In this chapter, we investigate hydro-mechanical effects in the Opalinus Clay, especially those l... more In this chapter, we investigate hydro-mechanical effects in the Opalinus Clay, especially those leading to cracking. We present a methodology comprising laboratory and field scale experiments, as well as the development and application of numerical approaches.
IOP Conference Series: Earth and Environmental Science
At the interface between the steel canister and the bentonite in a nuclear waste repository, we e... more At the interface between the steel canister and the bentonite in a nuclear waste repository, we expect generation of hydrogen gas because of corrosion processes. The pressurized gas might fracture the engineered or natural clay barrier system, enhancing radionuclide transport into the geosphere. To assess the long-term integrity of the clay host rock under various conditions and scenarios, we need a large number of numerical simulations. However, a simulation tool for complex fracture propagation is often prohibitively expensive to run many realizations. Here, we developed a risk analysis tool based on the Design of Experiments to overcome the computational challenges by generating a computationally inexpensive proxy fracture model using a set of critical factors known as heavy hitters. We provided parameters and their probability distributions that are subject to uncertainty, as well as an objective function that assesses the risk of fracturing due to high gas pressures. Through va...
Computer Methods in Applied Mechanics and Engineering, 2021
Abstract This paper proposes a diffused approach to approximate failure at interfaces that physic... more Abstract This paper proposes a diffused approach to approximate failure at interfaces that physically occupy negligible space compared to the bulk material. For an interface diffused over a certain length, we derive an effective interface fracture toughness based on the diffused length and the fracture toughness of the bulk and the interface. Our derivation ensures the energetic equivalence between the sharp and the diffused representations of interface. We verified the critical energy release rates in a steadily propagating tensile fracture example as well as the critical fracture pressure and the crack length evolution in toughness dominated hydraulic fracturing . The proposed model does not require any changes in existing implementation of phase-field models. The only requirement is to assign the analytically calculated effective interface fracture toughness over a diffused sub-domain.
Journal of Natural Gas Science and Engineering, 2019
Abstract We propose a phase field model to simulate CO2 fracturing under an isothermal condition.... more Abstract We propose a phase field model to simulate CO2 fracturing under an isothermal condition. We take advantage of the ability of the phase field approach in predicting fracture initiation and branching, and also to avoid tracking the fracture path. We model the CO2 as a compressible fluid by modifying Darcy's law. In particular, we assume the permeability is correlated to the phase field value by an exponential function. The dependence of the CO2 density as a function of the pressure is captured by the Span-Wagner equation of state. The computed breakdown pressure values show good agreement with analytical solutions and experimental results.
An efficient method to model the fluid lag in hydraulic fracturing has been developed based on th... more An efficient method to model the fluid lag in hydraulic fracturing has been developed based on the Elrod–Adams model. The main feature of this method is the absence of the need to explicitly track the free end of the fracturing fluid, but rather, the fluid front is obtained by solving the pressure field (zero for the lag) and an auxiliary field for the entire fracture. An important advantage of this method is that no change of formulation, and hence no contact detection, is needed when the fluid reaches the fracture tip. Moreover, the method works for both the injection phase and the liquid withdrawal phase. Based on the latter case studies can be developed to investigate the quantity of the remaining fluid after the fracturing process in order to assess the environmental impact of fracturing. The method applies to both 2D and 3D problems.
Journal of Shanghai Jiaotong University (Science), 2018
Phase field description of fracture is a very promising approach for simulating crack initiation,... more Phase field description of fracture is a very promising approach for simulating crack initiation, propagation, merging and branching. This method greatly reduces the implementation complexity, compared with discrete descriptions of cracks. In this work, we provide an overview of phase field models for quasistatic and dynamic cases. Afterward, we present useful vectors and matrices for the implementation of this method in two and three dimensions.
In this chapter, we investigate hydro-mechanical effects in the Opalinus Clay, especially those l... more In this chapter, we investigate hydro-mechanical effects in the Opalinus Clay, especially those leading to cracking. We present a methodology comprising laboratory and field scale experiments, as well as the development and application of numerical approaches.
IOP Conference Series: Earth and Environmental Science
At the interface between the steel canister and the bentonite in a nuclear waste repository, we e... more At the interface between the steel canister and the bentonite in a nuclear waste repository, we expect generation of hydrogen gas because of corrosion processes. The pressurized gas might fracture the engineered or natural clay barrier system, enhancing radionuclide transport into the geosphere. To assess the long-term integrity of the clay host rock under various conditions and scenarios, we need a large number of numerical simulations. However, a simulation tool for complex fracture propagation is often prohibitively expensive to run many realizations. Here, we developed a risk analysis tool based on the Design of Experiments to overcome the computational challenges by generating a computationally inexpensive proxy fracture model using a set of critical factors known as heavy hitters. We provided parameters and their probability distributions that are subject to uncertainty, as well as an objective function that assesses the risk of fracturing due to high gas pressures. Through va...
Computer Methods in Applied Mechanics and Engineering, 2021
Abstract This paper proposes a diffused approach to approximate failure at interfaces that physic... more Abstract This paper proposes a diffused approach to approximate failure at interfaces that physically occupy negligible space compared to the bulk material. For an interface diffused over a certain length, we derive an effective interface fracture toughness based on the diffused length and the fracture toughness of the bulk and the interface. Our derivation ensures the energetic equivalence between the sharp and the diffused representations of interface. We verified the critical energy release rates in a steadily propagating tensile fracture example as well as the critical fracture pressure and the crack length evolution in toughness dominated hydraulic fracturing . The proposed model does not require any changes in existing implementation of phase-field models. The only requirement is to assign the analytically calculated effective interface fracture toughness over a diffused sub-domain.
Journal of Natural Gas Science and Engineering, 2019
Abstract We propose a phase field model to simulate CO2 fracturing under an isothermal condition.... more Abstract We propose a phase field model to simulate CO2 fracturing under an isothermal condition. We take advantage of the ability of the phase field approach in predicting fracture initiation and branching, and also to avoid tracking the fracture path. We model the CO2 as a compressible fluid by modifying Darcy's law. In particular, we assume the permeability is correlated to the phase field value by an exponential function. The dependence of the CO2 density as a function of the pressure is captured by the Span-Wagner equation of state. The computed breakdown pressure values show good agreement with analytical solutions and experimental results.
An efficient method to model the fluid lag in hydraulic fracturing has been developed based on th... more An efficient method to model the fluid lag in hydraulic fracturing has been developed based on the Elrod–Adams model. The main feature of this method is the absence of the need to explicitly track the free end of the fracturing fluid, but rather, the fluid front is obtained by solving the pressure field (zero for the lag) and an auxiliary field for the entire fracture. An important advantage of this method is that no change of formulation, and hence no contact detection, is needed when the fluid reaches the fracture tip. Moreover, the method works for both the injection phase and the liquid withdrawal phase. Based on the latter case studies can be developed to investigate the quantity of the remaining fluid after the fracturing process in order to assess the environmental impact of fracturing. The method applies to both 2D and 3D problems.
Journal of Shanghai Jiaotong University (Science), 2018
Phase field description of fracture is a very promising approach for simulating crack initiation,... more Phase field description of fracture is a very promising approach for simulating crack initiation, propagation, merging and branching. This method greatly reduces the implementation complexity, compared with discrete descriptions of cracks. In this work, we provide an overview of phase field models for quasistatic and dynamic cases. Afterward, we present useful vectors and matrices for the implementation of this method in two and three dimensions.
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Papers by Mostafa Mollaali