Investigation of coupled hydraulic–geomechanical processes at the KTB site: pressure‐dependent characteristics of a long‐term pump test and elastic interpretation using a geomechanical facies model

The German Continental Deep Drilling Program comprising a pilot borehole down to 4000 m and a main borehole down to 9101 m in southeast Germany (KTB) is continuing to provide a unique opportunity for the identification of important factors and processes in deep‐seated fluid and energy transfer. In situ stress conditions significantly impact flow, transport and exchange characteristics of fracture networks that dominate the permeability of crystalline reservoir rocks. In this paper, several scales of information are combined to present a fully three‐dimensional hydraulic finite element model of the principal KTB fault zones, and linked to a geomechanical model describing the alteration of the hydraulic parameters with stress changes caused by fluid extraction. The concept of geomechanical facies is introduced to define and characterize architectural elements in the subsurface system. Evaluation of a long‐term pump test in the KTB pilot hole, June 2002–July 2003, coupled with a geomechanical model gives an insight into some of the elastic and nonelastic processes controlling hydraulic transport in the basement rocks. Trends in the decline of the permeability and the degree of storage in the system could only partially be explained by elastic processes, clearly indicating the importance of nonelastic processes. A number of inelastic processes are suggested as areas for further research.     

Hydraulic observations from a 1 year fluid production test in the 4000 m deep KTB pilot borehole

A long‐term pump test was conducted in the KTB pilot borehole (KTB‐VB), located in the Oberpfalz area, Germany. It produced 22 300 m³ of formation fluid. Initially, fluid production rate was 29 l min^?1 for 4 months, but was then raised to an average of 57 l min^?1 for eight more months. The aim of this study was to examine the fluid parameters and hydraulic properties of fractured, crystalline crusts as part of the new KTB programme ‘Energy and Fluid Transport in Continental Fault Systems’. KTB‐VB has an open‐hole section from 3850 to 4000 m depth that is in hydraulic contact with a prominent continental fault system in the area, called SE2. Salinity and temperature of the fluid inside the borehole, and consequently hydrostatic pressure, changed significantly throughout the test. Influence of these quantities on variations in fluid density had to be taken into account for interpretation of the pump test. Modelling of the pressure response related to the pumping was achieved assuming the validity of linear Darcy flow and permeability to be independent of the flow rate. Following the principle ‘minimum in model dimension’, we first examined whether the pressure response can be explained by an equivalent model where rock properties around the borehole are axially symmetric. Calculations show that the observed pressure data in KTB‐VB can in fact be reproduced through such a configuration. For the period of high pumping rate (57 l min^?1) and the following recovery phase, the resulting parameters are 2.4 × 10^?13 m³ in hydraulic transmissivity and 3.7 × 10^?9 m Pa^?1 in storativity for radial distances up to 187 m, and 4.7 × 10^?14 m³ and 6.0 × 10^?9 m Pa^?1, respectively, for radial distances between 187 and 1200 m. The former pair of values mainly reflect the hydraulic properties of the fault zone SE2. For a more realistic hydraulic study on a greater scale, program FEFLOW was used. Parameter values were obtained by matching the calculated induced pressure signal to fluid‐level variations observed in the KTB main hole (KTB‐HB) located at 200 m radial distance from KTB‐VB. KTB‐HB is uncased from 9031 to 9100 m and shows indications of leakage in the casing at depths 5200–5600 m. Analysis of the pressure record and hydraulic modelling suggest the existence of a weak hydraulic communication between the two boreholes, probably at depths around the leakage. Hydraulic modelling of a major slug‐test in KTB‐HB that was run during the pumping in KTB‐VB reveals the effective transmissivity of the connected formation to be 1 to 2 orders of magnitude lower than the one determined for the SE2 fault zone.     

Evaluation of recoverable energy potential from enhanced geothermal systems: a sensitivity analysis in a poro‐thermo‐elastic framework

This study presents application of an efficient approach to simulate fluid flow and heat transfer in naturally fractured geothermal reservoirs. Fluid flow is simulated by combining single continuum and discrete fracture approaches. The local thermal nonequilibrium approach is used to simulate heat transfer by conduction in the rock matrix and convection (including conduction) in the fluid. Fluid flow and heat transfer models are integrated within a coupled poro‐thermo‐elastic framework. The developed model is used to evaluate the long‐term response of a geothermal reservoir with specific boundary conditions and injection/production schedule. A comparative study and a sensitivity analysis are carried out to evaluate the capability of the integrated approach and understand the degree by which different reservoir parameters affect thermal depletion of Soultz geothermal reservoir, respectively. Also observed, there exists an optimum fracture permeability after which the reservoir stimulation does not change the recovery factor significantly. Estimation of fluid temperature by the assumption of local thermal nonequilibrium heat transfer between the fracture fluid and the rock matrix gives fluid temperature of about 3°C less than that of estimated by thermal equilibrium heat transfer at early stage of hot water production.     

Pore pressure evolution and fluid flow during visco‐elastic single‐layer buckle folding

Pore pressure and fluid flow during the deformational history of geologic structures are directly influenced by tectonic deformation events. In this contribution, 2D plane strain finite element analysis is used to study the influence of different permeability distributions on the pore pressure field and associated flow regimes during the evolution of visco‐elastic single‐layer buckle folds. The buckling‐induced fluid flow regimes indicate that flow directions and, to a lesser degree, their magnitudes vary significantly throughout the deformation and as a function of the stratigraphic permeability distribution. The modelling results suggest that the volumetric strain and the permeability distribution significantly affect the resulting flow regime at different stages of fold development. For homogeneous permeability models (k > 10^?21 m²), low strain results in a mostly pervasive fluid flow regime and is in agreement with previous studies. For larger strain conditions, fluid focusing occurs in the buckling layer towards the top of the fold hinge. For low permeabilities (<10^?21 m²), local focused flow regimes inside the buckling layer emerge throughout the deformation history. For models featuring a low‐permeability layer embedded in a high‐permeability matrix or sandwiched between high‐permeability layers, focused flow regimes inside the folded layer result throughout the deformation history, but with significant differences in the flow vectors of the surrounding layers. Fluid flow vectors induced by the fold can result in different, even reversed, directions depending on the amount of strain. In summary, fluid flow regimes during single‐layer buckling can change from pervasive to focused and fluid flow vectors can be opposite at different strain levels, that is the flow vectors change significantly through time. Thus, a complete understanding of fluid flow regimes associated with single‐layer buckle folds requires consideration of the complete deformation history of the fold.     

Main and trace elements in KTB-VB fluid: composition and hints to its origin

The pilot hole (VB) of the German Continental Deep Drilling Program (KTB) was drilled to a depth of 4000 m, where large amounts of free fluids were met. The KTB‐VB 4000 m fluid can be related to either Mesozoic seawater or formation water from Permo‐Carboniferous sedimentary rocks of the Weiden embayment. During the Upper Cretaceous uplift of the Bohemian Massif both fluids could have passed organic‐rich Triassic to Carboniferous formations of the Weiden embayment before invading the uplifted and fractured basement rocks of Devonian amphibolites and metagabbros, where the chemical composition of the fluids was changed by albitization, adularization, and chloritization. Results of chemical mass balances for both sources are presented. In order to concentrate the formation water from the Weiden embayment significant amphibolitization has to be assumed. During a 1‐year pumping test the chemical composition of the 4000 m fluids remained constant. The accuracy of chemical analyses is critically reviewed. An improved preconcentration method of rare earth elements and yttrium in high‐Ca‐bearing saline fluids is described.     

The upper continental crust, an aquifer and its fluid: hydaulic and chemical data from 4 km depth in fractured crystalline basement rocks at the KTB test site

Detailed information on the hydrogeologic and hydraulic properties of the deeper parts of the upper continental crust is scarce. The pilot hole of the deep research drillhole (KTB) in crystalline basement of central Germany provided access to the crust for an exceptional pumping experiment of 1‐year duration. The hydraulic properties of fractured crystalline rocks at 4 km depth were derived from the well test and a total of 23100 m³ of saline fluid was pumped from the crustal reservoir. The experiment shows that the water‐saturated fracture pore space of the brittle upper crust is highly connected, hence, the continental upper crust is an aquifer. The pressure–time data from the well tests showed three distinct flow periods: the first period relates to wellbore storage and skin effects, the second flow period shows the typical characteristics of the homogeneous isotropic basement rock aquifer and the third flow period relates to the influence of a distant hydraulic border, probably an effect of the Franconian lineament, a steep dipping major thrust fault known from surface geology. The data analysis provided a transmissivity of the pumped aquifer T = 6.1 × 10^?6 m² sec^?1, the corresponding hydraulic conductivity (permeability) is K = 4.07 × 10^?8 m sec^?1 and the computed storage coefficient (storativity) of the aquifer of about S = 5 × 10^?6. This unexpected high permeability of the continental upper crust is well within the conditions of possible advective flow. The average flow porosity of the fractured basement aquifer is 0.6–0.7% and this range can be taken as a representative and characteristic values for the continental upper crust in general. The chemical composition of the pumped fluid was nearly constant during the 1‐year test. The total of dissolved solids amounts to 62 g l^?1 and comprise mainly a mixture of CaCl₂ and NaCl; all other dissolved components amount to about 2 g l^?1. The cation proportions of the fluid (X_Ca approximately 0.6) reflects the mineralogical composition of the reservoir rock and the high salinity results from desiccation (H₂O‐loss) due to the formation of abundant hydrate minerals during water–rock interaction. The constant fluid composition suggests that the fluid has been pumped from a rather homogeneous reservoir lithology dominated by metagabbros and amphibolites containing abundant Ca‐rich plagioclase.     

Permeability of continental crust influenced by internal and external forcing

The permeability of continental crust is so highly variable that it is often considered to defy systematic characterization. However, despite this variability, some order has been gleaned from globally compiled data. What accounts for the apparent coherence of mean permeability in the continental crust (and permeability–depth relations) on a very large scale? Here we argue that large‐scale crustal permeability adjusts to accommodate rates of internal and external forcing. In the deeper crust, internal forcing – fluxes induced by metamorphism, magmatism, and mantle degassing – is dominant, whereas in the shallow crust, external forcing – the vigor of the hydrologic cycle – is a primary control. Crustal petrologists have long recognized the likelihood of a causal relation between fluid flux and permeability in the deep, ductile crust, where fluid pressures are typically near‐lithostatic. It is less obvious that such a relation should pertain in the relatively cool, brittle upper crust, where near‐hydrostatic fluid pressures are the norm. We use first‐order calculations and numerical modeling to explore the hypothesis that upper‐crustal permeability is influenced by the magnitude of external fluid sources, much as lower‐crustal permeability is influenced by the magnitude of internal fluid sources. We compare model‐generated permeability structures with various observations of crustal permeability.     

Fluid sinks within the earth's crust

The Urach 3 research borehole in south‐west (SW) Germany has been drilled through the sedimentary cover, and the gneisses of the Variscian crystalline basement at 1600 m below the surface (Black Forest basement) has been reached. An additional 2800 m has been drilled through the fractured crystalline rocks, and the borehole has been used for a number of hydraulic tests in the context of a ‘hot‐dry rock’ (HDR) project exploring for geothermal energy. The fracture system of the basement is saturated with a NaCl brine with about 70 g L^?1 dissolved solids. Water table measurements in the borehole cover a period of 13 years of observation, during which the water table continuously dropped and did not reach a steady‐state level. This unique set of data shows that the hydraulic potential decreases with depth, causing a continuous flow of fluid to the deeper parts of the upper continental crust. The potential decrease and the associated downward migration of fluid is an evidence for the progress of water (H₂O)‐consuming reactions in the crystalline rocks. Computed stability relations among relevant phases at the pressure temperature (PT) conditions in the fracture system and documented fossil fracture coatings in granites and gneisses suggest that the prime candidate for the H₂O‐consuming reaction is the zeolitization of feldspar. The potential of the gneisses to chemically bind H₂O matches the estimated amount of migrating H₂O.     

Analysis of strain‐induced ground‐water fluctuations at Devils Hole,Nevada

The pool in Devils Hole is a sensitive indicator of crustal strain and fluctuates in response to changes in atmospheric pressure, earth tides, earthquakes, large‐scale tectonic activity and ground‐water development. Short‐term and cyclic water‐level fluctuations caused by atmospheric pressure and earth tides were found to be on the order of millimeters to centimeters. The 1992 Landers/Little Skull Mountain earthquake sequence and the 1999 Hector Mine earthquake induced water‐level offsets of greater than ?12 and ?3.6 cm, respectively. The results of a dislocation model used to compute volumetric strain for each earthquake indicates that the coseismic water‐level offsets are consistent in magnitude and sense with poroelastic responses to earthquake‐induced strain. Theoretical postseismic fluid‐flow modeling indicates that the diffusivity of the system is on the order of 0.03 m² sec^?1, and identified areas of anomalous water‐level fluctuations. Interpretation of model results suggests that while the persistent post‐Landers rise in water‐level can be attributed to deformation‐induced channeling of fluid to the Devils Hole fault zone, the cause of the pre‐Hector Mine water‐level rise may be related to postseismic excess fluid pressures or preseismic strain accumulation.     

Modelling the Lost City hydrothermal field: influence of topography and permeability structure

The Lost City hydrothermal field (LCHF) is hosted in serpentinite at the crest of the Atlantis Massif, an oceanic core complex close to the mid‐Atlantic Ridge. It is remarkable for its longevity and for venting low‐temperature (40–91°C) alkaline fluids rich in hydrogen and methane. IODP Hole U1309D, 5 km north of the LCHF, penetrated 1415 m of gabbroic rocks and contains a near‐conductive thermal gradient close to 100°C km^?1. This is remarkable so close to an active hydrothermal field. We present hydrothermal modelling using a topographic profile through the vent field and IODP site U1309. Long‐lived circulation with vent temperatures similar to the LCHF can be sustained at moderate permeabilities of 10^?14 to 10^?15 m² with a basal heatflow of 0.22 W m^?2. Seafloor topography is an important control, with vents tending to form and remain in higher topography. Models with a uniform permeability throughout the Massif cannot simultaneously maintain circulation at the LCHF and the near‐conductive gradient in the borehole, where permeabilities <10^?16 m² are required. A steeply dipping permeability discontinuity between the LCHF and the drill hole is required to stabilize venting at the summit of the massif by creating a lateral conductive boundary layer. The discontinuity needs to be close to the vent site, supporting previous inferences that high permeability is most likely produced by faulting related to the transform fault. Rapid increases in modelled fluid temperatures with depth beneath the vent agree with previous estimates of reaction temperature based on geochemical modelling.     

Geodynamically induced variations in the emission of CO2 gas at San Faustino (Central Apennines,Italy)

The Central Apennines are affected by frequent earthquakes of moderate magnitude that occur mainly within the upper part of the crust at depths of <15 km. A large number of cold gas emissions that are rich in CO₂ are also found in the region. One particular vent with a high rate of degassing was equipped with a sensor to measure flow rates, which were recorded for a number of different periods between 2005 and 2010. Factors that could affect potentially CO₂ flow rates include barometric pressure, atmospheric temperature, precipitation and local seismicity. Our analysis indicates that the periods of anomalous flow rate were related not to the environmental factors but probably to the deformative processes of the crust associated with the local seismicity. Local seismic events as expression of geodynamic processes occurred always before and during these anomalous gas flow periods. This correlation exists only for events that occurred eastwards of the gas emission site close to the Martana fault zone. We herein consider this correlation as indication for a continuous interaction between the field of static strain and the deep fluid pressure. An approximation of the fluid pressure transmission towards the gas emission site gives reasonable values of 1–10 m² sec^?1. To make comparisons with the long‐term effects of the static strain, we also recorded the short‐term effects of the dynamic release of strain induced by the series of strong earthquakes that took place in L’Aquila in 2009. We detected a significant anomalous flow rate that occurred at the same time as this seismic sequence, during which widespread degassing was induced around the focal zone.     

Fluid effect on hydraulic fracture propagation behavior: a comparison between water and supercritical CO2‐like fluid

The initiation of hydraulic fractures during fluid injection in deep formations can be either engineered or induced unintentionally. Upon injection of CO₂, the pore fluids in deep formations can be changed from oil/saline water to CO₂ or CO₂ dominated. The type of fluid is important not only because the fluid must fracture the rock, but also because rocks saturated with different pore fluids behave differently. We investigated the influence of fluid properties on fracture propagation behavior by using the cohesive zone model in conjunction with a poroelasticity model. Simulation results indicate that the pore pressure fields are very different for different pore fluids even when the initial field conditions and injection schemes (rate and time) are kept the same. Low viscosity fluids with properties of supercritical CO₂ will create relatively thin and much shorter fractures in comparison with fluids exhibiting properties of water under similar injection schemes. Two significant times are recognized during fracture propagation: the time at which a crack ceases opening and the later time point at which a crack ceases propagating. These times are very different for different fluids. Both fluid compressibility and viscosity influence fracture propagation, with viscosity being the more important property. Viscosity can greatly affect hydraulic conductivity and the leak‐off coefficient. This analysis assumes the in‐situ pore fluid and injected fluid are the same and the pore space is 100% saturated by that fluid at the beginning of the simulation.     

Cementation and the hydromechanical behavior of siliciclastic aquifers and reservoirs

Progressive cementation and lithification significantly influence the mechanical and hydrologic properties of granular porous media through elastic stiffening and permeability reduction. We use published data that quantify the effect of grain‐bridging cement distribution in granular porous media at the grain scale to investigate the influence of variable cement content on the competing roles of hydrologic and mechanical effects on fluid flow and deformation at the reservoir scale. The impact of quartz overgrowths in natural samples was quantified using a bond‐to‐grain ratio, allowing a geologically meaningful interpretation of percent cement in conceptual models of quartz cementation. An increase in the bond‐to‐grain ratio from 1 to 2.2 (~1–15% cement by volume) results in a 1.4‐fold increase in Young's modulus and an ~1000‐fold decrease in permeability. The hydromechanical properties of a suite of variably cemented natural samples are used as input into two‐dimensional, kilometre‐scale, axially symmetric poroelastic models of an isotropic confined aquifer. Models isolating the hydrologic and mechanical effects of cementation indicate that the hydrologic properties dominate the overall mechanical response, controlling both the volume and magnitude of deformation. Incorporation of changes in hydrologic properties due to cementation is therefore essential to capturing the first‐order physics of coupled aquifer behavior.     

Real‐time quadrupole mass spectrometer analysis of gas in borehole fluid samples acquired using the U‐tube sampling methodology

Sampling of fluids in deep boreholes is challenging because of the necessity of minimizing external contamination and maintaining sample integrity during recovery. The U‐tube sampling methodology was developed to collect large volume, multiphase samples at in situ pressures. As a permanent or semi‐permanent installation, the U‐tube can be used for rapidly acquiring multiple samples or it may be installed for long‐term monitoring applications. The U‐tube was first deployed in Liberty County, TX to monitor crosswell CO₂ injection as part of the Frio CO₂ sequestration experiment. Analysis of gases (dissolved or separate phase) was performed in the field using a quadrupole mass spectrometer, which served as the basis for determining the arrival of the CO₂ plume. The presence of oxygen and argon in elevated concentrations, along with reduced methane concentration, indicates sample alteration caused by the introduction of surface fluids during borehole completion. Despite producing the well to eliminate non‐native fluids, measurements demonstrate that contamination persists until the immiscible CO₂ injection swept formation fluid into the observation wellbore.     

Comparing closed system, flow-through and fluid infiltration geochemical modelling: examples from K-alteration in the Ernest Henry Fe-oxide–Cu–Au system

Potassic alteration of rocks adjacent to, and within the Ernest Henry Fe‐oxide–Cu–Au deposit is used here as a test case to investigate fluid–rock interactions using various equilibrium dynamic geochemical modelling approaches available in the HCh code. Reaction of a simple K–Fe–(Na,Ca) brine (constrained by published fluid inclusion analysis) with an albite‐bearing felsic volcanic rock, resulted in predicted assemblages defined by (i) K‐feldspar–muscovite–magnetite, (ii) biotite–K‐feldspar–magnetite, (iii) biotite–quartz–albite and (iv) albite–biotite–actinolite–pyroxene with increasing rock buffering (decreasing log w/r). Models for isothermal–isobaric conditions (450°C and 2500 bars) were compared with models run over a T–P gradient (450 to 200°C and 2500 to 500 bars). Three principal equilibrium dynamic simulation methods have been used: (i) static closed system, where individual steps are independent of all others, (ii) flow‐through and flush, where a part of the result is passed as input further along the flow line, and (iii) fluid infiltration models that simulate fluid moving through a rock column. Each type is best suited to a specific geological fluid–rock scenario, with increasing complexity, computation requirements and approximation to different parts of the natural system. Static closed system models can be used to quickly ascertain the broad alteration assemblages related to changes in the water/rock ratio, while flow‐through models are better suited to simulating outflow of reacted fluid into fresh rock. The fluid infiltration model can be used to simulate spatially controlled fluid metasomatism of rock, and we show that, given assumptions of porosity relationships and spatial dimensions, this model is a first‐order approximation to full reactive transport, without requiring significant computational time. This work presents an overview of the current state of equilibrium dynamic modelling technology using the HCh code with a view to applying these techniques to predictive modelling in exploration for mineral deposits. Application to the Ernest Henry Fe‐oxide–Cu–Au deposit demonstrates that isothermal fluid–rock reaction can account for some of the alteration zonation around the deposit.     

Stress‐dependent transport properties of fractured arkosic sandstone

We measure the fluid transport properties of microfractures and macrofractures in low‐porosity polyphase sandstone and investigate the controls of in situ stress state on fluid flow conduits in fractured rock. For this study, the permeability and porosity of the Punchbowl Formation sandstone, a hydrothermally altered arkosic sandstone, were measured and mapped in stress space under intact, microfractured, and macrofractured deformation states. In contrast to crystalline and other sedimentary rocks, the distributed intragranular and grain‐boundary microfracturing that precedes macroscopic fracture formation has little effect on the fluid transport properties. The permeability and porosity of microfractured and intact sandstone depend strongly on mean stress and are relatively insensitive to differential stress and proximity to the frictional sliding envelope. Porosity variations occur by elastic pore closure with intergranular sliding and pore collapse caused by microfracturing along weakly cemented grain contacts. The macroscopic fractured samples are best described as a two‐component system consisting (i) a tabular fracture with a 0.5‐mm‐thick gouge zone bounded by 1 mm thick zones of concentrated transgranular and intragranular microfractures and (ii) damaged sandstone. Using bulk porosity and permeability measurements and finite element methods models, we show that the tabular fracture is at least two orders of magnitude more permeable than the host rock at mean stresses up to 90 MPa. Further, we show that the tabular fracture zone dilates as the stress state approaches the friction envelope resulting in up to a three order of magnitude increase in fracture permeability. These results indicate that the enhanced and stress‐sensitive permeability in fault damage zones and sedimentary basins composed of arkosic sandstones will be controlled by the distribution of macroscopic fractures rather than microfractures.     

Fracture spacing during hydro‐fracturing of cap‐rocks

Layered low permeability rock units, like shales, represent seals or ‘cap‐rocks’ in a variety of geological settings. A continuous increase in the fluid pressure gradients across a virtually impermeable rock layer will ultimately lead to hydro‐fracturing. Depending on the boundary conditions, such fracturing may lead to the formation of a set of sub‐parallel cracks oriented more or less perpendicular to the cap‐rock layer. In this article, we propose a new numerical model that describes interactions between multiple cross‐cutting fractures in an elastic low permeability rock layer. The width of each fracture and the spacing between them are modeled as a force balance between the fluid pressure and the elastic forces in the cap‐rock and between each fracture. The model indicates that the system of fractures evolves toward a spatially periodic steady‐state distribution with a fixed fracture spacing and aperture. The results are similar for incompressible and compressible fluids. The steady‐state conditions depend on only two dimensionless parameters, and the fracture spacing is only weakly dependent on the cap‐rock thickness. This is in contrast to fracturing produced by simple extension of an elastic rock layer beyond the fracture strength, in which case fracture spacing is proportional to layer thickness.     

Precipitation of fracture fillings and cements in the Buntsandstein (NW Germany)

The relationship between fracturing and fracture filling in opening‐mode fractures in the Triassic Buntsandstein in the Lower Saxony Basin (LSB; NW Germany) has been studied by an integration of petrographic and structural analysis of core samples, strontium isotope analysis and microthermometry on fluid inclusions. This revealed the relationship between the timing of the fracturing and the precipitation of different mineral phases in the fractures by constraining the precipitation conditions and considering the possible fluid transport mechanisms. The core was studied from four different boreholes, located in different structural settings across the LSB. In the core samples from the four boreholes, fractures filled with calcite, quartz and anhydrite were found, in addition to pore‐filling calcite cementation. In boreholes 2 and 3, calcite‐filled fractures have a fibrous microstructure whereas in borehole 1, fractures are filled with elongate‐blocky calcite crystals. Anhydrite‐filled fractures have, in all samples, a blocky to elongate‐blocky microstructure. Fractures that are filled with quartz are observed in borehole 2 only where the quartz crystals are ‘stretched’ with an elongated habit. Fluid inclusion microthermometry of fracturing‐filling quartz crystals showed that quartz precipitation took place at temperatures of at least 140°C, from a fluid with NaCl–CaCl₂–H₂O composition. Melting phases are meta‐stable and suggest growth from high salinity formation water. Strontium isotopes, measured in leached host rock, indicate that, in boreholes 2 and 3, the fluid which precipitated the calcite cements and calcite‐filled fractures is most likely locally derived whereas in borehole 1, the ⁸⁷Sr/⁸⁶Sr ratios from the pore‐filling cements and in the elongate‐blocky calcite‐filled fracture can only be explained by mixing with externally derived fluids. The elongate‐blocky anhydrite‐filled fractures, present in boreholes 1, 3 and 4, precipitated from a mixture of locally derived pore fluids and a significant quantity of fluid with a lower, less radiogenic, ⁸⁷Sr/⁸⁶Sr ratio. Taking into account the structural evolution of the basin and accompanying salt tectonics, it is likely that the underlying Zechstein is a source for the less radiogenic fluids. Based on the samples in the LSB, it is probable that fibrous fracture fillings in sedimentary rocks most likely developed from locally derived pore fluids whereas elongate‐blocky fracture fillings with smooth walls developed from externally derived pore fluids.     

The application of failure mode diagrams for exploring the roles of fluid pressure and stress states in controlling styles of fracture-controlled permeability enhancement in faults and shear zones

Permeability enhancement associated with deformation processes in faults and shear zones plays a key role in facilitating fluid redistribution between fluid reservoirs in the crust. Especially in high fluid flux hydrothermal systems, fracture-controlled permeability can be relatively short-lived, unless it is repeatedly regenerated by ongoing deformation. Failure mode diagrams in pore fluid factor and differential stress space, here termed λ–σ failure mode diagrams, provide a powerful tool for analysing how fluid pressure and stress states drive failure, associated permeability enhancement and vein styles during deformation in faults and shear zones. During fault-valve behaviour in the seismogenic regime, relative rates of recovery of pore fluid factor, differential stress and fault cohesive strength between rupture events impact on styles of veining and associated, fracture-controlled permeability enhancement in faults and shear zones. Examples of vein-rich fault zones are used to illustrate how constraints can be placed, not just on fluid pressure and stress states at failure, but also on the fluid pressurization and loading paths associated with failure and transitory permeability enhancement in faults and shear zones. This provides insights about when, during the fault-valve cycle, various types of veins can form. The use of failure mode diagrams also provides insights about the relative roles of optimally oriented faults and misoriented faults as hydraulically conductive structures. The analysis highlights the dynamics of competition between fluid pressures and loading rates in driving failure and repeated permeability regeneration in fracture-controlled, hydrothermal systems.     

Pore‐scale investigation on stress‐dependent characteristics of granular packs and the impact of pore deformation on fluid distribution

Understanding the effect of changing stress conditions on multiphase flow in porous media is of fundamental importance for many subsurface activities including enhanced oil recovery, water drawdown from aquifers, soil confinement, and geologic carbon storage. Geomechanical properties of complex porous systems are dynamically linked to flow conditions, but their feedback relationship is often oversimplified due to the difficulty of representing pore‐scale stress deformation and multiphase flow characteristics in high fidelity. In this work, we performed pore‐scale experiments of single‐ and multiphase flow through bead packs at different confining pressure conditions to elucidate compaction‐dependent characteristics of granular packs and their impact on fluid flow. A series of drainage and imbibition cycles were conducted on a water‐wet, soda‐lime glass bead pack under varying confining stress conditions. Simultaneously, X‐ray micro‐CT was used to visualize and quantify the degree of deformation and fluid distribution corresponding with each stress condition and injection cycle. Micro‐CT images were segmented using a gradient‐based method to identify fluids (e.g., oil and water), and solid phase redistribution throughout the different experimental stages. Changes in porosity, tortuosity, and specific surface area were quantified as a function of applied confining pressure. Results demonstrate varying degrees of sensitivity of these properties to confining pressure, which suggests that caution must be taken when considering scalability of these properties for practical modeling purposes. Changes in capillary number with confining pressure are attributed to the increase in pore velocity as a result of pore contraction. However, this increase in pore velocity was found to have a marginal impact on average phase trapping at different confining pressures.