Rapid microbial methanogenesis during CO2 storage in hydrocarbon reservoirs

Abstract

Carbon capture and storage (CCS) is a key technology to mitigate the environmental impact of carbon dioxide (CO₂) emissions. An understanding of the potential trapping and storage mechanisms is required to provide confidence in safe and secure CO₂ geological sequestration^1,2. Depleted hydrocarbon reservoirs have substantial CO₂ storage potential¹,³, and numerous hydrocarbon reservoirs have undergone CO₂ injection as a means of enhanced oil recovery (CO₂-EOR), providing an opportunity to evaluate the (bio)geochemical behaviour of injected carbon. Here we present noble gas, stable isotope, clumped isotope and gene-sequencing analyses from a CO₂-EOR project in the Olla Field (Louisiana, USA). We show that microbial methanogenesis converted as much as 13-19% of the injected CO₂ to methane (CH₄) and up to an additional 74% of CO₂ was dissolved in the groundwater. We calculate an in situ microbial methanogenesis rate from within a natural system of 73-109 millimoles of CH₄ per cubic metre (standard temperature and pressure) per year for the Olla Field. Similar geochemical trends in both injected and natural CO₂ fields suggest that microbial methanogenesis may be an important subsurface sink of CO₂ globally. For CO₂ sequestration sites within the environmental window for microbial methanogenesis, conversion to CH₄ should be considered in site selection.

Fig. 1. Map of study area showing locations of the Olla and Nebo-Hemphill oil fields as well as the Black Lake Oil Field, from which the injected CO₂ was sourced.

Only the Olla Oil Field contains injected CO₂. The inset on the right shows an expanded view with individual sample locations (nearby urban areas are denoted by stars) and a stratigraphic column showing the relevant lithologic units.

Fig. 2. The δ¹³C of CO₂ in the Olla (CO₂ injected field) samples.

a, The δ¹³C of CO₂ as a function of the CO₂/³He ratio. The dashed lines show endmember methanogenesis and dissolution (pH 7) fractionation trajectories. The tick marks represent the total amount of CO₂ trapping within the system, relative to sample O5. The shaded region represents trapping by the combination of both microbial methanogenesis and dissolution. The upper and lower methanogenesis:dissolution ratios (M:D) are 0.33 and 0.19, respectively, showing that dissolution accounts for approximately three times more CO₂ removal (M:D = 0.26) than microbial methanogenesis. The lines labelled ‘consumption’ show the portion of original injected CO₂ that has been removed by net microbial methanogenesis. b, The δ¹³C of CO₂ as a function of the δ¹³C of CH₄. The shaded region represents the CO₂ isotopic composition of the injectate from the Black Lake Oil Field into the Olla system. The Olla data are consistent with thermal re-equilibration with both reservoir temperatures (solid lines) and microbial methanogenesis (dotted lines). The 1σ level of uncertainty is within the symbol size.

Fig. 3. Δ¹²CH₂D₂ versus Δ¹³CH₃D of the measured Olla and Nebo-Hemphill samples.

The clumped isotopologue space illustrates whether measured CH₄ is at internal thermodynamic equilibrium (black line) or not. The thermodynamic equilibrium curve is calculated following ref. . The shaded cross represents thermal equilibration to current reservoir temperatures in each isotopologue. The arrows represent the theoretical trends for methanogenesis (dark grey) and AOM (light grey) . The 1σ level of uncertainty is shown on the measured samples.

Extended Data Fig. 1. Relationship between noble gas isotopic ratios in the Olla (blue circles) and Nebo-Hemphill (orange triangles) oil fields.

Both fields show a resolvable mantle component that is significantly greater in the Olla Oil Field. One standard deviation error bars are within symbol size. a, He isotopic ratios versus ⁴He/²⁰Ne ratio. Air-corrected He isotopes are reported relative to air (R_A = 1.38 × 10⁻⁶ (ref. )). All samples are in excess of typical radiogenic production (0.02R_A), requiring a mantle contribution. High ⁴He/²⁰Ne indicates negligible air contamination in the samples. b, Three neon isotope plot. The crustal production line reflects the observed trend in typical crustal fluids. The mantle-air line represents mixing between air and a MORB-like endmember. Data are consistent with mixing between air and a mantle-rich component. In both He and Ne isotopes, the mantle signal is stronger in the Olla Oil Field. Source data

Extended Data Fig. 2. CO₂/³He variation vs.

a, b, ⁴He (a) and ²⁰Ne (b) abundances in the Olla (blue circles) and Nebo-Hemphill (orange triangles) oil fields. 1σ level of uncertainty is shown on the measured samples. A negative correlation between ⁴He, which accumulates in formation waters as a function of time and ²⁰Ne (which is groundwater derived) and CO₂/³He, has been interpreted in other CO₂-rich natural gas fields to support the importance of the formation water in controlling CO₂ removal. Source data

Extended Data Fig. 3. Hydrocarbon isotopic composition in the Olla (blue circles) and Nebo-Hemphill (orange triangles) oil fields.

a, Comparison of δ¹³C-CH₄ and δD-CH₄, with typical CH₄ provenances after Whiticar. One standard deviation error bars are within symbol size. In isolation, the Olla samples (blue circles, this study; unfilled circles, from ref. ) are consistent with a mainly thermogenic origin, whereas Nebo-Hemphill (orange triangles, this study; unfilled triangles, from ref. ) appears to be a microbial-thermogenic mix. Clumped isotopologues nevertheless provide clear evidence that the Olla CH₄ is microbial in origin (Fig. 3). b, Plot of δ¹³C of C_n versus 1/C_n variation in natural gases. One standard deviation error bars are within symbol size. Both the Olla (blue circles) and Nebo-Hemphill (orange triangles) fields show an enrichment in their isotopic signature especially in propane; however, this is most pronounced within the Olla Oil Field. Source data

Extended Data Fig. 4. Schematic of the processes occurring within the Olla Oil Field and resulting changes in the isotopic composition within the field.

The first panel shows CO₂ injection into the 2,800-ft sandstone (sst) and subsequent lateral and vertical migration (yellow arrows) to all the producing formations within the field. The middle panel demonstrate the different processes (microbial methangonesis, AOM and dissolution), which are occurring in the field and their effects on the isotopic composition. Yellow circles represent CO₂ dissolution and green circles are the net CH₄ production from microbial activity. Hydrogen for microbial methanogenesis is sourced from the hydrocarbons and water. The final panel shows the current state of the reservoir.

Extended Data Fig. 5

Microbial community analysis of most abundant taxa from the SSU rRNA gene-sequence region. Bar charts represent the most resolved taxa from amplicon sequence variants generated in QIIME 2^,, for archaeal primers (a) and bacterial primers (b). Hydrogenotrophic methanogenesis-capable taxa are represented with bold red font. Source data

Extended Data Fig. 6. Carbon isotopic composition of the Olla samples as a function of CO₂ concentration for data from this study (dark blue circles) and those collected without ³He analysis (unfilled circles).

One standard deviation error bars are within symbol size. Dashed lines show endmember methanogenesis and dissolution fractionation (at pH 7) trajectories. Tick marks represent the total amount of CO₂ trapping within the system, relative to sample O5. The shaded region represents trapping by the combination of both microbial methanogenesis and dissolution. The same combination of dissolution and microbial methanogenesis has been modelled (grey region, cf. Fig. 2). Lines labelled ‘consumption’ at 5%, 13% and 19 % show the proportion of original CO₂ that has been removed by net microbial methanogenesis. One sample from ref. has a CO₂ concentration greater than our ‘most pristine’ CO₂/³He sample (O5). This illustrates that by using sample O5 as our least altered composition we calculate a conservative (minimum) amount of CO₂ consumption, and that initial CO₂ concentrations were probably greater. Source data

Extended Data Fig. 7. Carbon composition of the Pannonian Basin samples.

a, Szegholm South^, (red circles), Szegholm North^, (purple triangles), Ebes^, (blue diamonds) and Hajduszoboszlo^, (grey squares) gas field samples as a function of their CO₂ concentrations and CO₂/³He. Dashed lines represent the projection of CO₂ trapping in the fields. b, The Szegholm South field had multiple CO₂ isotope data and this field has been used to investigate the processes controlling this pattern using the carbon isotopic composition as a function of CO₂/³He. Dashed lines show endmember methanogenesis and dissolution fractionation (at pH 7) trajectories. Tick marks represent the total amount of CO₂ trapping within the system. The shaded region represents trapping by the combination of both microbial methanogenesis and dissolution. The upper and lower methanogenesis:dissolution ratios (M:D) are 0.050 and 0.060, respectively, showing dissolution accounts for approximately 16 times (M:D = 0.055) more CO₂ removal than microbial methanogenesis. Lines labelled ‘consumption’ show the proportion of original CO₂ that has been removed by net microbial methanogenesis.