First-Contact Miscibility Development in a High-Temperature, Ultra-Sour Reservoir Fluid System Offshore Abu Dhabi

First-contact miscibility (FCM) occurs when an original injection gas is miscible with a reservoir oil regardless of the mixing ratio. As a consequence, the displacement will remain single-phase and the microscopic displacement efficiency will be 100%. For the vast majority of field applications, FCM requires a pressure, which is prohibitively high. Some notable exceptions include ultra-sour fluids, such as the fluid system encountered in this reservoir, offshore Abu Dhabi. The exceptionally favorable miscibility conditions may help unlock further value from this challenging, ultra-sour, high-temperature, offshore field. The swelling experiment provides a key source of information for inferring the FCM pressure. The swelling pressure curve exhibits a maximum versus the volume of injected gas and this maximum pressure is the FCM pressure. Typically, most lab experiments are terminated once the transition point from a bubble-point system to a dew-point system is observed, but for this application, much larger gas volumes were added to the system to reduce uncertainty on the inflection point. We performed equation of state (EOS) predictions to guide the experimental design in terms of the selection of gas additions. We then used the EOS formulation to conduct numerical simulations to compare the efficiency of a first-contact miscible displacement against a multi-contact miscible displacement, which can take place at lower pressures. With numerical simulation, we show that a first-contact miscible displacement yields higher recovery compared to a multi-contact miscible displacement. We attribute this difference to the development of the compositional transition zone observed during multi-contact miscible displacement which gives rise to a gas-oil interfacial tension and a bending of the gas-oil relative permeability curves for that zone. The impact of IFT on relative permeability would depend on model parameters in the simulator. The gas utilization factor, however, reaches its minimum if gas re-injection is delayed by a couple of years, as a result of the impact of pressure on formation volume factor of reservoir oil and injection gas, respectively.