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Geologic carbon capture and storage (CCS) is an option for reducing CO2 emissions, but leakage to the surface is a risk factor. Natural CO2 reservoirs that erupt from abandoned oil and gas holes leak to the surface as spectacular cold geysers in the Colorado Plateau, United States. A better understanding of the mechanisms of CO2‐driven cold‐water geysers will provide valuable insight about the potential modes of leakage from engineered CCS sites. A notable example of a CO2‐driven cold‐water geyser is Crystal Geyser in central Utah. We investigated the fluid mechanics of this regularly erupting geyser by instrumenting its conduit with sensors and measuring pressure and temperature every 20 sec over a period of 17 days. Analyses of these measurements suggest that the timescale of a single‐eruption cycle is composed of four successive eruption types with two recharge periods ranging from 30 to 40 h. Current eruption patterns exhibit a bimodal distribution, but these patterns evolved during past 80 years. The field observation suggests that the geyser's eruptions are regular and predictable and reflect pressure and temperature changes resulting from Joule–Thomson cooling and endothermic CO2 exsolution. The eruption interval between multiple small‐scale eruptions is a direct indicator of the subsequent large‐scale eruption.

[1] Injection of CO 2 may perturb subsurface temperatures, leading to a dynamic temperature system in the storage formation and adjacent seal strata. In most cases, the individual effects from wellbore dynamics, solvation reactions, and phase changes are incremental, but collectively these relevant processes may cause significant temperature changes compared to ambient conditions. In this work, we evaluated several potential nonisothermal effects resulting from CO 2 injection activity. These include the Joule-Thomson (heating and cooling) effect, exothermic CO 2 dissolution, and heat changes associated with concomitant water vaporization. Results suggest that three effects: a) the adiabatic (de-) compression of CO 2 , b) the frictional energy losses, and c) conductive heat exchange between the injected CO 2 and surrounding fluid/rock, govern the resulting CO 2 thermal profiles within an injection well. In addition, as supercritical-phase CO 2 comes into contact with formation brine, the CO 2 will dissolve into the aqueous phase, and such dissolution is exothermic at typical conditions for CO 2 sequestration. However, we still seek a better understanding of heat effects associated with water vaporization into the supercritical-phase CO 2 . Finally, sensitivity studies, simulating supercritical-phase CO 2 injection into a 1-D radially symmetric domain, are conducted to evaluate the magnitude of different heat disequilibrium potentials and spatial location in the CO 2 plume affected by thermal processes. In addition, time-scales associated with migration rates of temperature fronts, pressure pulses, and dissolved-and supercritical-phase CO 2 profiles are investigated with a function of heat capacities of rock, different effective thermal conductivities, permeabilities, and porosities. Our results demonstrate that adiabatic CO 2 compression occurring in injection wells could have the most significant impact on the temperature change whilst the exothermic CO 2 dissolution occurred at the largest spatial domain.

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Available online xxxx Editor: P. Shearer Keywords: geyser CO 2 -driven eruptions faults carbon sequestration wellbore leakageThe CO 2 bubble volume fraction, eruption velocity, flash depth and mass emission of CO 2 were determined from multiple wellbore CO 2 -driven cold-water geysers (Crystal and Tenmile geysers, in Utah and Chimayó geyser in New Mexico). At shallow depths the bubble volume fraction ranges from 0 to 0.8, eruption velocities range from 2 to 20 m/s and flash depths are predominately shallow ranging from 5 to 40 m below the surface. Annual emission of CO 2 is estimated to be (4.77 ± 1.92) × 10 3 , (6.17 ± 1.73) × 10 1 , (6.54 ± 0.57) × 10 1 t/yr for Crystal, Tenmile and Chimayó geysers, respectively.These estimates are coherent with Burnside et al. (2013) showing that the rate of CO 2 leakage from wellbores is greater than fault-parallel or diffuse CO 2 leakage. The geyser plumbing geometry consists of a vertical wellbore which allows for the upward migration of CO 2 -rich fluids due to artesian conditions. The positive feedback system of a CO 2 -driven eruption occurs within the well. Active inflow of CO 2 into the regional aquifers through faulted bedrock allows geysering to persist for decades. Crystal geyser erupts for over 24 h at a time, highlighting the potential for a wellbore in a natural environment to reach relatively steady-state high velocity discharge. Mitigating high velocity CO 2 -driven discharge from wellbores will, however, be easier than mitigating diffuse leakage from faults or into groundwater systems.