Characterisation and modelling of stress-dependent permeability and flow in shallow fractured rock aquifers.

Author: Luke Mortimer

Mortimer, Luke, 2011 Characterisation and modelling of stress-dependent permeability and flow in shallow fractured rock aquifers., Flinders University, School of the Environment

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Abstract

In situ stress can exert a significant control on fluid flow patterns in fractured rocks with relatively low matrix permeability. In addition to overall reduction in rock mass permeability, fracture deformation results in preferential flow along fractures oriented orthogonal to the minimum principal stress direction (due to low normal stress) or inclined ~30o to the maximum principal stress direction (due to dilation). As fracture void geometries and the connectivity of a flow network change in response to changing in situ stress, the storage, permeability and flow pattern should also be expected to change in magnitude, heterogeneity and/or anisotropy. These influences of in situ stress on the absolute and relative magnitudes and spatial distribution of the components of the permeability tensor are well documented for applications at depth but have received little attention at shallow to near surface settings. Groundwater flow modelling of shallow (<200 m) fractured rock aquifers is typically conducted under the assumption that permeability is independent of the stress state i.e. fluid flow is taking place within an effectively non-deforming medium. The potential influence of stress on fracture permeability at shallow depths might also be considered weak owing to the relatively low overburden pressures. This is perhaps the main reason why the effects of stress fields are largely ignored in shallow depth hydrogeological investigations. The question as to what extent this assumption holds at shallow to near surface depths is the main focus of this body of research. Stress-related effects on groundwater flow at shallow depths are difficult to identify and characterise due to the complex interactions between all of the inherent properties of a fractured rock aquifer. These properties include the factors that dominantly control groundwater flow: fracture network density, geometry, connectivity and infill. Furthermore, surface processes such as weathering, erosion and unloading alter the original hydraulic nature (connectivity, transmissivity) of fractured rock masses resulting in higher degrees of spatial heterogeneity within shallow flow systems. These processes and interactions often mask the influence of in situ stress fields on fracture network permeability and groundwater flow. The multitude of the influential factors means that no one method can adequately map the spatial distribution of hydraulic properties that control advective groundwater flow at shallow depths. Therefore, any stress-dependent fracture permeability investigations require a multi-parameter, multi-disciplinary methodology including hydromechanical (HM) modelling. This research began with a detailed geological and hydrogeological characterisation of fractured rock aquifers within the Clare Valley, South Australia. The study area is situated within a near horizontal, WNW-ESE directed regional compressional stress field that is seismically active and undergoing uplift and erosion. This area was the subject of several previous geological, geophysical and hydrogeological investigations that provided invaluable background field observations which improved model constraints and reduced the level of uncertainty. This research built on these previous studies through an integrated analysis of local area fracture networks from outcrop mapping, geotechnical drill core logging, surface and borehole geophysical surveys, borehole groundwater flow analyses and representative HM models. It demonstrated how in situ stress affects groundwater flow in shallow (<200 m) fractured rock aquifers and to what extent fracture hydraulic aperture distributions, fracture network connectivity and groundwater flow rates are modified via fracture deformation processes. The inclusion of representative HM models was important as field techniques such as outcrop mapping, borehole hydraulic and geophysical surveys do not fully account for sub-surface fracture deformation and HM response of a fracture network as a whole. In particular, comparison between deformed (stressed state) and undeformed (zero stress state) HM models enabled the effects of in situ stress to be qualified and quantified through evaluation of the measurable changes in the groundwater flow system of the original (pre-existing) versus deformed (contemporary) state of a fracture network. This research adopted a unique philosophy and approach that demonstrates how to generate information complementary to standard hydrogeological observations, especially in areas where field hydrogeological data are limited. This concept was extended to preliminary fault stress state modelling to improve the probability of identifying zones of enhanced fault permeability, which in turn, could potentially increase productivity and reduce the uncertainty in locating fault-related fluid production targets, particularly in early stage exploration projects or in areas of unknown or complex geology. Ultimately, this research contributes to the knowledge and understanding of the role in situ stress plays and of its interactions with other influential factors in determining groundwater flow in fractured rock aquifers. It also provides guidance on what are the critical datasets and how they can be measured and practically applied in the field as well as incorporated within groundwater flow models over local to regional scales.

Keywords: fractured rock aquifer,fracture permeability,in situ stress,hydromechanical modelling,Clare Valley,South Australia
Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2011
School: School of the Environment
Supervisor: Prof Craig Simmons