结理论对压裂液渗流进入岩石孔隙时渗流力的作用机理进行了研究. 然后以裸眼井为例分析了渗流力作用形成的应力场, 推导了考虑渗流力作用的地层破裂压力解析解公式, 揭示了渗流力作用对裸眼井地层破裂压力的影响规律. 研究结果表明: 孔隙压差等于一个大气压时, 单位立方厘米岩样所受体积力渗流力的大小远超同等大小岩样所受体积力重力的大小, 渗流力对储层岩石的作用不可忽视. 压裂液渗流进入储层岩石孔隙时, 渗流力作用可以显著降低裸眼井筒周围的有效周向应力, 增大井壁发生拉伸破坏的可能. Biot有效应力系数越大, 渗流力作用越强, 井筒周围应力场被渗流力作用影响的范围越大. 相比较不可渗透的储层, 渗流力作用显著降低了裸眼井的地层破裂压力. 地层深度越大, 两向应力差越小时渗流力作用对裸眼井地层破裂压力的影响越显著.

     

    Abstract: When fluid seepage enters the pore throat of reservoir rocks, it exerts a seepage force on the rock matrix, breaking the original stress balance state of the reservoir rock and affecting its deformation and failure. Although a large number of experiments and numerical simulation studies have confirmed the significant impact of seepage forces on rock failure, research on seepage forces in the field of petroleum engineering is scarce. The mechanism of seepage force on the initiation and expansion of hydraulic fracturing cracks remains unclear. Based on this, this paper first studied the mechanism of seepage force when fracturing fluid seeps into the rock pores, using the definition of geomechanical seepage force and Biot consolidation theory. Then, taking an open hole as an example, we analyzed the stress field formed by the seepage force, derived a formation breakdown pressure analytical solution formula considering the effect of seepage force, and revealed the influence of seepage force on the formation breakdown pressure of the open hole. The results show that when the pore pressure difference is equal to one atmospheric pressure, the volumetric seepage force per cubic centimeter of rock sample greatly exceeds the gravitational force on rock samples of the same size. Therefore, the effect of seepage force on reservoir rocks is significant and cannot be ignored. When fracturing fluid seeps into the pore throats of reservoir rocks, it exerts a seepage force on the rock matrix, which can significantly reduce the effective circumferential stress around the open hole and increase the likelihood of tensile failure on the well wall. The greater the Biot effective stress coefficient, the stronger the effect of seepage force, and the wider the range of the stress field around the wellbore affected by seepage force. Compared to impermeable reservoirs, seepage force substantially reduces the formation breakdown pressure of open hole. The impact of seepage force on the formation breakdown pressure of unconventional reservoirs with deeper depths and smaller bi-directional stress differences is more significant.

     

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