Changes in Pore Structure and Connectivity Induced by CO2 ...

Energy Procedia 37 ( 2013 ) 5367 – 5378

1876-6102 © 2013 The Authors. Published by Elsevier Ltd.Selection and/or peer-review under responsibility of GHGTdoi: 10.1016/j.egypro.2013.06.455

GHGT-11

Changes in Pore Structure and Connectivity Induced by CO2Injection in Carbonates: a Combined Pore-Scale Approach.

Oussama Gharbia *, Branko Bijeljica, Edo Boekbk , Martin J. Blunta

aDepartement of Earth Science and Engineering, Imperial College, London SW7 2AZ UKa

bDepartement of Chemical Engineering, Imperial College, London SW7 2AZ UKb

Abstract

We investigate at the pore scale fluid-rock interactions that occur in the context of carbon dioxide (CO2) storage in saline carbonate aquifers. Brine saturated with super critical CO2 is injected into two carbonatesamples (Estaillades limestone and an aquifer sample) at typical storage conditions (9MPa and 50oC). Dryhigh resolution micro-computed tomography scans are obtained prior to and after the experiments and thepore structure, connectivity and computed flow fields are compared using image analysis and pore-scalemodeling techniques. We perform direct simulations of transport properties and velocity fields on the 3Dscans and we extract representative pore-throat networks to compute average coordination number andassess changes in pore and throat size distributions. In this study, we experimentally mimic near wellbore region conditions by injecting fluids at relatively high flow rates. For high Péclet and Damk hlernumbers, experimental observations confirm the formation of highly conductive channels i.e wormholes.A significant increase in porosity and permeability is found with fewer pore and throats after dissolution while the average coordination number does not change significantly. Flow becomes concentrated in thewormhole regions after reactions although a very wide range of velocities is still observed.© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: reactive transport; carbonates; pore scale; tomograpghy; wormholes

1. IntroductionIt is well established that more than half of the hydrocarbon reserves are contained in

carbonate reservoirs [1]. Therefore, carbonate formations such as depleted oil and gas reservoirs and deepsaline aquifers potentially represent huge storage volumes for Carbon and Capture Storage (CCS)technology. However, due to complex pore structures, micro-porosities, wide heterogeneities at different scales, and high reactivity, carbonate porous media remain very challenging to characterize [2-4]. Prior to

* Corresponding author. Tel.: +44 (0) 207 594 7332E-mail address: [email protected].

Available online at www.sciencedirect.com

© 2013 The Authors. Published by Elsevier Ltd.Selection and/or peer-review under responsibility of GHGT

5368 Oussama Gharbi et al. / Energy Procedia 37 ( 2013 ) 5367 – 5378

wide-scale implementation of CCS in carbonates, it is vital to investigate experimentally and numerically different phenomena that occur during carbon dioxide (CO2) injection. In this context, pore-scale experimental and numerical studies can add valuable in some cases predictive - information regarding the physics of single and multi-phase flow and transport properties [5-8].

During CO2 injection in deep saline carbonate aquifers, a series of physical and chemical phenomena

take place. At typical storage conditions the CO2 is injected at depths below 800m in a supercritical phase (P> 7.39MPa and T> 31.04oC) [9-11]. First, a portion of the injected supercritical-CO2 (sc-CO2) dissolves in the saline brine (solubility trapping) and a plume of the CO2-saturated/enriched brine simultaneously flows along with the sc-CO2 phase throughout the aquifer [12]. However, due to the formation and dissociation of the formed carbonic acid (see reactions 1-4 below), the pH decreases; consequently, the host formation is exposed to dynamic fluid-rock interactions where dissolution or precipitation may occur. Dissolution of the host formation can lead to an increase in porosity possibly increasing injectivity and storage capacity; however this dissolution could compromise the integrity of any seals, allowing the CO2 to escape. On the other hand, precipitation of carbonate can effectively transform the CO2 into solid minerals contributing to another important trapping mechanism (mineral trapping) [13, 14]. The reaction kinetics is strongly dependent on the pH, salinity, temperature and sc-CO2 partial pressures [15]. Characterizing transport properties and reaction regimes that are caused by the plume migration is a reactive transport problem where, advection is coupled with diffusion and chemical reaction [16]. This is a challenging phenomenon to describe since reaction rates and transport properties vary over different time and spatial scales [17-20]. This complexity is exacerbated in carbonates where complex pore structures, wide heterogeneities and non-uniform mineralogical compositions are present. In this paper we start at the pore scale to obtain an understanding of dissolution processes.

Several experimental investigations have reported calcite dissolution reaction rates using high pressure

and temperature CO2 batch reactors and rotating disks [21-26]. However, only a few experimental studies were conducted at the pore scale in order to mimic reactive transport in the context of CO2 injection in carbonate rocks. Noiriel et al. (2008) investigated the impact of CO2 injection on reactive surface area, permeability and porosity changes on cylindrical limestone samples (9mm diameter and 15mm length) that were mainly composed of micrite and sparite [27]. The injections were performed at ambient temperatures (T=20oC) and pressures below the supercritical threshold. Micro-computed tomography and chemical analysis of effluent concentrations showed a non-uniform increase of porosity, connectivity and reactive surface area induced by dissolution. Luquot and Gouze (2009) reported experimental results at typical storage conditions (100oC and 12MPa) where sc-CO2 was used [28]. A set of four core flooding experiments with varying sc-CO2 partial pressures were performed on cylindrical carbonate samples (9mm diameter and 18mm length). Results showed the changes in permeability and porosity induced by dissolution and precipitation. The measured experimental porosity evolution with time was matched by a simplified form of the Bleasdale-Nelder equation.

The aim of this paper is to address fluid-rock interactions induced by CO2 injection at typical storage

conditions by combining experimental core flooding, with high resolution micro-computed tomography and pore-scale modeling calculations. First, we briefly present the chemical reactions and describe the dissolution regimes. Then, the experimental apparatus and methodology will be presented. Subsequently, first imaging and modeling results will be discussed. Finally, we will highlight future work directions.

Oussama Gharbi et al. / Energy Procedia 37 ( 2013 ) 5367 – 5378 5369

2. Theory and experimental methodology 2.1 Chemical reactions and reactive transport theory In a CO2(g) H2O calcite (CaCO3: calcium carbonate) system, a series of chemical reactions occur [29]: CO2 (g) 2 (aq) (1) CO2 (aq) + H2 2CO3 (2) H2CO3

+ + HCO32- (3)

HCO3- 3

2- (4) CaCO3 + H+ 2+ + HCO3

- (5) CaCO3 + H2CO3 2+ + 2HCO3

- (6) CaCO3 2+ + CO3

2- (7) In reactive transport problems, calcite dissolution chemical reactions (Equations 5, 6 and 7) are coupled with transport (advection and diffusion) of reactants and products. The understanding and modeling of reactive transport in general and wormholing phenomenon in particular has been widely investigated in the context of acid stimulation in carbonate oilfields. Calcite dissolution patterns have been experimentally shown to depend on two dimensionless numbers namely the Péclet and Damk hler numbers (and/or the Péclet-Damk hler number). The Péclet number is defined as the ratio of advection rate to diffusion rate. Pe = ul /D, where u (m.s-1) is the interstitial velocity (the Darcy velocity v divided by the porosity), l (m) is a typical length scale (a representative pore size) and D (m2.s-1) is the molecular diffusion coefficient. The Damk hler number is here defined as the ratio of the reaction rate to the advection rate: Da = kl /u.[30] where k is the reaction rate (s-1). In our system, the Péclet number is equal to 3.5 (Estaillades) and 4.7 (aquifer sample). The diffusion coefficient for Ca2+ cations is D = 7.5 x 10-10 m2.s-1 [28]. For the typical length scale, we use the average pore radius which is obtained from the network extraction. Damk hler number was estimated from Luquot and Gouze (2009) where, for similar experimental conditions measured Damk hler numbers ranged from 1 to 6 [28].

Figure 1 Schematic diagram representing the dependence of dissolution regimes and patterns on Péclet (Pe) and Damkh ler (Da) numbers obtained from Golfier et al. [31]. Note that the transition values were computed for a particular system and specific reactants. Changes in the system and chemical composition will shift the transitions values. The blue point is an indication of the area investigated in this study.


2.2 Experimental methodology Injection of single aqueous phase brine saturated with sc-CO2 was performed at 50oC and 9MPa using a specially built core flood apparatus which is shown Figure 2. An equilibration reactor (1000 mL C276 autoclave with a gas entrainment stirrer Parr instruments CO., IL, USA) was used to generate equilibrium between the brine and sc-CO2 phases and produce the reactant fluid - brine saturated with sc-CO2. High precision syringe pumps (Teledyne ISCO 1000D and 100D, Lincoln, NE, USA) were used to maintain pressure and inject fluids at constant pressures and flow rates. The temperature of the system was controlled at the target value using water heating jackets for the pumps and insulation throughout the lines and valves. In addition, an electrical oven was used to accommodate the flow cell and maintain the temperature. The brine composition used was 5wt. % NaCl and 1wt. % KCl. The cylindrical samples have a diameter of 5mm and a length of 20mm. Injections of brine saturated with sc-CO2 were performed at a constant flow rate Q = 1.667 x 10-9 m3.s-1 (v=8.49 x 10-5 m.s-1). This is quite high flow rate (around 10m/day) representing the near well-bore region.

Figure 2 High pressure and temperature reactive transport experimental apparatus. The apparatus consisted of five precision pumps, a high pressure and temperature mixing unit used to generated sc-CO2 enriched brines, an oven and a specific flow cell that allow the injections at pressures and temperatures up to 20MPa and 800C. Sampling vessels are used to collect effluent samples and monitor changes in chemical concentrations.

The core samples were drilled in a way that allows the bedding lines to be perpendicular to the length of the core. The samples were wrapped with aluminum and were contained in Vitton sleeves. The confining pressure applied here was constant and equal to 110% of the injection pressure i.e. 10 MPa. In a reactive transport type core flood it is important to control the radial confining pressure in order to maintain the


core integrity. Table 1 summarizes the petro-physical and mineralogical compositions of the dry native samples note that permeability and porosity were measured on larger cores drilled from the same block of stone (37.84 mm diameter and 76.49mm length for Estaillades and 25mm diameter and 39.2mm length for the aquifer sample). Mineralogical composition was obtained from X-Ray Diffraction (XRD) measurements (Weatherford laboratories) on grains obtained from the same samples.

Sample

Porosity Permeability

XRD- Mineralogical composition

(%) (mD) (%)

Estaillades 29.5 151 Calcite: 97.9 Quartz: 2.1

Aquifer sample 28 217 Calcite: 97.2 Quartz: 2.8

Table 1 Experimental measurements of porosity and permeability and mineralogical characterizations for the two samples studied: Estaillades limestone and the aquifer sample.

Estailades is a quarry limestone with heterogonous pore size. The aquifer sample is a carbonate obtained from a deep saline aquifer in the Middle East. In brief, the main steps of the experimental procedures are: 1- The system (including the core holder and sample) is set under vacuum for 24 hours. 2- Carbonate pre-equilibrated brine is injected to saturate the core. More than 1,000 pore volumes (PV) are injected. 3- Brine saturated with CO2 was prepared using the high pressure and temperature reactor. The same experimental procedure as El Maghraby et al. (2012) is used [32]. 4- Brine saturated with sc CO2 is injected through the core at constant flow rate, (100±15) PV are injected. 5- Clean the samples by flushing out the liquids using a solvent (iso-propanol) and allow the samples to dry using a vacuum oven.


2.2.2 Micro-Computed Tomography using a Synchrotron Dry scans of the selected carbonate samples (prior to and after the core flooding) were made at the SYRMEP (Synchrotron Radiation for Medical Physics) beamline at the Elettra synchrotron in Trieste, Italy [33]. The images were obtained using energies between 27-30 keV. A detector of 3.85 μm was used and the images were recorded with a CCD camera located at a distance of 50 cm from the sample. The CCD camera binned the results, giving a final voxel size 7.7 μm. 3. Results 3.1 Image Analysis At the target flow rate, dissolution is not uniform and as expected wormholes are observed in the scans. During approximately the first 300 -350μm, face dissolution is observed where the rock is subject to significant reaction. Then the wormhole forms and propagates - in most cases around the center of the sample and through the most permeable pores. The length of the wormhole varies with the sample. Therefore, during the segmentation, only the region corresponding to the wormhole propagations were selected the face dissolution region (300-350 μm) was ignored. Figure 3 represents the methodology that was used to select the 3D areas for image analysis, network extraction and direct computations.

Figure 3 Example schematic of the cropping methodology for the aquifer sample. Important face dissolution occurs at the inlet of the sample. The remaining solid structure is cropped and segmented into a 3D binary representation of the pore space. Blue indicates the pore space and green represents the solid.

Figure 4 shows 2D sections of 3D micro-computed tomography scans of the Estaillades carbonate prior to and after injection of brine saturated with sc-CO2. Here, the same 2D slice is compared. The propagation of the wormhole is clearly visible. In order to better compare the pore space, the cropped segmented images of the same slices are shown in Figure 5. In addition to the wormhole propagation along the sample, we notice the increase of porosity in the regions away from the wormholes. However, this is a local effect which is observed on a single 2D slice - to assess the impact of dissolution on overall pore space connectivity, representative pore-throat networks will be extracted in order to compute the coordination number, and this will be discussed in the following sections.


Figure 4 2D images of 3D micro-computed tomography scans of Estaillades prior to (left) and after (right) reaction. The same physical location is shown in both cases. Images are representing the same slices. The presence of the wormhole can be clearly seen in the image to the right.

Figure 5 Binary 2D images of Estaillades limestone prior to (a) and after (b)reaction. The pore space is represented in black and the solid in white. The size of the images is: 429 x 365 pixels ant the resolution is 7.7μm. The generation of new connections in the regions away from the wormwholes are highlighted by the red boxes.

3.2 Connectivity analysis 3D scans representing the same physical volume were processed i.e. cropped and segmented in order to generate 3D binary images of the pore space. Topologically representative pore and throat networks were extracted in order to assess the impact of reaction on the pore structure. The maximal ball algorithm developed by Dong and Blunt (2009) was used to extract the networks [34]. A summary of the network properties is presented in Table 2. Overall, the number of pores and throats decreases as a result of the dissolution process: individual pores and throats are merged to form the wormhole and this effect dominates over the formation of new connections in the rest of the pore space. However, the average coordination number (average number of throats connected to a single pore) does not change significantly. The maximum coordination number increases dramatically since this represents the wormhole. Figure 6 shows the comparison of coordination number distributions for native and altered cases and both samples.


Number of

pores Number of

throats

Average coordination

number

Standard deviation coordination

number Maximum

coordination number Estaillades native 8649 13053 2.98 2.81 37 Estaillades altered 5853 8084 2.72 3.36 141 Aquifer sample native 7564 14328 3.74 3.82 46 Aquifer sample altered 5092 8763 3.39 4.44 112

Table 2 Description of the extracted networks.

Figure 6 Distribution of coordination numbers for the native (continous line) and the altered (dashed line) cases. Coordination number distribution does not change significantly, although the maximum representing the wormhole is much larger in the altered samples.

Figure 7 shows a comparison of the pore and throat size distributions of the native and altered networks. The shape of the distributions remains very similar; however fewer pores and throats are present in the altered networks. The shift in the distributions is more pronounced in Estaillaldes with a greater fraction of larger pores in the altered sample.


Figure 7 Pore and throat size distributions of the native (continuous line) and altered carbonate (dashed line) samples.

3.2 Transport properties We simulate incompressible viscous steady state flow directly on the pore space images by solving the volume conservation and momentum balance equations. Figure 8 shows the normalised velocity distribution computed at the centre of each void voxel in the image - for the native and altered Estiallades limestone. In both cases a wide range of velocities is observed. However, in the altered case, the flow is more concentrated in the wormhole regions. Non uniform dissolution leads to a sharpening of velocities within the large channels.

Figure 8 Normalised velocity distributions for native (solids line) and altered (dashed line) Estaillades limestone. The vertical axis represents the probability density function of (u/uav) where u is the local velocity magnitude at the centre of a single pore voxel and uav is the image average pore velocity.

00.10.20.30.40.50.60.70.80.9

1

1.0E-10 1.0E-08 1.0E-06 1.0E-04 1.0E-02 1.0E+00 1.0E+02

PDF

( u

/ uav

)

Normalised velocity u / uav


4. Conclusions and future work A specific experimental apparatus was used to mimic at the pore scale dynamic fluid-rock interactions that occur near the well-bore during CO2 injection in deep saline carbonate aquifers. The samples were imaged at a resolution of around 8μm before and after reaction and the results were analysed using network modelling and direct computation of the flow field in the pore space. At relatively high Péclet and Damk hler numbers, we noticed the formation and propagation of wormholes along the samples. The generation of new connectivity in the regions away from the wormhole is evident, although the total number of pores and throats as connections merge, while the distribution of coordination number remains largely unaffected. The flow field becomes more concentrated in the wormhole, but a very wide range distribution of local velocities spanning more than eight orders of magnitude is predicted. Future work will investigate dissolution regimes at lower Damk hler numbers and will will predict transport properties. 5. Acknowledgments We would like to acknowledge funding from the Qatar Carbonates and Carbon Storage Research Centre, QCCSRC, which is supported jointly by Qatar Petroleum, Shell and Qatar Science & Technology Park.


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