Stress influence on fracture aperture and permeability of fragmented rocks

The influence of far-field stresses on fracture apertures in a fragmented rock layer is investigated using finite element analysis of a three-dimensional mechanical model. The model implements realistic boundary conditions, interactions between the fragmented layer and neighboring plastic rock layers, and frictional interfaces between the rock blocks. Stress-strain analysis is conducted to obtain stress variations within the fragmented rock layer and the block displacements and rotations. The fracture apertures are calculated using the local stress states instead of the far-field stresses simply being projected on the fractures. It is observed that fracture apertures can vary for the fracture segments over the individual blocks. Ensemble permeability is calculated by running a single-phase flow analysis considering the obtained fracture apertures for fracture segments. The influence of the rock block displacements, rotations and deformations, difference between the mechanical properties of the rock layers, and the orientation of the horizontal stresses is investigated on the ensemble permeability. It is demonstrated that the compressibility of the neighboring layers and block rotations and deformations have significant influence on the permeability of the fragmented rock layer. These effects, which may be ignored in simpler aperture calculation models, can result in considerable inaccuracies in the estimation of fracture apertures and ensemble permeability. Hence, such methods may only be used as indicative tools

A comparative study of stress influence on fracture apertures in fragmented rocks

This study compares the calculated fracture apertures in a fragmented rock layer under different stress scenarios using two different approaches. Approach 1 is a simplified method using a two-dimensional (2D) mapping of the fracture network and projects the far-field stresses to individual fractures, and calculates the dilation, normal and shear displacements using experimental stiffnesses available in the literature. Approach 2 employs a three-dimensional (3D) finite element method (FEM) for the mechanical analysis of the fragmented rock layer considering the interaction with the neighbouring rock layers, frictional interfaces between the rock blocks, stress variations within the fragmented rock layer, and displacements, rotations and deformations of rock blocks. After calculating the fracture apertures using either of the approaches, the permeability of the fragmented rock layer is calculated by running flow simulations using the updated fracture apertures. The comparison between the results demonstrates an example of the inaccuracies that may exist in methods that use simplified assumptions such as 2D modelling, ignoring the block rotations and displacements, projected far-field stresses on fractures, and the stress variations within the rock layer. It is found that for the cases considered here, the permeability results based on apertures obtained from the simplified approach could be 40 times different from the results from apertures calculated using a full mechanical approach. Hence, 3D mechanical modelling implementing realistic boundary conditions, while considering the displacements and rotations of rock blocks, is suggested for the calculation of apertures in fragmented rocks

Aperture modelling for the flow-based determination of fracture-matrix ensemble saturation functions for naturally fractured reservoirs

DFN (fracture-only) and DFM (fracture and rock matrix) modelling is a rapidly growing field. While more and more geometrically realistic models get published, fracture aperture is often treated as single-valued or as set-by-set constant parameter. However, this is incompatible with field observations indicating variable apertures, lognormal or multimodal aperture distributions, and-or partial sealed fractures in naturally fractured hydrocarbon reservoirs. This presentation explores how realistic aperture variations across multiple sets of intersecting fractures can be modelled taking into account geometry (orientation, length versus frequency distributions, abutting relationships), mechanical rock properties, in situ stress, and pore pressure. New algorithms are used to account for fracture dilatation, asperity gliding, asperity crushing, and dissolution-precipitation. They are used in concert to produce physically realistic aperture models. These techniques are already part of a fracture modeling and upscaling workflow that has been applied in the field, and flow simulation highlights the first-order control that the ensuing variable apertures exert on permeability, anisotropy and flow localisation. The key remaining challenge, however, is the modeling of mechanical interactions between fractures and rock fragments. As an important aspect of this, here we address whether far-field-stress-based fracture aperture computations are applicable to rock fragmented by multiple fracture sets

Tensile behavior of groups of anchored blind bolts within concrete-filled steel square hollow sections

In this paper, the tensile behavior of groups of Ajax anchored blind bolts used within concrete-filled steel square hollow sections is investigated. Using Ajax anchored blind bolts moment-resisting bolted connections to concrete-filled steel hollow section columns will be possible. Extensive experimental and numerical studies were undertaken. Bolt sizes and section sizes suitable for medium-rise commercial buildings were used. It was concluded that the groups of Ajax anchored blind bolts can reach the ultimate capacity of equivalent groups of standard structural bolts. The location of an Ajax anchored blind bolt relative to the section side walls has a significant influence on its behavior. For bolts located close to the side walls of a section, concrete struts developed and transferred the loads to the corner of the sections. Bolt diameter, concrete grade, and strut angle were found to be the most influential factors in the stiffness of an Ajax anchored blind bolt. A simple theoretical model, based on finite-element (FE) parametric studies, was developed to estimate the tensile behavior of groups of Ajax anchored blind bolts. This was essential for the application of Ajax anchored blind bolts in building construction practice