Earth and Ocean Sciences (Conference Papers)

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  • Publication
    Tectonic structure and permeability in the Taupô Rift: new insights from analysis of LiDAR derived dems
    (New Zealand Geothermal Workshop, 2017-11-22) Villamor, P.; Nicol, A.; Seebeck, H.; Rowland, J.; Townsend, D.; Massiot, C.; McNamara, David D.; Mil, Sarah D.; Ries, W.; Alcaraz, S.
    The location of permeable zones, and preliminary quantification of a geothermal well’s production or injection capacity are routinely interpreted from well pressure, temperature, and flow measurements made at different injection rates and during heating after shut-in (also referred to as PTS or completion test data). The spatial resolution of feed zones interpreted from completion test data typically ranges from 10 – 100 m thick. By itself, the completion test dataset cannot inform on the nature of the permeability, i.e. fracture versus formation permeability, and is subject to uncertainties. On the other hand, borehole image logs provide a direct description of the fractures (location, morphology and orientation) and of rock textures intersected by the borehole, to a spatial resolution of ~0.01 – 1 m. Traditionally, analyses of completion test and borehole image log data are done independently by the reservoir engineer and geologist, respectively. We present a joint interpretation of completion test data and borehole image logs (acoustic and resistivity) from two boreholes in the Wairakei Geothermal Field, New Zealand, highlighting the advantages of a cross-discipline interpretation of a borehole’s permeable zones. In the two studied boreholes, we show that feed zones generally correlate with (1) fractures of low acoustic amplitude and visible on the traveltime image of the acoustic image log, and (2) low resistivity fractures with a high resistivity halo on the resistivity image log. In the latter case, fractures with haloes coincide with a reservoir-scale fault modelled independently from 3D modeling of stratigraphic offsets. However, not all fractures of such appearance observed on the image logs correlate with permeable zones. The integrated interpretation of completion test data and image log increases the robustness in the feed zone interpretation (location, extent, fracture or matrix permeability components), and thus can contribute to improving fluid flow numerical models used to sustainably manage geothermal resources.
  • Publication
    Discussion between a reservoir engineer and a geologist: permeability identification from completion test data and borehole image logs integration
    (39th New Zealand Geothermal Workshop, 2017-11-22) Massiot, Cécile; McLean, K.; McNamara, David D.; Sepulveda, F.; Milicich, Sarah D.
    The location of permeable zones, and preliminary quantification of a geothermal well’s production or injection capacity are routinely interpreted from well pressure, temperature, and flow measurements made at different injection rates and during heating after shut-in (also referred to as PTS or completion test data). The spatial resolution of feed zones interpreted from completion test data typically ranges from 10 – 100 m thick. By itself, the completion test dataset cannot inform on the nature of the permeability, i.e. fracture versus formation permeability, and is subject to uncertainties. On the other hand, borehole image logs provide a direct description of the fractures (location, morphology and orientation) and of rock textures intersected by the borehole, to a spatial resolution of ~0.01 – 1 m. Traditionally, analyses of completion test and borehole image log data are done independently by the reservoir engineer and geologist, respectively. We present a joint interpretation of completion test data and borehole image logs (acoustic and resistivity) from two boreholes in the Wairakei Geothermal Field, New Zealand, highlighting the advantages of a cross-discipline interpretation of a borehole’s permeable zones. In the two studied boreholes, we show that feed zones generally correlate with (1) fractures of low acoustic amplitude and visible on the travel-time image of the acoustic image log, and (2) low resistivity fractures with a high resistivity halo on the resistivity image log. In the latter case, fractures with haloes coincide with a reservoir-scale fault modelled independently from 3D modeling of stratigraphic offsets. However, not all fractures of such appearance observed on the image logs correlate with permeable zones. The integrated interpretation of completion test data and image log increases the robustness in the feed zone interpretation (location, extent, fracture or matrix permeability components), and thus can contribute to improving fluid flow numerical models used to sustainably manage geothermal resources.
  • Publication
    Volcanic texture identification and influence on permeability using a borehole resistivity image log in the Whakamaru Group Ignimbrite, Wairakei Geothermal Field, New Zealand
    (Workshop on Geothermal Reservoir Engineering, 2018-02-12) Milicich, Sarah D.; Massiot, Cécile; McNamara, David D.
    Discerning the contributions to fluid flow in a geothermal reservoir from intrinsic and structural permeability components is an important, yet difficult task. High-quality, resistivity borehole image log data (Formation MicroImager; FMI) collected from the Whakamaru Group ignimbrite in well WK271, Wairakei Geothermal Field, New Zealand, has been used to investigate the textural characteristics of volcanic rocks in the geothermal reservoir, with a view to improving reservoir model inputs. Textural analysis of the FMI log in the Whakamaru Group ignimbrite was able to provide an improved internal stratigraphy compared to that derived solely from drill-cuttings, and offers insights into the volcanic processes that generated it. Based on volcanic textures identified on the WK271 FMI image log, seven individual flow units in the Whakamaru Group ignimbrite are recognized, and are separated by texturally characteristic intervals of airfall tephra. Comparison of fracture density and rock type (including welding intensity) shows a low fracture density in non-welded ignimbrites and tuff beds. The logged interval occurs over a fault zone where permeability has previously been inferred to be strongly influenced by fractures, precluding an accurate assessment of the ignimbrites intrinsic permeability.
  • Publication
    Feasibility of storing carbon dioxide on a tectonically active margin: New Zealand
    (American Association of Petroleum Geologists, 2015-09-13) Field, B.D.; Lawrence, M.J.; Nicol, A.; McNamara, David D.; Arnot, M.J.; Coyle, F.; Higgs, K.E.; Mountain, B.; Gerstenberger, M.; Daniel, R.; Bunch, M.A.; Barton, B.; |~|
    New Zealand's sedimentary basins was found to have available several gigatonnes of CO2 storage capacity. However, CO2 storage is currently untested in New Zealand. The country's position on an active Neogene plate boundary raises additional key factors that will influence final site selection. Some risk factors will also influence the relationship between social acceptance and the design of regulations. Despite the risks, hydrocarbon producing fields in Taranaki indicate that viable reservoir-seal pairs are likely to be present.
  • Publication
    Exploring structure and stress from depth to surface in the Wairakei Geothermal Field, New Zealand
    (International Geothermal Association, 2016-02-22) McNamara, David D.; Bannister, Stephen; Villamor, Pilar; Sepúlveda, Fabian; Milicich, Sarah D.; Alcaraz, Samantha; Massiot, Cécile; |~|
    Structures such as fractures and faults have an important role as fluid flow pathways in geothermal fields, as the reservoir rocks hosting geothermal resources can often have little to no intrinsic permeability. As such, understanding and characterizing this structural network is vital to developing reservoir models and field operation and development plans that will maximize the potential of a geothermal resource. Presented here are the preliminary results of three recent studies, micro-earthquake analysis, borehole logging, and active fault mapping, carried out in the Wairakei Geothermal Field to determine the structural character of the system, if and how it contributes to fluid flow, and how the structural observations from these studies inform and relate to each other. Across all three techniques a dominant NE-SW structure strike orientation is observed with lesser population of N-S, E-W and NW-SE, consistent with the broad Taupo Volcanic Zone observed trend. Further analysis of the data is required to resolve important structural questions around the Wairakei Geothermal Field including: whether the data supports the model of the Wairakei Geothermal Field being an expression of enhanced permeability due to its location in an inferred rift accommodation zone, how the links between observed structures at the surface and subsurface can be resolved, and what role to these structures play in geothermal fluid flow from depth to surface?
  • Publication
    Geothermal structural geology in New Zealand: Innovative characterisation and micro-analytical techniques
    (International Geothermal Association, 2016-11-23) McNamara, David D.; Massiot, Cécile; |~|
    Many of New Zealand's geothermal reservoirs are hosted in rocks with low intrinsic permeability. As such, successful development of these resources relies on understanding the role subsurface structures, such as fractures and faults, play in reservoir permeability. Further complexity is added to this understanding due to the constantly evolving permeable nature of these geothermal reservoir structures. The same fractures and faults which operate as interconnected, open, fluid flow pathways, can also behave as fluid flow barriers due to geothermal mineral precipitation over time. Increased industry application of borehole logging technology, and the development of innovative geothermal data processing and interpretation, has allowed structural geologists to make advances in characterising the subsurface structure of the Taupo Volcanic Zone. These novel data reveal structural heterogeneity at a variety of scales, from changing dominant orientations across the Taupo Volcanic Zone, to decimetre changes in fracture orientation within a single well. Additionally, these techniques allow observation of the variability in in situ horizontal stress directions for the first time, revealing active subsurface structures. At a much smaller scale, the application of novel, advanced, microscopy techniques to analyse the micro-structure of geothermal vein minerals provides information on evolving geothermal reservoir fluid properties, and stress conditions. Crystallographic analysis of microstructures found in geothermal calcite veins can provide insight into the differential stress history of the reservoir, while the operation of temperature dependent, calcite crystal slip systems, may provide a tool to record evolving geothermal reservoir temperatures.
  • Publication
    Fracture width and spacing distributions from borehole televiewer logs and cores in the Rotokawa Geothermal Field, New Zealand
    (International Geothermal Association, 2015-04-19) Massiot, Cécile; McNamara, David D.; Nicol, Andrew; Townend, John; |~|
    The successful targeting of permeable fractures in geothermal fields is aided by understanding the spatial and geometric characteristics of fracture populations. Studies of numerous outcrop, and a limited number of geothermal reservoirs using cores and borehole logs, indicate that fracture frequency and width most commonly follow power-law distributions, with exponential, log-normal, gamma, and power-exponential distributions also reported. This paper presents the first statistical analysis of fracture width and spacing in the high-temperature Rotokawa Geothermal Field, Taupo Volcanic Zone, New Zealand. The fracture dataset comprises: (1) c. 3.6 km of acoustic borehole televiewer (BHTV) logs from three wells and, (2) c. 33 m of core. Statistical distributions have been fitted to the BHTV data using a maximum likelihood estimation method and statistical models selected using the Schwarz Bayesian Criterion. Fracture widths observed on BHTV logs range between c. 1 -105 mm. Image resolution and sampling bias reduce the useable range of fracture width to less than one order of magnitude (c. 8 -50 mm). Over this range, considering the sampling effects and core observations, the fracture width is best modelled by an exponential distribution with coefficients between 0.13±0.01 and 0.29±0.02, which should be treated as a lower bound. Analysis of fracture spacing of the four fracture sets identified on BHTV logs indicates that the dominant set (striking NE -SW) is best modelled by a log-normal distribution, while power-law, power-exponential and gamma are also possible for individual wells. These spacing distributions indicate the presence of a characteristic scale which has not been observed in other geothermal reservoirs hosted in crystalline formations. The characteristic scale may be associated with mechanical interfaces associated with stratigraphic layering, faults, or cooling joints and/or sub-horizontal flow-banding in andesitic formations. Stratigraphic layering can consist of a succession of lava flows with intercalated breccia layers in the andesites, welding variations in tuffs and sedimentary layering in the sedimentary formations sampled by the BHTV logs. The subordinate fracture set striking N -S is best modelled by a pareto (power-law) distribution which suggests that the spacing is more likely to be controlled by tectonic processes than by layering. This N -S fracture set is predominant in only one of the wells studied which may indicate a structural control on their occurrence in the vicinity of this well. Low fracture spacing (
  • Publication
    Modelling fluid flow through fractured rock: Examples using TVZ geothermal reservoirs
    (New Zealand Geothermal Workshop, 2015-11-18) Kissling, W.M.; Ellis, S.E.; McNamara, David D.; |~|
    Geothermal resources are often hosted within volcanic, plutonic, and basement-type lithologies. As such their matrix permeability can be very low and the transport and circulation of geothermal fluids will be dominated by fractures. Understanding the flow of hydrothermal fluid through fractured rock is thus essential to the efficient utilisation of New Zealand's deeper geothermal resources which are hosted in greywacke basement and lavas. In this study we investigate the nature of fluid flow for a variety of simple, single-and multiple-fracture arrangements and widths within Taupo Volcanic Zone (TVZ) greywacke basement and andesite hosted reservoirs, making use of known permeabilities. We present preliminary numerical models of fluid flow through a metre-scale sample of rock for a variety of simple fracture networks, with emphasis on the perturbation of the total flow across the sample with respect to that of a non-fractured sample. Later work will focus on the changes in fracture properties that influence the flow through these samples due to far field stresses and varying mechanical behaviour of the reservoir rock and its fractures, and upscaling these results from metre-scale experiments to kilometre-scale reservoirs. Future research will also test the effect on fluid flow of known fracture spacing and width distributions of TVZ geothermal reservoirs.
  • Publication
    Comparing borehole televiewer logs with continuous core: An example from New Zealand
    (International Geothermal Association, 2015-04-19) Milloy, Sophie Frances; McLean, Katie; McNamara, David D.; |~|
    The use of borehole televiewer logging is a recent addition to the well logging toolkit available to the geothermal industry in New Zealand. The information acquired from borehole televiewer (BHTV) equipment, such as the Acoustic Formation Imaging Technology (AFIT) tool provides valuable geological information about the geothermal reservoir. This paper investigates the accuracy of acoustic borehole imaging in a geothermal environment by comparing the information from a continuous core to a BHTV log acquired using the AFIT LT tool. This study is unique in the New Zealand geothermal industry as the data presented here has been obtained from the only well to have both a borehole image and continuous core over the imaged well interval.
  • Publication
    New Zealand geothermal power plants as critical facilities: an active fault avoidance study in the Wairakei Geothermal Field, New Zealand
    (International Geothermal Association, 2015-04-19) Villamor, Pilar; Clark, Kate; Watson, Matt; Rosenberg, Mike; Lukovic, Biljana; Ries, Willam; González, Álvaro; Milicich, Sarah D.; McNamara, David D.; Pummer, Bernd; Sepulveda, Fabian; |~|
    Active faults in rifts commonly provide high crustal permeability and control geothermal fluid pathways. However, active faults can also pose surface deformation hazards to geothermal power plants and associated infrastructure. The New Zealand Ministry for the Environment (MfE) guidelines recommend avoidance of active faults for construction of new buildings based on building importance and the rate of fault activity. Power plants, which are classed as 'high Building Importance Category', are permitted on faults with a rupture recurrence interval greater than 10,000 years. We present a site feasibility study for the Te Mihi Power Plant (Wairakei Geothermal Field), used to determine if there is recent major active faulting at the proposed site. The initial Power Plant site was proposed in an area exhibiting complex surface patterns of active faults with two closely spaced (few metres to hundreds of metres), intersecting, normal fault sets. Detailed aerial photo review and field mapping was undertaken to improve the accuracy of previously mapped fault traces, and to potentially identify previously undocumented faults. Fault scarps were assigned different geomorphic expression (from "clear" to "inferred"). In the study area, recurrence intervals of active faulting can be difficult to estimate because fault scarps are frequently blanketed by tephra from late Quaternary eruptions of the nearby Taupo caldera, and detailed paleoseismic studies are absent. Because of the potential burial of geomorphic fault scarps, GPR surveys and paleoseismic trenching were carried out to investigate the apparent lack of active faulting at the plant footprint, and to better understand fault activity rates close to the newly proposed Power Plant site. Displacements of several post-25 ka tephra marker horizons, and fault planes, were analysed in the trench to assess the presence or absence of recent fault activity, and to calibrate the reflectors observed in the GPR images. The trench study also allowed accurate estimation of fault slip rate and recurrence interval. This study has provided a first calibration for a correlation between geomorphic expression of faults and fault activity in this area. The study revealed that: a) some of the subtle features initially suspected as fault scarps were indeed active faults; b) deep paleoseismic excavations are needed in sites located in close proximity to frequently active volcanoes (due to thick cover beds) even when assessing faults with clear geomorphic expression; and c) GPR is useful when assessing activity of faults with large offsets (in this area, usually faults with recurrence interval less than 5,000 years), but the resolution of GPR might not allow evaluation of minor faulting which should then be assessed at the site during construction. This investigation allowed the Te Mihi Power Plant to be re-sited in an area outside the identified construction-avoidance envelope that conformed to the recommendations of the MfE guidelines.
  • Publication
    A damage mechanics approach to modeling failure in Greywacke rock
    (New Zealand Geothermal Workshop, 2014) Pogacnik, Justin; McNamara, David D.; O’Sullivan, Mike; O’Sullivan, John; |~|
    Fracture networks within greywacke basement rocks often control fluid flow in geothermal reservoirs in New Zealand. Thermal, hydrological, chemical, and mechanical processes affect the evolution of these fracture networks. Damage mechanics offers a framework that can be used in the numerical modeling of failure and permeability evolution of geothermal rocks. However, no damage mechanics model has been utilized to describe the complex mechanical behavior of greywacke basement rock. In this work, we apply a damage mechanics THM simulation technique to model stress-strain data for greywacke rock from laboratory uniaxial compression and confined compression tests. Calibration of solid mechanics properties for greywacke rock in THM modeling is a necessary step in the simulation of damage and permeability evolution for heat transport in enhanced geothermal systems.
  • Publication
    Fracture geometries and processes in andesites at Mt Ruapehu, New Zealand: implications for the fracture modelling of the Rotokawa Geothermal Field
    (Le Consortium Gocad, 2014-09-16) McNamara, David D.; Nicol, Andrew; Archibald, Garth; Townend, John; Massiot, Cecile; |~|
    Fluid flow in the high-temperature (300 C), andesite-hosted Rotokawa geothermal reservoir (Taupo Volcanic Zone (TVZ), New Zealand) is largely controlled by fractures and faults but their geometries are still poorly understood. The aim of this study is to measure and derive geometric parameters characterising fractures in andesitic formations in order to use these as input for dis-crete fracture network models (DFN) and predictive fluid flow models of the Rotokawa geothermal reservoir. We make use of two complementary fracture datasets. (1) The fracture geometry in-trinsic to andesitic formations are studied on outcrops at Mt Ruapehu (TVZ volcano), with the measurement of c. 200 fractures along a 100 m long scanline, and the acquisition of a Terrestrial Laser Scanner (TLS) scan acquired over the entire outcrop. (2) Fracture orientation, width and spacing are determined for three acoustic borehole televiewer (BHTV) logs and 33 m of cores from the Rotokawa Geothermal Field. Two types of fractures are observed at the Mt Ruapehu outcrop. The majority of fractures form sub-vertical cooling joints. The TLS scan samples six dip directions suggesting an hexagonal section typical of basaltic lava flows. The scanline survey did not fully sample the six directions. The preliminary analysis of fracture length on the scanline survey highlights the high degree of fracture connectivity and a weak spatial clustering. A subset of the fractures are sub-horizontal, highly clustered and are aligned with possible changes of crystallinity, viscosity and flow banding within the flow. Further analysis is required to make firm conclusions about the fracture length and spacing. Fractures are conchoidal which enhances the fracture linkages, which cannot be easily quantified from scanline surveys and will be evaluated on the TLS scans. The BHTV and core analysis reveals that fractures within the reservoir are predominantly steeply dipping and NE SW-striking, parallel to the trend of the maximum horizontal compressive stress (S Hmax) and the rift axis. Fractures in the reservoir are preferentially oriented with respect to the in-situ stress and the tectonic faults but may be locally inherited from cooling joints and fractures associated with the internal fabric of the lava flows. BHTV logs indicate that the 8 50 mm wide fractures follow an exponential distribution. The log-normal, power-exponential or power-law distributions have similar likelihoods for fracture spacing of 0.005 50 m. Low spacing are best fitted by either an exponential, gamma or power-exponential distribution. This change at c. 1 m spacing may correspond to the threshold at which fracture interaction occurs. The lithological controls on the fractures is observed at both the outcrop and core scale, with fracture being less numerous and more tortuous in breccias than in massive lava. The breccias are typically more permeable than the massive interior, and offer lateral and vertical connectivity in reservoirs. Breccias also affect the propagation of the fractures due to their heterogeneity. Integrating these observations into fracture models will be fundamental to the prediction capability of the fracture models of the Rotokawa andesitic reservoir. Observations made at Ruapehu and Rotokawa have wide implication for the successful development of geothermal resources in volcanic-hosted geothermal reservoirs.
  • Publication
    Rock properties of Greywacke Basement hosting geothermal reservoirs, New Zealand: preliminary results
    (Stanford University, 2014-02-24) McNamara, David D.; Faulkner, D.; McCarney, E.; |~|
    Geothermal resources in New Zealand are known to be hosted in greywacke basement rocks. Fluid flow in these reservoirs and the wells that access them is controlled by fracture networks. As such it is of vital importance to understand how these structures are impacted by the mechanical and thermal properties of this basement rock, and how these in turn are affected by changing temperature and pressure conditions at depth. This paper details the results of an initial set of laboratory tests of two New Zealand greywacke basement terranes, in which geothermal reservoirs are known to be hosted. The aim of the study was to provide an initial understanding of the rocks mechanical properties so that further, more refined testing could be applied. Low permeability and porosity measurements are consistent with the current understanding that fractures control fluid flow in the basement. Preliminary mechanical testing suggests a systematic difference between greywacke from the Waipapa and Torlesse Terranes, with the Waipapa Terrane being mechanically stronger, potentially as a result of a coarser grain size and/or composition differences. Tensile strength testing of whole rock and fractures in Waipapa greywacke rock show lower tensile strengths for the fractures. This indicates that geothermal fluid flow makes use of existing fracture networks via crack-seal mechanisms rather than through the generation of new fractures. Further work, including mechanical testing at; high temperature conditions, variable grain size, and variable composition will provide us with a better insight into the role increasing pressure and temperature conditions play in fracture controlled geothermal fluid flow as we approach the change from brittle to ductile deformation. This in combination with a rigorous investigation of the effects of the character and heterogeneity of greywacke basement in New Zealand will inform us on how best New Zealand can explore for and utilize deep (>3km), fracture dominated, geothermal resources.
  • Publication
    Statistical corrections of fracture sampling bias in boreholes from acoustic televiewer logs
    (New Zealand Geothermal Workshop, 2012) Massiot, C.; Lewis, B.; Price, L.; Bignall, G.; McNamara, David D.; |~|
    Targeting structurally controlled permeability remains a challenge in high temperature geothermal fields, because of the difficulties in characterising faults and fractures and their behaviour within the reservoir. The large-scale structural framework of a reservoir is usually well defined from offsets of key marker stratigraphic units intersected by wells. Some of these large-scale faults significantly contribute to reservoir permeability. Smaller-scale structures, particularly inferred active fractures, are also of major importance for the vertical and lateral flow of fluid within fractured formations. To identify the structures directly within the formations, acoustic televiewer logs are acquired in New Zealand geothermal fields with the advent of the Acoustic Formation Imaging Technology (AFIT) tool, which is rated to 300°C. This wireline logging tool acquires a full 360° acoustic image of the inside of the borehole. Typically, fractures have different acoustic impedances from the wall-rock formation and appear as discordant features on the image, which can be systematically picked during image analysis. Each fracture has its true orientation (dip/dip direction) calculated in-situ taking into account image orientation and well deviation. The detailed analysis of these wireline logs provides insights on the nature, distribution, aperture and orientation of the fractures directly at the borehole wall. This information can be correlated to other logs to identify which structures may be open to fluid flow. However, fractures sub-parallel to the borehole axis will be under-sampled as fewer are intersected by the well. Here we describe a technique which we use to statistically correct for the natural bias involved when counting fractures intersected by a borehole at various angles. We demonstrate the impact that this bias can have on the structural characterisation of a fractured reservoir from acoustic televiewer images, using examples from four AFIT log intervals acquired in the Rotokawa Andesite, Rotokawa Geothermal Field (New Zealand). This correction provides a more accurate representation of the true structural character of the reservoir. The resultant, improved dataset allows for greater confidence in reservoir characterisation, future well targeting, as well as fracture and reservoir modelling.
  • Publication
    Quantifying the stress distribution at the Rotokawa Geothermal Field, New Zealand
    (New Zealand Geothermal Workshop, 2012) Davidson, Jonathan; Siratovich, Paul; Wallis, Irene; Gravley, Darren; McNamara, David D.; |~|
    Knowledge of the orientation and magnitude of the principal stresses can be used to model the behavior of faults and fractures, and determine how they may influence fracture hosted permeability in geothermal reservoirs. The permeability of the Rotokawa geothermal reservoir is dominantly fracture hosted and tectonic stresses are largely responsible for maintaining fluid flow in the reservoir. Reactivation of a fault or fracture depends on its orientation relative to the orientation of the stress field and the magnitude of the principle stresses. The purpose of this study is to determine the magnitude of the three principal stress axes at Rotokawa, and how they vary spatially. This will help our understanding of the distribution of fracture-hosted permeability in the reservoir. In the extensional tectonic settings, such as the Taupo Volcanic Zone, the magnitude of the vertical stress is dominated by the weight of the overburden. Previous rock density studies on core from Rotokawa wells and on rock from other geothermal fields are used here, along with variable thicknesses of different geologic units, to model the vertical stress. Leak-off tests and acoustic images that contain stress induced features are used to quantify aspects of the minimum and maximum horizontal stresses. We show that the differential stress between the vertical and minimum horizontal is near the threshold for frictional failure. More importantly, preliminary results of our study indicate that spatial variation in the vertical stress magnitude may be an important factor in fracture permeability. This study highlights some of the difficulties faced when attempting to estimate stress magnitudes in a geothermal reservoir hosted in a complex volcanic terrain.
  • Publication
    The nature of fracture permeability in the basement greywacke at Kawerau Geothermal Field, New Zealand
    (Stanford University, 2012-01-30) Wallis, I.; McNamara, David D.; Rowland, J.; Massiot, C.; |~|
    The Mesozoic basement at Kawerau Geothermal Field comprises well indurated, inter-bedded sandstones and argillites with a complex structural history. These rocks have very low matrix porosity but nonetheless host both geothermal production and injection. Fluid flow therefore is localized in fault and fracture networks. The geometry of, and potential controls on, these fluid pathways is revealed by an integrated study of borehole acoustic image logs, geologic, drilling and reservoir data from two deep geothermal injection wells. Well permeability, as interpreted from pressure, temperature, and fluid velocity logs acquired during well completion testing, correlates with large aperture fractures and zones where cross-cutting fractures are densely distributed. The occurrence of large aperture fractures also correlates with the occurrence of high sandstone proportions in the drill cuttings. A similar spatial relationship between fracture aperture and rock type occurs in exposed basement greywacke hosted Kuaotunu epithermal deposit, Coromandel Peninsula, New Zealand. These observations demonstrate the importance of understanding material properties when exploiting and stimulating fracture permeability. Despite the complex structural history of the basement greywacke, nearly all large aperture fractures identified from image logs were found to be optimally orientated for reactivation within the modern stress field. Comparison between the orientations of fractures observed down-hole and the orientation of field-scale faults interpreted from vertical displacement between wells reveals a structural relationship across scales. An understanding of the relationship between the modern stress field, field-scale structures and fractures that contribute to wellbore flow can be applied to the mapping of reservoir fracture permeability.
  • Publication
    Fractures interpreted from acoustic formation imaging technology: Correlation to permeability
    (Stanford University, 2011-01-31) McLean, K.; McNamara, David D.; |~|
    Permeable feed zones in geothermal wells are commonly identified using well profiles of temperature, pressure and fluid velocity measured at different injection rates during well completion testing and heat-up. While this data gives some indication of the depth and relative strength of the feed zones it does not give any information on the nature of the permeability in those zones, be it primary or secondary. Fracturing is thought to contribute to permeability in areas targeted for deep reinjection in the Wairakei geothermal system, within the Tahorakuri and Waikora Formations. By characterizing those deep fractures in terms of orientation, density and aperture, as well as determining the orientation of the horizontal stress field it is possible to interpret the fracture component of the well permeability. This has implications both for well targeting and reservoir modelling. The recent use of high temperature acoustic formation imaging technology (AFIT) can provide the necessary fracture and stress data to assess the contribution of fractures to feed zone permeability. As part of an ongoing AFIT logging program at Wairakei, data has been collected from the open hole of a number of deep wells in the southern part of the field. The location of feed zones in these wells has been interpreted from the completion test data and then correlated with AFIT fracture density and aperture data to provide more accurate feed zone depths and to characterise the nature of the permeability. Only fractures with optimal orientation within the local stress field are considered as potentially open to fluid flow. While the correlation between feed zones and fracture density is poor, good correlation is observed with the location of individual wide-aperture fracture zones. These zones may represent significant flow paths in the reservoir.
  • Publication
    Electron backscatter diffraction (EBSD) in the SEM: applications to microstructures in minerals and rocks and recent technological advancements
    (Sociedad Española de la Mineralogía, 2008-09-16) Mariani, Elisabetta; Prior, D. J.; McNamara, David D.; Pearce, M.A.; Seaton, N.; Seward, G.; Tatham, D.; Wheeler, J.; |~|
    Electron backscatter diffraction (EBSD) is based on the principle that a beam of electrons generated in the scanning electron microscope (SEM) is the source of randomly scattered electrons in a specimen. The backscattered electrons (BSE) that escape the sample generate a Kikuchi pattern on a phosphor screen, which is linked to the specimen crystal structure. Different crystal orientations generate different EBSPs. EBSD provides orientation, misorientation and boundary measurements from a small area (>1 μm) in a crystal and automated EBSD analysis is applied to an increasingly large number of rock-forming minerals. Excellent results are obtained for example on calcite and quartz and recent achievements include successful automated indexing of low symmetry minerals such as feldspars and omphacite. The effective application of EBSD to mineralogy and petrology has increased and includes detailed studies of microstructures, twin boundaries, deformation mechanisms and metamorphic processes. A technique for in-situ high temperature tensile deformation of minerals and rocks is currently being developed.