Pressure Prediction and Application Considering Shale Weak Surface Effects and Anisotropic Characteristics

The existing wellbore stability models do not account for the effects of fracture seepage and pressure transmission on wellbore stability, leading to inaccurate predictions of collapse pressure. In this paper, we investigate factors such as wellbore physical parameters, anisotropy, seepage, and pressure transmission for horizontal drilling in shale reservoirs and propose a new collapse pressure prediction model that incorporates weak plane effects and mechanical anisotropy. This model revises the safe mud weight window for drilling shale gas horizontal wells. Taking a shale gas horizontal well in southern Sichuan as an example, the application of the model in the field and its improved prediction accuracy are verified. The research results show that the drilling fluid seepage action and pressure transfer under the condition of positive differential pressure in the wellbore reduce the effective radial stress around the well; with the increase in the number of shale fracture groups, the degree of well wall fragmentation increases accordingly, which reduces the effective support effect of drilling fluids on the well wall; compared with the traditional Moore–Cullen model and the weak face model, the new model of slump pressure prediction by introducing the indicator function has an average error rate of only 7.4%, with a high degree of agreement; the prediction value of the established formation fracture pressure calculation model has an error of less than 5% with the experimental results. The comparison of the field measurement data verifies the applicability and reliability of the pressure prediction model.