27-Ord

Hydrothermal brecciation: particle modeling.

A. Ord,1,2,3 B. E. Hobbs,1,2,3 & S. Mikula1

1 CSIRO E l ti & Mi i P th WA A t li1 CSIRO Exploration & Mining, Perth, WA, Australia2 predictive mineral discovery Cooperative Research Centre,3 School of Earth and Geographical Sciences, University of WesternAustralia, Perth, WA, Australia

1st International FLAC/DEM Symposium 25 August 2008

Purpose To test the hypothesis that brecciation could

be driven by a strong initial pressure gradient.y g p g To find conditions under which rock will behave in ways that resemble or could lead tobehave in ways that resemble or could lead to

brecciation. To explore the collective motion of particles To explore the collective motion of particles

in fluidised beds in order to determine whether weak rock masses exhibit a sequencewhether weak rock masses exhibit a sequence

of behaviours when a fluid flux is applied to them.


them.

A brecciais a rock

composed of pangular fragments

cemented in a matrix.

The formational processes are not well understood


are not well understood.

Current hypotheses for ypthe formation of breccias suggest the involvement of fluids under high pressurehigh pressure gradients underground.underground.


Brecciation Simulations are carried out to explore what

conditions may produce breccias resembling those observed.

Fracturing behaviour of rock is investigated ith f t fl idi d b d twith reference to a fluidised bed to

determine whether a process like fracturing and fluidisation of a rock could produceand fluidisation of a rock could produce

such formations. A particle code is used because it allows A particle code is used because it allows

for the development of microstructures forming significant cracks in particle

bli1st International FLAC/DEM Symposium 25 August 2008

assemblies.

Rock is modelled as a collection of particles bonded together with a finite cohesion. g


The particle flow code PFC2D from pITASCA incorporates a Fixed Coarse-Grid Fluid Scheme that couples fluid flow and deformation, and allows for large material

displacements.

(Stop Press: Is this also a suitable problem for the(Stop Press: Is this also a suitable problem for the computational fluid dynamics scheme now coupled in

with PFC4.0? Cao & Satoh, speaking now!

Consider also the stress blasting work.)


Fluidised bed15cm x 60cm; 2400 cohesionless particles15cm x 60cm; 2400 cohesionless particles.

Shimizu, Y., 2004

Fluidised Bed

Flow regime maps have been prepared for unfocused and focused

flow situations that detail relationships between

strength, fluid flow rates, and fracturing behaviors

and patterns.


Parameters of InterestParameters of Interest

Cohesive rock strength (parallel bondCohesive rock strength (parallel bond normal and shear strengths).

Fluid flow rate. Fluid mechanism: introduced uniformly or

from a point source.


SolidLift

Fracture & Lift

Disintegrate & Lift

g

t

h

(

P

a

)

o

n

a

r

y

DisintegrateSt

r

e

n

g

S

t

a

t

i

Fluid velocity (m/s)

Generic flow regime map showing the different behavior types according to strength (Pa) and applied fluid velocity (m/s).

Unfocused FlowThe effect of varying fluid velocity

through 0.1m/s, 0.2m/s, 0.4m/s at t t b d t th f 430Pconstant bond strength of 430Pa.

xxxxxxxxx

g

t

h

(

P

a

)

xxxxxx

SolidFracture & LiftDisintegrate &

LiftDi i t t

S

t

r

e

n

g

xxxxxxxxxxDisintegrate

Flow regime map

Unfocused Flow - Behavioural types

Solid rock remainsSolid rock remains unbroken.

Fracturing and liftingxxxxxxxxx

h

(

P

a

)

xxxxxxxxx

S

t

r

e

n

g

t

h

Unfocused Flow - Behavioural types

L lLateral fracturingDi i t tiDisintegration

xxxxxxxxx

t

r

e

n

g

t

h

(

P

a

)

S

t

LateralfracturingFracturing

Solid Disintegratesh

(

P

a

)

S

t

r

e

n

g

t

h


Flow regime map for unfocused flow

Focused flowExample of varying fluid velocity, from

1 5 t 5 5 / t1.5 to 5.5 m/s, at constant bond

strength of 4.3kPa.

Flow i ( P

a

)

regime map for focused

S

t

r

e

n

g

t

h

(

flow.

S

Behavioural types

SolidSolid

F t iFracturing and lifting

Behavioural types

FracturingFracturing leading to fluidisation Instant

fluidisation


Fracture & disintegrateFracture & lift

Solid Disintegrate

n

g

t

h

(

P

a

)

S

t

r

e

n


Flow regime map for focused flow

Heterogeneous bondsBond strengths applied according to a Gaussian distribution about the

mean value of 3.4e6 N, with standard deviation equal to a given percentage of this mean.

Little change is observed in the 1, 5 and 10 % deviationand 10 % deviation models, with only

the 25% model showing any

i ifi tsignificant differences in initial fracture patterning.

Heterogeneous bonds

Prolonged observation of gthe model shows that

there is no enhancement of

fracturing throughout the heterogeneous

assembly compared to y pthe homogeneous. This

modification to bond strengths is ineffective g

at providing a particularly easy path for the fluid to exploit.


p

ObObserve a cohesive strength-

fluid flow rate relationship where

rock has the capacity to break

Disintegrates

capacity to break into breccia-

resembling pieces i t f i Lifts & Fractures Lifts Disintegratesin terms of size and/or shape.

disintegrates & lifts solidlyLifts solidl

Lifts solidl

Lifts solidl

Lifts solidl

Method of fluid introduction affectsintroduction affects migration patterns,

with unfocused flow (left) resulting in

sheet-like uniform motion,

& focused flow (right) encouraging(right) encouraging

turbulence and migration of

fragments to the bottom of the box.

VEINS

n

g

t

h

(

P

a

)

VE NBRECCIASS t r

e

n

Figure demonstrates the consequences of the fluid flow being less than (veins) or greater than (breccias) the fluid g ( ) g ( )

flow predicted through Darcys Law for a given permeability and fluid pressure gradient.

Veins versus BrecciasVeins versus Breccias

It appears feasible to considerIt appears feasible to consider evolutionary paths for RVEs

(representative volume elements) as(representative volume elements) as the rock strength changes with damage as a function of fluid velocity and putas a function of fluid velocity and put deformation (strain? strain-rate?) as a

3rd axis3 axis.


Evolutionary paths

VEINS

m

a

g

e

g

t

h

+

d

a

m

BRECCIASo ck

s

t

r

e

n

g

BRECCIAS

R

o

No With

unsaturated

saturated


No Fluid

With Fluid

Conclusions The purpose of the study was to find conditions The purpose of the study was to find conditions that would cause fracturing into breccia-resembling pieces, & result in transport/reorientation of these p , p

pieces. What is gained from these unconstrained models

is an illustration of the distribution of fracture patterns of the assembly for a given range of flow

rates and cohesive strengths Mechanisms forrates and cohesive strengths. Mechanisms for completely fracturing a rock using introduced fluid

are not revealed. Some mixing and reorientation of fragments was

observed with the use of fluid from a point source; this could be relevant to breccias that showthis could be relevant to breccias that show

evidence of transport.

Further ConclusionsThe preparation of flow regime maps is

fundamental to the interpretation offundamental to the interpretation of breccia textures in the field since one can position a particular texture within p pa rock strength/fluid velocity/porosity

field on the map. The geological applications are

considerable; development of this area would benefit mineral formation

predictions and understanding of fracture formation


fracture formation.

Thank youThank you


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27-Ord