Study On Wellbore Stability Of Anisotropic Shale

Controlled by its fabrics and later historical sedimentation,structural weakness planes such as micro-cracks and layers develop abundantly in shale formation,which causes strongly anisotropy for permeability and mechanical properties.Hence,homogeneous medium model is no longer suitable for wellbore stability problems of shale formation.Meanwhile,transverse isotropy model used commonly does not consider the influence of weakness planes fully enough,and the anisotropic model involves complex parameters which are not easy to obtain accurately in laboratory,so they cannot analysis the practical engineering problems of shale stratum effectively.Based on the analysis of basic fabric and a series of mechanical property experiments of shales,this paper studies the fabric characteristics and its effects on the degree of mechanical anisotropy,which concludes that the main factors affecting the anisotropy of shale properties are the lamellate structural characteristics of high content clay minerals and their preferential orientation array in space.Through the experimental studies of water-rock evolution under pre-stressing,it can conclude that the strength of structural weakness planes in shale will gradually weaken under pre-stressing conditions.The micro-cracks primary closed or newly initiated will also gradually expand-evolve-connect into macro-fracture planes and run through the whole rock mass with the increase of loading stress,which seriously affects the strength of shale formation.Meanwhile,the mechanical properties of shale formations have obvious longitudinal distribution characteristics through the research on the development characteristics of micro-cracks and structural weakness planes.Based on the above research knowledge,this paper establishes an improved model considering the structural weakness planes and the longitudinal distribution characteristics of mechanical properties on the basis theory of conventional transverse isotropic model by means of ABAQUS finite element numerical software.Based on the above-improved model and combined with the actual geological data of X gas field,the distribution characteristics of pore pressure field,effective stress field and the stability law of area around well was studied under the conditions of structural weakness planes permeability,strength characteristics,horizontal stress anisotropy and different pore pressure level conditions.The results reveal there are distributed features that field quantities always distribute along the layers and extend to the deeper depth of formation include the pore pressure,effective stress and plastic zone around wells.Meanwhile,with the lower strength of structural weakness planes,the greater degree of horizontal stress anisotropy and the lower pore pressure levels of formation,the greater of differential stress value along the horizontal minimum in-situ stress direction around the wells is.Moreover,the location of the highest value point will shift from the wellbore to the depth of the formation gradually,which increases the probability of instability in the vicinity well of horizontal direction and the extent and severity of the instability areas around wellbore apparently.