Study on the influence of drilling fluid filtration on stability of coalbed wellbore

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.