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Mineralogical and Textural Controls on Shear Strength, Slip Stability and Permeability of Faults.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Mineralogical and Textural Controls on Shear Strength, Slip Stability and Permeability of Faults./
作者:	Wang, Chaoyi.
出版者:	Ann Arbor : ProQuest Dissertations & Theses, : 2019,
面頁冊數:	177 p.
附註:	Source: Dissertations Abstracts International, Volume: 80-12, Section: B.
Contained By:	Dissertations Abstracts International80-12B.
標題:	Geophysics. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=13917967
ISBN:	9781392319000

Mineralogical and Textural Controls on Shear Strength, Slip Stability and Permeability of Faults.
Wang, Chaoyi.

Mineralogical and Textural Controls on Shear Strength, Slip Stability and Permeability of Faults. - Ann Arbor : ProQuest Dissertations & Theses, 2019 - 177 p.

Source: Dissertations Abstracts International, Volume: 80-12, Section: B.

Thesis (Ph.D.)--The Pennsylvania State University, 2019.

Induced seismicity resulting from fluid injection into the subsurface related to water and CO2 disposal, hydraulic fracturing and the stimulation of geothermal reservoirs present an important societal concern. These human activities involve the injection of large volumes of pressurized fluid into the subsurface, potentially at high rates, raising local pore pressures and disturbing the pristine local stress regime by lowering effective normal stress on pre-existing faults and fractures. The reduction of effective normal stress may trigger fault/fracture reactivation and in some cases result in hazardous seismic ruptures. Effective management and engineering of anthropogenic seismic events requires substantial understanding in the mechanisms, especially for the controlling factors on coupled rheological and transport response, including fault shear strength, slip stability, and permeability evolution during such events. In this study, we explore the coupled rheological and transport response of faults and fractures during reactivation as controlled by two fundamental controlling properties, viz., mineralogy and textural features. We approach this problem through shear experiments on analog faults and fractures via laboratory and numerical experiments. Specifically, we investigate: (1) the influence of frictionally weak minerals (talc) in mixtures of mineral analogs featuring contrasting frictional properties, (2) the influence of iron oxide grain coatings on quartz aggregates, and (3) the influence of fracture roughness in mated fractures on the ensemble shear strength, slip stability, and permeability evolution during reactivation events. We address the following questions in this study: (1) how much and what distribution of frictionally weak minerals is required to induce significant weakening in faults consisting of a matrix of frictionally strong minerals, (2) how does a pre-imposed weak mineral layer influence the rheological and transport behavior of faults, (3) what is the influence of a trace amount of grain coating materials introduce on the coupled behavior of faults, and finally (4) how do asperity height and wave length control the ensemble behavior of faults. These questions are explicitly answered in the following.Chapter 1 explores the impact of phyllosilicate (weak but velocity-strengthening) in a majority tectosilicate (strong but velocity-weakening) matrix in bulk shear strength and slip stability of faults. Numerical simple-shear experiments using a Distinct Element Model (DEM) are conducted on both uniform mixtures of quartz and talc analogs and on textured mixtures consisting of a talc layer embedded in a quartz matrix. The mechanical response of particles is represented by a linear-elastic contact model with a slip weakening constitutive relation representing the essence of rate-state friction. The weight percentage of the talc in the uniform mixtures and the relative thickness of the talc layer in the textured mixtures are varied to investigate the transitional behavior of shear strength and slip stability. Specifically, for uniform mixtures, ~50% reduction on bulk shear strength is observed with 25% talc present, and a dominant influence of talc occurs at 50%; for textured mixtures, a noticeable weakening effect is shown at a relative layer thickness of 1-particle, ~50% shear strength reduction is observed with 3-particles, and a dominant influence occurs at 5-particles. In terms of slip stability, a transition from velocity-weakening to velocity-strengthening is observed with 10% to 25% talc present in the uniform mixtures or with 3-particles to 5-particles in the textured mixtures. In addition, further analysis suggest that quartz has a high tendency towards dilation, potentially promoting permeability; while talc dilates with increased slip rate, but compacts rapidly when slip rate is reduced, potentially destroying permeability. The simulation results match well with previous laboratory observations.Chapter 2 elaborates numerical shear reactivation experiments on analog mixtures of quartz and talc gouge using a three-dimensional (3D) distinct element model (DEM). We follow the evolution of shear strength, slip stability, and permeability of the gouge mixture during dynamic shear and explore the mesoscopic mechanisms. A modified slip-weakening constitutive law is applied at contacts. We perform velocity-stepping experiments on both uniform, and layered mixtures of quartz and talc analogs. We separately vary the proportion of talc in the uniform mixtures and talc layer thickness in the layered mixtures. Shear displacements are cycled through shear velocities of 1 and 10μm/s. Simulation results show that talc has a strong weakening effect on shear strength - a thin shear-parallel layer of talc (~8.1 wt%) can induce significant weakening. However, the model offsets laboratory derived strong weakening effects of talc observed in uniform mixtures, implying the governing mechanisms may be the strong shear localization effect of talc, which is enhanced by its natural platy shape. Ensemble stability can be enhanced by increasing talc content in uniform talc-quartz mixtures. No apparent influence of increasing talc layer thickness on is observed in layered mixtures. Talc enhances compaction at velocity down-steps, potentially reducing fault permeability. Additionally, we show that dimensionality significantly impacts the resolution of dynamic responses. 3D simulations are more representative of laboratory observed behavior. Numerical noise is shown to be of the order of ~0.1 of previous 2D counterparts. Evolution trends of stability parameters regarding the composition and structure of the fault gouge can be straightforwardly obtained from the 3D simulation. Our study elaborates a DEM approach to mechanistically investigate the mechanical and rheological response of faults during shearing and enhances the understanding of fault weakening mechanism.Chapter 3 involves investigating the influence of CO2-transformed iron oxide coatings on the coupled behavior of faults. Fugitive emissions of CO2 along faults may significantly influence their rheology and permeability by altering cementation and transforming gouge components. We conduct laboratory double direct shear experiments on pristine hematite-, and CO2-transformed goethite-coated quartz gouge to investigate the evolution of shear strength, slip stability and permeability. The gouge samples are synthesized in the laboratory and are characterized by particle size distribution and through SEM imaging both before and after shear-permeability experiments. Shear strength (at 3 MPa), a-b stability values, frictional healing and creep rates and (fault parallel) permeability are measured in velocity-stepping and slide-hold-slide loading modes. Hematite-coated quartz exhibit the highest peak shear strength, followed by goethite- then un-coated quartz. Coated and un-coated gouge samples exhibit similar residual shear strength. Hematite-coated quartz may undergo potential seismic slip, suggesting by negative (a-b) values. Goethite-coated quartz shows velocity-strengthening behavior by featuring positive (a-b) values but higher frictional healing rate and creep rate. All samples show an initial increase in permeability followed by a decline. However, goethite-coated samples show much less reduction in permeability than others. Characterization suggests that the liberation, transport and clogging of coating particles and shear-produced wear products can be the main mechanism for permeability evolution. These observations suggest CO2-transformed goethite-coated quartz-rich faults feature reduced risk of seismic reactivation, while greater loss of inventory in the long-term containment of CO2 may be expected.Chapter 4 investigates the influences of roughness on the ensemble mechanical and rheological behavior of mated fractures during reactivation.

ISBN: 9781392319000Subjects--Topical Terms:

535228
Geophysics.

Mineralogical and Textural Controls on Shear Strength, Slip Stability and Permeability of Faults.
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245 1 0 $a Mineralogical and Textural Controls on Shear Strength, Slip Stability and Permeability of Faults.
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300 $a 177 p.
500 $a Source: Dissertations Abstracts International, Volume: 80-12, Section: B.
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500 $a Advisor: Elsworth, Derek.
502 $a Thesis (Ph.D.)--The Pennsylvania State University, 2019.
520 $a Induced seismicity resulting from fluid injection into the subsurface related to water and CO2 disposal, hydraulic fracturing and the stimulation of geothermal reservoirs present an important societal concern. These human activities involve the injection of large volumes of pressurized fluid into the subsurface, potentially at high rates, raising local pore pressures and disturbing the pristine local stress regime by lowering effective normal stress on pre-existing faults and fractures. The reduction of effective normal stress may trigger fault/fracture reactivation and in some cases result in hazardous seismic ruptures. Effective management and engineering of anthropogenic seismic events requires substantial understanding in the mechanisms, especially for the controlling factors on coupled rheological and transport response, including fault shear strength, slip stability, and permeability evolution during such events. In this study, we explore the coupled rheological and transport response of faults and fractures during reactivation as controlled by two fundamental controlling properties, viz., mineralogy and textural features. We approach this problem through shear experiments on analog faults and fractures via laboratory and numerical experiments. Specifically, we investigate: (1) the influence of frictionally weak minerals (talc) in mixtures of mineral analogs featuring contrasting frictional properties, (2) the influence of iron oxide grain coatings on quartz aggregates, and (3) the influence of fracture roughness in mated fractures on the ensemble shear strength, slip stability, and permeability evolution during reactivation events. We address the following questions in this study: (1) how much and what distribution of frictionally weak minerals is required to induce significant weakening in faults consisting of a matrix of frictionally strong minerals, (2) how does a pre-imposed weak mineral layer influence the rheological and transport behavior of faults, (3) what is the influence of a trace amount of grain coating materials introduce on the coupled behavior of faults, and finally (4) how do asperity height and wave length control the ensemble behavior of faults. These questions are explicitly answered in the following.Chapter 1 explores the impact of phyllosilicate (weak but velocity-strengthening) in a majority tectosilicate (strong but velocity-weakening) matrix in bulk shear strength and slip stability of faults. Numerical simple-shear experiments using a Distinct Element Model (DEM) are conducted on both uniform mixtures of quartz and talc analogs and on textured mixtures consisting of a talc layer embedded in a quartz matrix. The mechanical response of particles is represented by a linear-elastic contact model with a slip weakening constitutive relation representing the essence of rate-state friction. The weight percentage of the talc in the uniform mixtures and the relative thickness of the talc layer in the textured mixtures are varied to investigate the transitional behavior of shear strength and slip stability. Specifically, for uniform mixtures, ~50% reduction on bulk shear strength is observed with 25% talc present, and a dominant influence of talc occurs at 50%; for textured mixtures, a noticeable weakening effect is shown at a relative layer thickness of 1-particle, ~50% shear strength reduction is observed with 3-particles, and a dominant influence occurs at 5-particles. In terms of slip stability, a transition from velocity-weakening to velocity-strengthening is observed with 10% to 25% talc present in the uniform mixtures or with 3-particles to 5-particles in the textured mixtures. In addition, further analysis suggest that quartz has a high tendency towards dilation, potentially promoting permeability; while talc dilates with increased slip rate, but compacts rapidly when slip rate is reduced, potentially destroying permeability. The simulation results match well with previous laboratory observations.Chapter 2 elaborates numerical shear reactivation experiments on analog mixtures of quartz and talc gouge using a three-dimensional (3D) distinct element model (DEM). We follow the evolution of shear strength, slip stability, and permeability of the gouge mixture during dynamic shear and explore the mesoscopic mechanisms. A modified slip-weakening constitutive law is applied at contacts. We perform velocity-stepping experiments on both uniform, and layered mixtures of quartz and talc analogs. We separately vary the proportion of talc in the uniform mixtures and talc layer thickness in the layered mixtures. Shear displacements are cycled through shear velocities of 1 and 10μm/s. Simulation results show that talc has a strong weakening effect on shear strength - a thin shear-parallel layer of talc (~8.1 wt%) can induce significant weakening. However, the model offsets laboratory derived strong weakening effects of talc observed in uniform mixtures, implying the governing mechanisms may be the strong shear localization effect of talc, which is enhanced by its natural platy shape. Ensemble stability can be enhanced by increasing talc content in uniform talc-quartz mixtures. No apparent influence of increasing talc layer thickness on is observed in layered mixtures. Talc enhances compaction at velocity down-steps, potentially reducing fault permeability. Additionally, we show that dimensionality significantly impacts the resolution of dynamic responses. 3D simulations are more representative of laboratory observed behavior. Numerical noise is shown to be of the order of ~0.1 of previous 2D counterparts. Evolution trends of stability parameters regarding the composition and structure of the fault gouge can be straightforwardly obtained from the 3D simulation. Our study elaborates a DEM approach to mechanistically investigate the mechanical and rheological response of faults during shearing and enhances the understanding of fault weakening mechanism.Chapter 3 involves investigating the influence of CO2-transformed iron oxide coatings on the coupled behavior of faults. Fugitive emissions of CO2 along faults may significantly influence their rheology and permeability by altering cementation and transforming gouge components. We conduct laboratory double direct shear experiments on pristine hematite-, and CO2-transformed goethite-coated quartz gouge to investigate the evolution of shear strength, slip stability and permeability. The gouge samples are synthesized in the laboratory and are characterized by particle size distribution and through SEM imaging both before and after shear-permeability experiments. Shear strength (at 3 MPa), a-b stability values, frictional healing and creep rates and (fault parallel) permeability are measured in velocity-stepping and slide-hold-slide loading modes. Hematite-coated quartz exhibit the highest peak shear strength, followed by goethite- then un-coated quartz. Coated and un-coated gouge samples exhibit similar residual shear strength. Hematite-coated quartz may undergo potential seismic slip, suggesting by negative (a-b) values. Goethite-coated quartz shows velocity-strengthening behavior by featuring positive (a-b) values but higher frictional healing rate and creep rate. All samples show an initial increase in permeability followed by a decline. However, goethite-coated samples show much less reduction in permeability than others. Characterization suggests that the liberation, transport and clogging of coating particles and shear-produced wear products can be the main mechanism for permeability evolution. These observations suggest CO2-transformed goethite-coated quartz-rich faults feature reduced risk of seismic reactivation, while greater loss of inventory in the long-term containment of CO2 may be expected.Chapter 4 investigates the influences of roughness on the ensemble mechanical and rheological behavior of mated fractures during reactivation.
520 $a Subsurface fluid injections can disturb the effective stress regime by elevating pore pressure and potentially reactivate faults and fractures. Laboratory studies indicate that fracture rheology and permeability s in such reactivation events are linked to the roughness of the fracture surfaces. We construct discrete element method (DEM) models to explore the influence of fracture surface roughness on the shear strength, slip stability, and permeability evolution during such slip events. For each simulation, a pair of analog rock coupons (3D bonded quartz-particle analogs) representing a mated fracture are sheared under a velocity-stepping scheme. The roughness of the fracture is defined in terms of asperity height and asperity wavelength. Results show that (1) samples with larger asperity heights (rougher), when sheared, exhibit a higher peak strength which quickly devolves to a residual strength after a threshold shear displacement; (2) these rougher samples also exhibit greater slip stability due to a high degree of asperity wear and resultant production of wear products; (3) long-term suppression of permeability is observed with rougher fractures, which is plausibly due to the removal of asperities and redistribution of wear products, which locally reduces porosity in the dilating fracture. This study provides insights into the understanding of the mechanisms of frictional and rheological evolution of rough fractures anticipated during reactivation events.
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