Pressure Solution Creep Research Articles

Fluid distribution in grain boundaries of natural fine-grained rock salt deformed at low differential stress (Qom Kuh salt fountain, central Iran): Implications for rheology and transport properties

We used a combination of broad ion beam cross-sectioning and cryogenic SEM to image polished surfaces and corresponding pairs of fractured grain boundaries in an investigation of grain boundary microstructures and fluid distribution in naturally deformed halite from the Qom Kuh salt glacier (central Iran). At the scale of observations, four types of fluid-filled grain boundary can be distinguished by morphology (from straight to wavy), thickness (from 5000 to 50 nm) and the presence of fluid inclusions. The mobility of the brine is shown after cutting the inclusions by broad ion beam (BIB) in vacuum and fine-grained halite forms efflorescence and precipitates on internal walls of inclusions. At cryogenic temperature, grain boundary brine is shown either as continuous film or in isolated inclusions. The halite–halite grain boundary between isolated fluid inclusions is interpreted to have formed by fluid-assisted grain boundary healing. Preliminary experiments on the samples at shear stress conditions of natural salt glacier show very slow strain rates (7.4 × 10−10 s−1 and 1 × 10−9 s−1), which are less than expected for pressure solution creep. Both microstructures and deformation experiments suggest interfacial energy-driven grain boundary healing and therefore rendering inactive the pressure solution creep in our samples. This result disagrees with previous microstructural studies of the same sample, which showed microstructural evidence for pressure solution (and dislocation creep). Different explanations are discussed, which imply that both healing and reactivation of grain boundaries are important in salt glaciers, leading to heterogeneous distribution of deformation mechanisms and strain rates in both space and time.

Settling and compaction of olivine in basaltic magmas: an experimental study on the time scales of cumulate formation

A series of centrifuge-assisted settling experiments of 30 vol % olivine in 70 vol % basaltic melt was conducted to elucidate the formation mechanisms and time scales of gravitational cumulates. The settling experiments were performed in a centrifuging piston cylinder at 200–1,500g, 1,270–1,280 °C, and 0.8–1.1 GPa on previously annealed and texturally equilibrated samples. The mechanical settling of the dense olivine suspension occurs at about 1/6 the speed of simple Stokes settling, resulting in a sedimentation exponent n = 4.1(6) in agreement with predictions from analogue systems. The porosity (φ m ) of the orthocumulate resulting from gravitational settling of crystals is about 54 % and formation times of olivine orthocumulates result to 0.1–10 m day−1 (for an initial crystal content of the melt of 1–5 % and grain sizes of 2–10 mm). After mechanical settling, olivine grains rest on each other, and further compaction occurs through pressure dissolution at grain contacts, olivine reprecipitation where olivine is in contact with melt, and concomitant expulsion of excess liquid from the cumulate layer. With centrifugation at 400g for 50 h, porosities as low as 30.3 vol % were achieved. The olivine content at the bottom of the gravitational cumulate is 1 − φm ~ log(Δρ · h · a · t), where Δρ is the density difference between crystals and melt, h the crystal layer thickness, a the acceleration, and t the time of centrifuging. Compaction is hence proportional to effective stress integrated over time indicating that pressure dissolution is the dominant mechanism for chemical compaction. The compaction limit, that is the lowermost porosity to be reached by this mechanism, is calculated by equating the lithostatic and hydraulic pressure gradients in the cumulate and results to 3–5 % porosity for the experiments. Crystal size distribution curves and a growth exponent n of 3.1(3) indicate that diffusion-controlled Ostwald ripening is the dominant crystal growth mechanism. The above relationship, combined with a linear scaling for grain size as appropriate for reaction-controlled pressure solution creep, allows calculation of formation times of adcumulates. If chemical compaction is dissolution–reprecipitation limited, then single layers of natural olivine adcumulates of ½ m thickness with 70–75 vol % olivine at the base (as observed in the Rhum layered intrusion) would have typical formation times of 0.4–3 years for grain sizes of 2–10 mm. This time scale compares favourably with characteristic cooling times of sills. If a greater than 20-m-thick series of cumulate layers pressurizes a base layer with the porosity still filled by a melt, then compaction proceeds to the compaction limit within a few years. It can thus be expected that in layered mafic intrusions where cumulates are continuously deposited from a large magma chamber and which characteristic cooling times of more than decades, a compaction zone of several tens of metres forms with adcumulates only maintaining porosities in the order of 5 %. In conclusion, gravitational settling and gravitation-driven chemical compaction are feasible cumulate-forming processes for dense mafic minerals in basaltic magmas and in particular in large layered intrusions.

Aseismic sliding of active faults by pressure solution creep: Evidence from the San Andreas Fault Observatory at Depth

Active faults in the upper crust can either slide steadily by aseismic creep, or abruptly causing earthquakes. Creep relaxes the stress and prevents large earthquakes from occurring. Identifying the mechanisms controlling creep, and their evolution with time and depth, represents a major challenge for predicting the behavior of active faults. Based on microstructural studies of rock samples collected from the San Andreas Fault Observatory at Depth (California), we propose that pressure solution creep, a pervasive deformation mechanism, can account for aseismic creep. Experimental data on minerals such as quartz and calcite are used to demonstrate that such creep mechanism can accommodate the documented 20 mm/yr aseismic displacement rate of the San Andreas fault creeping zone. We show how the interaction between fracturing and sealing controls the pressure solution rate, and discuss how such a stress-driven mass transfer process is localized along some segments of the fault.

Influence of CO2on the long-term chemomechanical behavior of an oolitic limestone

[1] In order to study the long-term mechanical and petrographical evolutions of a carbonate rock (oolitic limestone) during storage of CO2, CO2 injection tests were performed in triaxial cells at temperature and mechanical stresses (isotropic and deviatoric) corresponding to the depth of the Dogger carbonate reservoirs of the Paris basin (∼800 m). We used a specific “flow-through” triaxial cell which allowed us to measure very low strain rates in both axial and lateral directions, while ensuring the sealing of the samples during the injection of CO2. Under isotropic loading, neither the dynamic percolation (i.e., flow-through tests) of dry supercritical/gaseous CO2, nor the diffusion of CO2, into initially saturated samples was shown to produce significant axial compaction and calcite dissolution. Indeed, even though the interstitial aqueous fluid becomes acidic, the progressive increase in dissolved species induces the H2O-CO2-calcite re-equilibrium. The dynamic injection of CO2-saturated solution induced significant axial compaction due to the dissolution of calcite at the sample/piston interface only under open flow conditions (i.e., the injected acidic solution is continuously renewed). Under closed flow conditions (i.e., acidic solution recirculation or no-flow conditions) which reproduce the physicochemical conditions of CO2 storage at the field scale better, the rapid H2O-CO2-calcite re-equilibrium inhibits calcite dissolution. Under deviatoric loading and closed conditions, the diffusion of CO2 induced a very small increase in the PSC (pressure solution creep) process which was stopped by the H2O-CO2-calcite re-equilibrium inside the sample. Therefore, a significant compaction of limestone samples was obtained only under open conditions and is mainly due to a purely chemical mechanism (calcite dissolution), while the contribution of the chemo-mechanical mechanism (PSC) was found to not be of any great importance. In the context of massive injection of CO2 at the field scale, if the reservoir can be considered as a closed system from a hydrodynamic point of view (i.e., the brine circulates in the aquifer but is not renewed by any groundwater), CO2 will not play a significant role in the chemistry of carbonate reservoirs due to the H2O-CO2-calcite re-equilibrium and will not induce reservoir compaction and affect its long-term storage capacity, whatever the stress state (isotropic or deviatoric).

Experimental calcite dissolution under stress: Evolution of grain contact microstructure during pressure solution creep

For the first time, nanometer resolution techniques both in situ and ex situ were compared in order to study calcite dissolution under stress. The obtained results enabled identification of the relative importance of pressure solution driven by normal load and free surface dissolution driven by strain energy. It is found that pressure solution of calcite crystals at the grain scale occurred by two different mechanisms. Diffusion of the dissolved solid took place either at a rough calcite/indenter interface, or through cracks that propagated from the contact toward the less stressed part of the crystal. It is also found that strain rates are mostly a function of the active process, i.e., pressure solution associated or not with cracks, rather than being influenced by stress variations. Strain rates obtained in this study are in agreement with published data of experimental calcite and carbonate dissolution under stress.

Constraints on compaction rate and equilibrium in the pressure solution creep of quartz aggregates and fractures: Controls of aqueous concentration

A relationship is developed to examine dissolution precipitation creep in crustal rocks with implicit coupling of the dissolution‐diffusion‐precipitation system and without requiring the iterative solution of a linear equation system. Implicit control is maintained over aqueous silica concentrations within hydrated solid contacts and in open pore space. For arbitrary conditions of temperature, pressure, and mechanical stress, the simple equation system conforms to a polynomial solution for aqueous concentrations set within a small iterative compaction scheme. Equilibrium (long‐term) pressure solution compaction, previously ill constrained, is explored with two alternate methods: (1) a modified form of critical stress and (2) rate‐controlled growth of diffusion limiting cement at the periphery of solid contacts. Predictions are compared to previous experimental results that allow compaction equilibrium to be achieved. Only the modified critical stress is capable of reproducing these results. In this case the agreement is strong across a range of conditions (400°C–500°C, 20–150 MPa, and 3–120 μm mean particle diameter). Compaction rates are overestimated in very early times in a manner suggesting the importance of plastic flow during this period. Predictions are also compared to concentration independent simplifications at general conditions of 350°C and 50 MPa. Compared to the implicit coupling, these methods represent the mean behavior, slightly underestimating rates in dissolution control and slightly overestimating in diffusion control. Aqueous concentration is influential in either regime. The solution is applicable to open and closed systems, is extended to systems with boundary influx, and may be applied to granular media or fractures, differing only in the method defining evolving contact geometry.

Stress state interpretation in light of pressure-solution creep: Numerical modelling of limestone in the Eastern Paris Basin, France

This paper presents an interpretation of the present stress state in the eastern Paris sedimentary Basin (France) based on considerations that regard the long-term limestone rheology. A comprehensive stress profile was acquired in this region, and it was determined that the minimum horizontal stress in an argillite layer is much larger than that in the under- and overlying, stiffer limestone layers, while the major horizontal stress seems almost insensitive to lithology. In a previous study, the genesis of this intriguing stress state was modelled by considering the limestone to be elastic. However, the calculation implied the use of unrealistic mechanical characteristics for this rock (extremely low long-term elastic modulus and Poisson's ratio). From the study, it was concluded that some dissipative, viscous process of unknown origin might be at work in the limestone. Herein, we propose that the deformation micro-mechanism that is responsible for this time-dependent behaviour is pressure-solution creep (PSC). We show that this hypothesis is mechanically realistic by adapting an existing model of PSC, which is based on the description of mass transfer within grain-to-grain contacts (dissolution, diffusion and precipitation). We deduce a set of long-term rheological characteristics for the limestone and use them in the numerical model of the stress genesis in the Paris Basin. The results show that PSC is a possible explanation for the present stresses and total strains in this region. The fact that limestone should be considered as visco-elastic over geological time scales (with a viscosity ranging between 10 17 and 10 21 Pa s), and not as purely elastic, is at the core of this work. Although it is probably ubiquitous in nature, PSC is notoriously difficult to assess in the laboratory. Thus, our effort of coupling small-scale processes in a quantitative manner to large-scale deformation and in-situ measurements provides a useful tool for gaining insight into the role of PSC in the long-term behaviour of carbonated rocks and its effect on sedimentary basin evolution.

Pressure solution creep under cyclic loading

Pressure solution creep was studied on sodium chloride, calcium carbonate and ammonium nitrate in respective saturated aqueous solutions under static loading and cyclic unloading. Ball indentation and powder compaction curves show that each transition from static to cyclic regime produces an increase—sometimes manifold—in creep rate which lasts over the whole time of cyclic impact. After returning to static regime, the initial creep rate reappears. Over longer-term tests, both in static and cyclic regime, the creep gradually slows down. Increasing impact frequency enhances the effect. Any noticeable changes in strain rate are absent in a pure inert medium (paraffin oil). Possible mechanisms of the cyclic unloading effect are discussed.

Apparent activation energy and rate-limiting process estimation from natural shale deformed by pressure solution in shallow subduction zone

Pressure solution is one of the main deformation mechanisms for lithify and stress release in the rocks from shallow subduction zones. We observed temperature-dependent pressure solution development in naturally deformed shale in Shimanto accretionary complex in southwest Japan. The pressure solution develops with shear-dominated or co-axial-dominated deformation. We evaluated apparent activation energy by applying the constitutive equation of pressure solution creep to the temperature-dependent relations. The activation energy of each deformation type was estimated as 18 kJ mol − 1 for shear-dominated and 45 kJ mol − 1 for co-axial-dominated shale. The energies enable us to speculate rate-limiting processes of pressure solution i.e. dissolution, diffusion and precipitation, by comparing the energies obtained in this study with energies had been measured by experiments. The lower activation energy estimated here was similar with that of diffusion. The similarity indicates that possible rate-limiting process of shale deformation in shallow subduction zone would be diffusion. The difference of energy between deformation types can be explained by distinctive grain boundary structure.

Stress Transfer During Pressure Solution Compression of Rigidly Coupled Axisymmetric Asperities Pressed Against a Flat Semi-Infinite Solid

In a previous work, we developed a numerical model of compression by pressure solution (PS) of a single axisymmetric asperity pressed against a flat semi-infinite solid. The dissolution rate at any point along the contact and at any time t was determined by (1) computing the normal stress distribution from the current shape of the asperity, and (2) solving the diffusion equation inside the fluid-saturated solid-solid interface, including local dissolution source terms corresponding to the stress field previously determined. The change in shape of the asperity during an infinitesimal time interval δt can then be calculated and the entire procedure repeated as many times as desired. Our results showed that, as the contact flattens and grows during PS, the initial elastic deformation is partially relaxed and the stress transferred from the contact center to the edge. Our goal in the present paper is to demonstrate that, among a population of asperities, stress can also be transferred from one contact to another and that the overall compaction rate can be significantly affected by this process. For this purpose we extended our previous numerical model to simulate PS of two rigidly coupled spherical asperities simultaneously pressed against a flat semi-infinite solid. We considered two end-member cases: 1) transfer of stress to a newly created, not initially present contact, 2) transfer of stress between asperities with different sizes. In both cases, stress was transferred from the most stressed asperity to the least, and, the overall PS displacement rate was reduced. Thus, formation of new contacts and heterogeneous distribution of asperity sizes, which are both expected to exist in rough fractures with self-affine aperture or in heterogeneous granular materials with variable grain-packing geometry, may significantly slow down PS creep compaction.

Deformation mechanisms of deeply buried and surface-piercing Late Pre-Cambrian to Early Cambrian Ara Salt from interior Oman

We compared microstructures of Late Pre-Cambrian to Early Cambrian Ara Salt diapirs from the deep subsurface (3.5–5 km) of the South Oman Salt Basin and from surface-piercing salt domes of the Ghaba Salt Basin. Laterally, these basins are approximately 500 km apart but belong to the same tectono-sedimentary system. The excellent data situation from both wells and outcrops allows a unique quantification of formation and deformation mechanisms, spanning from sedimentation to deep burial, and via re-activated diapir rise to surface piercement. Microstructures of gamma-irradiated and etched thin sections indicate dislocation creep and fluid-assisted grain boundary migration as the main deformation mechanisms operating in the deep subsurface. Microstructures from the surface are characterised by large ‘old’ subgrain-rich crystals. These ‘old’ grains are partly replaced by ‘new’ subgrain-free and subgrain-poor crystals, which show gamma irradiation-decorated growth bands and fibrous microstructures, indicative of pressure solution creep and static recrystallisation, most likely due to surface piercement and exposure. Using subgrain size piezometry, the maximum differential stresses for the subsurface salt is 1.7 MPa and those for the surface-piercing salt is 3.4 MPa, the latter value displaying the high stress conditions in the diapir ‘stem’ as the salt rises on its way to the surface.

Exposed plate interface in the European Alps reveals fabric styles and gradients related to an ancient seismogenic coupling zone

We present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central Alps of Europe related to Late Cretaceous‐early Tertiary subduction and accretion of the South Penninic lower plate underneath the Adriatic upper plate. The material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation, thus reflecting a multistage evolution. Fabric formation and metamorphism, however, chiefly record the deformation conditions of the precollisional setting along the plate interface. We identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300°C. This zone coincides with a domain of intense veining in the subduction mélange with mineral growth into open cavities, indicating fast, possibly seismic, rupture. Evidence for transient near‐lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes, possibly reflecting a zone of conditionally stable slip. The zone above the unstable slip area is devoid of veins but displays ample evidence of fluid‐assisted processes similar to the deeper zone: solution‐precipitation creep and dehydration reactions in the mélange matrix, hydration, and sealing of the base of the upper plate. Seismic rupture here is possibly expressed by ubiquitous localized deformation zones. We hypothesize that trenchward sealing of parts of the plate interface as well as reaction‐enhanced destruction of upper plate permeability is an important component, localizing the unstable slip zone. This relation may result from the competition of the pervasive, presumably interseismic, pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure.

Kinetic effects of microscale plasticity at grain boundaries during pressure solution

It is generally assumed in kinetic models for pressure solution in materials such as quartz that the effective dissolution rate coefficient in grain boundaries is equal to the conventional geochemical dissolution rate coefficient on a free surface. However, predictions based on this assumption usually overestimate both natural and experimental pressure solution rates even when evidence for dissolution rate control is strong. A possible explanation for this discrepancy is that grain boundary structure and dissipative processes such as microscale plasticity in the grain boundary can decrease this effective rate coefficient. On the basis of a simple grain boundary model assuming an island‐channel structure, we have derived a preliminary model for the effect that dissipation by plastic deformation (work‐hardening flow and creep) of grain boundary islands has on dissolution‐controlled pressure solution rates. Comparing the predictions of this model with the experimental data on quartz pressure solution rates, we see that microscale plasticity at grain boundary islands does slow down pressure solution and can help explain the discrepancies between observed and theoretical pressure solution rates. When applied to pressure solution creep of sandstones or fault rocks in nature, our model predicts that grain boundary plastic deformation in quartz might have a significant effect at depths beyond ∼9–10 km.

A pressure solution creep law for quartz from indentation experiments

Indenter experiments have been performed on quartz crystals in order to establish a pressure solution creep law relevant at upper to middle crustal conditions. This deformation mechanism contributes to Earth's crust geodynamics, controlling processes as different as fault permeability, strength, and stress evolution during interseismic periods or mechanochemical differentiation during diagenesis and metamorphism. Indenter experiments have been performed at 350°C and 20–120 MPa during months with differential stress varying from 25 to 350 MPa. Several experimental parameters were varied: nature of quartz (synthetic or natural), nature of fluid, manner in which the solid/solution/solid interface was filled, and orientation of the indented surfaces versus quartz crystallographic c axis. Significant strain rates could only be obtained when using high‐solubility solutions (NaOH 1 mol L−1). Displacement rates of the indenter were found activated by differential stress, with exponential dependence, as theoretically predicted. The mean thickness of the trapped fluid phase below the indenter was estimated in the range 2–10 nm. Moreover, the development of this trapped fluid phase was relatively fast and allowed fluid penetration into previously dry contact regions by marginal dissolution. The indenter displacement rate was driven by differential stress, and its kinetics was controlled by diffusion along the trapped fluid and the development of a morphological roughness along the interface. Conversely, marginal strain energy driven dissolution was observed around the indenter, and its kinetics was controlled by free‐surface reaction. These experimental results are applied to model the interactions between pressure solution and brittle processes in fault zones, providing characteristic time scales for postseismic transitory creep and sealing processes in quartz‐rich rocks.

Long term friction: From stick‐slip to stable sliding

We have devised an original laboratory experiment where we investigate the frictional behaviour of a single crystal salt slider over a large number of deformation cycles. Because of its physical properties, salt, an analogue for natural faults, allows for frictional processes plastic deformation and pressure solution creep to operate on the same timescale. During the same experiment, we observe a continuous change of the frictional behaviour of the slider under constant conditions of stiffness, temperature and loading velocity. The stick‐slip regime is progressively vanishing, eventually reaching the stable sliding regime. Concomitantly, the contact interface, observed under the microscope, develops a striated morphology with contact asperities increase in length and width, arguing for an increase in the critical slip distance dc. Complementary experiments including velocity jumps show that the frictional parameters of the rate and state friction law, a and b, progressively vanish with accumulated slip. The ultimate stage of friction is therefore rate and state independent under our experimental conditions.

Enhanced deformation of limestone and sandstone in the presence of high fluids

Geological repositories subject to the injection of large amounts of anthropogenic carbon dioxide will undergo chemical and mechanical instabilities for which there are currently little experimental data. This study reports on experiments where low and high (8 MPa) aqueous fluids were injected into natural rock samples. The experiments were performed in flow‐through triaxial cells, where the vertical and confining stresses, temperature, and pressure and composition of the fluid were separately controlled and monitored. The axial vertical strains of two limestones and one sandstone were continuously measured during separate experiments for several months, with a strain rate resolution of 10−11 s−1. Fluids exiting the triaxial cells were continuously collected and their compositions analyzed. The high fluids induced an increase in strain rates of the limestones by up to a factor of 5, compared to the low fluids. Injection of high fluids into the sandstone resulted in deformation rates one order of magnitude smaller than the limestones. The creep accelerating effect of high fluids with respect to the limestones was mainly due to the acidification of the injected fluids, resulting in a significant increase in solubility and reaction kinetics of calcite. Compared to the limestones, the much weaker response of the sandstone was due to the much lower solubility and reactivity of quartz in high fluids. In general, all samples showed a positive correlation between fluid flow rate and strain rate. X‐ray tomography results revealed significant increases in porosity at the inlet portion of each core; the porosity increases were dependent on the original lithological structure and composition. The overall deformation of the samples is interpreted in terms of simultaneous dissolution reactions in pore spaces and intergranular pressure solution creep.

Grain-scale deformation in the Palaeoproterozoic Dongargarh Supergroup, central India: implications for shallow crustal deformation mechanisms from microstructural analysis

Analysis of the grain-scale deformation mechanisms in folded rocks of the Dongargarh Supergroup, central India, reveals that deformation was accomplished by a combination of pressure solution, microfracturing and dislocation creep processes. The finite strain was assessed using the Rf/ϕ method (X/Z ≈ 2). Partitioning of strain into various deformation mechanisms revealed that dislocation creep and pressure solution were the major contributors to the finite strain, followed by microfracturing. Analyses of microstructures suggest a sequence of dislocation creep followed by pressure solution and microfracturing, that ultimately gave way to microfracturing and limited crystallization or recrystallization. Overall constancy in volume during deformation is suggested from the balance between fracture-related grain-scale dilatancy and solution-related volume loss. Observations on cleavage spacing within various lithologies in a specific structural setting suggest that lithology played a vital role in cleavage development. Cleavage development in sandstones of the Dongargarh Supergroup required thin shale interbeds (for competency contrast) and grain size <0.036 mm (4.75 ϕ).

The influence of water on deformation microstructures and textures in synthetic NaCl measured using EBSD

Wet NaCl with >10–15 ppm water shows weakening behaviour compared with dry NaCl containing <5 ppm of water. At strains greater than about 0.1 this weakening is associated with recrystallization but at lower strains there is also considerable weakening that is thought to be associated with pressure solution creep. The development of textures and microstructures in wet, synthetic NaCl polycrystals deformed at elevated temperatures has been investigated using electron backscattered diffraction (EBSD). At very low natural strains (0.07), textures, grain shapes and average misorientations of subgrain boundaries in wet NaCl differ to those found in dry NaCl deformed under similar conditions. In wet NaCl, cube shaped grains, oriented in a hard orientation for slip on the 〈110〉{110) system, grow and produce a well defined sub texture, with 〈100〉 poles at 45° to the compression axis. An estimation of strain in wet NaCl was made using average misorientation values of subgrain boundaries. We estimate about 55% of the total strain was accommodated by dislocation creep in wet NaCl at 0.07 strain, the remaining strain being accommodated by pressure solution. At higher strains dynamic recrystallization occurs forming a 〈100〉 fibre texture. This texture can be explained by preferential nucleation of initially strain free grains, which occurs after a critical strain in the most highly deformed, soft-orientation grains, that is grains which have 〈100〉 parallel to the compression axis.

Numerical Modeling of the Effect of Carbon Dioxide Sequestration on the Rate of Pressure Solution Creep in Limestone: Preliminary Results

When carbon dioxide (CO2 ) is injected into an aquifer or a depleted geological reservoir, its dissolution into solution results in acidification of the pore waters. As a consequence, the pore waters become more reactive, which leads to enhanced dissolution-precipitation processes and a modification of the mechanical and hydrological properties of the rock. This effect is especially important for limestones given that the solubility and reactivity of carbonates is strongly dependent on pH and the partial pressure of CO2 . The main mechanism that couples dissolution, precipitation and rock matrix deformation is commonly referred to as intergranular pressure solution creep (IPS) or pervasive pressure solution creep (PSC). This process involves dissolution at intergranular grain contacts subject to elevated stress, diffusion of dissolved material in an intergranular fluid, and precipitation in pore spaces subject to lower stress. This leads to an overall and pervasive reduction in porosity due to both grain indentation and precipitation in pore spaces. The percolation of CO2 -rich fluids may influence on-going compaction due to pressure solution and can therefore potentially affect the reservoir and its long-term CO2 storage capacity. We aim at quantifying this effect by using a 2D numerical model to study the coupling between dissolution-precipitation processes, local mass transfer, and deformation of the rock over long time scales. We show that high partial pressures of dissolved CO2 (up to 30 MPa) significantly increase the rates of compaction by a factor of ~50 to ~75, and also result in a concomitant decrease in the viscosity of the rock matrix.

Single-contact pressure solution creep on calcite monocrystals

Abstract Pressure solution creep rates and interface structures have been measured by two methods on calcite single crystals. In the first kind of experiments, calcite monocrystals were indented at 40 °C for six weeks using ceramic indenters under stresses in the 50–200 MPa range in a saturated solution of calcite and in a calcite-saturated aqueous solution of NH 4 Cl. The deformation (depth of the hole below the indenter) is measured ex situ at the end of the experiment. In the second type of experiment, calcite monocrystals were indented by spherical glass indenters for 200 hours under stresses in the 0–100 MPa range at room temperature in a saturated aqueous solution of calcite. The displacement of the indenter was continuously recorded using a specially constructed differential dilatometer. The experiments conducted in a calcite-saturated aqueous solution of NH 4 Cl show an enhanced indentation rate owing to the fairly high solubility of calcite in this solution. In contrast, the experiments conducted in a calcite-saturated aqueous solution show moderate indentation rate and the dry control experiments did not show any measurable deformation. The rate of calcite indentation is found to be inversely proportional to the indenter diameter, thus indicating that the process is diffusion-controlled. The microcracks in the dissolution region under the indenter dramatically enhance the rate of calcite indentation by a significant reduction of the distance of solute transport in the trapped fluid phase. This result indicates that care should be taken in extrapolating the kinetic data of pressure solution creep from one mineral to another.