Wallula Basalt Pilot Demonstration Project: Post-injection Results and Conclusions

Deep underground geologic formations are being evaluated for long-term storage of CO₂, including large continental flood basalt formations. At the GHGT-11 and GHGT-12 conferences, progress was reported on the initial phases for Wallula Basalt Pilot demonstration test (located in Eastern Washington state), where nearly 1,000 metric tons of CO₂ was injected over a 3-week period during July/August 2013. The target CO₂ injection intervals were two permeable basalt interflow reservoir zones with a combined thickness of ∼20 m that occur within a layered basalt sequence between a depth of 830-890 m below ground surface. During the two-year post-injection monitoring period, downhole fluid samples were periodically collected, coupled with limited wireline borehole logging surveys that provided indirect evidence of on-going chemical geochemical reactions/alterations and CO₂ distribution. A detailed pre-closure field characterization program included downhole fluid sampling, and performance of hydrologic tests and wireline geophysical surveys. Side-wall cores also were retrieved from within the targeted injection zones. Visual observations of the core material identified small globular nodules, translucent to yellow in color, residing within vugs and small cavities, which were not evident in pre-injection side-wall cores obtained from the native basalt formation. Characterization by x-ray diffraction identified these nodular precipitates as ankerite, the identical iron and calcium rich carbonate observed to form in laboratory tests with Columbia River basalts. Isotopic characterization (δ¹³C, δ¹⁸O) conducted on the ankerite nodules indicate a distinct isotopic signature that is closely aligned with that of the injected CO₂. Final post-injection wireline geophysical logging results also indicate the presence of free-phase CO₂ at the top of the two injection interflow zones, with no vertical migration of CO₂ above the injection horizons. These findings support previous assumptions regarding storage feasibility and rapid mineralization of CO₂ injected into a suitable basalt formation.