Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Direct Observation of Coupled Geochemical and Geomechanical Impacts on Chalk Microstructure Evolution under Elevated CO2 Pressure Y. Yang,*,† S. S. Hakim,† S. Bruns,† M. Rogowska,† S. Boehnert,†,‡,§ J. U. Hammel,∥ S. L. S. Stipp,† and H. O. Sørensen† †
Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark Maersk Oil and Gas A/S, Esplanaden 50, 1263 Copenhagen, Denmark ∥ Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, 51502 Geesthacht, Germany ‡
S Supporting Information *
ABSTRACT: Dissolution in natural porous media by injected CO2 can undermine the mechanical stability of the formation before carbon mineralization can take place. The geomechanical impact of geologic carbon storage therefore affects the structural integrity of the formation. Here, using in situ Xray imaging, we show the coupled geochemical and geomechanical processes in natural chalk in the presence of aqueous CO2. We first measured the chalk dissolution rate in a closed, free drift system and obtained a phenomenological correlation between the rate and evolving aqueous calcium concentration. We then used this rate correlation in a segregated flow model to estimate the visual pattern of chalk microstructure dissolution. The model predicted a homogeneous pattern, which resulted from an increase in the reactive subvolume. This prediction was validated using in situ X-ray tomography. The imaging technique further revealed three typical mechanical impacts during microstructure disintegration in an imposed flow field: material compaction, fracturing, and grain relocation. These impacts differ but are strongly coupled with CO2-induced geochemical reactions and provide different types of feedback to the dissolution front migration. These observations led us to conclude that the presence of dissolved CO2 makes the migration of reactive fluid less sensitive to perturbations in the coupled geochemical and geomechanical processes. KEYWORDS: in situ X-ray imaging, microstructure evolution, porous media, mineral dissolution, chalk, geochemical−geomechanical coupling, geologic carbon storage
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INTRODUCTION
precipitation, which modify the microstructure. This modification, in turn, feeds back into the patterns of fluid migration that transports the reactants and products of these reactions.5 The coupled evolution of the fluid flow field and the microstructure is not yet well understood as a result of the difficulties in direct experimental observation.6 Geomechanical effects further complicate the issue.7−9 If a porous rock is weakened by the water−rock interaction and cannot withstand the stress exerted by the surrounding rock or the flow field, it collapses and forms a new structure to regain its mechanical stability. This constant change of the microstructure alters the flow field and, in the long run, determines the fate of sequestered CO2. The ability to model such processes and
The ratification of the Paris Agreement by the European Union (EU) in November 2016 has once more pushed the global agenda for geologic carbon storage (GCS).1 Concerns remain, however, regarding the consequences of GCS for society and the environment.2 Is it safe? How much CO2 can we store? For how long? Can we decrease the costs? Answers to these questions depend at least in part upon the structural changes in the rock formation after CO2 is injected. The opening or closing of flow pathways determines whether buoyancy can drive the escape of sequestered CO2.3 The microstructure of the storage formation, its initial changes upon CO2 injection, and its evolution with time in the years and centuries that follow control migration from the injection well and, therefore, the efficiency of solubility, residual, and mineral trapping.4 CO2 dissolves in the formation water and disrupts the preexisting chemical equilibrium between the pore fluid and the solid matrix. This disequilibrium is the driving force for geochemical reactions, such as mineral dissolution and © XXXX American Chemical Society
Received: January 29, 2018 Revised: April 20, 2018 Accepted: April 23, 2018
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DOI: 10.1021/acsearthspacechem.8b00013 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
ACS Earth and Space Chemistry
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include them in risk analysis relies on our ability to characterize them, understand them, and predict them. In this study, we focus on the interaction of chalk with CO2 dissolved in water, with an emphasis on the physical changes to the microstructure imposed by a flow field. Chalk is composed predominantly of biogenic calcite, primarily coccoliths, which are microscopic disks produced by some species of marine algae. These are usually preserved as micrometer-scale platelets or fragmented as the sub-micrometer-wide elements.10 Chalk formations are found in many localities worldwide, e.g., in the Gulf of Mexico, the Mediterranean Sea, Southern England, and the North Sea Basin, serving as groundwater aquifers as well as oil and gas reservoirs.11 Limestone, also dominated by calcite, is abundant in many other locations. In some countries, chalk and limestone formations are candidates for GCS as a result of their availability and the associated possibilities for CO2 enhanced oil recovery,12 which contributes to the financial feasibility of GCS. Despite the large number of publications over the past decade about water−rock interactions in GCS,13−20 studies on chalk reservoirs remain scarce, especially on the topic of morphological evolution at the micro- and sub-micrometer scale. In chalk, there is no overabundance of cations in the pore fluids or from Mg, Ca, and Fe silicates that can permanently mineralize CO2 as carbonate minerals, as there is in basalt.21 Therefore, GCS in carbonate reservoirs relies primarily on stratigraphic, solubility, and residual trapping.2,4 As supercritical CO2 or water equilibrated with gas containing a high partial pressure of CO2 migrates away from the injection well, CO2 is taken up by solubility and residual trapping. The rate and extent of migration are determined by the existence or the generation of flow pathways in the formation.22,23 Accompanying CO2 migration is a plume of cations, with an increase in the local cation concentration (e.g., [Ca2+]), that moves with the fluid flow. This plume results because cations are first released near the injection well by proton-mediated dissolution and can then be consumed again by carbon mineralization downstream.24−27 Although such a cation plume does not result in net CO2 consumption, it has a significant impact on the mechanical properties of the reservoir because it constantly relocates solid materials away from the injection point. At the same time, hydraulic pressure decreases as fluid overcomes resistance from the porous medium, resulting in CO2 degassing. Degassed CO2 forms dispersed bubbles, which can be trapped in the very fine pores of chalk by capillary forces. All of these processes are controlled by geochemically induced microstructural changes, but these are still poorly understood in chalk formations. Our goal with this work was to fill the information gaps by combining an analysis of the cumulative surface (CS) with in situ X-ray imaging, to study the dissolution of chalk samples in an imposed flow field under elevated CO2 pressure. CS is a concept devised to determine the reactive volume of a porous medium and predict its dissolution pattern.28 We first measured the kinetics of chalk dissolution in a closed, free drift system. The correlation between the reaction rate and fluid composition thus obtained is needed to calculate the CS. We then use the CS to identify the morphological features resulting from chemical reactions and, hence, to distinguish between geochemical and geomechanical impacts on structural evolution. In situ X-ray tomography records the evolution and disintegration of chalk samples in three-dimensional (3D) images, in real time, with high fidelity and, thus, provides direct evidence for the complex coupling between the processes.
Article
MATERIALS AND METHODS
Synthetic calcite particles and ground chalk samples collected from core plugs from the Maastrichtian Formation of the North Sea Basin were used for the powder dissolution experiments. Seven core plugs from the same location, nearly the same depth, were combined to obtain a large enough sample and were homogenized so that the material would be representative. A detailed analysis of the surface composition of samples from the same location can be found in the study by Okhrimenko et al.29 Synthetic calcite was synthesized by mixing 1 M CaCl2 and 1 M Na2CO3 solutions (both made from reagent-grade powders, Merck, Denmark) and stirring for 3 days to ensure that all vaterite had transformed to calcite, which was confirmed by X-ray diffraction (XRD). Scanning electron microscopy (SEM) images showed that synthetic calcite had rhombohedral particles. The particles had a homogeneous size distribution and a typical size of 30 μm.30 For chalk, it was difficult to define a uniform grain shape based on which size distribution could be calculated. The homogenized chalk powers were sieved using a vibratory sieve shaker (Analysette 3 Spartan, Fritsch GmbH, Germany), and the fraction between 50 and 100 μm was used in the cyclic dissolution experiments. This screen did impose an upper limit in the particle size (100 μm). However, it was not effective in specifying the lower limit or controlling the grain size distribution. We therefore resorted to the Brunauer− Emmett−Teller (BET) measurement for an estimate of the overall surface area. This limit in grain characterization was considered in the formulation of data processing and led to the lumping of three evolving quantities, surface area, surface site density, and solid−liquid ratio, into a time-dependent variable, i.e., total number of surface reactive sites (H), explained below. Duplicated dissolution experiments were conducted for both natural chalk and the synthetic calcite at 1 bar CO2 and under room temperature (∼25 °C). Each dissolution experiment consisted of 3 cyclic runs. The starting mass of the synthetic calcite powder and natural chalk was 7.0 g for the first run, 4.7 g for the second run, and 2.0 g for the third run. The solid remaining from the previous run was used as the starting material for the next. The specific surface area (SSA) of the material, determined before and after each dissolution cycle using the BET method,31 is listed in Table S-1 of the Supporting Information. A total of 600 mL of ultrapure deionized water (resistivity of >18.2 MΩ cm) equilibrated with 1 bar of CO2 (>99%, Air Liquide, Denmark) at room temperature was used as the starting solution in each run, and all experiments were conducted in closed, temperaturecontrolled, glass, double-jacketed, reaction vessels with Teflon lids and glass overhead propeller stirring (320 rpm). Solution samples (0.1 mL) were taken with syringes, filtered (polypropylene, 0.22 μm pore size), preserved, and diluted with a solution of 2% HNO3 (diluted from analytical grade, Merck, Denmark) and 0.1% KCl (reagent grade, Merck, Denmark), and then analyzed with atomic absorption spectroscopy (AAS, AAnalyst 800, PerkinElmer, Waltham, MA, U.S.A.) (Table S-2 of the Supporting Information). Over the course of the experiment, pH was constantly monitored. We measured the pH with a METROHM 827 pH lab meter (Metrohm AG, Switzerland) that was calibrated at the beginning of each experiment using buffers of pH 4, 7, and 9 (from Metrohm AG). The probe stayed in the solution during the experiments. The device was connected to a computer, which recorded the data every 5 min. B
DOI: 10.1021/acsearthspacechem.8b00013 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry The percolation experiments with in situ X-ray imaging used samples from a chalk outcrop near Aalborg, Denmark (Rørdal Quarry, Maastrichtian age). They were predominantly CaCO3 coccoliths and skeletal debris with an average porosity of ∼45% and permeability in the range of 3−5 mD.32 The silica content of the samples was
