Mechanism of gas migration through microstructure of cemented annulus in deep-water environment

Abstract

Gas migration has long been recognized as one of the most difficult problems in the oil and gas industry. Due to the complicated interactions between temperature, pressure, and cement hydration reaction, the temperature and pressure in a deep-water environment makes gas migration in cement a complicated process, by effecting the driving forces and channels of gas migration. Accordingly, a transient prediction model for cement temperature and pressure based on cement hydration kinetics was established in this study. Using this model, cement microstructure under different temperatures and pressures was studied. In addition, a mathematical model for gas seepage through the microstructure of a cement sheath was also suggested. The proposed models were validated by comparing the results obtained from those of experimental studies from the literature, where a good agreement was obtained. Furthermore, numerical simulations were conducted by applying the proposed models to a simulation well. The simulation results demonstrated that the microstructure of the cement sheath varied with time and space under the effect of temperature and pressure. The cement pressure first decreased with the deepening of cement hydration, and then increased as a result of gas migration. In addition, the gas migration resulted in a sustained casing pressure at the wellhead as high as 14.26 MPa. This would pose a threat to safety and subsequent oil and gas production.