Since fluid transport is limited in shales or seals, due to very low permeability, then any chemical reaction will not proceed very far.
The Gippsland Basin has long been considered to have the potential as a major CO2 storage site in Victoria, Australia. Some of the possible storage sites are characterised as storage complexes; having large anticlinal structures with four-way dip closure, a highly permeable reservoir and low permeability intra-formational seals and a regional top seal. One of the main leakage risks is likely to be the geochemical and petrophysical influence of the injectate on the seal strata. The cap rock seal efficiency evaluation is a vital part of the assessment of any CO2 storage site. The main goal of this study was to examine dynamic seal capacity of several Latrobe Group intra-formational shales in the Gippsland Basin and to characterise their cap rock sealing efficiency before and after exposure to supercritical CO2 (scCO2 ).
In this study, brine-saturated shale samples were exposed to scCO2 under reservoir conditions for a limited time of approximately 3 months. The study was a laboratory-based core analysis research program focusing on examining any changes to the cap rock mineralogical composition, capillary threshold pressure, pore size distribution and specific surface area before and after being exposed to scCO2 . Several analysis methods typically applied in the petrophysical assessment of seal rocks were used, including; x-ray diffraction (XRD), scanning electron microscopy (SEM) associated with energy dispersive spectrometry (EDS), mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), low pressure surface area measurements, and micro-CT scanning.
- Scanning electron microscopy (SEM) examinations showed precipitation of kaolinite, gypsum and minerals from jarosite group after exposure to scCO2 .
- Mercury injection capillary pressure results show a distinct shift toward smaller capillary pressure values for samples exposed to scCO2. This is in agreement with a general shift on the MICP curves toward larger pore and pore throat sizes for most of the samples analysed.
- Reduction of NMR signal after exposure to scCO2 indicates that some of the waters inside the samples were reduced by CO2.
- The low pressure nitrogen adsorption analysis shows the pore structure changed after shale samples were exposed to scCO2. In general, for most of the samples studied there was an increase in the pore surface area and pore volume, whereas a reduction in the pore diameter can be noted.
- These results suggest that when some of the shale samples studied in this project come in contact with scCO2/brine mixtures they may lose their original integrity as a cap rock and their seal efficiency may reduce. But it has to be noted that since fluid transport is limited in shales, due to very low permeability, then any chemical reaction will not proceed very far. Therefore, the reactions only penetrate a few centimeters and the seal capacity will be unaltered for a thick cap rock.
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