New paper: Mohr–Coulomb plasticity for sands incorporating density effects without parameter calibration

Our paper on simple Mohr–Coulomb plasticity for capturing density effects on sand has been accepted for publication in International Journal for Numerical and Analytical Methods in Geomechanics.

Read the paper: Choo, IJNAMG 2018.

Abstract: A simple approach is proposed for enabling the conventional Mohr–Coulomb plasticity to capture the effects of relative density on the behavior of dilative sands. The approach exploits Bolton's empirical equations to make friction and dilation angles state variables that depend on the current density and confining pressure. In doing so, the material parameters of Mohr–Coulomb plasticity become void ratios for calculating the initial relative density and the critical state friction angle, all of which are measurable without calibration. A Mohr–Coulomb model enhanced in this way shows good agreement with experimental data of different sands at various densities and confining pressures. In this regard, the proposed approach permits a significant improvement in the conventional Mohr–Coulomb plasticity for sands, without compromising its practical merits.

New paper: Liquid CO2 fracturing: Effect of fluid permeation on the breakdown pressure and cracking behavior

Our paper on liquid CO2 fracturing, in collaboration with Prof. Tae Sup Yun at Yonsei University, has been accepted for publication in Rock Mechanics and Rock Engineering.

Read the paper: Ha et al., RMRE 2018. 

Abstract: Liquid CO2 fracturing is a promising alternative to hydraulic fracturing since it can circumvent problems stemming from the use of water. One of the most significant differences between liquid CO2 and hydraulic fracturing processes is that liquid CO2 permeates into matrix pores very rapidly due to its low viscosity. Here we study how this rapid permeation of liquid CO2 impacts a range of features during the course of the fracturing process, with a focus on the breakdown pressure and cracking behavior. We first conduct a series of laboratory fracturing experiments that inject liquid CO2, water, and oil into nominally identical mortar specimens with various pressurization rates. We quantitatively measure the volumes of fluids permeated into the specimens and investigate how these permeated volumes are related to breakdown and fracture initiation pressures and pressurization efficiency. The morphology of the fractures generated by different types of fluids are also examined using 3D X-ray computed tomographic imaging. Subsequently, the cracking processes due to injection of liquid CO2 and water are further investigated by numerical simulations employing a phase-field approach to fracture in porous media. Simulation results show that rapid permeation of liquid CO2 gives rise to a substantial pore pressure buildup and distributed microcracks prior to the major fracture propagation stage. The experimental and numerical results commonly indicate that significant fluid permeation during liquid CO2 fracturing is a primary reason for its lower breakdown pressure and more distributed fractures compared with hydraulic fracturing.

Simulation of liquid CO2 fracturing in a heterogeneous brittle porous material. The red zone denotes fully cracked regions, whereas the blue zone denotes intact regions. The green zone denotes microcracked regions. The video shows that the injection of liquid CO2 — which involves a huge amount of fluid permeation into the matrix — gives rise to a lot of microcracks prior to fracture propagation.

Simulation of hydraulic (water) fracturing in the same porous material. The video shows that when the same material is fractured by the injection of water, virtually no microcrack develops in the matrix before and during the fracture propagation stage. As a result, this process manifests a higher breakdown pressure, as observed from experiments of our collaborator and others.

New project: Waterless fracturing for unconventional energy production: Coupled geomechanics–flow modeling and investigations

Our research project entitled "Waterless fracturing for unconventional energy production: Coupled geomechanics–flow modeling and investigations" will be supported by the Research Grants Council of Hong Kong under the Early Career Scheme. We deeply appreciate the support and look forward to research findings that this project will enable.

Students interested in working on the project are advised to send an email to jchoo@hku.hk

New paper: Large deformation poromechanics with local mass conservation: An enriched Galerkin finite element framework

Our paper on locally mass conservative finite element framework for large deformation poromechanics has been accepted for publication in International Journal for Numerical Methods in Engineering.

Read the paper: Choo, IJNME 2018.

Abstract: Numerical modeling of large deformations in fluid-infiltrated porous media must accurately describe not only geometrically nonlinear kinematics but also fluid flow in heterogeneously deforming pore structure. Accurate simulation of fluid flow in heterogeneous porous media often requires a numerical method that features the local (element-wise) conservation property. Here we introduce a new finite element framework for locally mass conservative solution of coupled poromechanical problems at large strains. At the core of our approach is the enriched Galerkin discretization of the fluid mass balance equation, whereby element-wise constant functions are augmented to the standard continuous Galerkin discretization. The resulting numerical method provides local mass conservation by construction, with a usually affordable cost added to the continuous Galerkin counterpart. Two equivalent formulations are developed using total Lagrangian and updated Lagrangian approaches. The local mass conservation property of the proposed method is verified through numerical examples involving saturated and unsaturated flow in porous media at finite strains. The numerical examples also demonstrate that local mass conservation can be a critical element of accurate simulation of both fluid flow and large deformation in porous media.

This paper proposes the first locally mass conservative finite element framework for large deformation poromechanical problems. The formulation builds on our previous work on enriched Galerkin (EG) methods. This figure shows local (element-wise) mass residuals in a synthetic land subsidence problem due to groundwater withdrawal. It can be seen that the local mass residuals are significant in the conventional continuous Galerkin (CG) solutions, but they become nearly zero in the EG solutions. This difference also affects the predicted subsidence results. The property of local mass conservation can become far more important once the flow is coupled with a transport phenomenon.

This paper proposes the first locally mass conservative finite element framework for large deformation poromechanical problems. The formulation builds on our previous work on enriched Galerkin (EG) methods. This figure shows local (element-wise) mass residuals in a synthetic land subsidence problem due to groundwater withdrawal. It can be seen that the local mass residuals are significant in the conventional continuous Galerkin (CG) solutions, but they become nearly zero in the EG solutions. This difference also affects the predicted subsidence results. The property of local mass conservation can become far more important once the flow is coupled with a transport phenomenon.