Faults In Sandstones Research Articles

The effect of temperature on injection-induced shear slip of laboratory faults in sandstone

Fluid injection into subsurface reservoirs may cause existing faults/fractures to slip seismically. To study the effect of temperature on injection-induced fault slip, at a constant confining pressure of 10 MPa, we performed a series of injection-induced shear slip experiments on critically stressed sandstone samples containing saw-cut fractures (laboratory-simulated faults) under varying fluid pressurization rates (0.1 and 0.5 MPa/min, respectively) and temperatures (25, 80, and 140 °C, respectively). At 25 °C, slow fault slip events with a peak slip velocity of about 0.13 μm/s were observed on a tested sample in response to a low fluid pressurization rate of 0.1 MPa/min. In contrast, fluid injection with a high pressurization rate of 0.5 MPa/min caused fault slip events with a peak slip rate up to about 0.38 μm/s. In response to a given fluid pressurization rate, several episodes of slip events with a higher slip velocity were induced at an elevated temperature of 140 °C, indicating an appreciable weakening effect at elevated temperatures. We also experimentally constrained the rate-and-state frictional (RSF) parameters at varying effective normal stresses and temperatures by performing velocity-stepping tests. The obtained RSF parameters demonstrate that for a relatively high normal stress, increasing temperature tends to destabilize fault slip. Post-mortem microstructural observations reveal that elevated temperatures promote the generation of abundant fine-grained gouge particles associated with injection-induced shear slip. Our experiments highlight that injection-induced fault slip is affected by temperature-related wear production over the fault surface.

Characterization of two-phase flow in a faulted sandstone

Fault zones present in some underground CO2 storage formations can impact on injectivity, storage capacity, and the path taken by migrating CO2, including possible leakage to overlying formations and the escape of a plume beyond the area of a demarcated storage lease licence. A thorough understanding of the 2-phase flow in fault zones is therefore essential to de-risk any storage location where faults could be present. Many previous studies have addressed top-seal risks from fault reactivation, but the influence of intra-reservoir faults on flow within the target storage formations has received less attention. Studies of intraformational faults in sandstones have typically used plugs cored from outcrops or from subsurface boreholes, where important aspects of the geological history such as the stress conditions during faulting, and the degree of post-faulting compaction and cementation may be poorly constrained. Moreover, only intact fault rocks can be cored and re-loaded under stress with this approach, so that it is very difficult to establish the interacting effects or rock fractures and cataclastic gouge in a single experiment using borehole/outcrop samples.This study investigates newly created fault rock in sandstone that has been deformed under controlled conditions and without the effects of secondary cementation. We used a large direct shear rig to deform a large, water-saturated Bentheimer sandstone block to sufficient displacement (40mm) and under enough overburden pressure (25 MPa) to create a zone of cataclastic gouge several mm thick bounded by a damage zone of fractured rock. The sheared block was cored at an inclined angle while deep frozen to extract an oriented plug containing a complete and intact section of fault rock that could be reloaded under hydrostatic conditions in an x-ray transparent core holder. The faulted plug of Bentheimer was used for running steady-state drainage and imbibition 2-phase flow experiments. The fluid saturation and its spatial distribution in 3D inside the faulted plug were dynamically monitored during all stages of the experiment in a medical x-ray CT scanner.Analysis of the flow patterns revealed from the X-ray CT images collected over time enables us to characterize in detail, the influence of recently formed fault core and damage zones on fluid flow both during drainage as gas breaks across the fault zone, and as water re-invades during imbibition. The fault core acts as a semi-permeable membrane that is only permeable to the wetting phase (water) under pressure differentials less than the capillary entry pressure (Pce) of the membrane. During drainage stages, the high Pce of the fault core led to backfilling of the upstream end of the core and almost to compartmentalization of the sample. The fracture network that exists in the damage zone played a critical role in keeping the upstream side of the fault core connected to the downstream side. During imbibition stages, the high Pce of the fault core led to significant gas trapping at the upstream side of the fault core adjacent to the fault core-damage zone interface. While the fault core formed a barrier to flow, the fracture network can provide bypass pathways for gas flow. In this particular sample bypassing through fractures was sufficient such that for a particular value of average gas saturation the flow rate was faster in the faulted rock than in the unfaulted reservoir rock.

The influence of CO2-transformed iron oxide grain coatings on the frictional stability and transport properties of simulated faults in sandstones

Carbon sequestration involves long-term containment of CO2 in ideally sealed reservoirs. However, CO2 migration can weaken rocks and faults by geochemical alteration, elevate risks of seismic hazards, and loss of inventory. Recent studies show that CO2 bleaching can alter the iron oxide grain coating of sand-sized quartz in sandstones, which may impose a significant influence on frictional stability and permeability evolution of faults in sandstones. This study investigates the influence of iron oxide grain coatings via coupled shear-flow experiments on uncoated, hematite-coated, and CO2-transformed goethite-coated synthetic sand gouge. Shear strength, frictional stability, healing/relaxation, and shear-parallel permeability are measured in velocity-stepping and slide-hold-slide loading modes. Hematite-coated sand exhibits the highest shear strength, followed by goethite-coated and uncoated sand. All samples, both coated and uncoated, show similar residual shear strength. Frictional stability measurements suggest hematite-coated sand may undergo potential seismic slip (negative (a − b) values); goethite-coated sand is aseismic (positive (a − b) values) but features higher frictional healing and relaxation. Shear-parallel permeability enhances during initial shear in all samples, followed by a sharp decline after the peak strength, except for goethite-coated sand, for which permeability reduction is moderate. SEM characterizations pre- and post-shear suggest that the competitive liberation, transport, and clogging of coating particles and shear-produced wear products can be an important mechanism in permeability evolution.

Polygonal deformation bands

We report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are shear deformation bands that developed under shallow burial conditions via strain hardening in dm-wide zones. The edges of the polygons are 1–5 m long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain <0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes.

Comparison of scaling relationships of extensional fault cores in tight carbonate and porous sandstone reservoirs

This paper presents a comparative analysis of the thicknesses–displacement relationships of fault cores in non-/low-porous carbonate and porous sandstone reservoirs. Fault thickness data were collected from extensional faults in mainly Cretaceous continental sandstones and Late Cretaceous–Paleogene marine carbonates exposed along the eastern flank of the Suez Rift, Egypt. The dataset consists of 730 thickness measurements, of which 313 are from 68 faults in carbonate rocks, and 417 are from 120 deformation band/microfaults and faults in sandstones. These data show that the increase in fault core thickness with displacement for the two lithologies is, overall, similar in log–log space, consistent with a power-law trend with an exponent of 0.5. However, for smaller faults, cores in carbonates are generally thicker than those in sandstones. The observation of nearly equal core thickness–displacement relationships for larger faults (&gt;10 m displacement) suggests that increased displacement reduces the geomechanical influence of lithology. In a statistical analysis of bins of fault displacement, small displacement faults seem to follow a skewed distribution towards smaller fault core thicknesses, moderate displacement faults have a normal distribution, while large displacement faults have a distribution skewed towards large core thicknesses. Each thickness distribution can be estimated with a mean and standard deviation. Fault core thickness variations are caused by interplaying factors such as fault geometric development, spatial and temporal positioning of shear and volumetric strains, and rheological changes. The variability in core thickness and intrinsic architecture of fault cores for any given location will cause significant variation in across-fault fluid flow, even for the same juxtaposition style.

Laboratory simulation of fluid-driven seismic sequences in shallow crustal conditions

[1] We report new laboratory simulations of fluid-induced seismicity on pre-existing faults in sandstone. By introducing pore pressure oscillations, faults were activated or reactivated to generate seismic sequences. These sequences were analysed using a slip-forecast model. Furthermore, field data from the Monticello reservoir was used to verify the model. Our results suggest that short-term forecasting is reliant upon the final stages when crack communication begins, limiting reservoir-induced seismicity (RIS) forecasting strategies to short periods. In addition, our laboratory data confirms the general accuracy and robustness of short-term forecast techniques dealing with natural crack-linkage processes, whether strain driven or fluid driven, ranging from volcanic hazard mitigation to episodic tremors and slips. Finally, oscillating pore pressure can prolong the period of fluid-induced seismicity, and the aftershock decay rate is slower than that without oscillations.

On the patterns of wing cracks along an outcrop scale flaw: A numerical modeling approach using complementarity

The 2D Displacement Discontinuity Method (DDM) is combined with a complementarity algorithm to model the quasi‐static formation and patterns of wing cracks that emanate from regions of stress concentration along a sliding frictional flaw in an otherwise homogeneous and isotropic elastic material. Because stress states and geometry change with sliding on the flaw and wing crack propagation, one cannot specify the boundary conditions a priori. Under these circumstances complementarity is superior to other well‐known contact algorithms. We focus on meter scale phenomena where mineralogical heterogeneity (common to centimeter‐scale laboratory samples) and 3D geometry (common to kilometer‐scale crustal structures) reasonably can be ignored. Analytical solutions to the elastic boundary value problem of the closed sliding flaw include those that assume no friction, uniform friction, and a cohesive end zone (CEZ), and those that assume infinitesimal or straight wing cracks. Here we generalize the problem to consider linearly varying friction in the CEZ and curved wing cracks, and we allow the sliding flaw to open when mechanical interaction with the wing crack dictates that it should. Trace lengths of 135 strike‐slip faults in sandstone are linearly related to wing crack lengths ranging from 0.16 to 72 m and correspond to a range of remote principal stress ratios: 0.06 ≤σ2/σ1≤ 0.2. Opening displacement profiles of wing cracks from the numerical model can be significantly different from analytical solutions. These solutions may produce significant errors in stable crack length for curved propagation paths. The smeared out stress concentration in the CEZ and the heterogeneity in strength of rock suggest that multiple wing cracks may form in one slip event. The mechanical interactions of these cracks leads to kink angles that increase with distance from the flaw tip, a relationship commonly observed in nature.

Aridic crusts and vein mineralisations in the playa Areg el Makrzene, South Tunisia

The playa Areg el Makrzene is situated in the north of the escarpment of the Djebel Rehach in southern Tunisia. It is characterised by more than 70 spring mounds which are bound to faults in the Triassic sandstone, and by (sub-) recent calcareous and iron hydroxide precipitations. The artesian spring water contains high amounts of Na +, Ca 2+, Mg 2+ and Cl −, SO 4 2−, and HCO 3, shows slightly alkaline character and a temperature of 25 °C. The spring mounds consist of porous travertine (calcite), goethite, gypsum and of Na–Mg evaporites and are up to 10 m high. Few metres away from the springs the amorphous iron hydroxides have altered to badly ordered goethite and ferrihydrite and finally to well ordered goethite. The muddy precipitate has altered to hard crusts (calcrete, ferricrete). The source of the precipitated material is discussed. The faults in the sandstone are filled with (hydr-)oxides of iron and manganese, e.g. with goethite, hollandite and pyrolusite. The amount of Mn-oxides may reach 60 mass%.

Structural evolution of fault zones in sandstone by multiple deformation mechanisms: Moab fault, southeast Utah

Faults in sandstone are frequently composed of two classes of structures: (1) deformation bands and (2) joints and sheared joints. Whereas the former structures are associated with cataclastic deformation, the latter ones represent brittle fracturing, fragmentation, and brecciation. We investigated the distribution of these structures, their formation, and the underlying mechanical controls for their occurrence along the Moab normal fault in southeastern Utah through the use of structural mapping and numerical elastic boundary element modeling. We found that deformation bands occur everywhere along the fault, but with increased density in contractional relays. Joints and sheared joints only occur at intersections and extensional relays. In all locations, joints consistently overprint deformation bands. Localization of joints and sheared joints in extensional relays suggests that their distribution is controlled by local variations in stress state that are due to mechanical interaction between the fault segments. This interpretation is consistent with elastic boundary element models that predict a local reduction in mean stress and least compressive principal stress at intersections and extensional relays. The transition from deformation band to joint formation along these sections of the fault system likely resulted from the combined effects of changes in remote tectonic loading, burial depth, fluid pressure, and rock properties. In the case of the Moab fault, we conclude that the structural heterogeneity in the fault zone is systematically related to the geometric evolution of the fault, the local state of stress associated with fault slip, and the remote loading history. Because the type and distribution of structures affect fault permeability and strength, our results predict systematic variations in these parameters with fault evolution.

Overprinting faulting mechanisms in high porosity sandstones of SE Utah

Normal faults in sandstone of the Jurassic Entrada Formation, South-East Utah formed by two mechanisms: (1) deformation band faulting overprinted by (2) jointing and subsequent shearing along joints. Fundamental structural elements of deformation band faults are single deformation bands, zones of deformation bands, and slip surfaces. Joint-based faults are composed of joints, sheared joints, splay fractures, fragmentation zones, breccia, and fine-grained fault rock. We demonstrate that both mechanisms contribute to slip in a single fault zone with joint-based faulting consistently postdating deformation band faulting at a given location along a fault. The occurrence, distribution, and geometric arrangement of structures formed by the two mechanisms resulted in faults with distinct fault architecture. This fault architecture is related to the relative contributions of each deformation mechanism to the total offset and to their relative timing. Overprinting of a deformation band-based fault by a joint-based mechanism introduces extensive localized structural heterogeneity with a distinct hydraulic signature. Whereas deformation bands tend to act as fluid baffles, joints may act as preferred fluid conduits. Therefore, fluid flow properties such as the permeability of the faults with overlapping mechanisms are expected to change over time accompanying the overprinting process.

Fluid-flow properties of faults in sandstone: The importance of temperature history

Research Article| November 01, 2003 Fluid-flow properties of faults in sandstone: The importance of temperature history Quentin J. Fisher; Quentin J. Fisher 1Rock Deformation Research Group, School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Martin Casey; Martin Casey 1Rock Deformation Research Group, School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Simon D. Harris; Simon D. Harris 1Rock Deformation Research Group, School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Robert J. Knipe Robert J. Knipe 1Rock Deformation Research Group, School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Geology (2003) 31 (11): 965–968. https://doi.org/10.1130/G19823.1 Article history received: 13 May 2003 rev-recd: 04 Aug 2003 accepted: 06 Aug 2003 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Quentin J. Fisher, Martin Casey, Simon D. Harris, Robert J. Knipe; Fluid-flow properties of faults in sandstone: The importance of temperature history. Geology 2003;; 31 (11): 965–968. doi: https://doi.org/10.1130/G19823.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Sandstone rheology and deformation style are often controlled by the extent of quartz cementation, which is a function of temperature history. Coupling findings from deformation experiments with a model for quartz cementation provide valuable insights into the controls on fault permeability. Subsiding sedimentary basins often have a transitional depth zone, here referred to as the ductile-to-brittle transition, above which faults do not affect fluid flow or form barriers and below which faults will tend to form conduits. The depth of this transition is partly dependent upon geothermal gradient. In basins with a high geothermal gradient, fault-related conduits can form at shallow depths in high-porosity sandstone. If geothermal gradients are low, and fluid pressures are hydrostatic, fault-related conduits are only formed when the sandstones have subsided much deeper, where their porosity (and hence fluid content) is low. Mineralization of faults is more likely to occur in areas with high geothermal gradients because the rocks still have a high fluid content when fault-related fluid-flow conduits form. The interrelationship between rock rheology and stress conditions is sometimes a more important control on fault permeability than whether the fault is active or inactive. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Characteristics of Faults in Sandstone and Their Effect on Fluid Flow: ABSTRACT

Characteristics of Faults in Sandstone and Their Effect on Fluid Flow: ABSTRACT

Petrophysical study of faults in sandstone using petrographic image analysis and X-ray computerized tomography

Petrographic image analysis (PIA) and X-ray computerized tomography (CT) provide local determinations of porosity in sandstone. We have investigated small faults called deformation bands in porous sandstones using these techniques. Because the petrophysical properties of the fault rock vary at a small scale (mm scale), the ability of PIA and CT to determine porosity in small volumes of rock and to map porosity distribution in two and three dimensions is crucial. This information is used to recognize the processes involved in fault development and the different kinds of microstructures associated with dilatancy and compaction. The petrophysical study of fault rock in sandstone permits one to make predictions of the hydraulic properties of a fault and thereby evaluate the sealing or fluid transmitting characteristics of faulted reservoirs and aquifers. The results of this study indicate that faulting in sandstone alters the original porosity and permeability of the host rock: the porosity is reduced by an order of magnitude and the permeability is reduced by one to more than seven orders of magnitude for faults associated with compaction.

Small Faults as Barriers to Permeability in Sandstone Gas Reservoirs, Pattani Basin, Thailand: ABSTRACT

A set of small conjugate faults in a sandstone conventional core from a Pattani basin, Thailand, gas well has reduced the horizontal permeability of the rock by about 45%. The faults recovered in core have displacements of several millimeters to several centimeters, and the associated deformation zones are 1.2-9 mm wide. Fault traces consist of zones of disturbed sandstone that contain fragmented sand grains, collapsed pores, and abundant admixed detrital clay, silt, carbonaceous debris, and micas. Sandstone within the disturbed zone is finer grained, more poorly sorted, and contains smaller pores than sandstone outside the fault zones. Sedimentary textures indicate that comminution and rotation of quartz and feldspar grains have taken place along the small faults. Locally, authigenic kaolinite fills fractures within deformation zones. Similar faults of somewhat larger displacement may act as barriers to permeability within Pattani basin gas reservoirs, a contention supported by the rapid pressure decline and early depletion of some reservoirs. Permeability impairment has been associated with small faults in sandstones from other areas, most notably those of the Simpson Group and the New Red sandstone. In some Pattani basin gas fields, major trap-forming normal faults (those having displacements greater than 10 m) are closely spaced.more » As many as eight faults per mile are present in some areas. The spacing of small seismically unresolvable faults may be even closer in some areas, perhaps attaining tens of faults per mile. Some of these small faults may be permeability barriers, and their presence may explain the small size of some Pattani basin gas reservoirs.« less