oa Complex Flow and Composition Path in CO2 Injection Schemes from Density Effects

CO2 injection has been used to improve oil recovery for the last four decades. In recent years, CO2 injection has become more attractive because of the dual effect; injection in the subsurface 1) allows reduction of CO2concentration in the atmosphere to reduce global warming, and 2) improves the oil recovery.

One of the screening criteria for CO2 injection as an enhanced oil recovery method is based on measurement of CO2 minimum miscibility pressure (MMP) in a slim tube. The slim tube data are used for the purpose of field evaluation and for the tuning of the equations of state. The slim tube represents one-D (1D) horizontal flow.

When CO2 dissolves in the oil, the density often increases. The density increase has not been accounted for in the modeling. The increase in density changes the flow path from 1D to 2D and 3D (downward flow). As a result of this density effect, the compositional path in a reservoir can be radically different from the flow path in a slim tube.

In this work, we study the density effect from CO2 dissolution in modeling of CO2 injection. We present a method to model the increase in oil density with CO2 dissolution using the Peng-Robinson equation of state and the Pedersen viscosity correlation. We apply this method to model the observed increase in oil density with CO2 dissolution in a West Texas oil sample. We perform compositional simulation of CO2 injection in a 2D vertical cross section with the density effect. Our results show that the density increase from CO2 dissolution may have a drastic effect onCO2 flow path and recovery performance. One main conclusion from this work is that there is a need to have accurate density data for CO2/oil mixtures at different CO2 concentrations to model properly CO2 injection studies.