Biosurfactant as an Enhancer of Geologic Carbon Storage: Microbial Modification of Interfacial Tension and Contact Angle in Carbon dioxide/Water/Quartz Systems

Injecting and storing of carbon dioxide (CO(2)) in deep geologic formations is considered as one of the promising approaches for geologic carbon storage. Microbial wettability alteration of injected CO(2) is expected to occur naturally by microorganisms indigenous to the geologic formation or microorganisms intentionally introduced to increase CO(2) storage capacity in the target reservoirs. The question as to the extent of microbial CO(2) wettability alteration under reservoir conditions still warrants further investigation. This study investigated the effect of a lipopeptide biosurfactant—surfactin, on interfacial tension (IFT) reduction and contact angle alteration in CO(2)/water/quartz systems under a laboratory setup simulating in situ reservoir conditions. The temporal shifts in the IFT and the contact angle among CO(2), brine, and quartz were monitored for different CO(2) phases (3 MPa, 30°C for gaseous CO(2); 10 MPa, 28°C for liquid CO(2); 10 MPa, 37°C for supercritical CO(2)) upon cultivation of Bacillus subtilis strain ATCC6633 with induced surfactin secretion activity. Due to the secreted surfactin, the IFT between CO(2) and brine decreased: from 49.5 to 30 mN/m, by ∼39% for gaseous CO(2); from 28.5 to 13 mN/m, by 54% for liquid CO(2); and from 32.5 to 18.5 mN/m, by ∼43% for supercritical CO(2), respectively. The contact angle of a CO(2) droplet on a quartz disk in brine increased: from 20.5° to 23.2°, by 1.16 times for gaseous CO(2); from 18.4° to 61.8°, by 3.36 times for liquid CO(2); and from 35.5° to 47.7°, by 1.34 times for supercritical CO(2), respectively. With the microbially altered CO(2) wettability, improvement in sweep efficiency of injected and displaced CO(2) was evaluated using 2-D pore network model simulations; again the increment in sweep efficiency was the greatest in liquid CO(2) phase due to the largest reduction in capillary factor. This result provides novel insights as to the role of naturally occurring biosurfactants in CO(2) storage and suggests that biostimulation of biosurfactant production may be a feasible technique for enhancement of CO(2) storage capacity.