This study presents two field pulselike CO2-release tests to demonstrate CO2 leakage detection in a shallow aquifer by monitoring groundwater pH, alkalinity, and dissolved inorganic carbon (DIC) using the periodic groundwater sampling method and a fiber-optic CO2 sensor for real-time in situ monitoring of dissolved CO2 in groundwater. Measurements of groundwater pH, alkalinity, DIC, and dissolved CO2 clearly deviated from their background values, showing responses to CO2 leakage. Dissolved CO2 observed in the tests was highly sensitive in comparison to groundwater pH, DIC, and alkalinity. Comparison of the pulselike CO2-release tests to other field tests suggests that pulselike CO2-release tests can provide reliable assessment of geochemical parameters indicative of CO2 leakage. Measurements by the fiber-optic CO2 sensor, showing obvious leakage signals, demonstrated the potential of real-time in situ monitoring of dissolved CO2 for leakage detection at a geologic carbon sequestration (GCS) site. Results of a two-dimensional reactive transport model reproduced the geochemical measurements and confirmed that the decrease in groundwater pH and the increases in DIC and dissolved CO2 observed in the pulselike CO2-release tests were caused by dissolution of CO2 whereas alkalinity was likely affected by carbonate dissolution.
This study presents a combined use of site characterization, laboratory experiments, single-well push-pull tests (PPTs), and reactive transport modeling to assess potential impacts of CO2 leakage on groundwater quality and leakage-detection ability of a groundwater monitoring network (GMN) in a potable aquifer at a CO2 enhanced oil recovery (CO2 EOR) site. Site characterization indicates that failures of plugged and abandoned wells are possible CO2 leakage pathways. Groundwater chemistry in the shallow aquifer is dominated mainly by silicate mineral weathering, and no CO2 leakage signals have been detected in the shallow aquifer. Results of the laboratory experiments and the field test show no obvious damage to groundwater chemistry should CO2 leakage occur and further were confirmed with a regional-scale reactive transport model (RSRTM) that was built upon the batch experiments and validated with the single-well PPT. Results of the RSRTM indicate that dissolved CO2 as an indicator for CO2 leakage detection works better than dissolved inorganic carbon, pH, and alkalinity at the CO2 EOR site. The detection ability of a GMN was assessed with monitoring efficiency, depending on various factors, including the natural hydraulic gradient, the leakage rate, the number of monitoring wells, the aquifer heterogeneity, and the time for a CO2 plume traveling to the monitoring well.
Reactive transport modeling plays a critical role in predicting and quantifying impacts of potential CO2 leakage into aquifers at geological carbon sequestration sites. However, a numerical approach generally requires significant computation. This study presents a semi‐analytical approach to reactive transport of CO2 leakage into an aquifer through decoupling transport equations of reactive aqueous species with algebraic manipulation and then solving the algebraic equation sets representing the mass action law of geochemical reactions with the Newton‐Raphson method. The semi‐analytical approach was implemented in a simulation tool, SASCO2M, and verified against a numerical approach for 2‐D synthetic cases. Verification shows that the semi‐analytical approach matched reasonably well the results simulated with the numerical approach. The semi‐analytical approach was applied to simulate pulse‐like CO2 release tests which were used to demonstrate CO2 leakage detection in a shallow aquifer. The semi‐analytical approach reproduced the overall trends of groundwater pH, dissolved inorganic carbon, alkalinity, and concentrations of Ca and Br observed in the testing well. The semi‐analytical approach was further applied to assess the efficiency of a groundwater monitoring network for CO2 leakage detection in a shallow aquifer at a CO2‐enhanced oil recovery site. This study demonstrates that the semi‐analytical approach is simple and efficient and can be followed as a strategic procedure for assessing risks of CO2 leakage on groundwater quality and efficiency of groundwater monitoring networks for leakage detection at geological CO2 sequestration sites. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd
A new distributed intrinsic fibre optic sensor has been developed by Intelligent Optical Systems (IOS) for in situ measurement of dissolved CO 2 in brines at conditions relevant for applications in groundwater analysis, Carbon Capture and Storage reservoir surveillance, environmental monitoring of sweet/sea water systems and general water remediation applications. Our tests have shown that the sensor can withstand exposure to increased total pressures (up to 350 psi) and provide a stable signal response proportional to dissolved CO 2 concentrations in an aqueous solution. This is the first implementation of a distributed optical sensor, which is sensitive to dissolved CO 2 concentration or partial pressure of CO 2 and also exhibits feasibility to commercially produce lengths relevant for actual reservoir monitoring. In addition, first promising results from high-pressure experiments suggest that the sensor has potential for eventually being used in long-term installations for measuring dissolved CO 2 concentrations at different reservoir depths. With regard to carbon sequestration, these first high-pressure experiments represent a first step in the development of a new fibre optic sensor system that can eventually be employed to continuously monitor the reservoir and overlying rock layers for CO 2 leaks, quantify the extend of the potential leak and help to take informed decisions regarding remediation and further leak-containment measures.
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