Analyzing of Multi Field Coupling Borehole Collapse Time for Shale

Based on the theory of semi-permeable diaphragm equivalent pore pressure, the fluid-chemistry coupling diffusivity equation of pore pressure is deduced, and then fluid-solid-chemistry coupling model for porous media is founded. Based on theory deduction and laboratory experiment, a new model which represents the relation between the pore pressure and the rock intensive parameters (cohesion stress and angle of internal friction) is founded. A new model which can calculate the collapse time for shale is developed by using the Coulomb-Mohr failure criteria, and then the finite element simulation program is developed by using the software of FEPG. According to the research, the drilling fluid of low activity can inhabit the increasing trend of pore pressure nearby the borehole effectively, and the drilling fluid of low activity can also inhabit the decreasing trend of cohesion stress effectively under the influence of chemical field and seeping field, which favors the borehole stability. The lithological character and drilling fluid performance should be considered in the process of evaluation and forecast of collapse time, and the drilling fluid can be optimized by using this multi field coupling model.