Water-saturated CO2 injection in a carbonate rock – Effect of pore heterogeneity

Water-saturated CO₂ (wsCO₂) injection is a novel technology that provides better CO₂ storage compared to pure CO₂ injection, due to the superior sweep efficiency of wsCO₂ injection. Previous wsCO₂ injection studies investigated only into sandstones, which have homogeneous pore size distribution. However, wsCO₂ injection into carbonate rock should also be investigated because its substantial heterogeneity hinders sweep efficiency.

In this study, we use an Indiana limestone core and classify its pores by diameter: large (>200 μm), medium (21–200 μm), and small (<21 μm). The core is saturated with brine first, then oil -in the form of either pure decane (C₁₀) or crude oil - is injected to reduce water saturation. Subsequently, injections of 16 pore volumes (PVs) of either pure CO₂ or wsCO₂ are performed under near-miscible conditions (70 °C and 12 MPa).

When C₁₀ is used, wsCO₂ injection and pure CO₂ injection are found to have similar CO₂ stored, oil recovery and pressure difference. But when crude oil is used, wsCO₂ injection yields additional CO₂ stored (3%–8.8%) and oil recovery (5%–14%), and greater pressure difference (19–36 times) than does pure CO₂ injection. This study observes that wsCO₂ injection creates emulsion because water vapor from wsCO₂ might be condensed into the oil phase, which consequently increases both flow resistance and pressure difference. Additionally, pure CO₂ could only enter large and medium pores (>21 μm), whereas wsCO₂ was able to enter pores up to 10 times smaller.