Wellbore Stability Analysis for Deep Carbonate Formations: A Case Study of the Dengying Formation, Penglai Area

Wellbore instability remains a critical challenge over the entire service life of oil and gas wells, especially in deep carbonate reservoirs. This study integrates triaxial rock mechanics experiments and acoustic emission (AE) Kaiser effect tests to characterize the mechanical properties and in situ stress regime of the Dengying Formation. By comparing the adaptability of three typical rock failure criteria based on published true triaxial test data for dolomite, the Mogi–Coulomb criterion is selected as the optimal failure criterion for the evaluated dolomite dataset. Integrating this criterion with the wellbore circumferential stress model applicable to arbitrary well inclinations, a novel wellbore stability evaluation model is established. The proposed model is applied to Well PS-1 to systematically investigate the influences of minimum horizontal principal stress, production pressure drawdown, Poisson’s ratio, wellbore inclination, reservoir pressure, and wellbore azimuth on near-wellbore stress redistribution and wellbore stability. The results indicate that, within the investigated parameter ranges, the minimum horizontal principal stress (σh) exerts the most significant influence on shear failure. The production pressure differential (ΔP) exceeds 65 MPa, which exposes the wellbore to a high risk of shear failure. Wellbore instability begins once the reservoir pressure drops to 61.9–81.9 MPa. Deviations of wellbore inclination from the optimal near-90° angle (i.e., high-angle and horizontal wells) markedly aggravate wellbore instability. Poisson’s ratio and wellbore azimuth show relatively minor effects on wellbore stability. The findings provide quantitative guidance for drilling and completion optimization in the Dengying Formation.