Jennifer A. Roberts, University of Kansas
The Arbuckle dolomite is currently being evaluated for CO2 storage and sequestration. Some critical parameters to qualify a reservoir as a feasible site is a large storage capacity, injectivity and an effective, regionally extensive seal. Therefore, it is imperative to characterize the reservoir system and demonstrate the integrity of the reservoir and seals. This research used batch laboratory experiments to investigate the impact of microbial biomass on rock:brine:CO2 interactions under reservoir temperatures and pressures.
Using materials recovered from a continuous borehole (KGS #1-28) drilled through the Arbuckle and its immediate and regional seals, a series of batch experiments were performed to evaluate the geochemical and microbiological influences on reservoir materials and seal integrity. We performed experiments with the Cherokee and Chattanooga Shales, which are currently considered the primary seals for the Arbuckle. Single mineral experiments using dolomite (Wards Scientific #49 V 5871; the primary mineralogy of the Arbuckle) and pyrite (Wards Scientific #46 V 6418; a reactive component of both the reservoir and seals) were also performed. Pore fluids with native microbial consortia were collected from the Arbuckle and Mississippian aquifers from a series of drill-stem-tests during drilling of boreholes KGS #1-28 and KGS #1-32. Abiotic brines were accomplished by filtration through 0.2 um filters. Ten grams of powdered rock or mineral was combined with 250-ml of brine and placed into a 1.3-liter steel autoclave reactor vessel lined with a Teflon liner, which was then pressurized and heated to average reservoir temperature and pressure (2500 psi and 50°C). After purging, the vessels were filled with 100% pCO2 super-critical CO2 using a high-precision Teledyne ISCO pump and allowed to sit at constant temperature and pressure for the duration of each experiment. Five time-sequence experiments were conducted for each shale; 5, 7, 14, 30, and 45 days and the single-mineral experiments were conducted for 15 days with either pyrite or dolomite. All experiments were conducted under biotic and abiotic conditions.
Aqueous geochemistry indicated dissolution reactions in all experiments, with significant reduction in pH (ranging from 1-3 pH units lower depending on the solid phase). XRD and SEM confirmed gypsum precipitated in experiments containing pyrite and Chattanooga Shale (which also contains pyrite) after CO2 exposure. Mineralogical changes were not detected in experiments with dolomite or Cherokee Shale (non-pyrite bearing), although aqueous geochemical data shows considerable dissolution of mineral phases occurred. It is likely that materials in these experiments incompletely dissolved or precipitates were below detection using XRD. No significant differences between biotic and abiotic experiments were observed, possibly due to low biomass in the reservoir. Mineralogic changes to seal and reservoir minerals induced by CO2 exposure significantly impact rock:brine:microbe interactions that have implications for fluid geochemistry and reservoir porosity and permeability. Our results suggest that pyrite-bearing phases, in particular, may precipitate secondary gypsum in the presence of CO2 and oxygenated brine fluids. Gypsum precipitation under these conditions could fill existing pore space or micro-fractures within the seals to create an even better seal, or conversely clog CO2 storage pore space, lowering storage capacity of the reservoir. More detrimental would be precipitation of these minerals near the site of CO2 injection, a likely location for oxygenation, resulting in a decrease of injection capability. Experiments are currently underway with other seal materials, higher microbial biomass, and anaerobic brines, which we suspect will alter the changes in mineralogy observed in this study.