Effect of CO2 injection on compaction of carbonate rocks
Carbon dioxide sequestration in depleted oil/gas reservoirs and saline aquifers is one of the mos... more Carbon dioxide sequestration in depleted oil/gas reservoirs and saline aquifers is one of the most promising long-term carbon dioxide management strategies. Possible reservoir formations include limestones, chalks and carbonate- cemented sandstones. To evaluate possible CO2 escape scenarios, and to model the long- term behavior of such reservoirs, data concerning the response of carbonate rocks to carbon dioxide introduction are needed.
Physics and Chemistry of the Earth, Parts A/B/C, 2013
Microcracks within the excavation damaged or disturbed zone (EDZ) in a salt-based radioactive was... more Microcracks within the excavation damaged or disturbed zone (EDZ) in a salt-based radioactive waste repository (or an energy storage facility) can heal/seal by mechanical closure driven by compaction creep, by surface-energy-driven processes like diffusive mass transfer, and by recrystallization. It follows that permeability evolution in the excavation damaged zone around a backfilled or plugged cavity will in the short term be dominated by mechanical closure of the cracks, while in the longer term diffusive mass transfer effects are expected to become more important. This paper describes a contribution to assessing the integrity of radioactive waste repositories sited in rocksalt formations by developing a microphysical model for single crack healing in rocksalt. More specifically, single crack healing models for cracks containing a thin adsorbed water film are developed. These microphysical models are compared with single crack healing experiments, which conclusively demonstrate diffusion controlled healing. Calibration of unknown model parameters, related to crack surface diffusivity, against the experimental data enable crack healing rates under repository conditions to be estimated. The results show that after the stress re-equilibration that follows repository sealing, crack disconnection can be expected on a timescale of a few years at laboratory humidity levels. However, much longer times are needed under very dry conditions where adsorbed aqueous films are very thin.
In order to investigate directly the structure and properties of grain boundaries in silicate mat... more In order to investigate directly the structure and properties of grain boundaries in silicate materials undergoing pressure solution, in situ measurements of these properties are required. We report electrical impedance spectroscopy measurements, performed, under hydrothermal conditions, on individual glass-glass and glass-quartz contacts undergoing pressure solution. Resulting estimates of the average grain boundary diffusivity product (Z ¼ Dd av C Ã) for silica transport and of the average grain boundary fluid film thickness (d av) fall in the ranges 6.3 ± 1.4 9 10-18 m 3 s-1 and 350 ± 210 nm, respectively. However, the average values for Z and d av obtained were likely influenced by cracking and irregular dissolution of the dissolving contact surfaces, rather than representing uniformly wetted grain boundary properties. Post-mortem SEM observations indicate that the contact surfaces were internally rough. Taken together, our data support the notion that during pressure solution of quartz, grain boundary diffusion is rapid, and interface processes (dissolution and precipitation) are more likely to be ratelimiting than diffusion.
High shear strain behaviour of synthetic muscovite fault gouges under hydrothermal conditions
Journal of Structural Geology, 2010
Major continental fault zones typically contain phyllosilicates and have long been recognised as ... more Major continental fault zones typically contain phyllosilicates and have long been recognised as zones of persistent weakness. To establish whether the presence of micas can explain this weakness, we studied the frictional behaviour of simulated muscovite fault ...
Journal of Geophysical Research: Solid Earth, 2012
Rock salt offers an attractive host rock for geological storage applications, because of its natu... more Rock salt offers an attractive host rock for geological storage applications, because of its naturally low permeability and the ability of excavation-induced cracks to heal by fluid-assisted diffusive mass transfer. However, while diffusive transport rates in bulk NaCl solution are rapid and well characterized, such data are not directly applicable to storage conditions where crack walls are coated with thin adsorbed water films. To reliably predict healing times in geological storage applications, data on mass transport rates in adsorbed films are needed. We determined the surface diffusivity in such films for conditions with absolute humidities (AH) ranging from 1 to 18 g/m 3 (relative humidities (RH) of 4%-78%) by measuring the surface impedance of single NaCl crystals. We use the impedance results to calculate the effective surface diffusivity S = DdC using the Nernst-Einstein equation. The S values obtained lie in the range 1 Â 10 À27 m 3 s À1 at very dry conditions to 1 Â 10 À19 m 3 s À1 for the deliquescence point at 296 K, which is in reasonable agreement with existing values for grain boundary diffusion under wet conditions. Estimates for the diffusivity D made assuming a film thickness d of 50-90 nm and no major effects of thickness on the solubility C lie in the range of 1 Â 10 À14 to 8 Â 10 À12 m 2 s À1 for the highest humidities studied (14-18 g/m 3 AH, 60%-78% RH). For geological storage systems in rock salt, we predict S values between 1 Â 10 À22-8 Â 10 À18 m 3 s À1. These imply crack healing rates 6 to 7 orders of magnitude lower than expected for brine-filled cracks.
Journal of Geophysical Research: Solid Earth, 1999
The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain... more The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain size of-1 pm has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850 ø to 1000øC, strain rates of 10-7 to 10-5 s-l and a confining pressure of 600 MPa. Nominally dry material behaved purely elastically. In contrast, samples with 0.5 wt% added water flowed at stresses between 9 and 81 MPa reaching strains up to 12%. Fitting a Dom-type power law to the wet data yielded stress exponents (n) of 1.7 _+ 0.4 and apparent activation energies in the range 302 _+ 22 kJ mo1-1. The wet-deformed samples showed polygonal grains, no crystallographic preferred orientation, no subgrains, low dislocation densities, and evidence for grain boundary cavitation. Grain growth was minor. It is concluded that the wet samples probably deformed by a grain size sensitive, grain boundary sliding dominated mechanism, though the nature of the accommodation process remains unclear, and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated. Application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere. The behavior observed in experiments is generally described using a power law creep equation of the form: • = Aexp(-Qc/RT)d•/d p (1) where •: represents axial strain rate (s-t), o is the differential stress ((si-•3; MPa), A is a preexponential constant (MPa-n s-I l amp), Qc is an apparent activation energy (J mol-l), R is the gas Copyright 1999 by the American Geophysical Union. Paper number 1999JB900075. 0148-0227/99/1999JB900075509.00 constant (J mol-• K-•), T is the absolute temperature (K), n is the stress exponent or stress sensitivity, d is grain size (lam), and p is the grain size exponent with p = 0 for GSI mechanisms. Previous deformation studies on olivine single crystals at high pressures (0.3 to 1.5 GPa) and/or high temperature (1300 ø to 1650øC) have allowed detailed investigation of crystal plasticity and the influence of oxygen fugacity (fO2), oxide activity, and H20 content on dislocation creep mechanisms [see Ricoult and
A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state... more A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (o) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D--101-12exp[29.3/RT]o -1-23 and demonstrated that D (•tm) decreases with increasing o (MPa) and increasing temperature (T) in a manner which is in agreement with the field botlndary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical D-o relations, used in paleo-piezometry, will need to be modified to account for temperature effects.
Geological storage of CO 2 in depleted oil and gas reservoirs is one of the most promising option... more Geological storage of CO 2 in depleted oil and gas reservoirs is one of the most promising options to reduce atmospheric CO 2 concentrations. Of great importance to CO 2 mitigation strategies is maintaining caprock integrity. Worldwide many current injection sites and potential storage sites are overlain by anhydrite-bearing seal formations. However, little is known about the magnitude of the permeability change accompanying dilatation and failure of anhydrite under reservoir conditions. To this extent, we have performed triaxial compression experiments together with argon gas permeability measurements on Zechstein anhydrite, which caps many potential CO 2 storage sites in the Netherlands. Our experiments were performed at room temperature at confining pressures of 3.5-25 MPa. We observed a transition from brittle to semi-brittle behaviour over the experimental range, and peak strength could be described by a Mogi-type failure envelope. Dynamic permeability measurements showed a change from 'impermeable' (<10)21 m 2) to permeable (10)16 to 10)19 m 2) as a result of mechanical damage. The onset of measurable permeability was associated with an increase in the rate of dilatation at low pressures (3.5-5 MPa), and with the turning point from compaction to dilatation in the volumetric versus axial strain curve at higher pressures (10-25 MPa). Sample permeability was largely controlled by the permeability of the shear faults developed. Static, postfailure permeability decreased with increasing effective mean stress. Our results demonstrated that caprock integrity will not be compromised by mechanical damage and permeability development.
An unresolved issue in the study of pressure solution in rock materials is the dependence of grai... more An unresolved issue in the study of pressure solution in rock materials is the dependence of grain boundary structure and diffusive properties on the mutual orientation of neighbouring grain lattices. We report electrical measurements yielding the diffusivity of differently oriented halite-glass and halite-halite contacts loaded in the presence of brine. The haliteglass contact experiments show pressure solution of the halite and an effect of halite lattice orientation on grain boundary transport. Post-mortem observations show an orientation-dependent grain boundary texture controlled by the periodic bond chains in the halite structure. It is inferred that this texture determines the internal grain boundary structure and properties during pressure solution. In the halite-halite experiments neck-growth occurred, its rate depending on twistmisorientation. The results imply that deformation by pressure solution may lead to lattice-preferred orientation development, and that polymineralic rocks may deform faster at lower stresses than monomineralic rocks.
A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state... more A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (o) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D-101-12exp[29.3/RT]o-1-23 and demonstrated that D (• tm) decreases with increasing o (MPa) and increasing temperature (T) in a manner which is in agreement with the field botlndary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical Do relations, used in paleo-piezometry, will need to be modified to account for temperature effects.
Carbon dioxide sequestration in depleted oil/gas reservoirs and saline aquifers is one of the mos... more Carbon dioxide sequestration in depleted oil/gas reservoirs and saline aquifers is one of the most promising long-term carbon dioxide management strategies. Possible reservoir formations include limestones, chalks and carbonate- cemented sandstones. To evaluate possible CO2 escape scenarios, and to model the long- term behavior of such reservoirs, data concerning the response of carbonate rocks to carbon dioxide introduction are needed.
Proceedings 76th EAGE Conference and Exhibition 2014, 2014
ABSTRACT As part of a study to investigate methods to enhance pore/crack connectivity between the... more ABSTRACT As part of a study to investigate methods to enhance pore/crack connectivity between the shale matrix and the induced fractures, we have investigated the matrix microstructure of an exposed analogue of the Jurassic Posidonia shales in the Dutch sub-surface. A combination of Precision-Ion-Polishing and Scanning Electron Microscopy has been used to image the in-situ porosity and mineralogy in shale samples from Whitby (UK), which are an analogue for the Dutch Posidonia shale. First results show a fine-grained mudstone with cm-sub-mm scale stratification. The section at Whitby can be divided into a clay matrix dominated upper half and a coarser grained, calcite-rich, lower half of the section. Commonly occurring minerals are pyrite, calcite (fossils, grains and cement), quartz, mica and dolomite. Organic matter content varies from 0 – 2 % in the calcite dominated layers to 5 – 18% in the clay matrix dominated layers. The most porous phases are the clay matrix and calcite fossils. This microstructural study shows which intervals within Posidonia shale contain the largest porosity and organic matter contents for sweet spot analyses and forms a basis for future research on enhancing connectivity between the pores and induced fractures
A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state... more A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (o) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded
Phyllosilicates are common constituents of mid-crustal fault zones and are widely believed to exe... more Phyllosilicates are common constituents of mid-crustal fault zones and are widely believed to exert a strong influence on the strength of fault rocks, in particular around the brittle-ductile transition. The present study aims to experimentally determine the mechanical strength and frictional behaviour of muscovite gouge, in order to establish whether the presence of muscovite might contribute to the long term weakness often inferred to hold for large scale crustal fault zones. Rotary shear experiments in the temperature range 20-700°C, at 100 MPa fluid pressure, have been performed on synthetic muscovite gouges (average grain size 13 mum, < 10% quartz). The effects of sliding velocity, normal stress and shear strain on the frictional behavior of the muscovite gouges have been studied. Microstructural analysis of the experimentally deformed samples has been done using SEM and XED in order to obtain insight in the operating microphysical processes. The mechanical behavior under th...
The effect of varying enstatite content on the deformation behavior of synthetic, finegrained (1 ... more The effect of varying enstatite content on the deformation behavior of synthetic, finegrained (1 to 2 gm) forsterite-enstatite rock with --,0.5 wt % added water was investigated at temperatures of 900 ø to 1000øC, strain rates between 10 -7 and 10 -5 s -l, and a confining pressure of --,600 MPa. The samples exhibited approached steady state flow at stresses <_60 MPa. The results show that, at constant strain rate, increasing enstatite content is associated with a sharp decrease in flow strength in the range 0-2.5% vol % enstatite, with little further change up to 20 vol %. The observed power law n value of ~ 1.7 and microstructures are similar to those obtained in previous work on material with 2.5 wt % enstatite and are consistent with a water-enhanced grain boundary sliding (GBS) dominated deformation mechanism. Significantly, a negative correlation was found between grain size and enstatite content, indicating that enstatite content played a role in controlling the grain size of the starting materials. Moreover, a high correlation between measured flow strength and grain size was found, consistent with a grain size exponent of-3 in a conventional grain size sensitive flow equation. A water-enhanced deformation mechanism involving GBS accommodated probably by grain boundary diffusion and/or dislocation activity is therefore implied, with the effect of enstatite content on flow strength being an indirect physical effect caused by grain size control. Other effects of second-phase content. such as weakening caused by interphase boundary diffusion and/or migration processes, seem to be unimportant under the conditions investigated.
The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain... more The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain size of--1 pm has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850 ø to 1000øC, strain rates of 10 -7 to 10 -5 s -l and a confining pressure of 600 MPa. Nominally dry material behaved purely elastically. In contrast, samples with 0.5 wt% added water flowed at stresses between 9 and 81 MPa reaching strains up to 12%. Fitting a Dom-type power law to the wet data yielded stress exponents (n) of 1.7 _+ 0.4 and apparent activation energies in the range 302 _+ 22 kJ mo1-1. The wet-deformed samples showed polygonal grains, no crystallographic preferred orientation, no subgrains, low dislocation densities, and evidence for grain boundary cavitation. Grain growth was minor. It is concluded that the wet samples probably deformed by a grain size sensitive, grain boundary sliding dominated mechanism, though the nature of the accommodation process remains unclear, and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated. Application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere. • = Aexp(-Qc/RT)d•/d p (1) where •: represents axial strain rate (s-t), o is the differential stress ((si -•3; MPa), A is a preexponential constant (MPa -n s -I l amp), Qc is an apparent activation energy (J mol -l), R is the gas Paper number 1999JB900075. 0148-0227/99/1999JB900075509.00 constant (J mol -• K-•), T is the absolute temperature (K), n is the stress exponent or stress sensitivity, d is grain size (lam), and p is the grain size exponent with p = 0 for GSI mechanisms. Previous deformation studies on olivine single crystals at high pressures (0.3 to 1.5 GPa) and/or high temperature (1300 ø to 1650øC) have allowed detailed investigation of crystal plasticity and the influence of oxygen fugacity (fO2), oxide activity, and H20 content on dislocation creep mechanisms [see Ricoult and Kohlstedt, 1985; Mackwell et al., 1985; Bai el al., 1991; Bai and Kohlstedt, 1992a, b]. Many workers have also observed GSI dislocation creep behavior and associated water weakening effects in experiments carried out on olivine-rich polycrystalline aggregates. in the earlier studies, natural rocks, mainly dunites. were used [e.g., Carter and Ave'Lallemant. 1970; Blacic, 1972; Kirby and Raleigh, 1973; Post, 1977; Ewms and Goetze, 1979; Chopra and Paterson, 1981, 1984]. More recent studies have focused on aggregates made from either natural mineral separates [e.g., Zeuch and Green, 1979, 1984a, b; Chopra, 1986; Karato et al., 1986; Hitchings et al., 1989; Botch and Green, 1989: Hirth and Kohlstedt 1995a, b] or synthetic olivine [e.g., Relandeau, 1981; Beeman and Kohlstedt, 1993], allowing greater control of bulk chemistry, grain size, second phase, and melt content. Concentrating on the better constrained data, obtained in experiments using gas or molten salt confining media, dislocation creep of dry olivine polycrystals is characterized by stress exponents (n) in the range 3.1 to 4.5 and apparent activation energies of-536 + 35 kJ mol -t. Wet olivine polycrystals (i.e., containing generally between 0.1 to 0.5 wt% H20) deformed in the dislocation creep field exhibit n values of 3.4 to 5.1, apparent activation energies 17,823 17,824 MCDONNELL ET AL.' FLOW OF SYNTHETIC DUNITE of 462 + 60 kJ mol -I [Post, 1977' Chopra and Paterson, 1981, 1984; Karate et at., 1986; Botch and Green, 1989; Hirth and Kohlstedt, 1995a, 1996] and flow stresses between -2.5 [Karate et al, 1986] and -6.5 times [Hirth and Kohlswdt, 19961 lower than obtained for dry material at comparable strain rates and temperatures. GSS diffusion creep behavior has been identified in a number of experimental investigations performed on fine-grained polycrystalline elivine under both confined and unconfined conditions [Schwenn and Goetze, 1978' Relandealt, 1981' Karate el a/., 1986; Chopra, 1986; Wolfi'nstine and Kohlstedt, 1994; Hirth and Kohlstedt, 1995a]. However, the range of conditions investigated is limited. In the case of dry material at 1300 to 1540 øC, n = 1 [Wo!/•'nstine and Kohlstedt, 1994' Hirth and Kohlstedt, 1995b] and p = 2 to 3 [Karate et al., 1986' Wo(fenstine and Kohlstedt, 1994; Hirth and Kohlstedt, 1995b]. For wet systems, deformed at 300 MPa and assumed to be largely melt free [Chopra, 1986; Karate et al. 1986' Rutter and Brodie 1988], the stress exponent (n) in the GSS field falls between 1 and 2. Karate e! al. [1986] obtained a value of n = 1.4 + 0.3 and a grain size exponent p of 3.4 + 0.6 at 1300 øC. Experimentally determined values of the apparent activation energy for wet diffusion creep are poorly constrained with values of 190 and 240 kJ reel -I reported by Chopra [1986] and 240 _ 40 kJ mol -• by Rtltter and Brodie [1988]. Karate el al. [1986] estimate the activation energy to be 250 kJ mol -•, taking a value equal to 2/3 that for lattice difi'usion of O and Si in elivine [.laottl el al.. 198(). 1981' Karate and ()gawa, 1982] and assuming grain boundary diffusion control. Note that in the GSS regime wet samples are 1() to 20 times weaker than dry samples at comparable conditions I Karato et al., 1986]. The above data have been widely used in recent years to model extension of the continental lithosphere [Bassi and nin, 1988' Braun and Beaumont, 1989' (lovers and Wortel, 1993: CIoetingh el al., 1995], assuming upper mantle flow to occur in a zone of symmetrically distributed pure shear [McKenzie, 1978]. Most studies assume that the rheology of the upper mantle is controlled by a fi'ictional sliding law plus a wet or dry dislocation creep law for elivine [e.g., Lynch & Morgan, 1987; Bassi and Bonnin, 1988' Braun and Beaumonl, 1989; Dunbar and Sawyer,
Geological storage of CO 2 in clastic reservoirs and aquifers is expected to have a variety of co... more Geological storage of CO 2 in clastic reservoirs and aquifers is expected to have a variety of coupled chemical-mechanical effects. To investigate the effects of CO 2 injection on creep phenomena, we performed uniaxial compaction experiments on granular aggregates of quartz and feldspar under both wet and dry control conditions. The experiments were performed in constant stress mode. Grain size, temperature, CO 2 partial pressure, and effective stress were varied in order to determine their individual effect. Pore fluid pH was varied by the injection of CO 2 and by addition of acidic and alkaline additives. Pore fluid salinity was increased by the addition of NaCl. Wet samples showed instantaneous compaction upon load application, followed by time-dependent creep. From the mechanical data and microstructures, the main compaction mechanism was inferred to be chemically enhanced microcracking in both quartz and feldspar, with subcritical crack growth, i.e. stress corrosion cracking, controlling deformation in the creep stage. The injection of CO 2 and the concomitant acidification of the pore fluid inhibited microcracking in both the quartz and feldspar samples in line with known effects of pH on stress corrosion cracking. We infer that the injection of CO 2 into quartz-and plagioclase-bearing Hangx, S. J. T., C. J. Spiers, and C. J. Peach (2010), Creep of simulated reservoir sands and coupled chemical-mechanical effects of CO 2 injection, 2 sandstones will inhibit grain-scale microcracking process, and that related geomechanical effects, such as reservoir compaction and surface subsidence, will be negligible compared with the poro-elastic response.
Effect of CO2 injection on compaction of carbonate rocks
Carbon dioxide sequestration in depleted oil/gas reservoirs and saline aquifers is one of the mos... more Carbon dioxide sequestration in depleted oil/gas reservoirs and saline aquifers is one of the most promising long-term carbon dioxide management strategies. Possible reservoir formations include limestones, chalks and carbonate- cemented sandstones. To evaluate possible CO2 escape scenarios, and to model the long- term behavior of such reservoirs, data concerning the response of carbonate rocks to carbon dioxide introduction are needed.
Physics and Chemistry of the Earth, Parts A/B/C, 2013
Microcracks within the excavation damaged or disturbed zone (EDZ) in a salt-based radioactive was... more Microcracks within the excavation damaged or disturbed zone (EDZ) in a salt-based radioactive waste repository (or an energy storage facility) can heal/seal by mechanical closure driven by compaction creep, by surface-energy-driven processes like diffusive mass transfer, and by recrystallization. It follows that permeability evolution in the excavation damaged zone around a backfilled or plugged cavity will in the short term be dominated by mechanical closure of the cracks, while in the longer term diffusive mass transfer effects are expected to become more important. This paper describes a contribution to assessing the integrity of radioactive waste repositories sited in rocksalt formations by developing a microphysical model for single crack healing in rocksalt. More specifically, single crack healing models for cracks containing a thin adsorbed water film are developed. These microphysical models are compared with single crack healing experiments, which conclusively demonstrate diffusion controlled healing. Calibration of unknown model parameters, related to crack surface diffusivity, against the experimental data enable crack healing rates under repository conditions to be estimated. The results show that after the stress re-equilibration that follows repository sealing, crack disconnection can be expected on a timescale of a few years at laboratory humidity levels. However, much longer times are needed under very dry conditions where adsorbed aqueous films are very thin.
In order to investigate directly the structure and properties of grain boundaries in silicate mat... more In order to investigate directly the structure and properties of grain boundaries in silicate materials undergoing pressure solution, in situ measurements of these properties are required. We report electrical impedance spectroscopy measurements, performed, under hydrothermal conditions, on individual glass-glass and glass-quartz contacts undergoing pressure solution. Resulting estimates of the average grain boundary diffusivity product (Z ¼ Dd av C Ã) for silica transport and of the average grain boundary fluid film thickness (d av) fall in the ranges 6.3 ± 1.4 9 10-18 m 3 s-1 and 350 ± 210 nm, respectively. However, the average values for Z and d av obtained were likely influenced by cracking and irregular dissolution of the dissolving contact surfaces, rather than representing uniformly wetted grain boundary properties. Post-mortem SEM observations indicate that the contact surfaces were internally rough. Taken together, our data support the notion that during pressure solution of quartz, grain boundary diffusion is rapid, and interface processes (dissolution and precipitation) are more likely to be ratelimiting than diffusion.
High shear strain behaviour of synthetic muscovite fault gouges under hydrothermal conditions
Journal of Structural Geology, 2010
Major continental fault zones typically contain phyllosilicates and have long been recognised as ... more Major continental fault zones typically contain phyllosilicates and have long been recognised as zones of persistent weakness. To establish whether the presence of micas can explain this weakness, we studied the frictional behaviour of simulated muscovite fault ...
Journal of Geophysical Research: Solid Earth, 2012
Rock salt offers an attractive host rock for geological storage applications, because of its natu... more Rock salt offers an attractive host rock for geological storage applications, because of its naturally low permeability and the ability of excavation-induced cracks to heal by fluid-assisted diffusive mass transfer. However, while diffusive transport rates in bulk NaCl solution are rapid and well characterized, such data are not directly applicable to storage conditions where crack walls are coated with thin adsorbed water films. To reliably predict healing times in geological storage applications, data on mass transport rates in adsorbed films are needed. We determined the surface diffusivity in such films for conditions with absolute humidities (AH) ranging from 1 to 18 g/m 3 (relative humidities (RH) of 4%-78%) by measuring the surface impedance of single NaCl crystals. We use the impedance results to calculate the effective surface diffusivity S = DdC using the Nernst-Einstein equation. The S values obtained lie in the range 1 Â 10 À27 m 3 s À1 at very dry conditions to 1 Â 10 À19 m 3 s À1 for the deliquescence point at 296 K, which is in reasonable agreement with existing values for grain boundary diffusion under wet conditions. Estimates for the diffusivity D made assuming a film thickness d of 50-90 nm and no major effects of thickness on the solubility C lie in the range of 1 Â 10 À14 to 8 Â 10 À12 m 2 s À1 for the highest humidities studied (14-18 g/m 3 AH, 60%-78% RH). For geological storage systems in rock salt, we predict S values between 1 Â 10 À22-8 Â 10 À18 m 3 s À1. These imply crack healing rates 6 to 7 orders of magnitude lower than expected for brine-filled cracks.
Journal of Geophysical Research: Solid Earth, 1999
The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain... more The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain size of-1 pm has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850 ø to 1000øC, strain rates of 10-7 to 10-5 s-l and a confining pressure of 600 MPa. Nominally dry material behaved purely elastically. In contrast, samples with 0.5 wt% added water flowed at stresses between 9 and 81 MPa reaching strains up to 12%. Fitting a Dom-type power law to the wet data yielded stress exponents (n) of 1.7 _+ 0.4 and apparent activation energies in the range 302 _+ 22 kJ mo1-1. The wet-deformed samples showed polygonal grains, no crystallographic preferred orientation, no subgrains, low dislocation densities, and evidence for grain boundary cavitation. Grain growth was minor. It is concluded that the wet samples probably deformed by a grain size sensitive, grain boundary sliding dominated mechanism, though the nature of the accommodation process remains unclear, and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated. Application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere. The behavior observed in experiments is generally described using a power law creep equation of the form: • = Aexp(-Qc/RT)d•/d p (1) where •: represents axial strain rate (s-t), o is the differential stress ((si-•3; MPa), A is a preexponential constant (MPa-n s-I l amp), Qc is an apparent activation energy (J mol-l), R is the gas Copyright 1999 by the American Geophysical Union. Paper number 1999JB900075. 0148-0227/99/1999JB900075509.00 constant (J mol-• K-•), T is the absolute temperature (K), n is the stress exponent or stress sensitivity, d is grain size (lam), and p is the grain size exponent with p = 0 for GSI mechanisms. Previous deformation studies on olivine single crystals at high pressures (0.3 to 1.5 GPa) and/or high temperature (1300 ø to 1650øC) have allowed detailed investigation of crystal plasticity and the influence of oxygen fugacity (fO2), oxide activity, and H20 content on dislocation creep mechanisms [see Ricoult and
A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state... more A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (o) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D--101-12exp[29.3/RT]o -1-23 and demonstrated that D (•tm) decreases with increasing o (MPa) and increasing temperature (T) in a manner which is in agreement with the field botlndary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical D-o relations, used in paleo-piezometry, will need to be modified to account for temperature effects.
Geological storage of CO 2 in depleted oil and gas reservoirs is one of the most promising option... more Geological storage of CO 2 in depleted oil and gas reservoirs is one of the most promising options to reduce atmospheric CO 2 concentrations. Of great importance to CO 2 mitigation strategies is maintaining caprock integrity. Worldwide many current injection sites and potential storage sites are overlain by anhydrite-bearing seal formations. However, little is known about the magnitude of the permeability change accompanying dilatation and failure of anhydrite under reservoir conditions. To this extent, we have performed triaxial compression experiments together with argon gas permeability measurements on Zechstein anhydrite, which caps many potential CO 2 storage sites in the Netherlands. Our experiments were performed at room temperature at confining pressures of 3.5-25 MPa. We observed a transition from brittle to semi-brittle behaviour over the experimental range, and peak strength could be described by a Mogi-type failure envelope. Dynamic permeability measurements showed a change from 'impermeable' (<10)21 m 2) to permeable (10)16 to 10)19 m 2) as a result of mechanical damage. The onset of measurable permeability was associated with an increase in the rate of dilatation at low pressures (3.5-5 MPa), and with the turning point from compaction to dilatation in the volumetric versus axial strain curve at higher pressures (10-25 MPa). Sample permeability was largely controlled by the permeability of the shear faults developed. Static, postfailure permeability decreased with increasing effective mean stress. Our results demonstrated that caprock integrity will not be compromised by mechanical damage and permeability development.
An unresolved issue in the study of pressure solution in rock materials is the dependence of grai... more An unresolved issue in the study of pressure solution in rock materials is the dependence of grain boundary structure and diffusive properties on the mutual orientation of neighbouring grain lattices. We report electrical measurements yielding the diffusivity of differently oriented halite-glass and halite-halite contacts loaded in the presence of brine. The haliteglass contact experiments show pressure solution of the halite and an effect of halite lattice orientation on grain boundary transport. Post-mortem observations show an orientation-dependent grain boundary texture controlled by the periodic bond chains in the halite structure. It is inferred that this texture determines the internal grain boundary structure and properties during pressure solution. In the halite-halite experiments neck-growth occurred, its rate depending on twistmisorientation. The results imply that deformation by pressure solution may lead to lattice-preferred orientation development, and that polymineralic rocks may deform faster at lower stresses than monomineralic rocks.
A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state... more A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (o) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D-101-12exp[29.3/RT]o-1-23 and demonstrated that D (• tm) decreases with increasing o (MPa) and increasing temperature (T) in a manner which is in agreement with the field botlndary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical Do relations, used in paleo-piezometry, will need to be modified to account for temperature effects.
Carbon dioxide sequestration in depleted oil/gas reservoirs and saline aquifers is one of the mos... more Carbon dioxide sequestration in depleted oil/gas reservoirs and saline aquifers is one of the most promising long-term carbon dioxide management strategies. Possible reservoir formations include limestones, chalks and carbonate- cemented sandstones. To evaluate possible CO2 escape scenarios, and to model the long- term behavior of such reservoirs, data concerning the response of carbonate rocks to carbon dioxide introduction are needed.
Proceedings 76th EAGE Conference and Exhibition 2014, 2014
ABSTRACT As part of a study to investigate methods to enhance pore/crack connectivity between the... more ABSTRACT As part of a study to investigate methods to enhance pore/crack connectivity between the shale matrix and the induced fractures, we have investigated the matrix microstructure of an exposed analogue of the Jurassic Posidonia shales in the Dutch sub-surface. A combination of Precision-Ion-Polishing and Scanning Electron Microscopy has been used to image the in-situ porosity and mineralogy in shale samples from Whitby (UK), which are an analogue for the Dutch Posidonia shale. First results show a fine-grained mudstone with cm-sub-mm scale stratification. The section at Whitby can be divided into a clay matrix dominated upper half and a coarser grained, calcite-rich, lower half of the section. Commonly occurring minerals are pyrite, calcite (fossils, grains and cement), quartz, mica and dolomite. Organic matter content varies from 0 – 2 % in the calcite dominated layers to 5 – 18% in the clay matrix dominated layers. The most porous phases are the clay matrix and calcite fossils. This microstructural study shows which intervals within Posidonia shale contain the largest porosity and organic matter contents for sweet spot analyses and forms a basis for future research on enhancing connectivity between the pores and induced fractures
A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state... more A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (o) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded
Phyllosilicates are common constituents of mid-crustal fault zones and are widely believed to exe... more Phyllosilicates are common constituents of mid-crustal fault zones and are widely believed to exert a strong influence on the strength of fault rocks, in particular around the brittle-ductile transition. The present study aims to experimentally determine the mechanical strength and frictional behaviour of muscovite gouge, in order to establish whether the presence of muscovite might contribute to the long term weakness often inferred to hold for large scale crustal fault zones. Rotary shear experiments in the temperature range 20-700°C, at 100 MPa fluid pressure, have been performed on synthetic muscovite gouges (average grain size 13 mum, < 10% quartz). The effects of sliding velocity, normal stress and shear strain on the frictional behavior of the muscovite gouges have been studied. Microstructural analysis of the experimentally deformed samples has been done using SEM and XED in order to obtain insight in the operating microphysical processes. The mechanical behavior under th...
The effect of varying enstatite content on the deformation behavior of synthetic, finegrained (1 ... more The effect of varying enstatite content on the deformation behavior of synthetic, finegrained (1 to 2 gm) forsterite-enstatite rock with --,0.5 wt % added water was investigated at temperatures of 900 ø to 1000øC, strain rates between 10 -7 and 10 -5 s -l, and a confining pressure of --,600 MPa. The samples exhibited approached steady state flow at stresses <_60 MPa. The results show that, at constant strain rate, increasing enstatite content is associated with a sharp decrease in flow strength in the range 0-2.5% vol % enstatite, with little further change up to 20 vol %. The observed power law n value of ~ 1.7 and microstructures are similar to those obtained in previous work on material with 2.5 wt % enstatite and are consistent with a water-enhanced grain boundary sliding (GBS) dominated deformation mechanism. Significantly, a negative correlation was found between grain size and enstatite content, indicating that enstatite content played a role in controlling the grain size of the starting materials. Moreover, a high correlation between measured flow strength and grain size was found, consistent with a grain size exponent of-3 in a conventional grain size sensitive flow equation. A water-enhanced deformation mechanism involving GBS accommodated probably by grain boundary diffusion and/or dislocation activity is therefore implied, with the effect of enstatite content on flow strength being an indirect physical effect caused by grain size control. Other effects of second-phase content. such as weakening caused by interphase boundary diffusion and/or migration processes, seem to be unimportant under the conditions investigated.
The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain... more The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain size of--1 pm has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850 ø to 1000øC, strain rates of 10 -7 to 10 -5 s -l and a confining pressure of 600 MPa. Nominally dry material behaved purely elastically. In contrast, samples with 0.5 wt% added water flowed at stresses between 9 and 81 MPa reaching strains up to 12%. Fitting a Dom-type power law to the wet data yielded stress exponents (n) of 1.7 _+ 0.4 and apparent activation energies in the range 302 _+ 22 kJ mo1-1. The wet-deformed samples showed polygonal grains, no crystallographic preferred orientation, no subgrains, low dislocation densities, and evidence for grain boundary cavitation. Grain growth was minor. It is concluded that the wet samples probably deformed by a grain size sensitive, grain boundary sliding dominated mechanism, though the nature of the accommodation process remains unclear, and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated. Application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere. • = Aexp(-Qc/RT)d•/d p (1) where •: represents axial strain rate (s-t), o is the differential stress ((si -•3; MPa), A is a preexponential constant (MPa -n s -I l amp), Qc is an apparent activation energy (J mol -l), R is the gas Paper number 1999JB900075. 0148-0227/99/1999JB900075509.00 constant (J mol -• K-•), T is the absolute temperature (K), n is the stress exponent or stress sensitivity, d is grain size (lam), and p is the grain size exponent with p = 0 for GSI mechanisms. Previous deformation studies on olivine single crystals at high pressures (0.3 to 1.5 GPa) and/or high temperature (1300 ø to 1650øC) have allowed detailed investigation of crystal plasticity and the influence of oxygen fugacity (fO2), oxide activity, and H20 content on dislocation creep mechanisms [see Ricoult and Kohlstedt, 1985; Mackwell et al., 1985; Bai el al., 1991; Bai and Kohlstedt, 1992a, b]. Many workers have also observed GSI dislocation creep behavior and associated water weakening effects in experiments carried out on olivine-rich polycrystalline aggregates. in the earlier studies, natural rocks, mainly dunites. were used [e.g., Carter and Ave'Lallemant. 1970; Blacic, 1972; Kirby and Raleigh, 1973; Post, 1977; Ewms and Goetze, 1979; Chopra and Paterson, 1981, 1984]. More recent studies have focused on aggregates made from either natural mineral separates [e.g., Zeuch and Green, 1979, 1984a, b; Chopra, 1986; Karato et al., 1986; Hitchings et al., 1989; Botch and Green, 1989: Hirth and Kohlstedt 1995a, b] or synthetic olivine [e.g., Relandeau, 1981; Beeman and Kohlstedt, 1993], allowing greater control of bulk chemistry, grain size, second phase, and melt content. Concentrating on the better constrained data, obtained in experiments using gas or molten salt confining media, dislocation creep of dry olivine polycrystals is characterized by stress exponents (n) in the range 3.1 to 4.5 and apparent activation energies of-536 + 35 kJ mol -t. Wet olivine polycrystals (i.e., containing generally between 0.1 to 0.5 wt% H20) deformed in the dislocation creep field exhibit n values of 3.4 to 5.1, apparent activation energies 17,823 17,824 MCDONNELL ET AL.' FLOW OF SYNTHETIC DUNITE of 462 + 60 kJ mol -I [Post, 1977' Chopra and Paterson, 1981, 1984; Karate et at., 1986; Botch and Green, 1989; Hirth and Kohlstedt, 1995a, 1996] and flow stresses between -2.5 [Karate et al, 1986] and -6.5 times [Hirth and Kohlswdt, 19961 lower than obtained for dry material at comparable strain rates and temperatures. GSS diffusion creep behavior has been identified in a number of experimental investigations performed on fine-grained polycrystalline elivine under both confined and unconfined conditions [Schwenn and Goetze, 1978' Relandealt, 1981' Karate el a/., 1986; Chopra, 1986; Wolfi'nstine and Kohlstedt, 1994; Hirth and Kohlstedt, 1995a]. However, the range of conditions investigated is limited. In the case of dry material at 1300 to 1540 øC, n = 1 [Wo!/•'nstine and Kohlstedt, 1994' Hirth and Kohlstedt, 1995b] and p = 2 to 3 [Karate et al., 1986' Wo(fenstine and Kohlstedt, 1994; Hirth and Kohlstedt, 1995b]. For wet systems, deformed at 300 MPa and assumed to be largely melt free [Chopra, 1986; Karate et al. 1986' Rutter and Brodie 1988], the stress exponent (n) in the GSS field falls between 1 and 2. Karate e! al. [1986] obtained a value of n = 1.4 + 0.3 and a grain size exponent p of 3.4 + 0.6 at 1300 øC. Experimentally determined values of the apparent activation energy for wet diffusion creep are poorly constrained with values of 190 and 240 kJ reel -I reported by Chopra [1986] and 240 _ 40 kJ mol -• by Rtltter and Brodie [1988]. Karate el al. [1986] estimate the activation energy to be 250 kJ mol -•, taking a value equal to 2/3 that for lattice difi'usion of O and Si in elivine [.laottl el al.. 198(). 1981' Karate and ()gawa, 1982] and assuming grain boundary diffusion control. Note that in the GSS regime wet samples are 1() to 20 times weaker than dry samples at comparable conditions I Karato et al., 1986]. The above data have been widely used in recent years to model extension of the continental lithosphere [Bassi and nin, 1988' Braun and Beaumont, 1989' (lovers and Wortel, 1993: CIoetingh el al., 1995], assuming upper mantle flow to occur in a zone of symmetrically distributed pure shear [McKenzie, 1978]. Most studies assume that the rheology of the upper mantle is controlled by a fi'ictional sliding law plus a wet or dry dislocation creep law for elivine [e.g., Lynch & Morgan, 1987; Bassi and Bonnin, 1988' Braun and Beaumonl, 1989; Dunbar and Sawyer,
Geological storage of CO 2 in clastic reservoirs and aquifers is expected to have a variety of co... more Geological storage of CO 2 in clastic reservoirs and aquifers is expected to have a variety of coupled chemical-mechanical effects. To investigate the effects of CO 2 injection on creep phenomena, we performed uniaxial compaction experiments on granular aggregates of quartz and feldspar under both wet and dry control conditions. The experiments were performed in constant stress mode. Grain size, temperature, CO 2 partial pressure, and effective stress were varied in order to determine their individual effect. Pore fluid pH was varied by the injection of CO 2 and by addition of acidic and alkaline additives. Pore fluid salinity was increased by the addition of NaCl. Wet samples showed instantaneous compaction upon load application, followed by time-dependent creep. From the mechanical data and microstructures, the main compaction mechanism was inferred to be chemically enhanced microcracking in both quartz and feldspar, with subcritical crack growth, i.e. stress corrosion cracking, controlling deformation in the creep stage. The injection of CO 2 and the concomitant acidification of the pore fluid inhibited microcracking in both the quartz and feldspar samples in line with known effects of pH on stress corrosion cracking. We infer that the injection of CO 2 into quartz-and plagioclase-bearing Hangx, S. J. T., C. J. Spiers, and C. J. Peach (2010), Creep of simulated reservoir sands and coupled chemical-mechanical effects of CO 2 injection, 2 sandstones will inhibit grain-scale microcracking process, and that related geomechanical effects, such as reservoir compaction and surface subsidence, will be negligible compared with the poro-elastic response.
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