Evolution of Mechanical Parameters in Fractured Carbonate Rocks Under Simulated High-Stress Conditions of Ultra-Deep Reservoirs

The ultra-deep carbonate reservoirs of the Fuman Oilfield in the Tarim Basin are characterized by intense fracture development. The coupled effects of high in-situ stress and fracture structures significantly deteriorate the mechanical properties of the rock mass, thereby constraining wellbore stability evaluation and safe drilling and completion operations. Existing studies have primarily focused on medium- to low-confining-pressure conditions and isolated fracture parameters, making it difficult to characterize the mechanical response of fractured rock masses under the high-stress conditions of ultra-deep reservoirs. To address this issue, limestone from the Yingshan Formation of the target reservoir was selected as the research object, and fractured specimens with varying fracture angles, widths, and densities were prepared. Uniaxial compression tests and triaxial compression tests under high confining pressures of 90 MPa and 120 MPa were conducted to systematically reveal the evolution of rock strength, deformation parameters, shear strength parameters, and failure modes under the coupled influence of fracture geometric parameters and confining pressure. On this basis, a confining-pressure–fracture coupled damage prediction model was established, and wellbore stability around the reservoir was analyzed using Finite Difference Method. The results indicate that fracture angle causes the peak strength and Young’s modulus to first decrease and then increase, with an inclination angle near 45° representing the most unfavorable fracture orientation. Increases in fracture width and density lead to continuous degradation of strength and stiffness. Although high confining pressure can close fractures and enhance load-bearing capacity, it cannot eliminate the controlling influence of fractures on failure pathways. Sensitivity analysis shows that the Young’s modulus and Poisson’s ratio are most sensitive to fracture width; cohesion is mainly governed by fracture width and density; and the internal friction angle is most sensitive to fracture density. Numerical simulations of wellbore stability further confirm that medium-inclination, large-aperture, and high-density fractures significantly increase the risk of wellbore instability. The findings provide experimental and theoretical support for mechanical-parameter correction, wellbore stability assessment, and construction-risk control in ultra-deep fractured carbonate reservoirs.