annual biogenic particle fluxes to the interior of the north atlantic ocean studied at 34 n 21 w and 48 n 21 w abstract in order to clarify the annual quality quantity and export processes of biogenic particles from the euphotic zone to the deep ocean interior an array of automated time series sediment traps were deployed for 1 year from 4 april 1989 to 17 april 1990 at 34 n 21 w and 48 n 21 w as part of the joint global ocean flux program jgofs north atlantic bloom experiment nabe three sediment traps with 13 time series sediment collectors were placed at both stations approximately 1 and 2 km below the surface and 0 7 km above the bottom they collected settling particles during 26 14 day intervals for 376 days with an 20 day hiatus in september october 1989 for changeover of the trap moorings the collection periods of the six traps were synchronized forming a spatio temporal matrix of 156 samples the annual mass flux at about 2 km deep during this experiment was 22 and 27 g m 2 y 1 at the 34 and 48 n stations consisting of biogenic particles with traceable quantities of lithogenic particle flux the spring particle bloom characterized by the sedimentation of particles relatively enriched by norg began in january at the 34 n station and in march at the 48 n station the bloom continued for 4 5 and 3 months and provided 62 and 50 of the annual biogenic particle mass flux at 2 km at the 34 and 48 n stations the surface bloom penetrated to the ocean interior within a few weeks with apparently accelerated settling speed at deeper layers the order of susceptibility of biogenic elements to mineralization while settling in the 0 7 1 km a b water column was from least to most resistant p norg corg si and ca the c n p ratio at 0 7 km a b was 154 18 1 at the 34 n station and 148 18 1 at the 48 n station

can a fractured caprock self heal 2015 elsevier b v the ability of geologic seals to prevent leakage of fluids injected into the deep subsurface is critical for mitigating risks associated with greenhouse gas sequestration and natural gas production fractures caused by tectonic or injection induced stresses create potential leakage pathways that may be further enhanced by mineral dissolution we present results from reactive flow experiments in fractured caprock dolomitic anhydrite where additional dissolution occurs in the rock matrix adjacent to the fracture surfaces preferential dissolution of anhydrite left a compacted layer of dolomite in the fractures at lower flow rate rock fluid reactions proceeded to near equilibrium within the fracture with preferential flow paths persisting over the 6 month duration of the experiment and a negligible change in permeability at higher flow rate permeability decreased by a dramatic two orders of magnitude this laboratory scale observation of self healing argues against the likelihood of runaway permeability growth in fractured porous caprock composed of minerals with different solubilities and reaction kinetics however scaling arguments suggest that at larger length scales this self healing process may be offset by the formation of dissolution channels our results have relevance beyond the greenhouse gas sequestration problem chemical disequilibrium at waste injection sites and in hydrothermal reservoirs will lead to reactive flows that may also significantly alter formation permeability

contribution of peat compaction to relative sea level rise within holocene deltas 1 modern and forecasted flooding of deltas is accelerated by subsidence of holocene deposits subsidence caused by tectonics isostasy sediment compaction and anthropogenic processes combined with eustatic sea level rise results in drowning and increased flood risk within densely populated deltas many deltaic sedimentary successions include substantial amounts of peat which is highly compressible compared to clay silt and sand peat compaction therefore may contribute considerably to total delta subsidence existing studies are inadequate for quantifying peat compaction across deltas we present a numerical peat compaction model calibrated with an extensive field dataset the model quantifies spatial and temporal trends in peat compaction within fluvial dominated holocene flood basin sequences of different compositions subsidence due to peat compaction is highly variable in time and space with local rates of up to 15 mm yr depending on sedimentary sequence this is extremely important information for developing sound delta management strategies artificial groundwater table lowering may cause substantial additional subsidence subsidence due to peat compaction might even exceed estimates of relative sea level rise and thus may seriously increase the risk of delta drowning and human vulnerability to flooding

mechano chemical interactions in sedimentary rocks in the context of co2 storage weak acid weak effects abstract due to the corrosive nature of dissolved co2 the potential short or long term alteration of rock properties represents a major issue in several sites where natural co2 circulation is observed as well as in reservoirs targeted for storage of anthropogenic co2 to date this has been primarily studied from a transport chemical perspective with laboratory evidence of microstructural modifications together with the consequences for flow properties compared to the transport chemical aspects the mechanical chemical aspects have been less investigated though it is to be expected that mechanical properties e g elastic properties failure parameters and time dependent mechanical behaviour could potentially be affected in a similar manner to hydraulic parameters yet since co2 is a weak acid the ph drop is expected to be moderate with a likely lower limit close to 4 0 the buffering of ph by calcite minerals present in most reservoirs targeted for storage may further limit the ph drop as well as confining it to a localized rock volume around the injection well this leads to the question of the magnitude and time spatial scales of chemically mediated mechanical processes during co2 sequestration the authors propose to address this issue by reviewing recent laboratory based studies restricted to sedimentary rocks namely reservoir rocks carbonate or sandstone intact or fractured caprocks and fault rocks key findings include the following 1 the short term impact on the elastic and inelastic behaviour of intact caprocks remains limited 2 shear strength weakening is likely to be respectively low and low to moderate for shale clay rich and anhydrite rich faults but without modifying slip stability in either case 3 the largest impact is located within carbonate reservoirs but with a broad range of reported responses depending on hydrodynamic conditions closed or open and on dissolution regime uniform or channelling and 4 creep experiments confirm that co2 induced dissolution may enhance long term compaction of carbonate reservoirs but the magnitude of acceleration varying from non significant to 50 times depends to a large extent on site specific conditions grain size ph temperature effective stress state etc which renders any direct extrapolation from laboratory to reservoir scale difficult finally some directions for future research studies are discussed

no geologic evidence that seismicity causes fault leakage that would render large scale carbon capture and storage unsuccessful in a recent perspective 1 zoback and gorelick argued that carbon capture and storage ccs is likely not a viable strategy for reducing co2 emissions to the atmosphere they argued that maps of earthquake epicenters portray earthquakes occurring almost everywhere suggesting that earth s crust is near a critical state so that increments in fluid pressure from injecting co2 at 1 to 3 km depth will likely trigger earthquakes within the reservoir and caprock that would be expected to result in leakage of co2 from the reservoirs to the surface

creep of simulated reservoir sands and coupled chemical mechanical effects of co2 injection 1 geological storage of co2 in clastic reservoirs and aquifers is expected to have a variety of coupled chemical mechanical effects to investigate the effects of co2 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 co2 partial pressure and effective stress were varied in order to determine their individual effect pore fluid ph was varied by the injection of co2 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 co2 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 co2 into quartz and plagioclase bearing 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 poroelastic response

laboratory measurements of elastic properties of carbonate rocks during injection of reactive co2 saturated water 1 to investigate the coupled physical and chemical effects of injecting co2 rich water into carbonate rock samples we monitor the elastic and transport properties of six carbonate rocks along with the calcium content and ph of the pore fluid under constant confining pressure carbonate samples range from calcite limestones containing dolomite to pure calcite mudstones which allow us to study how mineral composition and microstructure affect the magnitude of the observed changes the elastic properties of both the saturated and dry rock i e rock frame show a gradual loss of sstrength upon injection as testified by the continuous decrease in the dry p and s wave velocity and by proxy the bulk and shear moduli the magnitude of the observed changes in the elastic properties varies among the samples with a maximum relative change of 24 7 871 m s and 21 2 443 m s in p and s wave velocities respectively the decrease is also accompanied by a relative increase in permeability up to 495 and porosity up to 19 the observed changes likely derive from a change in microstructure which is monitored over time via scanning electron microscopy the variation in porosity results from two competing and interdependent processes the chemical dissolution of calcite and the resulting mechanical compaction under pressure the results of this study show that upon injection of fluids that are in chemical disequilibrium with the hosting rock the variation of the elastic properties cannot be described by existing rock physics models

effects of sinking velocities and microbial respiration rates on the attenuation of particulate carbon fluxes through the mesopelagic zone the attenuation of sinking particle fluxes through the mesopelagic zone is an important process that controls the sequestration of carbon and the distribution of other elements throughout the oceans case studies at two contrasting sites the oligotrophic regime of the bermuda atlantic time series study bats and the mesotrophic waters of the west antarctic peninsula wap sector of the southern ocean revealed large differences in the rates of particle attached microbial respiration and the average sinking velocities of marine particles two parameters that affect the transfer efficiency of particulate matter from the base of the euphotic zone into the deep ocean rapid average sinking velocities of 270 150 m d 1 were observed along the wap whereas the average velocity was 49 25 m d 1 at the bats site respiration rates of particle attached microbes were measured using novel respire respiration of sinking particles in the subsurface ocean sediment traps that first intercepts sinking particles then incubates them in situ respire experiments yielded flux normalized respiration rates of 0 4 0 1 day 1 at bats when excluding an outlier of 1 52 day 1 while these rates were undetectable along the wap 0 01 0 02 day 1 at bats flux normalized respiration rates decreased exponentially with respect to depth below the euphotic zone with a 75 reduction between the 150 and 500 m depths these findings provide quantitative and mechanistic insights into the processes that control the transfer efficiency of particle flux through the mesopelagic and its variability throughout the global oceans

reaction and diffusion at the reservoir shale interface during co2 storage impact of geochemical kinetics abstract we use a reactive diffusion model to investigate what happens to co2 injected into a subsurface sandstone reservoir capped by a chlorite and illite containing shale seal the calculations simulate reaction and transport of supercritical sc co2 at 348 15 k and 30 mpa up to 20 000 a given the low shale porosity 5 chemical reactions mostly occurred in the sandstone for the first 2000 a with some precipitation at the ss sh interface from 2000 to 4000 a ankerite dolomite and illite began replacing mg fe chlorite at the sandstone shale interface transformation of chlorite to ankerite is the dominant reaction occluding the shale porosity in most simulations from 4000 to 7500 a this carbonation seals the reservoir and terminates reaction overall the carbonates calcite ankerite dolomite chlorite and goethite all remain close to local chemical equilibrium with brine quartz is almost inert from the point of its dissolution precipitation however the rate of quartz reaction controls the long term decline in aqueous silica activity and its evolution toward equilibrium the reactions of feldspars and clays depend strongly on their reaction rate constants microcline is closer to local equilibrium than albite the timing of porosity occlusion mostly therefore depends on the kinetic constants of kaolinite and illite for example an increase in the kaolinite kinetic constant by 0 25 logarithmic units hastened porosity closure by 4300 a the earliest simulated closure of porosity occurred at approximately 108 a for simulations designed as sensitivity tests for the rate constants these simulations also emphasize that the rate of co2 immobilization as aqueous bicarbonate solubility trapping or as carbonate minerals mineral trapping in sandstone reservoirs depends upon reaction kinetics but the relative fraction of each trapped co2 species only depends upon the initial chemical composition of the host sandstone for example at the point of porosity occlusion the fraction of bicarbonate remaining in solution depends upon the initial na and k content in the host rock but the fraction of carbonate mineralization depends only on the ca mg fe content since ankerite is the dominant mineral that occludes porosity the dissolved concentration of ferrous iron is also an important parameter future efforts should focus on cross comparisons and ground truthing of simulations made for standard case studies as well as laboratory measurements of the reactivities of clay minerals

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

interaction between co2 rich sulfate solutions and carbonate reservoir rocks from atmospheric to supercritical co2 conditions experiments and modeling abstract a test site for co 2 geological storage is situated in hontomin burgos northern spain with a reservoir rock that is mainly composed of limestone during and after co 2 injection the resulting co 2 rich acid brine gives rise to the dissolution of carbonate minerals calcite and dolomite and gypsum or anhydrite at depth may precipitate since the reservoir brine contains sulfate experiments using columns filled with crushed limestone or dolostone were conducted under different p p co 2 conditions atmospheric 1 10 3 5 bar subcritical 10 10 bar and supercritical 150 34 bar t 25 40 and 60 c and input solution compositions gypsum undersaturated and gypsum equilibrated solutions we evaluated the effect of these parameters on the coupled reactions of calcite dolomite dissolution and gypsum anhydrite precipitation the crunchflow and phreeqc v 3 numerical codes were used to perform reactive transport simulations of the experiments within the range of p p co 2 and t of this study only gypsum precipitation took place no anhydrite was detected and this only occurred when the injected solution was equilibrated with gypsum under the p p co 2 t conditions the volume of precipitated gypsum was smaller than the volume of dissolved carbonate minerals yielding an increase in porosity up to 4 a decrease in t favored limestone dissolution regardless of p co 2 owing to increasing undersaturation with decreasing temperature however gypsum precipitation was favored at high t and under atmospheric p co 2 conditions but not at high t and under 10 bar of p co 2 conditions the increase in limestone dissolution with p co 2 was directly attributed to ph which was more acidic at higher p co 2 limestone dissolution induced late gypsum precipitation long induction time in contrast to dolostone dissolution which promoted rapid gypsum precipitation moreover owing to the slow kinetics of dolomite dissolution with respect to that of calcite both the volume of dissolved mineral and the increase in porosity were larger in the limestone experiments than in the dolostone ones under all p co 2 conditions 10 3 5 and 10 bar by increasing p co 2 carbonate dissolution occurred along the column whereas it was localized in the very inlet under atmospheric conditions this was due to the buffer capacity of the carbonic acid which maintains ph at around 5 and keeps the solution undersaturated with respect to calcite and dolomite along the column 1d reactive transport simulations reproduced the experimental data carbonate dissolution and gypsum precipitation for different p p co 2 t conditions drawing on reaction rate laws in the literature we used the reactive surface area to fit the models to the experimental data the values of the reactive surface area were much smaller than those calculated from the geometric areas

compaction creep of simulated anhydrite fault gouge by pressure solution theory v experiments and implications for fault sealing the sealing and healing behaviour of faults filled with anhydrite gouge by processes such as pressure solution is of interest in relation both to the integrity of faults cutting geological storage systems sealed by anhydrite caprocks and to seismic events that may nucleate in anhydrite bearing sequences such as those present in the seismogenic zone beneath the apennines we have developed a detailed series of kinetic models for pressure solution in anhydrite fault gouge allowing for dissolution diffusion and precipitation control to estimate the time scale on which such sealing and healing effects occur we compare the models obtained with previously reported experimental data on compaction creep rates in simulated anhydrite fault gouge tested under wet upper crustal conditions the results confirm earlier indications that compaction under these conditions likely occurs by diffusion controlled pressure solution applying our most rigorous model for diffusion controlled pressure solution constrained by the fit to the experimental data we infer that anhydrite fault sealing will occur in a few decades at most which is rapid compared with both co2 storage time scales and with the recurrence interval for seismicity in the apennines

effects of temperature and co2 on the frictional behavior of simulated anhydrite fault rock the frictional behavior of anhydrite bearing faults is of interest to a the safety and effectiveness of co2 storage in anhydrite capped reservoirs b seismicity induced by hydrocarbon production and c natural seismicity nucleated in evaporite formations we performed direct shear experiments on simulated anhydrite fault gouges at a range of temperatures 80 150 c and sliding velocities 0 2 10 ms 1 under a fixed effective normal stress of 25 mpa four types of experiments were conducted 1 dry experiments 2 experiments pressurized with water 3 dry experiments pressurized with co2 and 4 wet experiments pressurized with co2 fluid pressures of 15 mpa were used when applied over the temperature range investigated water saturated samples were found to be up to 15 frictionally weaker than dry equivalents wet samples containing co2 were also up to 15 weaker than co2 free equivalents dry sample strength without co2 was independent of temperature whereas wet samples without co2 strengthened 10 from 80 to 150 c samples containing co2 weakened by 4 dry and 10 wet from 80 to 150 c under the p t conditions investigated only dry anhydrite fault gouge showed velocity weakening behavior above 120 c required for faults to potentially generate earthquakes assuming natural fault gouges are wet in situ seismicity is unlikely to nucleate in anhydrite rich faults though the presence of dolomite or reaction produced calcite may change seismic potential co2 penetration into wet anhydrite rich faults may trigger slip on critically stressed faults due to the observed short term co2 weakening effects excluding possible formation of secondary minerals but is not expected to influence slip stability

fault controlled co2 leakage from natural reservoirs in the colorado plateau east central utah abstract the study investigated a natural analogue for soil co 2 fluxes where co 2 has naturally leaked on the colorado plateau east central utah in order to identify various factors that control co 2 leakage and to understand regional scale co 2 leakage processes in fault systems the total 332 and 140 measurements of soil co 2 flux were made at 287 and 129 sites in the little grand wash lgw and salt wash sw fault zones respectively measurement sites for co 2 flux involved not only conspicuous co 2 degassing features e g co 2 driven springs geysers but also linear features e g joints fractures and areas of diffusive leakage around a fault damage zone co 2 flux anomalies were mostly observed along the fault traces specifically co 2 flux anomalies were focused in the northern footwall of the both lgw and sw faults supporting the existence of north plunging anticlinal co 2 trap against south dipping faults as well as higher probability of the north major fault traces as conduits anomalous co 2 fluxes also appeared in active travertines adjacent to co 2 driven cold springs and geysers e g 36 259 g m 2 d 1 at crystal geyser ancient travertines e g 5 917 g m 2 d 1 joint zones in sandstone e g 120 g m 2 d 1 and brine discharge zones e g 5 515 g m 2 d 1 these observations indicate that co 2 has escaped through those pathways and that co 2 leakage from these fault zones does not correspond to point source leakage the magnitude of co 2 flux is progressively reduced from north i e the lgw fault zone 36 259 g m 2 d 1 to south i e the sw fault zone 1 428 g m 2 d 1 despite new inputs of co 2 and co 2 saturated brine to the northerly sw fault from depth this discrepancy in co 2 flux is most likely resulting from the differences in fault zone architecture and associated permeability structure co 2 rich fluids from the lgw fault zone may become depleted with respect to co 2 during lateral transport resulting in an additional decrease in co 2 fluxes within the sw fault zone in other words co 2 and co 2 charged brine originating from the lgw fault zone could migrate southward over 10 20 km through a series of high permeable aquifers e g entrada navajo kayenta wingate and white rim sandstones these co 2 rich fluids could finally reach the southernmost tumbleweed and chaffin ranch geysers across the sw fault zone the potential lateral transport of both co 2 and co 2 laden brine can be further supported by similar co 2 3 he and 3 he 4 he ratios of gas and a systematic chemical evolution of water emitted from the regional springs and geysers which suggest the same crustal origins of co 2 and co 2 rich brine for the region

influences of chemical degradation on mechanical behaviour of a limestone this paper is devoted to the experimental investigation of chemical degradation effects on mechanical behaviour of a porous limestone hydrostatic and triaxial compression tests are first performed respectively on sound and chemically degraded samples it is found that the chemical degradation induces the diminution of pore collapse limit stress elastic modulus and material cohesion coupled triaxial creep chemical degradation tests are also performed the chemical degradation enhances time dependant deformation and can generate significant increase in permeability through the formation of wormholes networks

frictional properties of sedimentary rocks and natural fault gouge from the longmen shan fault zone sichuan china in this paper we report friction experiments performed on samples collected from the region hit by the 2008 wenchuan earthquake in the longmen shan fault zone lfz of sichuan southwestern china the materials tested consisted of simulated gouges prepared from intact clay rich mudstone and sandstone a calcite limestone plus a natural fault gouge from a trenched surface rupture cutting the mudstone and sandstone the clay rich samples including the natural gouge were dominated by illite and quartz in our experiments we sheared 1 mm thick gouge layers between saw cut driver blocks using a triaxial testing machine at conditions corresponding to 2 km depth in the lfz temperature was varied from 25 c to 150 c and to investigate the velocity dependence of friction we stepped the shear displacement rate between 1 22 and 0 122 m s our results show that the natural gouge was more illite rich and much weaker than the protolith mudstone and sandstone and showed a steady state friction coefficient of 0 4 compared with 0 6 for the latter the limestone gouge displayed values of 0 6 0 7 all samples except the limestone showed stable velocity strengthening slip the limestone showed velocity strengthening at 25 c 50 c but quasi static oscillations at 100 c 150 c along with velocity weakening behavior at 150 c we apply our results to discuss the role of the sedimentary rocks studied during events such as the wenchuan earthquake and argue that the clay rich sediments of the region may have a damping effect upon ruptures propagating from depth whereas the limestone may accelerate propagation producing significant stress drops

geologic factors controlling co2 storage capacity and permanence case studies based on experience with heterogeneity in oil and gas reservoirs applied to co2 storage a variety of structural and stratigraphic factors control geological heterogeneity inferred to influence both sequestration capacity and effectiveness as well as seal capacity structural heterogeneity factors include faults folds and fracture intensity stratigraphic heterogeneity is primarily controlled by the geometry of depositional facies and sandbody continuity which controls permeability structure the permeability structure in turn has implications for co2 injectivity and near term migration pathways whereas the long term sequestration capacity can be inferred from the production history examples of gulf coast oil and gas reservoirs with differing styles of stratigraphic heterogeneity demonstrate the impact of facies variability on fluid flow and co2 sequestration potential beach and barrier island deposits in west ranch field in southeast texas are homogeneous and continuous in contrast seeligson and stratton fields in south texas examples of major heterogeneity in fluvial systems are composed of discontinuous channel fill sandstones confined to narrow sinuous belts these heterogeneous deposits contain limited compartments for potential co2 storage although co2 sequestration effectiveness may be enhanced by the high number of intraformational shale beds these field examples demonstrate that areas for co2 storage can be optimized by assessing sites for enhanced oil and gas recovery in mature hydrocarbon provinces

frictional properties of shale reservoir rocks 1 we performed laboratory friction experiments on shale samples from three hydrocarbon reservoirs to assess compositional controls on fault slip behavior accompanying hydraulic stimulation the samples span a range of clay and total organic content from 10 to 60 by weight and demonstrate fine scale heterogeneity within each reservoir friction measurements demonstrate strong dependence of strength and rate state constitutive properties on shale composition shale samples with clay and organic content above 30 by weight show coefficient of friction 0 4 and consistently velocity strengthening behavior while those below this threshold show increased strength and velocity weakening behavior this transition in frictional strength and stability suggests a change in the shale grain packing framework from rigid clast supported to clay mineral and organic matter supported measurements of gouge dilatancy further support these relationships showing a negative correlation with clay and organic content critical slip evolution distance exhibits similar dependence on composition implying common micromechanical mechanisms for the observed transition in frictional behavior we performed microstructural characterization of the experimental samples and observe changes in the gouge load bearing framework and shear localization features consistent with these mechanical data while these results can be applied generally to infer slip stability on faults in clay bearing sedimentary rock we employ the experimental data to place constraints on microearthquake magnitudes and the occurrence of slow shear slip on preexisting faults during hydraulic stimulation in shale reservoirs

frictional properties of exhumed fault gouges in dfdp 1 cores alpine fault new zealand principal slip zone gouges recovered during the deep fault drilling project dfdp 1 alpine fault new zealand were deformed in triaxial friction experiments at temperatures t of up to 350 c effective normal stresses n of up to 156 mpa and velocities between 0 01 and 3 m s chlorite white mica bearing dfdp 1a blue gouge 90 62 m sample depth is frictionally strong friction coefficient 0 61 0 76 across all experimental conditions tested t 70 350 c n 31 2 156 mpa it undergoes a transition from positive to negative rate dependence as t increases past 210 c the friction coefficient of smectite bearing dfdp 1b brown gouge 128 42 m sample depth increases from 0 49 to 0 74 with increasing temperature and pressure t 70 210 c n 31 2 93 6 mpa the positive to negative rate dependence transition occurs as t increases past 140 c these measurements indicate that in the absence of elevated pore fluid pressures dfdp 1 gouges are frictionally strong under conditions representative of the seismogenic crust

geographical seasonal and depth variation in sinking particle speeds in the north atlantic particle sinking velocity is considered to be a controlling factor for carbon transport to the deep sea and thus carbon sequestration in the oceans the velocities of the material exported to depth are considered to be high in high latitude productive systems and low in oligotrophic distributions we use a recently developed method based on the measurement of the radioactive pair 210po 210pb to calculate particle sinking velocities in the temperate and oligotrophic north atlantic during different bloom stages our estimates of average sinking velocities asvs show that slowly sinking particles 100 m d 1 contribute significantly to carbon flux at all the locations except in the temperate regions during the bloom asvs appear to vary strongly with season which we propose is caused by changes in the epipelagic community structure our results are the first field data to confirm the long standing theory that particle sinking velocities increase with depth with increases of up to 90 between 50 and 150 m depth

effect of sub core scale heterogeneities on acoustic and electrical properties of a reservoir rock a co2 flooding experiment of brine saturated sandstone in a computed tomography scanner the effect of sub core scale heterogeneity on fluid distribution pattern and the electrical and acoustic properties of a typical reservoir rock was studied by performing drainage and imbibition flooding tests with co2 and brine in a laboratory moderately layered rothbach sandstone was used as a test specimen two core samples were drilled one perpendicular and the other parallel to the layering to allow injection of fluids along and normal to the bedding plane during the test 3d images of fluid distribution and saturation levels were mapped by an industrial x ray ct scanner together with simultaneous measurement of electrical resistivity ultrasonic velocities as well as amplitudes the results showed how the layering and the flooding direction influenced the fluid distribution pattern and the saturation level of the fluids for a given fluid saturation level the measured changes in the acoustic and electrical parameters were affected by both the fluid distribution pattern and the layering orientation relative to the measurement direction the p wave amplitude and the electrical resistivity were more sensitive to small changes in the fluid distribution patterns than the pwave velocity the change in amplitude was the most affected by the orientation of the layering and the resulting fluid distribution patterns in some instances the change due to the fluid distribution pattern was higher than the variation caused by the change in co2 saturation as a result the gassmann relation based on uniform or patchy saturation pattern was not suitable to predict the p wave velocity variation overall the results demonstrate the importance of core imaging to improve our understanding of fluid distribution patterns and the associated effects on measured rock physics properties

the effects of lateral variations in rock composition and texture on anhydrite caprock integrity of co2 storage systems abstract we investigated the effect of rock texture and composition on the mechanical strength and volumetric behaviour of anhydrite rich caprock conventional triaxial experiments were performed at 80 c confining pressures of 1 5 35 mpa and strain rates of 10 5 s 1 both dry and in the presence of fluids we determined the failure envelope and the effect of fluids upon it for anhydrite displaying an equigranular texture consisting of euhedral grains we observed a general pattern of increasing peak compressive strength with increasing confining pressure consistent with a previous study on anhydrite from the same formation which showed an irregular texture consisting of interlocking acicular grains embedded in a fine grained matrix the peak strength of euhedral anhydrite was found to be 15 65 lower than that of acicular anhydrite no chemical effects were observed for the fluids tested hydraulic fracturing experiments showed the tensile strength to be approximately 4 mpa for euhedral and 6 mpa for acicular anhydrite we combined the failure envelopes with simple analytical calculations on the effect of cooling on in situ stress showing that tensile caprock failure due to cooling would be unlikely for both types of anhydrite assuming a hypothetical reservoir at a depth of 3800 m however it should be noted that tensile failure may pose an issue for shallower reservoirs or weaker caprock formations

changes in seal capacity of fractured claystone caprocks induced by dissolved and gaseous co2 seepage 1 claystone caprocks are often the ultimate seal for co2 underground storage when residual co2 gas reaches the reservoir top due to buoyancy permeability changes of a fractured claystone due to seepage of co2 enriched brine and water vapor saturated co2 gas are investigated results show that brine flow induces a large porosity increase up to 50 in the vicinity of the fracture due to dissolution of calcite and quartz while permeability remains unchanged conversely cyclic flows of co2 brine and co2 gas increase the fracture aperture abruptly after each gas flow period producing a progressive decrease of the caprock seal capacity aperture increase is controlled by decohesion of the clay framework within a micrometer scale thick layer induced by co2 gas acidification results show that hydraulic aperture increases linearly with duration of the preceding co2 brine flow period emphasizing the kinetic control of the quartz grains dissolution during the brine flow periods

gas geochemistry of natural analogues for the studies of geological co2 sequestration geological sequestration of anthropogenic co2 appears to be a promising method for reducing the amount of greenhouse gases released to the atmosphere geochemical modelling of the storage capacity for co2 in saline aquifers sandstones and or carbonates should be based on natural analogues both in situ and in the laboratory the main focus of this paper has been to study natural gas emissions representing extremely attractive surrogates for the study and prediction of the possible consequences of leakage from geological sequestration sites of anthropogenic co2 i e the return to surface potentially causing localised environmental problems these include a comparison among three different italian case histories i the solfatara crater phlegraean fields caldera southern italy is an ancient roman spa the area is characterised by intense and diffuse hydrothermal activity testified by hot acidic mud pools thermal springs and a large fumarolic field soil gas flux measurements show that the entire area discharges between 1200 and 1500 tons of co2 per day ii the panarea island aeolian islands southern italy where a huge submarine volcanic hydrothermal gas burst occurred in november 2002 the submarine gas emissions chemically modified seawater causing a strong modification of the marine ecosystem all of the collected gases are co2 dominant maximum value 98 43 vol iii the tor caldara area central italy located in a peripheral sector of the quiescent alban hills volcano along the faults of the ardea basin transfer structure the area is characterised by huge co2 degassing both from water and soil although the above mentioned areas do not represent a storage scenario these sites do provide many opportunities to study near surface processes and to test monitoring methodologies

analysis of co2 leakage through low permeability faults from natural reservoirs in the colorado plateau east central utah the numerous co 2 reservoirs in the colorado plateau region of the united states are natural analogues for potential geologic co 2 sequestration repositories to better understand the risk of leakage from reservoirs used for long term underground co 2 storage we examine evidence for co 2 migration along two normal faults from a reservoir in east central utah co 2 charged springs geysers and a hydrocarbon seep are localised along these faults these include natural springs that have been active for long periods of time and springs that were induced by recent drilling the co 2 charged spring waters have deposited travertine mounds and carbonate veins the faults cut siltstones shales and sandstones and the fault rocks are fine grained clay rich gouge generally thought to be barriers to fluid flow the geologic and geochemical data are consistent with these faults being conduits for co 2 to the surface consequently the injection of co 2 into faulted geologic reservoirs including faults with clay gouge must be carefully designed and monitored to avoid slow seepage or fast rupture to the biosphere

the long term fate of co2 in the subsurface natural analogues for co2 storage abstract co 2 is a common gas in geological systems so that planned storage of co 2 in the subsurface may do no more than mimic nature natural co 2 has a wide number of sources that can be at least partly identified by carbon stable isotope geochemistry three pairs of case studies with different reservoir characteristics and co 2 contents have been examined to assess the natural impact of adding co 2 to geological systems carbonate minerals partially dissolve when co 2 is added simply because the co 2 dissolves in water and forms an acidic solution therefore carbonate minerals in the subsurface are not capable of sequestering secondary co 2 the addition of co 2 to a pure quartz sandstone or a sandstone in which the supply of reactive aluminosilicate minerals has been exhausted by excess natural co 2 addition will have no consequences the co 2 will simply saturate the water and then build up as a separate gas phase the addition of co 2 to carbonate cemented sandstone without reactive aluminosilicate minerals will induce a degree of carbonate mineral dissolution but no solid phase sequestration of the added co 2 when co 2 is naturally added to sandstones it will induce combined aluminosilicate dissolution and carbonate cementation if the aluminosilicate minerals contain calcium or magnesium or possibly iron examination of a co 2 filled porous sandstone with abundant reactive aluminosilicate minerals that received a huge co 2 charge about 8000 to 100 000 years ago reveals minimal evidence of solid phase sequestration of the added co 2 this indicates that either dissolution of reactive aluminosilicates or precipitation of carbonate minerals is relatively slow it is very likely that the slow dissolution of reactive aluminosilicates is the rate limiting step solid phase sequestration of co 2 occurs only when reactive aluminosilicates are present in a rock and when the system has had many tens to hundreds of thousands of years to equilibrate the two critical aspects of the behaviour of co 2 when injected into the subsurface are 1 that the rock must contain reactive ca and mg aluminosilicates and 2 that reaction to produce carbonate minerals is extremely slow on a human timescale the reactive minerals include anorthite zeolite smectite and other fe and mg clay minerals such minerals are absent from clean sandstones and limestones but are present in dirty standstones lithic arenites which are mineralogically immature and some mudstones the analysis of geological analogues shows that injection of co 2 into carbonate bearing rocks that do not contain reactive minerals will induce dissolution of the carbonate whether it is a matrix cement rock fragment fault seal or part of a top sealing mudstone

frictional properties of megathrust fault gouges at low sliding velocities new data on effects of normal stress and temperature abstract friction data used in modelling studies of subduction zone seismogenesis are often poorly representative of in situ conditions we investigated the influence of in situ effective stresses and temperatures on the frictional properties of simulated fault gouges prepared either from nankai odp material or illite shale at sliding velocities approaching those relevant for earthquake nucleation and sses biaxial double direct shear experiments were performed at room temperature normal stresses of 5 30 mpa and sliding velocities of 0 16 18 m s all materials exhibited velocity strengthening under these conditions along with an increase in the friction coefficient and slip hardening rate with increasing normal stress illite gouge showed increased velocity strengthening towards higher normal stresses the effect of temperature was investigated by means of ring shear experiments on illite gouge at 200 300 c an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa and sliding velocities of 1 100 m s these experiments showed a transition from velocity strengthening to velocity weakening at 250 c our results provide a possible explanation for the updip seismogenic limit within subduction zone megathrusts and imply an enhanced tendency for earthquake nucleation and sses at low effective normal stresses

rock physics analysis and time lapse rock imaging of geochemical effects due to the injection of co2 into reservoir rocks the fundamental concept of time lapse seismic monitoring is that changes in physical parameters such as saturation pore fluid pressure temperature and stress affect rock and fluid properties which in turn alter the seismic velocity and density increasingly however time lapse seismic monitoring is called upon to quantify subsurface changes due in part to chemical reactions between injected fluids and the host rocks this study springs from a series of laboratory experiments and high resolution images assessing the changes in microstructure transport and seismic properties of fluid saturated sandstones and carbonates injected with co2 results show that injecting co2 into a brine rock system induces chemo mechanical mechanisms that permanently change the rock frame injecting co2 into brine saturated sandstones induces salt precipitation primarily at grain contacts and within small pore throats in rocks with porosity lower than 10 salt precipitation reduces permeability and increases p and s wa

percolation of co2 rich fluids in a limestone sample evolution of hydraulic electrical chemical and structural properties percolation of co2 rich fluids in limestones causes the dissolution and eventual reprecipitation of calcium carbonate minerals which affect the rock microstructure and change the rock petrophysical properties i e hydraulic electrical and elastic properties in addition microstructural changes further feed back to affect the chemical reactions to better understand this coupled problem and to assess the possibility of geophysical monitoring we performed reactive percolation laboratory experiments on a well characterized carbonate sample 35 cm in length and 10 cm in diameter in a comprehensive study we present integrated measurements of aqueous chemistry ph calcium concentration and total alkalinity petrophysical properties permeability electrical formation factor and acoustic velocities and x ray tomography imaging the measured chemical and electrical parameters allowed rapid detection of the dissolution of calcite in the downstream fluid after circulating fluids of various salinities at 5ml min 1 for 32 days about 290 pore sample volumes at a pco2 of 1 atm ph 4 porosity increased by 7 from 0 29 to 0 31 permeability increased by 1 order of magnitude from 0 12 d to 0 97 d and the electrical formation factor decreased by 15 from 15 7 to 13 3 x ray microtomography revealed the creation of wormholes these along with the convex curvature of the permeability porosity relationship are consistent with a transport controlled dissolution regime for which advection processes are greater than diffusion processes confirming results from previous numerical studies this study shows that nonseismic geophysical techniques i e electrical measurements are promising for monitoring geochemical changes within the subsurface due to fluid rock interactions

undrained poroelastic response of sandstones to deviatoric stress change deformation of porous crustal rock through diagenesis tectonic loading or other processes can change pore volume and affect fluid pressure the largest stress induced pore pressure changes occur when fluid is trapped in pores in an undrained condition we have measured the undrained poroelastic response of two sandstones to changes in mean and deviatoric stress pore pressure was found to respond to mean stress m in the usual manner p b m b ranging from 0 4 to 0 7 nearly independent of the ambient deviatoric stress state however variations in deviatoric stress d 1 3 2 at constant mean stress were also found to induce a reversible elastic pore pressure response to stress levels up to and exceeding 80 percent failure stress i e p d m const the coefficient became more negative with increasing deviatoric stress level in sandstone and ottawa sand samples that is represents a dilatant response where increased deviatoric stress causes a decrease in pore pressure the poroelastic response to deviatoric stress is explained in terms of anisotropic matrix stiffening due to closure of crack like pore space or flattening of grain contacts at high ambient stress levels and can be important in calculations of earthquake stress transfer

high export via small particles before the onset of the north atlantic spring bloom sinking organic matter in the north atlantic ocean transfers 1 3 gt carbon year 1 from the surface ocean to the interior the majority of this exported material is thought to be in form of large rapidly sinking particles that aggregate during or after the spring phytoplankton bloom however recent work has suggested that intermittent water column stratification resulting in the termination of deep convection can isolate phytoplankton from the euphotic zone leading to export of small particles we present depth profiles of large 0 1mm equivalent spherical diameter esd and small 300m depth leading to deep mixing of particles as deep as 600m subsequent re stratification could trap these particles at depth and lead to high particle fluxes at depth without the need for aggregation mixed layer pump overall we suggest that pre bloom fluxes to the mesopelagic are significant and the role of small sinking particles requires careful consideration this article is protected by copyright all rights reserved

the initial stages of cave formation beyond the one dimensional paradigm article i nfo the solutional origin of limestone caves was recognized over a century ago but the short penetration length of an undersaturated solution made it seem impossible for long conduits to develop this is contradicted by field observations where extended conduits sometimes several kilometers long are found in karst environments however a sharp drop in the dissolution rate of caco3 near saturation provides a mechanism for much deeper penetration of reactant the notion of a kinetic trigger a sudden change in rate constant over a narrow concentration range has become a widely accepted paradigm in speleogenesis modeling however it is based on one dimensional models for the fluid and solute transport inside the fracture assuming that the dissolution front is planar in the direction perpendicular to the flow here we show that this assumption is incorrect a planar dissolution front in an entirely uniform fracture is unstable to infinitesimal perturbations and inevitably breaks up into highly localized regions of dissolution this provides an alternative mechanism for cave formation even in the absence of a kinetic trigger our results suggest that there is an inherent wavelength to the erosion pattern in dissolving fractures which depends on the reaction rate and flow rate but is independent of the initial roughness in contrast to one dimensional models two dimensional simulations indicate that there is only a weak dependence of the breakthrough time on kinetic order localization of the flow tends to keep the undersaturation in the dissolution front above the threshold for non linear kinetics

influence of pore fluid salt content on compaction creep of calcite aggregates in the presence of supercritical co2 abstract we have performed uniaxial compaction experiments on simulated porous carbonate rocks crushed and sieved carrara marble using pore fluids of varying salinity saturated with co2 in order to investigate the long term chemically coupled effects of co2 on creep of such rock materials experiments were performed at reservoir conditions equivalent to 3 km depth which is up to 30 mpa effective pressure and a temperature of 80 c on samples with mean grain sizes of 28 37 50 and 106 m the tests were carried out using saline pore fluid solutions containing 0 0 6 1 2 and 3 m nacl or 0 1 and 2 m mgcl2 with and without supercritical co2 at a pressure of 10 mpa addition of either salt nacl or mgcl2 to the pore fluid in samples compacted with co2 caused a reduction in compaction creep rate at low salt concentrations 0 6 1 m as salt concentration was increased to values of 2 3 m creep rates increased to approach or exceed values obtained in salt free experiments by comparison of our data on the compaction creep of carbonate aggregates in the presence of added salts and supercritical co2 with microphysical models and previous work on fluid enhanced deformation mechanisms we suggest that the effect of increasing salt concentration in samples tested with supercritical co2 is the net result of progressive inhibition of pressure solution and enhancement of the role of subcritical crack growth the present results imply that storage of co2 in highly porous carbonate formations increases creep rates by up to ten times depending on the salt concentration however it remains difficult to quantify the rate of compaction creep of specific carbonate reservoirs in the context of co2 storage as compaction behaviour is strongly influenced by porosity and other microstructural and compositional factors as well as by salinity

the effect of micrite content on the acoustic velocity of carbonate rocks abstractwe have demonstrated the effect of micrite content on the acoustic properties of well controlled microstructures that are created in the laboratory to closely mimic those of carbonate rocks in particular we examined the effect of micrite content on the acoustic velocity sensitivity of velocity to pressure and changes in velocity resulting from dissolution upon saturation with a reactive fluid we followed dunham s classification and fabricated the samples by mixing coarse sand sized and very fine micrite sized calcite grains in different ratios and then cold compressing the mixture the acoustic velocities were measured under benchtop conditions and as functions of confining pressure before and after the injection of a co2 aqueous solution the benchtop measurements indicated that the addition of micrite made the frame of the carbonate samples stiffer because the sensitivity of velocity to pressure decreases as the content of micrite increases we hypothesized a stiffer pore structure in

recent advances in time lapse laboratory rock physics for the characterization and monitoring of fluid rock interactions abstractmonitoring thermo chemo mechanical processes geophysically e g fluid disposal or storage thermal and chemical stimulation of reservoirs or natural fluids simply entering a new system raises numerous concerns because of the likelihood of fluid rock chemical interactions and our limited ability to decipher the geophysical signature of coupled processes one of the missing links is understanding the evolution of seismic properties together with reactive transport because rock properties evolve as a result of chemical reactions and vice versa capturing this coupling experimentally is one of the missing elements in the existing literature this paper describes recent advances in rock physics experiments to understand the effects of dissolution induced compaction on acoustic velocity porosity and permeability this paper has a dual aim understanding the mechanisms underlying permanent modifications to the rock microstructure and providing a richer set of experimental information to inform th

an experimental study of acoustic responses on the injection of supercritical co2 into sandstones from the otway basin quantitative knowledge of the acoustic response of rock from an injection site on supercritical co2 scco2 saturation is crucial for understanding the feasibility of time lapse seismic monitoring of co2 plume migration a suite of shaley sandstones from the injection interval of the crc 2 well otway basin australia is tested to reveal the effects of supercritical co2 injection on acoustic responses co2 is first injected into dry samples flushed out with brine and then injected again into brine saturated samples such a suite of experiments allows us to obtain acoustic velocities of the samples for a wide range of co2 brine saturations from 0 to 100 on injection of scco2 into brine saturated samples the rocks exhibit a decrease of compressional velocities by about 7 with the increase of co2 saturation from 0 to a maximum of about 50 anisotropy of the shaley sandstones from the otway basin must be taken into account as the difference in the velocities normal and parallel to bedding is comparable with the perturbation due to co2 injection and the samples of different orientations exhibit transition from gassmann hill to gassmann wood bound at different scco2 saturations changes of the dry samples before and after the co2 injection if any are not traceable by acoustic methods

influence of co2 on 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

co2 sequestration in a uk north sea analogue for geological carbon storage the fizzy discovery a southern north sea uk gas accumulation with 50 natural co 2 content provides an opportunity to study the long term quantity of co 2 related mineral reaction as an analogue for engineered co 2 storage the reservoir contains diagenetic dolomite typical of the formation to identify and quantify any sequestration related dolomite is challenging to this end co 2 was extracted by stepwise extraction from dolomite from both the fizzy discovery and equivalent sandstones from a low co 2 location between 0 and 22 of the dolomite in the fizzy discovery precipitated due to the high co 2 concentration this corresponds to 11 8 of the recent high co 2 charge sequestered as dolomite a relatively low proportion after 50 m y of potential co 2 water rock interaction

frictional properties of a low angle normal fault under in situ conditions thermally activated velocity weakening the zuccale fault is a regional low angle normal fault exposed on the isle of elba in central italy that accommodated a total shear displacement of 6 8 km the fault zone structure and fault rocks formed at 8 km crustal depth the present day fault structure is the final product of several deformation processes superposed during the fault history in this study we report results from a series of rotary shear experiments performed on 1 mm thick powdered gouges made from several fault rock types obtained from the zuccale fault the tests were done under conditions ranging from room temperature to in situ conditions i e at temperatures up to 300 c applied normal stresses up to 150 mpa and fluid saturated the ratio of fluid pressure to normal stress was held constant at either 0 4 or 0 8 to simulate an overpressurized fault the samples were sheared at a constant sliding velocity of 10 m s for at least 5 mm after which a velocity stepping sequence from 1 to 300 m s was started to determine the velocity dependence of friction this can be represented by the rate and state parameter a b which was determined by an inversion of the data to the rate and state equations friction of the various fault rocks varies between 0 3 and 0 8 similar to values obtained in previous studies and decreases with increasing phyllosilicate content friction decreases mildly with temperature whereas normal stress and fluid pressure do not affect friction values systematically all samples exhibited velocity strengthening conditionally stable behavior under room temperature conditions and a b increased with increasing sliding velocity in contrast velocity weakening accompanied by stick slips was observed for the strongest samples at 300 c and sliding velocities below 10 m s an increase in fluid pressure under these conditions led to a further reduction in a b for all samples so that they exhibited unstable stick slip behavior at low sliding velocity the results suggest that phyllosilicate bearing fault rocks can deform by stable aseismic creep at low resolved shear stress and low shear rate conditions an increase in fluid pressure or loading of stronger portions could lead to a runaway instability the runaway instability might be limited in size because of 1 the fault heterogeneity 2 the observed strengthening at higher sliding velocities and 3 a co seismic drop in pore fluid pressure

slip weakening as a mechanism for slow earthquakes slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake laboratory simulations of slow slip in rock samples taken from the nankai subduction zone japan reveal similar characteristics to fast earthquakes implying that some slow slip events could be prematurely arrested earthquakes

healing and sliding stability of simulated anhydrite fault gouge effects of water temperature and co2 abstract anhydrite bearing faults are currently of interest to 1 co2 storage sites capped by anhydrite caprocks such as those found in the north sea and 2 seismically active faults in evaporite formations such as the italian apennines in order to assess the likelihood of fault reactivation the mode of fault slip and or fault leakage it is important to understand the evolution of frictional strength during periods of no slip and upon reloading healing and relaxation behavior and of the velocity dependence of friction of anhydrite fault gouge therefore we performed slide hold slide experiments combined with a velocity stepping sequence using simulated anhydrite fault gouge 95 wt caso4 vacuum dry and water wet experiments were performed at temperatures ranging from 20 to 150 c and at an effective normal stress of 25 mpa we also performed tests using dry co2 water wetted co2 and co2 saturated water as pore fluid but only at 120 c if pore fluid was present a fluid pressure of 15 mpa was present vacuum dry samples exhibit similar frictional healing to samples containing lab air but healing is significantly enhanced in wet samples dry samples exhibit velocity weakening behavior at t 120 c and wet samples exhibit velocity strengthening behavior over the full temperature range the presence of co2 does not influence the healing behavior or the velocity dependence of friction samples containing water wetted co2 exhibit behavior similar to wet samples we infer that the healing in dry samples is controlled by plastic asperity creep dieterich type possibly through dislocation creep and or twinning in wet samples healing is inferred to be controlled by increases in contact area and cohesion by pressure solution using a pressure solution rate model to extrapolate healing by contact area growth indicates that the maximum re strengthening through such a mechanism will only take days to tens of days

diagenetic compaction experiments on simulated anhydrite fault gouge under static conditions faults that crosscut subsurface co2 storage systems offer potential leakage pathways especially if fault reactivation and dilation occur after reactivation however newly formed fault gouge is expected to gradually compact and seal as a function of time to estimate the time scale on which this occurs the processes that control compaction must be understood we performed uniaxial compaction experiments on simulated anhydrite fault gouge to investigate the deformation mechanisms that operate under postslip conditions in faulted anhydrite caprocks this involved constant stress 5 12 mpa and stress stepping experiments 5 7 5 10 mpa performed at 80 c under dry and wet conditions on fault gouge samples prepared from crushed natural anhydrite sieved into different grain size fractions in the range 20 500 m dry samples showed little to no compaction creep whereas wet samples i e flooded with presaturated caso4 solution showed rapid compaction our mechanical data and microstructural observations on wet samples suggest that for fine grain sizes 50 m and low stresses gouge compaction is controlled by diffusion controlled pressure solution with increasing grain size and stress fluid assisted subcritical microcracking becomes dominant pressurizing solution flooded samples with co2 15 mpa led to no significant effect on compaction rates in fine grained material but it decreased compaction rates in coarse samples since fine grain sizes are expected in reactivated faults we infer that pressure solution will dominate in anhydrite cap rocks with extrapolation of our lab data to reservoir conditions suggesting sealing time scales of a few decades

influence of subduction zone conditions and gouge composition on frictional slip stability of megathrust faults to understand the temperature depth distribution of destructive earthquakes in subduction megathrusts and the mechanisms of nucleation of these events data on the frictional behaviour of phyllosilicate quartz rich megathrust fault gouges under in situ conditions are needed we performed rotary shear friction experiments at effective normal stresses of 25 200 mpa pore fluid pressures of 50 200 mpa at 140 600 c and sliding velocities of 1 100 m s using gouge mixtures with an illite quartz ratio between 65 35 and zero experiments on 65 35 mixtures deformed at an effective normal stress neff of 170 mpa a pore fluid pressure pf of 100 mpa and 150 500 c provided a reference dataset this showed three temperature dependent slip stability regimes with potentially unstable velocity weakening behaviour at 250 400 c and velocity strengthening at lower and higher temperatures the velocity weakening regime was found to shift towards higher temperatures with decreasing neff being located at 350 600 c at 50 mpa increasing quartz content and decreasing sliding velocity also displaced the velocity weakening regime towards lower temperatures increasing pf increased a b at all temperatures narrowing the temperature extent of the velocity weakening regime we explain our results qualitatively in terms of a microphysical model in which changes in friction coefficient and a b with velocity and temperature are brought about by changes in the relative importance of deformation of the clast phase by thermally activated stress corrosion cracking and pressure solution versus athermal granular flow of the mixture accompanied by dilatation our results imply that the depth extent of the seismogenic zone on subduction megathrusts depends not only on temperature and that to predict its location it is essential to have well constrained depth profiles for pore fluid pressure and effective normal stress

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

effect of fluid salinity on subcritical crack propagation in calcite abstract the slow propagation of cracks also called subcritical crack growth is a mechanism of fracturing responsible for a ductile deformation of rocks under crustal conditions in the present study the double torsion technique was used to measure the effect of fluid chemistry on the slow propagation of cracks in calcite single crystals at room temperature time lapse images and measurements of force and load point displacement allowed accurate characterization of crack velocities in a range of 10 8 to 10 4 m s velocity curves as a function of energy release rates were obtained for different fluid compositions varying nh 4 cl and nacl concentrations our results show the presence of a threshold in fluid composition separating two regimes weakening conditions where the crack propagation is favored and strengthening conditions where crack propagation slows down we suggest that electrostatic surface forces that modify the repulsion forces between the two surfaces of the crack may be responsible for this behavior

3d analysis of geometry and flow changes in a limestone fracture during dissolution summary the effects of reactive transport on fracture geometry and fluid flow were investigated through an integrated experimental and modelling approach a fractured limestone sample 90 calcite was injected with an acidic co2 rich solution over a period of 55 h to induce carbonate dissolution the changes in fracture geometry and related parameters are reported for six data sets obtained from synchrotron x ray micro tomography experiments a series of algorithms was used to extract the aperture and fracture walls from 3d images and allowed quantification of the geometry changes with an optical resolution of 4 91 m in addition measurement of fluid chemistry hydraulic tests and computation of navier stokes flow constrained the characterisation of the dissolution process the effects of reactive transport on fracture geometry and fluid flow were then discussed the presence of silicates in the rocks led to heterogeneous dissolution at the micro scale despite dissolution appearing to be quite homogeneous at sample scale no formation of preferential flow pathways was noticed although heterogeneous dissolution at the micro scale led to fracture walls and aperture decorrelation and to modification of the flow velocity profiles in the fracture

experimental investigation into the sealing capability of naturally fractured shale caprocks to supercritical carbon dioxide flow geological storage of co2 is considered a solution for reducing the excess co2 released into the atmosphere low permeability caprocks physically trap co2 injected into underlying porous reservoirs injection leads to increasing pore pressure and reduced effective stress increasing the likelihood of exceeding the capillary entry pressure of the caprocks and of caprock fracturing assessing on how the different phases of co2 flow through caprock matrix and fractures is important for assessing co2 storage security fractures are considered to represent preferential flow paths in the caprock for the escape of co2 here we present a new experimental rig which allows 38 mm diameter fractured caprock samples recovered from depths of up to 4 km to be exposed to supercritical co2 scco2 under in situ conditions of pressure temperature and geochemistry in contrast to expectations the results indicate that scco2 will not flow through tight natural caprock fractures even with a differential pressure across the fractured sample in excess of 51 mpa however below the critical point where co2 enters its gas phase the co2 flows readily through the caprock fractures this indicates the possibility of a critical threshold of fracture aperture size which controls co2 flow along the fracture

measurements of the elastic and anelastic properties of sandstone flooded with supercritical co2 the aim of this study was to investigate the effects of supercritical co2 scco2 injection on the elastic and anelastic properties of sandstone at seismic and ultrasonic frequencies we present the results of the low frequency and ultrasonic experiments conducted on water saturated sandstone donnybrook western australia flooded with scco2 the sandstone was cut in the direction perpendicular to a formation bedding plane and tested in a hoek triaxial pressure cell during the experiments with scco2 the low frequency and ultrasonic systems and the pump dispensing scco2 were held at a temperature of 42 c the elastic parameters obtained for the sandstone with scco2 at seismic 0 1 hz 100 hz and ultrasonic 0 5 mhz frequencies are very close to those for the dry rock the extensional attenuation was also measured at seismic frequencies for the dry water saturated and scco2 injected sandstones the applicability of gassmann s fluid substitution theory to obtained results was also tested during the experiments

mechanical properties of organic matter in shales mapped at the nanometer scale abstract the mechanical properties of organic matter strongly affect the way shales deform and fracture however the way organic matter responds to mechanical stresses is poorly understood representing a critical obstacle to assessing oil and gas production in shale formations little is known about the mechanical properties of organic matter in fine grained rocks primarily because it often occupies tiny nanometer scale voids between the mineral grains which cannot be accessed using standard mechanical testing techniques here we use a new atomic force microscopy technique peakforce qnm to map the mechanical properties of organic and inorganic components at the nanometer scale we find that the method is able to distinguish between different phases such as pyrite quartz clays and organic matter moreover within the organic component young s modulus values ranged from 0 to 25 gpa in 3 different samples all of which come from thermally mature type ii iii source rocks in the dry gas window a modal value of 15 16 gpa was measured with additional peaks measured at 10 gpa in addition the maps suggest that some porous organic macerals possess a soft core surrounded by a harder outer shell 50 100 nm thick thus our results demonstrate that the method represents a powerful new petrographic tool with which to characterize the mechanical properties of organic rich sedimentary rocks

fluid mineral reactions and trace metal mobilization in an exhumed natural co2 reservoir green river utah red sandstones near green river utah united states have been bleached by diagenetic fluids field relationships modeling fluid inclusion and isotopic data suggest that the causal fluid was a co2 charged brine distinguishing this site from hydrocarbon related bleaching elsewhere on the colorado plateau mineralogical and chemical profiles from unbleached to bleached sandstone show that bleaching is related to hematite dissolution and precipitation of a 1 2 cm band of secondary oxide and carbonate at the reaction front trace metals are mobilized by the fluid and concentrated near the reaction front high flux fluid pathways are more heavily altered with large scale secondary calcite and iron oxide precipitation changes may be modeled by a reaction with stoichiometry 20fe2o3 5ch4 64co2 19h2o 11h 30fe2 10fehco3 59hco3 the fe rich reduced fluid precipitates iron oxides and carbonate at the reaction front between bleached and unbleached sandstone these findings make the site an analogue for processes occurring over long time scales in geological carbon storage projects trace metals moblized by co2 charged brines are likely to be rapidly re precipitated at reaction fronts

investigation of porosity and permeability effects from microstructure changes during limestone dissolution we studied experimentally the dissolution of a porous limestone core during co2 enriched water injection we measured the changes in porosity and permeability arising from modifications of the pore network geometry and the fluid rock interface a methodology based on periodic x ray microtomography imaging was implemented to record the evolution of the time and scale dependent microstructures with a spatial resolution of 4 91 m two processes were successively involved in the rapid permeability increase of the sample as documented from microscale to core scale measurements

permeability impairment of a limestone reservoir triggered by heterogeneous dissolution and particles migration during co2 rich injection 1 a co2 rich brine core flood experiment in calcite limestone for conditions representative of underground storage p 12 mpa and t 100 c was performed in order to explore the dissolution mechanisms arising at moderate co2 partial pressure 0 3 mpa an increase of the total porosity t accompanied by a persistent permeability k decrease was measured the mechanisms controlling this atypical anticorrelated k t relationship were investigated from the analysis of high resolution x ray microtomography images of the sample acquired before and after the experiment all the evidences converge to the conclusion that the ubiquitous decrease of permeability measured during the 44 h of dissolution is due to the clogging of a fraction of the macroporosity by microporous material triggered by the rearrangement of the detached undissolved particles this mechanism results in the development of low permeability zones bridging the macroporosity

influence of hydrocarbon injection on the compaction by pressure solution of a carbonate rock an experimental study under triaxial stresses abstract the preservation of good petrophysical properties high porosity high permeability at great depth in carbonate rocks may lead to the existence of a deeply buried reservoir dbr a target of interest for the oil industry one of the key processes controlling diagenesis of the burial environment is pressure solution creep psc an efficient compaction process responsible for the evolution of porosity and permeability in many carbonate reservoirs in this experimental study we examine the effect of i the presence of oil in the pore space and ii its timing of injection on the psc process and the petrophysical properties of a carbonate rock the experiments were performed using a flow through high pressure cell allowing the simulation of the pressure stresses and temperature conditions of a dbr multi disciplinary data mechanical chemical petrographical and petrophysical demonstrate that without oil in the pore space the main diagenetic process is the psc a process reducing by three the initial porosity but having no influence on intrinsic water permeability an early injection of oil prior to water circulation causes the inhibition of psc by the coating of the grains leading to the preservation of porosity conversely a late injection of oil does not preserve initial porosity the dataset obtained from these experiments show the importance of the timing of oil charging in a reservoir in the preservation of initial porosity at great depth by the inhibition of psc however the coating of grains by hydrocarbons may also inhibit further diagenetic processes leading to a creation of secondary porosity at depth

evolution of fault permeability during episodic fluid circulation evidence for the effects of fluid rock interactions from travertine studies utah usa faults are known to be important pathways for fluid circulation within the crust the transfer properties along faults can evolve over time and space the little grand wash and salt wash normal faults located on the colorado plateau are well known examples of natural co2 leakage systems from depth to surface previous studies dated and established a chronology of co2 enriched fluid source migration along the fault traces and linked the aragonite veins observed close to crystal geyser to co2 pulses however multiple circulation events recorded along a given fault segment deserve to be studied in minute detail in order to unravel the chronology of these events precipitation processes and associated mechanisms a combination of structural geology petrography u th dating oxygen and carbon isotope analysis were used to study the fault related co2 enriched paleo circulations in order to build a conceptual model of co2 circulation along the faults this study resulted in the precise descriptions of the features attesting co2 enriched fluid circulation by a characterization of their relationship and architecture at the outcrop scale these features are witnesses of a large range of circulation sealing mechanisms as well as changes in fluid chemistry and thermodynamic state of the system providing evidence for i the evolution of the fluid through a pathway from depth to the surface and ii different cycles of fault opening and sealing large circulation events linked with fault opening sealing are observed and calibrated in nature with millennial circulation and sealing time lapses numerical modelling indicates that such sealing timescale can be explained by the introduction of a fault sealing factor that allows modifying permeability with time and that is calibrated by the natural observations

variability of el nino southern oscillation related noise in the equatorial pacific ocean 1 this study documents the variability of noise in the equatorial pacific associated with the el nino and la nina events based on the ensemble retrospective forecasts of the climate forecast system it is found that the noise measured by the ensemble spread in the western equatorial pacific zonal wind stress is enhanced around and before the peak of el nino events and weakened around 2 3 months after the peak of el nino events the change in the wind stress noise is communicated to the thermocline depth in the eastern equatorial pacific with about 1 month time lag the eastern equatorial pacific sst noise however decreases during the decay stages of el nino events while the corresponding noise in the surface zonal wind and heat flux increases this decrease in the sst noise is related to the weakening of the sst front and the suppression of the tropical instability waves along the flanks of the equatorial pacific cold tongue that is associated with the sst warming due to el nino events similar relations are seen during la nina events except that the changes are in opposite sense as such the signal to noise ratio and thus the predictability for the eastern equatorial pacific sst is relatively high during warm events compared to cold events

seal integrity and feasibility of co 2 sequestration in the teapot dome eor pilot geomechanical site characterization this paper reports a preliminary investigation of co2 sequestration and seal integrity at teapot dome oil field wyoming usa with the objective of predicting the potential risk of co2 leakage along reservoir bounding faults co2 injection into reservoirs creates anomalously high pore pressure at the top of the reservoir that could potentially hydraulically fracture the caprock or trigger slip on reservoir bounding faults the tensleep formation a pennsylvanian age eolian sandstone is evaluated as the target horizon for a pilot co2 eor carbon storage experiment in a three way closure trap against a bounding fault termed the s1 fault a preliminary geomechanical model of the tensleep formation has been developed to evaluate the potential for co2 injection inducing slip on the s1 fault and thus threatening seal integrity uncertainties in the stress tensor and fault geometry have been incorporated into the analysis using monte carlo simulation the authors find that even the most pessimistic risk scenario would require 10 mpa of excess pressure to cause the s1 fault to reactivate and provide a potential leakage pathway this would correspond to a co2 column height of 1 500 m whereas the structural closure of the tensleep formation in the pilot injection area does not exceed 100 m it is therefore apparent that co2 injection is not likely to compromise the s1 fault stability better constraint of the least principal stress is needed to establish a more reliable estimate of the maximum reservoir pressure required to hydrofracture the caprock

co2 wettability of seal and reservoir rocks and the implications for carbon geo sequestration we review the literature data published on the topic of co2 wettability of storage and seal rocks we first introduce the concept of wettability and explain why it is important in the context of carbon geo sequestration cgs projects and review how it is measured this is done to raise awareness of this parameter in the cgs community which as we show later on in this text may have a dramatic impact on structural and residual trapping of co2 these two trapping mechanisms would be severely and negatively affected in case of co2 wet storage and or seal rock overall at the current state of the art a substantial amount of work has been completed and we find that sandstone and limestone plus pure minerals such as quartz calcite feldspar and mica are strongly water wet in a co2 water system oil wet limestone oil wet quartz or coal is intermediate wet or co2 wet in a co2 water system the contact angle alone is insufficient for predicting capillary pressures in reservoir or seal rocks the current contact angle data have a large uncertainty solid theoretical understanding on a molecular level of rock co2 brine interactions is currently limited in an ideal scenario all seal and storage rocks in cgs formations are tested for their co2 wettability achieving representative subsurface conditions especially in terms of the rock surface in the laboratory is of key importance but also very challenging

impact of coating development on the hydraulic and transport properties in argillaceous limestone fracture 1 results are reported for an acidic water flow through experiment conducted in a fractured argillaceous limestone sample 73 carbonates the change in fracture geometry and related parameters is reported for six data sets obtained from synchrotron x ray microtomography experiments high resolution three dimensional images of the sample allowed quantification of the changes in fracture morphology at a spatial resolution of 6 m mineral mass loss and permeability changes in the sample were also determined several physico chemical phenomena were identified during the experiment initial smooth fracture surfaces evolved rapidly toward rough surfaces with uneven clay coverage due to the preferential dissolution of carbonate minerals compared to clay minerals whose dissolution rate is about 106 slower a microporous clay coating progressively developed at the fluid rock interface during heterogeneous dissolution of the fracture while the global dissolution rate of the fracture walls exponentially decreased the increase in surface roughness and the presumed reorganization of clays caused a progressive reduction in permeability during the last flow through stage a large decrease in sample permeability was attributed to the large removal of clay particles this process was responsible for a dramatic collapse of the fracture walls near the sample inlet and led to the development of preferential flow pathways the development of the clay coating also acted as a barrier to flow and mass transfer between calcite grains and bulk solution and affected transport processes within the fracture

attenuation of particulate organic carbon flux in the scotia sea southern ocean is controlled by zooplankton fecal pellets the southern ocean so is an important co2 reservoir some of which enters via the production sinking and remineralization of organic matter recent work suggests that the fraction of production that sinks is inversely related to production in the so a suggestion that we confirm from 20 stations in the scotia sea the efficiency with which exported material is transferred to depth transfer efficiency is believed to be low in high latitude systems however our estimates of transfer efficiency are bimodal with stations in the seasonal ice zone showing intense losses and others displaying increases in flux with depth zooplankton fecal pellets dominated the organic carbon flux and at stations with transfer efficiency 100 fecal pellets were brown indicative of fresh phytodetritus we suggest that active flux mediated by zooplankton vertical migration and the presence of sea ice regulates the transfer of organic carbon into the oceans interior in the southern ocean

gas water rock interactions in frio formation following co2 injection implications for the storage of greenhouse gases in sedimentary basins to investigate the potential for the geologic storage of co2 in saline sedimentary aquifers 1600 t of co2 were injected at 1500 m depth into a 24 m thick sandstone section of the frio formation a regional brine and oil reservoir in the u s gulf coast fluid samples obtained from the injection and observation wells before co2 injection showed a na cacl type brine with 93 000 mg l total dissolved solids tds at near saturation with ch4 at reservoir conditions following co2 breakthrough samples showed sharp drops in ph 6 5 5 7 pronounced increases in alkalinity 100 3000 mg l as hco3 and fe 30 1100 mg l and significant shifts in the isotopic compositions of h2o dissolved inorganic carbon dic and ch4 geochemical modeling indicates that brine ph would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides this rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for co2 and brine leakage dissolution of minerals especially iron oxyhydroxides could mobilize toxic trace metals and where residual oil or suitable organics are present the injected co2 could also mobilize toxic organic compounds environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater the d 18 o values for brine and co2 samples indicate that supercritical co2 comprises 50 of pore fluid volume 6 mo after the end of injection postinjection sampling coupled with geochemical modeling indicates that the brine gradually will return to its preinjection composition

geomechanical and geochemical effects on sandstones caused by the reaction with supercritical co2 an experimental approach to in situ conditions in deep geological reservoirs the geochemical and geomechanical behaviour of reservoir rocks from deep saline aquifers during the injection and geological storage of co2 is studied in laboratory experiments a combination of geochemical and geomechanical studies was carried out on various sandstones from the north german basin after the mineralogical geochemical and petrophysical characterization a set of sandstone samples was exposed to supercritical sc co2 and brine for 2 4 weeks in an autoclave system one sample was mineralogically and geochemically characterised and then loaded in a triaxial cell under in situ pressure and temperature conditions to study the changes of the geomechanical rock properties after treatment in the autoclaves geochemical alterations mainly in the carbonate but also in the sheet silicate cements as well as in single minerals of the sandstones were observed affecting the rocks granular structure in addition to partial solution effects during the geochemical experiments small grains of secondary carbonate and other mineral precipitations were observed within the pore space of the treated sandstones results of additional geomechanical experiments with untreated samples show that the rock strength is influenced by the saturation degree the confining pressure the pore fluid pressure and temperature the exposure to pure scco2 in the autoclave system induces reduced strength parameters modified elastic deformation behaviour and changes of the effective porosity in comparison to untreated sandstone samples experimental results show that the volume of pore fluid fluxing into the pore space of the sandstones clearly depends on the saturation level of the sample

frictional properties and slip stability of active faults within carbonate evaporite sequences the role of dolomite and anhydrite seismological observations show that many destructive earthquakes nucleate within or propagate through thick sequences of carbonates and evaporites for example along the apennines range italy carbonate and evaporite sequences are present at hypocentral depths for recent major earthquakes 5 0 mw 6 3 however little is known about the frictional properties of these rocks which are dominated by dolomite and anhydrite and differ from typical quartzofeldspathic rocks here we report on laboratory experiments designed to illuminate friction constitutive properties and associated microstructures of anhydrite and dolomite fault rocks we performed frictional sliding experiments on powdered samples for a range of normal stresses n from 10 to 150 mpa and sliding velocities from 1 to 300 m s experiments employed the double direct shear configuration and were conducted both dry at room temperature and water saturated at 75 c under true triaxial stresses at 25 c sliding friction is 0 65 for all samples and conditions studied at 75 c samples develop cohesive strength and sliding friction ranges from 0 4 to 0 5 for n from 10 to 35 mpa velocity step tests were used to assess rate state frictional behavior at 25 c the slip behavior is velocity strengthening neutral for all samples microstructural observations indicate localized deformation on r1 riedel and incipient y shear planes with marked grain size reduction at 75 c and larger strain we find that rate state friction behavior correlates with velocity dependent layer dilation and compaction and this is associated with the development of through going y shear planes we conclude that elevated temperature strain and the presence of pore fluids enhance shear localization into continuous y shear planes which could promote frictional instability at seismogenic depth within dolomite anhydrite sequences

compaction creep of wet granular calcite by pressure solution at 28 c to 150 c 1 uniaxial compaction experiments have been carried out on wet calcite powders prepared from milled limestone analytical grade calcite and superpure calcite the tests were performed at 28 c 150 c effective stresses of 20 47 mpa and a pore pressure of 20 mpa using presaturated caco3 solution as the pore fluid sample grain sizes ranged from 12 to 86 m the aim was to determine if creep occurs by intergranular pressure solution ips under these conditions and to identify the rate controlling process the wet samples showed significant creep while dry and oil flooded samples did not wet samples were also characterized by microstructures such as sutured grain contacts grain indentations and overgrowths suggesting the operation of ips the behavior of the wet samples at low strains 4 5 agreed well with the predictions of standard models for diffusion controlled pressure solution however at higher strains 5 10 creep slowed down compared with the model predictions transient flow through tests performed in this regime led to an acceleration of creep consistent with models for precipitation controlled ips and demonstrated an increase in the impurity content of the pore fluid toward higher strains since some of these impurities notably mg2 retard precipitation in calcite we suggest that creep occurred by diffusion controlled ips at low strain giving way to precipitation control at larger strains as impurities accumulated fitting of the diffusion controlled ips model to the low strain data yielded values of the grain boundary diffusion product ds in calcite of 10 19 m3 s 1 at 150 c

chemical mechanical coupling observed for depleted oil reservoirs subjected to long term co2 exposure a case study of the werkendam natural co2 analogue field geological storage of co2 is one of the most promising technologies to rapidly reduce anthropogenic emissions of carbon dioxide in order to ensure storage integrity it is important to understand the effect of long term co2 brine rock interactions on the mechanical behaviour of a storage complex as most of these reactions are too slow to reproduce on laboratory timescales we studied a natural co2 analogue reservoir the rot fringe sandstone werkendam field the netherlands 125 135 ma of co2 exposure and its unreacted counterpart we focused on co2 induced mineralogical and porosity permeability changes and their effect on mechanical behaviour of both intact rock and simulated fault gouge overall co2 exposure did not lead to drastic mineralogical changes the co2 exposed material shows a stronger dependence of permeability on porosity which is attributed to differences in diagenesis closed system diagenesis and hydrocarbon emplacement taking place before co2 charging the limited extent of reaction was in part the result of bitumen coatings protecting specific mineral phases from reaction in local mm sized zones displaying significant anhydrite cement dissolution enhanced porosity was observed for most of the reservoir the long term mechanical behaviour after co2 exposure could be described by the behaviour of the unreacted sandstone while these more porous zones had a lower rock strength in addition co2 exposure did not affect the fault friction behaviour and slip is expected to result in stable sliding simple stress path calculations predict that reservoir failure due to depletion and injection is unlikely even for the porous zones nor will fault reactivation occur for realistic injection scenarios

compaction creep of quartz sand at 400 600 c experimental evidence for dissolution controlled pressure solution intergranular pressure solution ips is an important compaction and deformation mechanism in quartzose rocks but the kinetics and rate controlling process remain unclear the aim of the present study is to test microphysical models for compaction creep by ips against isostatic hot pressing experiments performed on quartz sand under conditions expected to favor pressure solution confining pressure 300 mpa pore water pressure 150 250 mpa temperature 400 600 c microstructural observations revealed widespread intergranular indentation features and confirmed that intergranular pressure solution was indeed the dominant deformation mechanism under the chosen conditions for porosities down to 15 the mechanical data agree satisfactorily with a microphysical model incorporating a previously determined kinetic law for dissolution of loose granular quartz suggesting that the rate limiting mechanism of ips was dissolution the model also predicts ips rates within one order of magnitude of those measured in previous experiments at 150 350 c and thus seem robust enough to model sandstone compaction in nature such applications may not be straightforward however as the present evidence for dissolution control implies that the compositional variability of natural pore fluids may strongly influence ips rates in sandstones

rapid porosity and permeability changes of calcareous sandstone due to co2 enriched brine injection reservoir injectivity and storage capacity are the main constraints for geologic co2 sequestration subject to safety and economic considerations brine acidification following co2 dissolution leads to fluid rock interactions that alter porosity and permeability thereby affecting reservoir storage capacity and injectivity thus we determined how efficiently co2 enriched brines could dissolve calcite in sandstone cores and how this affects the petrophysical properties during computerized tomography monitored flow through reactor experiments calcite dissolved at a rate largely determined by the rate of acid supply even at high flow velocities which would be typical near an injection well the porosity increase was accompanied by a significant increase in rock permeability larger than that predicted using classical porosity permeability models this chemically driven petrophysical change might be optimized using injection parameters to maximize injectivity and storage

enhanced deformation of limestone and sandstone in the presence of high fluids 1 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

the influence of water and supercritical co2 on the failure behavior of chalk abstract reduction of compressive strength by injection of water into chalk is a well known mechanism responsible for increased compaction in chalk reservoirs this raises the question of whether such effects might be enhanced in the context of long term storage of co 2 or of co 2 injection for enhanced oil and gas recovery eor egr purposes therefore data regarding the effect of supercritical co 2 on the mechanical behavior of chalk are needed the effect of supercritical co 2 on the short term failure behavior of wet chalk was accordingly investigated by means of conventional triaxial deformation experiments performed on maastrichtian chalk cores under dry conditions in the presence of saturated chalk solution and using co 2 saturated solution at temperatures simulating reservoir conditions 20 80 c and effective confining pressures up to 7 mpa increasing temperature from 20 to 80 c did not show any significant effects on the strength of the dry samples addition of aqueous solution to the samples led to drastic weakening of the chalk the effect being more pronounced at high effective confining pressures peff 3 mpa addition of 10 mpa supercritical co 2 to wet samples did not produce any significant additional effect in comparison with the wet samples all samples showed a yield strength envelope characterized by shear failure at low effective mean stresses giving way to a compaction cap at high mean stresses the weakening effect of aqueous solution was explained in terms of a reduction in frictional resistance of the material due to water enhanced grain contact cracking and perhaps pressure solution with a possible contribution by disjoining pressure effects caused by water adsorption while co 2 does not seem to reduce short term failure strength of wet chalk processes such as intergranular pressure solution have to be considered for assessing mechanical stability of chalk in the context of long term co 2 storage or eor egr operations