Fluid injection and surface deformation at the KTB location: modelling of expected tilt effects

This investigation is indented to explore the relationship between changes in pore fluid pressure and deformation of the land surface induced by a large‐scale injection experiment at the KTB site. Deformation will be monitored by ASKANIA borehole tiltmeters at five locations. During the year 2003, a network of borehole tiltmeters was installed, data transmission links established and tested, and recording of tilt data started. Our first main interest was to receive data sets of all stations well before the injection experiment to start in May 2004, to be able to evaluate local site effects. Thus, the separation of injection‐induced effects will be more reliable. Principal 3D numerical modelling (poro‐elastic modelling and investigations, using the finite element method, FEM) of poro‐elastic behaviour showed that significant tilt amplitudes can be expected during controlled fluid injection. Observed deformation will be investigated within the framework of the fluid flow behaviour and resulting deformation. Two models have been used: a coupled hydro geomechanical finite element model (abaqus ) and, as a first step, also a multi‐layered poro‐elastic crust (poel ). With the numerical model two effects can be quantified: (i) the deformation of the upper crust (tilt measurements) and (ii) the spatial distribution and the changes of material properties in the KTB area. The main aim of the project is to improve the knowledge of coupled geomechanic–hydraulic processes and to quantify important parameters. Thus, the understanding of fracture‐dominated changes of the hydrogeological parameters will be enhanced, geomechanical parameter changes and the heterogeneity of the parameter field quantified. In addition, the induced stress field variation can be explained, which is believed to be mainly responsible for the increase of local seismic activity. Here, we introduce the tiltmeter array at the KTB site, the modelling for a poro‐elastic crust and the preliminary FEM modelling.     

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.     

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.     

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.     

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.     

Physical properties of carbonate fault rocks,fucino basin (Central Italy): implications for fault seal in platform carbonates

We documented the porosity, permeability, pore geometry, pore type, textural anisotropy, and capillary pressure of carbonate rock samples collected along basin‐bounding normal faults in central Italy. The study samples consist of one Mesozoic platform carbonate host rock with low porosity and permeability, four fractured host rocks of the damage zones, and four fault rocks of the fault cores. The four fractured samples have high secondary porosity, due to elongated, connected, soft pores that provide fluid pathways in the damage zone. We modeled this zone as an elastic cracked medium, and used the Budiansky–O'Connell correlation to compute its permeability from the measured elastic moduli. This correlation can be applied only to fractured rocks with large secondary porosity and high‐aspect ratio pores. The four fault rock samples are made up of survivor clasts embedded in fine carbonate matrices and cements with sub‐spherical, stiff pores. The low porosity and permeability of these rocks, and their high values of capillary pressure, are consistent with the fault core sealing as much as 77 and 140 m of gas and oil columns, respectively. We modeled the fault core as a granular medium, and used the Kozeny–Carmen correlation, assigning the value of 5 to the Kozeny constant, to compute its permeability from the measured porosities and pore radii. The permeability structure of the normal faults is composed of two main units with unique hydraulic characteristics: a granular fault core that acts as a seal to cross‐fault fluid flow, and an elastic cracked damage zone that surrounds the core and forms a conduit for fluid flow. Transient pathways for along‐fault fluid flow may form in the fault core during seismic faulting due to the formation of opening‐mode fractures within the cemented fault rocks.     

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.     

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.     

Evolution of the transport properties of fractures subject to thermally and mechanically activated mineral alteration and redistribution

Strong feedbacks link temperature (T), hydrologic flow (H), mechanical deformation (M), and chemical alteration (C) in fractured rock. These processes are interconnected as one process affects the initiation and progress of another. Dissolution and precipitation of minerals are affected by temperature and stress, and can result in significant changes in permeability and solute transport characteristics. Understanding these couplings is important for oil, gas, and geothermal reservoir engineering, for CO2 sequestration, and for waste disposal in underground repositories and reservoirs. To experimentally investigate the interactions between THMC processes in a naturally stressed fracture, we report on heated (25°C up to 150°C) flow‐through experiments on fractured core samples of Westerly granite. These experiments examine the influence of thermally and mechanically activated dissolution of minerals on the mechanical (stress/strain) and transport (permeability) responses of fractures. The evolutions of the permeability and relative hydraulic aperture of the fracture are recorded as thermal and stress conditions' change during the experiments. Furthermore, the efflux of dissolved mineral mass is measured periodically and provides a record of the net mass removal, which is correlated with observed changes in relative hydraulic fracture aperture. During the experiments, a significant variation of the effluent fluid chemistry is observed and the fracture shows large changes in permeability to the changing conditions both in stress and in temperature. We argue that at low temperature and high stresses, mechanical crushing of the asperities and the production of gouge explain the permeability decrease although most of the permeability is recoverable as the stress is released. While at high temperature, the permeability changes are governed by mechanical deformation as well as chemical processes, in particular, we infer dissolution of minerals adjacent to the fracture and precipitation of kaolinite.     

On the geochemistry of gases and noble gas isotopes (including 222Rn) in deep crustal fluids: the 4000 m KTB-pilot hole fluid production test 2002–03

The concentrations of H₂, O₂, CO₂, and concentrations and isotopic composition of the noble gases (including ²²²Rn), N₂, CH₄, and higher hydrocarbons dissolved in 4000 m deep‐seated fluids from a 12‐month fluid production test in the KTB pilot hole were analyzed. This determination of the gas geochemistry during the test in combination with the knowledge of the hydraulic data provides relevant information about the fluid hydraulics of the deep system. All gas concentrations and isotopic signatures, except for ²²²Rn, showed constancy during the course of the test. This, in combination with large fluid flow rates at a moderate water table drawdown, imply an almost infinite fluid reservoir in 4000 m depth. From the change in ²²²Rn‐activity as a function of pump rate, the contribution of smaller and wider pores to the overall fluid flow in an aquifer can be deduced. This ²²²Rn‐activity monitoring proved therefore to be a valuable instrument for the qualitative observation of the scavenging of pore and fracture surfaces, a hydraulic feature invisible to standard hydraulic testing tools. The observance of this scavenging effect is due to (i) the continuous on‐line geochemical monitoring, (ii) the durability of the test, (iii) a change in pump rate during the course of the test, and (iv) due to the short half‐life of ²²²Rn. The fluids have a 5.9% mantle He component, and a δ²¹Ne excess of 14%, and a noble gas model age of about (5.5–6.2) ± 2.0 Myr. The mean N₂/Ar‐ratio of 516 and δ¹⁵N‐data of about +1.5‰ indicates sedimentary or metamorphic origin of N₂. The hydrocarbons, amounting to 33 vol.% in the gas phase, are derived from thermal decomposition of marine organic matter of low maturity. But a key question, the identification of the potential source region of the fluids and the migration pathway, is still unidentified.     

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.     

Numerical modelling of 3D fluid flow and oxygen isotope exchange in fractured media: spatial distribution of isotope patterns

An understanding of fluid flow, mass transport and isotopic exchange in fractured rock is required to understand the origin of several geological processes including hydrothermal mineral deposits. The numerical model HydroGeoSphere simulates 3D advection, molecular diffusion, mechanical dispersion and isotopic exchange in a discretely fractured porous media, and can be used to better understand the processes of mass transport and isotopic exchange in fractured rocks. Study of ¹⁸O isopleth patterns for different types of fractures and fracture networks with a range of structural complexity and hydraulic properties shows that fracture properties and geometry control mass transport and isotopic exchange. The hydraulic properties, as well as the density, spacing, and connectivity of fractures determine the isotopic patterns. Asymmetries in the geometry of oxygen isotope patterns could be used to determine the direction of hydrothermal fluid flow.     

A vector of high‐temperature paleo‐fluid flow deduced from mass transfer across permeability barriers (quartz veins)

Quartz veins acted as impermeable barriers to regional fluid flow and not as fluid‐flow conduits in Mesoproterozoic rocks of the Mt Painter Block, South Australia. Systematically distributed asymmetric alteration selvedges consisting of a muscovite‐rich zone paired with a biotite‐rich zone are centered on quartz veins in quartz–muscovite–biotite schist. Geometric analysis of the orientation and facing of 126 veins at Nooldoonooldoona Waterhole reveals a single direction along which a maximum of all veins have a muscovite‐rich side, irrespective of their specific individual orientation. This direction represents a Mesoproterozoic fluid‐flow vector and the veins represent permeability barriers to the flow. The pale muscovite‐rich zones formed on the downstream side of the vein and the dark biotite‐rich zones mark the upstream side. The alteration couplets formed from mica schist at constant Zr, Ga, Sc, and involved increases in Si, Na, Al and decreases in K, Fe, Mg for pale alteration zones, and inverse alteration within dark zones. The asymmetry of the alteration couplets is best explained by the pressure dependence of mineral–fluid equilibria. These equilibria, in combination with a Darcian flow model for coupled advection and diffusion, and with permeability barriers imposed by the quartz veins, simulate the pattern of both fluid flow and differential, asymmetric metasomatism. The determined vector of fluid flow lies along the regional foliation and is consistent with the known distribution of regional alteration products. The presence of asymmetric alteration zones in rock containing abundant pre‐alteration veins suggests that vein‐rich material may have generally retarded regional fluid flow.     

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.     

Reactive transport and thermo‐hydro‐mechanical coupling in deep sedimentary basins affected by glaciation cycles: model development,verification, and illustrative example

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 CO₂ 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 MIN3P reactive transport code (MIN3P‐THCm) 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 Paleozoic sedimentary basins in North America, 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.     

Effects of episodic fluid flow on hydrocarbon migration in the Newport‐Inglewood Fault Zone,Southern California

Fault permeability may vary through time due to tectonic deformations, transients in pore pressure and effective stress, and mineralization associated with water‐rock reactions. Time‐varying permeability will affect subsurface fluid migration rates and patterns of petroleum accumulation in densely faulted sedimentary basins such as those associated with the borderland basins of Southern California. This study explores the petroleum fluid dynamics of this migration. As a multiphase flow and petroleum migration case study on the role of faults, computational models for both episodic and continuous hydrocarbon migration are constructed to investigate large‐scale fluid flow and petroleum accumulation along a northern section of the Newport‐Inglewood fault zone in the Los Angeles basin, Southern California. The numerical code solves the governing equations for oil, water, and heat transport in heterogeneous and anisotropic geologic cross sections but neglects flow in the third dimension for practical applications. Our numerical results suggest that fault permeability and fluid pressure fluctuations are crucial factors for distributing hydrocarbon accumulations associated with fault zones, and they also play important roles in controlling the geologic timing for reservoir filling. Episodic flow appears to enhance hydrocarbon accumulation more strongly by enabling stepwise build‐up in oil saturation in adjacent sedimentary formations due to temporally high pore pressure and high permeability caused by periodic fault rupture. Under assumptions that fault permeability fluctuate within the range of 1–1000 millidarcys (10^?15–10^?12 m²) and fault pressures fluctuate within 10–80% of overpressure ratio, the estimated oil volume in the Inglewood oil field (approximately 450 million barrels oil equivalent) can be accumulated in about 24 000 years, assuming a seismically induced fluid flow event occurs every 2000 years. This episodic petroleum migration model could be more geologically important than a continuous‐flow model, when considering the observed patterns of hydrocarbons and seismically active tectonic setting of the Los Angeles basin.     

Large‐scale chemical stratification of fluids in the crust: hydraulic and chemical data from the geothermal research site Urach,Germany

The Urach 3 research borehole in SW Germany has been drilled through a sedimentary cover sequence and reached gneisses of the Variscan crystalline basement at 1604 m below surface. An additional 2840 m has been drilled through fractured basement rocks. The borehole has been used for hydraulic tests in the context of a ‘hot dry rock’ (HDR) project. The sedimentary cover ranges from the Carboniferous to the Middle Jurassic (Dogger) in age and comprises mostly clastic sediments in the Paleozoic and limestone and shale in the Mesozoic. Water composition data from 10 different depths include samples from all major lithological units. The total dissolved solids (TDS) increases from the surface to about 650 m where it reaches 4.1 g l^?1 in Triassic limestone. In lower Triassic sandstones, TDS increases very sharply to 28.5 g l^?1 and the water is saturated with pure CO₂ gas. With increasing depth, TDS does not change much in the clastic sediments of the Permian and Carboniferous. The crystalline basement is marked by a very sharp increase in TDS to 55.5 g l^?1 at about 1770 m depth. TDS increases within the basement to more than 78.5 g l^?1 at about 3500 m depth. The data suggest that there is limited vertical chemical communication over long periods of time. The CO₂ gas cap in the lower Triassic sandstones requires a gastight cover. The chemical stratification of the fluids relates to the permeability structure of the crust at the Urach site and fits well with hydraulic and thermal data from the site.     

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.     

Salt tectonics and shallow subseafloor fluid convection: models of coupled fluid‐heat‐salt transport

Thermohaline convection associated with salt domes has the potential to drive significant fluid flow and mass and heat transport in continental margins, but previous studies of fluid flow associated with salt structures have focused on continental settings or deep flow systems of importance to petroleum exploration. Motivated by recent geophysical and geochemical observations that suggest a convective pattern to near‐seafloor pore fluid flow in the northern Gulf of Mexico (GoMex), we devise numerical models that fully couple thermal and chemical processes to quantify the effects of salt geometry and seafloor relief on fluid flow beneath the seafloor. Steady‐state models that ignore halite dissolution demonstrate that seafloor relief plays an important role in the evolution of shallow geothermal convection cells and that salt at depth can contribute a thermal component to this convection. The inclusion of faults causes significant, but highly localized, increases in flow rates at seafloor discharge zones. Transient models that include halite dissolution show the evolution of flow during brine formation from early salt‐driven convection to later geothermal convection, characteristics of which are controlled by the interplay of seafloor relief and salt geometry. Predicted flow rates are on the order of a few millimeters per year or less for homogeneous sediments with a permeability of 10^?15 m², comparable to compaction‐driven flow rates. Sediment permeabilities likely fall below 10^?15 m² at depth in the GoMex basin, but such thermohaline convection can drive pervasive mass transport across the seafloor, affecting sediment diagenesis in shallow sediments. In more permeable settings, such flow could affect methane hydrate stability, seafloor chemosynthetic communities, and the longevity of fluid seeps.     

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.