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

Water-saturated CO2 (wsCO(2)) injection is a novel technology that provides better CO2 storage compared to pure CO2 injection, due to the superior sweep efficiency of wsCO(2) injection. Previous wsCO(2) injection studies investigated only into sandstones, which have homogeneous pore size distribution. However, wsCO(2) 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 mu m), medium (21-200 mu m), and small (<21 mu m). The core is saturated with brine first, then oil -in the form of either pure decane (C-10) or crude oil - is injected to reduce water saturation. Subsequently, injections of 16 pore volumes (PVs) of either pure CO2 or wsCO(2) are performed under near-miscible conditions (70 degrees C and 12 MPa). When C-10 is used, wsCO(2) injection and pure CO2 injection are found to have similar CO2 stored, oil recovery and pressure difference. But when crude oil is used, wsCO(2) 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 wsCO(2) injection creates emulsion because water vapor from wsCO(2) 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 mu m), whereas wsCO(2) was able to enter pores up to 10 times smaller.