Carbon Capture and Storage
Global warming is the most urgent threat that the world encounters today. Anthropogenic CO2 emissions into the atmosphere is the main driver of many climate extremes around the globe. Several countries are already battling unprecedented deadly heatwaves, droughts, wildfires, and floods. The global response to climate disruptions centers around limiting the temperature increase to well below 2 ºC above pre-industrial levels according to the Paris Agreement, which entails approaching net zero emissions by the mid-century. It is widely accepted that CO2 capture and storage (CCS), mainly in deep geological formations at the scale of several gigatonnes per year, is central to effective decarbonization and achieving climate neutrality. The Intergovernmental Panel on Climate Change (IPCC) introduces CCS as a credible and feasible climate change mitigation technique with cumulative CO2 stored through 2100 ranging from 550 to 1017 gigatonnes (IPCC, 2018). Accordingly, many countries have considered CCS as a key tool able to drastically cut CO2 emissions in their climate mitigation plans and confirmed the need to accelerate CCS deployment at large scales.
Geologic carbon storage aims at permanently storing CO2 deep underground (Bachu, 2003). Large volumes of CO2 injected into the subsurface alter the initial state of equilibrium in temperature, pore pressure, stress state and chemical composition of geological formations, which trigger a series of coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) processes operating over different spatial and temporal scales (Zhang et al., 2016). Among them, CO2-brine-rock chemical interactions are of first-order significance. As CO2 dissolves into the resident pore fluid, carbonic acid is formed, which promotes chemical reactions of rock mainly in terms of dissolution and precipitation of the rock-forming minerals (Gaus, 2010). These reactions alter the pore structure of the rock and potentially cause considerable time-dependent alterations in its hydraulic (porosity, permeability and two-phase flow) and mechanical (stiffness, poroelastic and strength) properties (Luquot and Guoze, 2009; Rohmer et al., 2016). The type and extent of geochemical interactions depend primarily on the mineralogical composition of rocks and thermodynamic conditions (salinity of the resident pore fluid and in-situ pressure and temperature), which control CO2 dissolution in the pore fluid. The fastest reactions take place with carbonate minerals, such as calcite, dolomite, and magnesite, while reactions with siliciclastic minerals like feldspar, illite-smectite, kaolinite-quartz groups are characterized by much slower rates (Dai et al., 2019). Therefore, the interactions may vary significantly among rock types and across a reservoir as the propagation of the injected CO2 plume forms different zones of CO2 saturation.
The induced changes in HM properties of rock cause redistribution of pressure and stresses that may affect the reservoir injectivity, storage capacity and geomechanical response of the storage system in terms of (i) fault reactivation, compromising the caprock integrity and sealing capacity and potentially leading to contamination of near-surface aquifers, (ii) perceivable induced seismicity, harming public perception of CCS, and (iii) differential ground surface uplift, damaging infrastructure (Rutqvist, 2012; Vilarrasa et al., 2019). These phenomena could put at stake the successful deployment of CCS at the gigatonne scale in a nature-friendly fashion. Particularly, the risks of CO2 leakage and induced seismicity are two obstacles that should be resolved before the widespread implementation of CCS occurs. CO2 leakage to shallow sediments would likely degrade potable water resources by releasing heavy metals (Delkhahi et al., 2020), and in the case of reaching the surface, it may harm vegetation (Zhao et al., 2017), animals (Farrar et al., 1995) and people (Hill, 2000) by asphyxia under special circumstances, while challenging the CCS effectiveness upon seepage back to the atmosphere at rates exceeding 0.01% of the stored CO2 per year (Hepple & Benson, 2005). On the other hand, although CCS at the megatonne scale has reportedly caused little seismicity, debate still persists on seismic hazards in the course of gigatonne-scale CO2 injections. At these scales, large volumes of CO2 will be injected into a target formation through a grid of closely-spaced injection sites. The interfering pressure perturbations of the neighboring sites may give rise to basin-wide pressurization of several megapascals, increasing the likelihood of induced earthquakes. Therefore, to improve our understanding of the short- and long-term response of the subsurface to CO2 injection and related uncertainties and safety issues, accurate analysis of coupled THMC processes both in the laboratory and at the field scale is necessary.
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