The Potential of CO2 Leakage Along De-Bonded Cement-Rock Interface

Leakage along wellbores is one of the key concerns for geological storage of CO2. De-bonding of annular cement - casing/rock interfaces during wellbore construction or operations could result in formation of microannuli as leakage pathways. A field scale reactive transport model was developed in TOUGHREACT to evaluate potential leakage of supercritical CO2 along microannuli. The model is axisymmetric, consisting of a cemented casing and surrounding rock formations: the CO2 reservoir, impermeable caprock and overlying permeable aquifer. Between the annular cement and surrounding rock formation, a microannulus with 100 micron aperture is implemented from the reservoir towards the surface. Simulated leakage rates with and without considering chemical reactions between CO2, formation water and cement minerals are compared to analytical equations which are often used to predict well leakage. The results show that the analytical equations highly overestimate leakage rates because they oversimplify the transport processes and do not consider relevant parameters such as gas saturation, relative permeabilities and even gravitational effects. If chemistry is excluded, the numerical simulations predict leakage rates which are one to two orders of magnitude lower than the analytical equations. In addition, the chemical interaction between the rock formation waters with the cement minerals result in relatively fast cement mineral dissolution and precipitation of calcite within the microannulus, with bicarbonate and calcium supplied by respectively the formation water and the dissolved cement minerals (mainly portlandite). Only if both the reservoir pressure and microannulus permeability are high and the resulting upward migration rate of the supercritical CO2 is fast enough, leakage of CO2 into the aquifer overlying the caprock is predicted to take place before calcite precipitation can seal the leak path. Future studies should focus on improving aperture-permeability relationships to be used in the numerical simulations. With reliable aperture and permeability input, numerical simulations can be used by operators to define maximum CO2 injection pressures for their storage sites and minimize leakage risks.