Propagation of CO2 desaturation front in near wellbore region

Abstract

Understanding the development and propagation of CO2 desaturated front is important for managing the potential of CO2 leakage via injection wells after cessation of CO2 injection. The CO2 desaturation front is the front at which oil becomes fully devoid, referred to as “desaturated”, of CO2. In this research, water injection into an oil reservoir containing CO2 is investigated to understand the effect of CO2 component transfer between brine and oil phases in the near wellbore region, as well as to monitor the development of the CO2 desaturation front. CMG-GEM compositional simulator is used to model the process. A 1D cartesian model, including homogenous properties, is utilised for the modelling. The volume of water needed to be injected to desaturate the oil from the CO2 in the vicinity of the wellbore is determined. Additionally, the velocity of the CO2 desaturation front propagation is calculated, and it is compared with the calculations of the Buckley-Leverett saturation front displacement. The study also examines how different parameters such as salinity, pressure, and temperature affect the velocity of the CO2 desaturation front, as CO2 solubility in water is influenced by these parameters. Results show that while varying the injection water salinity affects the velocity at which the CO2 desaturation front propagates, variation in the initial reservoir temperature and pressure, as well as the formation water salinity, has a minor impact, although they do alter the CO2 solubility and thus the mobilities at the front. The results also indicate that compared to the temperature front, which generally propagates at about one third of the saturation front velocity, the CO2 desaturation front in this specific case study travels at about one tenth of the saturation front velocity, so it takes much longer for the near wellbore region to completely desaturate from CO2. Other sensitivities, such as varying the residual oil saturation, affected the propagation of the CO2 desaturation front. The desaturation front travels at a slower speed at greater residual oil saturations due to the greater mass of immobilised CO2 to be displaced by this mechanism.