Numerical modelling of pore-fluid-enhanced  thermal spallation in granitic rock

This paper considers numerically the effect of pore-fluid on thermal spallation of granitic rock. For this end, a numerical model based on the embedded discontinuity finite element approach to rock fracture and an explicit scheme to solve the underlying thermo-mechanical problem is developed. In the present implementation, a displacement discontinuity (crack) is embedded perpendicular to the first principal direction in a linear triangle element upon violation of the Rankine criterion. In the thermo-mechanical problem, the heating due to mechanical dissipation is neglected as insignificant in comparison to the external heat flux. This leads to an uncoupled thermo-mechanical problem where the only input from the thermal part to the mechanical part is thermal strains. This problem is solved with explicit time marching using the mass scaling to speed up the solution. Finally, the fluid trapped into the micro-pores is modelled as a material that can bear only volumetric compressive stresses. A thermal spallation problem of a rock sample under axisymmetry is simulated as a numerical example.

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Volatile-consuming reactions fracture rocks and self-accelerate fluid flow in the lithosphere

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2110776118 ◽

2022 ◽

Vol 119 (3) ◽

pp. e2110776118

Author(s):

Masaoki Uno ◽

Kodai Koyanagawa ◽

Hisamu Kasahara ◽

Atsushi Okamoto ◽

Noriyoshi Tsuchiya

Keyword(s):

Fluid Flow ◽

Reaction Rate ◽

Rock Fracture ◽

Local Stress ◽

Confining Pressure ◽

Pore Fluid ◽

Reaction Products ◽

Permeability Enhancement ◽

High Connectivity ◽

Hydromechanical Behavior

Hydration and carbonation reactions within the Earth cause an increase in solid volume by up to several tens of vol%, which can induce stress and rock fracture. Observations of naturally hydrated and carbonated peridotite suggest that permeability and fluid flow are enhanced by reaction-induced fracturing. However, permeability enhancement during solid-volume–increasing reactions has not been achieved in the laboratory, and the mechanisms of reaction-accelerated fluid flow remain largely unknown. Here, we present experimental evidence of significant permeability enhancement by volume-increasing reactions under confining pressure. The hydromechanical behavior of hydration of sintered periclase [MgO + H2O → Mg(OH)2] depends mainly on the initial pore-fluid connectivity. Permeability increased by three orders of magnitude for low-connectivity samples, whereas it decreased by two orders of magnitude for high-connectivity samples. Permeability enhancement was caused by hierarchical fracturing of the reacting materials, whereas a decrease was associated with homogeneous pore clogging by the reaction products. These behaviors suggest that the fluid flow rate, relative to reaction rate, is the main control on hydromechanical evolution during volume-increasing reactions. We suggest that an extremely high reaction rate and low pore-fluid connectivity lead to local stress perturbations and are essential for reaction-induced fracturing and accelerated fluid flow during hydration/carbonation.

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Numerical Modeling of Temperature Effect on Tensile Strength of Granitic Rock

Applied Sciences ◽

10.3390/app11104407 ◽

2021 ◽

Vol 11 (10) ◽

pp. 4407

Author(s):

Timo Saksala

Keyword(s):

Tensile Strength ◽

Temperature Effect ◽

Curie Point ◽

Granitic Rock ◽

Critical Time ◽

Slow Heating ◽

Single Element ◽

Target Temperature ◽

Time Step ◽

Mass Scaling

The aim of this paper is to numerically predict the temperature effect on the tensile strength of granitic rock. To this end, a numerical approach based on the embedded discontinuity finite elements is developed. The underlying thermo-mechanical problem is solved with a staggered method marching explicitly in time while using extreme mass scaling, allowed by the quasi-static nature of the slow heating of a rock sample to a uniform target temperature, to increase the critical time step. Linear triangle elements are used to implement the embedded discontinuity kinematics with two intersecting cracks in a single element. It is assumed that the quartz mineral, with its strong and anomalous temperature dependence upon approaching the α-β transition at the Curie point (~573 °C), in granitic rock is the major factor resulting in thermal cracking and the consequent degradation of tensile strength. Accordingly, only the thermal expansion coefficient of quartz depends on temperature in the present approach. Moreover, numerically, the rock is taken as isotropic except for the tensile strength, which is unique for each mineral in a rock. In the numerical simulations mimicking the experimental setup on granitic numerical rock samples consisting of quartz, feldspar and biotite minerals, the sample is first heated slowly to a target temperature below the Curie point. Then, a uniaxial tension test is numerically performed on the cooled down sample. The simulations demonstrate the validity of the proposed approach as the experimental deterioration of the tensile strength of the rock is predicted with agreeable accuracy.

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3D Numerical Prediction of Thermal Weakening of Granite under Tension

Geosciences ◽

10.3390/geosciences12010010 ◽

2021 ◽

Vol 12 (1) ◽

pp. 10

Author(s):

Timo Saksala

Keyword(s):

Tensile Strength ◽

Rock Fracture ◽

Critical Time ◽

Heterogeneous Material ◽

Initial Boundary ◽

Initial Boundary Value Problem ◽

Numerical Approach ◽

Numerical Prediction ◽

Time Step ◽

Mass Scaling

This paper deals with numerical prediction of temperature (weakening) effects on the tensile strength of granitic rock. A 3D numerical approach based on the embedded discontinuity finite elements is developed for this purpose. The governing thermo-mechanical initial/boundary value problem is solved with an explicit (in time) staggered method while using extreme mass scaling to increase the critical time step. Rock fracture is represented by the embedded discontinuity concept implemented here with the linear (4-node) tetrahedral elements. The rock is modelled as a linear elastic (up to fracture by the Rankine criterion) heterogeneous material consisting of Quartz, Feldspar and Biotite minerals. Due to its strong and anomalous temperature dependence upon approaching the α-β transition at the Curie point (~573 °C), only Quartz in the numerical rock depends on temperature in the present approach. In the numerical testing, the sample is first volumetrically heated to a target temperature. Then, the uniaxial tension test is performed on the cooled down sample. The simulations demonstrate the validity of the proposed approach as the experimental deterioration, by thermally induced cracking, of the rock tensile strength is predicted with a good accuracy.

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Dilatancy stabilises shear failure in rock

10.5194/egusphere-egu21-5626 ◽

2021 ◽

Author(s):

Franciscus Aben ◽

Nicolas Brantut

Keyword(s):

Pore Pressure ◽

Fault Zone ◽

Shear Failure ◽

Rock Fracture ◽

Fluid Pressure ◽

Fault Slip ◽

Pore Fluid ◽

Lithostatic Pressure ◽

Strengthening Effect ◽

Pore Fluids

<p>Failure and fault slip in crystalline rocks is associated with dilation. When pore fluids are present and drainage is insufficient, dilation leads to pore pressure drops, which in turn lead to strengthening of the material. We conducted laboratory rock fracture experiments with direct in-situ fluid pressure measurements which demonstrate that dynamic rupture propagation and fault slip can be stabilised (i.e., become quasi-static) by such a dilatancy strengthening effect. We also observe that, for the same effective pressures but lower pore fluid pressures, the stabilisation process may be arrested when the pore fluid pressure approaches zero and vaporises, resulting in dynamic shear failure. In case of a stable rupture, we witness continued after slip after the main failure event that is the result of pore pressure recharge of the fault zone. All our observations are quantitatively explained by a simple spring-slider model combining slip-weakening behaviour, slip-induced dilation, and pore fluid diffusion. Using our data in an inverse problem, we estimate the key parameters controlling rupture stabilisation, fault dilation rate and fault zone storage. These estimates are used to make predictions for the pore pressure drop associated with faulting, and where in the crust we may expect dilatancy stabilisation or vaporisation during earthquakes. For intact rock and well consolidated faults, we expect strong dilatancy strengthening between 4 and 6 km depth regardless of ambient pore pressure, and at greater depths when the ambient pore pressure approaches lithostatic pressure. In the uppermost part of the crust (<4 km), we predict vaporisation of pore fluids that eliminates dilatancy strengthening. The depth estimates where dilatant stabilisation is most likely coincide with geothermal energy reservoirs in crystalline rock (typically between 2 and 5 km depth) and in regions &#160;where slow slip events are observed (pore pressure that approaches lithostatic pressure).&#160;</p>

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Three-Dimensional Propagation Simulation and Parameter Analysis of Rock Joint with Displacement Discontinuity Method

Mathematical Problems in Engineering ◽

10.1155/2019/3164817 ◽

2019 ◽

Vol 2019 ◽

pp. 1-11

Author(s):

Ke Li ◽

Xinghong Jiang ◽

Hao Ding ◽

Xuebing Hu

Keyword(s):

Rock Fracture ◽

Rock Joint ◽

Three Dimensional ◽

Propagation Model ◽

Parameter Analysis ◽

Displacement Discontinuity ◽

Joint Surface ◽

Rate Theory ◽

Distribution Law ◽

Nonlinear Joint

Displacement discontinuities method (DDM) is convenient for and efficient at dealing with discontinuity problems such as fracture and fault. However, the known method of ease iteration is not available for nonlinear joint surface problems. This paper introduces Barton-Bandis nonlinear joint deformation failure criterion, figures out the propagation model of joint through the maximum energy release rate theory of rock fracture mechanics, and establishes three-dimensional nonlinear displacement discontinuity model for rock joint. This paper gives results of the joint propagation pattern and its distribution law under tension and compression with different parameters and side pressure coefficients via compiled program.

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Numerical Modelling of Microwave Heating Assisted Rock Fracture

Rock Mechanics and Rock Engineering ◽

10.1007/s00603-021-02685-8 ◽

2021 ◽

Author(s):

Martina Pressacco ◽

Jari J. J. Kangas ◽

Timo Saksala

Keyword(s):

Tensile Strength ◽

Microwave Heating ◽

Solution Method ◽

Numerical Study ◽

Rock Fracture ◽

Model Performance ◽

Mechanical Problem ◽

Mechanical Parts ◽

Tension Tests ◽

Rock Specimens

AbstractThis paper presents a numerical study on the effects of microwave irradiation on the mechanical properties of hard rock. More specifically, the weakening effect of microwave heating induced damage on the uniaxial compressive and tensile strength of granite-like rock is numerically evaluated. Rock fracture is modelled by means of a damage-viscoplasticity model with separate damage variables for tensile and compressive failure types. We develop a global solution strategy where the electromagnetic problem is solved first separately in COMSOL multiphysics software, and then provided into a staggered implicit solution method for the thermo-mechanical problem. The thermal and mechanical parts of the problem are considered as uncoupled due to the dominance of the microwave-induced heat source. The model performance is tested in 2D finite element simulations of heterogeneous numerical rock specimens subjected first to heating in a microwave oven and then to uniaxial compression and tension tests. According to the results, the compressive and tensile strength of rock can be significantly reduced by microwave heating pretreatment.

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