Long term assessment of the geochemical integrity of offshore wellbore cement– results from numerical modeling

Abstract

Oil and gas wells across different offshore fields are currently experiencing plugging and abandonment. With this comes the risk associated with leakage of fluids from wellbores. One area of the long-term integrity of oil and gas wells that has received less attention is the geochemical assessments of cement and cement-rock integrity. This study investigates rock-fluid-cement interactions over long time scales using numerical models of coupled heat and fluid flow and reactive solute transport. Four different kinds of brine compositions from the Gulf of Mexico formations are used to simulate chemical interactions in wellbore plug, cement, and adjacent rock. Porosity changes are used as indicators of the geochemical integrity of the plugging and cement materials. Results demonstrate the impact of temperature, pH, and depth of cement placement in integrity predictions. At greater depths, higher temperatures and pressure will be encountered leading to the precipitation of gypsum and the dissolution of ettringite, calcite, and portlandite which adversely affects the integrity of the wells. The cement-rock interface is a zone of vulnerability and subject to greater loss of integrity. Other modeling parameters from our study which influenced integrity include reactive areas, diffusion coefficients, flux rates, boundary conditions, and the presence of mechanical damage. This study highlights the importance of brine specific geochemical assessments as well as the importance of thermodynamic properties when investigating geochemical wellbore integrity, and the approach used can be applied by practitioners for predictive analysis of geochemical longevity of cement and plugs in subsurface conditions.