Abstract:

Geological sequestration is a potential technology for the long term storage of carbon dioxide (CO2) emissions from stationary sources such as fossil fuel fired power plants. CO2 injected into geological formations such as saline aquifers can be effectively immobilized by structural trapping, residual trapping, solution trapping and mineralization. In this work, previously developed optimization methods were modified and used for the optimization of CO2 sequestration process. The optimization was based on the conjugate gradient (CG) method and used a commercial simulator as a “black box” for the calculation of numerical gradients. The main objective of the optimization was to determine optimal valve settings/injection rates for wells that maximize residual trapping of CO2, so as to minimize the amount of CO2 that is structurally trapped. This would mitigate the risk of leakage of the CO2 to the atmosphere due to a loss in integrity of the formation cap rock.

First, 2-D simulations were carried out to study factors such as injection rates and aquifer properties that affect residual trapping and to verify that these factors were being captured by our models. Then, optimizations were carried out on a mildly heterogeneous, 2-D model for a variety of cases. It was shown that the optimization acted to increase the amount of residual trapping of CO2 in the aquifer. When compared with an unoptimized two-well case, the optimization led to a decrease of 43% in the amount of structurally trapped CO2. Optimization was also carried out on a 3-well case, which again produced significant improvements over the base case. Cases with high aquifer heterogeneity, a case with capillary pressure hysteresis and a case with a higher number of optimization steps were also studied to understand the effect of these factors on the optimized solution. The convergence of the optimization procedure to sub-optimal solutions was observed, however, so there is scope for improvements in algorithmic performance. There is also a need for enhanced computational efficiency, as the current algorithm is only suitable for cases with relatively few wells and well setting updates.

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Copyright 2007, Deepanshu Kumar: Please note that the reports and theses are copyright to their original authors. Authors have given written permission for their work to be made available here. Readers who download reports from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the author, Deepanshu Kumar.