Implementing geological carbon sequestration at a large scale to mitigate anthropogenic emissions involves the injection of carbon dioxide (CO2) into deep brine-filled structures. An alternative to injecting CO2 as a buoyant phase is to dissolve it into brine extracted from the storage formation, then inject the CO2-saturated brine into the storage formation. The CO2-concentration front shape, when it reaches the saturation-pressure contour, defines the maximal areal extent of CO2-saturated brine and thus the aquifer-utilization efficiency.
The heterogeneity of the aquifer reduces the utilization efficiency significantly. We illustrate by comparing the utilization efficiency in a homogeneous-permeability field with that in uncorrelated and correlated heterogeneous fields under the same well control. The example cases yield significant reductions of the utilization efficiency.
We develop an optimal-control strategy of the injection/extraction rates to maximize the utilization efficiency for heterogeneous aquifers. We propose two objective functions: One seeks to improve the areal sweep by minimizing the mismatch between the CO2-concentration front and the saturation-pressure contour; the other directly formulates the utilization efficiency while penalizing zones that contain gas-phase CO2. Both approaches have improved the aquifer-utilization efficiency by delaying the arrival of the dissolved CO2 front at the contour of saturation pressure.
Heterogeneity plays an important role in determining the location of the saturation-pressure contour within the storage formation. For a simple example domain, we propose a well-pattern orientation strategy by placing line-drive injectors in a high-permeability zone and extractors in a low-permeability zone, so that the saturation-pressure contour is closer to the extractors and thus increases the aquifer utilization efficiency. Illustration of this concept in the correlated heterogeneous field shows an improvement of the utilization efficiency. When combined with the optimal control of injection/extraction rates, the increase in the utilization efficiency almost compensates the reduction because of the heterogeneity.