Crack Tunneling in Wellbore Cement Sheath Driven by Thermal Loads

With the booming of unconventional oil/gas exploitation in harsh downhole conditions, the cemented hydrocarbon wells may experience extremely high temperature variation during construction and production stages. An excessive thermal load will drive small defects in the cement sheath to grow and tunnel along the length of the well, leading to the failure of zonal isolation function. In this work, we investigate the crack tunneling in cement sheath under steady-state and transient thermal loads. We calculate the energy release rate at tunneling front as a function of the width of tunnel and time. As long as the maximum energy release rate remains below the fracture energy of the cement, tunnels of any size will not form at any time, which provides a conservative failsafe criterion for the cement sheath without requiring the knowledge of crack information. The failsafe criterion predicts that the cement sheath is more prone to cracking under thermal shock, and the thermal constants of cement only have limited effect on the critical condition. This work provides guidelines for cement design and downhole operation conditions to maintain the integrity of cement sheath.