PNNL

Abstract

Deep underground geologic formations are emerging as a reasonable option 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 were 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 period, downhole fluid samples were periodically collected during this post-injection monitoring phase, coupled with limited wireline borehole logging surveys that provided indirect evidence of on-going chemical geochemical reactions/alterations and CO2 disposition. A final detailed post-closure field characterization program that included downhole fluid sampling, and performance of hydrologic tests and wireline geophysical surveys. Included as part of the final wireline characterization activities was the retrieval of side-wall cores from within the targeted injection zones. These cores were examined for evidence of in-situ mineral carbonization. Visual observations of the core material identified small globular nodules, translucent to yellow in color, residing within vugs and small cavities of the recovered basalt side-wall cores, 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, a commonly occurring iron and calcium rich carbonate. Isotopic characterization (d13C, d18O) conducted on the ankerite nodules indicate a distinct isotopic signature that is closely aligned with that of the injected CO2. Both the secondary mineral nodules and injected CO2 are measurably different from the isotopic content of basalt, injection zone groundwater and for naturally occurring calcite. 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 are significant and demonstrate the feasibility of sequestering CO2 in a basalt formation.