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

Abstract

CO2 injection has been used to improve oil recovery for the last 4 decades. In recent years, CO2 injection has become more attractive because of the dual effect: injection in the subsurface (1) allows for reduction of the CO2 concentration 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 the 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-dimensional (1D) horizontal flow. When CO2 dissolves in the oil, the density may increase. The effect of the density increase in high-permeability reservoirs when CO2 is injected from the top has not been modeled in the past. The increase in density changes the flow path from 1D to two-dimensional (2D) and three-dimensional (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 account for the increase in oil density with CO2 dissolution using the Peng–Robinson equation of state. The viscosity is modeled based on the Pedersen–Fredenslund viscosity correlation. 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 on the CO2 flow path and recovery performance. One 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. Our main conclusion is that the downward flow of the CO2 and oil mixture may not be gravity-stable, despite the widespread assumption in the literature.

Onset of convection with fluid compressibility and interface movement

“…However, the linear density equation of state (5) is expected to be invalid for very high solubility hydrocarbons since c is required to be sufficiently small, and we cannot perform a linear stability analysis for such mixtures. Another complication is that CO 2 dissolution may significantly change the viscosity 2,3 and the partial molar volumes 37 of the hydrocarbon phase, so that µ and α cannot be approximated as constants. The second key point is that compressibility may be important for CO 2 sequestration even if it does not noticeably affect the onset of convection in the aqueous phase.…”

“…Carbon dioxide has been employed for over four decades in improved oil recovery. 2,3 Dissolution of CO 2 in oil may reduce the viscosity by over an order of magnitude and may increase the volume of the resulting mixture by up to 60%. The volume expansion helps to expel the oil from smaller porous cavities.…”

Three‐phase compositional modeling of CO2 injection by higher‐order finite element methods with CPA equation of state for aqueous phase

“…Upon dissolution, CO 2 may swell the oil and water phases. CO 2 dissolution may also increase the density , which may start density‐driven mixing due to gravity effects [ Simon et al , 1978; Ashcroft and Ben Isa , 1997; Ahmed et al , 2012]. Under some reservoir conditions, CO 2 has a higher density than the reservoir oil, making injection from the bottom more efficient.…”

Suggesting a numerical pressure-decay method for determining CO2 diffusion coefficient in water

“…CO 2 can be injected in oil reservoirs for the purpose of enhancing oil recovery as well as sequestration. Even in this case dissolution can progress under the mechanisms of diffusion and convection [29].…”