Pore‐scale modeling of deformation and shear band bifurcation in porous crystalline rocks

SummaryIn this paper, the analytical dual-porosity dual-permeability poromechanics solution for saturated cylinders is extended to account for electrokinetic effects and material transverse isotropy, which simulate the responses of chemically active naturally fractured shale under time-dependent mechanical loading and ionic solution exposure.The solution addresses the stresses, fracture pore pressure, matrix pore pressure, fluid fluxes, ion concentration evolution, and displacements due to the applied stress, pore pressure, and solute concentration difference between the sample and the circulation fluid. The presented solution will not only help validate numerical simulations but also assist in calibrating and interpreting laboratory results on dual-porosity dual-permeability shale. It is recommended that the analytical solutions of radial and axial displacements be used to match the corresponding laboratory-recorded data to determine shale dual permeability and chemo-electrical parameters including membrane coefficient, ions diffusion coefficients, and electro-osmotic permeability.KEYWORDS chemically active, dual-poro-chemo-electro-elasticity, electro-osmotic permeability, natural fractures, shale | INTRODUCTIONThe recent boom in oil and gas production from shale reservoirs around the world has produced a tectonic shift in the global energy landscape. However, despite the promising potentials, there are still many challenges in understanding, analyzing, and predicting the behaviors of these unconventional reservoirs during drilling, stimulation, and production. Some of these challenges come from the complex geomechanics properties of shale. Most shales have higher degree of mechanical anisotropy than conventional reservoir rocks. In addition, shale displays swelling and shrinking behaviors when exposed to aqueous drilling and stimulation fluids because of the electrochemical interaction between its clay content and the solutions. Furthermore, many shales are naturally fractured, as illustrated in Figure 1, resulting in complex pore pressure distribution and evolution in the dual-porosity dual-permeability system. It is noted that shales with dual-porosity system that might result from different scales of pore structures 1 can be also modeled using the current approach.This study aims to investigate the behavior of dual-porosity dual-permeability shale rocks under laboratory conditions. The original dual-porosity dual-permeability concept for fractured/fissured rocks was proposed by Barenblatt et al. 2 The 2 overlapping continua, primary porosity and secondary porosity, possess their own fluid pressure fields. Warren and Root 3 proposed an idealized dual-porosity model, assuming that the secondary porosity consists of an orthogonal system of uniform fractures and that fluid can communicate between primary and secondary porosities, but not among the primary porosity elements. Later on,

“…Such fracture property differences are not studied in this paper. For the case of open and closed fractures, numerical methods can be found in previous studies …”

Section: Numerical Examplementioning

confidence: 99%

Responses of chemically active and naturally fractured shale under time‐dependent mechanical loading and ionic solution exposure

Liu

Hoang

Abousleiman

2017

“…Although the continuum‐type method has no model size problem, it is not very good at simulating microscopic mechanisms (such as the evolution of microcracks). The computational framework presented by Tjioe and Borja can accommodate the presence of pores in the solid and the stress concentrations that develop in the edges and corners of the pores and capture the microfracture processes . Pore‐scale modeling approaches are indeed very good methods that can address not only large‐scale engineering but also micromechanisms.…”

Section: Discussionmentioning

confidence: 99%

“…The computational framework presented by Tjioe and Borja can accommodate the presence of pores in the solid and the stress concentrations that develop in the edges and corners of the pores and capture the microfracture processes. 41,42 Pore-scale modeling approaches are indeed very good methods that can address not only large-scale engineering but also micromechanisms. We will pay close attention to this method and combine discrete element method and porescale modeling approaches to establish the multiscale model of a fractured reservoir to study the fracturing problem for future study.…”

Section: Discussionmentioning

confidence: 99%

Numerical analysis of the effect of natural microcracks on the supercritical CO₂ fracturing crack network of shale rock based on bonded particle models

Peng

Wang

et al. 2017

Summary The problem of predicting the geometric structure of induced fractures is highly complex and significant in the fracturing stimulation of rock reservoirs. In the traditional continuous fracturing models, the mechanical properties of reservoir rock are input as macroscopic quantities. These models neglect the microcracks and discontinuous characteristics of rock, which are important factors influencing the geometric structure of the induced fractures. In this paper, we simulate supercritical CO2 fracturing based on the bonded particle model to investigate the effect of original natural microcracks on the induced‐fracture network distribution. The microcracks are simulated explicitly as broken bonds that form and coalesce into macroscopic fractures in the supercritical CO2 fracturing process. A calculation method for the distribution uniformity index (DUI) is proposed. The influence of the total number and DUI of initial microcracks on the mechanical properties of the rock sample is studied. The DUI of the induced fractures of supercritical CO2 fracturing and hydraulic fracturing for different DUIs of initial microcracks are compared, holding other conditions constant. The sensitivity of the DUI of the induced fractures to that of initial natural microcracks under different horizontal stress ratios is also probed. The numerical results indicate that the distribution of induced fractures of supercritical CO2 fracturing is more uniform than that of common hydraulic fracturing when the horizontal stress ratio is small.

“…Also, more complex loading paths have been simulated using the material models presented, where the PSO algorithm shows consistent and robust performance. Because the PSO algorithm is able to handle optimization of very complex objective functions, it could potentially be applied to even more complex material models, for example, materials with extreme incremental nonlinearity , which will be looked into in future investigations.…”

Section: Numerical Examplesmentioning

confidence: 99%

Particle swarm optimization for numerical bifurcation analysis in computational inelasticity

Lai

Chen

2016

Summary An efficient and robust algorithm to numerically detect material instability or bifurcation is of great importance to the understanding and simulation of material failure in computational and applied mechanics. In this work, an intelligence optimizer, termed the particle swarm optimization, is introduced to the numerical solution of material bifurcation problem consisting of finding the bifurcation time as well as the corresponding bifurcation directions. The detection of material bifurcation is approached as a constrained minimization problem where the determinant of the acoustic tensor is minimized. The performance of the particle swarm optimization method is tested through numerical bifurcation analysis on both small and finite deformation material models in computational inelasticity with increasing complexity. Compared with conventional numerical approaches to detect material bifurcation, the proposed method demonstrates superior performance in terms of both computational efficiency and robustness. Copyright © 2016 John Wiley & Sons, Ltd.