Issue 27

X. Ran et alii, Frattura ed Integrità Strutturale, 27 (2014) 74-82; DOI: 10.3221/IGF-ESIS.27.09 77 P ERMEABILITY EVOLUTION EQUATION n the stage of elastic deformation, rock permeability is decreasing when the compression stress increases. When plastic deformation takes place, rock permeability will increase slowly first and then sharply with the new crack extending and connecting. The permeability evolution equation of shale under different stress states can be indicated as follows [15]:       / 3 3 1/3 0 0 0 0 0 1 1 1 1 , 0 10 , 0 v v d m Ae B n k n n k k                                           (7) where 3 m  , which means shale permeability will rise about 3 orders of magnitude when rock fractured; 0 k is permeability of shale with no damage. v  is elastic strain. The porosity evolution equation is [16]: 0 3/2 0 1 1 , 0 0.61 , 0 v d n n n               (8) where 0 n is porosity of shale with no damage. N UMERICAL SIMULATION Modelling or the shale drilling at the depth of 3500 m in China western oilfield, the physical model is established by using porous media fluid-solid coupling unit (CPE4P) in software ABAQUS (Fig.1). By considering the disturbance damage and rock permeability changes during drilling, the stress distribution and influence of drilling fluid density is studied. The physical model parameters ( Ⅰ ) are indicated in Tab. 1. The parameter w r is the borehole radius;  is the average density of rock; H  is the maximum horizontal ground stress; h  is the minimum horizontal ground stress; v  is the vertical ground stress; p P is the pore pressure; i P is the drilling fluid pressure; m  is the drilling fluid density;  is the effective stress coefficient. In boundary conditions setting process, the wall is permeable boundary and its displacement is unconstrained. The computational process is divided into three steps: (1) balance the initial stress field; (2) kill the borehole unit and load drilling fluid pressure; (3) calculate seepage-stress-damage coupling in 20 days. Figure 2 : Finite element model. I F