Abstract
Injecting and storing of carbon dioxide (CO2) in deep geologic formations is considered as one of the promising approaches for geologic carbon storage. Microbial wettability alteration of injected CO2 is expected to occur naturally by microorganisms indigenous to the geologic formation or microorganisms intentionally introduced to increase CO2 storage capacity in the target reservoirs. The question as to the extent of microbial CO2 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 CO2/water/quartz systems under a laboratory setup simulating in situ reservoir conditions. The temporal shifts in the IFT and the contact angle among CO2, brine, and quartz were monitored for different CO2 phases (3 MPa, 30°C for gaseous CO2; 10 MPa, 28°C for liquid CO2; 10 MPa, 37°C for supercritical CO2) upon cultivation of Bacillus subtilis strain ATCC6633 with induced surfactin secretion activity. Due to the secreted surfactin, the IFT between CO2 and brine decreased: from 49.5 to 30 mN/m, by ∼39% for gaseous CO2; from 28.5 to 13 mN/m, by 54% for liquid CO2; and from 32.5 to 18.5 mN/m, by ∼43% for supercritical CO2, respectively. The contact angle of a CO2 droplet on a quartz disk in brine increased: from 20.5° to 23.2°, by 1.16 times for gaseous CO2; from 18.4° to 61.8°, by 3.36 times for liquid CO2; and from 35.5° to 47.7°, by 1.34 times for supercritical CO2, respectively. With the microbially altered CO2 wettability, improvement in sweep efficiency of injected and displaced CO2 was evaluated using 2-D pore network model simulations; again the increment in sweep efficiency was the greatest in liquid CO2 phase due to the largest reduction in capillary factor. This result provides novel insights as to the role of naturally occurring biosurfactants in CO2 storage and suggests that biostimulation of biosurfactant production may be a feasible technique for enhancement of CO2 storage capacity.
Highlights
Geological carbon storage (GCS) is being pursued as one of the promising solutions to stabilize the atmospheric CO2 concentration (White et al, 2003; Solomon et al, 2009)
We evaluated the variations of CO2-brine interfacial tension (IFT) and CO2-brinequartz contact angle at three different pressure and temperature (P/T) conditions: 37◦C and 3 MPa for gaseous CO2 phase, 28◦C and 10 MPa for liquid CO2 phase, and 37◦C and 10 MPa for supercritical CO2 phase, respectively
The similarity between the two Fourier transform-infrared spectroscopy (FT-IR) spectra confirmed that the biosurfactant produced by B. subtilis with the mineral salt medium had the similar structures and functional groups to surfactin, which is corroborated by previous studies by Joshi et al (2008) and Willenbacher et al (2015)
Summary
Geological carbon storage (GCS) is being pursued as one of the promising solutions to stabilize the atmospheric CO2 concentration (White et al, 2003; Solomon et al, 2009). Evaluating the stability of capillary trapping and the corresponding CO2 storage capacity in geologic formations is a challenging task For this purpose, the interfacial tension (IFT) between CO2 and water/brine and the wettability of CO2 to different minerals are the pre-requisites to answer relevant questions; for instance, how much brine will be swept by the CO2 invasion in brinesaturated porous networks and how far CO2 will propagate away from an injection well (Bachu et al, 1994; Juanes et al, 2010). Various methods, including the injections of acid gas, carbonated water, and surfactant into target formations, have been studied to increase the CO2 storage capacity and injectivity (Shah et al, 2008; Sohrabi et al, 2012; Kim and Santamarina, 2014). The lack of experimental data on the variations of interfacial properties by production of biosurfactants under such high-pressure conditions hinders us from developing a predictive model of CO2-brine flow dynamics in porous media
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