Upscaling Quantifying alteration in creating geothermal ... › EGU2020 ›...
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0.1 0.2 0.3 0.4 0.5 0 connected porosity [-] 0.6 10 -15 matrix permeability [m 2 ] 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 Upscaling laboratory measurements: Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources Michael Heap 1 ([email protected] @LDR_Strasbourg) Darren Gravley 2 , Ben Kennedy 2 , H. Albert Gilg 3 , Liz Bertolett 2 , and Shaun Barker 4 (1) Géophysique Expérimentale, Institut de Physique de Globe de Strasbourg (UMR 7516 CNRS, Université de Strasbourg/EOST), 5 rue René Descartes, 67084 Strasbourg cedex, France (2) University of Canterbury, Christchurch, New Zealand, (3) Technische Universität München, Munich, Germany, (4) University of Tasmania, Australia W e s h o w u s i n g l a b o r a t o r y p h y s i c a l p r o p e r t y m e a s u r e m e n t s , a n d s o m e s i m p l e u p s c a l i n g c o n s i d e r a t i o n s , t h a t h y d r o t h e r m a l a l t e r a t i o n c a n c r e a t e e c o n o m i c a l l y v i a b l e g e o t h e r m a l a n d e p i t h e r m a l m i n e r a l ( A u - A g ) r e s o u r c e s 2 Experimental materials 2 Experimental methods Porosity measured using a helium pycnometer Permeability measured using nitrogen gas at a con�ining pressure of 1 MPa at at room temperature Uniaxial compressive strength measured using a uniaxial loadframe Young's modulus calculated from stress-strain data 3 Results 74% glass 21% plagiolcase 5% quartz 4 Discussion (b) (a) permeant gas (N 2 ) confining gas (N 2 ) 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% adularia 47% quartz 23% plagioclase 6% quartz 29% cristobalite 42% smectite 86% glass 9% plagioclase 5% quartz 1 Introduction Highly-altered, dense rock close to the Ohakuri Dam Unaltered and unlithi�ied parent material Large-scale hydrothermal convection, driven by magmatic heat, can create economically viable geothermal and epithermal mineral resources. Geothermal energy exploitation is most ef�icient at high temperatures and high �low rates and requires a high-permeability reservoir and a low-permeability cap. High-grade epithermal deposits typically form in high-�low (high permeability) zones and, in particular, when the �low of hydrothermal �luids is focussed within fractures. Our goal here is to understand, from a physical property perspective, how geothermal and epithermal mineral resources can develop in an ignimbrite. Our case study site is the Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand), which hosts a palaeo-hydrothermal system that is now accessible for sampling. We provide physical property measurements for a range of variably altered samples and show how alteration can provide both a permeable reservoir and a low-permeability cap. Hydrothermal alteration (silici�ication) signi�icantly reduces matrix permeability Slight alteration or alteration to smectite does not signi�icantly change matrix permeability Our results also show that silici�ication increases the propensity for permeability-enhancing fracture formation. Indeed, we see many fractures in the highly-altered deposit, and essentially no fractures in the unaltered, parent material We use a simple two-dimensional model that considers �low in parallel layers (see Heap and Kennedy, 2016) to upscale our laboratory data We �ind that highly-altered rock masses are more permeable (despite their low matrix permeability) than moderately-altered rock masses or rock masses characterised by smectite alteration We therefore show, from a rock physical property perspective, how hydrothermal alteration can produce a high-permeability reservoir and a low-permeability cap required for a viable geothermal resource and the focussed �low required for a viable epithermal mineral resource 0.1 0.2 0.3 0.4 0.5 0 connected porosity [-] 0.6 0.1 0.2 0.3 0.4 0.5 0 total porosity [-] 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 axial stress [MPa] 0.2 0.4 0.6 0 0.8 1.0 1.2 1.4 1.6 axial strain [%] 2A 2D TF3 2C TF2 2B 0.1 0.2 0.3 0.4 0.5 0 connected porosity [-] 0.6 uniaxial compressive strength [MPa] 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 0.5 0 connected porosity [-] 0.6 Young's modulus [GPa] 0 2 4 6 8 10 12 14 16 18 20 highly altered (quartz/adularia) highly altered (clay) slightly altered 1 2 3 4 5 0 axial strain [%] 6 7 120 100 80 60 40 20 0 differential stress [MPa] Peff = 20 MPa 5 MPa 5 MPa highly altered (2A) slightly altered (2B) 5 10 15 0 fracture density [m -1 ] 20 10 -15 equivalent permeability [m 2 ] 10 -14 10 -13 10 -12 Pc = 1 MPa 10 -16 highly altered (2A) slightly altered (2B) 2A fracture density surface highly altered (clay) low fracture density lower permeability (~10 -14 m 2 ) highly altered (silicified) high fracture density higher permeability (~10 -13 m 2 ) S N channelised flow Maps showing the location of the Ohakuri ignimbrite (Gravley et al., 2007; doi: 10.1130/B25924.1) Permeameter (modi�ied from Heap and Kennedy, 2016; doi: 10.1016/j.epsl.2016.05.004) bottom plate LVDT for measuring axial displacement actuator piston sample perspex casing Uniaxial loadframe (Heap et al., 2020; doi: 10.1016/j.jvolgeores.2019.106684) There is no isolated porosity in the studied materials Permeability is reduced by up to four orders of magnitude as a result of alteration Young's modulus increases by up to a factor of ten as a result of alteration Uniaxial compressive strength increases by up to a factor of ten as a result of alteration We also performed triaxial deformation experiments Highly-altered samples are brittle over a range of pressures (depths) Slightly-altered samples are ductile at low pressure (depth) Large fractures in the highly-altered deposit Photo credit: M.J. Heap Heap, M. J., Gravley, D. M., Kennedy, B. M., Gilg, H. A., Bertolett, E., & Barker, S. L. (2020). Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: The Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand). Journal of Volcanology and Geothermal Research, 390, 106703 doi: 10.1016/j.jvolgeores.2019.106703 Unless otherwise stated, all images are from:
Transcript of Upscaling Quantifying alteration in creating geothermal ... › EGU2020 ›...
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Upscalinglaboratorymeasurements:QuantifyingtheroleofhydrothermalalterationincreatinggeothermalandepithermalmineralresourcesMichaelHeap1([email protected]@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
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(a)pe
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pressure transducer
gas flowmeter
valve
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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
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S Nchannelised flow
MapsshowingthelocationoftheOhakuriignimbrite(Gravleyetal.,2007;doi:10.1130/B25924.1)
Permeameter(modi�iedfromHeapandKennedy,2016;
doi:10.1016/j.epsl.2016.05.004)
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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:
