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.