Man-made versus natural CO2 leakage : a 400 k.y. history of an analogue for engineered geological storage of CO2

Burnside, Neil and Shipton, Zoe and Dockrill, Ben and Ellam, R.M. (2013) Man-made versus natural CO2 leakage : a 400 k.y. history of an analogue for engineered geological storage of CO2. Geology, 41 (4). pp. 471-474. ISSN 0091-7613 (

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To evaluate sites for long-term geological storage of CO2 and optimize techniques for monitoring the fate of injected CO2, it is crucial to investigate potential CO2 migration pathways out of a reservoir and surface leakage magnitudes. For the first time, we calculate CO2 leakage rates and volumes from ancient fault-related travertines and from an abandoned borehole. U-Th–dated travertine along two faults near Green River, Utah (western United States), shows that leakage has occurred in this area for over 400 k.y. and has switched location repeatedly over kilometer-scale distances. One individual travertine was active for at least 11 k.y. Modern leakage is predominantly through the active Crystal Geyser, which erupts from an abandoned exploration well. Using age data and travertine volume, we calculate magnitudes and rates of CO2 emission. Fault-focused leakage volume is twice as great than diffuse leakage through unconfined aquifers. The leakage rate from a poorly completed borehole is 13 times greater than the long-term time averaged fault-focused leakage. Although magnitudes and rates of any leakage from future storage sites will be highly dependent on local geology and pressure regime, our results highlight that leakage from abandoned wells is likely to be more significant than through faults. INTRODUCTION Carbon dioxide could potentially migrate from underground storage sites through boreholes, poor cap rocks, or faults and fractures. Storage formation integrity, and effects of leaking CO2 on the surface environment, is commonly investigated (Stevens et al., 2001; Kirk, 2011); however, the overburden between the storage formation and the surface is a poorly studied part of the CO2 storage system (Gaus, 2010). Understanding flow through potential migration pathways in the overburden, such as faults or high-permeability strata is crucial for evaluating long-term storage security. The integrity of well bores and their long-term ability to retain CO2 has also been recognized as a significant risk to the long-term security of geological storage (IEAGHG, 2005). Storage operators will be legally required to monitor the fate of injected CO2 (European Commission, 2009). Effective monitoring and engineered remediation depends on the likely nature of migration pathways through the overburden, and on the locations and likely fluxes of CO2 shallow leakage pathways (Cortis et al., 2008; Benson and Hepple, 2005). We investigate a unique location in the Paradox Basin, Utah, (western United States) where fossil travertine mounds allow us to compare ancient CO2 flux via fault-associated fracturing and diffuse leakage through an aquifer, with modern leakage from abandoned wells. The Paradox Basin hosts at least nine natural CO2 accumulations, most of which have contained CO2 for millennia (Gilfillan et al., 2008). CO2-charged springs along the Little Grand Wash (LGW) and Salt Wash Graben (SWG) normal faults (Fig. 1) demonstrate that CO2 is leaking to the surface through fault zones and abandoned wells (Doelling, 1994). The springs are fed from a shallow aquifer, overlain by low-permeability siltstones and shales and charged by CO2 from depth—making this setting analogous to heterogeneous and variably permeable overburden above a geological CO2 storage site. We use U-Th dating of travertine formed by out-gassing of CO2 from springwater to determine flow history along the faults. Combining travertine age and volume allows us to estimate volumes and rates of CO2 emission to the surface through time and compare fault-focused, diffuse and borehole leakage, in a single hydrogeological setting.