Arsenic mobilization in shallow aquifers due to CO2 and brine intrusion from storage reservoirs

A comprehensive review of remediation strategies for mitigating salt precipitation and enhancing CO2 injectivity during CO2 injection into saline aquifers

Geological CO2 sequestration is a proven method for mitigating climate change by reducing atmospheric CO2 levels. However, CO2 injection often induces salt precipitation, leading to decreased formation permeability, which in turn limits CO2 injectivity and storage capacity. Conventional approaches, such as freshwater and low-salinity water injection, have been employed to mitigate salt precipitation. Despite their widespread use, these methods provide only temporary improvement and can be ineffective in some scenarios, resulting in long-term issues such as salt recrystallization and clay swelling. Given the complexity and significance of this issue, a comprehensive review of salt precipitation mechanisms and remediation techniques is essential. This paper critically examines the processes of salt precipitation during CO2 injection in saline aquifers and evaluates various remediation techniques aimed at improving CO2 injectivity. The paper reviews the influence of CO2 flow dynamics, geochemical reactions, and fluid properties on salt precipitation and pore throat accumulation, assessing the efficacy and limitations of existing mitigation methods. Additionally, the paper explores alternative techniques with potential for long-term CO2 sequestration, analyzing their advantages and drawbacks. Based on insights from the reviewed sources, the paper recommends exploring alternative treatment measures and the integration of hybrid solutions to enhance CO2 injectivity. The findings presented serve as a valuable reference for advancing research and practice in this critical area, offering a deeper understanding of the challenges and potential solutions for effective CO2 sequestration in saline aquifers.

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.

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

Potential leakage of reservoir fluids is considered a key risk factor for geologic CO2 sequestration (GCS), with concerns of their chemical impacts on the quality of overlying underground sources of drinking water (USDWs). Effective risk assessment provides useful information to guide GCS activities for protecting USDWs. In this study, we present a quantified risk assessment case study of an active commercial-scale CO2-enhanced oil recovery (CO2-EOR) and sequestration field, the Farnsworth Unit (FWU). Specific objectives of this study include: (1) to quantify potential risks of CO2 and brine leakage to the overlying USDW quality with response surface methodology (RSM); and (2) to identify water chemistry indicators for early detection criteria. Results suggest that trace metals (e.g., arsenic and selenium) are less likely to become a risk due to their adsorption onto clay minerals; no-impact thresholds based on site monitoring data could be a preferable reference for early groundwater quality evaluation; and pH is suggested as an indicator for early detection of a leakage. This study may provide quantitative insight for monitoring strategies on GCS sites to enhance the safety of long-term CO2 sequestration.

Potential CO2 intrusion in near-surface environments: a review of current research approaches to geochemical processes

Carbon dioxide (CO₂) capture and storage (CCS) plays a crucial role in reducing carbon emissions to the atmosphere. However, gas leakage from deep storage reservoirs, which may flow back into near-surface and eventually to the atmosphere, is a major concern associated with this technology. Despite an increase in research focusing on potential CO₂ leakage into deep surface features and aquifers, a significant knowledge gap remains in the geochemical changes associated with near-surface. This study reviews the geochemical processes related to the intrusion of CO₂ into near-surface environments with an emphasis on metal mobilization and discusses about the geochemical research approaches, recent findings, and current knowledge gaps. It is found that the intrusion of CO₂(g) into near-surface likely induces changes in pH, dissolution of minerals, and potential degradation of surrounding environments. The development of adequate geochemical research approaches for assessing CO₂ leakage in near-surface environments, using field studies, laboratory experiments, and/or geochemical modeling combined with isotopic tracers, has promoted extensive surveys of CO₂-induced reactions. However, addressing knowledge gaps in geochemical changes in near-surface environments is fundamental to advance current knowledge on how CO₂ leaks from storage sites and the consequences of this process on soil and water chemistry. For reliable detection and risk management of the potential impact of CO₂ leakage from storage sites on the environmental chemistry, currently available geochemical research approaches should be either combined or used independently (albeit in a manner complementarily to one another), and the results should be jointly interpreted.