Research on wellbore stress in under-balanced drilling horizontal wells considering anisotropic seepage and thermal effects

Under-balanced drilling (UBD) is widely used in petroleum drilling engineering, and regarding wellbore safety, it is necessary to analyze stress distribution around the wellbore during drilling procedures. Aiming at seepage and thermal effects during UBD, a model of pore pressure distribution under anisotropic seepage of horizontal wells was derived using conformal transformation. Combined with elastic theory, a new model of seepage stress around wellbores has been constructed. Moreover, based on a heat transfer model solved by finite difference methods, this paper studied wellbore and formation temperature fields during horizontal drilling when heat sources (circulating pressure drops and mechanical friction) are considered. This study shows that both pore pressure and seepage stress are present in an elliptical manner under anisotropic seepage. The annulus temperature of the horizontal section is greater than the geothermal temperature, thus compressive thermal stress forms in the surrounding rock; in contrast, tensile thermal stress develops when heat sources are excluded. The two factors, therefore, should be considered sufficiently important. The heat sources with thermal effects and anisotropy of seepage, which are usually omitted, have been treated again, leading to more accurate results of horizontal UBD and providing a theoretical basis for subsequent wellbore stability analysis.