Numerical modeling of a multiphase water–oil–CO2 system using a water–CO2 system: Application to the far field of a U.S. Gulf Coast reservoir

Active oil and gas reservoirs and connected saline aquifers are among the immediate candidates for CO2 sequestration, owing to their proven sealing capabilities and good characterization. If CO2 sequestration is implemented in the connected saline aquifer, the active hydrocarbon reservoir will act as a varying boundary condition to the system. The two-step methodology presented in this paper deals with changes that must be made to reservoir-model input files of a numerical code for replacing hydrocarbons with brine above the water–oil contact (WOC) while maintaining identical boundary conditions for carbon-sequestration sites below the WOC. Advantages of this approach include faster compositional simulations with fewer components, simulating the geological storage (GS) domain below WOC without considering the oil phase and production specifics updip, and not having to depend on detailed oil-production data, which may not be available. We show that simulating a water–oil–CO2 system using a simpler water–CO2 system provided some straightforward modifications could be accomplished in two steps: (1) modification of the permeability field above WOC to ensure identical pressure-propagation speed in the reservoir and (2) adjustment of production rates above the WOC to ensure correct reproduction of actual bottom-hole pressure at producers. Validation is achieved by comparing two different modes: (1) an oil-production mode, in which we model the three phases that act as the truth, and (2) an equivalent brine-production mode, in which no oil is present in the model. Numerical simulations show that, in the far field, estimated pressure values in both modes differ only by 2% or less.

► A two-step method to convert a water–oil–CO2 system to an equivalent water–CO2 system.
► Permeability field above WOC is modified to match the pressure front speed.
► Oil production rate is replaced by equivalent water production rate to match BHP.
► After replacing the oil with water and modifications, pressure response in water leg has less than 2% error.