Sequestration of CO2 in geologic formations
such as
depleted oil reservoirs has emerged as one of the lead solutions to
tackle greenhouse gas emissions to reduce pollution and global warming.
Supercritical CO2 (sc-CO2) injection in oil
reservoirs has proven to be useful as an enhanced oil recovery (EOR)
technique along with the benefits of CO2 sequestration.
In this study, a tortuous microscopic pore scale model was used to
study and investigate the phenomena of water-alternating gas (WAG)
and surfactant-alternating gas (SAG) with sc-CO2. The study
scrutinizes the dynamics of the pore-level phenomenon in the multiphase
WAG and SAG flows at the pore level in detail. Transient computational
fluid dynamics (CFD) analysis was used to study the fluid flow characteristics
of oil, water, and sc-CO2 at different reservoir pressure
and temperature conditions in oil-wet conditions. Governing equations
were coupled with EOS (Helmholtz free energy equation) to capture
the viscous and intrinsic properties of sc-CO2 due to variations
in pressure and temperature conditions. It was found that higher oil
recovery does not necessarily indicate higher sc-CO2 sequestration
and that temperature harms the displacement mechanism due to unfavorable
mobility ratios. Comparing WAG and SAG for the first injection cycle,
SAG showed a more diffused interface between displaced and displacing
fluid. The additional oil recovery produced in patches was a result
of pressure oscillations near the blind pores. Moreover, high vorticity
promotes greater intermixing between the displacing and displaced
fluid by increasing the rate of interface length. In SAG cases, faster
sc-CO2 breakthroughs were observed due to reduced shear
stress along the fluid interfaces, which resulted in higher sequestration
values in a given time frame. The CO2 sequestration volume
in SAG cases was found to be approximately 40% more than in WAG experiments.
The study confirms that lower values of oil–water interfacial
tension aids in faster and more efficient sequestration of sc-CO2 along with additional oil gain from a given reservoir.
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