Potential of Co-practicing Enhanced Rock Weathering and Geologic Carbon Storage

Abstract

Geologic carbon storage (GCS) is an important technology for mitigating climate change, and hundreds to thousands of gigatons of CO₂ needs to be stored via GCS by 2100 to achieve the 2 °C or 1.5 °C target. (1) One challenge that hinders the large-scale deployment of GCS is the risk of the injected CO₂ migrating through abandoned wells and/or faults/fractures into overlying formations or the atmosphere. Assessments that considered extensive empirically measured and simulated data of GCS suggests that the surface seepage rate is highly likely to be less than 0.05% stored/year, (2) corresponding to CO₂ fluxes at the land surface on the order of 0.1–10⁴ μmol m^–2 s^–1 depending on the affected area (e.g., 1000 m² for a single leaky well and ∼100 km² for the CO₂ plume of a typical GCS project). Although leakage risks associated with GCS are low, the development of effective remediation measures is crucial to minimize the potential environmental impacts. Various remediation measures have been proposed, including halting CO₂ injection, brine management, repairing leaky wellbores, and plugging abandoned wells. We submit that practicing enhanced rock weathering (ERW), which involves surficial application of ground alkaline rocks and/or materials, in the vicinity of the injection sites may serve as another measure that can mitigate surface seepage of CO₂ should it occur, while otherwise capturing CO₂ from the atmosphere and/or soils (Figure 1). ERW is projected to sequester up to 2.5 Gt of CO₂/year by 2100 under the 1.5 °C scenario. (1) Figure 1. Conceptual diagram illustrating the co-practicing of ERW and GCS (and BECCS) (TEA: techno-economic analysis). ERW using minerals such as wollastonite and brucite can mitigate fluxes of hundreds of micromoles per square meter per second (up to 1000 μmol m^–2 s^–1) for days and weeks when applied at fine grain sizes (1–10 μm) and an application rate of 20 kg/m² (i.e., 200 t/ha), as estimated using the shrinking core model and reaction rates at pH 7. Experimental and numerical studies that considered different levels of realistic constraints reported averaged in situ absorption of CO₂ flux up to micromoles per square meter per second. (3) These analyses suggest that ERW can be effective for diffused low seepage (∼0.1 μmol m^–2 s^–1) or may provide a buffer of weeks to months before other remediation measures are implemented. For moderate and high seepage (1–10⁴ μmol m^–2 s^–1), although more unlikely, measures that boost in situ weathering rates, e.g., microbial interventions, or more frequent applications may be needed. ERW also has the potential to remediate leakage into groundwater. The major concern of the latter is the adverse effect of changes in water chemistry and water–rock interactions following a decreased pH, whereas one of the known outcomes of ERW is the increased pH and alkalinity. (3) Co-practicing ERW and GCS may have other co-benefits, such as the sharing of infrastructures and monitoring systems. Monitoring, verification, and accounting (MVA) is of significant importance for any carbon removal technology to be deployed at meaningful scales. Most of the near-surface monitoring techniques (including geochemical monitoring in soils and groundwater) required by GCS are ones that will be imperative for ERW to evaluate its environmental impacts. (4) In turn, regular examination of the applied rock materials necessary for ERW may serve as a prewarning of leakage when abnormal weathering rates are recorded. When GCS projects are coupled with bioenergy production, co-practicing ERW and GCS may also have the benefit of improving bioenergy crop production. Such co-practicing may be possible geographically. For example, the Decatur project in the Illinois Basin is the world’s first bioenergy carbon capture and storage (BECCS) project and is also located in the U.S. corn belt that has been used for testing the effectiveness of ERW in decarbonizing the farming sector. (3) There are, however, challenges for co-practicing ERW and GCS in addition to their individual hurdles. Of utmost concern is the clear allocation of the responsibilities and carbon credits, which calls for further regulatory intervention. To reduce capital investments and operational costs of shared infrastructures and MVA systems, effective communication and considerable coordination between stakeholders are essential. Geographical analysis of the storage sites and source rocks is necessary, as transportation of the rock materials can be a large energy penalty for ERW. In summary, there are opportunities and challenges for co-practicing ERW and GCS, and its prospect is worthy of more careful assessments with inputs from all engaged stakeholders. The scientific community can fill key research gaps for this purpose. First, investigations that devise scenarios considering co-practicing are needed to more accurately examine its effectiveness and environmental impacts and to develop co-optimization strategies. For example, (i) current studies of ERW do not include an additional source of CO₂ from the bottom, which would be the case when serving as a remediation measure for potential GCS leakage, (ii) whether operations (e.g., application depth and amount) that maximize the remediation effect by ERW can lead to maximized carbon removal when leakage is absent remains unclear, and (iii) the environmental impacts (e.g., heavy metal accumulation) of actions that boost weathering rates, necessary at medium to high seepage rates, have not been fully assessed or tested in the field. Second, life cycle assessment with adjusted system boundaries for GCS and ERW and integrated assessment models that monetize potential co-benefits and penalties of co-practicing are vital in evaluating its techno-economic feasibility and informing policy analyses. More broadly, as global decarbonization requires the implementation of a portfolio of carbon removal technologies, it is important to identify potential synergy and competition, particularly for engineered natural processes and/or systems, such as ERW-GCS, and the more recently introduced concept of orange hydrogen (i.e., hydrogen production from carbonation reactions in GCS), (5) because they take advantage of similar natural processes and can have simultaneous but differentiated impacts on biogeochemical cycling, the occurrence, fate, and transport of contaminants, etc. We believe that the Environmental Science & Technology community has a role to play to appraise the physicochemical, economic, and social impacts of co-practicing these technologies and to develop innovative interventions that maximize synergy. Hang Deng is an assistant professor at the College of Engineering and an adjunct professor at the Institute of Energy and Institute of Carbon Neutrality, Peking University, and an affiliated researcher at the Peking University Ordos Research Institute of Energy. Hang received her bachelor’s degrees in science and arts from Peking University and her Ph.D. from the Department of Civil and Environmental Engineering at Princeton University in 2015. Afterward, she worked at Lawrence Berkeley National Laboratory’s Earth and Environmental Sciences Area, first as a postdoc fellow and then as an Earth research scientist. Her research focuses on fundamental multiphysics and multiscale problems that underlie a multitude of applications, including geologic carbon storage, enhanced rock weathering, enhanced geothermal systems, waste disposal, and contaminant remediation. Hang’s recent research centers around reactive transport processes of various fluids and chemicals in fractured and porous media across scales, e.g., coprecipitation of calcium carbonate with heavy metals and pore-scale interactions between multiphase flow and geochemical reactions. This article references 5 other publications. This article has not yet been cited by other publications.