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Energy and the Environment 2
Permanent CO trapping through localized and chemical gradient-driven basalt carbonation Anne Holland Menefee, Daniel E. Giammar, and Brian R. Ellis Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01814 • Publication Date (Web): 07 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018
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Environmental Science & Technology
Permanent CO2 trapping through localized and chemical gradient-driven basalt carbonation Anne H. Menefee,1 Daniel E. Giammar,2 and Brian R. Ellis1,3* 1. Department of Civil and Environmental Engineering University of Michigan, Ann Arbor, MI, United States 2. Department of Energy, Environmental, and Chemical Engineering Washington University, St. Louis, MO, United States 3. ORISE at the National Energy Technology Laboratory Morgantown, WV, United States *Corresponding author 1351 Beal Ave, EWRE building Ann Arbor, MI 48109-2125 Phone: 734-763-5470; fax: 734-764-4292; email:
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Abstract
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Recent laboratory and field studies have demonstrated that basalt formations may present one of the most secure repositories for anthropogenic CO2 emissions through carbon mineralization. In this work, a series of high-temperature, high-pressure core flooding experiments was conducted to investigate how transport limitations, reservoir temperature, and brine chemistry impact carbonation reactions following injection of CO2-rich aqueous fluids into fractured basalts. At 100°C and [NaHCO3] representative of typical reservoir conditions (6.3 mM), carbonate precipitates were highly localized on reactive mineral grains contributing key divalent cations. Geochemical gradients promoted localized reaction fronts of secondary precipitates that were consistent with 2D reactive transport model predictions. Increasing [NaHCO3] to 640 mM dramatically enhanced carbonation in diffusion-limited zones, but an associated increase in clays filling advection-controlled flow paths could ultimately obstruct flow and limit sequestration capacity under such conditions. Carbonate and clay precipitation were further enhanced at 150°C, reducing the pre-reaction fracture volume by 48% compared to 35% at 100°C. Higher temperature also produced more carbonate-driven fracture bridging, which generally increased with diffusion distance into dead-end fractures. In combination, the results are consistent with field tests indicating that mineralization will predominate in buffered diffusion-limited zones adjacent to bulk flow paths and that alkaline reservoirs with strong geothermal gradients will enhance the extent of carbon trapping.
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Introduction
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The need to reduce greenhouse gas emissions quickly and effectively has compelled efforts to leverage the reactivity of basalt or ultramafic reservoirs for secure long-term carbon sequestration. Reactive silicate minerals dissolve rapidly under acidic conditions following CO2 injection, consuming H+ and releasing divalent metal cations (e.g. Ca2+, Mg2+, Fe2+) that can ultimately mineralize dissolved CO2 in the form of stable carbonate minerals. While sedimentary formations such as saline aquifers or depleted oil and gas reservoirs offer storage capacities most relevant to the scale of CO2 emission reduction targets, these reservoirs rely primarily on short-term solubility and physical trapping mechanisms that present leakage risks if CO2 is not structurally secured. Given the inherent advantage of mineral trapping for long-term storage security, basalts may be more readily deployable CO2 repositories in the near term.1 Recent field tests under the CarbFix project in Iceland2 and Wallula Basalt Pilot Project in Washington state3 have demonstrated that considerable CO2 mineralization occurs within 2 years of injection, motivating further investigations on microscale dissolutionprecipitation processes driving large-scale sequestration efficiency.
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Despite a need to understand how basalt responds to near-wellbore CO2 injection conditions, few studies have been carried out with advective flow. In percolation experiments on sintered dunite (160°C; 11 MPa PCO2; initial pH 6.7), Andreani et al.4 only observed significant carbonate-induced porosity alterations in low-flow zones, whereas high-flow zones promoted rapid dissolution followed by the formation of Si-rich passivating layers that created a barrier between fluids and reactive surfaces.4 Peuble et al.5 also conducted reactive percolation experiments on sintered olivine cores at a similar pH (6.6) and concluded that higher flow rates improve carbonation efficiency, as permeability reductions blocked transport in low-flow regimes. Luhmann et al.6,7 observed no carbonate precipitation in flow-through experiments with whole basalt cores (150°C; 15 MPa PCO2), likely due to the low pH (3.3) of the influent, but secondary Si- and Al-rich phases led to slight permeability reductions at lower flow rates. Adeoye et al.8 also found no evidence of carbonation in
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Environmental Science & Technology
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a series of flow-through experiments (45 and 100°C; 10 MPa PCO2) on serpentinized and unaltered basalt cores because low retention times promoted net dissolution. Wolff-Boenisch and Galeczka9 carried out mixed-flow experiments on glass and crystalline basalt (90°C; 0.6 MPa PCO2) using ammonium bicarbonate as a surrogate for CO2. Secondary silicates filled pore spaces unless synthetic seawater was injected as a quasi-infinite source of divalent cations, in which case Ca- and Mgcarbonates formed.9 Related reaction path modeling confirmed continuous fluid injection is necessary to avoid clays and zeolites clogging near-wellbore regions at higher pH.10
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Collectively, prior static and flow studies demonstrate that the extent of mineral carbonation is a complex function of initial mineralogy, fluid chemistry, and P/T conditions. While studies on whole natural basalts have been limited, early work on powdered olivine carbonation established optimal reaction conditions of 185°C and 14 MPa PCO2 in solutions of 640 mM NaHCO3 and 1 M NaCl.11,12 Theoretically, diffusive mass transfer across the crystal-fluid interface increases with temperature while the driving chemical potential decreases, resulting in maximum carbonation rates around 185°C.13 In 3-hour stirred batch experiments using 15 wt% solids, Gadikota et al.14 found the extent of olivine carbonation (with respect to Ca- and Mg-carbonates) increased from 3% at 90°C to 71% at 150°C and 85% at 185°C. The effect of CO2 partial pressure was less pronounced; an increase from 6.5 to 14.1 MPa under the optimal temperature increased carbonation from 39 to 85%.14 They also concluded the reaction-enhancing effects of the optimal solution were driven by NaHCO3 serving as both a buffer and source of carbonate ions, as an increase from 0 to 0.64M NaHCO3 improved the 3hr carbonation yield from 5.8% to 83% at the optimal T and P.14 The effect of NaCl was deemed negligible, although other work has demonstrated that NaCl enhances dissolution of forsterite15 and natural basalts.8 NaHCO3 may also inhibit the formation of reaction-inhibiting silica-rich passivation layers, which slow and could ultimately obstruct carbonation reactions.16 The effect of high-NaHCO3 solutions has not yet been studied in natural basalts; while NaHCO3 provides pH buffering necessary to initiate and sustain carbonate precipitation, the dissolution rates of the primary silicate minerals contributing divalent cations for carbonation decrease significantly with increasing pH. The roles of 'reaction-enhancing’ controls on long-term carbonation efficiency will likely be coupled with transport conditions but have only been closely studied in static systems, motivating the systematic flow-through studies developed here.
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The objective of this work is to evaluate how transport limitations in advection- or diffusioncontrolled zones influence the extent of carbonation and secondary mineral alteration in natural basalts exposed to flow of CO2-acidified fluids under relevant geologic storage conditions. Given their demonstrated ability to enhance olivine dissolution and carbonation rates14,17 but limited investigation in the context of different flow regimes, the roles of temperature and [NaHCO3] were explicitly targeted. A series of three core flooding experiments was carried out using serpentinized basalt cores, where saw-cut surfaces were milled with fractures designed to isolate advective flow channels and diffusion-limited dead-end fractures. Reaction products were characterized through a suite of non-destructive imaging techniques and experimental observations were supplemented with reactive transport models that demonstrated how geochemical gradients drive secondary alteration patterns along fractures. In combination, the results shed new light on conditions favoring permanent CO2 sequestration through mineral trapping in fractured basalts.
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Methods
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2.1. Basalt core preparation
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Serpentinized basalt samples from Valmont Butte, Colorado were supplied by Ward’s Science. Cores measuring 2.54 cm (1”) in diameter and 3.8 cm (1.5”) in length were cut in half lengthwise using a precision saw with a diamond blade. One side of each sample was polished with 400-grit sandpaper and the other was milled with a precision CNC milling machine (Roland Model MDX-40a) with a 0.5mm diamond burr to create the 100-µm deep fractures shown in Figures 2 and 4. These etched pathways were designed to simulate a main advection-controlled flow channel connected to four dead-end fractures of varying lengths and widths, which represent different diffusion length scales and water:rock ratios. While the aperture size is realistic, these milling patterns are not intended to be representative of natural fractures but rather to address our objectives by studying multiple transport regimes within the same experimental system. Given the low porosity of the bulk matrix (
