Based on the fractured-porous elastic and seepage theory, considering the orthotropic physical properties of coal cleats and different seepage characteristics in the orientation of the face and butt cleats, the orthotropic dual media mathematical model for fluid-solid coupling is established. The finite element equation is derived on the basis of the orthotropic dual media mathematical model, and also the relative two dimensional program of finite element method is developed. Taking the ZP-1well in Qinshui Basin as an example, the effect of the permeability orthotropic coefficient on the pressure distribution in the borehole wall is simulated. According to the mathematical model, the various parameters which effect the collapsed pressure for borehole stability are analyzed in detail, including non-uniform in-situ stress coefficient, hole size, permeability, pore pressure, internal friction angle and cohesion. The results shows that the pressure distribution in the boreholewall decreases with permeability orthotropic coefficient increasing between 0° and 45°, whereas it increases between 45° and 90°. Collapsed pressure increases with non-uniform in-situ stress coefficient, hole size, permeability and pore pressure increasing, and decreases with friction angle and cohesion increasing.
The formation fluid flows into the borehole unceasingly during the under balanced drilling. After the borehole is formed, the stress around the borehole will be distributed again accompanied by the percolation of the formation fluid which influences the borehole stability. Over the years, a lot of valuable results have been gained by many domestic and foreign scholars according to the large number of studies on borehole stability. Fjaer presents the elastic stress solution in the borehole wall under non-uniform insitu stress. Fonseca shows the porous linear elastic stress solution in the borehole wall subjected to fluidsolid coupling, considering the effect of the uniform in-situ stress and radial porous flow.