Design and Development of a High Pressure-High Temperature Wellbore Simulator for Investigation of the Impact of Cyclic Stresses on the Integrity of Wellbore Sections with Cement/Casing Interfaces

Abstract

Abstract Understanding the mechanisms of leakage-pathway formation are crucial for proper assessment, remediation, and in the end, prevention of leaking wells due to severe modern-day well stresses such as multistage hydraulic fracturing operations. Aiming to improve the understanding of the drivers behind fluid leakage from wells subject to modern-day well stresses, an experimental study of the cement sheath integrity under downhole stress conditions was conducted using a custom-designed and fabricated physical wellbore simulator. The physical wellbore simulator was designed and constructed to be capable of measuring the permeability (up to nanodarcy level) of the cemented annulus between two casings under cyclic temperature and pressure conditions (up to 43 MPa and 120°C). Permeability (nitrogen) of the casing-cement-casing system was measured under i-) non-cyclic temperature and pressure; ii-) non-cyclic temperature and cyclic pressure; iii-) cyclic temperature and non-cyclic pressure conditions. Potential leakage pathways were visually inspected after each experiment. The rough shear bonding strength between the inner casing and the cement sheath was also measured. Three factors were identified as having the most significant impacts on the permeability of the cemented annulus between two casings: i). Cement and/or casing shrinkage/expansion caused by the temperature change, ii). Casing shrinkage/expansion caused by the inner casing pressure change and, iii). Test duration time (time after curing and before each permeability measurement). The final permeability of the cemented annulus was controlled by the combined effects of these three factors. Overall, cement was extremely resilient to stresses, and slight increases in permeability were only observed after subjecting the cement sheath to significant stress. Once the debonding occurred at the cement/casing interface due to the initial change in pressure and/or temperature, applying cyclic pressure load did not significantly alter the permeability of the cemented wellbore section. The shear bonding strengths were on the high side, and the final permeabilities were below or around the critical permeability of 0.1 mD (Ozyurtkan et al. 2013). Through the development of the physical wellbore simulator the factors affecting the integrity of the cement-casing interfaces under representative real-world wellbore conditions of variable temperature and pressure conditions have been investigated. The improved understanding of the factors contributing to leakage-pathway formation are now being used to implement more reliable barrier technologies for effective mitigation of fugitive emissions.