Analyzing well instability mechanisms in broken formations of faulted zones based on discrete element method

Due to the effect of compressive tectonic movement, the Ordovician formation in Shunbei 5 faulted zone is broken, leading to serious well collapse and fallout in the process of drilling. The deformation and stress of broken formation rocks are discontinuous and there is no cohesion between broken rocks, so the conventional rock mechanical analysis method based on continuous medium is not applicable to this type of broken formation. The discrete element method assumes the rock mass with structural surface as the combination of several rigid bodies to depict the discontinuity of deformation and stress, so it is applicable to the modeling of broken formation. In this paper, a new indicator which can characterize the plugging capacity of drilling fluid more rationally by considering formation breaking features was proposed, namely pressure transfer coefficient, which can be obtained by means of simple experiment. Then, the discrete element model applicable to Shunbei faulted zone was established. Finally, the calibrated model was adopted to analyze the influence degree of hole deviation angle, drilling fluid density and pressure transfer coefficient on well collapse. And the following research results were obtained. First, the instability of broken formation is the result of the unbalanced stress on rock mass instead of the strength failure of the rock mass itself. The pressure transfer of drilling fluid leads to the increase of the pore pressure around the well and the decrease of inter-rock compressive force and friction coefficient, and thus the great decrease of friction force results in the instability of rock mass. Second, the rock size in the faulted zone has an influence on well stability. The more broken the rock mass is, the more easily the instability happens. Third, the risk of well instability increases sharply with the increase of hole deviation angle. Fourth, the simulation results of different pressure transfer coefficients indicate that the safety density window of drilling fluid is increased in different degrees with the increase of plugging capacity, and the fault tolerance of each influential factor is improved. The research results provide a guidance for the preparation of the engineering measures of safe and efficient drilling in broken formations.