Mechanisms of Degradation of Cement in CO2 Injection Wells: Maintaining the Integrity of CO2 Seals

Abstract

The degradation of cement due to CO2 exposure affects its transport and mechanical properties, resulting in potential fluid leakage from wells used for CCUS. This study focused on investigating the mechanisms of cement degradation in CO2 injection wells. We employ a fully integrated 3-D reservoir simulator that incorporates fluid flow, geomechanics, and geochemistry, along with a new model designed to accurately replicate the changes in rock properties resulting from cement degradation. Chemical reactions, including dissolution and precipitation, between CO2-rich brine and cement minerals are modeled, allowing for changes in rock and cement properties. Porosity is recalculated considering volume changes due to chemical reactions, and permeability is reevaluated using the Kozeny-Carman equation. Based on the simulation results, the chemo-mechanical composite layer model reassesses mechanical properties, considering the mineral composition of cement. According to the simulation results, the chemical changes in cement exhibited three stages: 1) dissolution of primary minerals, 2) precipitation of carbonates, and 3) re-dissolution of carbonates. While reactions 1 and 2 played a major role, they led to a decrease in rock porosity and a degradation of mechanical properties. However, as the dissolution of primary minerals diminished and the transition from stage 2 to stage 3 began, the porosity increased, accompanied by an increase in mechanical stiffness. The predicted values of porosity were compared to experimental data obtained from prior studies, confirming their consistency for short-term CO2 exposure, which can be reproduced in experiments. These mechanisms of cement degradation and the alteration of mechanical properties that occur in CO2 injection wells agree well with experiments. Our numerical simulator that fully integrates flow, geochemistry, and geomechanics with a chemical reaction model can be used to model more complex cement geometries to evaluate the risks of CO2 escape along the wellbore annulus.