Carbon Capture, Utilization, and Storage Conference
Todd Umstot, Principal Hydrogeologist, and Gregory Schnaar, PhD, PG, Director of Expert Services, Principal Environmental Scientist, co-authored “Risk Based Area of Review Delineation with MODFLOW and Groundwater Contaminant Fate and Transport Modeling” for presentation at the Carbon Capture, Utilization, and Storage Conference (CCUS) in Houston, Texas, on March 3 through 5, 2025.
Olivia Brown, Carbon Capture and Storage Market Leader, and Daniel Acevedo, Hydrogeologist, will also be in attendance. Learn more about DBS&A’s carbon management services here.
Risk Based Area of Review Delineation with MODFLOW and Groundwater Contaminant Fate and Transport Modeling
Authors: Todd Umstot, Gregory Schnaar, PhD, PG
Objectives and Scope: The Area of Review (AoR) for a Class VI project defines the area where abandoned wells must be assessed for corrective action and where regulators may require monitoring. The AoR is defined by the outermost extent of the carbon dioxide (CO2) plume and the area where increased pressure may pose a risk of leakage to overlying underground source of drinking water (USDW). The U.S. Environmental Protection Agency (EPA) Class VI guidance allows for risk-based AoR delineation, stating that numerical or analytical groundwater modeling can be used to estimate how additional fluid leakage caused by the injection project will be attenuated within USDWs. We have developed a suite of modeling techniques to simulate leakage from hypothetical abandoned boreholes or faults into USDWs, as well as the resulting contaminant transport within the USDW.
Methods, Procedures, Process: In cases where salinity in the reservoir does not significantly affect water density, we use the United States Geological Survey (USGS) MODFLOW 6 model to estimate brine flow into the USDW. MODFLOW is considered the international standard for simulating and predicting groundwater conditions. Key input parameters for MODFLOW include the stratigraphy overlying the reservoir, aquifer properties, reservoir pressure, wellbore or fault characteristics, and fluid properties. The simulated brine flux into USDWs is then used as input into an analytical groundwater modeling platform to estimate contaminant transport in the USDW. Alternatively, when reservoir brine has sufficiently high density, coupled numerical groundwater and solute transport modeling is conducted using the USGS MODFLOW-SEAWAT that accounts for variable-density groundwater flow. Both approaches account for contaminant dispersion, sorption, and decay, which are key attenuation processes. Additionally, both methods can model the transport of contaminants beyond salinity.
Results, Observations, Conclusions: The analytical modeling approach is likely sufficient for most projects and has the advantage of being easier to implement for sensitivity analysis. The results of groundwater transport modeling are verified against benchmark problems. A case study example demonstrates how groundwater modeling results are used to define the AoR based on risk-based considerations of impacts to USDWs.
Significance/Novelty: Compared to other leakage assessment platforms our method provides the advantages of (1) allowing for borehole leakage into dissipation zones; and (2) modeling contaminant transport and attenuation in USDWs. Use of MODFLOW allows for flexibility in stratigraphic assumptions compared to analytical borehole leakage models.