Dynamic development of hydrofracture

Many natural examples of complex joint and vein networks in layered sedimentary rocks are hydrofractures that form by a combination of pore fluid overpressure and tectonic stresses. In this paper, a two-dimensional hybrid hydro-mechanical formulation is proposed to model the dynamic development of natural hydrofractures. The numerical scheme combines a discrete element model (DEM) framework that represents a porous solid medium with a supplementary Darcy based pore-pressure diffusion as continuum description for the fluid. This combination yields a porosity controlled coupling between an evolving fracture network and the associated hydraulic field. The model is tested on some basic cases of hydro-driven fracturing commonly found in nature, e.g., fracturing due to local fluid overpressure in rocks subjected to hydrostatic and nonhydrostatic tectonic loadings. In our models we find that seepage forces created by hydraulic pressure gradients together with poroelastic feedback upon discrete fracturing play a significant role in subsurface rock deformation. These forces manipulate the growth and geometry of hydrofractures in addition to tectonic stresses and the mechanical properties of the porous rocks. Our results show characteristic failure patterns that reflect different tectonic and lithological conditions and are qualitatively consistent with existing analogue and numerical studies as well as field observations. The applied scheme is numerically efficient, can be applied at various scales and is computational cost effective with the least involvement of sophisticated mathematical computation of hydrodynamic flow between the solid grains

From fracture to fragmentation: discrete element modeling -- Complexity of crackling noise and fragmentation phenomena revealed by discrete element simulations

Discrete element modelling (DEM) is one of the most efficient computational approaches to the fracture processes of heterogeneous materials on mesoscopic scales. From the dynamics of single crack propagation through the statistics of crack ensembles to the rapid fragmentation of materials DEM had a substantial contribution to our understanding over the past decades. Recently, the combination of DEM with other simulation techniques like Finite Element Modelling further extended the field of applicability. In this paper we briefly review the motivations and basic idea behind the DEM approach to cohesive particulate matter and then we give an overview of on-going developments and applications of the method focusing on two fields where recent success has been achieved. We discuss current challenges of this rapidly evolving field and outline possible future perspectives and debates

Numerical modeling of thermal fracture development in unconventional reservoirs

Rapid depletion of hydrocarbons in conventional reservoirs and the availability of abundant oil and gas resources in unconventional forms demand new technology to economically produce these energy resources. From exploration to consumption of hydrocarbons, a great number of complex physical, chemical, mechanical, electrical, and thermal phenomena occurs in reservoir from reservoir rock to surface facilities. In addition to good rock samples and cores which could represent the reservoir rock for laboratory experiments, numerical tools should always be used to provide predictive capability of reservoir rock and fluid behavior. A great number of numerical methods have been developed and used over the past decades for rock mechanics problems. In this research the main focus is on thermal fracturing, heat transfer in rock, flow in porous media and stress-deformation analysis. We use three numerical methods for thermal fracturing of reservoir rock and investigate their capabilities in providing a solution for fracture propagation in rock. We finally propose the best numerical method among the ones we used in this work. We finally show two applications of thermal rock fracturing for improvement of hydrocarbon recovery in tight formations.Petroleum and Geosystems Engineerin

Nanoscale stacking fault-assisted room temperature plasticity in flash-sintered TiO2.

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO2 prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO2 can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO2 is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO2 deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed

Numerical modelling and in-situ experiment for self-sealing of the induced fracture network of drift into the Callovo-Oxfordian claystone during a hydration process

The excavation damage zone surrounding an underground tunnel/gallery, and in particular its evolution, is being studied for the performance assessment of a radioactive waste underground repository. This paper focuses on numerical analysis of the self-sealing of the damaged zone based on an in-situ CDZ experiment for exploring the self-sealing of excavation damage zone during a hydration process. A plastic damage model is employed to describe the mechanical behaviour of Callovo-Oxfordian claystone (COx), and an added deformation model coupled with the standard Biot's model to simulate the significant deformation of COx claystone during the change of water content. Crack estimation and permeability evaluation of unsaturated fractured COx claystone are carried out through a post-processing method based on the fracture energy regularization and the cubic law, respectively. The validation of the proposed model is performed by numerical simulation of: (1) COx claystone swelling and triaxial compression tests, (2) self-sealing of fractured COx claystone samples during hydration process, (3) self-sealing of the damaged zone during a hydration process. Comparisons between the numerical and experimental results demonstrate the reliability of the proposed model to accurately describe the self-sealing of the fractured COx claystone, and the global water permeability reduction in hydration illustrates the accomplishment of the self-sealing of damaged zone

Micromechanics of sea urchin spines

The endoskeletal structure of the Sea Urchin, Centrostephanus rodgersii, has numerous long spines whose known functions include locomotion, sensing, and protection against predators. These spines have a remarkable internal microstructure and are made of single-crystal calcite. A finite-element model of the spine's unique porous structure, based on micro-computed tomography (microCT) and incorporating anisotropic material properties, was developed to study its response to mechanical loading. Simulations show that high stress concentrations occur at certain points in the spine's architecture; brittle cracking would likely initiate in these regions. These analyses demonstrate that the organization of single-crystal calcite in the unique, intricate morphology of the sea urchin spine results in a strong, stiff and lightweight structure that enhances its strength despite the brittleness of its constituent material

Hydromechanical Frameworks for Assessing the Occurrence of Wellbore Bridging and Fracture Broaching During Blowouts

Rigorous hydromechanical frameworks needed for modeling wellbore bridging and broaching during uncontrolled production of oil and gas are developed in this work. First, two sources of sand production are identified: borehole breakout and erosion of the producing formation. Theoretical framework for predicting the morphology of type B breakout mode is developed for the first time in this study; both fracture mechanics and shear failure theories are used in predicting the breakout geometry. Furthermore, a framework for estimating the size of caving produced during breakout (type A or B) is presented. Using asymptotic analysis of crack-boundary interactions, the state of damage around the borehole during the breakout process is determined, and the limiting buckling lengths of the resulting wing-cracks are predicted based on plate buckling theory. Third, a three-phase erosion kinetic equations, coupled with an erosion constitutive law, which is based on virtual power principle, are used in modeling radial and axial erosion in the reservoir and along the wellbore respectively. The proposed erosion constitutive law identifies the limitation of the pressure-gradient phenomenological model, which is currently being used. For a rigorous investigation into the self-killing of the well, a thermodynamically multiphase field model is developed for the gas-liquid-solid flow. The model, which is the combination of Navier-Stokes and Cahn-Hilliard type equations, incorporates the hydrodynamic interactions among the different species of the mixture. Lastly, this work considers a faster means for estimating fracture propagation in heterogeneous media (layered or naturally fractured) in the event the well is shut-in

Modeling the spatio-temporal evolution of fracture networks and fluid-rock interactions in GPU : Applications to lithospheric geodynamics

In this thesis, I present the theory and modeling of poro-elasto-plastic rheology coupled to a non-linear diffusion equation with a step increase in permeability at the onset of slip. This theoretical model is implemented in the graphic processing unit (GPU) architecture and programmed using the nVidia CUDA programming language. The numerical models are benchmarked by investigating fracture orientation for the solid-mechanical aspects, and by using the Method of Manufactured solutions for the diffusion part. I find that the GPU platform is ideal for these models because very high resolution simulations can be performed on an explicit finite difference algorithm using a single GPU card, outperforming CPU by a factor of at least five. The inherent problem with these coupled systems is the wide range of time and length scales that needs to be considered, and the advantage of GPU is its inherent parallel architecture that allows to do so. In these models, numerical fractures develop and evolve in response to prevailing far-field stresses, to local stress heterogeneity and pore-elastic stresses resulting from fracture growth, dislocation slip and fluid pressure diffusion within the domain. The numerical models, once benchmarked, are used to understand a variety of important and diverse lithospherical geodynamical problems, including enhanced geothermal systems (EGS), volcano-tectonic interactions and aftershocks. Envisaged future applications include hydro-fracture (’Fracking’), CO2 sequestration, earthquake nucleation and nuclear waste isolation. The potential of this model is far-reaching, and future developments in 3 dimensions will open up countless new avenues of insight and understanding of fluid-rock interactions and lithospheric dynamics