Surface CO₂ leakage during two shallow subsurface CO₂ releases

Abstract

A new field facility was used to study CO2 migration processes and test techniques to detect and quantify potential CO2 leakage from geologic storage sites. For 10 days starting 9 July 2007, and for seven days starting 3 August 2007, 0.1 and 0.3 t CO2 d−1, respectively, were released from a ∼100‐m long, sub‐water table (∼2.5‐m depth) horizontal well. The spatio‐temporal evolution of leakage was mapped through repeated grid measurements of soil CO2 flux (FCO2). The surface leakage onset, approach to steady state, and post‐release decline matched model predictions closely. Modeling suggested that minimal CO2 was taken up by groundwater through dissolution, and CO2 spread out on top of the water table. FCO2 spatial patterns were related to well design and soil physical properties. Estimates of total CO2 discharge along with soil respiration and leakage discharge highlight the influence of background CO2 flux variations on detection of CO2 leakage signals.

Dynamics of CO2 fluxes and concentrations during a shallow subsurface CO2 release

“…These areas of focused CO 2 emission were typically observed above the relatively high elevation ends of well injection zones and above and only slightly offset from packer locations (Figure 1). Based on these observations, it is likely that CO 2 migrated from relatively low to high elevation within injection zones until it encountered the barriers of the packers (e.g., Lewicki et al, 2007). It probably then migrated vertically and relatively rapidly to the surface along high permeability pathways in the cobble and soil layers.…”

“…However, results presented here and by Lewicki et al (2007) specifically pertained to the case where the location of the CO 2 leakage source was known a priori. Potential pathways for CO 2 leakage at GCS sites will likely be poorly constrained and located within large, reservoir-scale areas.…”

“…Following the methods of Lewicki et al (2007), we estimated total CO 2 discharge (D tot , t CO 2 d -1 ) for each grid dataset by calculating the declustered mean CO 2 flux using GSLIB (Deutsch and Journel, 1998) and multiplying it by the grid area (7700 m 2 ). D tot therefore includes CO 2 discharge associated with background processes (i.e., ecosystem respiration) plus CO 2 leakage.…”

“…During summer 2007, two controlled releases of CO 2 were carried out over ten and eight days at different rates (0.1 and 0.3 t d -1 , respectively) from a shallow horizontal well. Based on repeated grid measurements of soil CO 2 flux, Lewicki et al (2007) characterized the spatiotemporal evolution and quantified the surface leakage rates 3 of the signals associated with these relatively short releases. During summer 2008, CO 2 was released from the horizontal well over a one month period at 0.3 t d -1 .…”

“…Building on the work of Lewicki et al (2007), during summer 2008 we made repeated measurements of soil CO 2 flux on a grid surrounding the well, while continuously monitoring soil and atmospheric CO 2 concentrations [CO 2 ] and ancillary environmental parameters at several locations. Based on these measurements, we (1) characterize the spatiotemporal evolution of both CO 2 leakage and background biological (ecosystem respiration) fluxes, (2) quantify surface CO 2 leakage rates and their change over time, (3) characterize the development and decay of the leakage signal as expressed in atmospheric and soil [CO 2 ] at varying distances from the well, and (4) investigate relationships between temporal variations in CO 2 fluxes and concentrations and environmental parameters.…”