The mechanism of wellbore instability in high-temperature fractured granite formation

Wellbore instability presents a formidable challenge in drilling engineering, significantly hindering the exploitation of geothermal energy and deep oil and gas resources. However, the current comprehension of wellbore instability in high-temperature fractured granite formation remains inadequate. This study establishes a numerical model based on the DFN of heterogeneous granite under THM coupling. The numerical model incorporates the rock failure criterion and the Mohr-Coulomb friction model for fractures. This investigation delves into the stress distribution and failure modes of wellbore surrounding rock and fractures, considering fracture degree (ranging from fractured to fragmented formation), mud pressure, and temperature. Results highlight that the intersection and number of fractures with the wellbore significantly influence wellbore instability more than fracture degree. A high fracture degree induces significant pore pressure, increasing the possibility of instability. Fractures cause stress concentration, leading to damage spreading. Mud pressures and temperature differences affect stress and failure modes. Preventing mud invasion and maintaining a temperature differential within 250 °C is conducive to wellbore stability. The wellbore stability has a tolerance range for high mud pressure, exceeding the initial formation pore pressure by approximately 10 MPa. Findings are crucial for understanding and safeguarding high-temperature fractured granite wellbore stability.