upscaling quantifying alteration in creating geothermal ... › egu2020 ›...

0.1 0.2 0.3 0.4 0.50

connected porosity [-]0.6

10-15

mat

rix p

erm

eabi

lity

[m2 ]

10-14

10-13

10-12

10-11

Pc = 1 MPa

10-16

10-17

10-18

highly altered (quartz/adularia)

highly altered (clay)

slightly altered

unaltered

Upscalinglaboratorymeasurements:QuantifyingtheroleofhydrothermalalterationincreatinggeothermalandepithermalmineralresourcesMichaelHeap1(heap@unistra.fr@LDR_Strasbourg)DarrenGravley2,BenKennedy2,H.AlbertGilg3,LizBertolett2,andShaunBarker4(1)GéophysiqueExpérimentale,InstitutdePhysiquedeGlobedeStrasbourg(UMR7516CNRS,UniversitédeStrasbourg/EOST),5rueRenéDescartes,67084Strasbourgcedex,France(2)UniversityofCanterbury,Christchurch,NewZealand,(3)TechnischeUniversitätMünchen,Munich,Germany,(4)UniversityofTasmania,Australia

Weshowusinglaboratoryphysicalpropertymeasurements,andsomesimpleupscalingconsiderations,thathydrothermalalterationcancreateeconomicallyviablegeothermalandepithermalmineral(Au-Ag)resources

2 Experimentalmaterials

2 Experimentalmethods

PorositymeasuredusingaheliumpycnometerPermeabilitymeasuredusingnitrogengasatacon�iningpressureof1MPaatatroomtemperature

Uniaxialcompressivestrengthmeasuredusingauniaxialloadframe

Young'smoduluscalculatedfromstress-straindata

3 Results

74%glass21%plagiolcase5%quartz

4 Discussion

(b)

(a)pe

rme

an

t g

as

(N2)

con

finin

gg

as

(N2)

pressure transducer

gas flowmeter

valve

3

permeant inlet

upstream endcap

spreader plate

pressure vessel

confining fluid inlet

sample

viton jacket

downstream endcap

permeant outlet

53%adularia47%quartz

23%plagioclase6%quartz

29%cristobalite42%smectite

86%glass9%plagioclase5%quartz

1 Introduction

Highly-altered,denserockclosetotheOhakuriDam Unalteredandunlithi�iedparentmaterial

Large-scalehydrothermalconvection,drivenbymagmaticheat,cancreateeconomicallyviablegeothermalandepithermalmineralresources.Geothermalenergyexploitationismostef�icientathightemperaturesandhigh�lowratesandrequiresahigh-permeabilityreservoirandalow-permeabilitycap.High-gradeepithermaldepositstypicallyforminhigh-�low(highpermeability)zonesand,inparticular,whenthe�lowofhydrothermal�luidsisfocussedwithinfractures.

Ourgoalhereistounderstand,fromaphysicalpropertyperspective,howgeothermalandepithermalmineralresourcescandevelopinanignimbrite.OurcasestudysiteistheOhakuriignimbrite(TaupōVolcanicZone,NewZealand),whichhostsapalaeo-hydrothermalsystemthatisnowaccessibleforsampling.Weprovidephysicalpropertymeasurementsforarangeofvariablyalteredsamplesandshowhowalterationcanprovidebothapermeablereservoirandalow-permeabilitycap.

Hydrothermalalteration(silici�ication)signi�icantlyreducesmatrixpermeability

Slightalterationoralterationtosmectitedoesnotsigni�icantlychangematrixpermeabilityOurresultsalsoshowthatsilici�icationincreasesthepropensityforpermeability-enhancingfractureformation.Indeed,weseemanyfracturesinthehighly-altereddeposit,andessentiallynofracturesintheunaltered,parentmaterialWeuseasimpletwo-dimensionalmodelthatconsiders�lowinparallellayers(seeHeapandKennedy,2016)toupscaleourlaboratorydata

We�indthathighly-alteredrockmassesaremorepermeable(despitetheirlowmatrixpermeability)thanmoderately-alteredrockmassesorrockmassescharacterisedbysmectitealteration

Wethereforeshow,fromarockphysicalpropertyperspective,howhydrothermalalterationcanproduceahigh-permeabilityreservoirandalow-permeabilitycaprequiredforaviablegeothermalresourceandthefocussed�lowrequiredforaviableepithermalmineralresource

0.1 0.2 0.3 0.4 0.50

connected porosity [-]0.6

0.1

0.2

0.3

0.4

0.5

0

tota

l por

osity

[-]

0.6

highly altered - quartz-adularia (0 wt.% glass)

highly altered - clay (0 wt.% glass)

slightly altered (74 wt.% glass)

1:1

0

10

20

30

40

50

60

70

80

axia

l str

ess

[MP

a]

0.2 0.4 0.60 0.8 1.0 1.2 1.4 1.6

axial strain [%]

2A

2D

TF32C

TF2

2B

0.1 0.2 0.3 0.4 0.50

connected porosity [-]0.6

unia

xia

l com

pres

sive

str

eng

th [M

Pa]

0

10

20

30

40

50

60

70

80

90

100

b)

a)

highly altered (quartz/adularia)

highly altered (clay)

slightly altered

unaltered

0.1 0.2 0.3 0.4 0.50

connected porosity [-]0.6

You

ng's

mod

ulu

s [G

Pa]

0

2

4

6

8

10

12

14

16

18

20

highly altered (quartz/adularia)

highly altered (clay)

slightly altered

1 2 3 4 50

axial strain [%]

6 7

120

100

80

60

40

20

0

diffe

rent

ial s

tres

s [M

Pa]

Peff = 20 MPa

5 MPa

5 MPa

highly altered (2A)

slightly altered (2B)

5 10 150

fracture density [m-1]20

10-15

equi

vale

nt p

erm

eabi

lity

[m2 ]

10-14

10-13

10-12

Pc = 1 MPa

10-16

highly altered (2A)

slightly altered (2B)

2A fracture den

sity

surface

highly altered (clay)

low fracture density

lower permeability (~10-14 m2)

highly altered (silicified)

high fracture density

higher permeability (~10-13 m2)

S Nchannelised flow

MapsshowingthelocationoftheOhakuriignimbrite(Gravleyetal.,2007;doi:10.1130/B25924.1)

Permeameter(modi�iedfromHeapandKennedy,2016;

doi:10.1016/j.epsl.2016.05.004)

bottomplate

LVDTformeasuringaxial

displacement

actuatorpiston

sample

perspexcasing

Uniaxialloadframe(Heapetal.,2020;

doi:10.1016/j.jvolgeores.2019.106684)

ThereisnoisolatedporosityinthestudiedmaterialsPermeabilityisreducedbyuptofourordersofmagnitudeasaresultofalteration

Young'smodulusincreasesbyuptoafactoroftenasaresultofalteration

Uniaxialcompressivestrengthincreasesbyuptoafactoroftenasaresultofalteration

Wealsoperformedtriaxialdeformationexperiments

Highly-alteredsamplesarebrittleoverarangeofpressures(depths)

Slightly-alteredsamplesareductileatlowpressure(depth)

Largefracturesinthehighly-altereddepositPhotocredit:M.J.Heap

Heap,M.J.,Gravley,D.M.,Kennedy,B.M.,Gilg,H.A.,Bertolett,E.,&Barker,S.L.(2020).Quantifyingtheroleofhydrothermalalterationincreatinggeothermalandepithermalmineralresources:TheOhakuriignimbrite

(TaupōVolcanicZone,NewZealand).JournalofVolcanologyandGeothermalResearch,390,106703

doi:10.1016/j.jvolgeores.2019.106703

Unless otherwise stated, all images are from: