Shale gas reservoirs like coalbed methane ( CBM ) reservoirs are promising targets for geological sequestration of carbon dioxide ( CO 2 ). However, the evolution of permeability in shale reservoirs on injection of CO 2 is poorly understood unlike CBM reservoirs. In this study, we report measurements of permeability evolution in shales infiltrated separately by nonsorbing (He) and sorbing ( CO 2 ) gases under varying gas pressures and confining stresses. Experiments are completed on Pennsylvanian shales containing both natural and artificial fractures under nonpropped and propped conditions. We use the models for permeability evolution in coal ( Journal of Petroleum Science and Engineering , Under Revision) to codify the permeability evolution observed in the shale samples. It is observed that for a naturally fractured shale, the He permeability increases by approximately 15% as effective stress is reduced by increasing the gas pressure from 1 MP a to 6 MP a at constant confining stress of 10 MP a. Conversely, the CO 2 permeability reduces by a factor of two under similar conditions. A second core is split with a fine saw to create a smooth artificial fracture and the permeabilities are measured for both nonpropped and propped fractures. The He permeability of a propped artificial fracture is approximately 2‐ to 3fold that of the nonpropped fracture. The He permeability increases with gas pressure under constant confining stress for both nonpropped and propped cases. However, the CO 2 permeability of the propped fracture decreases by between one‐half to one‐third as the gas pressure increases from 1 to 4 MP a at constant confining stress. Interestingly, the CO 2 permeability of nonpropped fracture increases with gas pressure at constant confining stress. The permeability evolution of nonpropped and propped artificial fractures in shale is found to be similar to those observed in coals but the extent of permeability reduction by swelling is much lower in shale due to its lower organic content. Optical profilometry is used to quantify the surface roughness. The changes in surface roughness indicate significant influence of proppant indentation on fracture surface in the shale sample. The trends of permeability evolution on injection of CO 2 in coals and shales are found analogous; therefore, the permeability evolution models previously developed for coals are adopted to explain the permeability evolution in shales.
Established techniques that have been successfully used to characterize pore systems in conventional reservoir rocks lack the resolution and scalability required to adequately characterize the nano‐ to micrometer scale pore systems found in shale and cannot be applied on stressed samples. We have therefore investigated the utility of K linkenberg gas slippage measurements for shale pore structure characterization. In contrast to other approaches, slippage measurements characterize the effective porosity of core samples and can be applied at stress conditions experienced in the reservoir during production. Slippage measurements on horizontally and vertically oriented samples from the E agle F ord S hale F ormation, T exas, USA , at a range of stress states revealed two orders of magnitude in slippage variation over five orders of magnitude permeability range. Slippage measurements are negatively correlated with permeability and follow similar trends to those found in other studies on higher permeability rocks. The samples had varying degrees of slippage anisotropy, which allowed interpretation of the relative contribution of tortuosity and pore size to permeability anisotropy. Slippage and therefore average effective pore size was found to vary up to one order of magnitude at a given permeability, warranting investigation of the significance this might have on flow properties and ultimately hydrocarbon production from shale. The heterogeneity and diversity inherent to shale lends itself to variable responses to stress, likely dependent on myriad of factors including fabric, composition, structure, and flow orientation. Slippage analysis yielded insight into the complex responses of these rocks to stress that other methods of pore structure analysis currently being employed to characterize shale pore systems are blind to.
Highly saline, deep‐seated basement brines are of major importance for ore‐forming processes, but their genesis is controversial. Based on studies of fluid inclusions from hydrothermal veins of various ages, we reconstruct the temporal evolution of continental basement fluids from the Variscan Schwarzwald (Germany). During the Carboniferous (vein type i), quartz–tourmaline veins precipitated from low‐salinity (20wt% NaCl + CaCl 2 , Cl/Br mass ratios = 60–110). Both fluids types were present during vein formation but did not mix with each other (because of hydrogeological reasons). Jurassic–Cretaceous veins (vein type iv) record fluid mixing between an older bittern brine (Cl/Br mass ratios ~80) and a younger halite dissolution brine (Cl/Br mass ratios >1000) of similar salinity, resulting in a mixed H 2 O‐NaCl‐CaCl 2 brine (50–140°C, 23–26wt% NaCl + CaCl 2 , Cl/Br mass ratios = 80–520). During post‐Cretaceous times (vein type v), the opening of the Upper Rhine Graben and the concomitant juxtaposition of various aquifers, which enabled mixing of high‐ and low‐salinity fluids and resulted in vein formation (multicomponent fluid H 2 O‐NaCl‐CaCl 2 ‐( SO 4 ‐ HCO 3 ), 70–190°C, 5–25wt% NaCl‐CaCl 2 and Cl/Br mass ratios = 2–140). The first occurrence of highly saline brines is recorded in veins that formed shortly after deposition of halite in the Muschelkalk Ocean above the basement, suggesting an external source of the brine's salinity. Hence, today's brines in the European basement probably developed from inherited evaporitic bittern brines. These were afterwards extensively modified by fluid–rock interaction on their migration paths through the crystalline basement and later by mixing with younger meteoric fluids and halite dissolution brines.
The formation of the world‐class, high‐grade unconformity‐related uranium deposits in the Athabasca Basin (Canada) requires circulation of large amounts of fluids, the mechanisms for which are still not well understood. Recent studies advocate thermal convection as a possible driving force for the fluid flow related to uranium mineralization; however, little is known regarding how basement faults, which are spatially associated with most unconformity‐related uranium deposits, influence fluid convection and how this may affect the localization of mineralization. This study addresses these questions through simulations of thermal convection with various configurations of basement faults using the FLAC 3D software. Modelling results indicate that the location, spacing, orientation and thermal conductivities of basement faults influence the size and location of thermally driven fluid convection. In a model with a single isolated fault, the fault coincides with an upwelling plume and the dip angle of the fault does not affect the fluid flow pattern; when the fault is moved laterally, the upwelling plume shifts accordingly. In the case of two vertical faults, the faults may either coincide with upwelling flow between two convection cells or be located below individual convection cells, depending on fault spacing. In the latter case, fluid may flow into and out of individual fault zones. Similar results were also obtained for models with two nonvertical (i.e. dipping) faults. Convective flow can penetrate the uppermost basement when the permeability is less than two orders of magnitude lower than that of the overlying sandstone. In this case, the basement faults not only can control the location of ascending flow, but also can passively act as fluid conduits of either flow from the basin into the basement (ingress), or flow from the basement into the basin (egress), depending on their thermal conductivities and relative locations in the models.
Faults in CO 2 storage reservoirs affect the migration and spatial distribution of injected CO 2 in reservoir formations. Based on geological data from the Ordos CO 2 geological storage demonstration project site, A 2D numerical model of the CO 2 storage reservoir was constructed to study the influence of high‐porosity and high‐permeability faults within a low‐porosity and low‐permeability reservoir. The results show that the faults had a significant effect on CO 2 migration and storage. If the permeability of the fault zone increased by 2–3 orders of magnitude (a common range of variation in fault zone) and porosity increased correspondingly compared to the base case (no fault in strata), the CO 2 migration distance in the reservoir after 100 years was approximately 1.18 times larger than that of the base case without faults, and the total CO 2 storage amount in the entire reservoir increased by 1.40–1.61 times. The faults weakened the sealing ability of the local interlayer caprocks (mudstone), which resulted in CO 2 migrating through mudstone and entering in the neighboring interlayer storage stratum. When highly permeable faults existed, the CO 2 migration distance and storage amount in the neighboring interlayer storage stratum were approximately two times and 19.41 times (maximum value) higher, respectively, compared with the case without a fault. Therefore, the faults distributed in strata should be given sufficient attention when selecting a CO 2 sequestration site.
We present the results of hydromechanical changes across and along the evolving rupture surface of carbonate rocks during direct shear experiments. Direct shear experiments were performed on a laminated travertine of continental, microbial origin with calcite content of 99 wt%, chosen as a lithological analogue for Aptian presalt oil reservoir rocks found in South Atlantic presalt basins. Medical X‐ray CT images show that the porosity (~9–13%) is mainly composed of subplanar pores and vugs. Permeability is high along the laminations (~50–200 mD ), controlled by interconnected pores and fractures, and extremely low across the laminations (≪1 mD ). Six intact samples of travertine were sheared across the bedding direction to short displacement (20 mm), and a further three samples were sheared to a much longer displacement (120 mm). A constant effective vertical stresses of either 35, 40 or 45 MP a was applied throughout the tests. Fluid flow response across and along the fault zone was monitored continuously during both shear deformation (dynamic transmissibility) and hold periods (static transmissibility), while keeping a constant pore pressure throughout the measurement. While the samples show some microstructural variability, the set of samples sheared to 20‐mm displacement followed the same early kinematics and show very similar mechanical response to that seen in the same period of the samples where shear was continued on to 120‐mm displacement. After 20‐mm displacement, the microstructures are dominated by fractures in a zone 10–20 mm wide resembling a typical fault damage zone, with only thin and patchy development of gouge on the principal displacement plane. In all of the samples sheared to a high displacement of 120 mm, a continuous layer of gouge (6.2–13 mm thick) was formed, defining a distinct core of the shear zone. The dynamic transmissibility across the fault decreases progressively in all the sheared samples regardless of the applied effective stress. In general, the volume decreases as well, though at a tapering rate, throughout shear. A small vertical dilation occurred near the peak strength for travertine sheared under 35 MPa vertical stress, but this was not accompanied by an increase in transmissibility. From the results, we conclude that once the gouge material is developed in the core of the fault zone, the dynamic transmissibility across the fault is permanently decreased.
Mineral precipitation in an open fracture plays a crucial role in the evolution of fracture permeability in rocks, and the microstructural development and precipitation rates are closely linked to fluid composition, the kind of host rock as well as temperature and pressure. In this study, we develop a continuum thermodynamic model to understand polycrystalline growth of quartz aggregates from the rock surface. The adapted multiphase‐field model takes into consideration both the absolute growth rate as a function of the driving force of the reaction (free energy differences between solid and liquid phases), and the equilibrium crystal shape ( W ulff shape). In addition, we realize the anisotropic shape of the quartz crystal by introducing relative growth rates of the facets. The missing parameters of the model, including surface energy and relative growth rates, are determined by detailed analysis of the crystal shapes and crystallographic orientation of polycrystalline quartz aggregates in veins synthesized in previous hydrothermal experiments. The growth simulations were carried out for a single crystal and for grain aggregates from a rock surface. The single crystal simulation reveals the importance of crystal facetting on the growth rate; for example, growth velocity in the c ‐axis direction drops by a factor of ~9 when the faceting is complete. The textures produced by the polycrystal simulations are similar to those observed in the hydrothermal experiments, including the number of surviving grains and crystallographic preferred orientations as a function of the distance from the rock wall. Our model and the methods to define its parameters provide a basis for further investigation of fracture sealing under varying conditions.
The Upper Triassic Mercia Mudstone is the caprock to potential carbon capture and storage ( CCS ) sites in porous and permeable Lower Triassic Sherwood Sandstone reservoirs and aquifers in the UK (primarily offshore). This study presents direct measurements of vertical ( k v ) and horizontal ( k h ) permeability of core samples from the Mercia Mudstone across a range of effective stress conditions to test their caprock quality and to assess how they will respond to changing effective stress conditions that may occur during CO 2 injection and storage. The Mercia samples analysed were either clay‐rich (muddy) siltstones or relatively clean siltstones cemented by carbonate and gypsum. Porosity is fairly uniform (between 7.4 and 10.7%). Porosity is low either due to abundant depositional illite or abundant diagenetic carbonate and gypsum cements. Permeability values are as low as 10 −20 m 2 (10nD), and therefore, the Mercia has high sealing capacity. These rocks have similar horizontal and vertical permeabilities with the highest k h / k v ratio of 2.03 but an upscaled k h / k v ratio is 39, using the arithmetic mean of k h and the harmonic mean of k v . Permeability is inversely related to the illite clay content; the most clay‐rich (illite‐rich) samples represent very good caprock quality; the cleaner Mercia Mudstone samples, with pore‐filling carbonate and gypsum cements, represent fair to good caprock quality. Pressure sensitivity of permeability increases with increasing clay mineral content. As pore pressure increases during CO 2 injection, the permeability of the most clay‐rich rocks will increase more than carbonate‐ and gypsum‐rich rocks, thus decreasing permeability heterogeneity. The best quality Mercia Mudstone caprock is probably not geochemically sensitive to CO 2 injection as illite, the cause of the lowest permeability, is relatively stable in the presence of CO 2 –water mixtures.
World‐class unconformity‐related U deposits in the Athabasca Basin (Saskatchewan, Canada) are generally located within or near fault zones that intersect the unconformity between the Athabasca Group sedimentary basin rocks and underlying metamorphic basement rocks. Two distinct subtypes of unconformity‐related uranium deposits have been identified: those hosted primarily in the Athabasca Group sandstones (sediment‐hosted) and those hosted primarily in the underlying basement rocks (basement‐hosted). Although significant research on these deposits has been carried out, certain aspects of their formation are still under discussion, one of the main issues being the fluid flow mechanisms responsible for uranium mineralization. The intriguing feature of this problem is that sediment‐hosted and basement‐hosted deposits are characterized by oppositely directed vectors of fluid flow via associated fault zones. Sediment‐hosted deposits formed via upward flow of basement fluids, basement‐hosted deposits via downward flow of basinal fluids. We have hypothesized that such flow patterns are indicative of the fluid flow self‐organization in fault‐bounded thermal convection (Transport in Porous Media, 110, 2015, 25). To explore this hypothesis, we constructed a simplified hydrogeologic model with fault‐bounded thermal convection of fluids in the faulted basement linked with fluid circulation in the overlying fault‐free sandstone horizon. Based on this model, a series of numerical experiments was carried out to simulate the hypothesized fluid flow patterns. The results obtained are in reasonable agreement with the concept of fault‐bounded convection cells as an explanation of focused upflow and downflow across the basement/sandstone unconformity. We then discuss application of the model to another debated problem, the uranium source for the ore‐forming basinal brines. Fault‐bounded geothermal convection provides conditions for focused upflow and downflow circulation of the basement and basinal fluids across the basement/sandstone unconformity at formation of the Athabasca Basin ‘egress‐style’ and ‘ingress‐style’ uranium deposits. The numerically estimated fluid flow rates show that at geologically realistic permeability values of the basement (about 10 −16 m 2 ) and of the sandstone aquifer (about 10 −14 m 2 ) industrial‐scale ore bodies could form in tens of thousands to a few hundred thousand years
We model pore‐pressure diffusion caused by pressurized waste‐fluid injection at two nearby wells and then compare the buildup of pressure with the observed initiation and migration of earthquakes during the early part of the 2010–2011 Guy–Greenbrier earthquake swarm. Pore‐pressure diffusion is calculated using MODFLOW 2005 that allows the actual injection histories (volume/day) at the two wells to diffuse through a fractured and faulted 3D aquifer system representing the eastern Arkoma basin. The aquifer system is calibrated using the observed water‐level recovery following well shut‐in at three wells. We estimate that the hydraulic conductivities of the Boone Formation and Arbuckle Group are 2.2 × 10 −2 and 2.03 × 10 −3 m day −1 , respectively, with a hydraulic conductivity of 1.92 × 10 −2 m day −1 in the Hunton Group when considering 1.72 × 10 −3 m day −1 in the Chattanooga Shale. Based on the simulated pressure field, injection near the relatively conductive Enders and Guy–Greenbrier faults (that hydraulically connect the Arbuckle Group with the underlying basement) permits pressure diffusion into the crystalline basement, but the effective radius of influence is limited in depth by the vertical anisotropy of the hydraulic diffusivity. Comparing spatial/temporal changes in the simulated pore‐pressure field to the observed seismicity suggests that minimum pore‐pressure changes of approximately 0.009 and 0.035 MPa are sufficient to initiate seismic activity within the basement and sedimentary sections of the Guy–Greenbrier fault, respectively. Further, the migration of a second front of seismicity appears to follow the approximately 0.012 MPa and 0.055 MPa pore‐pressure fronts within the basement and sedimentary sections, respectively.
To investigate the biases and trends in observations of the permeability structures of fault zones in various geoscience disciplines, we review and compile a database of published studies and reports containing more than 900 references. The global data are categorized, mapped, and described statistically. We use the chi‐square test for the dependency of categorical variables to show that the simplified fault permeability structure (barrier, conduit, barrier–conduit) depends on the observation method, geoscience discipline, and lithology. In the crystalline rocks, the in situ test methods (boreholes or tunnels) favor the detection of permeable fault conduits, in contrast to the outcrop‐based measurements that favor a combined barrier–conduit conceptual models. These differences also occur, to a lesser extent, in sedimentary rocks. We provide an estimate of the occurrence of fault conduits and barriers in the brittle crust. Faults behave as conduits at 70% of sites, regardless of their barrier behavior that may also occur. Faults behave as barriers at at least 50% of the sites, in addition to often being conduits. Our review of published data from long tunnels suggests that in crystalline rocks, 40–80% (median about 60%) of faults are highly permeable conduits, and 30–70% in sedimentary rocks. The trends with depth are not clear, but there are less fault conduits counted in tunnels at the shallowest depths. The barrier hydraulic behavior of faults is more uncertain and difficult to observe than the conduit.
A comprehensive dataset for discrete groundwater inflows to mines in the Poehla‐Tellerhaeuser Ore Field and the mining scale fault zones has been compiled from unpublished data recorded by eastern German and Soviet hydrogeologists at the Soviet‐German stock company ( SDAG ) Wismut. This dataset has been analyzed to provide novel insights into the 3D distribution of preferential groundwater pathways and the impacts of faulting on the distribution of hydraulic parameters in crystalline rocks at site scale. The sampled 1030 discrete inflows include flow rates ranging from 1.7E‐8 to 3.7E‐2 m 3 sec −1 , which were transformed into mesoscale fracture transmissivity values ranging between 3E‐13 and 2E‐4 m 2 sec −1 . These mesoscale fracture transmissivities were spatially correlated with fault zones exhibiting trace lengths between 0.3 and 30 km, which were mainly formed during and reactivated several times since Variscan orogeny. The statistical correlations are based on a 3D geological model composed of 14 litho‐stratigraphic units and 131 mining scale faults, separated into five main strike directions. These fault zones strongly overlap and cover about 90% of the investigated rock mass volume with a decreasing percentage of overlap in the investigated depth range (0–900 mbgs). 97% of all inflows are located within fault damage zones, and most of the flow occurs within the overlap of multiple fault damage zones. A dimensionless hydraulic model for the distribution of flow Q as a function of the position x within mining scale fault zones has been derived as Q = 1.1 e −4.5 x (where x decreases from the fault core to the protolith and the exponent varies as a function of fault orientation). 75–95% of the flow occurs within the inner 50% of the damage zone, and mainly NW ‐ SE and NE ‐ SW striking mining scale faults are transmissive. The orientations of conductive mesoscale fractures within these damage zones show a larger variability than the corresponding mining scale faults.
3 He and 4 He concentrations in excess of those in water in solubility equilibrium with the atmosphere by up to two and three orders of magnitude are observed in the shallow Glacial Drift and Saginaw aquifers in the Michigan Basin. A simplified He transport model shows that in situ production is negligible and that most He excesses have a source external to the aquifer. Simulated results show that 3 He and 4 He fluxes entering the bottom of the Saginaw aquifer are 7.5 × 10 −14 and 6.1 × 10 −7 cm 3 STP cm −2 yr −1 , both of which are lower than fluxes entering the underlying Marshall aquifer, 1.0 × 10 −13 and 1.6 × 10 −6 cm 3 STP cm −2 yr −1 for 3 He and 4 He, respectively. In contrast, He fluxes entering the Saginaw aquifer are higher than fluxes entering the overlying Glacial Drift aquifer of 5.2 × 10 −14 and 1.5 × 10 −7 cm 3 STP cm −2 yr −1 for 3 He and 4 He, respectively. The unusually high He fluxes and their decreasing values from the lower Marshall to the upper Glacial Drift aquifer strongly suggest the presence of an upward cross‐formational flow, with increasing He dilution toward the surface by recharge water. These fluxes are either comparable to or far greater than He fluxes in deeper aquifers around the world. Model simulations also suggest an exponential decrease in the horizontal groundwater velocity with recharge distance. Horizontal velocities vary from 13 to 2 myr −1 for the Saginaw aquifer and from 18 to 6 myr −1 for the Marshall aquifer. The highly permeable Glacial Drift aquifer displays a greater velocity range, from 250 to 5 myr −1 . While Saginaw 4 He ages estimated based on the simulated velocity field display an overall agreement with 14 C ages, 14 C and 4 He ages in the Glacial Drift and Marshall aquifers deviate significantly, possibly due to simplifications introduced in the He transport model leading to calculation of first‐order approximation He ages and high uncertainties in Glacial Drift 14 C ages.
Changes in water level are commonly reported in regions struck by a seismic event. The sign and amplitude of such changes depend on the relative position of measuring points with respect to the hypocenter, and on the poroelastic properties of the rock. We apply a porous media flow model (TOUGH2) to describe groundwater flow and water‐level changes associated with the first M L 5.9 mainshock of the 2012 seismic sequence in Emilia (Italy). We represent the earthquake as an instantaneous pressure step, whose amplitude was inferred from the properties of the seismic source inverted from geodetic data. The results are consistent with the evolution recorded in both deep and shallow water wells in the area and suggest that our description of the seismic event is suitable to capture both timing and magnitude of water‐level changes. We draw some conclusions about the influence of material heterogeneity on the pore pressure evolution, and we show that to reproduce the observed maximum amplitude it is necessary to take into account compaction in the shallow layer.
The quantitative assessment of COH fluids is crucial in modeling geological processes. The composition of fluids, and in particular their H 2 O/ CO 2 ratio, can influence the melting temperatures, the location of hydration or carbonation reactions, and the solute transport capability in several rock systems. In the scientific literature, COH fluids speciation has been generally assumed on the basis of thermodynamic calculations using equations of state of simple H 2 O–nonpolar gas systems (e.g., H 2 O– CO 2 – CH 4 ). Only few authors dealt with the experimental determination of high‐pressure COH fluid species at different conditions, using diverse experimental and analytical approaches (e.g., piston cylinder + capsule piercing + gas chromatography/mass spectrometry; cold seal + silica glass capsules + Raman). In this contribution, we present a new methodology for the synthesis and the analysis of COH fluids in experimental capsules, which allows the quantitative determination of volatiles in the fluid by means of a capsule‐piercing device connected to a quadrupole mass spectrometer. COH fluids are synthesized starting from oxalic acid dihydrate at P = amb and T = 250°C in single capsules heated in a furnace, and at P = 1 GP a and T = 800°C using a piston‐cylinder apparatus and the double‐capsule technique to control the redox conditions employing the rhenium–rhenium oxide oxygen buffer. A quantitative analysis of H 2 O, CO 2 , CH 4 , CO , H 2 , O 2 , and N 2 along with associated statistical errors is obtained by linear regression of the m / z data of the sample and of standard gas mixtures of known composition. The estimated uncertainties are typically <1% for H 2 O and CO 2 , and <5% for CO . Our results suggest that the COH fluid speciation is preserved during and after quench, as the experimental data closely mimic the thermodynamic model both in terms of bulk composition and fluid speciation.
Deep sedimentary basins are complex systems that over long time scales may be affected by numerous interacting processes including groundwater flow, heat and mass transport, water–rock interactions, and mechanical loads induced by ice sheets. Understanding the interactions among these processes is important for the evaluation of the hydrodynamic and geochemical stability of geological CO 2 disposal sites and is equally relevant to the safety evaluation of deep geologic repositories for nuclear waste. We present a reactive transport formulation coupled to thermo‐hydrodynamic and simplified mechanical processes. The formulation determines solution density and ion activities for ionic strengths ranging from freshwater to dense brines based on solution composition and simultaneously accounts for the hydro‐mechanical effects caused by long‐term surface loading during a glaciation cycle. The formulation was implemented into the existing MIN 3 P reactive transport code ( MIN 3 P ‐ THC m) and was used to illustrate the processes occurring in a two‐dimensional cross section of a sedimentary basin subjected to a simplified glaciation scenario consisting of a single cycle of ice‐sheet advance and retreat over a time period of 32 500 years. Although the sedimentary basin simulation is illustrative in nature, it captures the key geological features of deep P aleozoic sedimentary basins in N orth A merica, including interbedded sandstones, shales, evaporites, and carbonates in the presence of dense brines. Simulated fluid pressures are shown to increase in low hydraulic conductivity units during ice‐sheet advance due to hydro‐mechanical coupling. During the period of deglaciation, Darcy velocities increase in the shallow aquifers and to a lesser extent in deeper high‐hydraulic conductivity units (e.g., sandstones) as a result of the infiltration of glacial meltwater below the warm‐based ice sheet. Dedolomitization is predicted to be the most widespread geochemical process, focused near the freshwater/brine interface. For the illustrative sedimentary basin, the results suggest a high degree of hydrodynamic and geochemical stability.
Single‐ and two‐phase (gas/water) fluid transport in tight sandstones has been studied in a series of permeability tests on core plugs of nine tight sandstones of the southern North Sea. Absolute (Klinkenberg‐corrected) gas permeability coefficients ( k gas _ inf ) ranged between 3.8 × 10 −16 and 6.2 × 10 −19 m 2 and decreased with increasing confining pressure (10–30 MPa) by a factor 3–5. Klinkenberg‐corrected (intrinsic) gas permeability coefficients were consistently higher by factors from 1.4 to 10 than permeability coefficients determined with water. Non‐steady‐state two‐phase (He/water) flow experiments conducted up to differential pressures of 10 MPa document the dynamically changing conductivity for the gas phase, which is primarily capillary‐controlled (drainage and imbibition). Effective gas permeability coefficients in the two‐phase flow tests ranged between 1.1 × 10 −17 and 2.5 × 10 −22 m², corresponding to relative gas permeabilities of 0.03% and 10%. In the early phase of the nonstationary flow regime (before establishment of steady‐state conditions), they may be substantially (>50%) lower. Effective gas permeability measurements are affected by the following factors: (i) Capillary‐controlled drainage/imbibition, (ii) viscous–dynamic effects (iii) and slip flow. Single‐ and two‐phase (gas/water) permeability experiments were carried out on nine tight sandstones from the Southern North Sea. Intrinsic (water) permeability coefficients ranged from 10 −16 to 10 −19 m 2 . Relative gas permeability coefficients established during successive drainage ranged between 0.03% and 10% of the steady state Klinkenberg‐corrected gas permeability coefficients. Gas permeability coefficients after capillary breakthrough clearly increase with increasing pressure difference, confirming capillary pressure‐controlled change in gas saturation.
Fluid chemistry and microbial community patterns in chimney habitats were investigated in two hydrothermal fields located at the Central Indian Ridge. Endmember hydrothermal fluid of the Solitaire field, located ~3 km away from the spreading center, was characterized by moderately high temperature (307°C), Cl depletion (489 m m ), mildly acidic pH (≥4.40), and low metal concentrations (Fe ≤ 105 μ m and Mn = 78 μ m ). Chloride depletion indicates that the subseafloor source fluid had undergone phase separation at temperatures higher than ~390°C while the metal depletion was likely attributable to fluid alteration occurring at a venting temperature of around 307°C. These different temperature conditions suggested from fluid chemistry might be associated with an off‐spreading center location of the field that allows subseafloor fluid cooling prior to seafloor discharge. The microbial community in the chimney habitat seemed comparable to previously known patterns in typical basalt‐hosted hydrothermal systems. Endmember hydrothermal fluid of the Dodo field, standing on center of the spreading axis, was characterized by high H 2 concentration of 2.7 m m . The H 2 enrichment was likely attributable to fresh basalt–fluid interaction, as suggested by the nondeformed sheet lava flow expansion around the vents. Thermodynamic calculation of the reducing pyrite–pyrrhotite–magnetite ( PPM ) redox buffer indeed reproduced the H 2 enrichment. The quantitative cultivation test revealed that the microbial community associated with the hydrothermal fluid hosted abundant populations of (hyper)thermophilic hydrogenotrophic chemolithoautotrophs such as methanogens. The function of subseafloor hydrogenotrophic methanogenic populations dwelling around the H 2 ‐enriched hydrothermal fluid flows was also inferred from the 13 C‐ and D‐depleted signature of CH 4 in the collected fluids. It was observed that the hydrothermal activity of the Dodo field had ceased until 2013.
Transport properties of reduced carbonic fluid have been studied experimentally at P = 2 kbar and T = 700–1000°C in internally heated pressure vessel ( IHPV ). Synthetic Fe CO 3 and natural siderite were used to generate fluid during experiments using a platinum double‐capsule technique. A natural CaTiSiO 5 aggregate was placed into the inner capsule as an additional source of trace elements. The outer capsule was loaded with albite glass. No water was introduced to the system and oxygen fugacity was established near to graphite–oxygen ( CCO ) buffer due to transformation of Fe CO 3 into a magnetite aggregate during decarbonation to yield CO and CO 2 . The carbonates decomposed during initial heating of the experiments, causing their some constituent components to be dissolved in and transferred by the fluid to the pore space of the albite glass matrix. After temperature reached 1000°C glass, the shards annealed and then melted, as evidenced by a vesiculated glass in the quench products. Micro‐Raman investigation of the fluid in bubbles in the albite glass in experiments with decomposition of natural siderite yielded CO – CO 2 mixture where CO mole fraction was 0.15–0.16. We observe significant concentrations of Pt, Mn, P, and REE in the albite glass; in contrast, no Fe or Mg transfer was detected. LA ‐ ICP ‐ MS analysis of the albite glass product yielded the average Pt content of 2 ppm. Such high Pt signal came from Pt particles (100–500 nm in size), which were observed on the walls of the bubbles embedded in the glass. Olivines and aluminous spinel were observed in the Fe‐oxide aggregate, demonstrating transfer of SiO 2 and Al 2 O 3 from the albite melt by the reduced carbonic fluid from the albite glass (large capsule). Our results demonstrate that dry CO – CO 2 fluid can be important agents of dissolution and transport, especially for Pt and other metals. The data imply that metals are chiefly dissolved as carbonyl complexes.
Fluid injection in deep geological formations usually induces microseismicity. In particular, industrial‐scale injection of CO 2 may induce a large number of microseismic events. Since CO 2 is likely to reach the storage formation at a lower temperature than that corresponding to the geothermal gradient, both overpressure and cooling decrease the effective stresses and may induce microseismicity. Here, we investigate the effect of the stress regime on the effective stress evolution and fracture stability when injecting cold CO 2 through a horizontal well in a deep saline formation. Simulation results show that when only overpressure occurs, the vertical total stress remains practically constant, but the horizontal total stresses increase proportionally to overpressure. These hydro‐mechanical stress changes result in a slight improvement in fracture stability in normal faulting stress regimes because the decrease in deviatoric stress offsets the decrease in effective stresses produced by overpressure. However, fracture stability significantly decreases in reverse faulting stress regimes because the size of the Mohr circle increases in addition to being displaced towards failure conditions. Fracture stability also decreases in strike slip stress regimes because the Mohr circle maintains its size and is shifted towards the yield surface a magnitude equal to overpressure minus the increase in the horizontal total stresses. Additionally, cooling induces a thermal stress reduction in all directions, but larger in the out‐of‐plane direction. This stress anisotropy causes, apart from a displacement of the Mohr circle towards the yield surface, an increase in the size of the Mohr circle. These two effects decrease fracture stability, resulting in the strike slip being the least stable stress regime when cooling occurs, followed by the reverse faulting and the normal faulting stress regimes. Thus, characterizing the stress state is crucial to determine the maximum sustainable injection pressure and maximum temperature drop to safely inject CO 2 . Fracture stability represented by Mohr circles in the caprock and reservoir for normal faulting, strike slip and reverse faulting stress regime. The grey circles indicate the initial stress state, the orange circles correspond to the stress state when injecting CO 2 in thermal equilibrium with the storage formation, and the blue circles show the stress state when injecting cold CO 2 . The red lines represent the yield surface.