The Effect of Varying Damage History in Crystalline Rocks on the P- and S-Wave Velocity under Hydrostatic Confining Pressure

Blake, O. O.; Faulkner, Daniel R.; Rietbrock, Andreas

doi:10.1007/s00024-012-0550-0

Cited by 40 publications

(18 citation statements)

References 57 publications

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“…For gouge samples, dynamic Young's moduli and Poisson ratios are between 26 to 46 GPa and 0.16 to 0.30. These values are in the range of values obtained from boreholes and laboratory measurements comprised between 10 GPa to 80 GPa and 0.1 to 0.3, respectively [ Faulkner et al ., ; Wang and Ji , ; Jeppson et al ., ; Blake et al ., ].…”

Section: Resultsmentioning

confidence: 99%

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Development and maintenance of fluid overpressures in crustal fault zones by elastic compaction and implications for earthquake swarms

et al. 2015

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The ability of crustal faults to compact and to pressurize pore fluids is examined by combining geological observations, petrophysical measurements (permeability, P and S wave velocities, and porosity), and fully coupled hydromechanical modeling. A strike-slip fault located in the Argentera-Mercantour crystalline massif (southwestern French-Italian Alps) was analyzed in the field. This mature fault belongs to a large active fault system characterized by a recurrent seismic swarm activity (M w < 4) between 2 and 12 km depth. The studied exposure corresponds to a 50 m thick anastomosing fault composed of three types of rock: host-rock gneiss, damage-zone phyllonite, and core zone gouge. Laboratory measurements made at effective pressures ranging from 10 to 190 MPa show that the studied fault differs from the classical model and has a high-porosity, high-permeability, and low-rigidity core zone surrounded by a low-porosity, low-permeability, and high-rigidity damage zone with respect to the host rock. The hydraulic and elastic properties are controlled by different microstructures such as foliation, microcracks, and pores developed during the exhumation history of the massif and the reactivation of inherited low-friction mylonitic foliation. Hydromechanical modeling is then used to investigate the spatio-temporal evolution of the fluid overpressures across the fault zone elements in response to elastic compaction. Models demonstrate that fluid pressure can be developed and maintained temporally in the studied fault zone. This study concludes on the key role played by the hydromechanical properties of faults during compaction and provides an explanation for seismic swarm triggering and maintenance.

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Section: Resultsmentioning

confidence: 99%

“…For a description of the apparatus and techniques used, see Blake et al . []. A through‐transmission method was employed to measure the P and S wave velocity of the sample in the axial direction.…”

Section: Permeability Porosity and Elastic Wave Velocity Measurementsmentioning

confidence: 99%

Development and maintenance of fluid overpressures in crustal fault zones by elastic compaction and implications for earthquake swarms

et al. 2015

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“…We see a clear correlation of crack area per unit volume (Sv) to the observed compressional wave velocities ( Figure 8D) and interpret this to be attenuation of the compressional wave through the cracked intracrystalline and intercrystalline boundaries that are abundant in the andesite (e.g., Figures 3 and 4). Many authors (e.g., Vinciguerra et al 2005;Keshavarz et al 2010;Blake et al 2012; Heap et al 2014) have also shown that the elastic wave velocities can be highly attenuated by the presence of microcracks.…”

Section: Micromechanical Interpretationmentioning

confidence: 99%

Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand

et al. 2014

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Background: Geothermal systems are commonly hosted in highly altered and fractured rock. As a result, the relationships between physical properties such as strength and permeability can be complex. Understanding such properties can assist in the optimal utilization of geothermal reservoirs. To resolve this issue, detailed laboratory studies on core samples from active geothermal reservoirs are required. This study details the results of the physical property investigations on Rotokawa Andesite which hosts a significant geothermal reservoir. Methods:We have characterized the microstructure (microfracture density), porosity, density, permeability, elastic wave velocities, and strength of core from the high-enthalpy Rotokawa Andesite geothermal reservoir under controlled laboratory conditions. We have built empirical relationships from our observations and also used a classical micromechanical model for brittle failure. Further, we compare our results to a Kozeny-Carman permeability model to better constrain the fluid flow behavior of the rocks. Results: We show that the strength, porosity, elastic moduli, and permeability are greatly influenced by pre-existing fracture occurrence within the andesite. Increasing porosity (or microfracture density) correlates well to a decreasing uniaxial compressive strength, increasing permeability, and a decreasing compressional wave velocity. Conclusions:Our results indicate that properties readily measurable by borehole geophysical logging (such as porosity and acoustic velocities) can be used to constrain more complex and pertinent properties such as strength and permeability. The relationships that we have provided can then be applied to further understand processes in the Rotokawa reservoir and other reservoirs worldwide.Keywords: Geothermal; Uniaxial compressive strength; Permeability; Physical properties; Elastic modulus; Microstructure BackgroundFractures on multiple scales are the dominant control on fluid flow in most geothermal systems worldwide. Geothermal environments are prone to variable heat fluxes, dynamic fluid flow regimes, and active tectonics which impact the physical and mechanical properties of the reservoir rocks in which they are hosted. The influence of such a dynamic environment can render the host rocks highly altered, fractured, and microstructurally complex. As a result, the empirical correlation of physical properties to

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“…The elastic properties of granite have been similarly well studied (e.g. Sano et al 1992;Heap and Faulkner 2008;Blake et al 2013;Blake and Faulkner 2016).…”

Section: Introductionmentioning

confidence: 99%

Estimating in situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)

et al. 2018

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Knowledge of the strength and elastic modulus of a reservoir rock is important for the optimisation of a particular geothermal resource. The reservoir rock for many geothermal projects in the Upper Rhine Graben, such as those at Soultz-sous-Forêts and Rittershoffen (both France), is porphyritic granite. High fracture densities (up to ~ 30 fractures/m) in this reservoir rock require that we consider the strength and elastic modulus of the rock mass, rather than the intact rock. Here we use uniaxial and triaxial deformation experiments performed on intact rock coupled with Geological Strength Index assessments-using the wealth of information from core and borehole analyses-to provide rock mass strength and elastic modulus estimates for the granite reservoir at Soultz-sous-Forêts (from a depth of 1400 to 2200 m) using the generalised Hoek-Brown failure criterion. The average uniaxial compressive strength and elastic modulus of the intact granite are 140 MPa (this study) and 40 GPa (data from this study and the literature), respectively. The modelled strength of the intact granite is 360 MPa at a depth of 1400 m and increases to 455 MPa at 2200 m (using our estimate for the empirical m i term of 30, determined using triaxial and tensile strength measurements on the intact granite). Strength of the rock mass varies in accordance with the fracture density and the extent and nature of the fracture infill, reaching lows of ~ 40-50 MPa (in, for example, the densely fractured zones in EPS-1 at depths of ~ 1650 and ~ 2160 m, respectively) and highs of above 400 MPa (in, for example, the largely unfractured zone at a depth of ~ 1940-2040 m). Variations in rock mass elastic modulus are qualitatively similar (values vary from 1 to 2 GPa up to the elastic modulus of the intact rock, 40 GPa). Our study highlights that macrofractures and joints reduce rock mass strength and should be considered when assessing the rock mass for well stability and rock mass deformation due to stress redistribution in the reservoir. We present a case study to demonstrate how a simple and cost-effective engineering method can be used to provide an indication of the in situ strength and elastic modulus of reservoir rock masses, important for a wide range of modelling and stimulation strategies. We recommend that the effect of macrofractures on rock mass strength and stiffness be validated for incorporation into geomechanical characterisation for geothermal reservoirs worldwide.

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The Effect of Varying Damage History in Crystalline Rocks on the P- and S-Wave Velocity under Hydrostatic Confining Pressure

Cited by 40 publications

References 57 publications

Development and maintenance of fluid overpressures in crustal fault zones by elastic compaction and implications for earthquake swarms

Development and maintenance of fluid overpressures in crustal fault zones by elastic compaction and implications for earthquake swarms

Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand

Estimating in situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)

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