We used Icelandic basalt drill cores from the CarbFix site, collected at ∼ 350 m depth. The composition of Icelandic basalt has been identified as tholeiite and contains ∼ 25 wt % of calcium, magnesium and iron oxides (7 wt %–10 wt % Ca; 5 wt %–6 wt % Mg; 7 wt %–13 wt % Fe) with an average porosity of ∼ 8 % based on hydrological and tracer recovery modeling (Alfredsson et al., 2008, 2013; Aradóttir et al., 2012; Matter and Kelemen, 2009; Snæbjörnsdóttir and Gislason, 2016). The rock is formed by an aphanitic matrix that consists of crystals of feldspars, clinopyroxene, olivine, glass and secondary alteration minerals as shown in Figs. 1 and A1 in the Appendix. Our observations are consistent with previously reported petrographic analysis which shows that the primary minerals of the Icelandic basalt are predominantly plagioclase (An90-30), olivine (Fo90-80), clinopyroxene (augite), magnetite–ilmenite and interstitial glass, alteration of the basaltic lava flows commonly leads to smectite and zeolite precipitation (Alfredsson et al., 2013; Larsson et al., 2002). The fraction of crystal-to-glass ratio as well as crystal habitat is variable as documented in Fig. 1. Round pores with a mean diameter of ∼ 0.5 mm are randomly distributed throughout the matrix; some are filled with feldspar (primarily potassium feldspar) and some are voids with no filling (Fig. 1). Pore walls, as well as pre-existing crack walls, are coated by a thin layer of a phyllosilicate, as documented in Fig. 1d and e. The matrix is locally altered by dissolution of larger subhedral feldspar crystals and local replacement by phyllosilicate (see Fig. 1b and e). Cylindrical samples were ground to ∼ 40 mm in diameter and ∼ 80 mm in length (see Table 1). The samples were jacketed using copper foil of ∼ 0.05 mm thickness, joined to titanium end caps by Viton tubes and coated with Duralco 4538 epoxy. The end caps had a concentric hole which allows fluid access to the sample. Figure 2 shows the schematics of the sample configuration in this study. An internal force gauge was mounted below the sample inside the vessel, allowing direct measurement of the differential stress (Δσ=σ1-σ3). Displacement of the axial piston was measured externally using a linear variable differential transformer (LVDT). Variations of the sample length were measured using two internal LVDTs. Local axial (ϵa) and radial strains (ϵr) of the rock were measured using strain gauges affixed to the copper jacket around the sample. Piezoelectric sensors were installed around the sample for passive monitoring of acoustic emissions (AEs).

Because of the variation in structure and composition of the natural material and limited drill core material available, we adopted the “stress-stepping” experimental procedures to study creep deformation (Heap et al., 2009; Lockner, 1993). This method allows several creep experiments to be conducted on a single sample at different stress levels and minimizes the issue of intersample variability (see details in Sect. 2.2). Piezoelectric sensors allowing independent recording of compressional and shear waves were fabricated with PZT-5A ceramics with thickness of 3 to 5 mm and resonance frequency of ∼ 450 kHz to 1 MHz. The PZT-5A crystals were mounted on titanium spacers with one side concavely curved to match the sample surface, thus providing protection of the sensing crystals and optimal contact area. A backup element was epoxied to the back of the sensor to minimize ringing. We also used analogue low-pass filters (∼ 500 kHz) compatible with the frequency range of the employed PZT ceramics to reduce the electromagnetic interference (EMI) effect. Data were collected using two combined four-channel universal serial bus (USB) oscilloscopes, recording at 50 MS/s with a 12-bit resolution (TiePie HS4-50). Using low-noise amplifiers (ITASCA-60dB), we carefully selected the most sensitive sensor positions, preferably far from each other, as master channels. The data collection system was set such that, if the master channels detected a signal satisfying a sufficiently large signal / noise ratio in a moving time window, the event would be recorded in all channels. We amplified the two master channels with a flat gain of 60 dB in a frequency range of 50 kHz to 1.5 MHz. Frequencies from 1.5 to 15 MHz were amplified nonlinearly, the gain decreasing exponentially from 52 to 37 dB with increasing frequency (Ghaffari et al., 2021; Ghaffari and Pec, 2020). Considering the above limitations, the main frequency range of the recording system was between ∼ 50 and 500 kHz, although other frequencies could be recorded due to the exponential nature of the amplification filters.

All experiments were conducted at 50 MPa effective pressure, Peff, with pore fluid pressures, Pf, of either 0 or 5 MPa for dry and fluid-saturated experiments, respectively. The fluids used in this study were H2O and H2O + CO2. The H2O + CO2 fluid is prepared in the fluid mixing vessel (Fig. 2a) where deionized water is saturated with CO2 in the vessel under a gas pressure of 5 MPa. The fluid-saturated samples were first immersed in deionized water under vacuum for more than 30 d prior to the experiment. Details of the experimental conditions are listed in Table 1. The samples were inserted in the NER Autolab 3000 testing rig installed at Massachusetts Institute of Technology (MIT) and deformed under triaxial stress conditions, with the maximum principal stress (σ1) acting in the axial direction. The radial principal stresses (σ2 and σ3) were generated by the confining pressure, i.e., σ2=σ3=Pc. The effective pressure is calculated as Peff=Pc-Pf. During deformation, a constant pressure difference of 0.5 MPa was maintained between the inlet and outlet of the pore pressure system, while the mean pore pressure was kept at 5 MPa. We thus maintained fluid flow across the sample and measured the permeability evolution during deformation. In one H2O + CO2 experiment (OR2_M), we closed the fluid mixing vessel after the initial filling of the sample and thus formed a closed pore fluid loop (OR2_M was referred to as the H2O + CO2 closed experiment in the following discussion). In the other H2O + CO2 experiment (OR3_B), the pore fluid system was connected to the fluid mixing vessel during the entire experiment and therefore acted as a semi-open system since it was in constant communication with a large CO2 source (OR3_B was referred to as the H2O + CO2 open experiment in the following discussion).

We started the experiments by bringing the sample to an effective pressure of 50 MPa and subsequently to a temperature of ∼ 80 ∘C while holding the pressure constant. Heating the sample took ∼ 12 h, which was long enough to allow thermal equilibrium to be reached. After reaching the desired P–T conditions, the samples were deformed using a step-loading procedure. During a step, the differential stress was increased at a rate of ∼ 2 MPa/min, which corresponds to an axial strain rate of ∼ 1.1 × 10-6 s-1. Once the desired stress level was reached, we kept the load constant for ∼ 24 h, while monitoring the sample deformation. This step sequence was repeated until failure of the sample occurred – typically during the increase of differential stress. We recorded the stress level at which the failure occurred as “failure strength” and use this parameter to quantify the strength of the tested material. We point out that the definition of “failure strength” used here presents a lower bound for the commonly presented “ultimate strength” which is measured in short-term, constant displacement rate deformation experiments. The dry experiment is halted earlier due to failure of the strain gauges and LVDTs and therefore the strength estimate is only a lower bound of the failure strength. The total duration of the experiments ranged between 5 and 12 d. Details of the load steps are summarized in the Appendix (Fig. A2).

In this study, we focus on the transient creep evolution and only creep steps where final failure is not observed are analyzed. We use the term “phase I” to refer to the creep immediately following a stress change, during which the creep strain evolves rapidly (i.e., relatively higher strain rate). We call “phase II” the portion of the creep curve with an approximately constant or very slowly varying strain rate over a ∼ 24 h window (i.e., dε/dt=cte; see Figs. A3 and A4). For comparison with previous work on brittle creep, we calculate a characteristic creep strain rate using a least-squares fit to the slope of the creep strain vs. time curve during the identified phase II transient creep (Fig. A5; we will simply refer to it as “creep rate” in the following discussion).

To investigate the microstructural changes occurring during deformation, the rock samples were scanned before and after deformation using X-ray computed tomography with scan parameters set at ∼ 150 kV and ∼ 250 µA. The obtained X-ray images have a pixel size of ∼ 90 × 90 µm. Thin sections were prepared from selected samples and imaged using a field emission scanning electron microscope (SEM).

The evolution of fluid composition was evaluated by collecting fluid samples from the end of the pore fluid outlet (Fig. 2a) after each creep step. The concentrations of Mg2+ and Ca2+ in the fluid sample were analyzed using the inductively coupled plasma mass spectrometry (ICP-MS).

The creep deformation during each load step exhibited typical transient creep evolution (Brantut et al., 2013; Robertson, 1964; Scholz, 1968) with a transition from phase I, where rapid straining occurs, to a slowly varying phase II, which exhibits an approximately constant strain rate over our observation time (Fig. 3). This transition generally took place within the first 104 s (∼ 2.7 h) of the loading step. In the dry experiment, large variations in phase I creep strains were observed (Figs. 3a and 4c), and the creep rates measured during the slowly evolving phase II stages showed a neutral sensitivity to stress (Fig. 3e). In experiments where pore fluids were present (H2O and H2O + CO2), the strain accumulated during the phase I creep systematically increased with increasing stress, and the creep strain rate during the phase II creep displayed a clear exponential dependence on stress (Fig. 3e). This stress sensitivity of creep strain rate showed strong similarity in the different experiments irrespective of the pore fluid composition and can be adequately described by power-law (e.g., Atkinson, 1984; Meredith and Atkinson, 1983) as well as exponential functionals (Charles and Hillig, 1962; Hillig, 2006), but the exponential model seems to work slightly better with our data according to the R2 value (see Fig. A6).

In Fig. 4, we compare the strain accumulation during phase I and II of the transient creep as illustrated in Fig. A4a. We observe a universal power-law relationship of the accumulated creep strain during phase I (εi) to creep stain accumulated during phase II (εii) in all experiments irrespective of fluid presence or the composition of the fluid (Fig. 4a); i.e., the ratio between log⁡εi and log⁡εii remains constant. The accumulated creep strains during both phase I and phase II were exponentially dependent on creep stress (Fig. 4c and d). In Fig. 4b, we show that regardless of the creep stress level, the ratio between the logarithmic accumulated phase I and logarithmic phase II creep strain after ∼ 24 h was approximately constant, except for two outliers associated with two stress steps in the dry experiment, during which anomalously large phase I creep strains occurred (Fig. 4a and c).

Overall, the fluid-saturated samples crept faster than the dry sample during phase II stages in similar stress conditions. In spite of variations in failure strength, the fluid-saturated samples consistently showed stronger stress dependence of the creep rate than the dry sample. Comparing the fluid-saturated experiments, we observe that the sample saturated with H2O had the same creep rate as the H2O + CO2 closed experiment and a higher creep rate than the H2O + CO2 open experiment under similar stress level (Fig. 3e). Analysis of the fluid chemistry demonstrates that the H2O + CO2 closed and H2O experiment show the same fluid composition which we will describe in more detail in Sect. 3.6.

In all experiments, creep deformation was initially compactive as indicated by a positive change in the volumetric strain, ϵv, calculated from the strain gauge measurements (ϵv=ϵa+2ϵr). Shear-enhanced dilation (Brace et al., 1966) started 10–20 MPa before the failure strength of the sample was reached (highlighted by yellow arrowheads in Fig. 5). The onset of dilation generally occurred at a lower stress level in the fluid-saturated experiments than in dry conditions. The largest dilation was observed in H2O + CO2 open experiments, as shown in Fig. 5d. In the dry experiment, a large amount of dilation (ϵv>0.5 %) was also observed at creep stress of ∼ 90 and ∼ 105 MPa, which is significantly higher than in other steps (ϵv<0.1 %). Furthermore, the dilation at ∼ 90 MPa is also accompanied by a drop in stress (see Fig. 5). The strength of the tested samples seems to be correlated with the elastic modulus measurements; the stiffer the rock, the higher the strength (see Table 1).

In fluid-saturated experiments, permeability decreased with increasing effective pressure during hydrostatic loading (Fig. 6a, b and c). The largest decrease in permeability was observed in the water-saturated experiment, where permeability dropped by 3 orders of magnitude as effective pressure was raised from 15 to 50 MPa (Fig. 6a). Permeability reduction was much lower in both H2O + CO2 experiments, only ∼ 1 order of magnitude, over the same effective pressure range (Fig. 6b, c). Permeability variations after heating are shown in Fig. 6d, e and f, where the minimum permeability reached during hydrostatic loading is indicated for comparison (empty circles in Fig. 6d, e and f). The permeability change during heating was rather small in the H2O and H2O + CO2 closed experiment, while the H2O + CO2 open experiment exhibited more than an order of magnitude permeability reduction after heating.

During creep, permeability did not evolve much with time but did show a clear dependence with the stress level of the individual creep stages, first slightly decreasing with increasing differential stress and then substantially increasing when the onset of transition from compaction to dilatancy (C*′) was passed, shortly before failure (Fig. 6d and f).

Post-mortem examination of the samples reveals that fractures inside the fluid-saturated samples form a complex, wide system rather than a clearly defined, distinct shear fault plane (Figs. 10 and A8). The fluid-saturated samples exhibit bulging on the surface. In contrast, the dry sample shows a weakly developed fault plane and less bulging; however, it should be noted that this sample did not, in fact, reach failure strength.

X-ray tomographic images (Fig. 11) and BSE images (Fig. 12) of the deformed samples display abundant fractures, whereas cracks are much more rare in the pre-deformation CT scans and the BSE images (Fig. 1). The amount of visible cracks in each sample tends to scale with the cumulative AE count; the dry experiment has a lower fracture density than the experiments with H2O and H2O + CO2 despite the fact that the dry sample experienced a higher stress and accumulated a larger total strain. To illustrate these observations, we selected representative pairs of 2-D tomographic slices oriented parallel and perpendicular to the loading direction and traced the observable microcracks (Fig. 11). We quantified both the orientation and anisotropy of the microcracks using the “surfor” method that relies on the projection of an outline (Heilbronner and Barrett, 2014; Panozzo, 1984). As documented in Fig. 11, cracks are strongly aligned in the axial sections. The cracks are mainly oriented parallel to the maximum principal stress in the H2O + CO2 experiment, indicating Mode I cracking, but are aligned 20–30∘ to the maximum principal stress in the dry and H2O experiments, suggesting mixed Mode I + Mode II cracking. A weaker alignment is generally observed in radial sections.

Concentration of the Mg2+ and Ca2+ cations increased once heating started (Fig. 13). This increase in the Mg2+ and Ca2+ concentration reflects the dissolution of Mg and Ca bearing minerals during the reaction. In the H2O + CO2 closed experiment (OR2_M), the supply of CO2 was limited and led to a dissolution-dominated system that resulted in the high concentration of Mg2+ and Ca2+, similar to the H2O experiment (OR2_T). In the H2O + CO2 open experiment (OR3_B), the cation concentration was significantly lower than in the OR2_M and OR2_T experiments. This was likely caused by the potential precipitation uptake due to the continuous supply of CO2 in the semi-open setting of the pore fluid system. This interpretation is also supported by the ∼ 2 orders of magnitude drop in permeability observed in the CO2 open experiment after heating started since precipitation could potentially clog the pore throats and lead to permeability decrease.

Acoustic emission, microstructure analysis and mechanical data confirm that the observed deformation is a brittle process as is expected at the given P–T conditions. The strong similarity between the time evolution of cumulative AE counts and strain (Figs. 7 and 8) is consistent with observations from other creep deformation experiments using cemented and uncemented porous rocks (e.g., Brzesowsky et al., 2014; Heap et al., 2009). These considerations suggest that the creep deformation observed in this study is a result of a time-dependent brittle process such as subcritical cracking that can still generate AE activity (Chester et al., 2007, 2004).

Previous studies concluded that observable amount of brittle creep strain is unlikely to occur below 80 % of ultimate strength defined by the short-term, constant strain rate deformation experiments (Baud and Meredith, 1997; Heap et al., 2009). However, all our strain measurements (strain gauges, LVDTs, axial ram displacement) show that, in this study, creep did occur at stress levels of only ∼ 11 % of failure strength (and therefore even lower percentage of the ultimate strength). Similar creep deformation with measurable strain at low stress level has been previously observed in shale (e.g., Mighani et al., 2019). We also found that the strain rates measured during all creep steps could be fitted using the same exponential law derived from strain rate measurements. Furthermore, the amount of creep strain accumulated during phase I and phase II showed a consistent stress dependence across all stress conditions (Fig. 4c and d). Therefore, our experiments demonstrated that there does not seem to exist a threshold below which no creep strain will be observed. The creep deformation was likely governed by the same mechanism across our tested stress conditions, and the accumulated creep strain at a given time can be formulated as a function of stress.

Our AE statistics show that the b values were higher for the fluid-saturated experiments than the dry experiment, indicating a higher proportion of low-amplitude AEs (i.e., higher ratio of low- to high-amplitude events). This abundance of low-amplitude events in fluid-saturated rock is a direct evidence that aqueous fluids promoted creep deformation in basalt. As argued in previous studies, growth of small cracks and low-amplitude events are facilitated when stress corrosion is activated in the presence of aqueous fluids (Hatton et al., 1993). We also observed that the amplitude of the largest events increased with increasing stress. And this effect becomes more significant in the fluid-saturated experiments (H2O and H2O + CO2). This could be attributed to the increase in microcrack nucleation, consequently maximizing the likelihood of an “avalanche” of coalescing cracks, which, in turn, generates large-amplitude events. Overall, as more and more energy is dissipated through microcracking and the associated low-amplitude AEs, the macroscopic deformation becomes less dynamic, which is consistent with the increase in the Gutenberg–Richter b value with increasing stress.

Post-mortem examination of the fluid-saturated samples demonstrated the presence of a complicated network of fractures within the sample and absence of a well-defined major shear fracture plane. The samples also exhibited distributed deformation features such as bulging, likely caused by the bulk formation of dilation cracks in addition to the cataclastic shear during the final failure. These microstructural observations further support the idea that deformation during creep is diffuse and distributed rather than localized (Hatton et al., 1993; Heap et al., 2009), consistent with nucleation-controlled crack growth since the nucleation sites are normally randomly distributed in the sample.

Microstructure analysis of the deformed samples demonstrates that the presence of fluid resulted in more abundant Mode I cracks (Fig. 11). Larger amount of cracks oriented parallel to the maximum principal stress were observed in the H2O + CO2 samples, implying dominant Mode I cracking, while the dry experiment showed less cracking, with the cracks aligned 20–30∘ to the maximum principal stress, thus pointing to mixed Mode I + Mode II cracking. This observation is consistent with previous studies on strain localization, as they often proposed rock fracture models predicting that Mode II cracking takes place during the localization stage of fracture development (Lockner et al., 1992; Reches and Lockner, 1994; Wong and Einstein, 2009). Among the present experiments, the samples subjected to creep deformation under H2O + CO2 conditions exhibited the largest amount of Mode I cracks. The sample deformed under dry condition, despite having experienced similar differential stress and total accumulated strain, showed a lower amount of cracks. As stated in previous studies, Mode II cracks often propagate at velocities close to the Rayleigh velocity, which increases the probability of occurrence of high-amplitude events. On the other hand, Mode I cracks have significantly lower rupture velocities and tend to produce low-amplitude acoustic events (Broberg, 2006). Therefore, increased Mode I cracking should lead to an increase in the proportion of low-amplitude AEs, i.e., an increase in the Gutenberg–Richter b value.

We infer that the difference in creep rate of the dry and fluid-saturated experiments is a result of fluid-assisted subcritical crack growth. The fluid presence promotes stress corrosion, possibly related to hydrolytic weakening (Atkinson, 1984), accelerates crack growth, activates more crack nucleation sites and, consequently, leads to a distributed array of small microcracks. In contrast, crack growth under dry conditions is concentrated on fewer and larger cracks since activation of the nucleation sites is more difficult. Thus, it is easier to create localized deformation under dry condition.

Previous studies also suggested that intergranular pressure solution (IPS) could play a significant role as a deformation mechanism during creep (Liteanu et al., 2012; Zhang and Spiers, 2005; Zhang et al., 2010). The creep deformation by IPS involves dissolution and the presence of a fluid phase might be expected to affect creep deformation, generating additional strain accumulation apart from dilatant cracking. Importantly, because the driving process of IPS is not producing abrupt stress drops, it is not expected to produce acoustic emissions. Although we did see difference in creep strain between the dry and fluid-saturated experiments, it was likely caused by dynamic fracturing, as evidenced by the volumetric strain and AE observations (Figs. 5a and A1). We attribute the change in creep strain rate between dry and fluid-saturated experiments to fluid-assisted subcritical crack growth. We posit that under our experimental conditions, IPS was not a dominant creep mechanism; however, more detailed microstructural observations are needed.

Our experiments show that the time-dependent creep deformation was also strongly stress dependent. We observed that the creep strain accumulated during phase I was exponentially dependent on stress (Fig. 3c). Two exceptions are noted in the dry experiment. Both showed high strain accumulation during phase I transient creep and followed a sharp temporary stress drop during the creep step with a nominal differential stress of ∼ 90 MPa (Fig. 5). This temporary stress drop was accompanied by a swarm of large-amplitude AEs (Fig. A2), implying that the concurrent strong dilation was likely caused by local dynamic fracturing while the bulk of the sample remained mostly intact and still capable of supporting the applied load.

We also observed an exponential relationship between stress and creep rate. Interestingly, the fluid-saturated experiments yielded approximately equal stress sensitivities of the creep rates, ε˙∝ e0.02∼0.03σ, despite the variability in their absolute strengths (Fig. 3). The exponential stress dependence of strain rate in fluid-saturated experiments is consistent with brittle creep being the dominant deformation mechanism. Indeed, the values of the fitting constant (0.02–0.03) are comparable in order of magnitude to those reported in previous studies on other basaltic rocks (0.05 in Heap et al., 2011, from experiments using Etna basalt). Since the creep rate was exponentially dependent on stress, so should be the accumulated phase II creep strain. This inference is supported by our observation in Fig. 4d. Concerning the dry experiment, we attribute the slightly negative dependence of creep strain rate on stress (Fig. 3e) to statistical artifact due to large data fluctuations as suggested by the low R2 value of the exponential fitting (Fig. A6).

The fact that both cumulative phase I and phase II creep strains were exponentially dependent on stress (Fig. 4c and d) implies a power-law relationship between the accumulated phase I and phase II creep strain. This power-law relationship (i.e., the ratio between the logarithmic total phase I and logarithmic phase II creep strain), based on our experimental observation, is independent of the stress level and even the presence or absence of fluids. This empirical relationship can be formulated as 2log⁡(εt-εi)log⁡(εi)=log⁡(εii)log⁡(εi)=cte, where εt is the total strain accumulated at the end of an individual creep stage (∼ 24 h), εi the creep strain accumulated during phase I and εii the strain accumulated during phase II (see Fig. A5). This phenomenological power-law relationship is supported by our observation that the ratios in Eq. (2) were indeed approximately constant ∼ 0.8 (Fig. 4a and b). This power-law relationship expressed in Eq. (2) implies that the strain evolution with time can be predicted; some fundamental link between strain accommodated in phase I creep and strain rate in phase II creep exists.

The increase in concentration of both Mg2+ and Ca2+ occurring after heating in the H2O and H2O + CO2 closed experiment (Fig. 13) indicates that the system was dominated by dissolution of Mg- and Ca-bearing minerals. In the case of the H2O + CO2 open experiment, we observed a much smaller increase in cation concentration, implying that a significant amount of the released Mg2+ and Ca2+ cations reacted with the continuously supplied CO2 in the semi-open setting to form carbonate precipitates. These cation concentration trends appeared strongly correlated with the permeability evolution and creep strength of the rocks. The experiment with H2O + CO2 open showed a larger post-heating permeability decrease than the experiments with H2O and H2O + CO2 closed and was stronger (Figs. 3e and 6). The absolute creep rate was consistent for experiments with comparable fluid chemistry (H2O and H2O + CO2 closed) and about a factor of 3 faster than in the experiment where precipitation was dominant (H2O + CO2 open), indicating that dissolution associated with fluid presence weakens the rock, while precipitation reactions slightly strengthen the rock and partly compensate the effect of dissolution. This congruence of observations is a strong argument that precipitation occurred in the pore space of the CO2 open experiment. However, we could not directly resolve evidence of precipitation within the resolution of our microstructural observations and this requires further study. Interestingly, the strain rate sensitivity to stress was similar in all fluid-saturated experiments (Fig. 3), implying that creep rate sensitivity to stress was not significantly influenced by the fluid chemistry.

Our chemical data support the idea that carbonation of basalt is a kinetically favored reaction and are consistent with the fast rate of carbonation observed during the CarbFix field tests (Matter et al., 2016). We interpret the difference between the Mg2+ and Ca2+ concentrations measured in the H2O + CO2 open experiment and those in the H2O and H2O + CO2 closed experiments to be a result of the consumption of Mg2+ and Ca2+ in the formation of carbonate. This indicates that the supply of CO2 is more sufficient in the H2O + CO2 open experiment and the rate-limiting factor during carbonation under our experimental condition was the net supply of Mg2+ and Ca2+ cations, which is associated with dissolution.

Permeability was affected by both chemical and mechanical processes. The evolution of permeability during the experiments was generally consistent with previous observations of monotonic permeability decrease during hydrostatic loading of samples of limestone, sandstone and Etna basalt (Brantut, 2015; Fortin et al., 2011; Zhu and Wong, 1997). Comparison of the dissolution dominated experiments (H2O and H2O + CO2 closed) and the precipitation dominated experiment (H2O + CO2 open) shows that the carbonation reaction reduced permeability in our experiment. In low differential stress conditions, the samples compacted and this compaction was accompanied by a further permeability decrease, which was likely related to the pore volume reduction expected during compressive deformation. Shortly before failure strength was reached, volumetric dilation became dominant and coincided with permeability increase. Our observations of the permeability evolution demonstrate that, although the permeability might decrease due to compaction, formation and propagation of cracks can mitigate the permeability loss and even lead to an increase with further cracking. The effect of creep deformation on the long-term permeability evolution of reservoir rocks is therefore non-negligible. Increase in permeability, combined with other observations such as increasing volumetric strain and acoustic emissions, could potentially be used as a warning sign for impending failure during the long-term monitoring of reservoirs' integrity in GCS applications.

As our samples are taken from drill cores collected at depth at the CarbFix carbon mineralization site, the heterogeneity is larger than in rocks typically used in rock mechanics experiments. The samples investigated in this study exhibit variations in their initial porosity (5 %–15 %; see Table 1), failure strength (55–130 MPa) and Young's modulus (12–28 GPa). We observed a correlation between the failure strength and the elastic modulus of the samples where stiffer samples reach higher peak strengths, consistent with previous reports of an empirical relationship between the unconfined compressive strength and the elastic modulus of sedimentary rocks (see review in Chang et al., 2006). The peak strength however varied inversely with porosity; the dry sample (OR5), which has the highest initial porosity (15 %), shows a higher failure strength (>105 MPa) and exhibits the lowest creep rate compared to the fluid-saturated experiments where porosity measurements were available (H2O and H2O + CO2 closed). Remarkably, the stress sensitivity of the creep strain rate shows consistency (e0.02-0.03σ) in all the fluid-saturated experiments (H2O and H2O + CO2 open and closed) in spite of these variations in porosity, stiffness and failure strength. Moreover, the creep rate at individual stress steps is consistent for experiments with comparable fluid chemistry (H2O and H2O + CO2 closed) despite a variation in porosity by a factor of 2 in between the samples (Fig. 3 and Table 1). These results are a strong argument for the operation of chemical processes that contribute to creep. While variations in porosity resulted in variation in peak strength, they did not seem to affect the absolute creep rates or the sensitivity of creep rate to stress.