Abstract：The strength changing for fully coupled mechanical-chemical behavior of shale formation is studied. The drilling fluid in which there are many chemical reagents which exchange water and ions with rock nearby borehole while drilling；as the result that the strength parameters of shale nearby the borehole are depend on time and position. To analyze the strength changing affected by chemical effect of drilling fluid，a new model is presented. Based on experiment，the relation between strength parameters and water content is obtained；and based on hydrodynamics，the relation between pore pressure and strength parameters is obtained. At the same time，a new model that calculating pore pressure for fully coupled hydraulic-chemical behavior of the shale formation is developed. So using these models，the pore pressure profile and strength profile at different times and positions are investigated. Some conclusions are found that for given poison，with the water content increasing，the pore pressure increases，but the cohesion decreases that will induce the wellbore instability. However，the chemical effect can restrain the increase of water content，so that restrain the increase of pore pressure，and restrain the decrease of cohesion that will be of benefit to enhance the borehole stability. This paper will be of benefit to engineering application for analyzing borehole stability and borehole reinforcing.
