Hydraulic Properties Of Fault Zone Research Articles

Relationship between Earthquake-Induced Hydrologic Changes and Faults

Hydraulic properties of fault zones are important to understanding the pore pressure development and fault stability. In this work, we examined the relationship between water level changes caused by the 2008 Wenchuan Mw 7.9 earthquake and faults using four wells with the same lithology around the Three Gorges Dam, China. Two of the wells penetrating the fault damage zones recorded sustained water level changes, while the other two wells that are not penetrating any fault damage zones recorded transient water level changes. The phase shift and tidal factor calculated from water level, a proxy of permeability and storage coefficient, revealed that both the permeability and storage coefficient changed in the two wells penetrating the fault damage zones, while the other two wells not penetrating the fault damage zone did not show any change in permeability and storage coefficient. Thus, we tentatively suggest that faults may play an important controlling role on earthquake-induced hydrologic changes because the detrital or clogging components in the fractures may be more easily removed by seismic waves.

Exploring the shallow geothermal resources in the Chilean Southern Volcanic Zone: Insight from the Liquiñe thermal springs

Southern Volcanic Zone (SVZ) hosts numerous thermal springs (25–85 °C), related to Liquiñe-Ofqui Fault System (LOFS) and Andean Transverse Faults (ATFs), suggesting the feasibility of low- to high-enthalpy geothermal exploitation. However, the local understanding of processes and factors controlling the high-temperature groundwater circulation remained undefined, which limits its exploitation. Following a brief review of SVZ, we address the previous issue by a hydrogeochemical and isotopic analysis of 15 thermal springs at Liquiñe area (representative of SVZ), supplemented by geochemical simulations. We demonstrate that fault zones and derived hydraulic properties have a predominant control on the ascending high-temperature groundwater circulation and hydrogeochemical processes. Spring discharges and outflowing temperatures are higher along the LOFS damage-zone and ATF than those along the LOFS fault core-zone. Albite dissolution is the main water-rock interaction that is enhanced by absorbed geothermal gases (H2S and CO2 in LOFS; CO2 in ATF). Trace element contents (Li, Rb, Cs, As, Mo) are influenced by hydraulic properties and geothermal gases within faults. Intrinsic hydraulic properties of fault zones, where damage-zone is more permeable than the core zone, affect the Li, Rb, Cs contents. While As and Mo solely differ according to the gases presence. Water stable isotopes point that steam heating processes enhance the hydrogeochemical reactions through the LOFS damage-zone and ATF, whereas only low-temperature processes occur through the LOFS core-zone. LOFS damage-zone and ATFs are great targets for the shallow geothermal resource exploitation. But simulations indicate an elevated risk of scaling with consequences on the operation plants due to silicate mineral precipitations. Finally, Liquiñe thermal springs provide interesting insights to respond to the current and future energy challenges in Central-South Chile.

Evolution of the hydraulic properties of deep fault zone under high water pressure

Repeated water injection tests with varied injection flow rates are conducted on a fault zone under the roadway floor to study the evolution of the hydraulic properties of fault zone under high water pressure. Based on the analysis of test results, the evolution process of the hydraulic properties of fault zone under high water pressure can be divided into three successive stages: the initial infiltration stage, the splitting stage, and the scouring infiltration stage. It is found that in the splitting stage and the scouring infiltration stage, the hydraulic conductivity of fault zone increases rapidly under the condition of sufficient water supply, and this is likely to evolve into a large-flow-rate water inrush accident. Therefore, the safety factor e of fault zone should be defined as the ratio of the splitting pressure of fault zone Pf over the aquifer pressure Ph, i.e., e = Pf /Ph; when e < 1, water inrush may occur in the fault. Based on the results in this study, a new method is proposed for assessing the risk of fault.

The in situ hydraulic properties of fault zones in mine roadway floor

In this study, we explored the hydraulic properties of the fault zone at different positions of roadway floor using in situ high-pressure water injection tests under controlled water injection. During the test, injecting water flow rate and pressure as well as water pressure in the monitor borehole were continuously measured and recorded. The test results showed that changes of injecting water pressure in the fault zone obey the changing characteristics of the typical hydraulic fracturing pressure. Analysis of the test results showed that the hydraulic process could be divided into original and post-fracture stages at the breakdown pressure. In the original stage, the flow dimension (v) continuously increased. In the post-fracture stage, the equivalent hydraulic fracture aperture (b) also continuously increased even when the flow rate decreased. The fracturing-induced fractures were scoured during water injection. In addition, the original hydraulic conductivity (K) of the fault zone decreased while the hydraulic fracturing breakdown pressure (P0) increased with the distance (D) of the test point to the roadway floor increasing. The main cause for these phenomena was the difference in the stress states of the fault zone at different locations of the roadway floor.

Experimental investigation of the variations in hydraulic properties of a fault zone in Western Shandong, China

Coal remains the largest contributor to China’s energy structure. However, coal production has frequently been threatened by groundwater inrush incidents induced by fault zones. The variations in the hydraulic properties of fault zones under high water pressure in relation to groundwater inrush risk are poorly understood. This paper presents high-pressure fluid injection tests used to investigate the variations in hydraulic properties of a fault zone at depth. The analysis of experimental results shows that the injection rate and hydraulic conductivity increase as nonlinear functions of injection pressure. The evolution process induced by high water pressure can be divided into three flow phases, including a Darcy flow phase, a flow transition phase and an unstable flow phase. Two thresholds of injection pressure for the onset of nonlinear flow and the onset of substantial fracture dilation and hydraulic fracturing were identified. The hydraulic conductivity of the fault zone is closely related to the water pressure, with an evident increase in permeability resulting from fracture extension and enhancement of fracture connectivity, which were attributed to water injection. Finally, a conceptual model was developed to demonstrate the textural and hydraulic evolution of the fault zone.

Deformation mechanisms and petrophysical properties of chert and limestone fault rocks within slope-to-basin succession (Gargano Promontory, Southern Italy)

In this work, we examine faults that crosscut limestone and chert rocks pertaining to a slope-to-basin succession of the eastern Gargano Promontory (southern Italy). Based on field data, microstructural observations, and quantitative analysis of cataclastic fabric, two stages of faulting are recognized. The first one, the pre-lithification faulting stage, took place within partially lithified sediments prior to their complete lithification. Differently, the second one, the post-lithification faulting stage, occurred within cohesive, well-lithified rocks. The structural properties of pre-lithification faults were likely controlled by the competence contrast between limestone and chert sediments. In fact, due to their different lithification stages, faulting occurred when chert was still not completely lithified, and hence was dragged along the fault planes. As a consequence, the pre-lithification fault cores are mainly composed of chert clasts. On the contrary, post-lithification fault cores are mostly made up of limestone clasts. The results of both microstructural and image analyses show that the carbonate fault rock includes a higher percentage of bigger clasts with lower values of angularity than the chert fault rock. Mercury-intrusion porosimetry indicates that the chert fault rock is characterized by larger pore throats and a lower amount of total porosity with respect to the limestone fault rock. The permeability values obtained for the limestone fault rock are lower than those for the chert fault rock, probably because of the lower amount of pore connectivity within the former fault rock. Results of this multidisciplinary work highlight the role played by cherty layers present within well-layered, slope-to-basin carbonate successions on both microtextural and petrophysical fault rock properties. Furthermore, these results increase our ability to predict how lithological heterogeneities and amount of lithification influence the deformation mechanisms, hence the hydraulic properties of fault zones that crosscut sedimentary successions.

Hydraulic properties of fault zones in porous carbonates, examples from central and southern Italy

We present the results of in situ permeability measurements performed, using a portable field permeameter, on normal and strikeslip fault zones that crosscut high-porosity carbonate grainstones. The measurement sites expose in the Cretaceous Orfento Formation of the Majella Mountain (Abruzzo, Italy), and the Lower Pleistocene deposits of the Favignana Island (Sicily, Italy). Nine small-displacement, compactive shear banding-based fault zones have been tested in the field. The fault offset ranges between 10 and 200 centimeters. The acquired permeability data indicate a two orders of magnitude decrease of porosity and permeability from the host rock to the cataclastic fault cores. A clear dependence of the fluid circulation paths through porous carbonates is therefore inferred at depth due to orientation, density and connectivity of the fault zones. Moreover, this study indicates the key role played by the pore network characteristics (pore dimensions above all) of undeformed host rocks on determining extremely different permeability values of the faulted porous carbonate grainstones. Accordingly, the results presented in this study may be helpful in applications such as geofluids management for improving the forecasting of carbonate reservoir quality and understanding the extent of reservoir compartmentalization.

Fluid flow in fault zones from an active rift

The geometry and hydraulic properties of fault zones are investigated for Mesozoic greywacke basement and Miocene sandstone from ∼37 km of tunnels in the southern Taupo Rift, New Zealand. Localised groundwater inflows occur almost exclusively (≥∼90%) within, and immediately adjacent to, fault zones. Fault zones in the contrasting lithologies comprise fault rock, small-scale faults, and fractures with thicknesses of 0.01–∼110 m approximating power law distributions and bulk permeabilities of 10−9−10−12 m2. Variability in fault zone structure results in a highly heterogeneous distribution of flow rates and locations. Within basement ∼80% of the flow rate occurs from fault zones ≥10 m wide, with ∼30% of the total localised flow rate originating from a single fault zone (i.e. consistent with the golden fracture concept). No simple relationships are found between flow rates and either fault strike or hydraulic head, with ≤50% of fault zones in any given orientation flowing. A general positive relationship does however exist between fault zone thickness and maximum flow rate. Higher flow rates from larger fault zones may arise because these structures have greater dimensions and are more likely (than smaller faults) to be connected to other faults in the system and the ground surface.

Fault compaction and overpressured faults: results from a 3-D model of a ductile fault zone

SUMMARY A model of a ductile fault zone is incorporated into a forward 3-D earthquake model to better constrain fault-zone hydraulics. The conceptual framework of the model fault zone was chosen such that two distinct parts are recognized. The fault core, characterized by a relatively low permeability, is composed of a coseismic fault surface embedded in a visco-elastic volume that can creep and compact. The fault core is surrounded by, and mostly sealed from, a high permeability damaged zone. The model fault properties correspond explicitly to those of the coseismic fault core. Porosity and pore pressure evolve to account for the viscous compaction of the fault core, while stresses evolve in response to the applied tectonic loading and to shear creep of the fault itself. A small diffusive leakage is allowed in and out of the fault zone. Coseismically, porosity is created to account for frictional dilatancy. We show in the case of a 3-D fault model with no in-plane flow and constant fluid compressibility, pore pressures do not drop to hydrostatic levels after a seismic rupture, leading to an overpressured weak fault. Since pore pressure plays a key role in the fault behaviour, we investigate coseismic hydraulic property changes. In the full 3-D model, pore pressures vary instantaneously by the poroelastic effect during the propagation of the rupture. Once the stress state stabilizes, pore pressures are incrementally redistributed in the failed patch. We show that the significant effect of pressure-dependent fluid compressibility in the no in-plane flow case becomes a secondary effect when the other spatial dimensions are considered because in-plane flow with a near-lithostatically pressured neighbourhood equilibrates at a pressure much higher than hydrostatic levels, forming persistent high-pressure fluid compartments. If the observed faults are not all overpressured and weak, other mechanisms, not included in this model, must be at work in nature, which need to be investigated. Significant leakage perpendicular to the fault strike (in the case of a young fault), or cracks hydraulically linking the fault core to the damaged zone (for a mature fault) are probable mechanisms for keeping the faults strong and might play a significant role in modulating fault pore pressures. Therefore, fault-normal hydraulic properties of fault zones should be a future focus of field and numerical experiments.

Deformation mechanisms and hydraulic properties of fault zones in unconsolidated sediments; the Roer Valley Rift System, The Netherlands

In general, faults cutting through the unconsolidated sediments of the Roer Valley Rift System (RVRS), The Netherlands, form strong barriers to horizontal groundwater flow. The relationships between deformation mechanisms along fault zones and their impact on the hydrogeological structure of the fault zone are analyzed in a shallow (0–5 m below land surface) trench over one of the faults in the study area. Recently developed digital-image-analysis techniques are used to estimate the spatial distribution of hydraulic conductivity at the millimeter-scale and to describe the micromorphologic characteristics of the fault zone. In addition, laboratory measurements of hydraulic conductivity on core-plug samples show the larger-scale distribution of hydraulic conductivity in the damage zone flanking the main fault plane. Particulate flow is the deformation mechanism at shallow depths, which causes the damage zone around the fault, in the sand-rich parts, to have a relatively enhanced hydraulic conductivity. The fault core is characterized by reduced hydraulic conductivity due to clay smearing, grain-scale mixing, and iron-oxide precipitation.

Clay gouge

Clays are a common component of fault gouge, but their genesis and importance in fault evolution is poorly understood. We present preliminary evidence that clays participate in extensive mineral reactions and microfabric changes during faulting. Rather than thinking of clay reactions as a consequence of mechanical processes or fault localization following diagenetically altered horizons, we see the interplay between clay mineral reactions and mechanical processes as a single, integrated process. Furthermore, faulting may lower kinetic barriers to low-temperature (∼100°C) mineral reactions that are common in sedimentary rocks. Our most striking example of fault diagenesis-deformation is a profile of %illite in mixed-layer illite/smectite in shales beneath the Lewis Thrust, Canada. Whereas burial diagenesis caused minimal smectite-to-illite reaction, shales within meters of the thrust are almost completely converted to illite. The consequences of these changes are manifested in geochemical, geochronologic and microfabric characteristics of clay gouge. In this example, faulting has helped overcome a kinetic barrier in the smectite-to-illite reaction without appreciable addition of heat. In another example we infer that dissolution–precipitation reactions continue during faulting even when smectite has already been completely transformed to illite. If mineral reactions intimately interact with mechanical processes in shallow-crustal faults, then our current understanding of the mechanical and hydraulic properties of fault zones may be incomplete. Syndeformational mineral reactions and associated fabric changes could make faults much weaker than would be expected from evaluation of the static mineral assemblage of gouge and single crystal properties. Syndeformational mineral reactions may promote fault slip (affecting earthquake activity) in gouge-bearing faults under stress conditions considerably lower than predicted from static mineral properties. In addition, fault-induced dissolution-precipitation reactions may contribute to fault localization.