Effect of clay mineralogy and pore fluid chemistry on fluid flow in fault zones
Final Report Abstract
Phyllosilicates, such as clays and micas, are abundant throughout the upper crust. They are commonly found in upper crustal faults and constitute a large percentage of the sediments that make up accretionary wedges. Phyllosilicate-rich rocks are known to have low permeability, but the influence of the mineralogy, as well as pore fluid - mineral interaction on the permeability is poorly characterised. Phyllosilicates are also known for their frictional weakness, a property that might add to the observed weakness of some faults. Within this project, permeability and frictional strength of ten phyllosilicate powders were measured and the effect mineralogical differences have on these properties analysed. Phyllosilicates were selected to include 1:1 and 2:1 minerals with both di-octahedral and trioctahedral structures and cover a range of layer charges. Minerals included talc, pyrophyllite, kaolinite, lizardite, illite, montmorillonite, chlorite, muscovite, phlogopite, and biotite. Particle size was below 30 µm for all minerals. Permeability measurements were performed under hydrostatic conditions, with either water or argon as the pore fluid (constant pore pressure of 10 MPa) and varying confining pressure up to 160 MPa. All phyllosilicate powders in this study have very low permeabilities, from 10 -19 m2 (muscovite) to 10-24 m2 (montmorillonite), depending on mineral, pore fluid, and effective pressure. Compression of powders during cycling loading and unloading decreases permeability, and fully compacted powders (after 10 pressure cycles) show permeability to depend exponentially on effective pressure, with a pressure sensitive exponent of 0.004 MPa-1 to 0.015 MPa-1. A difference in permeability between water and argon was observed which can be related to the hydrophobicity of the mineral surface. The hydrophobic minerals talc and pyrophyllite showed the smallest difference in permeability between water and argon, while permeability of montmorillonite is 1.8 orders of magnitude higher for argon than for water. Particle size and shape might also affect permeability. A further correlation was found with frictional strength of minerals. Frictional strength of both over-dried and water-saturated phyllosilicate powders was measured for a range of effective normal stress (5 MPa – 100 MPa) using a triaxial deformation apparatus. Friction coefficients for phyllosilicates were between 0.22 – 0.44 (dry) and 0.12-0.38 (wet). All minerals were weaker when sheared wet than dry. Mineralogical differences were found to influence strength, in particular are tri-octahedral minerals weaker than di-octahedral minerals with otherwise similar crystal structures. Friction coefficients of dry samples are constant over most of the tested range of normal stress, apart from an increase in frictional strength for low normal stress <20 MPa, which might be related to experimental conditions such as rigidity of the jackets that are separating samples from the confining medium. Wet samples show an increase of friction coefficients, especially towards the high-pressure end of the range of normal stress used, which might indicate an increase in dry surface contacts between particles. The results of this project indicate that permeability of phyllosilicate-rich regions of the upper crust will be strongly affected by the mineralogy and particle size of phyllosilicates present, as well as by pore water composition and pore fluid interaction with mineral surfaces.
Publications
- Argon permeability of pure phyllosilicate minerals under high pressure. Deformation Mechanics, Rheology & Tectonics, Liverpool, 07.-09.09.2009
Behnsen, J., Faulkner, D.
- Frictional strength of sheet silicates under medium to high pressure. Gordon Research Conference on Rock Deformation, Tilton, NH, 08.08.-13.08.2010
Behnsen, J., Faulkner, D.
- Dry and wet frictional strength of clays at low to medium normal stress. Tectonic Studies Group 2011 Meeting. Durham, UK, 05.01-07.01.2011
Behnsen, J., Faulkner, D.
