Apr 1, 2001 - samples containing 10 % clays than for clay-free sam- ples. .... son, 1973]. Pressure .... A power law was fitted simultaneously to the entire.
GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 7, PAGES 1295-1298, APRIL 1,2001
Enhanced pressure solution creep rates induced by clay particles' Experimental evidence in salt aggregates Francois Renard, •'2DagDysthe, 3 JensFeder, 3 Knut Bjorlykke, 2 andB.iorn Jamtveit 2 Fluid-Rock Interactiongroup,Departmentsof Geologyand Physics,Universityof Oslo, P.O.box 1047 Blindern, 0316 Oslo, Norway
Abstract. Pressure solution is responsible fbr mechano-chemical compaction of sediments in the up-
1996]conditions. Theeffectof clayshasbeenarguedto be purelychemical[Oelker's et al., 2000]althoughno
per crust (2-10 km). This processalsocontrolsporosity mechanismhas been presented. Others argue that the effect of clay is mechanical;the clay particles propping up the contactsof the dissolvingparticles and sustaining an open contactat a micron scale.This allowsfor a higher diffusionaltransport rates along grain contacts [Tads et al., 1987;Dewersand Ortoleva,1991;Renard micron),the strain ratesbeing50 % to 200 % fasterin et al., 1997]. Anotherpossibleeffectis that clay partisamplescontaining10 % claysthan for clay-freesam- cles inhibit the formation of true grain boundariesbeples. Even the presenceof 1% clay increasesthe strain tween individual salt particles (analogueto crack healrate significantly. We proposethat clay particles en- ing). Grain boundaryformationat a fractionof the salt hance pressuresolution creepbecausethesemicroscopic particle contactswill stop pressuresolution and inhibit minerals are trapped within the salt particle contacts relative sliding of the particles at thosecontacts. variations in a fault gouge after an earthquake. We present experimental results from chemicalcompaction of aggregatesof halite mixed with clays. It is shownthat clay particles (1-5 microns)greatly enhancethe deformation by pressuresolutionin salt aggregates(100-200
where they allow faster diffusion of solutes from the particle contacts to the pore space and inhibit grain boundary formation.
Little experimental work has been performed on the effectof clay during mechano-chemical compaction.On one hand, large mica sheets added at halite singlecrystal interfacesdo not significantlyenhancethe rate
Introduction
of pressure solution [Martin et al., 1999].On the other hand, Hickman and Evans [1995]have observedthat
The deformation of rocks by dissolution-transport-
the presenceof a film of montmorillonite clays at the precipitation mechanisms(i.e., pressuresolution) has interface between two halite lenses increases the rate of been known for many years, particulm'ly in sandstones shortening of these lensesby a factor 3. Rutter and and limestones[Heald,1955]. Suchprocesses are resWanten [2000] have recently publishedexperimental ponsible for a mechano-chemicaltime dependent compaction of sedimentsduring disgenesisand also controls porosity variations in fault gouges. The main controversy surroundingpressuresolution is to what extent it is affectedby increasingeffective stress,increasingtemperature, and by the presenceof clay particles along
work on the compaction of illire, muscovite and quartz at 400øC that again indicates that phyllosilicatespromote time-dependent compaction. It is outside the scope of this paper to present a detailed microscopic model of the salt particle contacts with and without clay particles and experimentallyvergrain interfaces(or a combinationof these) [Worden ify this. The aim of this paper is to test experimentally and Morad,2000]. if there is any macroscopiceffect of clayson pressuresoGeological observationsindicate that clay or mica lution and to examine whether this effect must be chepm'ticlescould greatly enhancemechano-chemicalcommical or not. For this purposewe have performed experpaction. Clay particlesare typically concentratedalong imentson a systemwhere a chemicaleffectof the clay on
stylolites [Heald,1955]and mica flakeshavebeenob- the dissolution/precipitationof the compactingmateservedto indent quartz grainsin diagenetic [Oelkers rial will not have any effecton the compactionrate. The et al., 2000]andmetamorphic[SchwartzandStockherr, compactionby pressuresolutionof halite is known to be •LGIT-CNRS-Observatoire, University Joseph Fourier, Grenoble F-38041, France 2Institute of Geology,Universityof Oslo aDepartment of Physics,University of Oslo Copyright2001 by theAmericanGeophysical Union. Papernumber2000GL012394. 0094-8276/01/2000GL012394505.00
diffusionlimited and will thereforenot be affectedby local alterationsof solubility or dissolution/precipitation kinetics at clay-halite contacts. We present results of experimental compaction of aggregatesof salt and clay and quantify the effectsof variousparameterssuch as grain size, pressure, and clay content. In the remainder of the paper we will term halite particles in the aggregate"grains" and other particles (for exam-
1295
1296
RENARD
ET AL.: ENHANCEMENT
OF PRESSURE
pie clays)will be termed "particles".The halite grains changeshapedueto dissolution/precipitation and possibly plastic flow and they can form grain boundaries, the clay particles are inert and are largely shapeconserving.
The mechanism of pressure solution The driving force for pressuresolution is related to
SOLUTION
CREEP
BY CLAYS
are: stress(1, 5, 40 bar); salt grain size (106-250microns);and clay content(0, 1, and 10% in weight). During the whole experiment,the load is constantand the fluid pressureis atmospheric.The vertical shortening of the cylindricalsampleis measuredduringseveral days. We have verifiedthat the compactionis homogeneouseverywherein the sample. The height, x(t), is measured with a micrometer
or with a CCD
cam-
era (resolution-•50 micron). In the latter case,one the variationsof stressalongthe grainsurface[Pater- picture is taken each hour during 7 to 15 days, resultson,1973].Pressuresolutionoccursas a response to a ing in a very accuratemeasurements of uniaxial strain nonzero effective stress,i.e., the axial stresscomponent e(t) = (xo- x(t))/xo where x0 standsfor the initial
ernacrossa grain contact is greater than the stresson the grain due to the normal stressin the adjacentpore. Thus, the chemical potential of a mineral can be seento vary along the surfaceof a grain, driving the diffusive flux of solutes from the contact area to the free pore
height of the sample. The accuracyis limited by the reproducibility of the sample preparation, not by the resolution of the measurement technique. Compaction curvesindicate that the presenceof clay increasesthe creeprate (Fig. 2). For pure salt experiments,the resurface. producibility is very good whereasfor salt and clay agThe nature of the grain-grain interfaceis crucial begregatesthe spreadof the compactioncurvesis larger. cause it is the place where the dissolutionof the minWhen usingsiliconcarbide(SiC flakes,3-6 micron) ineral and the transport of solutes to the surrounding stead of clays,preliminary resultsindicate that there is pore space occurs. A film of confined water can be alsoan enhancementof pressuresolution(Fig. 2). trapped between the sheets of minerals such as clays Compaction curves are then analyzed to obtain staandmicas[PashleyandIsraelachvili, 1984]or sapphires tistical quantification of the effectsof grain size, stress, [Horn et al., 1989].The thickness of this film hasbeen and clay content. Test experimentswere performedin measured or calculated for different minerals and varies dry conditionsand the deformationwas two to three from a few tenthsto severalnanometers[Pescheland times smaller, indicating that plasticity or grain sliding Aidfinger,1971;Renardand Ortoleva,1997]. are less important than pressuresolution creep in our For diffusion-controlledpressure solution, the transexperimental conditions. port properties(thickness,diffusioncoefficient)of this film control the rate at which solutes are expelled from the stressed contact to the pore. When •nicroparticles are initially present in the rock, they could remain dead weight trapped at grain contactsand enhancelocal diffusion rates by increasingthe thicknessof the contact between glass piston grains. This couldbe a major effectwhen pressuresolu-
tioncreepislimitedbydiffusion at graincontacts asit is the casefor salt at lowtemperatures[SpiersandBrzesowsky,1993]or quartzat hightemperatures[Gratier and Guiguet,1986].
Experiments
with salt and clays
The principle of the compaction technique is shown in Fig. 1: the aggregate of salt and clays is contained in a transparent and hollow glasscylinder. The salt is
brass plate
spring brass plate sample
base pure halite (NaC1) and the grain size (106-250micron) is controlledby sieving. The clay is a pottery bentonite 2 cm (Black Ball Clay, grain size 1-5 micron). The powders of clay and salt are carefully mixed in the presenceof Figure 1. The sample of salt and clay aggregate•s a halite saturated brine. The mixture is poured into compactedin a transparentglasscylinder (internal dithe compaction cell before a dead weight is applied for ameter 15 ram). The vertical stressis imposedby a loading and the cell is sealed. The aggregateswere pre- dead weight on a cylindrical piston and the stressis pared in brine to avoid segregationof salt and clay that transmitted through two brass cylinders and a spring would otherwise occur in dry mixing and to allow clay As the cell is transparent, the deformation is followed by accuratelymeasuringthe heightof the sample.The particlesto attach to the salt grain surfacesbefore com- measurementswere performed during 7 to 15 days. Dif-
paction.
ferent parametersof deformationwere varied: salt grain Fifty of suchexperimental deviceswere built and the size (106 to 250 micron), verticalstress(1, 5, and 40 independentparametersvaried duringthe experiments bar), and claycontent(0, 1, 10 % weight).
10
l0
......
1297
vertical stress: 4.7 bar
temperature: 22 ø C .
•8
salt grain size: 180-250 micro/
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100
150
200
250
300
t [hour]
ß I
0
20
40
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i dry salt I
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60 80 100 120 140 1•0 180 time [hour]
Figure 2. Compactioncurves•(t) -- (x0- x(t))/xo for differentsampleswithout clay (square),with 10% clay (circle),10 % SiC (opencircle). Evenif the reproducibility is not very good for the salt clay aggregates, the compaction curves indicate a significant effect of clays:the strain after 180 hom'sis 1.5 to 3 timesgreater for samplescontaining10 % clay. The effectof SiC particles seemsto be similar to that of clays.
Figure 3. Common behavior during compaction. All the data have been collapsed and fitted with a power law in time with an exponent • = 0.4 that is similar for all the experimental conditions.
paction rate increasesby 5% for a 1% increasein the weight fraction of clay in the aggregate. Several samples were flushed with hexadecane after deformation to remove water fi'om the pore space. The sampleswere then opened and observedin a scanning electron microscope. The initially cubic salt grains comemore sphericalwith lesssharp edgesafter pressure
solution(Fig. 4). For the samplescontainingclay,ob-
Effects of stress, grain size and clay content
.
.
. rtl• micro n':" ' . ' .-i...
A powerlaw was fitted simultaneously to the entire
setof compaction curves •j(t) = c•jtœ•,j E [1,40]at 5 m•d 40 bar. This yieldeda singleexponent,fi = 0.4,
andindividualcompaction ratesc•jforeachexperiment. One major advantagewith this method is that we ob-
tain a singletime independent compaction rate c•j for each experimentas opposedto the usual compaction
rate•j(t) = Oe(t)/Ot.Thedatacollaps ej
