Numerical simulation of CO2 migration into cement sheath of oil/gas wells

Abstract

It is generally recognized that the CO2 dissolved in formation water poses a serious threat to the cement sheath in oil and gas wells. A numerical model was developed in this study to investigate CO2 migration into oil/gas well cement sheath through pore space and cracks. The convective velocities were formulated on the basis of balance of capillary and frictional forces. Case studies and sensitivity analyses with the model indicate that CO2 invasion into cement sheaths through the pore space increases with pore size, diffusion coefficient, interfacial tension, and decreases with cement-CO2 contact angle. It should take over 400 years for CO2 to fully penetrate a 3.5 cm-thick cement sheath through the pore space of cement, suggesting that CO2 attack on cement sheath through pore space is not a concern during the life (<50 years) of oil/gas wells. For CO2 migration into cement sheaths through cracks, it will be accelerated by the increase in crack width, diffusion coefficient, interfacial tension, and reduced by the decrease in cement-CO2 contact angle. It will take 9 months at most for CO2 breakthrough the cement sheath, indicating that special attention should be paid to CO2 migration through cracks. Cement crack width is a dominating factor affecting CO2 migration velocity compared to other factors. Therefore, oil/gas wells should be completed/operated in safe ranges of pressure to prevent formation of cement cracks.