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Modeling, monitoring, and controlling tools for risk management of shallow aquifer contamination by far-field brine leakage from CO2 storage formations: intermediate-scale laboratory testing
Askar, Ahmad Hamed
Askar, Ahmad Hamed
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2021
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2022-10-14
Abstract
Carbon-dioxide geological sequestration (CGS) into deep saline formations is accepted as a promising technology to reduce CO2 emissions in the atmosphere. Injected super-critical CO2 pressurizes and displaces native brine to significant lateral distances within the storge formation. Far-field caprock discontinuities (e.g., faults and fractures) can act as conductive pathways posing a contamination risk of brine leakage into drinking water aquifers. In response to such potential risk and other adverse impacts by CGS operations, the EPA requires a multidimensional risk management plan to be submitted prior to CO2 injection. The key requirements of this plan include the development of a risk assessment study, a monitoring system design, and an emergency remedial response strategy. Developing and validating tools for enhancing the management of CGS activities are essential for secure and permanent geologic storage of CO2 and safe CGS operations.
Due to technical and cost constraints on field testing, this research incorporated intermediate-scale laboratory experimentation with numerical modeling to (1) provide insights about brine leakage simulations necessary for improving risk assessment and (2) validate developed approaches to design leakage monitoring and control systems. In this study, an ~8m long two-dimensional soil tank was constructed to simulate brine leakage from a storage zone to a shallow aquifer under a multidimensional flow field and complex geological settings mimicking field conditions. A series of experiments were conducted in this tank to generate high-resolution data on brine leakage propagation under various scenarios to support different investigations. For example, data from the first three of five experiments were collected to validate a FEFLOW-based transport model that was used as a testbed to evaluate the impacts of source condition uncertainties on brine leakage predictions. The fourth experiment was performed to test a developed framework for designing cost-efficient leakage monitoring systems. In this analysis, predictions made by the calibrated model using monitoring data were compared with experimental measurements to validate the framework’s applicability. The final experiment was performed to test the efficacy of using deep extraction wells to control brine leakage concentrations into a shallow aquifer. As handling extracted brine can be extremely costly, an optimization algorithm was employed to place and design the wells’ pumping rates. The brine extraction technique was also numerically tested on a hypothetical leakage problem at San Joaquin Basin, California, to assess its practicality under field-scale settings.
The results of the integrated experimental and numerical analyses suggest that the storage zone's heterogeneity and pressure field are the most influential factors on leakage pathway predictions. Optimally integrating less-expensive shallow observation wells with deep sensors can minimize the monitoring cost while allowing for early leakage detection and providing useful data to improve model predictions. The analysis showed also that brine leakage can be controlled by extracting a brine volume less than half that of the injected CO2 by volume. Further, utilizing the dilution capacity of the overlaying formations can significantly reduce the required extraction rate to control brine leakage through a fracture. This research provides designers and operators with tested and validated tools to enhance the management of CGS operations, meet the EPA requirements, and improve site-selection. The findings of this study could be extended to shed light on managing waste fluid leakage from other deep injection applications such as hydraulic fracturing and deep waste disposal.
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