b'CO 2stability FeatureDoes surface temperature and climate change affect CO 2stability?Bhavik Harish Lodhia University of New South Wales Sydney b.lodhia@unsw.edu.auCO 2storage is an important factor in the drive towardsFigure 1.Phase diagram for CO 2 , methane calculated at normal geological decarbonisation and the energy transition. It is a well- conditions and geothermal gradient = 25 oC /km. Blue and red lines represent established field, and there have been many advancementsPT paths calculated at Ts = 0 oC and 20 oC, respectively. Black lines and points in the last two decades on the geological storage of CO 2 .represent phase diagram for CO 2 . Tp = triple point, Cp = critical point. Modified This is dependent on the existence of conditions required tofrom Lodhia and Clark, 2022.keep CO 2stable on human timescales and prevent leakage that will undoubtedly cause environmental problems. These conditions include existence and effectiveness of geological seals to prevent leakage and absence of features, e.g., faults, which may act as conduits for CO 2 . However, given the fundamental chemistry of CO 2and the fact that the triple point is at conditions encountered at shallow depths (P c= 7.38 MPa, T c= 30.98oC), there is an obvious question: do variations in surface temperature affect the condition of CO 2in the subsurface? If so, can surface or near-surface temperature variations caused by climate change or urbanisation affect the depths for which CO 2will remain stable for storage in rocks?As we know, it can get very hot in Australia and in many places around the world. Low-frequency temperature signals (e.g., decadal climate change) may be retained at depth ( 100 m)Figure 2.Fluid mobility for CO 2and methane calculated according to Eq. while high-frequency temperature signals (e.g. monthly) are(1). Fluid mobilities are calculated at Ts = 0 oC (top) and Ts = 20 oC (bottom), generally retained at shallow depths of 10 m (Lesperancerespectively. Units for fluid mobility are m2 Pa1 S1 and buoyancy are kg m2 etal., 2010). Recent research indicates that fluids migratingS2. Yellow and blue lines represent fluid mobility in sandstone and carbonate, along fracture networks through rock volumes may berespectively. Grey lines represent buoyancy and shaded regions represent significant propagators of heat, and the combined effectsrange of values calculated using different Equations of State and lithological of urbanisation and global warming may reach more thanparameters. Dashed lines represent phase transitions: G gas, L liquid, DL dense 100 m below the surface (Kurylyk and MacQuarrie, 2014;liquid and SC super-critical. Gas-fluid phase transitions are shown in bold. Taniguchi etal., 2007; Westaway and Younger, 2016). UnderVertical fluid velocity may be calculated by multiplying mobility and buoyancy. normal geological conditions (Pressure-Temperature gradientModified from Lodhia and Clark, 2022.of 0.5 MPa/K), an increase in temperature of 20oC corresponds to change in the phase boundary between CO 2gas to liquid/supercritical fluid of 0.4 km to 0.6 km, respectively (Figure 1).Application of basic chemical equations of state and determination of rock properties with depth for sandstones and carbonates indicates that at these depths, vertical CO 2velocities may increase by over an order of magnitude, due to the significant step in buoyancy when CO 2changes from liquid or supercritical fluid to gas (Figure 2, Lodhia and Clark 2022). This raises the prospect of increased surface or near-surface temperatures and the subsequent transportation of heat through shallow conduits (e.g., faults) influencing the depth of CO 2gas-fluid phase transitions on human timescales. This is intriguing as most experiments and test wells for CO 2injection are typically drilled at depths of less than 1 km. Increased subsurface temperatures due to long-term climate changeFigure 3.Schematic diagram of the potential effects of surface or near-surface and urbanisation may mean that CO 2must be injected deepertemperature variations on CO 2stability.AUGUST 2022 PREVIEW 40'