钻井液滤失对煤岩井壁稳定性的影响
Study on the influence of drilling fluid filtration on stability of coalbed wellbore
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摘要: 煤岩具有割理、孔隙等多孔特性,钻井液易侵入,导致井壁围岩失稳。考虑钻井液滤失对煤岩的物理作用,以Mohr-Coulomb塑性模型为煤岩破坏准则,建立了煤岩井壁稳定评价的流固耦合模型,通过数值模拟,对比分析了钻井液滤失对煤岩应力场和渗流场的影响。结果表明:不均匀地应力的挤压作用使钻井液容易沿着最大水平地应力方向滤失;而钻井液滤失会导致孔隙压力增大,继而使井壁煤岩的应力发生改变,应力场最大值由最大水平地应力方向转移到了最小水平地应力方向,且应力极值由15 MPa增至17 MPa,不利于煤岩稳定;在正向压差条件下,钻井液会侵入煤岩孔隙,使孔隙压力增大,导致煤岩应力增大,引起煤岩失稳;因此,钻井液密度设计时应综合考虑井筒液柱支撑井壁和孔隙增压破坏井壁的影响。
Abstract: Coal rock has many porous characteristics, such as cleavage, pore and so on. Drilling fluid is easy to invade into coal bed rock, which is easy to cause wellbore instability. Considering the physical effect of drilling fluid loss on coal rock, a fluid-solid coupling model for evaluating the stability of coal wellbore is established by using Mohr-Coulomb plastic model as the failure criterion of coal bed rock. Through numerical simulation, the effects of drilling fluid loss on the stress field and seepage field of coal rock are compared and analyzed. The results show that the extrusion of non-uniform in-situ stress makes drilling fluid easy to leak along the direction of maximum horizontal in-situ stress, and the loss of drilling fluid will lead to the increase of pore pressure, which will change the stress of coal rock surrounding the wellbore. The maximum stress field shifts from the direction of maximum horizontal in-situ stress to the direction of minimum horizontal in-situ stress, and the maximum stress increases from 15 MPa to 17 MPa, which is not conducive to the stability of coal rock. Under the condition of positive pressure difference, drilling fluid will invade into the pore of coal rock, increase the pore pressure, cause the stress of coal and rock to increase, and cause wellbore instability. Therefore, the influence of drilling fluid pressure supporting wellbore and pore pressurization to damage wellbore should be considered comprehensively in the design of drilling fluid density.
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