CO2–water–rock interactions in low-salinity reservoir systems

Abstract

Carbon dioxide (CO 2) storage in geological media has attracted global interest as a mitigation strategy for reducing anthropogenic CO 2 emissions. Aquifers are particularly attractive CO 2 sinks, in part due to their extensive storage capacities, favourable petrophysical characteristics, and their proximity to CO 2 source nodes. To date, studies of CO 2-water-rock interactions have almost exclusively focussed on CO 2 storage in deep saline aquifers. However, in locations lacking suitable saline reservoirs, such as in southeast Queensland, Australia, where aquifers are generally of low salinity, alternative storage options are being considered. One such option under active assessment is the lower interval of the Jurassic-aged Surat Basin, comprising the Precipice Sandstone, a potential low-salinity primary reservoir, the overlying Evergreen Formation, an interformational seal, and the uppermost Hutton Sandstone, an internally-baffled unit with mixed reservoir-seal characteristics. A detailed mineralogical study of the targeted reservoir system, utilising and comparing several investigatory techniques, including optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and hyperspectral logging (HyLogger), revealed differing lithologies for the three units and provided insight into their CO 2 storage characteristics. The Precipice Sandstone is geochemically poorly-reactive, with a clean, quartzose mineralogy dominated by quartz and kaolins. The Evergreen Formation is geochemically reactive, containing abundant feldspar, smectite, chlorite, carbonate and kaolinite. Nonetheless, the fine-grained nature and generally poor reservoir quality indicate an effective interformational seal. The Hutton Sandstone is a heterogeneous unit with mixed reservoir-seal characteristics and reactive intervals containing smectite, chlorite, feldspars and lithics, in addition to common CaFe -Mn-Mg carbonates. Variable clay matrix content and porosity, and locally-significant cementation, indicate a heterogeneous unit with the capacity to buffer emplaced CO 2 , providing an additional vertical component of capture and storage to the Surat Basin reservoir system. The CO 2-water-rock interactions in low-salinity host formations remain largely unexplored for conditions relevant to CO 2 injection and storage. In particular, there are uncertainties surrounding the impact of these interactions on mineralogy and rock quality characteristics of both reservoir and seal lithologies in the near-wellbore environment. Batch-reaction experiments conducted under simulated reservoir conditions (T = 60 °C, P = 120 bar) for 16-18 days, coupled with highresolution mineralogical and microstructural studies of rock samples, established that injection of CO 2 into low-salinity formations will result in geochemical reactions that are highly dependent on lithology. Changes in aqueous fluid chemistry are attributed to the dissolution and desorption of i material into the fluid phase. Where present, carbonates dissolved rapidly in the early ...

Experimental and Modelled Reactions of CO2 and SO2 with Core from a Low Salinity Aquifer Overlying a Target CO2 Storage Complex

“…Clay minerals were additionally observed to have precipitated in that natural system, mainly kaolinite from alteration of plagioclase, in agreement with the current study, and additionally smectite/illite from alteration of K-feldspar. The predictions of mineral trapping of CO 2 as siderite from reaction of Fe-rich chlorite clay in the Hutton Sandstone 724.1 m presented here are reasonable given the above and other studies of natural systems [33,36,[45][46][47]. A reactive transport modelling study of an arkose (20 vol% plagioclase), saline reservoir predicted Ca-Na-plagioclase (oligoclase) and chlorite alteration to ankerite, dawsonite, and siderite on reaction with CO 2 and 1% SO 2 at 75 • C. Predicted mineral trapping was 40-50 kg/m 3 over 1000-10,000 y, with dawsonite predicted to be formed from the Na supplied by plagioclase dissolution.…”

“…CO 2 fugacity was calculated at 12 MPa and 60 • C from Duan and Sun (2003), with SO 2 gas added by mass, models were also run with the CO 2 fugacity at half of the full fugacity to test the effect on pH [32]. Saturated minerals were allowed to precipitate based on observations of experiments and natural analogue systems, e.g., the carbonates siderite and ankerite/dolomite have been observed to precipitate in natural systems along with kaolinite, smectites, silica, and pyrite [33][34][35][36].…”

A combined geochemical and μCT study on the CO2 reactivity of Surat Basin reservoir and cap-rock cores: Porosity changes, mineral dissolution and fines migration

“…Increases in concentrations of dissolved ions from 568 carbonate and silicate minerals were reported initially followed by decreasing concentrations Kazsuba and Wdowin also reported precipitation of minerals including montmorillonite, 581 chalcedony, kaolinite and siderite. Fe-rich chlorite has been observed in natural analogue 582 situations over geologic timescales to be altered to siderite and kaolinite mineral trapping CO2 583 (Farquhar, 2016;Watson et al, 2004). The dissolved Fe from chlorite (as measured in the 584 current experiments) has the potential to lead to mineral trapping as ferroan carbonates such as 585 ankerite and siderite in the clay rich packages overlying the lower Precipice Sandstone.…”

Experimental Determination of Impure CO2 Alteration of Calcite Cemented Cap-Rock, and Long Term Predictions of Cap-Rock Reactivity

“…Future work is suggested on coupled geochemical, petrophysical and mechanical parameter changes to reservoir or cap-rocks via experimental CO 2 -fluid reactions, field injection studies, and natural analog studies, especially for O 2 co-injection Further studies of natural analogue sites, especially those where S or O 2 bearing fluids where present with CO 2 , are suggested to understand alterations on a geological timescale. The Surat, Bowen, and Eromanga Basins have recently been shown to have previously undergone natural CO 2 alteration especially around faults, this deserves further work to understand the long term potential for CO 2 storage mineral trapping and metal sequestration [61,62]. The models performed here were limited by the availability of data on the Evergreen Formation in the central and southern Surat Basin.…”