Chemical Impacts of Potential CO2 and Brine Leakage on Groundwater Quality with Quantitative Risk Assessment: A Case Study of the Farnsworth Unit

Risk Assessment of Selected CCS Wells through Feature, Event, and Process Method and Comparison of the Barrier Effect

To reduce the CO2 release in the atmosphere, the carbon capture and sequestration (CCS) technique presents a solution in which the CO2 is captured from the emitting source and injected into a suitable geological formation in the subsurface. For the CCS project to be successful, CO2 must be trapped underground for hundreds of years. In that respect, good integrity plays an important role as it ensures that the injected CO2 remains sequestrated in the subsurface. Hence, this study presents a risk assessment technique with the help of which critical elements that can compromise the integrity of the CCS well can be identified. The approach taken for the risk assessment is based on the feature, event, and process (FEP). However, this method gives a qualitative analysis, and to convert it to a semiquantitative one, FEP is integrated with an interaction matrix, incident potential matrix (IPM), and cause-effect plot diagram. In this paper, risk assessment was conducted on two fictitious wells with different well configurations. It was found that cement and casing were the most vulnerable components, while formation water and subsidence were the most problematic elements in the given well system. It was also concluded that the number of plugs and their location in the well could increase or decrease the intensity of the risk levels in the CCS wells.

Chemical impacts of subsurface CO2 and brine on shallow groundwater quality

Leakage from geologic CO₂ sequestration (GCS) sites to overlying shallow drinking water aquifers is a tangible risk. A primary purpose of this study is to assess the potential impacts of CO₂ leakage into a fresh-water aquifer with associated CO₂-water-sediment interactions. The study site is the Ogallala aquifer overlying an active demonstration-scale GCS site in north Texas, USA. Using the results of combined batch experiments and reactive transport simulations, we discuss the effects of salinity on potential trace metal release and the potential for groundwater quality recovery after leakage ceases. RESULTS: suggest that trace metals are released from sediment due to impure carbonate mineral dissolution and cation exchange with exposure to aqueous CO₂. Concentrations of Mn, Zn and Sr might exceed the U.S. Environmental Protection Agency's (EPA) limits. After CO₂ leakage stops, most cation concentrations decrease to levels observed before leakage quickly, suggesting that water quality may not be a long-term concern. However, saline water that co-leaks with CO₂ may increase salinity of a shallow aquifer and induce more trace metals release from the sediment. In most cases, pH is sensitive to even small increases of CO₂, suggesting that pH may be a sufficiently sensitive parameter for detecting CO₂ leakage.

Underground sources of drinking water chemistry changes in response to potential CO2 leakage

The purpose of this study was to quantify changes to underground sources of drinking water (USDW) quality in response to potential CO₂ leakage from geologic CO₂ sequestration (GCS) reservoirs. We developed a framework of combined laboratory experiments and reactive transport simulations and used this framework to evaluate the Ogallala aquifer overlying the Farnsworth Unit (FWU), an active GCS site, as a case study. Using chemical reaction parameters obtained from laboratory experiments and numerical simulations, site-specific mechanisms of CO₂-water-sediment interactions at the USDW aquifer were interpreted. Long-term risks of potential CO₂ leakage were then evaluated with field-scale numerical models using the regional hydrogeological characteristics and reaction parameters obtained from our experiments and simulations. Results suggest that carbonate mineral impurity and cation exchange are key mechanisms for interactions between CO₂ and the aquifer sediment. Additionally, for a large leakage rate of 0.1 % injection from one leaky well, the leakage plume might impact an area of 300 m in diameter and significantly affect the local water quality by changing pH and cation concentrations (e.g., Zn, Ba and Sr). After leakage ceases, the zone of impacted fluids would not migrate significantly in subsequent decades due to a low regional groundwater flowrate (for this case study). The relatively small area of impact might not be detected in a monitoring well given the broader spacing in a typical field scenario. Effective early leakage detection may require additional tools, e.g., borehole CO₂ movement, four-dimensional seismicity, CO₂ soil flux, samples from deeper aquifers, etc., to ensure effective leakage detection and long-term safety of GCS projects.