In vitro screening identifies candidate wells and nutrient profiles for MEOR
In order to evaluate which candidate wells would respond to nutrient supplementation, in vitro microcosms were created in an anaerobic environment using a mixture of nine-parts oil well effluent and one-part crude nutrient source stock [40% corn syrup or molasses]. In addition to culture optical density, we monitored the microbial production of gases and organic acids that may enhance oil recovery by tracking microcosm headspace pressure and pH, respectively, over the course of 1 week (Figure 1 ). While two of the wells (Wells B, C,) did not respond to nutrient supplementation, microcosms from Well D and G responded to both molasses and corn syrup supplementation. One replicate from Well S demonstrated a moderate gas production on molasses while all replicates showed a drop in pH. The responses of Wells D and G were more reproducible, generating up to 13 PSI of pressure as well as a 2 pH-unit-drop in the culture pH. These findings are consistent with other biostimulation36 results and further supports the notion that not all wells respond to nutrient supplementation and may respond in different ways (e.g., more acids than gases or vice versa). Overall, cultures grown on molasses, a commonly used nutrient source in MEOR,9 produced more pressure and had larger pH drops suggesting that it had stronger microbe-activating potential for MEOR. Therefore, molasses was chosen as the crude nutrient source for our future screening efforts. Bacteria from the orders Lactobacillales, Enterobacterales, Bacteroidales, and Campylobacterales dominated the molasses microcosms from Well D and G comprising ≥ 96% of the total reads (Supplemental Figure 2 ). These bacteria are collectively known to produce CO2 and H2gases as well as acetic, lactic, butyric, and propionic acids that have roles in MEOR9,19,31. The increasing pressure and drop in pH imply that the production of some or all of these was stimulated by nutrient supplementation. However, production of H2S by Campylobacteria, such as Sulfospirillium andMalaciobacter, 33,38 may have also been stimulated. While our results indicated at least two responsive wells for MEOR intervention, successful MEOR biostimulation requires limited production of corrosive H2S while also increasing the gas, acid, and/or solvent production for oil recovery.