A risk-based approach to evaluating the Area of Review and leakage risks at CO2 storage sites

https://doi.org/10.1016/j.ijggc.2019.102884Get rights and content

Highlights

  • The NRAP-IAM-CS was used to determine risk-based leakage impacts and the Area of Review at three potential CO2 storage sites.

  • The risk-based Area of Review was smaller than that determined from analytical calculations for an over-pressurized reservoir.

  • The NRAP-IAM-CS showed minimal influx of CO2 and brine through legacy wells with no impact to the USDW.

  • This study provided an invaluable opportunity for testing and improving the NRAP-IAM-CS for quantifying risks at CCS sites.

Abstract

The U.S. Environmental Protection Agency’s Class VI regulations for underground carbon dioxide (CO2) injection require owners and operators of storage projects to identify (1) an Area of Review (AoR) that represents the region that may be affected by the injection of CO2, and (2) leakage risks that might impact the quality of underground sources of drinking water (USDWs). This article describes how such risks can be determined by accounting for the physical and chemical properties of all components of a CO2 storage site using elements of the National Risk Assessment Partnership Integrated Assessment Model for Carbon Storage (NRAP-IAM-CS). We used practical data from three sites that are part of two CarbonSAFE (Carbon Storage Assurance Facility Enterprise) projects to demonstrate application of the NRAP-IAM-CS toolset to determine project risk areas.

NRAP-IAM-CS was used to estimate the project risk area that could represent the AoR and the impact of leakage through legacy wells to overlying drinking waters at these candidate CO2 storage sites. Our study shows that risk to USDWs is minimal despite (1) the presence of multiple legacy wells at these sites where the AoR approximates the maximum extent of injected CO2 or (2) the presence of a much larger AoR approximating the pressure front caused by the injection of CO2.

Introduction

U.S. Environmental Protection Agency (EPA) Class VI regulations require that owners and operators of carbon storage projects identify an Area of Review (AoR) that represents the region whose underground sources of drinking water (USDWs) might be endangered by the injection of carbon dioxide (CO2). The AoR serves as a project footprint that can be used to develop monitoring plans to ensure the protection of USDWs. According to Title 40 of the Code of Federal Regulations Part 146, Section 84 (40 CFR 146.84, 2019), delineation of the AoR needs to be derived from computational modeling which accounts for the physical and chemical properties of all phases of the injected CO2 stream, and be based on available site characterization, monitoring, and operational data. Site permitting also requires an understanding of the risks associated with leakage pathways, such as wells and/or faults connecting the storage reservoir with any overlying USDWs. The EPA Class VI Rule requires groundwater geochemistry monitoring above the lowermost confining zone overlying the storage reservoir to detect changes in aqueous geochemistry caused by fluid leaking out of the injection zone (40 CFR 146.90, 2019(d); USEPA, 2013).

The EPA recommends the AoR be delineated by the maximum extent of the separate-phase CO2 plume or the pressure front, whichever is greater, over the lifetime of the project as defined by numerical model simulations. The pressure front is defined as the extent of the minimum pressure within the injection zone needed to cause fluid to flow from the injection zone into the USDW through a hypothetical conduit. A number of analytical approaches are recommended for calculating this maximum pressure threshold for sites where the pressure regime is hydrostatic or the storage reservoir is underpressured with respect to the USDW (USEPA, 2013). More sophisticated methods are required for over-pressured sites. Regardless of how the pressure threshold is calculated, the AoR delineation must encompass the maximum extent of the CO2 plume or the pressure front, whichever is larger (USEPA, 2013).

Wells are considered high-risk pathways for fluid leakage from geologic CO2 storage reservoirs because breaches in the engineered system can connect the reservoir to USDWs and the atmosphere (Gasda et al., 2004). The lack of well data often makes assessing well integrity difficult because potential avenues for upward fluid migration cannot be adequately identified. In such cases, the National Risk Assessment Partnership Integrated Assessment Model for Carbon Storage (NRAP-IAM-CS) can be used to evaluate the probability of CO2 and brine leakage and its impact on drinking water quality from known well locations using default permeability distributions based on oil and gas wells in the Alberta and Gulf Coast basins and the greenfield FutureGen 2.0 site (Pawar et al., 2016).

NRAP-IAM-CS is a science-based toolset developed by the U.S. Department of Energy (DOE) for quantitative risk assessment of the geologic sequestration of CO2 (Pawar et al., 2016). The toolset uses a stochastic approach in which predictions address uncertainties in storage reservoirs, leakage scenarios, and shallow groundwater impacts. It is derived from detailed physics and chemistry simulation results that are used to train more computationally efficient models—reduced-order models (ROMs)—for each component of the system. Use of NRAP-IAM-CS can help regulators and operators define the risk-based AoR and better understand the expected extents and longevity of changes in water quality caused by CO2 and brine leakage from a storage reservoir into drinking water aquifers.

This paper presents the results derived from some of the first applications of the NRAP toolset for the screening of potential CO2 storage sites to evaluate the project risk area that could be used to estimate the AoR and the area that would be affected by leakage of CO2 or brine through legacy wells. The NRAP-IAM-CS toolset was used to estimate the project risk area and the impact of leakage through legacy wells to overlying drinking waters at three Carbon Storage Assurance Facility Enterprise (CarbonSAFE) project sites: two saline reservoir storage sites evaluated as part of a Central Appalachian Basin project in Ohio (the primary and secondary selected sites), and one St. Peter Sandstone saline reservoir storage site evaluated as part of a project in Michigan. While the EPA makes the final determination of a project’s AoR, operators are required to submit as part of their permit application a proposed AoR based on site-specific data and modeling. Therefore, for the purposes of this study and for conciseness in this paper, we are using the term AoR to mean our estimate of the project risk area that could represent the project’s AoR. For each of the three sites mentioned above, we calculated (1) an AoR using the EPA critical pressure method; (2) a risk-based AoR using the NRAP-IAM-CS tool to predict the impacts of leakage from hypothetical open and uncemented wells on groundwater quality in a shallow drinking water aquifer overlying the storage reservoir; and (3) the cumulative mass and impact of CO2 and brine leaked from existing legacy wells using the NRAP-IAM-CS toolset.

Section snippets

Approach

The NRAP-IAM-CS toolset was applied to three saline reservoir sites to assess the risk to USDWs from leakage of CO2 or brine along potential leakage pathways. The NRAP-IAM-CS is designed to perform probabilistic simulations related to the long-term fate of a CO2 sequestration operation by modeling the following components of a carbon storage project: a primary CO2 injection reservoir, potential leakage pathways, and receptors such as shallow aquifers. The model’s stochastic framework at the

AoR delineation

The risk-based AoRs for all three sites were calculated using spatial and temporal distributions of CO2 saturations and pressures within the storage reservoir that were generated by a multiphase numerical model. The model layers chosen to be extracted were selected because they had the highest pressure buildup and largest CO2 plume for their respective sites. Once CO2 saturations and pressures were translated from the reservoir model to the NRAP-IAM-CS grid, the open wellbore component was used

Discussion

In this paper, we use the NRAP-IAM-CS toolset to provide an alternative method for calculating the project AoR to assess risk and to develop monitoring plans to ensure the protection of USDWs above prospective CO2 storage sites that have a history of oil and gas operations. This work represents some of the first applications of the tools at potential CO2 storage sites. The toolset provides a consistent framework for not only defining the risk-based AoR assuming CO2 and brine were to move

Conclusions

The AoR represents the region where USDWs might be endangered by the injection of CO2 and serves as a project footprint that can be used to develop monitoring plans to ensure the protection of USDWs. Estimates of the AoR are required to use computational modeling to account for the physical and chemical properties of all phases of the injected CO2 stream, and must be based on available site characterization, monitoring, and operational data. Site permitting also requires an understanding of the

Funding sources

This material is based upon work supported by the Department of Energy (DOE) under Award Number DE-FE0029276 with co-funding by Core Energy, LLC and other team members. The Project Team is led by Battelle and includes Core Energy, LLC, Pacific Northwest National Laboratory, Los Alamos National Laboratory, Lawrence Livermore National Laboratory, Wade LLC, PKM Energy, the Loomis, Ewert, Parsley, Davis & Gotting, P.C., and Western Michigan University.

This material is based upon work supported by

Declaration of Competing Interest

None.

Acknowledgements

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344, by Los Alamos National Laboratory (LANL) under Contract DE-AC52-06NA25396, and by Pacific Northwest National Laboratory under Contract DE-AC06-76RLO1830. The authors acknowledge Traci Rodosta (NETL Carbon Storage Program) and Mark Ackiewicz (DOE Office of Fossil Energy) for their programmatic guidance, direction, and support. Section 4.0

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