Mechanical and physical properties of hydrothermally ...

Journal of Volcanology and Geothermal Research 288 (2014) 76–93

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Mechanical and physical properties of hydrothermally altered rocks, Taupo Volcanic Zone, New Zealand L.D. Wyering a,⁎, M.C. Villeneuve a, I.C. Wallis b, P.A. Siratovich a, B.M. Kennedy a, D.M. Gravley a, J.L. Cant a a b

Department of Geological Sciences, University of Canterbury, P O Box 4800, Christchurch 8140, New Zealand Mighty River Power, 283 Vaughan Road, P O Box 245, Rotorua 3040, New Zealand

a r t i c l e

i n f o

Article history: Received 15 August 2014 Accepted 8 October 2014 Available online 29 October 2014 Keywords: Geothermal Mechanical properties Physical properties Uniaxial compressive strength Hydrothermal alteration

a b s t r a c t Mechanical characterization of hydrothermally altered rocks from geothermal reservoirs will lead to an improved understanding of rock mechanics in a geothermal environment. To characterize rock properties of the selected formations, we prepared samples from intact core for non-destructive (porosity, density and ultrasonic wave velocities) and destructive laboratory testing (uniaxial compressive strength). We characterised the hydrothermal alteration assemblage using optical mineralogy and existing petrography reports and showed that lithologies had a spread of secondary mineralisation that occurred across the smectite, argillic and propylitic alteration zones. The results from the three geothermal fields show a wide variety of physical rock properties. The testing results for the non-destructive testing shows that samples that originated from the shallow and low temperature section of the geothermal field had higher porosity (15 – 56%), lower density (1222 – 2114 kg/m3) and slower ultrasonic waves (1925 – 3512 m/s (vp) and 818 – 1980 m/s (vs)), than the samples from a deeper and higher temperature section of the field (1.5 – 20%, 2072 – 2837 kg/m3, 2639 – 4593 m/s (vp) and 1476 – 2752 m/s (vs), respectively). The shallow lithologies had uniaxial compressive strengths of 2 – 75 MPa, and the deep lithologies had strengths of 16 – 211 MPa. Typically samples of the same lithologies that originate from multiple wells across a field have variable rock properties because of the different alteration zones from which each sample originates. However, in addition to the alteration zones, the primary rock properties and burial depth of the samples also have an impact on the physical and mechanical properties of the rock. Where this data spread exists, we have been able to derive trends for this specific dataset and subsequently have gained an improved understanding of how hydrothermal alteration affects physical and mechanical properties. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Alteration produces significant changes in almost all the mineralogical, chemical and physical properties of rocks (Lumb, 1983; Arel and Tugrul, 2001; Begonha and Sequeira Braga, 2002; Arikan et al., 2007). Two types of alteration are observed in volcanic environments: weathering and hydrothermal (Ceryan et al., 2008; Yıldız et al., 2010; Pola et al., 2012). Weathering occurs when the Earth’s atmosphere and waters interact with the rock system; while in a hydrothermal context alteration is caused by the movement of hot, dissolved-ion rich fluids through reservoir rocks causing dissolution, mineral deposition, clay mineral formation producing secondary mineralisation (Frank, 1995; Finizola et al., 2002; Hurwitz et al., 2002; Hase et al., 2005; Pola et al., 2012, 2014). Several factors, in a geothermal field, affect the formation of alteration minerals; pressure, permeability, rock type, temperature, duration, and these minerals vary in relative abundance both within a system and between systems (Browne, 1978, 1989; ⁎ Corresponding author at: Department of Geological sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. Tel.: +64 27 427 1955. E-mail address: [email protected] (L.D. Wyering).

http://dx.doi.org/10.1016/j.jvolgeores.2014.10.008 0377-0273/© 2014 Elsevier B.V. All rights reserved.

Mehegan et al., 1982; Cox and Browne, 1998; Robb, 2005; Pola et al., 2012). Relationships between strength and porosity, density or mineralogy for a specific rock formation (Chang et al., 2006), and the influence of secondary mineralisation on the physical and mechanical properties of rock has been studied by many authors (Ulusay et al., 1994; Kahraman et al., 2005; Sousa et al., 2005; Chang et al., 2006; Tamrakar et al., 2007; Ceryan et al., 2008; Yıldız et al., 2010; Rajesh Kumar et al., 2011; Pola et al., 2012, 2014). Studies to this extent are of limited interest to the conventional geothermal industry because few rocks in a hot, dynamic, liquid and/or steam filled reservoir are unaltered or exposed at the surface. However, the results/relationships developed are mainly for sedimentary, granitic and metamorphic rocks and cannot be applied ubiquitously to all lithologies, especially hydrothermally altered volcanic rocks at depth. Only recently have studies investigated the physical and mechanical properties of volcanic rocks (Vinciguerra et al., 2005; Smith et al., 2009; Nara et al., 2011; Pola et al., 2012, 2014; Heap et al., 2014). However, these studies are focused on site-specific lithologies and on rock properties that are not directly relevant to the sub-surface reservoir rocks of the Taupo Volcanic Zone (TVZ) geothermal fields, which are examined in this study. Mass transfer resulting primarily

L.D. Wyering et al. / Journal of Volcanology and Geothermal Research 288 (2014) 76–93

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Fig. 1. Digital elevation map with the geologic setting of geothermal activity in the Taupo Volcanic Zone (TVZ), showing the positions of geothermal systems (red, purple, orange), the active and inferred caldera boundaries and the Taupo Rift. The yellow names represent the geothermal fields that are being addressed in this study. Abbreviations are named calderas: KA = Kapenga, MO = Mangakino, OH = Ohakuri, OK = Okataina, RE = Reporoa, RO = Rotorua, TA = Taupo, WH = Whakamaru. The map is split up into the main volcanic activity is the TVZ and outlined by the boundary of the young TVZ (b0.34 Ma) (Adapted from Wilson et al., 1995; Bibby et al., 1995; Rowland and Sibson, 2004; Kissling and Weir, 2005; Rowland and Simmons, 2012).

from dissolution of reservoir rocks minerals and precipitation of alteration products (Ferry, 1979; Giggenbach, 1984; Henneberger and Browne, 1988; Simmons and Browne, 2000; Pochee, 2010; Esmaeily et al., 2012) cause the bulk geochemistry and mineralogy of the reservoir rocks to differ from its initial primary rock mineralogy, leading to a partial or wholesale changes in the rock composition and subsequent physical and mechanical behaviour (Lumb, 1983; Arel and Tugrul, 2001; Begonha and Sequeira Braga, 2002; Robb, 2005; Arikan et al., 2007; Yıldız et al., 2010). The aims of this paper are: to produce a detailed description of the altered lithologies used in this study and to physically and mechanically characterize hydrothermally altered volcanic rocks. We then examine the relationships between physical and mechanical properties and lithology to quantify the effect of hydrothermal alteration on these lithologies. The purpose of this analysis is to support on-going research into the development of a new geotechnical model to aid in the understanding of the primary controls of rock properties in a young igneous geothermal system, and how these properties may be utilised for bit selection in drilling (Wyering et al., in Preparation). 2. Geological Setting The rocks in this study originate from the Taupo Volcanic Zone (TVZ), which is located at the southern end of the Tonga-Kermadec arc in the central North Island of New Zealand, in a 300 km long (200 km on land) and 60 km wide belt, defined by caldera structural

boundaries, vent positions and geothermal fields (Fig. 1: Wilson et al., 1995; Rowland and Sibson, 2001), and in its modern form coincides with a structurally and magmatically segmented rift system (Taupo rift). The oblique subduction of oceanic crust from the Pacific plate beneath the Indian-Australian plate caused the back arc/arc basin producing the TVZ (Cole, 1990; Bibby et al., 1995, 2008; Darby et al., 2000; Rowland and Sibson, 2004; Cole and Spinks, 2009; Seebeck et al., 2010; Rowland et al., 2012). The geothermal systems in the TVZ are developed by the transportation of meteoric waters, which percolate down through fractures, faults and textures in lithologies, and then rise when heated by deep magma or intrusive bodies (Henneberger and Browne, 1988; Hochstein, 1995; Rowland and Sibson, 2004; Rowland and Simmons, 2012). These circulating geothermal fluids become rich in dissolved minerals, as they percolate through the stratigraphy (Henneberger and Browne, 1988) and precipitate minerals in the reservoir rocks (Goff and Janik, 2000). The samples were taken from core sourced from the Ngatamariki, Rotokawa and Kawerau geothermal fields. Ngatamariki and Rotokawa comprise volcanic and sedimentary lithologies overlying a Mesozoic metasedimentary (greywacke) basement. The shallow formations contain sediments, tuffs and tuffaceous breccias, siltstones, ignimbrites, and brecciated/tuffaceous rhyolite lava. Deeper formations contain the Tahorakuri Formation, which is divided into two sections: a mix of sedimentary layers, and tuff or pyroclastic volcaniclastics with andesitic lava or breccia on top of the basement rock. Ngatamariki also contains an intrusive material in the northern end of the field (Fig. 2: Krupp

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Fig. 2. Generalised stratigraphy of the three geothermal fields, approximate thickness of each unit and generalised alteration zones (argillic – blue and propylitic - green); the highlighted units are tested in this study (adapted from Rae et al., 2009; Milicich et al., 2013; Chambefort et al., 2014).

and Seward, 1987; Browne, 1989; Browne et al., 1992; Brotheridge et al., 1995; Arehart et al., 2002; Rae, 2007; Chambefort et al., 2014). At Kawerau the shallow formations consist of a mix of extrusive rhyodacite, ignimbrites, sedimentary lithologies mixed with breccia tuffs and rhyolite lava. The deep formations comprise of welded tuffs, sedimentary tuffacious breccia, andesite lavas, and sedimentary tuffs overlying the basement rock (Fig. 2: Milicich et al., 2011, 2013). 3. Characterization of samples We used polarized light microscopes, utilizing plane polarized light (PPL) and cross polarized light (CPL) to identify the primary and

secondary minerals based on optical properties in each of the samples to recognize the alteration zones in which the lithologies originated; along with identifying veining, vein infill, microfractures and natural planes of weaknesses. We noted the primary and secondary textures in the samples if they could be identified. There are three zones of alteration expected in a conventional geothermal system: smectite, argillic and propylitic alteration (Fig. 3: Robb, 2005; Stimac et al., 2008; Cumming, 2009; Esmaeily et al., 2012). Smectite alteration is characterized by the formation of smectite clay and low to atmospheric temperature alteration minerals. Argillic alteration is characterized by the formation of illite and other low to moderate temperature minerals (Robb, 2005; Lutz et al., 2010, 2011;


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3.1. Thin section Mineralogy

Fig. 3. Conceptual model of a conventional, hot, liquid dominated geothermal field. The model has been split into the alteration zones typical for a geothermal field, with temperature profiles and surface expressions (adapted from Stimac et al., 2008; Cumming, 2009).

Table 1 Geology from the three geothermal fields split into their respective sections of the field with the type of alteration present in each sample. Shallow Formations

Deep Formations

Welded dense rhyolitic ignimbrite – Tahorakuri Formation, Te Teko Formation (argillic to propylitic alteration). Andesite Lavas and breccia – Rotokawa Andesite, Kawerau Andesite, Andesite Breccia (argillic to propylitic alteration). Crystal rich quartz bearing siltstone/ Porphyritic and equigranular tonalite sandstone – Tahuna Formation intrusive – Tonalite Igneous intrusions (argillic alteration). (argillic alteration) Crystal poor pumacious rhyolitic Ignimbrite – Matahina Ignimbrite (smectite alteration). Rhyolitic Lava – Caxton Formation (argillic alteration).

Pola et al., 2012); and propylitic alteration has high temperature minerals resembling chlorite, epidote and quartz, with lesser quantities of calcite and albite (Browne, 1978; Robb, 2005). To understand how each lithology has been impacted by the alteration in their environments, we divided the formations into shallow and deep lithologies based on secondary mineralisation, burial depths and primary lithologies (Table 1).

3.1.1. Ngatamariki Thin Section mineralogy The Andesite Breccia is an intensely altered pale green to dark green clast supported breccia. Under the polarized light microscope we observed that the breccia contains clasts of greywacke, granite, andesitic lava, rhyolite lava and siltstone. It has undergone propylitic alteration, as shown by the main alteration assemblage of epidote, chlorite and quartz, with minor calcite, pyrite, albite, adularia, and titanium oxide. The breccia contains small veins (b1-2 mm) filled with calcite, epidote, and quartz (Fig. 4). The fractures in the Andesite Breccia samples are predominately transgranular (affecting more than one mineral), however, some samples contain intragranular fractures (within a mineral). The Tahorakuri Formation contains strongly altered light greenish/ greyish grey-white ignimbrites, although one core is an intensely altered, light grey breccia with abundant clay. All of the Tahorakuri Formation samples are predominately altered to clay and fine-grained quartz. However, one core only contains these two minerals along with quartz – pyrite veins. The alteration products in the samples are quartz, calcite, chlorite, albite, adularia, and wairakite, pyrite, epidote, muscovite, and titanium oxide. The abundance of each of these minerals varies between samples with the top three minerals typically being quartz, clay and a split between calcite and epidote. The veins in the samples mainly contain quartz, pyrite and some rare smaller calcite or illite veins (b 0.2-0.3 mm) (Fig. 5). The Tahorakuri Formation has undergone argillic to propylitic alteration, which is evident in the high percentages of illite and chlorite in the samples. The fractures in the Tahorakuri Formation are transgranular. The intrusive body at Ngatamariki is a moderately altered light and/or dark coloured porphyritic and equigranular tonalite. In the polarized light microscope studies the light/white tonalite contains primary plagioclase and quartz phenocrysts in a fine-grained matrix. The secondary mineralisation consists of common calcite, muscovite, minor albite and rare titanium oxide, pyrite and quartz. The dark/grey tonalite has the same primary mineralogy and contains minor secondary pyrite, albite, calcite, chlorite, muscovite, anhydrite and rare titanium oxide (Fig. 6) The tonalite samples have experienced argillic alteration, as shown by the presence of illite alteration in large abundance, with transgranular fractures throughout the samples. 3.1.2. Rotokawa Thin Section Mineralogy The Rotokawa Andesite is a moderately to intensely altered lava/ breccia. The matrix and primary minerals (pyroxene and plagioclase) have been altered predominately to calcite, chlorite, quartz and minor to common epidote or hematite with minor to rare albite, adularia, titanium oxide and pyrite (Fig. 7). Due to the depth of burial and resulting high temperatures, the Rotokawa Andesite appears to have experienced intense propylitic alteration as shown by the presence of illite, chlorite, albite and calcite. Additionally, due to brittle behaviour of the Rotokawa

Fig. 4. Photomicrographs of Andesite Breccia thin sections. A (PPL) and B (CPL) = Porphyritic andesite containing epidote (Epi), titanium oxide (TiO), quartz (Qtz), chlorite (Chl), albite (Alb) and clay. The right hand image is a vug that has been infilled with first chlorite, then epidote and finally quartz.

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Fig. 5. Photomicrographs of different samples from the Tahorakuri Formation. A = Crystal lithic bearing volcaniclastic rock that has been altered by clay (illite), calcite, chlorite, calcite, titanium oxide and quartz. B = Pumiceous lithic tuff that has been altered by secondary epidote, quartz, clay, chlorite and albite. The groundmass of the sample contains fine-grained quartz, clay and chlorite. The fracture in the samples has been infilled by chlorite and the albite has altered primary plagioclase. C = Intense clay-altered volcanic rock with pervasive clay and quartz replacement (F-G Qtz, fine-grained quartz). The sample has intense silicification, with disseminated pyrite, quartz and clay. D = Clay altered ignimbrite with sections of wairakite (Wai) and a pyrite (Py) crystal, which is overgrown by radial clinozoisite (Clin). The thin section has illite alteration in the groundmass on the right and fine-grained quartz on the left.

Andesites the fractures found in the thin sections are both intragranular and transgranular.

3.1.3. Kawerau Thin Section Mineralogy The Matahina Ignimbrite is a moderately altered light brown to light greyish cream ignimbrite. The primary minerals of plagioclase and quartz have been altered to abundant clay, quartz, titanium oxide, calcite, albite and pyrite, showing that the samples have undergone smectite to argillic alteration. Glass shards are still evident in some of the welded ignimbrite samples (Fig. 8). Samples in the Matahina Ignimbrite have intragranular fractures. The Caxton Formation is white and yellow altered rhyolitic lava that is highly fractured with transgranular defects and predominately

consists of clay, quartz, anhydrite, wairakite and titanium oxide, which is typically present in argillic alteration (Fig. 9). The Kawerau Andesite is a moderately to intensely altered pale to dark green andesitic lava that has been altered to chlorite, calcite, albite, illite, titanium oxide and quartz. A large number of calcite veins occur in transgranular fractures and vugs in-filled with calcite, quartz and chlorite are seen in the sample illustrated in (Fig. 10) The Kawerau andesite has undergone argillic to propylitic alteration. The Tahuna Formation is a dark grey moderately to intensely altered mudstone, siltstone or tuffaceous sandstone (Fig. 11). The primary minerals in the tuffaceous pebbly sandstones (plagioclase and quartz) have undergone argillic to propylitic alteration with calcite, pyrite, chlorite, titanium oxide, illite, quartz and albite. The mudstone/siltstone consists of quartz and plagioclase as its primary minerals that have been strongly

Fig. 6. Photomicrographs of Tonalite (porphyritic and equigranular). A = Embayed quartz with local calcite, muscovite and illite and pyrite (opaque). The black arrow is pointing out prominent mymekitic textures. The red arrow is pointing out relict plagioclase altered by illite and calcite. B = Tonalite altered by anhydrite (anh), calcite and pyrite. Primary quartz is seen in the top right hand corner.


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Fig. 7. Photographs of porphyritic andesite lava with phenocrysts of plagioclase (plag) and pyroxene (pyr). A and B (CPL/PPL) = A vug in the centre of the thin section has been infilled with chlorite. The plagioclase minerals have been replaced by secondary chlorite, calcite and quartz. The pyroxene minerals have been replaced by calcite. The groundmass consists of finegrained quartz and illite. C = A CPL image of plagioclase minerals replaced by albite, calcite and chlorite, with a majority of the samples being replaced by quartz, illite, calcite, chlorite, titanium oxide and minor epidote. D = A PPL image of what looks like a illite vein around the bottom end of a vug that has been infilled by quartz and chlorite.

altered to argillic alteration with clay, quartz, calcite, albite, adularia and titanium oxide. The Te Teko Formation is a pale cream crystal lithic tuff with specks of green. It has undergone intense argillic to propylitic alteration where

the primary minerals (plagioclase, ferromagnesian minerals and quartz) have been replaced by chlorite, clay, calcite, pyrite, quartz, albite, wairakite and titanium oxide. The samples show strong veining with clay and a few calcite veins (Fig. 12).

Fig. 8. Photomicrographs of the Matahina Ignimbrite. All images show that the ignimbrites groundmass is derived of clay (smectite), fine-grained quartz and same of the samples (A) have relict glass shards. The primary minerals, plagioclase and quartz and been replaced by albite, calcite, clay and quartz.

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Fig. 9. Photomicrograph of the Caxton Formation rhyolitic lava. The thin sections illustrate the common clay and fine grained quartz minerals in the groundmass, with primary quartz being replaced by anhydrite and titanium oxide.

Fig. 10. Photomicrograph of the Kawerau Andesite lava. In both samples the primary mineral (plagioclase) has been replaced by albite, quartz, chlorite and calcite. The samples also have a large amount of veining present that is predominately infilled by calcite (A). In image B the red boxes outline amygdales in the andesite that are being infilled y secondary quartz that is needle shaped, and a few sections of calcite.

4. Physical and Mechanical Properties

4.2. Ultrasonic pulse Velocities

4.1. Porosity and Density Testing

We determined the compressional (vp) and shear wave (vs) velocities using the GCTS Testing Systems Computer Aided Testing System Ultrasonic Velocity Testing System (CATS ULT-100) with axial P and S wave piezoelectric crystals inside platens (Table 3). The pulse frequency rate was 20 MHz, and the resonance frequency of the transducer was 900 kHz; 48 waveforms were captured during each cycle. Petroleum jelly was used as a coupling agent along with an axial load of 5 MPa to provide good acoustic coupling between the platens and the core sample and ensures a consistent waveform along the sample. The selected vp and vs were used to derive the dynamic Poisson’s ratio and Young’s moduli for each sample using Eq. (1) and Eq. (2), respectively (Christaras et al., 1994). This process was then repeated 3–5 times to

We used the suggested method from the International Society of Rock Mechanics (ISRM) (Ulusay and Hudson, 2007a,b) to determine the effective porosity (ηe) and density of the hydrothermally altered samples using cylindrical cores (Table 2). We used dichloromethane to determine the saturated density of any hydrothermally altered rocks with lower than ~150 °C alteration (smectite/argillic alteration) as this non-polar saturation fluid did not activate the swelling clays (Frolova et al., 2010). The mean effective porosity increases as the density decreases and effective porosity of the deep lithologies is lower than the shallow lithologies, while the density is greater in the deep lithologies (Fig. 13).

Fig. 11. Photographic of The Tahuna Formation. A = mudstone sample being altered to calcite (light brown) with quartz and clay present. B = CPL image of the tuffaceous pebbly sandstone with the minerals being altered to calcite, albite, titanium oxide, chlorite and quartz. The tuffaceous sandstone has large vugs that have predominately not started to be infilled by secondary minerals.


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Fig. 12. Photomicrographs of the Te Teko Formation. A = CPL image of the crystal lithic tuff with the primary minerals being altered to calcite, albite, titanium oxide, chlorite and quartz. Vugs are present in the sample and have not been infilled by secondary minerals. B = CPL image of a crystal lithic tuff that has the same alteration as A, but this thin section has calcite infilled fractures that are cutting the sample and primary quartz that is being dissolved.

produce a range of compressional and shear wave velocities from which to draw an average. Dynamic Poisson’s Ratio Calculation Vd ¼

5. Discussion

2 2 2 2 vp −2 vs =2 vp –vs

ð1Þ

Dynamic Young’s Modulus Calculation Ed ¼

2 2 2 2 2 ρ vs 3vp −4vs = vp −vs

ð2Þ

Where vp and vs are in m/s and density is in kg/m3 we obtain the Young’s Modulus in Pa (Ed) and the dynamic Poisson’s ratio (νd). The compressional wave velocity increases with the shear wave velocity (Fig. 14). Shallow lithologies have slower velocity end than the deep lithologies, with a large overlap of the two end members. 4.3. Uniaxial Compressive Strength Testing Uniaxial compressive strength (UCS) tests are one of the standard methods used to determine the strength of a sample and a key variable in our study. The samples sourced from the geothermal fields had a mean diameter of 39.6 mm and were cut and ground to within the length to diameter ratio of 2:1 to allow for the validation of UCS testing using the ISRM suggested methods (Ulusay and Hudson, 2007a, b) and the American Society for Testing and Materials (ATSM International, 2010). The samples were tested using a Technotest 3000 kN, servocontrolled loading frame and loaded at a constant rate that allowed failure to occur between 5–10 minutes after initial loading. Tokyo Sokki Kenkyujo Co. Ltd (TML) 20 mm strain gauges with a factor of 2.12 were glued to the samples, two axial and two radial. The samples were tested at ambient laboratory temperature and humidity conditions. The results from the UCS tests have a range of strengths from 2

Table 2 Mean and standard deviation of effective porosity and density, ratio for all lithologies from the Ngatamariki, Rotokawa and Kawerau geothermal fields. Results for each sample are in Appendix A, Table A.1. Shallow lithologies

Porosity (%)

Bulk Dry Density (kg/m3)

Number of samples

Rhyolitic ignimbrite Rhyolite lava Sedimentary sandstone and siltstone

32.8 ± 13.1 19.5 ± 0 17.9 ± 2.0

1571.0 ± 320.7 1819.3 ± 0 1960.2 ± 79.2

N =27 N =1 N =13

12.3 ± 5.3 2.4 ± 0.63 5.6 ± 3.9

2325.2 ± 161.5 2651.6 ± 130.1 2561.3 ± 110.7

N =47 N =14 N =29

Deep Lithologies Rhyolitic ignimbrite Intrusive Andesite lava/breccia

to 211 MPa for the three geothermal fields (Table 4). The shallow lithologies have mean strengths lower than the deep lithologies.

The relationships between physical and mechanical rock properties are discussed below with relation to lithology, alteration zones and secondary minerals to supply information on the effects of hydrothermal alteration at depth in rock types that are encountered in conventional geothermal fields. This information will enable the development of geotechnical models that encounter these alteration zones, which can improve exploration or development drilling, through improving drill bit selection, by providing rock property results that enable improved understanding of rock behaviours due to alteration. 5.1. Porosity and density There typically exists a negative relationship between porosity and density and for this dataset there is no systematic deviation from this relationship according to either original composition or alteration type (Fig. 13). Frolova et al. (2010) found that lithologies exposed to high temperature systems have higher density (N2.3 g/cm3), and lower porosity (b15%) than the low temperature systems (density b1.2 g/cm3, and porosity N20%). These results agree with our study where the deep lithologies exposed to high temperatures have a higher density and lower porosity (mean 2.5 g/cm3 and 7%, respectively) than shallow lithologies exposed to low temperature (mean 1.8 g/cm3 and 24%, respectively). Volcanic rocks typically have a wide range of primary matrix porosity, due to the wide variety of depositional processes, producing variations in the original rock textures, grain size distribution, and this initial porosity can be enhanced or reduced by alteration (Ferry, 1979; Giggenbach, 1984; Moon, 1993; Rejeki et al., 2005; Frolova et al., 2010; Esmaeily et al., 2012; Pola et al., 2012, 2014;). Our porosity and density results vary widely within the shallow ignimbrites, even though they all originate from one field and unit (Fig. 13). This variability could be due to some of the samples sourced from the welded section of ignimbrite and others sourced from the semi-welded sections of the same ignimbrite, indicating that the depositional processes have caused a natural variation in the effective porosity and have also caused the large standard deviation in the dataset (Moon, 1993). Variability in the porosity and density could also be caused by mass transfer. Hydrothermal alteration at high temperatures (N200 °C) can typically cause a decrease in porosity, and an increase in density (Frolova et al., 2010), by minerals being deposited and infilling the intercrystal and intergranular micropore space, causing consolidation and hardening of the lithology (Nasimov et al., 2005; Frolova et al., 2010), which could be the case for the deep ignimbrites and explains the small standard deviation. Hydrothermal alteration at low temperatures (b 150 °C) is complex and diverse as it can cause an increase or decrease in the

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Table 3 Mean and standard deviation of compressional and shear wave velocities, dynamic Young’s moduli and dynamic Poisson’s ratio for all lithologies from the Ngatamariki, Rotokawa and Kawerau geothermal fields. Results for each sample are in Appendix A, Table A.1. Shallow lithologies

vp (m/s)

vs (m/s)

Youngs (GPa)

Poisson’s Ratio

Number of samples

Rhyolitic ignimbrite Rhyolite lava Sedimentary sandstone and siltstone

2363 ± 460 2941 ± 0 3187 ± 362

1216 ± 334 1734 ± 0 1790 ± 277

7.4 ± 4.5 14.0 ± 0 16.3 ± 4.3

0.3 ± 0.04 0.23 ± 0 0.27 ± 0.03

N =27 N =1 N =13

Deep Lithologies Rhyolitic ignimbrite Intrusive Andesite lava/breccia

3375 ± 537 3235 ± 334 4165 ± 165

1863 ± 272 1755 ± 191 2478 ± 157

21.8 ± 7.1 21.5 ± 5.8 38.0 ± 6.3

0.27 ± 0.04 0.29 ± 0.02 0.22 ± 0.03

N =47 N =14 N =29

porosity or density depending on the initial primary lithology, pressure, and fluid interaction (Frolova et al., 2010) and would produce a wider range of data and a high standard deviation, as seen with the shallow ignimbrites. The deep ignimbrites have a lower porosity and higher density than the shallow ignimbrites. The depositional history of both the shallow and deep ignimbrites would have been similar; producing welded and unwelded material with a wide, but comparable, range of porosities and densities (Moon, 1993) but as the ignimbrites became buried and exposed to hydrothermal alteration they started to be altered, causing their properties to become less comparable. This shows that although the lithologies started out similar, differing alteration histories and burial depth impacted the resulting porosity and density (Table 2 and Fig. 13). The porosity and density plot (Fig. 13) has been subdivided into smectite, illite and chlorite dominated mineralogy. We see that lithologies containing abundant smectite or illite tend to have a higher porosity and lower density than lithologies that have abundant chlorite. Rejeki et al. (2005) showed that alteration mineral assemblage and clay type play a significant role in enhancing or reducing primary porosity; such that the samples that contain abundant illite should have a lower porosity and higher density than smectite dominated samples. The andesite samples from the three geothermal fields experienced moderate to high temperature alteration, leading to abundant illite and chlorite being present, with the samples from Kawerau having more illite present, while those from Ngatamariki and Rotokawa have abundant chlorite present. Andesites are likely to have had a wide range of primary porosities depending on clast arrangement and depositional history (Dobson et al., 2003). However, in our data, the

samples with an overall high temperature alteration (Ngatamariki and Rotokawa andesites) have similar porosities and densities, while the Kawerau andesites have porosities around 60% higher and densities around 10% lower, which indicates that the alteration, and burial depth matched the trends seen in Nasimov et al. (2005); Frolova et al. (2010), where the porosities of the samples increased and the densities decreased from high temperature alteration to low temperature alteration. Compaction by grain rotation/crushing (which can occur during faulting) and infilling of large primary voids by early alteration minerals both work to reduce porosities. When looking at the Rotokawa Andesite data in Fig. 15 there may be a trend occurring with a small porosity change with depth, as in Stimac et al. (2004). However, it is important to note that typically the deep lithologies in our dataset have low primary porosities when deposited, compared to the shallow lithologies. A clear compaction/diagenesis trend cannot be applied across lithologies, and trends should only be assigned to individual lithologies. 5.2. Ultrasonic Wave Velocities Ultrasonic wave velocities are one of the non-destructive geophysical methods used by engineers in various fields (Martínez-Martínez et al., 2006, 2007, 2011). Our research uses ultrasonic wave velocities to determine a relationship between wave propagation and hydrothermal alteration. The ultrasonic wave velocity graph has been split into areas where samples contained abundant smectite, illite or chlorite. There is no regular divergence from the expected positive linear relationship of compressional and shear waves velocities according to

Fig. 13. Relationship between effective porosity and density from the three geothermal fields. The graph has been split into shallow and deep lithologies. The change in colour represents the different geothermal fields (shallow is all from Kawerau, deep: Ngatamariki = black, Rotokawa = dark grey, Kawerau = medium grey). Plot is split into three approximate alteration zones based on observed mineralogy.


85

Fig. 14. Relationship between compressional wave velocities and shear wave velocities (ultrasonic wave velocities) from the three geothermal fields. The graph has been split into shallow and deep lithologies. The change in colour represents the different geothermal fields (shallow is all from Kawerau, deep: Ngatamariki = black, Rotokawa = dark grey, Kawerau = medium grey). Plot is split into three approximate alteration zones based on observed mineralogy. Plot is split into three approximate alteration zones based on observed mineralogy.

either original composition or alteration type. Data from the three geothermal fields do show that those rocks with chlorite alteration, have lower porosity and faster compressional wave velocities (N3500 m/s), compared to samples with smectite alteration and higher porosities (b3000 m/s), as shown by Ladygin et al. (2000) on three geothermal systems from the Kuril-Kamchatsky Arc, Russia. Ultrasonic wave velocities generally decrease with increasing porosity and microfractures. Wyllie et al. (1958) demonstrated that if possible, a wave would preferably travel through intact rock avoiding fractures and voids thus slowing the transit time through rock. The relationship of compressional wave velocity with porosity (Fig. 16), which has been previously shown by several authors (Wyllie et al., 1958; Gardner et al., 1974; Sousa et al., 2005; Binal, 2009; Heap et al., 2014), in our data shows that although the lithologies have been hydrothermally altered, which includes the infilling of pores and voids, the relationship between vp and effective porosity remains as expected, whereby the higher porosity samples have slower compressional wave velocities.

differing alteration mineral assemblages present in the two ignimbrites, along with changes in mineralogy from predominately low temperature alteration minerals (mostly clay and some quartz) to high temperature alteration minerals (mostly calcite and quartz along with some illite, albite, chlorite, epidote and pyrite), cause the strength of the altered ignimbrites to increase by nearly 50%. Vutukuri et al. (1974) found that there is a positive relationship between the strength of minerals present within the whole rock mineralogy and the resulting compressional strength. Therefore, samples containing larger quantities of ‘strong’ minerals e.g. quartz and epidote (deep/ high temperature samples), will be stronger than samples containing larger quantities of clay minerals (shallow/low temperature samples). This is exemplified by the andesites that come from Ngatamariki, Rotokawa and Kawerau. The primary lithology is the same across the samples, however the

5.3. Compressive Strength The samples representing the shallow lithologies have lower compressive strengths than the samples representing the deep lithologies (Table 4). The difference in the peak compressive strengths for the material from different sections of the geothermal field is related to the primary rock type, however, the deep ignimbrites have a higher mean strength than the shallow ignimbrites, suggesting that alteration also plays a role. The varying porosities and densities, caused by the

Table 4 Mean and standard deviation of uniaxial compressive strength (MPa) for shallow and deep lithologies from Ngatamariki, Rotokawa and Kawerau. Results for each sample are in Appendix B, Table B.1. Shallow lithologies Rhyolitic ignimbrite Rhyolite lava Sedimentary sandstone and siltstone Deep Lithologies Rhyolitic ignimbrite Intrusive Andesite lava/breccia

Mean UCS (MPa)

Number of samples

24.1 ± 19.1 23.4 35.6 ± 11.8

N =27 N =1 N =13

43.3 ± 22.1 93.7 ± 23.7 117.5 ± 45.9

N = 33 N = 14 N = 29

Fig. 15. Relationship between the Ngatamariki, Rotokawa and Kawerau effective porosity (%) and measured depth (mD). The samples have been divided into there respective lithologies. The change in colour represents the different geothermal fields (shallow is all from Kawerau, deep: Ngatamariki = black, Rotokawa = dark grey, Kawerau = medium grey).

86


Fig. 16. Relationship between compressional wave velocities (vp) vs. effective porosity (n).

differing hydrothermal alteration minerals present has caused the mean strength of the samples to be dramatically different, where the Andesite Breccia and Rotokawa Andesite, which are of argillic to propylitic alteration (common epidote, quartz, albite, chlorite and rare calcite) have mean strengths of 113.9 MPa and 129.5 MPa, respectively, while the Kawerau Andesite that had argillic alteration (common illite, quartz, calcite and rare epidote or chlorite) has a mean strength of 68.2 MPa (Fig. 17). This again shows that the high temperature alteration leads to a higher mean strength than a rock with similar primary lithologies overprinted by low to moderate temperature alteration. In our study samples with low porosity (andesite and intrusive samples) tended to fail along a fracture or fractures that coalesced from pre-existing fractures or failed explosively as fractures nucleated and propagated from inclusions, microcracks and other defects during compression. The samples that have high porosity tended to fail from

compaction of the pore space leading the sample to crumble, instead of failing along a failure surface. In strong, indurated, low porosity rocks, compressive failure is preceded by the growth of cracks from the border of existing inclusions, microcracks and other defects. These cracks coalesce into growing cracks finally extending to the sample edges after which failure occurs (Romana and Vásárhelyi, 2007). In a sample that has pre-existing macrofractures the resulting compressive strength tends to be lower than a unfractured sample as the fracture tends to propagate or slide quickly from initial compression, leading to failure (Lama and Vutukuri, 1978; Ashby and Sammis, 1990). Therefore, the resulting peak strength of a rock is more affected by a few long cracks than numerous microcracks because the amount of energy required to cause failure along a large crack is less than the energy required to produce failure through expanding and connecting numerous small cracks (Walsh, 1961; Ashby and Sammis, 1990). A few of the

Fig. 17. Box plot of uniaxial compressive strength of the shallow and deep lithologies. The Andesite represents the Ngatamariki and Rotokawa andesite and the Kawerau label is the Kawerau andesite. The top whisker = Max, bottom whisker = min, upper box = upper quartile, middle line = median, lower box = lower quartile.


87

Fig. 18. Relationship between dynamic Young’s modulus (Ed) and effective porosity (n).

andesite samples we tested had pre-existing fractures and failed at stresses of around 25% of the mean, in addition to a few of the intrusive samples with pre-existing fractures failing at 30% of the mean failure stress. Therefore, strength will be highest for rock with no voids, followed by rock with pores/voids, than rock with distributed pre-existing macrofractures having the lowest strength (Ashby and Sammis, 1990; Heap et al., 2014). Compressibility of rocks is dependent upon the ability of individual grains, pores and cracks to compress. Compressibility of porous rocks is greater than that of solid material of the same composition and for any pore shape or concentration (Lama and Vutukuri, 1978). It is expected that rocks with high porosity have higher compressibility than those with low porosity (Fig. 18), and even though the lithologies used in our study are altered, and the voids/vugs in the samples could have been infilled, they follow the same trend as shown for unaltered rocks (Lama and Vutukuri, 1978; Carmichael, 1982; Binal, 2009). The relationship between Young’s modulus and UCS (Fig. 19.) is expected to increase because as the rocks stiffness increases so should the peak strength of the rock (Lama and Vutukuri, 1978; Carmichael, 1982;

Begonha and Sequeira Braga, 2002; Chang et al., 2006). Our data show the same trend, even though the samples have been hydrothermally altered, proving that the hydrothermal alteration is not modifying the expected relationship. 6. Conclusion We tested samples of a variety of lithologies that are typical in New Zealand geothermal fields from the Ngatamariki, Rotokawa and Kawerau geothermal fields for physical and mechanical rock properties. 1. Through thin section analysis we identified a wide variety of minerals that occur across the smectite, argillic and propylitic alteration zones. The shallow lithologies: ignimbrite, rhyolite, and sedimentary samples in our study contain low temperature (smectite to argillic alteration) alteration minerals like smectite/illite, calcite, and quartz. The deep lithologies: ignimbrite, andesite and intrusive material contain higher temperature (argillic to propylitic alteration) alteration minerals like epidote, chlorite, albite, pyrite and quartz. This shows

Fig. 19. Relationship between dynamic Young’s modulus (Ed) and UCS.

88

2.

3.

4.

5.


that the samples obtained cover the range of typical alteration found in a conventional liquid/gas geothermal field. The type of alteration impacts the porosity and density of the rocks in this dataset. The shallow lithologies in the low to moderate temperature section of the field have a higher porosity (23.5%) and lower density (1783 kg/m3) than lithologies from deep higher temperature sections of the field (6.8% and 2512 kg/m3, respectively), which is evident in the two ignimbrites. Changes in primary mineralogy, due to alteration, leads to distinctive porosities and densities depending on the alteration mineral assemblage. No deviation from the systematic trend was observed in this dataset. There exists a relationship between porosity and burial depth in these rocks. However, the effect of burial itself cannot be isolated in these rocks because the alteration type and primary lithologies also have a major influence on porosity. The shallow lithologies in these geothermal systems are typically ignimbrites and sedimentary material, which naturally have higher porosities, while the deep lithologies are typically extrusive or intrusive volcanic rock, which naturally have lower porosities. Hydrothermal alteration and secondary mineralisation impacts ultrasonic wave propagation. Rocks with chlorite alteration have lower porosity and faster compressional wave propagation (3500 – 5000 m/s), compared to rocks with smectite alteration (1700–3000 m/s). Rock type, texture, density, porosity, water content, temperature, pore structure, pore frequency and fracturing will also affect the velocity of the ultrasonic waves. Hydrothermal alteration and the development of secondary minerals and changes in physical properties leads to differing mechanical behaviour. The samples in our study from the shallow, low temperature regions of the geothermal fields have lower UCS (27.7 ± 10.3 MPa)

compared to samples from deep, high temperature regions (84.8 ± 30.6 MPa). However, the relationships between Young’s modulus, porosity, and UCS show the expected inverse trend that fresh and unaltered samples follow. 6. With respect to conclusions 2, 4 & 5, it is clear that hydrothermal alteration does not cause a significant deviation from the relationships that would be expected between physical and mechanical properties. This was previously an unconfirmed hypothesis in a geothermal field with relation to drilling. 7. The research presented in this paper and on-going research provides a framework to make informed estimates of the key physical and mechanical parameters that are used for bit selection where samples have not yet been physically extracted and tested, through development of a geotechnical model of the geothermal field.

Acknowledgements The authors wish to thank Mighty River Power, Rotokawa Joint Venture Limited; a joint venture between the Tauhara North No.2 trust and Mighty River Power Company Limited, and Ngati Tuwharetoa Geothermal Assets Limited, for the use of core supplied for this study. The staff of the Department of Geological Sciences at the University of Canterbury was invaluable in assisting in all aspects of this research. The Brian Mason Trust also provided for funding for the collection and transportation of the core to the University of Canterbury. The Callaghan Innovation (contract number: MRPR1201/32965) and Source to Surface, a multiyear research incentive between the University of Canterbury and Mighty River Power who provided funding for the completion of fieldwork and core collection.

Appendix A Table A.1 Complete dataset of the porosity, density, ultrasonic wave velocities, Young’s Modulus and Poisson’s Ratio from the Ngatamariki, Rotokawa and Kawerau geothermal fields.

Ngatamariki Andesite Breccia

Tahorakuri

Sample ID

Depth

Porosity

Dry Density

Vp

Vs

Young’s

Poisson’s

NM 7-1 NM 7-2 NM 7-3 NM 7-4 NM 7-5 NM 7-6 NM 7-7 NM 7-8 NM 7-9 NM 7-10 NM 7-11 NM 7-12 NM 1-1 NM 1-2 NM 2-1 NM 2-2 NM 2-3 NM 3-1 NM 3-2 NM 3-3 NM 3-4 NM 3-5 NM 3-6 NM 3-7 NM 4-1 NM 4-2 NM 5-1 NM 5-2 NM8A 1-1 NM8A 1-2 NM8A 1-3 NM8A 1-5 NM8A 1-6 NM8a B NM8a C

−2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −1305.5 −1305.5 −1350 −1350 −1350 −1243 −1243 −1243 −1243 −1243 −1243 −1243 −1224 −1224 −1775 −1775 −2525.5 −2525.5 −2525.5 −2525.5 −2525.5 −2525.5 −2525.5

1.50 1.69 1.63 1.63 2.27 1.45 1.80 1.76 1.67 1.95 1.66 1.74 4.7 6.17 18.6 20.07 20.28 9.52 9 10.9 9.79 9.97 9.79 9.1 5.44 6.17 9.9 12.1 4.03 3.17 3.57 3.58 4.09 2.57 3.37

2701.7 2660.9 2676.9 2697.5 2642.9 2665.7 2670.2 2616.0 2672.1 2641.2 2646.3 2660.8 2454.2 2469.9 2127.1 2115.3 2084.3 2472.0 2490.2 2435.7 2460.9 2460.5 2344.1 2479.6 2440.5 2391.6 2361.8 2289.2 2569.9 2589.5 2563.3 2575.2 2550.0 2598.7 2578.6

4202 4101 4155 4411 4466 4238 4157 4070 4124 4194 4165 4160 4017 4399 3256 3493 3484 3192 3282 3632 3798 3589 3327 3740 4593 3913 4015 3819 3775 4233 4119 3930 3829 4141 4149

2497 2415 2441 2686 2557 2521 2414 2365 2379 2414 2347 2448 2040 2229 1765 1861 1866 1699 1744 1879 2006 1913 1644 1973 2258 2099 2089 2062 2143 2295 2147 2079 2047 2464 2488

41.56 34.41 49.92 27.81 52.09 29.80 39.25 37.60 42.04 33.23 34.91 37.57 33.75 36.47 17.89 16.95 16.88 22.44 25.84 23.82 20.18 25.10 16.74 23.32 37.34 32.45 24.18 28.26 29.80 35.29 31.08 29.11 27.77 38.67 38.92

0.24 0.24 0.25 0.23 0.21 0.22 0.24 0.24 0.24 0.24 0.24 0.23 0.30 0.30 0.31 0.33 0.30 0.30 0.31 0.29 0.30 0.31 0.33 0.31 0.32 0.31 0.30 0.31 0.26 0.29 0.31 0.31 0.30 0.23 0.22


89

Table A.1 A.1 (continued) (continued)

Tonalite

Rotokawa Andesite

Kawerau Matahina Ignimbrite

Te Teko

Sample ID

Depth

Porosity

Dry Density

Vp

Vs

Young’s

Poisson’s

NM11 1-1 NM11 1-2 NM11 1-3 NM11 1-4 NM11 1-5 NM11 A NM11 B NM11 C NM11 D NM11 A-1 NM11 B-1 NM11 C-1 NM11 D-1 NM11 E-1 NM9 1-1 NM9 1-2 NM9 1-3 NM9 1-4 NM9 1-5 NM9 1-6 NM9 1-7 NM9 1-8 NM9 1-9 NM9 2-2 NM9 2-3 NM9 2-4 NM9 2-5 NM9 2-6 NM8A C2 1 21.1 C .8a .5B 3.2 A 20.4B 10.6A 10.8C 10.9B 13.2A 10.6C 21.0A 22.3B 22.4A 21.1B 21.7B G1Box131-1 G1Box131-2 G1Box131-3 G1Box132-1 G1Box132-2 G1Box132-3 G1Box132-4 G1Box111-1 G1Box111-2 G1Box111-3 G1Box111-4 G1 Box 11 2 G1 Box 17 1 G1Box172-1 G1Box172-2 G1 Box 17 3 G1 Box 17 4 G2 Box 15 1 G2Box152-1 G2Box152-2 G2 Box 15 3 G2 Box 15 4 G2 1-1 G2 1-2 G2 1-3 G2 2-1 G2 2-2 KA37 1-1 KA37 1-2 KA37 1-3 KA37 1-4

−2083.3 −2083.3 −2083.3 −2083.3 −2083.3 −2087.4 −2087.4 −2087.4 −2087.4 −2083.3 −2083.3 −2083.3 −2083.3 −2083.3 −3200.1 −3200.1 −3200.1 −3200.1 −3200.1 −3200.1 −3200.1 −3200.1 −3200.1 −3201.9 −3201.9 −3201.9 −3201.9 −3201.9 −3280.1 −2121.1 −2121.8 −2121.5 −2123.2 −2120.4 −2310.6 −2310.8 −2310.9 −2313.2 −2310.6 −2321 −2322.3 −2322.4 −2321.1 −2321.7 −77.4 −77.4 −77.4 −78.85 −78.85 −78.85 −78.85 −72.3 −72.3 −72.3 −72.3 −72.75 −89.1 −89.25 −89.25 −89.66 −89.45 −77.9 −78.6 −78.6 −78.85 −79.64 −106.5 −106.5 −106.5 −106.84 −106.84 −1102.2 −1102.2 −1102.2 −1102.2

14.80 15.29 15.74 14.78 14.65 13.17 14.87 14.58 13.82 14.73 15.27 14.72 15.79 15.17 2.82 2.70 3.06 2.83 2.99 2.71 2.67 2.87 2.76 1.81 1.67 1.65 1.59 1.57 3.40 13.10 13.49 10.72 6.61 4.37 5.85 6.72 7.42 6.97 5.97 6.84 7.51 6.49 7.47 6.28 25.64 25.14 24.75 25.51 24.79 25.40 26.51 19.80 17.58 18.37 18.79 18.15 46.32 46.06 47.48 46.73 48.03 25.33 25.28 24.97 26.29 24.88 54.66 56.56 47.00 50.15 47.91 17.21 17.60 17.53 17.68

2290.1 2292.5 2264.3 2282.9 2295.4 2348.6 2308.6 2320.2 2344.1 2297.7 2284.8 2286.6 2273.9 2288.4 2574.3 2585.7 2574.4 2573.8 2560.9 2579.8 2585.3 2576.5 2590.8 2806.3 2831.8 2819.1 2824.2 2837.8 2454.7 2366.0 2329.1 2441.3 2560.9 2557.7 2505.5 2532.6 2512.5 2547.8 2503.3 2555.2 2533.1 2560.7 2539.8 2594.9 1777.0 1794.7 1787.2 1758.2 1764.6 1770.3 1754.5 1894.8 1896.6 1912.6 1907.5 1933.4 1262.4 1235.9 1233.9 1245.2 1222.1 1757.5 1738.7 1728.6 1715.8 1812.2 1047.1 1094.4 1105.8 1115.9 1150.5 2138.2 2115.6 2148.5 2125.4

2757 2711 2693 2816 2758 3284 3175 3319 3182

1578 1502 1526 1524 1554 1967 1914 1884 1968

14.43 13.25 13.38 13.74 14.09 22.18 20.53 20.76 21.59

0.26 0.28 0.26 0.29 0.27 0.22 0.22 0.26 0.19

3147 3212 3124 3270 2971 3082 3058 3024 2952 3020 2992 2972 3064 3686 3710 3627 3718 3703 2954 3877 3850 4005 4182 4556 4305 4147 4285 4013 4350 4070 4133 4181 4352 4154 1925 1963 1982 1983 2058 2035 1981 3015 3006 3037 2969 2987

1947 1989 1935 2057 1606 1599 1564 1639 1652 1687 1655 1628 1659 1948 1957 2025 2051 2070 1588 2405 2363 2443 2490 2752 2602 2537 2615 2531 2652 2508 2550 2582 2659 2588 934 1055 979 978 968 999 946 1767 1725 1739 1720 1674

20.62 21.50 20.22 22.69 17.22 17.40 16.69 17.89 17.82 18.73 18.15 17.59 18.46 27.88 28.41 29.53 30.53 31.02 16.06 32.50 31.16 35.07 38.91 45.94 41.13 39.15 41.53 37.19 42.40 38.39 39.22 40.70 43.17 41.11 4.41 5.38 4.79 4.73 4.59 4.90 4.52 14.90 14.24 14.68 14.18 13.91

0.19 0.19 0.19 0.17 0.29 0.31 0.32 0.29 0.27 0.27 0.28 0.28 0.29 0.31 0.31 0.27 0.28 0.27 0.30 0.19 0.20 0.20 0.23 0.21 0.21 0.20 0.20 0.21 0.20 0.19 0.19 0.19 0.20 0.18 0.35 0.30 0.34 0.34 0.36 0.34 0.35 0.24 0.25 0.26 0.24 0.27

2642 2671 2592 2565 2826 1763 1879 1976 2051 2081 2785 2796 2639 2776

1411 1370 1304 1279 1426 818 872 924 957 917 1560 1551 1477 1528

9.42 8.93 8.07 7.70 10.06 2.12 2.42 2.75 2.98 2.77 13.25 13.02 11.94 12.74

0.30 0.32 0.33 0.33 0.33 0.36 0.36 0.36 0.36 0.38 0.27 0.28 0.27 0.28

(continued on next page)

90


Table A.1 A.1 (continued) (continued)

Tahuna

Caxton Andesite

Sample ID

Depth

Porosity

Dry Density

Vp

Vs

Young’s

Poisson’s

KA37 1-5 KA37 1-6 KA37 1-7 KA37 1-8 KA37 2-1 KA37 2-2 KA30 1-1 KA30 1-2 KA30 2-1 KA30 2-2 KA30 3-1 KA30 3-2 KA30 4-1 KA30 4-2 KA30 4-3 KA30 4-4 KAW17 1 KAW17 3-2 KAW17 4 KAM 11 KA3 1-2 KA3 2

−1102.2 −1102.2 −1102.2 −1102.2 −1103.1 −1103.1 −489.5 −489.5 −489.6 −489.6 −490.3 −490.3 −491.9 −491.9 −491.9 −491.9 −458.5 −458.6 −394.2 −155.3 −641.86 −669.95

17.49 17.72 17.79 16.99 18.73 17.86 17.40 16.99 16.10 16.10 17.43 17.27 18.82 18.87 19.44 19.86 14.95 17.47 23.03 19.54 12.35 12.92

2126.3 2121.6 2110.3 2132.9 2072.8 2114.1 1993.1 1963.5 2040.0 2114.3 1928.9 1965.6 1919.6 1933.8 1911.4 1904.9 1968.4 2046.4 1793.2 1819.3 2339.8 2347.2

2701 2812 2800 2896 2970 2924 3427 3354 3420 3512 3386 3370 3373 3248 3331 3276 2604 2781 2360 2941 3908 3988

1500 1541 1594 1567 1656 1638 1973 1963 1980 1931 1946 1945 1897 1873 1928 1892 1316 1433 1194 1735 2056 2051

12.24 12.96 13.52 13.60 14.49 14.45 19.56 18.89 20.07 19.53 18.43 18.68 17.61 17.03 17.81 17.12 9.10 11.20 6.85 14.00 26.00 26.20

0.28 0.28 0.26 0.29 0.27 0.27 0.25 0.24 0.25 0.28 0.25 0.25 0.27 0.25 0.25 0.25 0.33 0.33 0.32 0.23 0.31 0.32

Table B.1 Complete dataset of the uniaxial compressive strength from the Ngatamariki, Rotokawa and Kawerau geothermal fields.

Ngatamariki Andesite Breccia

Tahorakuri

Tonalite

Sample ID

Depth (mD)

UCS (MPa)

NM 7-1 NM 7-2 NM 7-3 NM 7-4 NM 7-5 NM 7-6 NM 7-7 NM 7-8 NM 7-9 NM 7-10 NM 7-11 NM 7-12 NM 1-1 NM 1-2 NM 2-1 NM 2-2 NM 2-3 NM 3-1 NM 3-2 NM 3-3 NM 3-4 NM 3-7 NM 4-1 NM 4-2 NM 5-1 NM 5-2 NM8A 1-1 NM8A 1-2 NM8A 1-3 NM8A 1-5 NM8A 1-6 NM11 1-2 NM11 1-3 NM11 1-4 NM11 1-5 NM9 1-1 NM9 1-2 NM9 1-3 NM9 1-4 NM9 1-5 NM9 1-6 NM9 1-7 NM9 1-8 NM9 1-9 NM9 2-2 NM9 2-3 NM9 2-4

−2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −2180 −1305.5 −1305.5 −1350 −1350 −1350 −1243 −1243 −1243 −1243 −1243 −1224 −1224 −1775 −1775 −2525.5 −2525.5 −2525.5 −2525.5 −2525.5 −2083.34 −2083.34 −2083.34 −2083.34 −3200.12 −3200.12 −3200.12 −3200.12 −3200.12 −3200.12 −3200.12 −3200.12 −3200.12 −3201.9 −3201.9 −3201.9

130.2 188.1 135.3 107.0 36.7 123.0 113.1 23.9 174.4 127.1 160.8 36.6 80.9 114.3 20.8 30.9 29.6 22.7 23.0 22.9 24.9 16.0 35.3 22.6 62.1 39.4 51.7 62.3 65.6 100.5 53.9 34.5 35.3 35.4 33.0 67.3 81.7 65.8 84.3 108.2 83.5 77.0 65.6 76.0 111.1 113.1 87.8


91

Table B.1 B.1 (continued) (continued)

Rotokawa Andesite

Kawerau Matahina Ignimbrite

Te Teko

Tahuna

Caxton Kawerau Andesite

Sample ID

Depth (mD)

UCS (MPa)

NM9 2-5 NM9 2-6 NM8A C2 1 21.1 C .8a .5B 3.2 A 20.4B 10.6A 10.8C 10.9B 13.2A 10.6C 21.0A 22.3B 22.4A 21.1B 21.7B G1Box131-1 G1Box131-2 G1Box131-3 G1Box132-1 G1Box132-2 G1Box132-3 G1Box132-4 G1Box111-1 G1Box111-2 G1Box111-3 G1Box111-4 G1 Box 11 2 G1 Box 17 1 G1Box172-1 G1Box172-2 G1 Box 17 3 G1 Box 17 4 G2 Box 15 1 G2Box152-1 G2Box152-2 G2 Box 15 3 G2 Box 15 4 G2 1-1 G2 1-2 G2 1-3 G2 2-1 G2 2-2 KA37 1-1 KA37 1-2 KA37 1-3 KA37 1-4 KA37 1-5 KA37 1-6 KA37 1-7 KA37 1-8 KA37 2-1 KA37 2-2 KA30 1-1 KA30 1-2 KA30 2-1 KA30 2-2 KA30 3-1 KA30 3-2 KA30 4-1 KA30 4-2 KA30 4-3 KA30 4-4 KAW17 1 KAW17 3-2 KAW17 4 KAM 11 KA3 1-2 KA3 2

−3201.9 −3201.9 −3280.1 −2121.1 −2121.8 −2121.5 −2123.2 −2120.4 −2310.6 −2310.8 −2310.9 −2313.2 −2310.6 −2321 −2322.3 −2322.4 −2321.1 −2321.7 −77.4 −77.4 −77.4 −78.85 −78.85 −78.85 −78.85 −72.3 −72.3 −72.3 −72.3 −72.75 −89.1 −89.25 −89.25 −89.66 −89.45 −77.9 −78.6 −78.6 −78.85 −79.64 −106.5 −106.5 −106.5 −106.84 −106.84 −1102.25 −1102.25 −1102.25 −1102.25 −1102.25 −1102.25 −1102.25 −1102.25 −1103.1 −1103.1 −489.5 −489.5 −489.66 −489.66 −490.3 −490.3 −491.93 −491.93 −491.93 −491.93 −458.55 −458.66 −394.29 −155.3 −641.86 −669.95

124.0 138.1 123.1 69.5 79.9 86.0 105.3 211.0 92.1 109.9 137.3 146.2 146.2 126.5 138.0 148.4 157.9 162.7 29.4 33.9 28.4 13.7 21.7 25.0 28.5 49.7 50.8 35.4 57.7 74.7 2.8 2.1 2.1 2.2 2.2 26.8 33.1 30.1 30.6 34.6 5.2 7.7 6.7 7.6 10.0 41.2 39.4 49.7 43.7 28.8 36.3 37.4 34.7 44.9 55.5 45.4 29.9 58.1 46.4 34.0 44.8 36.6 28.7 35.4 37.3 22.1 32.9 11.1 23.5 63.2 71.4

92


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