Abstract
In this study, we advance the understanding of three-dimensional (3-D) electrical resistivity tomography (ERT) for monitoring long-term CO2 storage by analyzing two previously published field time-lapse data sets. We address two important aspects of ERT inversion—the issue of resolution decay, a general impediment to the ERT method, and the issue of potentially misleading imaging artifacts due to 2-D model assumptions. The first study analyzes data from a shallow dissolved-CO2 injection experiment near Escatawpa (Mississippi), where ERT data were collected in a 3-D crosswell configuration. We apply a focusing approach designed for crosswell configurations to counteract resolution loss in the inter-wellbore area, with synthetic studies demonstrating its effectiveness. The 3-D field data analysis reveals an initially southwards-trending flow path development and a dispersing plume development in the downgradient inter-well region. The second data set was collected during a deep (over 3km) injection of supercritical CO2 near Cranfield (Mississippi). Comparative 2-D and 3-D inversions reveal the projection of off-planar anomalies onto the cross-section, a typical artifact introduced by 2-D model assumptions. Conforming 3-D images from two different algorithms support earlier hydrological investigations, indicating a conduit system where flow velocity variations lead to a circumvention of a close observation well and an onset of increased CO2 saturation downgradient from this well. We relate lateral permeability variations indicated by an independently obtained hydrological analysis to this consistently observed pattern in the CO2 spatial plume evolution.
Highlights
Investigations are ongoing to evaluate the feasibility of geologic sequestration of carbon dioxide (CO2)to mitigate climatic effects due to its accumulation in the atmosphere
Gamma logging, and electrical resistivity tomography (ERT) were performed using an array of four monitoring wells
By analyzing ERT time-lapse data from two pilot injection experiments, along with select synthetic data studies, we have demonstrated important aspects of 3D ERT inversions for monitoring subsurface
Summary
Investigations are ongoing to evaluate the feasibility of geologic sequestration of carbon dioxide (CO2). For the first focus area, resolution issues, we investigate a case where, despite the presence of an array of four monitoring wells, central target resolution remains weak due to a large well separation relative to the actual vertical reservoir extent. This kind of problem is common in monitoring scenarios where one wants to maximize the investigation volume with a limited number of wells. One result of this issue is that reconstructed resistivity magnitudes of the inter-well region are underestimated (Ramirez et al, 2003; Kiessling et al, 2010). For further indicators of their consistency, we draw on the potential of hydrological inversion results for supplying complementary information that aids a more comprehensive ERT data interpretation (e.g. Koch, 2009; Kowalsky et al, 2011)
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