Study of Interactions between Induced and Natural Fracture Effects on Hydraulic Fracture Propagation Using a Discrete Approach

The interaction mode of induced fracture and natural fracture plays an important role in prediction of hydraulic fracture propagation. In this paper, a two-dimensional hydromechanical coupled discrete element model is first introduced in the framework of particle flow simulation, which can well take into account mechanical and hydraulic properties of rock samples with natural fracture. The model’s parameters are strictly calibrated by conducting numerical simulations of uniaxial compression test and direct tensile and shear tests, as well as fluid flow test. The effectiveness of coupled model is also assessed by describing hydraulic fracture propagation in two representative cases, respectively, rock samples with and without preexisting fracture. With this model in hand, the effects of interaction between induced and natural fractures with different approach angles and differential stresses on fluid injection pressure and fracture propagation patterns are investigated and discussed. Results suggest that the interaction modes mainly involve three basic behaviors including the arrested, captured with offset, and directly crossing. For a given differential stress, the captured offset of hydraulic fracture by natural fracture gradually decreases with the approach angle increase, while for a fixed approach angle, that captured offset increases with differential stress decrease.

Evolution of Quartz and Calcite Microstructures Exhumed From Deep Brittle-Ductile Shear Zones in the Southern Alps of New Zealand

<p><b>Arrays of brittle-ductile shears exposed in the Southern Alps of New Zealand, haveprovided a superb natural laboratory for insight into the microstructural evolution of lowercrustal shear zones during exhumation. Shears are exposed in the central section of theSouthern Alps at Sam Peak, Chancellor Ridge, and Baumann Glacier in a zone ~2 kmwide that is located 6–8 km structurally above the Alpine Fault. An array ofsystematically spaced shear zones that formed by embrittlement and faulting ofquartzofeldspathic schist took place at the same time as ductile shearing of quartzcarbonateveins embedded within the schist. This study has used field-based structuralmapping along with optical microscopy and universal stage measurements ofcrystallographic preferred orientations (CPO) to resolve the shear zone kinematics andrheology. On the basis of these data, the strain path can be reconstructed for the shearedveins during their progressive deformation. This began with their incidence as backshearsat the base of the Alpine Fault ramp and ended with their subsequent recrystallisation,uplift, and exhumation.</b></p> <p>The near-vertical shear planes have mean orientation of 221@89 NW ± 1o (n =780). They are inferred to have formed as backshears accommodating uplift of the PacificPlate as it was translated onto the oblique footwall ramp of the Alpine Fault during lateCenozoic oblique convergence. Detailed fault offset transect surveys across the shears atChancellor Ridge and Baumann Glacier reveal a mean spacing between the shear zones of25 ± 5 cm (n = 410). Quartz-carbonate marker veins are displaced in a dextral west-sideupshear sense. Fault offset geometry and a consistent arrangement of mineral fibrelineations that decorate fault surfaces, indicate that the mean displacement vector pitches35o SW in the shear plane (trend and plunge of: 262, 35 ± 7o). Ductilely deformed markerveins have been subject to a mean displacement of 9.9 ± 1.4 cm (n = 344) and a meanfinite ductile shear strain of 4.8 ± 0.3 (n = 219). A strain-rate for the ductile deformationof the veins is estimated at 3 x 10-11 sec-1 based on the observed finite ductile shear strain,an escalator kinematic model, and assumptions about the width of the deforming zone.</p> <p>Five deformation phases have affected the sheared veins during their transport upthe fault ramp: 1) initial brittle faulting and ductile shearing; 2) grain boundary sliding ofmylonitic quartz in response to a post-ramping differential stress drop; 3) recrystallisationand grain growth; 4) renewed late-stage dislocation creep; and 5) semibrittle deformationand exhumation. In the schist, the shears initiated as planar brittle faults at lower crustal depths of~21 km at a temperature of 450 ± 50oC. They developed in a zone of transiently highshear strain-rates near the base of the Alpine Fault ramp. Dislocation creep caused a CPOof quartz and calcite to develop in sheared veins. Using the flow law of Hirth et al. (2001)and the estimated strain-rate, a differential stress of ~165 MPa is inferred for ductiledeformation of the veins. Near-lithostatic (λ = 0.85) fluid pressures would have causedthe rocks to undergo brittle failure, a situation that is confirmed by a late component ofbrittle deformation that over prints the ductilely sheared veins. Syntectonic quartz-calciteveins infill the shear fractures, and these themselves have been sheared. The deformationof the veins was not a simple shear process but one with triclinic flow symmetry. This isinferred from discordance between the shear direction and the near-vertical principleextension direction that is revealed by the pattern and symmetry of quartz and calcite CPOfabrics.</p> <p>After the shears move away from the ramp-step, grain boundary sliding (GBS)accommodated by solid-state diffusion creep is inferred to have affected quartz veins.</p> <p>This deformation mechanism takes place because of 1) the small 8 μm grain size inheritedfrom Phase 1; 2) the presence of fluid in the shear zone; and 3) a stress drop to ~22 MPathat followed the initial up-ramping. Quartz CPO fabrics in the sheared veins areremarkably weak considering their large shear strains. GBS is inferred to have been achief deformation mechanism that caused the weakening of quartz CPO fabrics in thehighly sheared sections of deformed veins. Calcite has also affected the quartz fabricstrength as those veins containing >5% calcite have very weak quartz CPO fabrics. Incontrast to quartz, the CPO fabrics for the co-existing calcite remained strong andcontinued to develop by dislocation creep.</p> <p>The third phase of deformation, a process that may have contributed to subsequentweakening of quartz CPO fabrics, was recrystallisation and grain growth to 126 μm and anequigranular-polygonal grain shape fabric. This fabric was overprinted by late-stagedislocation creep microstructures in the fourth deformation phase in response increaseddifferential stress encountered by the rocks at lower temperatures in the upper crust. Thefinal phase of deformation to affect the sheared veins was semibrittle deformation atdifferential stresses of <189 MPa and temperatures of 200–280oC as the rocks passedthrough the steady-state brittle-ductile transition zone at depths of 8–10 km before beingexhumed at the surface.</p>

Evolution of Quartz and Calcite Microstructures Exhumed From Deep Brittle-Ductile Shear Zones in the Southern Alps of New Zealand

<p><b>Arrays of brittle-ductile shears exposed in the Southern Alps of New Zealand, haveprovided a superb natural laboratory for insight into the microstructural evolution of lowercrustal shear zones during exhumation. Shears are exposed in the central section of theSouthern Alps at Sam Peak, Chancellor Ridge, and Baumann Glacier in a zone ~2 kmwide that is located 6–8 km structurally above the Alpine Fault. An array ofsystematically spaced shear zones that formed by embrittlement and faulting ofquartzofeldspathic schist took place at the same time as ductile shearing of quartzcarbonateveins embedded within the schist. This study has used field-based structuralmapping along with optical microscopy and universal stage measurements ofcrystallographic preferred orientations (CPO) to resolve the shear zone kinematics andrheology. On the basis of these data, the strain path can be reconstructed for the shearedveins during their progressive deformation. This began with their incidence as backshearsat the base of the Alpine Fault ramp and ended with their subsequent recrystallisation,uplift, and exhumation.</b></p> <p>The near-vertical shear planes have mean orientation of 221@89 NW ± 1o (n =780). They are inferred to have formed as backshears accommodating uplift of the PacificPlate as it was translated onto the oblique footwall ramp of the Alpine Fault during lateCenozoic oblique convergence. Detailed fault offset transect surveys across the shears atChancellor Ridge and Baumann Glacier reveal a mean spacing between the shear zones of25 ± 5 cm (n = 410). Quartz-carbonate marker veins are displaced in a dextral west-sideupshear sense. Fault offset geometry and a consistent arrangement of mineral fibrelineations that decorate fault surfaces, indicate that the mean displacement vector pitches35o SW in the shear plane (trend and plunge of: 262, 35 ± 7o). Ductilely deformed markerveins have been subject to a mean displacement of 9.9 ± 1.4 cm (n = 344) and a meanfinite ductile shear strain of 4.8 ± 0.3 (n = 219). A strain-rate for the ductile deformationof the veins is estimated at 3 x 10-11 sec-1 based on the observed finite ductile shear strain,an escalator kinematic model, and assumptions about the width of the deforming zone.</p> <p>Five deformation phases have affected the sheared veins during their transport upthe fault ramp: 1) initial brittle faulting and ductile shearing; 2) grain boundary sliding ofmylonitic quartz in response to a post-ramping differential stress drop; 3) recrystallisationand grain growth; 4) renewed late-stage dislocation creep; and 5) semibrittle deformationand exhumation. In the schist, the shears initiated as planar brittle faults at lower crustal depths of~21 km at a temperature of 450 ± 50oC. They developed in a zone of transiently highshear strain-rates near the base of the Alpine Fault ramp. Dislocation creep caused a CPOof quartz and calcite to develop in sheared veins. Using the flow law of Hirth et al. (2001)and the estimated strain-rate, a differential stress of ~165 MPa is inferred for ductiledeformation of the veins. Near-lithostatic (λ = 0.85) fluid pressures would have causedthe rocks to undergo brittle failure, a situation that is confirmed by a late component ofbrittle deformation that over prints the ductilely sheared veins. Syntectonic quartz-calciteveins infill the shear fractures, and these themselves have been sheared. The deformationof the veins was not a simple shear process but one with triclinic flow symmetry. This isinferred from discordance between the shear direction and the near-vertical principleextension direction that is revealed by the pattern and symmetry of quartz and calcite CPOfabrics.</p> <p>After the shears move away from the ramp-step, grain boundary sliding (GBS)accommodated by solid-state diffusion creep is inferred to have affected quartz veins.</p> <p>This deformation mechanism takes place because of 1) the small 8 μm grain size inheritedfrom Phase 1; 2) the presence of fluid in the shear zone; and 3) a stress drop to ~22 MPathat followed the initial up-ramping. Quartz CPO fabrics in the sheared veins areremarkably weak considering their large shear strains. GBS is inferred to have been achief deformation mechanism that caused the weakening of quartz CPO fabrics in thehighly sheared sections of deformed veins. Calcite has also affected the quartz fabricstrength as those veins containing >5% calcite have very weak quartz CPO fabrics. Incontrast to quartz, the CPO fabrics for the co-existing calcite remained strong andcontinued to develop by dislocation creep.</p> <p>The third phase of deformation, a process that may have contributed to subsequentweakening of quartz CPO fabrics, was recrystallisation and grain growth to 126 μm and anequigranular-polygonal grain shape fabric. This fabric was overprinted by late-stagedislocation creep microstructures in the fourth deformation phase in response increaseddifferential stress encountered by the rocks at lower temperatures in the upper crust. Thefinal phase of deformation to affect the sheared veins was semibrittle deformation atdifferential stresses of <189 MPa and temperatures of 200–280oC as the rocks passedthrough the steady-state brittle-ductile transition zone at depths of 8–10 km before beingexhumed at the surface.</p>

Brittle Creep and Viscoelastic Creep in Lower Palaeozoic Shales from the Baltic Basin, Poland

The paper analyses the mechanical properties of shales from the Baltic Basin, focusing on creep strain in conditions of variable stress and elevated temperature (85 °C). Rock samples were collected from drill cores from various depths between 3600–4000 m. A series of creep tests was performed using a triaxial apparatus in simulated pressure and temperature conditions in the reservoir. The creep tests were conducted at variable levels of differential stress in variable time intervals. The laboratory experiments were performed in order to study brittle and viscoelastic creep proceeding in time in shales rich in organic matter and clay minerals. Creep compliance of shale formations rich in organic matter influences the success of hydraulic fracturing procedures, as well as migration of natural gas during exploitation. Laboratory characteristics of geomechanical properties (compressive strength, strain and elastic moduli) is crucial for planning natural gas exploitation from unconventional resources. The results indicate that the level of constant differential stress and creep time significantly influence the mechanical properties of shales. The paper presents the differences between brittle and viscoelastic strain registered during creep tests at variable stress conditions and time intervals. In viscoelastic creep tests, creep strain is over two times larger in the second stage of creep in comparison to the magnitude of strain registered in the first stage. In brittle creep tests, axial strain in the first creep stage is two times larger than in viscoelastic creep tests in the second stage. Based on the experiments, elastic parameters, i.e., Young’s modulus and Poisson’s ratio, have been determined for each of the analysed samples. In brittle creep tests, Young’s modulus is smaller than in viscoelastic creep tests. In viscoelastic creep tests Young’s modulus increases in successive stages. Whereas Poisson’s ratio is larger for samples from brittle creep tests than for samples from viscoelastic creep tests and does not change with subsequent creep stages in viscoelastic creep tests.

Lower crustal earthquake associated with highly pressurized frictional melts

Earthquakes at lower crustal depths are common during continental collision. However, the coseismic weakening mechanisms required to propagate an earthquake at high pressures are poorly understood. Transient high-pressure fluids or melts have been proposed as a viable mechanism, but verifying this requires direct in situ measurement of fluid or melt overpressure along fault planes that have hosted dynamic ruptures. Here, we report direct measurement of highly overpressurized frictional melts along a seismic fault surface. Using Raman spectroscopy, we identified high-pressure quartz inclusions sealed in dendritic garnets that grew from frictional melts formed by lower crustal earthquakes in the Bergen Arcs, Western Norway. Melt pressure was estimated to be 1.8–2.3 GPa on the basis of an elastic model for the quartz-in-garnet system. This is ~0.5 GPa higher than the pressure recorded by the surrounding pseudotachylyte matrix and wall rocks. The recorded melt pressure could not arise solely from the volume expansion of melting, and we propose that it was generated when melt pressure approached the maximum principal stress in a system subject to high differential stress. The associated palaeostress field demonstrates that a strong lower crust accommodated up to 1 GPa differential stress during the compressive stage of the Caledonian orogeny.

Stress: conceptualization, framing, and induction

Past research has often conceptualized stress from a deficit-oriented approach. This approach is unbalanced, often associating stress with negative events and outcomes. The current study examined stress from the Transactional Model, and proposed alternative ways to conceptualize stress. In addition, both psychosocial and physiological stressors were utilized to induce stress. Study aims were threefold: (1) to examine beliefs about stress and effects of framing on changes to perceptions of stress, (2) to examine the comparative effects of two different stress-induction methods, and (3) to explore the interactive effects of framing and stress-induction on subjective perceptions of stress and measures of stress reactivity. Results confirmed a deficit-orientation of stress within the sampled population. Comparative effects of both stressors highlighted differential stress responses based on task demands and appraisal. Finally, interactive effects of framing and stress-induction provided support for alternative conceptualizations of stress in adaptive coping.

Stress: conceptualization, framing, and induction

Past research has often conceptualized stress from a deficit-oriented approach. This approach is unbalanced, often associating stress with negative events and outcomes. The current study examined stress from the Transactional Model, and proposed alternative ways to conceptualize stress. In addition, both psychosocial and physiological stressors were utilized to induce stress. Study aims were threefold: (1) to examine beliefs about stress and effects of framing on changes to perceptions of stress, (2) to examine the comparative effects of two different stress-induction methods, and (3) to explore the interactive effects of framing and stress-induction on subjective perceptions of stress and measures of stress reactivity. Results confirmed a deficit-orientation of stress within the sampled population. Comparative effects of both stressors highlighted differential stress responses based on task demands and appraisal. Finally, interactive effects of framing and stress-induction provided support for alternative conceptualizations of stress in adaptive coping.