The Analytical Model for Horizontal Wellbore Stability in Anisotropic Shale Reservoir

Wellbore collapse is a frequent problem during drilling in shale gas reservoir, restricting efficient exploration of shale gas. Due to the presence of bedding plane, conventional homogeneity model is inappropriate to conduct wellbore stability analysis in shale reservoir. Therefore, in this study, under the influence of bedding plane, stress distributions including in situ stress components, pore pressure, seepage stress have been calculated. Meanwhile, shale strength is expressed using the Jaeger's criterion, which assumes shale is isotropic body with one group of weak planes. In combination with stress distribution and shale strength, an analytical model has been developed to investigate wellbore failure region. By using this model, influence factors of wellbore stability have been fully analyzed. Results demonstrate that the difference between bedding plane and rock matrix will modify in situ stress components, transport equations and rock mechanical properties around wellbore. As a result, there are heterogeneous distributions of shale strength and stress state. When the shear failure along bedding plane happens, shale strength has clearly reduction and breakout region has increment, changed from two-lobed failure to four-lobed failure shape. The different intersections between bedding plane and wellbore make variable anisotropy in wellbore plane, modifying the pore pressure propagation and seepage stress distribution. When the bedding plane angle in wellbore plane is low or high value, shear failure along bedding plane doesn’t happen and wellbore stability is depended on rock matrix strength. For middle bedding plane angle, failure region clearly increases and shear failure along bedding plane occurs, aggravating the instability. Furthermore, drilling fluid reduces shale strength, thus increasing region area and changing failure mode. This drilling fluid impact is strong in initial stage and becomes stable after 96 h. By having comprehensive investigation on bedding plane influence, findings in this study are advantageous for drilling operations in shale reservoir.