The purpose of this paper is to (i) review field data on stress‐induced permeability changes in fractured rock; (ii) describe estimation of fractured rock stress‐permeability relationships through model calibration against such field data; and (iii) discuss observations of temperature and chemically mediated fracture closure and its effect on fractured rock permeability. The field data that are reviewed include in situ block experiments, excavation‐induced changes in permeability around tunnels, borehole injection experiments, depth (and stress) dependent permeability, and permeability changes associated with a large‐scale rock‐mass heating experiment. Data show how the stress‐permeability relationship of fractured rock very much depends on local in situ conditions, such as fracture shear offset and fracture infilling by mineral precipitation. Field and laboratory experiments involving temperature have shown significant temperature‐driven fracture closure even under constant stress. Such temperature‐driven fracture closure has been described as thermal overclosure and relates to better fitting of opposing fracture surfaces at high temperatures, or is attributed to chemically mediated fracture closure related to pressure solution (and compaction) of stressed fracture surface asperities. Back‐calculated stress‐permeability relationships from field data may implicitly account for such effects, but the relative contribution of purely thermal‐mechanical and chemically mediated changes is difficult to isolate. Therefore, it is concluded that further laboratory and in situ experiments are needed to increase the knowledge of the true mechanisms behind thermally driven fracture closure, and to further assess the importance of chemical‐mechanical coupling for the long‐term evolution of fractured rock permeability. This paper reviews stress‐induced permeability changes in fractured rock observed from field data, including effects of temperature and chemically mediated fracture closure. While the stress‐permeability relationship of a rock mass might be bounded from site specific field investigations, it is concluded that further laboratory and in situ experiments are needed to increase the knowledge of the true mechanisms underlying thermally driven fracture closure, and to further assess chemical‐mechanical coupling effects on the long‐term evolution of fractured rock permeability.
Thermal springs in the Southern Alps, New Zealand, originate through penetration of fluids into a thermal anomaly generated by rapid uplift and exhumation on the Alpine Fault. Copland hot spring (43.629S, 169.946E) is one of the most vigorously flowing, hottest of the springs, discharging strongly effervescent CO 2 ‐rich 56–58°C water at 6 ± 1 l sec −1 . Shaking from the Mw7.8 Dusky Sound (Fiordland) 2009 and Mw7.1 Darfield (Canterbury) 2010 earthquakes, 350 and 180 km from the spring, respectively, resulted in a characteristic approximately 1°C delayed cooling over 5 days. A decrease in conductivity and increase in pH were measured following the Mw7.1 Darfield earthquake. Earthquake‐induced decreases in Cl, Li, B, Na, K, Sr and Ba concentrations and an increase in SO 4 concentration reflect higher proportions of shallow‐circulating meteoric fluid mixing in the subsurface. Shaking at amplitudes of approximately 0.5% g Peak Ground Acceleration (PGA) and/or 0.05–0.10 MPa dynamic stress influences Copland hot spring temperature, which did not respond during the Mw6.3 Christchurch 2011 aftershock or other minor earthquakes. Such thresholds should be exceeded every 1–10 years in the central Southern Alps. The characteristic cooling response at low shaking intensities (MM III–IV) and seismic energy densities (approximately 10 −1 J m −3 ) from intermediate‐field distances was independent of variations in spectral frequency, without the need for post‐seismic recovery. Observed temperature and fluid chemistry responses are inferred to reflect subtle changes in the fracture permeability of schist mountains adjacent to the spring. Permanent 10 −7 –10 −6 strains recorded by cGPS reflect opening or generation of fractures, allowing greater quantities of relatively cool near‐surface groundwater to mix with upwelling hot water. Active deformation, tectonic and topographic stress in the Alpine Fault hanging wall, where orographic rainfall, uplift and erosion are extreme, make the Southern Alps hydrothermal system particularly susceptible to earthquake‐induced transient permeability. In response to large distant earthquakes Copland hot spring cooled approximately 1°C and changed fluid chemistry. Relatively low intensity shaking induced small permanent strains across the Southern Alps – opening fractures which enhanced mixing of relatively cool near‐surface groundwater with upwelling hot water. Active deformation, tectonic and topographic stress in the Alpine Fault hanging wall makes the Southern Alps hydrothermal system particularly susceptible to earthquake‐induced transience.
The initiation of hydraulic fractures during fluid injection in deep formations can be either engineered or induced unintentionally. Upon injection of CO 2 , the pore fluids in deep formations can be changed from oil/saline water to CO 2 or CO 2 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 CO 2 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. Hydraulic fractures can be induced during fluid injection. Thin fluids with properties of supercritical CO 2 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 at a constant injection flow rate: the time at which a crack ceases opening and the later time point at which a crack ceases propagating. These times are different for different fluids.
To quantify and rank gas wettability of coal as a key parameter affecting the extent of CO 2 sequestration in coal and CH 4 recovery from coal, we developed a contact angle measuring system based on a captive gas bubble technique. We used this system to study the gas wetting properties of an A ustralian coal from the S ydney B asin. Gas bubbles were generated and captivated beneath a coal sample within a distilled water‐filled ( pH 5.7) pressurised cell. Because of the use of distilled water, and the continuous dissolution and shrinkage of the gas bubble in water during measurement, the contact angles measured correspond to a ‘transient receding’ contact angle. To take into account the mixed‐gas nature ( CO 2 , CH 4 , and to a lesser extent N 2 ) of coal seam gas in the basin, we evaluated the relative wettability of coal by CH 4 , CO 2 and N 2 gases in the presence of water. Measurements were taken at various pressures of up to 15 MPa for CH 4 and N 2 , and up to 6 MPa for CO 2 at a constant temperature of 22°C. Overall, our results show that CO 2 wets coal more extensively than CH 4 , which in turn wets coal slightly more than N 2 . Moreover, the contact angle reduces as the pressure increases, and becomes < 90° at various pressures depending on the gas type. In other words, all three gases wet coal better than water under sufficiently high pressure. The gas wettability of coal affects the ease of adsorption and desorption of gases onto and from coal. Using a modified captive gas technique, the magnitude of the ‘receding’ contact angle of CH 4 , CO 2 and N 2 bubbles on a bituminous coal in presence of water was studied. The results show that gas wettability is in general higher for CO 2 than for CH 4 or N 2 . In most cases, increases in gas‐water pressure led to increases in gas wettability of coal.
Thermohaline convection of subsurface fluids strongly influences heat and mass fluxes within the Earth's crust. The most effective hydrothermal systems develop in the vicinity of magmatic activity and can be important for geothermal energy production and ore formation. As most parts of these systems are inaccessible to direct observations, numerical simulations are necessary to understand and characterize fluid flow. Here, we present a new numerical scheme for thermohaline convection based on the control volume finite element method (CVFEM), allowing for unstructured meshes, the representation of sharp thermal and solute fronts in advection‐dominated systems and phase separation of variably miscible, compressible fluids. The model is an implementation of the Complex Systems Modelling Platform CSMP ++ and includes an accurate thermodynamic representation of strongly nonlinear fluid properties of salt water for magmatic‐hydrothermal conditions (up to 1000°C, 500 MPa and 100 wt% NaCl). The method ensures that all fluid properties are taken as calculated on the respective node using a fully upstream‐weighted approach, which greatly increases the stability of the numerical scheme. We compare results from our model with two well‐established codes, HYDROTHERM and TOUGH2, by conducting benchmarks of different complexity and find good to excellent agreement in the temporal and spatial evolution of the hydrothermal systems. In a simulation with high‐temperature, high‐salinity conditions currently outside of the range of both HYDROTHERM and TOUGH2, we show the significance of the formation of a solid halite phase, which introduces heterogeneity. Results suggest that salt added by magmatic degassing is not easily vented or accommodated within the crust and can result in dynamic, complex hydrologies. We present a new numerical scheme for multiphase convection of salt water at magmatic‐hydrothermal conditions based on the control volume finite element method. In a series of benchmarks with HYDROTHERM and TOUGH2, we find very good agreement of the simulated hydrothermal systems. Simulations at high‐temperature, high‐salinity conditions outside of the range of these models show the influence of solid halite on dynamic flow behavior and suggest that salt from magmatic degassing is not easily vented or accommodated within the crust.
Lithium (Li) concentrations of produced water from unconventional (horizontally drilled and hydraulically fractured shale) and conventional gas wells in Devonian reservoirs in the Appalachian Plateau region of western Pennsylvania range from 0.6 to 17 mmol kg −1 , and Li isotope ratios, expressed as in δ 7 Li, range from +8.2 to +15‰. Li concentrations are as high as 40 mmol kg −1 in produced waters from Plio‐Pleistocene through Jurassic‐aged reservoirs in the Gulf Coast Sedimentary Basin analyzed for this study, and δ 7 Li values range from about +4.2 to +16.6‰. Because of charge‐balance constraints and rock buffering, Li concentrations in saline waters from sedimentary basins throughout the world (including this study) are generally positively correlated with chloride (Cl), the dominant anion in these fluids. Li concentrations also vary with depth, although the extent of depth dependence differs among sedimentary basins. In general, Li concentrations are higher than expected from seawater or evaporation of seawater and therefore require water–mineral reactions that remove lithium from the minerals. Li isotope ratios in these produced waters vary inversely with temperature. However, calculations of temperature‐dependent fractionation of δ 7 Li between average shale δ 7 Li (−0.7‰) and water result in δ 7 Li water that is more positive than that of most produced waters. This suggests that aqueous δ 7 Li may reflect transport of water from depth and/or reaction with rocks having δ 7 Li lighter than average shale. Lithium, compiled from oil and gas field fluids around the world, correlates generally with depth and with Cl, confirming water–rock interactions are its primary source. δ 7 Li of Appalachian Plateau and Gulf Basin (USA) water ranges from +4.4‰ to +16.6‰ and follows a coherent temperature trend, despite differences in tectonic histories of the two basins. Equilibrium isotope fractionation predicts a Li source material that is approximately 4‰ lighter than average shale δ 7 Li.
Hot springs can occur in amagmatic settings, but the mechanisms of heating are often obscure. We have investigated the origin of the Truth or Consequences, New Mexico low‐temperature (approximately 41°C) hot springs in the southern Rio Grande Rift. We tested two hypotheses that could account for this amagmatic geothermal anomaly: lateral forced convection in a gently dipping carbonate aquifer and circulation through high‐permeability crystalline basement rocks to depths of 8 km that is then focused through an overlying faulted hydrologic window. These hypotheses were tested using a regional two‐dimensional hydrothermal model. Model parameters were constrained by calibrating to measured temperatures, specific discharge rates, and groundwater residence times. We collected 16 temperature profiles, 11 geochemistry samples, and 6 carbon‐14 samples within the study area. The geothermal waters are Na + /Cl − ‐dominated and have apparent groundwater ages ranging from 5500 to 11 500 years. Hot‐spring geochemistry is consistent with water/rock interaction in a silicate geothermal reservoir, rather than a carbonate system. Peclet number analysis of temperature profiles suggests that specific discharge rates beneath Truth or Consequences range from 2 to 4 m year −1 . Geothermometry indicates maximum reservoir temperatures are around 170°C. Observed measurements were reasonably reproduced using the deep circulation permeable‐basement modeling scenario (10 −12 m 2 ) but not the lateral forced‐convection carbonate‐aquifer scenario. Focused geothermal discharge is the result of localized faulting, which has created a hydrologic window through a regional confining unit. In tectonically active areas, such as the Rio Grande Rift, deep groundwater circulation within fractured crystalline basement may play a more prominent role in the formation of geothermal systems than has generally been acknowledged. Geochemical analysis and two‐dimensional numerical modeling results strongly suggest that the Truth or Consequences, New Mexico geothermal system is the result of deep (2–8 km) groundwater circulation within highly permeable (10 −12 m 2 ) crystalline basement rocks. Geothermal waters discharge at the surface only through a hydrologic window in a regional overlying confining unit. This study provides additional evidence that the crystalline basement rocks beneath the Rio Grande Rift can be remarkably permeable in some places.
The aim of this experimental study is to investigate the impact of wetting characteristics on multiphase flow, sweep efficiency, and residual fluid distribution in unconsolidated porous media. A sequence of oil and water injections was performed on bead packs with uniform porosity and permeability, but different wettability characteristics. Uniform and mixed‐wet bead packs with varying degree of wettability were fabricated to analyze how the residual saturation profiles and the distribution of fluid phases at the pore scale respond to changes in wettability. X‐ray microtomography was used to visualize and analyze the fluid distribution in each bead pack at the end of oil and brine injection. It was found that sweep efficiency was high for the uniform, strongly wetting glass bead pack. For the intermediate‐wet plastic bead pack, we observed evidence of viscous fingering resulting in degenerating sweep efficiency after water injection. In media with mixed wetting surfaces, the spatial distribution of wettability influenced the topology of the saturation profiles and resulted in larger quantities of disconnected fluid blobs. Results also showed that the average blob size was independent of the average residual saturation. In addition, the difference in saturation conditions preceding each injection affected sweep efficiency. The residual saturation after the 1st displacement was higher than the residual saturation after the 2nd displacement. The difference in saturation conditions preceding each injection cycle affected sweep efficiency. The residual saturation after the 1st displacement was higher than the residual saturation after 2nd displacement. Sweep efficiency was high in the uniform, strongly wetting sample. The spatial distribution of wettability in mixed‐wet porous samples influenced the topology of residual saturation profiles.
We present a structural, microstructural, and stable isotope study of a calcite vein mesh within the Cretaceous Natih Formation in the Oman Mountains to explore changes in fluid pathways during vein formation. Stage 1 veins form a mesh of steeply dipping crack‐seal extension veins confined to a 3.5‐m‐thick stratigraphic interval. Different strike orientations of Stage 1 veins show mutually crosscutting relationships. Stage 2 veins occur in the dilatant parts of a younger normal fault interpreted to penetrate the stratigraphy below. The δ 18 O composition of the host rock ranges from 21.8‰ to 23.7‰. The δ 13 C composition ranges from 1.5‰ to 2.3‰. This range is consistent with regionally developed diagenetic alteration at top of the Natih Formation. The δ 18 O composition of vein calcite varies from 22.5‰ to 26.2‰, whereas δ 13 C composition ranges from −0.8‰ to 2.1‰. A first trend observed in Stage 1 veins involves a decrease of δ 13 C to compositions nearly 1.3‰ lower than the host rock, whereas δ 18 O remains constant. A second trend observed in Stage 2 calcite has δ 18 O values up to 3.3‰ higher than the host rock, whereas the δ 13 C composition is similar. Stable isotope data and microstructures indicate an episodic flow regime for both stages. During Stage 1, formation of a stratabound vein mesh involved bedding‐parallel flow, under near‐lithostatic fluid pressures. The 18 O fluid composition was host rock‐buffered, whereas 13 C composition was relatively depleted. This may reflect reaction of low 13 C CO 2 derived by fluid interaction with organic matter in the limestones. Stage 2 vein formation is associated with fault‐controlled fluid flow accessing fluids in equilibrium with limestones about 50 m beneath. We highlight how evolution of effective stress states and the growth of faults influence the hydraulic connectivity in fracture networks and we demonstrate the value of stable isotopes in tracking changes in fluid pathways. Carbon and oxygen stable isotope compositions are a valuable tool to track changes in fluid flow regimes in fracture and vein meshes. During Stage 1, the formation of a strata‐bound crack‐seal vein mesh involved bedding‐parallel flow, under near‐lithostatic fluid pressures. The relative depletion of 13C composition of veins may reflect influx of low 13C CO2 derived from detrital organic matter. Stage 2 fault vein formation is associated with fault‐controlled fluid flow accessing fluids from 50 m beneath the Stage 1 mesh.
In some cases, water–rock interactions in fault zones can affect radionuclide migration. Here, we analyzed the chemical compositions of well‐exposed fault rocks from the strike‐slip Atera Fault, Central Japan, in order to understand the variability and behavior of major and selected trace elements. The fault zone has a 1.2‐m‐wide, smectite‐rich fault core and paired damage zones that developed within welded tuff on one side of the core and within granite on the other side. The 30‐cm‐wide, kaolinite‐rich fault gouge is developed in granite cataclasite, and it shows indications of the latest fault activity, while the 1.2‐m‐wide fault core appears to be older. Hydrogen and oxygen isotope ratios in the clay‐rich fault gouges, and carbon and oxygen isotope ratios in carbonates indicate that the two major clay‐rich zones formed in bedrock near the surface, consistent with observed deformation structures. Based on chemical analyses, we identified (1) a slight depletion in SiO 2 , Na 2 O, K 2 O, and light rare earth elements at the edges of the 1.2‐m‐wide fault core, (2) a clear depletion in SiO 2 , Na 2 O, K 2 O, and all rare earth elements except Eu in the 30‐cm‐wide fault gouge, and (3) an increase in CaO, MnO, and heavy rare earth elements across the entire 1.2‐m‐wide fault core. Findings (1) and (2) reflect water–rock interactions in the 1.2‐m‐wide fault core and in the 30‐cm‐wide fault gouge that resulted in the formation of smectite and kaolinite. Finding (3) reflects carbonate precipitation caused by the addition of basalt fragments from a nearby site to the 1.2‐m‐wide fault core during faulting, and subsequent sorption reactions of heavy rare earth elements via processes such as complexation with the carbonates. Chemical composition was analyzed for well‐exposed fault rocks from the strike‐slip Atera Fault, Central Japan, to understand the variability and behavior of major and some selected trace elements. We identified increase of CaO, MnO, and HREEs across the fault core. The finding reflects carbonate precipitation caused by the addition of basalt fragments exposed at a nearby site to the fault core during fault activities, and subsequent sorption reactions of HREEs via processes such as complexation with the carbonates.
This study focuses on the mechanics of methane bubble phase behavior in the gas hydrate stability zone. The transformation of deep‐water methane bubbles into solid hydrate was investigated in Lake Baikal in situ . After being released from the lake bottom, methane bubbles were caught by different traps with transparent walls. When bubbles entered the internal spaces of the traps, the bubbles could be transformed into two different solid hydrate structures depending on the ambient conditions. The first structure was hydrate granular matter consisting of solid fragments with sizes on the order of 1 mm. The second structure was a highly porous solid foam consisting of solid bubbles with sizes on the order of 5 mm. The granular matter did not change as it was brought up to the top border of the gas hydrate stability zone, whereas in the solid foam, free methane rapidly exsolved from the sample during depressurization. We conclude that the decrease in depth and the decrease in the bubble flux rate were key factors in the formation of the hydrate granular matter, whereas the increase in the depth of bubble sampling and the increase in the bubble flux rate facilitated the conversion of bubbles into a highly porous solid hydrate foam. As a result of catching of the methane bubbles by the traps in the hydrate stability zone, the bubbles can be transformed into a hydrate granular matter or form hydrate solid foam. The type of transformation depends on the competition of influential factors: the current depth and the intensity of the bubble flux.
Thermal–hydrological–mechanical coupling processes suggest that fault permeability should undergo dynamic change as a result of seismic slip. In igneous rocks, a fault's slip surface may have much higher permeability than the surrounding rock matrix and therefore operate as a conduit for fluids. We conducted laboratory experiments to investigate changes in fracture permeability (or transmissivity) of a fault in granite due to shear slip and cyclic heating and cooling. Our experiments showed that high initial fracture transmissivity (>10 −18 m 3 ) was associated with a high friction coefficient and that transmissivity decreased during slip. We propose that this reduction in transmissivity reflects the presence of gouge in fracture voids, increasing the area of contact in the fault plane and reducing the hydraulic aperture. In contrast, when initial fracture transmissivity was low (<10 −18 m 3 ), we observed that friction was lower and transmissivity increased during slip. The high transmissivity and high friction may be explained by large areas of bare rock being in contact on the slip surface. Slip velocity had little influence on the evolution of permeability, probably because gouge produced at different slip velocities had similar grain size distributions, or because gouge leaked from the slip surface. Transmissivity decreased with increasing temperature in heating tests, probably due to thermal expansion increasing normal stress on the fracture. Frictional heating did not influence transmissivity during the shearing tests. Fault permeability should undergo dynamic change as a result of seismic slip. In this paper, we conducted laboratory experiments to investigate changes in fracture permeability of a fault in low permeable granite due to shear slip and cyclic heating. The results show low permeability is associated with high friction coefficient. High initial permeability was reduced by slip, whereas for small initial permeability, slip enhanced permeability. The production of gouge and the evolution of the geometry affect permeability.
The ascent of magmatic carbon dioxide in the western E ger ( O hře) R ift is interlinked with the fault systems of the V ariscian basement. In the C heb B asin, the minimum CO 2 flux is about 160 m 3 h −1 , with a diminishing trend towards the north and ceasing in the main epicentral area of the N orthwest B ohemian swarm earthquakes. The ascending CO 2 forms Ca‐Mg‐HCO 3 type waters by leaching of cations from the fault planes and creates clay minerals, such as kaolinite, as alteration products on affected fault planes. These mineral reactions result in fault weakness and in hydraulically interconnected fault network. This leads to a decrease in the friction coefficient of the C oulomb failure stress ( CFS ) and to fault creep as stress build‐up cannot occur in the weak segments. At the transition zone in the north of the C heb B asin, between areas of weak, fluid conductive faults and areas of locked faults with frictional strength, fluid pressure can increase resulting in stress build‐up. This can trigger strike‐slip swarm earthquakes. Fault creep or movements in weak segments may support a stress build‐up in the transition area by transmitting fluid pressure pulses. Additionally to fluid‐driven triggering models, it is important to consider that fluids ascending along faults are CO 2 ‐supersaturated thus intensifying the effect of fluid flow. The enforced flow of CO 2 ‐supersaturated fluids in the transitional zone from high to low permeability segments through narrowings triggers gas exsolution and may generate pressure fluctuations. Phase separation starts according to the phase behaviour of CO 2 ‐H 2 O systems in the seismically active depths of NW B ohemia and may explain the vertical distribution of the seismicity. Changes in the size of the fluid transport channels in the fault systems caused, or superimposed, by fault movements, can produce fluid pressure increases or pulses, which are the precondition for triggering fluid‐induced swarm earthquakes. Ascending mantle‐derived CO 2 induces mineral reactions between dissolved CO 2 and silicate minerals. The kaolinitic – montmorillonitic alteration products cause fault weakness by reducing the friction coefficient μ resulting in fault creep. Stress build‐up and seismicity occur at the transition zone to the unaltered fault segment with frictional strength at the northern border of the C heb B asin in the N ový K ostel area. In vertical regard, the seismicity starts with the phase separation according to the phase behaviour of CO 2 ‐H 2 O systems. The CO 2 ‐driven formation of phyllosilicates (kaolinite) reduces the friction coefficient η to η' and thus the angel of internal friction (shear line), inducing a fracture failure that is, fault creep in the weak fault segments between θ' and θ''.
The hydromechanical effects of Pleistocene glacial loading on the Michigan Basin are assessed using numerical analysis based on coupled stress and pore-water pressure. The two-dimensional model domain included the Basin cross section and extended 10km into the Precambrian. In the analysis, we considered the effects of the number of glacial loading cycles, the presence and connectedness of a deep Cambrian aquifer, the direction of glacial advance, the effect of a wet versus dry glacier/soil interface, topographic effects, density-driven flow effects, and lithosphere flexure on the development of anomalous pressures. The modeling results were compared with data collected from highly instrumented wells completed in the eastern margin of the Basin. The present-day results define regions of significant underpressure in the upper Ordovician and lower Silurian formations characterized by very low hydraulic conductivity and regions of overpressure where hydraulic conductivity is higher. To achieve the degree of underpressure observed in the instrumented wells using the model, a specific loading cycle applied over 100000years was repeated four times. As the number of loading cycles increased, the Paleozoic formations reached a state where the underpressures remain constant. Our results also illustrate the difference in poroelastic modeling between the application of mechanical loads on the land surface and the application of an equivalent hydraulic head, where the latter developed overpressures rather than the observed underpressures. The modeling also shows that the overpressures observed in the Cambrian formations are most likely to be the result of density-driven flow. Finally, the simulations show that the effects of lithosphere flexure in the hydromechanical models results in the development of lateral stresses that generate overpressures rather than underpressures in the southern half of the domain. As there are no suitable observation points, these results remain unconfirmed, and further study is warranted.
The hydromechanical effects of Pleistocene glacial loading on the Michigan Basin are assessed using numerical analysis based on coupled stress and pore‐water pressure. The two‐dimensional model domain included the Basin cross section and extended 10 km into the Precambrian. In the analysis, we considered the effects of the number of glacial loading cycles, the presence and connectedness of a deep Cambrian aquifer, the direction of glacial advance, the effect of a wet versus dry glacier/soil interface, topographic effects, density‐driven flow effects, and lithosphere flexure on the development of anomalous pressures. The modeling results were compared with data collected from highly instrumented wells completed in the eastern margin of the Basin. The present‐day results define regions of significant underpressure in the upper Ordovician and lower Silurian formations characterized by very low hydraulic conductivity and regions of overpressure where hydraulic conductivity is higher. To achieve the degree of underpressure observed in the instrumented wells using the model, a specific loading cycle applied over 100 000 years was repeated four times. As the number of loading cycles increased, the Paleozoic formations reached a state where the underpressures remain constant. Our results also illustrate the difference in poroelastic modeling between the application of mechanical loads on the land surface and the application of an equivalent hydraulic head, where the latter developed overpressures rather than the observed underpressures. The modeling also shows that the overpressures observed in the Cambrian formations are most likely to be the result of density‐driven flow. Finally, the simulations show that the effects of lithosphere flexure in the hydromechanical models results in the development of lateral stresses that generate overpressures rather than underpressures in the southern half of the domain. As there are no suitable observation points, these results remain unconfirmed, and further study is warranted. A hydromechanical model was applied using multiple glacial loading cycles to the Michigan Basin to investigate the generation of significant under and overpressures observed in the eastern margin. The results show that a particular glacial cycle of ~ 100 ka applied at least four times will generate the observed underpressures in low‐permeability Ordovician formations. The generation of the underpressure is attributed to the presence of a permeable Cambrian aquifer underlying the Ordovician units that acts to drain pore fluids during loading events.
Ongoing (1996–present) volcanic unrest near South Sister, Oregon, is accompanied by a striking set of hydrothermal anomalies, including elevated temperatures, elevated major ion concentrations, and 3 He/ 4 He ratios as large as 8.6 R A in slightly thermal springs. These observations prompted the US Geological Survey to begin a systematic hydrothermal‐monitoring effort encompassing 25 sites and 10 of the highest‐risk volcanoes in the Cascade volcanic arc, from Mount Baker near the Canadian border to Lassen Peak in northern California. A concerted effort was made to develop hourly, multiyear records of temperature and/or hydrothermal solute flux, suitable for retrospective comparison with other continuous geophysical monitoring data. Targets included summit fumarole groups and springs/streams that show clear evidence of magmatic influence in the form of high 3 He/ 4 He ratios and/or anomalous fluxes of magmatic CO 2 or heat. As of 2009–2012, summit fumarole temperatures in the Cascade Range were generally near or below the local pure water boiling point; the maximum observed superheat was <2.5°C at Mount Baker. Variability in ground temperature records from the summit fumarole sites is temperature‐dependent, with the hottest sites tending to show less variability. Seasonal variability in the hydrothermal solute flux from magmatically influenced springs varied from essentially undetectable to a factor of 5–10. This range of observed behavior owes mainly to the local climate regime, with strongly snowmelt‐influenced springs and streams exhibiting more variability. As of the end of the 2012 field season, there had been 87 occurrences of local seismic energy densities approximately ≥ 0.001 J/m 3 during periods of hourly record. Hydrothermal responses to these small seismic stimuli were generally undetectable or ambiguous. Evaluation of multiyear to multidecadal trends indicates that whereas the hydrothermal system at Mount St. Helens is still fast‐evolving in response to the 1980–present eruptive cycle, there is no clear evidence of ongoing long‐term trends in hydrothermal activity at other Cascade Range volcanoes that have been active or restless during the past century (Baker, South Sister, and Lassen). Experience gained during the Cascade Range hydrothermal‐monitoring experiment informs ongoing efforts to capture entire unrest cycles at more active but generally less accessible volcanoes such as those in the Aleutian arc. In 2009, the USGS began a systematic hydrothermal‐monitoring effort encompassing 25 sites and 10 of the highest‐risk volcanoes in the Cascade Range. Targets included summit fumarole groups and springs/streams that show clear evidence of magmatic influence (high 3 He/ 4 He ratios and/or anomalous fluxes of magmatic CO 2 or heat). The resulting hourly, multiyear records are suitable for retrospective comparison with continuous geophysical monitoring data, and inform ongoing efforts to capture unrest cycles at more active but less accessible volcanoes.
Wells located in the C olombian A ndean foreland often produce biodegraded hydrocarbons and relatively fresh water (total dissolved solids concentrations of 2000 mg l −1 dominate across most of the basin). The ratio of water to hydrocarbon in the produced fluids is high and has often been interpreted by explorationists and simulation engineers as being due to massive meteoric water invasion. To challenge this hypothesis, previously published and existing data are compiled here, along with 68 new water samples collected from both the surface (rivers) and subsurface (production wells), which are analyzed for major elements, salinity, and stable isotope compositions. The data indicate a mixed origin for the water, which involves formation water, meteoric water, and a third source that we interpret as being related to diagenetic processes (such as clay dehydration). Analyses of the variation of the parameters from north to south and west to east allow us to define the mixtures as coming from waters of different origins and make it possible to determine the relative contribution of each source. Geochemical characterization of formation waters in the Foreland Llanos Basin of Colombia shows a complex mixture of low‐salinity fluids. Shallow reservoirs reflex a variable influence of bicarbonate‐dominated meteoric waters associated with a topography‐driven recharge. Formation waters with depth show a transitional increase of δ 18 O associated with diagenetic reactions of the shale‐dominated column and probable fluids leakage from the foothills in the basin boundary.
This study assesses the displacement of coalbed methane by CO 2 migration along a fault into the coal seam in the Yaojie coalfield. Coal and gas samples were collected continuously at various distances in NO .2 coal seam from F19 fault. Vitrinite reflectance, maceral, and pore distributions and proximate analysis of fourteen coal samples were performed. Gas components, concentrations, carbon isotopes of 28 gas samples were determined. We examined the coal–gas trace characteristics of coalbed methane displaced away from the fault by CO 2 injection after geological ages. From east to west, away from the F19 fault, the CO 2 concentration decreased, whereas the CH 4 concentration increased gradually. The δ 13 C values for CO 2 varied between −9.94‰ and 1.12‰, suggesting a metamorphic origin. A wider range of δ 13 C CO 2 values (from −9.94‰ to 20‰) was associated with the mixing of microbial carbon dioxide, isotopic fractionation during CO 2 migration through the microporous structures of coals, and/or carbon isotope fractionation during gas–water exchange and dissolution of CO 2 . Away from the F19 fault, the volumes of micropores, mesopores and macropores decrease gradually. The Dubinin–Radushkevich ( DR ) micropore volume decreased from 0.0059 to 0.0037 cm 3 g ‐1 , and the mesopore and macropore volumes decreased from 0.066 to 0.026 cm 3 g ‐1 . The CO 2 injection can mobilize aromatic hydrocarbons and mineral matter from coal matrix, resulting in the decrease in the absorption peak intensity for coal samples after supercritical CO 2 treatment, which indicates that chemical reactions occur between coal and CO 2 , not only physical adsorption. Natural CO 2 injects the coal then security sequestration over long geological periods. In the migration, the pore volumes of micropores, mecropores, macropores, and the concentration of CO 2 decrease gradually due to the interaction between CO 2 and coal.
Large‐scale conical and saucer‐shaped sand injectites have been identified in the Upper Miocene sediments of the L ower C ongo B asin. These structures are evidenced on the 3 D high‐resolution seismic data at about 600 ms TWT (two‐way traveltime) beneath the seabed. The conical and saucer‐shaped anomalies range from 20 to 80 m in height, 50 to 300 m in diameter, and 10 to 20 ms TWT in thickness. They are located within a sedimentary interval of about 100 m in thickness and are aligned over 20 km in dip direction ( NE ‐ SW ), above the NW margin of an underlying U pper M iocene submarine fan. We have interpreted the conical and saucer‐shaped anomalies as upward‐emplaced sand injectites sourced from the U pper M iocene fan because of their discordant character, the postsedimentary uplifting of the sediments overlying the cones and saucer‐shaped bodies, the alignment with the lateral fringe of the U pper M iocene submarine fan, and the geological context. Sand injection dates from the M iocene– P liocene transition (approximately 5.3 Ma). The prerequisite overpressure to the sand injection process may be due to the buoyancy effect of hydrocarbons accumulated in the margins of the fan. Additionally, overpressure could have been enhanced by the lateral transfer of fluids operating in the inclined margins of the lobe. The short duration of sand injection and the presence of many sandstone intrusions suggested that the process of injection was triggered by an event, likely due to a nearby fault displacement related to diapiric movements. This is the first time that sand injectites of seismic scale have been described from the L ower C ongo B asin. The localized nature of these injectites has led to a change in the migration path of fluids through the sedimentary cover. Consequently, the sand intrusions are both evidence and vectors of fluid migration within the basin fill. Large‐scale conical and saucer‐shaped sandstone intrusions have been identified in the Upper Miocene sediments of the Lower Congo Basin. They are ranged from 20 to 80 m in height, 50 to 300 m in diameter, and 20 to 40 m in thickness. They are aligned over 20 km in dip direction, above the NW fringe of an underlying Upper Miocene submarine lobe, which is interpreted as the parent sand. Sand injection dates from the Miocene‐Pliocene boundary.