Compositional phase trapping in CO2 WAG simulation

A three-dimensional, three-phase, parallel compositional simulator has been developed to handle large computational efforts involving carbon dioxide (CO2) water-alternating-gas (WAG) injection. Algorithms have been developed to calculate three-phase relative permeability and capillary pressure, including the effects of phase trapping and relative permeability hysteresis for use in the simulator. Injecting CO2 in oil reservoirs has two advantages: increasing oil production by miscible and immiscible displacement and sequestering CO2 as an environmental measure. Typically a WAG injection scheme is used to control the mobility of the injected fluids. Reservoir simulation models allow us to predict the future performance of CO2 enhanced oil recovery (EOR) and sequestration projects which are of high interest in the Middle East and throughout the world. Test cases applicable to Abu Dhabi carbonate reservoirs were simulated. It is important to correctly account for trapped oil, water, and gas in reservoir simulation models. An incorrect estimate of trapped fluids in a sequestration project leads to over- or under-estimation of the CO2 stored in the reservoir. An incorrect estimate of trapped fluids in an EOR project leads to over- or under-estimation of CO2 required and inaccurate timing and amount of incremental oil recovery. Industry simulators assume that mobile and trapped oil and gas in a grid cell are in thermodynamic equilibrium while in this model they are not. The formulation presented in this paper allows tracking the changes in composition of the trapped and mobile fluids during different WAG cycles.