Mechano-chemical interactions in sedimentary rocks in the context of CO 2 storage: Weak acid, weak effects? J. Rohmer, A. Pluymakers, F. Renard PII: DOI: Reference:
S0012-8252(16)30060-5 doi: 10.1016/j.earscirev.2016.03.009 EARTH 2242
To appear in:
Earth Science Reviews
Received date: Revised date: Accepted date:
19 October 2015 29 February 2016 28 March 2016
Please cite this article as: Rohmer, J., Pluymakers, A., Renard, F., Mechano-chemical interactions in sedimentary rocks in the context of CO2 storage: Weak acid, weak effects?, Earth Science Reviews (2016), doi: 10.1016/j.earscirev.2016.03.009
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ACCEPTED MANUSCRIPT Mechano-chemical interactions in sedimentary rocks in the
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context of CO2 storage: weak acid, weak effects?
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Rohmer1,*, J., Pluymakers2, A., and Renard2,3, F.
BRGM, 3 avenue C. Guillemin, B.P. 36009, 45060 Orléans Cedex 2, France
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PGP, Department of Geosciences and Institute of Physics, University of Oslo, Postboks 1048
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ISTerre, Université Grenoble Alpes, CS 40700, 38058 Grenoble cedex 9, France
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Blindern, 0316 Oslo, Norway
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Abstract
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*Correspondence to: J. Rohmer (
[email protected]), Tel: (+33) 2 38 64 30 92
Due to the corrosive nature of dissolved CO2, the potential short or long term alteration of rock properties, represents a major issue in several sites where natural CO2 circulation is observed, as well as in reservoirs targeted for storage of anthropogenic CO2. To date, this has been primarily studied from a transport-chemical perspective, with laboratory evidence of microstructural modifications together with the consequences for flow properties. Compared to the transport-chemical aspects, the mechanical-chemical aspects have been less investigated, though it is to be expected that mechanical properties (e.g. elastic properties, failure parameters, and time-dependent mechanical behaviour) could potentially be affected in a similar manner to hydraulic parameters. Yet, since CO2 is a weak acid, the pH drop is 1
ACCEPTED MANUSCRIPT expected to be moderate with a likely lower limit close to 4.0. The buffering of pH by calcite minerals present in most reservoirs targeted for storage may further limit the pH drop, as well
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as confining it to a localized rock volume around the injection well. This leads to the question
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of the magnitude and time/spatial scales of chemically-mediated mechanical processes during
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CO2 sequestration. The authors propose to address this issue by reviewing recent laboratorybased studies restricted to sedimentary rocks, namely: reservoir rocks (carbonate or sandstone), intact or fractured caprocks and fault rocks. Key findings include the following: 1.
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the short-term impact on the elastic and inelastic behavior of intact caprocks remains limited;
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2. shear strength weakening is likely to be respectively low and low-to-moderate for shale/clay-rich and anhydrite-rich faults, but without modifying slip stability in either case; 3. the largest impact is located within carbonate reservoirs, but with a broad range of reported
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responses depending on hydrodynamic conditions (closed or open) and on dissolution regime (uniform or channelling); 4. creep experiments confirm that CO2-induced dissolution may
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enhance long-term compaction of carbonate reservoirs, but the magnitude of acceleration (varying from non-significant to 50 times) depends to a large extent on site-specific
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conditions (grain size, pH, temperature, effective stress state, etc.), which renders any direct extrapolation from laboratory to reservoir scale difficult. Finally, some directions for future research studies are discussed.
Keywords: CO2 sequestration; Rock deformation; Weakening; Acid attack; Short and Longterm; Laboratory experiments
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ACCEPTED MANUSCRIPT 1. Introduction CO2 capture and geological storage is seen as a promising technology in the portfolio of
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measures required to mitigate the effects of anthropogenic greenhouse gas emissions (as originally identified by Benson and Cook, 2005). Suitable geological targets, which present
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sufficient capacity and injectivity, mainly correspond to deep sedimentary formations, including oil and gas depleted reservoirs and deep saline aquifers (Bachu, 2008). Yet, a
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prerequisite for the large-scale industrial development of Carbon Capture and Storage (CCS) is the demonstration by the operators that the containment is effective and that the storage
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(aka sequestration) is safe in the long term (e.g., Bouc et al., 2009) so that leakage does not exceed 1% of the stored amount of CO2 in 1,000 years (Hepple and Benson, 2005).
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Compared to other engineered geological storage facilities (e.g. natural gas, liquid waste or
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nuclear solid wastes), one major difference is that the injected CO2 is prone to dissolve within
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the resident reservoir pore fluid. Although pure dry CO2 has low reactivity, once it comes into contact with brine, it forms H2CO3, a weak acid that will almost immediately dissociate (e.g.,
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Gaus et al., 2008):
(1)
This will cause an imminent drop in pH of the brine as shown by experimental studies (e.g., Rosenbauer et al., 2005), simulation-based investigations, (e.g. André et al., 2007), and field studies, (e.g. Kharaka et al., 2006), reducing the pH of the formation from near neutral values, to acid pH in the range 4-5. This also causes acidic reactions with the minerals of the different rock materials composing the “storage complex” as defined in the Guidance Document for Geological Storage by the European Communities (2011). This complex encompasses the reservoir host rock, the caprock formation, the operational well, and also potential leakage 3
ACCEPTED MANUSCRIPT pathways like existing faults, fractured zones within the caprocks or abandoned wells (see
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Figure 1a for an overview).
Figure 1: a) Schematic overview of the storage complex with different zones around the injection well depending on the spatial distribution of CO2 saturation Sg. b) Experimental conditions and rock types used in the laboratory studies indicated in Table 1.
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ACCEPTED MANUSCRIPT CO2-fluid-mineral interactions were thoroughly studied; see for instance the reviews by Gaus (2010), for a comprehensive overview, by Czernichowski-Lauriol et al. (1996), for host
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reservoirs, by Zhang and Bachu (2011), for wells, and by Song and Zhang (2013), for
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caprocks. Of the different common rock-forming minerals, calcite is frequently expected to be
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the most reactive, because of both high solubility and kinetics rates; see the studies on calcite solubility and on dissolution rates by Plummer et al. (1978) and by Pokrovsky et al. (2005).
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One major challenge for the storage performance assessment is CO2-induced alteration of rock properties, whether in the short term (during the injection period, i.e. over 25-50 years)
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or long term (during the storage phase with time scales over 100 years, up to 10 kyrs). To date, this has mainly been tackled from a transport-chemical perspective. Experimental
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studies at laboratory scale have outlined the CO2-induced microstructure modifications of the
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porous medium, (e.g. Lamy-Chappuis et al., 2014; Noiriel et al., 2004) or of fractures (e.g., Noiriel et al., 2013) and the consequences on macroscopic parameters, for example hydraulic
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(e.g., Smith et al., 2013; Carroll et al., 2013; Nover et al., 2013; Canal et al., 2014; Luquot and Gouze, 2009) or multiphase flow properties (see Chiquet et al., 2007 for CO2/water interfacial
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tensions).
Similarly to hydraulic parameters, mechanical properties are also expected to be affected by CO2-related mineral dissolution such as elastic properties, failure parameters or timedependent mechanical behaviour (e.g., Vialle and Vanorio, 2011; Le Guen et al., 2007; Grgic, 2011). These dissolution-induced changes may have different implications. Acid-induced mechanical degradation is considered to be beneficial when related to reservoir injectivity enhancement: reservoir stimulation using acid injection is a commonly practised technique in this field (Cohen et al., 2008). On the other hand, reservoir degradation may cause acceleration of reservoir compaction, potentially leading to large surface subsidence due to a
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ACCEPTED MANUSCRIPT significant alteration in stiffness (Wojtacki et al. 2015) or to caprock failure through bending (Kim and Santamarina, 2014). If dissolution-induced strength weakening affects caprock
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formations, the creation or reactivation of fractures may be expected (Rutqvist, 2012) with a
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potential for induced seismicity (Zoback and Gorelick, 2012), especially when faults are
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impacted (Pluymakers et al., 2014b; Samuelson and Spiers, 2012).
Though CO2 in contact with brine rapidly causes a pH drop (Eq. 1), interactions with
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carbonate minerals (expected to be present in most rock formations targeted for CO2 storage
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applications) will buffer the pH and ultimately make the brine less acidic (Gaus et al., 2008):
(2)
In the presence of carbonate minerals the acid attack is thus expected to be limited, especially
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compared to the effect that can be induced by a stronger acid such as ammonium nitrate solution. This was used to test a worst-case scenario in the study by Xie et al. (2011) for
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limestone. Moreover, it should be underlined that a large number of field tests involving CO2 flooding operations have been conducted in carbonate hydrocarbon reservoirs (e.g. Ambrose
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et al., 2008) and to date, no major adverse effects have been observed at such sites, as reported for instance by Liteanu and Spiers (2009). This raises questions concerning the magnitude and time/spatial scales of chemicallymediated mechanical processes during CO2 sequestration. More specifically, the following questions arise: Where do the effects primarily occur? Since the main evidence of such effects is of an experimental and/or a modelling nature, how can such effects be extrapolated to reservoir conditions? In other words, what are the in-situ conditions (for instance hydrodynamic conditions, rock microstructure, presence of impurities, etc.) that have the largest influence on the magnitude and extent of such processes?
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ACCEPTED MANUSCRIPT These questions are addressed below via a review of laboratory experimental studies performed in the past ten years related to the interplay between chemical and mechanical
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processes in the context of CO2 storage. Section 2 describes the experimental methods used to
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investigate these issues. The short- and long-term impact of CO2 on the mechanical behaviour
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of reservoir rocks (carbonates or sandstones, Section 3) is then addressed, followed by the mechanical degradation of intact or fractured caprocks (Section 4) and fault rocks in both the
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short and long term (Section 5). In the conclusion, future research directions are highlighted.
2. Experimental methods
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A large set of laboratory experiments has been developed to study the coupling between the injection of CO2 into a rock and the subsequent change in its petrophysical and mechanical
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properties (Figure 2). Table 1 gives a summary of the experimental conditions and main
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results of forty representative laboratory studies. The rock types and their location in the CO2
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storage complex are indicated on Figure 1b. Two main types of experiments were performed: 1) under static or low fluid flow rate conditions, which characterize the slow evolution of the rock; 2) under high fluid flow rate, greater than several hundred pore volumes, to study
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dissolution under acidic conditions.
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[See attached excel document]
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Table 1: List of representative laboratory studies of rock deformation in the context of CO2 geological sequestration. The numbers in the last column correspond to those on
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Figure 1b.
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ACCEPTED MANUSCRIPT The range of thermodynamic conditions explored in these experiments varies between shallow surface, with atmospheric fluid pressure, to reservoir conditions where the state of
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stress and the temperature can reach those prevailing at a depth of up to five kilometres depth.
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The duration is also a key factor: some experiments reproduce fast processes occurring mainly
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near the injection well. Other experiments have focused on the long-term effects of CO2 on the change in deformation and permeability of the samples. Various rock types were also
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used: those for which the interaction with CO2 may lead to enhanced fluid-rock reactions such as carbonates, and those for which the acidity of the injected fluid is expected to have only
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minor effects (anhydrite, clay-rich caprocks, and sandstone). Often, the chemistry of the fluid at the outlet is analysed and used for mass balance estimations.
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When considering the coupling between chemical and mechanical effects on rocks as a result
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of CO2 injection, two main types of studies were performed. On one hand, high injection rate
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experiments using acidic fluids show the development of wormholes in carbonate rocks, with alteration of the mechanical properties (yield stress, elastic parameters) accompanying the increase of porosity. On the other hand, slow injection rates were used in short- or long-term
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experiments to characterize how the creep and/or frictional properties of the rock are modified by CO2-rich fluids.
Finally, three types of samples were used: rock core samples to study the reservoir and its surroundings, aggregates to study either compaction of granular materials or fault gouge friction, and artificially pre-cracked core samples to study flow in fractures. Usually, the measured effect of CO2 is greater for the last two types of samples because of flow focusing and/or higher reactive surface area. In many of these experiments, tests start under dry CO2 injection conditions (gaseous or supercritical) to show that the effect of dry CO2 on deformation is negligible, usually below 9
ACCEPTED MANUSCRIPT the limit of resolution of the strain measurement device. Then, aqueous fluids – distilled water or brine – with controlled pH are injected and the deformation is measured. Finally, CO2-rich
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aqueous fluids are injected to characterize the effect of fluid acidification on rock
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deformation. Note that in many studies, the mechanical effect of CO2, if any, is also at the
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resolution limit of the strain measurement device. A few studies were performed with controlled partial saturation of CO2 (e.g. Sg less than 1 in Figure 1), whereas a greater number of experiments were performed by injecting aqueous fluids saturated with dissolved CO2. The
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sections below describe the main findings of the effect of CO2 on the mechanical properties of
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rocks in geological reservoirs, caprocks and fault zones, when using several kinds of experimental setups (Figure 2). Various kinds of experimental apparatuses were used and fall in three classes: those for which a fluid is left in contact or percolated into a sample (core or
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powder) without stress, those for which a triaxial stress is applied to the sample during percolation, and those for which a shear stress is applied on a sample to study its frictional
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properties. These three kinds of experiments are described in the Figure 2.
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Figure 2: Three kinds of experimental apparatus to study the effect of CO2 in rocks under various thermodynamic and loading conditions. a) Direct shear experiment where powders are sheared and slip under various conditions of stress, slip velocity, and fluid composition. In these experiments, the friction coefficient of these gouge-like samples is measured (after Pluymakers et al., 2014; Hangx et al. 2010c). b) Triaxial rig 11
ACCEPTED MANUSCRIPT experiments. A core sample is loaded and heated under thermodynamic conditions relevant to CO2 storage at depth. Fluids of various compositions can be injected. The
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short- or long-term mechanical response of the sample is then measured (after Le Guen
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et al., 2007). c) Flow-through experiments where a core sample or a powder is flushed
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with fluids under various conditions (after Noiriel et al., 2013). Temperature and fluid pressure are controlled. No axial load is applied, though, under some conditions, a
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confining pressure may be applied.
3. Mechano-chemical effects of CO2 on reservoir rocks
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Studies relying on numerical simulations of CO2 injection in reservoirs have shown that the volume where CO2-induced mineral reactions occur around the injection well can be divided
Zone I: The volume near the wellbore is expected to be (quasi-) fully saturated by
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-
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and as also indicated on Figure 1:
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into different zones depending on the gas saturation spatial distribution (André et al., 2007),
supercritical CO2, in which the drying effect may lead to salt precipitation; Zone II: A transition zone with the presence of a two-phase mixture of supercritical
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CO2 and brine with expected buffered pH as outlined by Gaus (2010); -
Zone III: A zone that is fully saturated with an acidified aqueous CO2 solution;
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Zone IV: The zone furthest from the wellbore is considered unaffected.
The CO2-enhanced dissolution/precipitation phenomena primarily occur in the two intermediate zones, zone II and III. Depending on the injection rates, dissolution kinetics and rock heterogeneity, localized dissolution patterns may initiate in these zones. This may lead to wormhole growth, especially in carbonate-rich rocks (Zinsmeister et al., 2013; Vialle et al., 2014). The short- and long-term impact of CO2 on rock mechanical properties was 12
ACCEPTED MANUSCRIPT investigated by experiments under static or low fluid flow rate conditions (Section 3.1), under a fluid flow rate with or without mechanical stresses (Section 3.2), and by compaction
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experiments specifically dedicated to long-term mechanical-chemical processes (Section 3.3).
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3.1 Experiments under static conditions
Reservoir rock formations targeted for hosting CO2 are mostly sandstones and carbonates, including chalk. Since carbonates are expected to be the most reactive to CO2, they were
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primarily investigated. For instance, Sterpenich et al. (2009) studied wet limestone samples of
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Lavoux formation exposed to supercritical CO2 and a saline aqueous solution at 80°C and 15 MPa fluid pressure, i.e. representative of conditions targeted for possible storage sites. This
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showed a limited dissolution of calcite of less than 1% in mass with only a slight change in
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microstructure, which resulted in a non-significant variation in porosity without any changes in velocity of ultrasonic waves, i.e. of the dynamic elastic moduli. They explained this minor
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effect by the opposing effects of CO2 and pH: due to the increase in partial pressure of CO2 (pCO2), the solution becomes more acidic due to dissolution of CO2 and dissociation of
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carbonic acid. On the other hand, the thermochemical stability of calcite increases as a result of the higher concentration of dissolved calcium and carbonate species. Moreover, these thermodynamic considerations showed that pH values below 4 cannot be reached for realistic calcium activities in closed systems regardless of the value of CO2 partial pressure. These findings are consistent with the results of Grgic (2011) and Rimmelé et al. (2010). To evaluate what could be the worst-case impact of acid attack on carbonate samples, several studies have applied the protocol described in Egermann et al. (2005) to homogeneously alter rock samples using a retarded acid solution, which is activated under specific temperature conditions (Bemer and Lombard, 2010; Nguyen et al., 2011; Zinsmeister et al., 2013). The
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ACCEPTED MANUSCRIPT alteration procedure includes the following stages: i) flushing of the sample with fresh retarded acid at ambient temperature (using about three times the sample porous volume); ii)
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activation of the acid by increasing temperature; iii) flushing of the sample with fresh brine at
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ambient temperature. The number of successive cycles controls the final degradation level.
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The great advantage of this procedure compared to the flow-through experiments (see Section 3.2) is that it avoids wormholing dissolution that could lead to erroneous measurements at the core scale. In case of heterogeneous dissolution, the samples cannot be considered as
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representative elementary volumes, see e.g. Egermann et al. (2005).
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Bemer and Lombard (2010) analysed the alteration of elastic moduli and strength parameters of a wackestone (matrix-supported carbonate rock that contains over 10% allochems in a
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carbonate mud matrix, based on Dunham’s classification (Dunham, 1962)) from the
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Comblanchian Formation, and of a packstone (grain-supported carbonate rocks that contain no micrite with spaces between grains filled with sparite cement) from the Lavoux Formation,
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both from the Paris basin. These petrophysical properties were measured on pristine core
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samples and on samples altered in the presence of CO2.
Figure 3: Trends of drained elastic moduli as a function of porosity for pristine and CO2-altered carbonate core samples (adapted from Bemer & Lombard, 2010). 14
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The results showed a change in porosity between 1.0 and 2.1% (porosity unit), though without
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any clear trend on permeability. Despite these small porosity changes, both carbonate systems
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are weakened, as evidenced by a decrease in the strength value and the elastic moduli (Figure
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3). However, Bemer et al. (2004) suggested that the change in elastic moduli with porosity cannot be explained by using empirical relationships fitted to natural samples data. The altered stiffness values of the Lavoux packstone remained in the range of values given by the
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empirically established trend, whereas the elastic moduli of the altered Comblanchian
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wackestones fell outside the allowed range, which indicated a higher sensitivity of this rock to chemical effects (Figure 3). Bemer and Lombard (2010) proposed a microstructural interpretation related to the differences in structure and mineralogy between the two
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carbonates studied. It suggests that more advanced models accounting for a full microstructural description with details other than only a single total porosity value (e.g.
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relative proportion of solid, distribution of calcite clasts, and different porosity distributions within the porous calcitic cement) are required to represent the potential for chemical
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alteration.
This line was further explored by Nguyen et al. (2011), who applied the same alteration cycle to Euville limestone, which yielded experimental results consistent with the experiments of Bemer and Lombard (2010). To obtain a better insight into the understanding of the weakening process, Nguyen et al. (2011) developed a micromechanical model, where the carbonate rock is viewed as a disordered assemblage of macropores and grains. The grains are made up of a calcite core surrounded by a cement layer. The cement is considered to be a medium made up of solid material and micropores. The model accounted for two categories of pores; intergranular pores (macroporosity) and microporosity, mainly present in the calcite core and in the cement. This model showed that the cement layer had a significant influence 15
ACCEPTED MANUSCRIPT on the effective mechanical behaviour. More specifically, the relative proportion of porosity increase between macroporosity and microporosity plays a major role. The model allows the
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mechanical weakening to be reproduced, suggesting it overcomes the limitations outlined by
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Bemer and Lombard (2010), but additional data, especially at the microscopic scale, are still
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required to validate the model assumptions.
Regarding the characterization procedures, limestone investigations were improved by
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Zinsmeister et al. (2013), who investigated the micro-mechanisms of strain localization at different levels of alteration, through a combination of standard triaxial compression tests
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with 2D and 3D digital image correlation techniques together with X-ray micro-computed tomography. They showed that the retarded-acid-based procedure leads to a porosity increase
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that is on average spatially homogeneous and linear. Locally, however, the alteration pattern
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is heterogeneous and controlled by the nature of the grains. They also showed that the failure regime can shift from a brittle fracture to a ductile regime depending on the amount of
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alteration. This finding is consistent with the empirical relationships reported by Wong and Baud (2012), which link the change in compactive yield cap with the porosity level of the
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different types of carbonates (such as wackestone vs. packstone). Wojtacki et al. (2015) further improved the protocol by combining numerical homogenisation techniques and X-ray computed tomography scans at different alteration levels to evaluate the impact on macroscale properties: this allowed highlighting the anisotropic degradation of hydraulic conductivity and of Young’s modulus. Static experiments were also conducted for carbonate-cemented and for silicate-cemented quartzose sandstones. In the case of carbonate-cement, microstructural changes included a slight increase in porosity (a few percent) and in hydraulic conductivity, interpreted to be caused by the relative high reactivity of the carbonate phase (Hangx et al., 2013; Marbler et
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ACCEPTED MANUSCRIPT al., 2013; Nover et al., 2013). Similar increases (but of smaller magnitude) were reported in the case of silicate-cement, although these were mainly due to interactions of CO2 with clay
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minerals (Nover et al., 2013; Rimmelé et al., 2010; Schütt et al., 2005; Zemke et al., 2010).
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Such effects are potentially mitigated by the precipitation of either secondary carbonate
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minerals (Yasuhara et al., 2014) or of salts, which may clog micropores (Marbler et al., 2013; Nover et al., 2013). The geomechanical response of a degraded sandstone is, however, more variable. The static experiments of Nover et al. (2013) and Rimmelé et al. (2010) reported no
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significant changes for silicate-cemented sandstones. In particular, the measured strength
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remained within the range of naturally bleached and unbleached samples (Nover et al., 2013). In contrast, Marbler et al. (2013) and Rathnaweera et al. (2015) showed that exposure to supercritical CO2 led to modified elastic deformation behaviour (linked with the elastic
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modulus) for both carbonate-cemented and silica-cemented sandstones as shown in Figure 4
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and to a lesser extent to reduced strength parameters as shown in Figure 10.
Figure 4: Elastic modulus of deformation (GPa) of different sandstones (silicate and carbonate cemented). This modulus is defined as the slope of the line drawn from the 17
ACCEPTED MANUSCRIPT origin of the stress-strain diagram to the point defined at 30-60% of (’1-’3) at failure. The saturation level as well as the pressure fluid are respectively indicated by “sat.” and
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“pf” (adapted from Table 2 of Marbler et al., 2013).
This alteration appeared to be enhanced by impurities (e.g. SO2, H2S, etc.), which may be present in the injected CO2 (Erickson et al., 2015). However, those effects cannot be
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attributed only to pure chemical effects: variations in elastic and strength parameters are also caused by varying saturation degrees and by the distribution of the different fluids in the pore
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space (as discussed by Lebedev et al. 2013).
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3.2 Flow-through experiments on core samples
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In the light of the limited effect of CO2 observed in batch-reactor experiments, the issue of
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CO2-induced degradation has been addressed more extensively using flow-through experiments. In these experiments the sample is maintained at reservoir pressure and temperature conditions while subjected to flow, either of CO2-saturated reactive brine
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equilibrated with a given value of partial pressure pCO2 or of a two-phase fluid mixture, i.e. liquid brine and supercritical CO2, at different injection rates, e.g. (Carroll et al., 2013; Luquot and Gouze, 2009; Smith et al., 2013). This allows mimicking of the mass transfers within the different zones from the injection well (e.g., André et al., 2007). Many of these systems can also be set under different loading conditions, either isotropic or deviatoric (e.g. Grgic, 2011; Le Guen et al., 2007; Neveux et al., 2014). In general, flow-through experiments result in a greater response, with larger porosity alterations, than batch reactor experiments. Several experiments showed that rock-fluid interactions are highly influenced by the initial microstructure, leading to a heterogeneous dissolution process at the scale of the core sample 18
ACCEPTED MANUSCRIPT (Carroll et al., 2013; Garcia-Rios et al., 2014; Le Guen et al., 2007; Luquot and Gouze, 2009; Smith et al., 2013). Under certain injection conditions, such heterogeneity can promote
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reactive flow focusing and triggering of reaction-infiltration instabilities in the dissolution
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front, which may eventually lead to the creation of wormholes, a process well-known in the
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petroleum industry (e.g. Cohen et al., 2008) and in karst studies (Szymczak and Ladd, 2011). An example of wormholes in a highly heterogeneous vuggy limestone is shown in Figure 5a, and can be compared to the more compact dissolution in a more homogeneous marly
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dolostone in Figure 5b. Such channelling processes may however be hampered during two-
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phase CO2-brine injections since supercritical CO2 preferentially occupies wormhole seeds,
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which prevents their growth (Ott and Oedai, 2015).
Figure 5: Observed void space via X-ray computed micro-tomography analysis after injection of brines equilibrated with pCO2=3MPa at T=60°C for a highly heterogeneous 19
ACCEPTED MANUSCRIPT vuggy limestone (a) and a more homogeneous marly dolostone (b) from the Weyburn-
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Midale field (adapted from Carroll et al. 2013).
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For carbonate rocks with a homogeneous microstructure, such as the Midale Marly dolostone
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of the Weyburn CO2 enhanced-oil-recovery site in Canada (Carroll et al. 2013), the impact on the permeability can be characterized by a power-law relationship linking porosity and
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permeability with an exponent of 3. On the other hand, for more heterogeneous carbonate rocks, like vuggy limestone (Smith et al., 2013), similar experiments are represented by a
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relationship with a higher exponent of the order of 8 to 10 (Smith et al., 2013; Carroll et al., 2013) or even larger with a possible dependence on time (Vialle et al., 2014). From a
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geomechanical perspective, a large increase in permeability is beneficial since it implies lower
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pore pressure changes, hence lower effective stress changes (see e.g., Rohmer and Seyedi, 2010). However, it should be noted that mineral precipitation (mainly Mg-calcite or salts, see
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e.g., Izgec et al., 2008; Luquot and Gouze, 2009) or particle clogging (Luquot et al., 2014; Mangane et al., 2013) may lead to more complex responses to the flow of CO2, and in some
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cases even to a negative correlation between porosity and permeability. Recently, Garing et al. (2015) showed that porosity and permeability were anti-correlated possibly related to porethroat clogging by the accumulation of partially dissolved carbonate particles. The mechanical response of such chemical alterations may be considerable, with a strong dependence on injection conditions and confining pressure. Reported changes in elastic moduli range from 10% to 60% in carbonate/sandstone rocks (Vanorio et al., 2011). This is in agreement with the reported decrease in P- and S-wave velocities of the order of 20-25% as shown in Figure 6 when considering different carbonate samples ranging from calcite limestones containing dolomite to pure calcite mudstones (Vialle and Vanorio, 2011). It is 20
ACCEPTED MANUSCRIPT interesting to note that the larger decrease in velocity occurs at the beginning of the experiment (after the injection of the first pore volumes of reactive fluids), whereas any
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further injection only leads to small-to-moderate changes (Vialle and Vanorio, 2011; Vanorio
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et al., 2011; 2014): this may be explained by the combination of two factors, namely lack of
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material being dissolved and connectivity of the flow paths, as proposed by Vanorio (2015).
Figure 6: Variation in VP- and VS- wave velocities, normalized with respect to their pre‐injection values, as a function of injected pore volume of CO2‐rich water in five different carbonate samples (corresponding to the five different colours of the curves) under dry and fully saturated conditions (adapted from Vialle and Vanorio, 2011).
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ACCEPTED MANUSCRIPT The study by Vialle and Vanorio (2011) clearly underlines the importance of conducting wave velocity measurements under loading, since the final porosity change results from several
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competing processes: dissolution (increased porosity), mechanical removal of particles
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(increased porosity) and mechanical compaction (decreased porosity). The magnitude and
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extent of those processes depend on the microstructure and fabric of the carbonate rock considered. Vanorio et al. (2011) showed that packstones exhibit selective dissolution of micrite (due to its higher surface area), leading to newly formed porosity after injection of
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aqueous CO2 (pH=2.8), but this is counterbalanced by compaction. The effect of micrite
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content on dissolution-induced velocity and elastic moduli changes was investigated in more detail by Husseiny El and Vanorio (2015) using well-controlled microstructures created in the laboratory. Compared to packstones, mudstones showed lower dissolution and compaction
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due to higher pore stiffness and the smaller available surface area that is in contact with the fluid. Despite this, considerable changes in permeability were observed in mudstones by
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Vanorio et al. (2014), which were attributed to enhanced connectivity. A mathematical model was developed and tested against observations by Arson and Vanorio (2015) to explain the
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differences in stiffness changes for both porous micrite and grain-supported carbonates by distinguishing the effects of microporosity and macroporosity. It should be noted, however, that the afore-described results are not fully in agreement with those of Bemer and Lombard (2010), who showed that the tested matrix-supported carbonate rock (wackestone) presents a higher decrease in elastic moduli compared to the one of a grain-supported carbonate rock (grainstone), but the differences may be related to the experimental conditions (use of a retarded acid instead of flushing the rock sample). Due to their high porosity, chalk formations were identified as good candidates for CO2 storage. However, this rock material also has high specific surface area, and hence a high
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ACCEPTED MANUSCRIPT degree of exposure to the fluid and potentially a high reactivity to CO2. Triaxial experiments were conducted with injection of supercritical CO2 into brine-saturated chalk samples (Alam
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et al., 2014; Liteanu et al., 2013; Madland et al., 2006). Only minor mechanical effects (the
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main result being a decrease in stiffness) were reported for short-term exposure (hours) with
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the greatest response being observed for chalk rock material with a small amount of silica and/or clay (Monzurul Alam et al., 2014).
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The study of Xie et al. (2011) can be considered a worst-case scenario: they flushed an oolitic limestone with a strong acid, namely ammonium nitrate solution (6 mol/l), for one year,
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which led to an increase in porosity from 23 to 27%. This was mostly related to degradation of the intergranular cementation, combined with the formation of connected wormholes. They
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showed that the elastic properties are significantly altered as the drained bulk modulus
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decreased from ~5GPa to ~1GPa, whereas the plastic pore collapse threshold was also reduced from ~30 to 20 MPa. This means that the degraded materials become more prone to
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collapse as shown on Figure 7. The impact of such degraded behaviour on reservoir compaction has been investigated from a theoretical standpoint by Stefanou and Sulem
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(2014), who showed that there are specific conditions (more particularly the strong coupling between chemical degradation of the solid skeleton and grain damage) for the triggering of compaction bands in carbonate reservoirs undergoing CO2-induced dissolution.
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Figure 7: Comparisons of pore collapse yield surfaces and shear failure surfaces between unaltered samples (red lines) and degraded samples (black lines). Filled squares
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and circles respectively correspond to failure stresses and to initial yield stress of unaltered samples. Open squares and circles respectively correspond to failure stresses
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and initial yield stress of degraded samples (adapted from Xie et al., 2011).
Finally, tests on sandstones with calcite cement showed that the circulation of fluid leads to calcite dissolution, with an associated increase in porosity and permeability (Lamy-Chappuis et al., 2014; Ross et al., 1982), but with limited effects on the mechanical properties (Canal et al., 2014; Hangx et al., 2013). One author performed flow-through experiments using CO2saturated solution on an anhydrite cemented sandstone, showing anhydrite dissolution and calcite precipitation and an associated increase in permeability, provided that an alkaline source was present (Kühn et al., 2009). Experiments on quartz-cemented sandstones showed
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ACCEPTED MANUSCRIPT no significant alteration, and changes in acoustic properties are attributed to saturation distribution effects (Alemu et al., 2013; Lebedev et al., 2013a). In quartz-cemented
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sandstones, the greatest effects reported are mainly due to salt precipitation (Vanorio et al.,
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2011; Zheng et al., 2015), but these are expected to be very local, i.e. in the vicinity of the
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well injection zone, as in zone I in Figure 1a (André et al., 2007).
3.3 Long-term effects
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Under constant mechanical stress, reservoirs rocks may undergo compaction, which can be
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purely of a mechanical nature due to poro-elastic compression (Lockner, 2002), although it may also involve time-dependent processes (i.e. where the strain depends on stress and time,
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commonly referred to as “compaction creep”). In the long term, such deformation processes
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may induce damage to the different sealing parts of the storage system such as the wellbore, caprock, and the bounding as well as cross-cutting faults. Even under upper crustal pressure
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and temperature conditions, time-dependent compaction can involve an interplay of mechanical and chemical processes including: i) elastic strain due to a modification of the
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mechanical properties of the porous medium, (e.g. Wojtacki et al., 2015); ii) cataclastic flow, such as grain crushing (Zhu et al., 2010); iii) grain failure and debonding by slow subcritical cracking, also called stress corrosion (Atkinson, 1984); and iv) stress-induced dissolution– precipitation processes such as intergranular pressure solution creep (Gratier et al., 2013; Niemeijer et al., 2002; Rutter, 1976). Due to the sensitivity of carbonate rocks to CO2 partial pressure, the question whether increased rates of dissolution (both in pore spaces and grain contacts) will lead to significantly higher strain rates has mostly been addressed experimentally. The laboratory investigations span a wide range of temperature, fluid pressure and stress state conditions, and are overall
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ACCEPTED MANUSCRIPT similar to those relevant to subsurface reservoirs targeted for CO2 storage. These are either based on flow-through 1-D compaction experiments performed on aggregates (Liteanu and
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Spiers, 2009; Liteanu et al., 2013, 2012) or on “flow-through” triaxial experiments (under
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isotropic and deviatoric stress state) performed on cohesive rock core samples (Grgic, 2011;
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Le Guen et al., 2007). These studies report enhanced strain in the presence of CO2–saturated solutions (see an example in Figure 8), although the amount of acceleration varies between insignificant (compared to pure chemical effect as discussed by Grgic 2011) to a maximum
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increase of ~50 times.
Figure 8: Long-term triaxial deformation experiments on Lavoux limestone core samples subjected to fluid circulation (low or high partial pressure of CO2) and
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ACCEPTED MANUSCRIPT measurement of creep (after Le Guen et al., 2007). The vertical axial strain measured (cyan line) is compared to a reference displacement sensor (dark blue line). a) Under dry
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conditions with applied stress no measurable compaction occurred. Injection of a low
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CO2 partial pressure saline fluid caused immediate compaction; the strain rate
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gradually decreased with time. Renewal of fluid injection caused an immediate increase in strain rate at end of experiment. b) Vertical axial strain deformation in the absence of fluid and during injection of high CO2 partial pressure saline fluid (cyan curve). Time
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periods with no data represent non-stable conditions associated with parameter changes.
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The red time period strain rate includes a short flow period. Note that the renewed injection of high CO2 partial pressure saline solution led to a considerable increase in strain rate, but after a time lag of around 40 days. The end of the experiment was
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marked by a sudden, rapid increase in strain and strain rate.
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The most significant increase was measured by Liteanu et al. (2012) using wet calcite aggregates with grain size 100 micron oolite
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7 to 9%
40
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on deformati on. overall permeabil ity remains stable or decreases due to pore clogging by particles dragged by the flow, slight increase of porosity due to dissolutio n Increase of porosity and permeabil ity due to dissolutio n, formation of wormhole s
Luquot et al., 2014
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Luquot and Gouze, 2009
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89
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37
45
3 to 5
6 to 20
0,5 to 1
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calcite (96%), silica (4%)
0,2 to 0,8
up to 90
brine saturated with CO2, up to 140 pore volumes injected
up to 18 days
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brine with dissolved CO2 (partial pressure 0.3 MPa), 20000 pore volumes injected
1.8 days
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Roerdal chalk
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CR
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Mondeville oolitic limestone
calcite (99% Ca, 1% Mg)
> 100 micron oolite
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12.6%
40
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Increase of compacti on creep deformati on of chalk when flooding with CO2rich water, presence of wormhole s, injection of gaseous CO2 has no effect on deformati on Increase of porosity with permeabil ity decrease, attributed to pore throat clogging by dissolved
Madland et al., 2006
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Mangane et al., 2013
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90
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IP CR US MA N Birkigt: 23–28 % Uder: 27% Bebertal: 12–23%
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70 mm diamet er, 140 mm long
Birkigt: 35 Uder: 400 Bebertal: 0,035
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