Polyaxial stress-dependent permeability of a three-dimensional fractured rock layer

Abstract

A study about the influence of polyaxial (true-triaxial) stresses on the permeability of a three-dimensional (3D) fractured rock layer is presented. The 3D fracture system is constructed by extruding a two-dimensional (2D) outcrop pattern of a limestone bed that exhibits a ladder structure consisting of a Bthrough-going^joint set abutted by later-stage short fractures. Geomechanical behaviour of the 3D fractured rock in response to in-situ stresses is modelled by the finite-discrete element method, which can capture the deformation of matrix blocks, variation of stress fields, reactivation of pre-existing rough fractures and propagation of new cracks. A series of numerical simulations is designed to load the fractured rock using various polyaxial in-situ stresses and the stress-dependent flow properties are further calculated. The fractured layer tends to exhibit stronger flow localisation and higher equivalent permeability as the far-field stress ratio is increased and the stress field is rotated such that fractures are preferentially oriented for shearing. The shear dilation of pre-existing fractures has dominant effects on flow localisation in the system, while the propagation of new fractures has minor impacts. The role of the overburden stress suggests that the conventional 2D analysis that neglects the effect of the out-of-plane stress (perpendicular to the bedding interface) may provide indicative approximations but not fully capture the polyaxial stress-dependent fracture network behaviour. The results of this study have important implications for understanding the heterogeneous flow of geological fluids (e.g. groundwater, petroleum) in subsurface and upscaling permeability for largescale assessments.

Is the permeability of naturally fractured rocks scale dependent?

“…[1985] model, appears as the next logical step toward physical realism, albeit a very challenging one [cf. Lei et al ., ]. The main difference between this and earlier studies is that we consider permeability anisotropy and matrix permeability.…”

Stress‐Induced Anomalous Transport in Natural Fracture Networks

“…In addition, this work studied 2‐D fracture system, and there could be important 3‐D effects. While several studies have investigated fluid flow in stressed 3‐D fracture networks (Garipov et al, ; Lei et al, ; McClure et al, ), the effects of geological stress on tracer transport through 3‐D fracture networks remain to be investigated. A recent study has shown that the Bernoulli CTRW model can capture particle transport through 3‐D fracture networks (Hyman et al, ), and the natural extension of this study will be the effects of geological stress on particle transport through stressed 3‐D fracture networks.…”

“…The E‐W striking set that formed in an early stage contains laterally persistent fractures, which tend to arrest the later developed N‐S striking set consisting of short fractures. This natural fracture network exhibiting a hierarchical, ladder pattern has been used as an analog to the subsurface system for studying single‐phase flow, multiphase flow, solute transport, and thermal conductivity (Edery et al, ; Geiger & Emmanuel, ; Geiger et al, ; Lei et al, ; Matthäi & Belayneh, ; Matthäi et al, ). In this paper, we represent the system with a two‐dimensional (2‐D) DFN (Geiger et al, ) and use this fracture network to explore the impact of geological stress on single‐phase flow and transport behavior.…”

Stress Influence on Fracture Aperture and Permeability of Fragmented Rocks

“…This effect cannot be considered in 2‐D modelings. Hence, as also mentioned by Lei et al (), results from 2‐D models may only provide indicative approximations. It is worth noting that in this study all the fractures were initially assumed to be closed.…”

“…Combining the geomechanical behavior of the fracture network with its fluid flow properties, they showed that some particular stress states can reactivate the preexisting fractures and can change the system's permeability. Lei et al () also studied the influence of triaxial stresses on the permeability of a 3‐D fractured rock layer, which was created by extruding a 2‐D outcrop pattern. They considered the influence of stress on the permeability of the fractured rock layer running a decoupled geomecahincal and flow analysis.…”