Physics of Failure Remaining Life Assessment for Safe Geothermal Repurposing of Legacy Wells

Abstract

The increasing population of legacy and orphaned oil and gas wells represents a mounting well-management challenge for industry, regulators, and society. Beyond age alone, the barrier system degrades through corrosion, debonding and micro-annulus formation, and fluid migration, raising methane-emission risk, groundwater concerns, and long-term monitoring and remediation costs. Current screening practice classifies wells by their present-day condition and therefore cannot predict when a well that presently meets integrity requirements will cease to do so. This study develops a physics-based framework to forecast wellbore barrier reliability and remaining useful life (RUL) through mechanistic degradation modeling rather than high-cost computational numerical simulation, which requires a long sequence of solutions for each considered well. Hence, the numerical solution is impractical within the time available for operational decisions across a large number of wells. The framework couples corrosion-driven casing degradation to a three-layer plane-strain Lamé solution for the casing-cement-formation system, solved through interface compatibility for the contact pressures at each time step. Time-dependent casing wall loss and the associated reduction in effective modulus are computed first; the degraded geometry then drives the stress evolution under thermal and pressure loading, from which cement failure indices, interface debonding, and micro-annulus aperture are evaluated. RUL is defined as the earliest time at which the cement safety factor falls below unity. The workflow is applied to 79 plugged and abandoned wells in the Piceance Basin, Colorado, using reported casing geometry, cement properties, and formation pressure. The physics-based model was validated against three-dimensional finite-element simulations, with a maximum mean absolute percentage error of 4%, and further verified using field data from three wells reported by the Colorado Oil & Gas Conservation Commission (COGCC). The results show that formation pressure has a non-linear effect on cement sheath integrity. As formation pressure increases, the stress in the cement first decreases and then increases. The lowest stress occurs near the equilibrium condition, where the pressure difference across the cement sheath is minimized. A larger temperature differential further lowers the safety factor, as the degraded cement is less able to accommodate the associated thermal effects. Applying the framework to a 79-well population over a 50-year study period in the Piceance Basin predicted one well to require intervention within ten years, five wells to require intervention within thirty years, and no intervention is required for the rest of the studied wells through year fifty. A statistical comparison of the predicted at-risk and intact populations identifies well depth as the dominant contributor to failure risk, with smaller casing size and lower casing weight further elevating it. This work advances well integrity assessment from condition monitoring to predictive life forecasting, integrating corrosion evolution, thermo-mechanical loading, interfacial debonding, and micro-annulus development within a single physics-based workflow. The method provides a practical basis for prioritizing aging well inventories, planning interventions, and evaluating long-term reuse opportunities such as geothermal repurposing.