Geochemical controls on fracture evolution in carbon sequestration

Transcription

1 ARMA Geochemical controls on fracture evolution in carbon sequestration Fitts, J.P., Ellis, B.R., Deng, H. and Peters, C.A. Dept. of Civil & Environmental Engineering, Princeton University, Princeton, NJ, USA Copyright 2012 ARMA, American Rock Mechanics Association This paper was prepared for presentation at the 46 th US Rock Mechanics / Geomechanics Symposium held in Chicago, IL, USA, June This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT: Stored supercritical CO 2 will acidify native brines in deep saline aquifers and promote mineral dissolution within the storage formation resulting in varying degrees of calcite saturation. Injection overpressure will force these reactive brines into existing and induced fractures in overlying caprock formations. We present examples from our experimental efforts to understand how these fluids might alter fracture geometry and leakage pathway permeability, with the ultimate goal of predicting caprock integrity. We use mineral-specific imaging analysis to correlate changes in fracture geometry with spatial maps of dissolution and precipitation. Synchrotron-based x-ray spectroscopy and diffraction imaging of thin section sub-samples of the cores from the flow-thru experiments are used to connect mineral-specific dissolution and precipitation processes with the geometric changes in fracture aperture observed with CT images. Results of µxrf, µxanes and µxrd analyses reveal that preferential calcite dissolution and the spatial distribution of relatively insoluble dolomite and silicate minerals produced the non-uniform aperture widening. These results clearly point to the need for predictive models of caprock integrity to consider coupled geochemical processes, mineralogical characterizations, and geometric alterations of flow paths. 1. INTRODUCTION One of the most important challenges to predicting the long-term integrity of caprock formations overlying geologic CO 2 storage reservoirs is evaluating how much CO 2 will flow through caprock fractures, potentially damaging other subsurface resources and eventually escaping to the atmosphere [1]. In addition to studies that quantify the spatial distribution and permeability of existing and induced fractures and faults, mounting experimental evidence suggests that predictive models of caprock integrity need to consider how CO 2 -acidified brines will react with fracture walls and thereby change the permeability of these flow paths with time [2]. This paper contributes to the growing body of experimental work aimed at identifying the conditions of fluid chemistry and caprock mineralogy that will result in fracture erosion [3-5]. These studies have identified dissolution of carbonate minerals as one of the primary geochemical drivers of permeability evolution in fractures of carbonaceous caprock formations. This observation is important because carbonate rocks are ubiquitous within the geologic record both as primary formation minerals and as secondary minerals, in which case carbonates most commonly occur as mineral cements. Furthermore, three geologic CO 2 demonstration projects have already been sited in geologic settings with carbonate caprock seals [6]. Finally, the large and distributed scale of geologic CO 2 storage needed to meaningfully offset greenhouse gas emissions will require that caprocks containing different quantities and assemblages of carbonate minerals be evaluated for their long-term integrity. Numeric simulations of reactive transport of CO 2 acidified brines have predicted significant geochemically induced changes of fracture permeability [7]. In situ x-ray computed tomography (x-ct) imaging experiments, however, have shown how dissolutiondriven changes in fracture geometry are non-uniform and depend on a complex relationship between brine chemistry, fluid flow velocity and mineral spatial distribution [4]. It is important to note that in situ imaging observations do introduce a myriad of experimental limitations including restrictions on sample size and geometry [8, 9], compromised imaging resolution [10] and limitations on characterizing the spatial distribution of minerals [11]. This paper is an extension of previously published work [4] that used x-ct imaging of a core-flooding experiment at 27 C and 10 MPa to observe fracture widening that was primarily the result of calcite dissolution by CO 2 -acidified brines. The new results presented here represent the first application of synchrotron-based x-ray imaging, spectroscopy and scattering methods to characterize the mineral spatial distribution in caprock fractures following reactions with

2 CO 2 -acidified brines. We describe some unique capabilities of these x-ray methods and demonstrate their complementarities with electron microscopy methods. Finally, we discuss how in situ and ex situ methods can be combined to identify the mineral-specific features needed to predict important aspects of permeability evolution. provides a schematic overview of the laboratory system used for the core-flooding experiment. 2. EXPERIMENTAL A brief summary of experimental methods follows and additional details of the CO 2 injection demonstration site, core sample and core-flooding experiments can be found in Ellis et al. [4]. a ) b) 25.4 mm Inlet 70.6 mm Outlet Fig. 1. Core samples from the Amherstburg limestone formation (a) and schematic of core subsample used in the core-flooding experiment with artificially induced fracture Caprock core samples A caprock specimen was sampled from the drilling core of the injection well at one of the CO 2 injection demonstration sites of the US Department of Energy. The site is the Midwest Regional Carbon Sequestration Partnership s project located in Otsego County, Michigan. Approximately 60,000 tons of CO 2 were injected into the Bass Islands Dolostone between 2008 and This formation is overlain by the Bois Blanc formation, a cherty carbonate, and above that by the Amherstburg formation, a dense fossiliforous dolomitic limestone. The Amherstburg formation was considered the primary caprock for this injection site [12] Flow-thru experiments Subsamples of the caprock cores were artificially fractured (see Fig. 1b) and epoxy was used to maintain the registry of the two halves of the core. Hydrostatic pressure and geothermal temperatures were maintained throughout the flow-through experiments. Figure 2 Fig. 2. Schematic of core flooding experimental system [4]. Sub-samples were sectioned from the core by sawing out slices perpendicular to the direction of flow through the fracture. The approximately 5 mm thick discs were then polished to 75±25 µm thick sections and mounted on glass slides. The rock sections were lifted off of the glass slide prior to x-ray spectromicrospy in order to eliminate background scattering from the silica and fluorescence from impurities within the slides X-ray spectromicrospy All x-ray measurements were performed at the National Synchrotron Light Source (NSLS) at Brookhaven National Lab (Upton, NY USA) using the x-ray microprobes at beamlines X27A and X26A. X-ray fluorescence maps were collected by raster flyscanning the approximately 7x10 (HxW) micron beam at (2) kev over regions of interest within the samples, centered on the fracture. Element maps of Ca, Fe and Sr were generated by gating their Kα emission lines and applying appropriate cutoffs to eliminate lowcount noise and spurious high counts. Calcium K-edge X-ray Absorption Near Edge Structure (XANES) spectra were collected at points of interest of the thin section. Beam energy was controlled with a Si(111) monochromator. A helium environment was used to limit air scattering losses in the beam path. Background subtraction, normalization and Linear Combination Fits (LCF) were performed using Athena software [13]. X-ray diffraction point spectra were collected at an x-ray beam energy of kev using either a Bruker or Rayonix CCD camera. The diffraction images were background subtracted and integrated into 1-D intensity versus 2-theta using FIT2D [14]

3 3. RESULTS The thin sections characterized in this work were subsamples from the core-flooding experiment reported by Ellis and co-workers [4]. Therefore, a brief summary of the core-flooding results is needed to provide context. Over the seven days of exposure to flowing CO2acidified brines (ph 4.4), reconstructed x-ct scans collected before and after flow showed aperture widening throughout the core. The change in fracture geometry produced a ~2.7x increase in void volume, which corresponded to a decrease in fluid velocity from 110 to 42 cm/hr under the constant-flow experimental regime. Aperture widening, however, was not uniform throughout the entire fracture. Electron microscopy results showed that calcite dissolution was primarily responsible for aperture widening and the non-uniform aperture widening was governed by a complex relationship between the spatial distribution of calcite in relation to the occurrence of relatively slow-dissolving dolomite and the thermodynamically stable silicate minerals including clays Element Distributions Figure 3 shows an x-ray fluorescence elemental map of calcium (Ca), iron (Fe) and strontium (Sr). The fracture aperture appears black and the section is perpendicular to flow direction. Calcium is distributed throughout the unaltered limestone as expected, while Fe and Sr show more irregular distributions. Iron-free Ca-containing minerals predominantly define the left fracture boundary. In two sections on the left wall of the fracture, however, Fe-containing minerals extend across the boundary of Ca-containing minerals and into the fracture. The ~200 micron gap in the Fe-containing region along the bottom of the image presumably marks the extent of the fracture aperture prior to reaction. The XRF image provides an indication of the complex relationship between mineralogy and the rate and extent of aperture widening at the spatial scale of 100 s of microns. Fig. 3. XRF image of a thin section specimen from reacted fractured caprock core subsamples sectioned perpendicular to fluid flow. XRF image combines Ca (red), Fe (green) and Sr (blue) elemental maps Calcium speciation and mineralogy The spatial variation of calcium nearest-neighbor coordination and bonding environments (i.e., speciation) was characterized by collecting Ca K-edge µxanes spectra at the select points labeled with yellow O in Figure 3. The x-ray sampling volume can be approximated by the 10x7 µm beam spot size and thickness of the sample (75±25 µm). The Ca µxanes spectra from the subset of points labeled A-F are shown in Fig. 4 along with end-member spectra for dolomite and calcite. The four points of comparison highlighted by the grey arrows in Fig. 4 show how calcite and dolomite spectra can be clearly distinguished qualitatively. The spectra at points A-D are consistent with the calcite end-member, and therefore these spectra are also consistent with calcite being the dominant mineral in the limestone. In contrast, the spectra at points E and F share the spectral features of the dolomite end-member spectrum.

4 Fig. 4. Ca µxanes spectra of points A-F shown in Figure 3, along with end-member (e-m) spectra for dolomite and calcite used in linear combination fit results reported in Table 1. The end-member spectra are consistent with the spectral shape of standard spectra collected on powdered calcite and dolomite samples. The absolute amplitudes of the Ca µxanes spectra, however, were not amenable to quantitative linear combination fitting (LCF) using spectra of powdered samples of model mineral compounds, according to the typical protocol. Instead, the end-member spectra were collected at points on the thin section where µxrd independently confirmed only the single crystalline carbonate phase was present. These end-member spectra match the amplitude envelope of the unknown point spectra because they share a common sample geometry. The linear combination fit results shown in Table 1 clearly distinguish regions of predominantly calcite, predominantly dolomite and mixed dolomite-calcite mineralogy. Calcite dominates all regions of the sample that have not been altered by the flowing CO 2 -acidified brines, even in the unaltered regions with mixed calcitedolomite. The only exceptions are altered Fe-containing region where the calcite has been dissolved leaving behind dolomite. This observation is consistent with the electron microscopy results from the same experiments reported by Ellis and coworkers [4]. The relatively high chi-square values for the spectra in this region suggest the presence of additional Ca-containing mineral phases other than calcite or dolomite. Table 1. Linear Combination Fits of Ca µxanes spectra Point spectrum % Dolomite % Calcite chi-square A unaltered 2.5(9) 97.5(9) B unaltered C unaltered 4.6(5) 95.4(5) D unaltered 4(1) 95(1) E altered 84(3) 15(3) F altered Mineral assemblages The mineralogical compositions within the Fecontaining regions in Fig. 3 were characterized further using µxrd in both point diffraction and mapping modes. The point diffraction mode is akin to the µxanes point spectra where µxrd patterns were collected while maintaining the x-ray beam at each yellow O in Fig. 3. The µxrd patterns at point C is shown in Figure 5. In contrast, the µxrd patterns D and F in Fig. 5 were collected in mapping mode, during which the x-ray beam was automatically raster stepscanned in a micron box around the point of interest. The mapping mode is useful for improving signal-to-noise. This mode also provides a method for recording and defining average mineralogy within larger areas of the sample. Fig. 5. Stack plot of µxrd diffractograms of points C, D and F shown in Fig. 3, along with diffractograms of calcite and dolomite powder standards. The patterns at points A and B only show the peaks observed in the calcite standard pattern (data not shown). The patterns at points C and D show peaks for calcite (C) and dolomite (D) minerals. The presence of dolomite and the absence of calcite peaks in pattern F is consistent with the preferential dissolution of calcite within this calcium-depleted region of the fracture boundary. 4. DISCUSSION The collective results of the µxrf, µxanes and µxrd analyses support the conclusion that preferential calcite dissolution and the spatial distribution of relatively insoluble dolomite and silicate minerals produced the non-uniform aperture widening observed in the x-ct images of the post-reacted fracture. These results are also consistent with the previously reported conclusions based on SEM EDX and BSE images of

5 sections from the same core-flooding experiment [4]. While the nanometer spatial resolution of the SEM cannot be matched, the synchrotron x-ray methods have spectroscopic capabilities that can provide unique insights into the mineralogical reasons underlying the variability of calcite dissolution rates, and the deeper penetrating x-rays provide a complementary method for categorizing regions of mixed mineralogy. Calcite incorporates impurities during precipitation and crystallization that alter the mineral structure, and as a result can accelerate or inhibit dissolution rates relative to pure calcite [15, 16]. The current study showed how Ca µxanes is sensitive to Mg substitution and the associated structural changes. Additional Ca µxanes studies of calcite minerals with different degrees and types of impurity substitution could be used to develop spectroscopic signatures for calcite minerals according to their dissolution rate. The microprobe x-rays penetrate calcite to a depth of approximately 50 µm at the Ca K-edge and 1.4 mm at the Mo K-edge where the µxrd were collected. In contrast, the SEM electrons penetrate less than 50 nm of calcite and are essentially surface sensitive. While the greater penetration depth limits the spatial resolution of the x-ray microprobe to the thickness of the sample, the method is ideal for deriving average mineral assemblages based on spectroscopic and diffraction characteristics. Average mineral assemblages can be used to segment µxrf (and SEM) images according to dissolution susceptibility. These mineral assemblages could then be correlated with density contrast within the x-ct 3-D reconstructions of whole cores. Finally, major technical advances in the x-ray sources, optics and detectors used in x-ct systems continue to improve in situ imaging of core-flooding experiments. These technical advances along with improved image processing methods will further efforts to use density contrast to characterize mineral heterogeneity [17]. Synchrotron-based x-ray methods, however, combine the energy tunability, high intensity beams and penetrating power needed to conduct unique in situ tomography, diffraction and full-field imaging measurements at P/T relevant to geologic storage. 5. CONCLUSIONS Here are the observations and conclusions drawn from this work. This work revealed the complex relationship between mineralogy and the rate and extent of aperture widening at the spatial scale of 100 s of microns. For example, carbonate dissolution can substantially widen an aperture in calcite-rich minerals or it can deplete a mineral but leave the aperture unaltered. Mineral spatial heterogeneity must be accounted for in models of permeability evolution caused by reactive fluid flow through fractures containing minerals such as calcite that are susceptible to dissolution. High-resolution benchtop x-ct systems are being developed with the large working distances and x-ray power needed to observe fractured rocks as they undergo both geochemical and geomechanical stress. Carbonate formations should be the caprock of last resort for geologic CO 2 storage, and guidance should be developed based on acceptable caprock mineral assemblages and petrographic features that are most susceptible to permeability evolution. 6. ACKNOWLEDGEMENTS The authors would like to thank Dr. Antonio Lanzirotti, William Rao and Dr. Ryan Tappero for operating the x-ray microprobe beamlines at the NSLS. Brookhaven National Lab and the NSLS operate under U.S. Department of Energy Contract No. DE-AC02-98CH This work was sponsored by NSF grant number CBET REFERENCES 1. Shukla, R., et al., A review of studies on CO 2 sequestration and caprock integrity. Fuel, (10): p Detwiler, R.L. and H. Rajaram, Predicting dissolution patterns in variable aperture fractures: Evaluation of an enhanced depthaveraged computational model. Water Resources Research, (4). 3. Detwiler, R.L., Experimental observations of deformation caused by mineral dissolution in variable-aperture fractures. Journal of Geophysical Research-Solid Earth, (B8). 4. Ellis, B.R., et al., Deterioration of a fractured carbonate caprock exposed to CO 2 -acidified brine flow. Greenhouse Gases Science and Technology, (3): p Gouze, P., et al., X-ray tomography characterization of fracture surfaces during dissolution. Geophysical Research Letters, (5). 6. Michael, K., et al., Geological storage of CO 2 in saline aquifers-a review of the experience from existing storage operations. International Journal of Greenhouse Gas Control, (4): p

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