Imperial College London

Dr Robin Thomas

Faculty of EngineeringDepartment of Earth Science & Engineering

Research Associate
 
 
 
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Contact

 

robin.thomas11

 
 
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Location

 

Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

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9 results found

Paluszny A, Thomas RN, Saceanu MC, Zimmerman RWet al., 2020, Hydro-mechanical interaction effects and channelling in three-dimensional fracture networks undergoing growth and nucleation, JOURNAL OF ROCK MECHANICS AND GEOTECHNICAL ENGINEERING, Vol: 12, Pages: 707-719, ISSN: 1674-7755

The flow properties of geomechanically generated discrete fracture networks are examined in the context of channelling. Fracture networks are generated by growing fractures in tension, modelling the low permeability rock as a linear elastic material. Fractures are modelled as discrete surfaces which grow quasi-statically within a three-dimensional (3D) volume. Fractures may have their locations specified as a simulation input, or be generated as a function of damage, quantified using the local variation in equivalent strain. The properties of the grown networks are shown to be a product of in situ stress, relative orientation of initial flaws, and competitive process of fracture interaction and growth. Fractures grow preferentially in the direction perpendicular to the direction of maximum tension and may deviate from this path due to mechanical fracture interaction. Flow is significantly channelled through a subset of the fractures in the full domain, consistent with observations of other real and simulated fractures. As the fracture networks grow, small changes in the geometry of the fractures lead to large changes in the locations and scale of primary flow channels. The flow variability and formation of channels are examined for two growing networks, one with a fixed amount of fractures, and another with nucleating fractures. The interaction between fractures is shown to modify the local stress field, and in turn the aperture of the fractures. Pathways for single-phase flow are the results of hydro-mechanical effects in fracture networks during growth. These are the results of changes to the topology of the network as well as the result of mechanical self-organisation which occurs during interaction leading to growth and intersection.

Journal article

Thomas RN, Paluszny A, Zimmerman RW, 2020, Permeability of three‐dimensional numerically grown geomechanical discrete fracture networks with evolving geometry and mechanical apertures, Journal of Geophysical Research: Solid Earth, Vol: 125, ISSN: 2169-9313

Fracture networks significantly alter the mechanical and hydraulic properties of subsurface rocks. The mechanics of fracture propagation and interaction control network development. However, mechanical processes are not routinely incorporated into discrete fracture network (DFN) models. A finite element, linear elastic fracture mechanics‐based method is applied to the generation of three‐dimensional geomechanical discrete fracture networks (GDFNs). These networks grow quasi‐statically from a set of initial flaws in response to a remote uniaxial tensile stress. Fracture growth is handled using a stress intensity factor‐based approach, where extension is determined by the local variations in the three stress intensity factor modes along fracture tips. Mechanical interaction between fractures modifies growth patterns, resulting in nonuniform and nonplanar growth in dense networks. When fractures are close, stress concentration results in the reactivation of fractures that were initially inactive. Therefore, GDFNs provide realistic representations of subsurface networks that honor the physical process of concurrent fracture growth. Hydraulic properties of the grown networks are quantified by computing their equivalent permeability tensors at each growth step. Compared to two sets of stochastic DFNs, GDFNs with uniform fracture apertures are strongly anisotropic and have relatively higher permeabilities at high fracture intensities. In GDFN models, where fracture apertures are based on mechanical principles, fluid flow becomes strongly channeled along distinct flow paths. Fracture orientations and interactions significantly modify apertures, and in turn, the hydraulic properties of the network. GDFNs provide a new way of understanding subsurface networks, where fracture mechanics is the primary influence on their geometric and hydraulic properties.

Journal article

Thomas R, Paluszny A, Zimmerman RW, 2020, Growth of three-dimensional fractures, arrays, and networks in brittle rocks under tension and compression, Computers and Geotechnics, Vol: 121, ISSN: 0266-352X

Concurrent growth of multiple fractures in brittle rock is a complex process due to mechanical interaction effects. Fractures can amplify or shield stress on other fracture tips, and stress field perturbations change continuously during fracture growth. A three-dimensional, finite-element based, quasi-static growth algorithm is validated for mixed mode fracture growth in linear elastic media, and is used to investigate concurrent fracture growth in arrays and networks. Growth is governed by fracture tip stress intensity factors, which quantify the energy contributing to fracture extension, and are validated against analytical solutions for fractures under compression and tension, demonstrating that growth is accurate even in coarsely meshed domains. Isolated fracture geometries are compared to wing cracks grown in experiments on brittle media. A novel formulation of a Paris-type extension criterion is introduced to handle concurrent fracture growth. Fracture and volume-based growth rate exponents are shown to modify fracture interaction patterns. A geomechanical discrete fracture network is generated and examined during its growth, whose properties are the direct result of the imposed anisotropic stress field and mutual fracture interaction. Two-dimensional cut-plane views of the network demonstrate how fractures would appear in outcrops, and show the variability in fracture traces arising during interaction and growth.

Journal article

Almulhim OA, Paluszny A, Thomas RN, Zimmerman RWet al., 2019, Fully-coupled three-dimensional finite element simulations of the interaction between a hydraulic fracture and a pre-existing natural fracture

Copyright 2019 ARMA, American Rock Mechanics Association. The effects of approach angle and matrix poroelasticity on three-dimensional fracture interactions are investigated by examining changes in stress intensity factors around hydraulic fracture tips. Additionally, the effect of the compressive stress induced by the hydraulic fracture (the “stress shadow effect”) on the natural fracture is explored. Fracture interaction is captured using three-dimensional interaction maps based on two interaction measures that quantify the magnitude and type of interaction. The results show that the stress conditions at the hydraulic fracture tip are more favorable for growth when interacting fractures have shallow approach angles, and the interaction reduces as the approach angle increases. For “high” permeability reservoirs, fluid leak-off causes the rock matrix to dilate, and generates a tensile stress that amplifies the local stress field. The increase in stress in the region ahead of the hydraulic fracture (the stress amplification zone) can cause natural fractures to open in tension before the hydraulic fracture intersects the natural fracture. The increase in matrix permeability causes earlier activation of natural fractures due to the additional stress induced by the dilated rock matrix.

Conference paper

Almulhim OA, Paluszny A, Thomas RN, Zimmerman RWet al., 2019, Fully-coupled three-dimensional finite element simulations of the interaction between a hydraulic fracture and a pre-existing natural fracture

The effects of approach angle and matrix poroelasticity on three-dimensional fracture interactions are investigated by examining changes in stress intensity factors around hydraulic fracture tips. Additionally, the effect of the compressive stress induced by the hydraulic fracture (the “stress shadow effect”) on the natural fracture is explored. Fracture interaction is captured using three-dimensional interaction maps based on two interaction measures that quantify the magnitude and type of interaction. The results show that the stress conditions at the hydraulic fracture tip are more favorable for growth when interacting fractures have shallow approach angles, and the interaction reduces as the approach angle increases. For “high” permeability reservoirs, fluid leak-off causes the rock matrix to dilate, and generates a tensile stress that amplifies the local stress field. The increase in stress in the region ahead of the hydraulic fracture (the stress amplification zone) can cause natural fractures to open in tension before the hydraulic fracture intersects the natural fracture. The increase in matrix permeability causes earlier activation of natural fractures due to the additional stress induced by the dilated rock matrix.

Conference paper

Thomas RN, Paluszny A, Hambley D, Hawthorne FM, Zimmerman RWet al., 2018, Permeability of observed three dimensional fracture networks in spent fuel pins, Journal of Nuclear Materials, Vol: 510, Pages: 613-622, ISSN: 0022-3115

The three-dimensional fracture network within a spent fuel pin is characterised using sequential grinding, and its permeability is numerically estimated. Advanced Gas-cooled Reactors (AGRs) produce spent fuel pins consisting of an outer steel cladding enclosing ceramic uranium dioxide (UO2) pellets. During irradiation, fuel pellets may undergo fracturing due to thermal, densification and swelling effects. Fracture patterns are usually observed on the surface of the pellet or through a cross section or longitudinal plane along the pellet. In this work, the three-dimensional fracture pattern within the pellet is characterised using an optical microscope. The pellet is progressively ground and polished, providing sequential cross sections, which together yield a three-dimensional discrete fracture pattern. Multiple large fractures grow to connect the cladding to the internal region of the pellet. Multiple surface fractures are observed that do not penetrate into the matrix of the pellet. The porosity of the UO2 and apertures of the fractures are estimated by sampling microscopic images. Darcy flow is numerically solved using the finite element method, computing flow through the matrix and fractures simultaneously. The equivalent tensorial permeability of the system is estimated for various approximate fracture apertures. The fracture network raises the permeability of the pellet by an order of magnitude.

Journal article

Thomas RN, Paluszny A, Zimmerman RW, 2018, Effect of fracture growth velocity exponent on fluid flow through geomechanically-grown 3d fracture networks, 2nd International Discrete Fracture Network Engineering Conference

Copyright 2018 ARMA. Geomechanical discrete fracture networks (DFNs) are grown using a 3D finite element-based fracture mechanics simulator. The influence of the fracture growth rate exponent (β) on the resulting fracture geometry and hydraulic properties of networks is investigated. Previous work has found that β has a complex relationship with the final geometry of geomechanically-grown 2D DFNs. Realistic features evolve during the growth of DFNs as a result of the orientation of the principal stress axis and fracture interaction. High values of β cause interaction effects to be more pronounced, and irregular shaped fractures to be more common. Low values of β are found to produce networks with a balance between selective growth on preferentially oriented and interacting fractures, and significant increases in fracture surface area with computation time. The permeability of DFNs is significantly influenced by anisotropy, which develops in the axes perpendicular to the principal stress direction. For fracture networks with different β values, permeabilities along the principal axes are similar for the same total fracture void space.

Conference paper

Paluszny A, Thomas RN, Zimmerman RW, 2018, Finite element-based simulation of the growth of dense three-dimensional fracture networks, 52nd US Rock Mechanics / Geomechanics Symposium

Copyright © 2018 ARMA, American Rock Mechanics Association. The growth of fractures within a quasi-brittle rock is computed numerically with the aim of generating high-density geomechanically realistic three-dimensional discrete fracture patterns. Patterns are generated with a finite element-based discrete fracture propagation simulator, in which deformation and flow are numerically computed. These detailed multi-fracture growth simulations study the emergence of patterns as a function of the interaction of fractures and the mechanical effects of pattern evolution on the distribution of apertures in response to in situ stresses.

Conference paper

Thomas RN, Paluszny A, Zimmerman RW, 2017, Quantification of fracture interaction using stress intensity factor variation maps, Journal of Geophysical Research. Solid Earth, Vol: 122, Pages: 7698-7717, ISSN: 2169-9356

Accurate and flexible models of fracture interaction are sought after in the fields of mechanics and geology. Stress intensity factors (SIFs) quantify the energy concentrated at the fracture tips and are perturbed from their isolated values when two fractures are close to one another. Using a three-dimensional finite element fracture mechanics code to simulate static fractures in tension and compression, interaction effects are examined. SIF perturbations are characterized by introducing three interaction measures: the circumferential and maximum SIF perturbation provide the “magnitude” of the effect of interaction, and the amplification to shielding ratio quantifies the balance between increased and decreased SIFs along the tip. These measures are used to demonstrate the change in interaction with fracture separation and to find the separation at which interaction becomes negligible. Interaction maps are constructed by plotting the values of the interaction measures for a static fracture as a second fracture is moved around it. These maps are presented for several common fracture orientations in tension. They explore interaction by highlighting regions in which growth is more likely to occur and where fractures will grow into nonplanar geometries. Interaction maps can be applied to fracture networks with multiple discontinuities to analyze the effect of geometric variations on fracture interaction.

Journal article

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