Carbonate rock dissolution during CO2 and brine injection – An experimental study applying in-situ imaging by PET and CT
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- Master theses 
Carbon Capture and Storage (CCS); capture of CO2 from point sources followed by permanent storage in subsurface geological formations; can contribute to mitigating anthropogenic CO2 emissions, and to the ongoing energy transition, by reducing climate impacts from petroleum production. CO2 that is injected into subsurface reservoirs will interact with the reservoir fluids; unavoidably dissolving in already present water (injected or formation). The result is acidic conditions in the reservoir, which can promote the dissolution of rock matrix. Dissolution may especially be an issue in reactive carbonate rocks. Carbonate reservoirs contain approximately 60% of the remaining hydrocarbon resources globally, and significant volumes of CO2 may be stored in saline aquifers within carbonate formations. Understanding geochemical interactions that occur when CO2 is injected into subsurface formations for storage is important: dissolution may, depending on location and pattern, benefit injectivity and/or threaten well integrity, cause geo-mechanical weakening, or create preferential flow paths within the reservoir which decreases the overall sweep efficiency. Dissolved matrix particles may flow within the reservoir and cause clogging of pores and throats, which may increase CO2 storage security by reducing CO2 migration or be detrimental to the rock flow and storage capacity. Investigation of dissolution on the core scale forms the basis for predictions of reactive transport and its effects on larger scales. The main objectives of this thesis were to 1) investigate dissolution of carbonate core samples during co-injection of supercritical CO2 and brine, and 2) utilize emerging in-situ imaging techniques to quantify reactive fluid flow, dissolution patterns, and changes in local rock structure. Co-injection was performed at reservoir conditions (40°C and 90bar), into five Edwards Yellow limestone core samples. The cores were fractured before co-injection, to localize the reactive fluid flow and promote dissolution and the formation of preferential pathways (wormholes) in and around the fracture. Two different fracture networks were utilized: a tight fracture network with a pre-existing longitudinal flow-channel along one side (A) and a closed fracture network with a tight longitudinal fracture (B). Global measurements of pressure and volumetric flow rates, and dynamic measurements of effluent pH were used to describe dynamic dissolution during co-injection of CO2 and brine. Computed Tomography (CT) imaging was used to gain insight into initial rock structure, and the dissolution pattern that had formed during co-injection. Micro-CT (µ-CT) provided high spatial resolutions (tens of µm scale) for detailed investigations of the fracture and matrix structure, while a preclinical CT module was used to characterize fracture and heterogeneities at mm-scale. CT combined with emerging imaging technique Positron Emission Tomography (PET) provided detailed insight into the relation between the evolving dissolution pattern (changes in pore structure) and local reactive flow regime. Co-injection of CO2 and brine into fractured carbonate core samples caused dissolution of the rock material in all five core samples. For fracture network (A) with a pre-existing high-conductivity channel, wormholes formed due to local dissolution within the conductive channel. Global measurements showed increased injectivity with time, but failed to predict the size and location of dissolution visualized by PET and CT. In tight fracture network (B) injectivity decreased during co-injection, estimated from global measurements. Visual observations, however, showed significant dissolution at the injection side of the core samples. In-situ visualization revealed that reactive transport and dissolution also had occurred outside of the fracture area, indicating that the tight fracture network had been partially or fully blocked by particles, and fluids diverted into the pore network. In-situ imaging was necessary to determine changes in structure and flow during CO2 injection, and revealed significant dissolution heterogeneities that could not be well captured with global measurements.