David D. McNamara

Heterogeneity of structure and stress in the Rotokawa Geothermal Field, New Zealand

Geometric characterization of a geothermal reservoirs structures, and their relation to stress field orientation, is vital for resource development. Subsurface structure and stress field orientations of the Rotokawa Geothermal Field, New Zealand, have been studied, for the first time, using observations obtained from analysis of three acoustic borehole televiewer logs. While an overall NE-SW fracture strike exists, heterogeneity in fracture dip orientation is evident. Dominant dip direction changes from well to well due to proximity to variously oriented, graben-bounding faults. Fracture orientation heterogeneity also occurs within individual wells, where fractures clusters within certain depth intervals have antithetic dip directions to the wells dominant fracture dip direction. These patterns are consistent with expected antithetic faulting in extensional environments. A general SHmax orientation of NE-SW is determined from induced features on borehole walls. However, numerous localized azimuthal variations from this trend are evident, constituting stress field orientation heterogeneity. These variations are attributed to slip on fracture planes evidenced by changes in the azimuth of drilling-induced tensile fractures either side of a natural fracture. Correlation of observed fracture properties and patterns to well permeability indicators reveal that fractures play a role in fluid flow in the Rotokawa geothermal reservoir. Permeable zones commonly contain wide aperture fractures and high fracture densities which have a dominant NE-SW strike orientation and NW dip direction. Studies of this kind, which show strong interdependency of structure and stress field properties, are essential to understand fluid flow in geothermal reservoirs where structural permeability dominates.

The nature of fracture permeability in the basement greywacke at Kawerau Geothermal Field, New Zealand

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.

Fractures interpreted from acoustic formation imaging technology: Correlation to permeability

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. 1.0 INTRODUCTION Recent deep drilling at Wairakei in the Karapiti South reinjection area has drilled beyond the relatively well-understood Waiora Formation and into the Tahorakuri formation, encountering previously unknown deep permeable zones around 2000-2500m depth. The nature of permeability in these zones and the controls on that permeability are of interest both for the reinjection strategy at Wairakei as well as understanding the nature of the connection between Wairakei and Tauhara geothermal systems. The acoustic formation imaging tool (AFIT) or ‘borehole televiewer’ has provided a fracture dataset for the deep sections of these wells. Of the hundreds of fractures imaged only a small number will be permeable as fractures in the well bore wall need to also extend a significant distance beyond the well bore and be interconnected. Correlation with feed zones identified from completion testing enables the identification of fractures associated with permeability. The spatial extent and orientation of these permeable fractures is providing new insights into the mechanics of the reservoir. 2.0 RESERVOIR SETTING Pressure interference between the neighbouring Wairakei and Tauhara geothermal fields and the fluid geochemistry indicate that they are separate fields with separate upflows which are hydrologically connected at more than one level. The deep wells WK317, WK404 and WK407 are located along the southern and eastern boundary of the Wairakei system, between the Wairakei and Tauhara systems (Figure 1). It is noteworthy that the overall permeability found in each of the wells discussed is in the “high” to “very high” range, even for Wairakei, with values in the order of 100 t/h per bar and greater. Figure 1: Well layout map for Wairakei-Tauhara. Wells in depth range 2500-3000m are in green, 1800-2500m in red, all others in yellow. As illustrated in Figure 2 the active extensional tectonic setting of the Taupo Volcanic Zone (TVZ) has produced normal faulting oriented NE-SW throughout the region (Bignall et al, 2010), perpendicular to the orientation of the connection between Wairakei and Tauhara (Figure 1). Figure 2: 3D geological model of the Wairakei system. Waiora Formation in pale blue and pink, Tahorakuri Formation in brown and rhyolite lavas in red. Wairakei and Tauhara have distinct pressure-depth profiles at shallow depths (Figure 3). While no deep drilling has been completed at Tauhara, projection of the shallow pressure gradient predicts intersection with the Wairakei pressure gradient around minus 2500mRL, implying a permeable connection around this level. Figure 3: Pressure-depth profiles for Wairakei and Tauhara fields. 3.0 GEOLOGIC SETTING The two major geologic formations relevant to this study are described below, from Bignall et al (2010) and the stratigraphic relationship between the formations is illustrated in Figure 4. 3.1 Waiora Formation This is a thick volcanic sequence of nonwelded/welded ignimbrite, tuff and breccia, with interlayered mudstones and siltstones, containing both rhyolite and andesite lavas. The current understanding of permeability in the Waiora Formation is that is it is controlled by flow unit boundaries, particularly rhyolite lava boundaries. 3.2 Tahorakuri Formation This is a pumiceous lithic tuff with intercalated partially welded ignimbrite. The tuff contains pumice, rhyolite lava and siltstone. Minor occurrences of the Waikora Formation greywackepebble conglomerate are intercalated with the Tahorakuri Formation. Figure 4: Cross-section in the vicinity of WK317 modified from Milicich et al (2010). 4.0 ACOUSTIC FORMATION IMAGING 4.1 Data acquisition The AFIT tool is an acoustic borehole televiewer that is capable of operation in conditions ≤300°C. Developed by Advanced Logic Technology (ALT) in Europe it is operated in New Zealand by Tiger Energy Services (TES). As the AFIT tool is lowered and raised in the well an acoustic transducer emits a sonic pulse. This pulse is reflected from a rotating, concave mirror in the tool head, focusing the pulse and sending it out into the borehole. The sonic pulse travels through the borehole fluid until it encounters the borehole wall. There the sonic pulse is attenuated and some of the energy of the pulse is reflected back toward the tool. This is reflected off the mirror back to the receiver and the travel time and amplitude of the returning sonic pulse is recorded. Through the use of the rotating mirror (≤5 rev/sec) 360° coverage of the inside of the borehole wall can be obtained. 4.2 Data set acquired Planar geological features such as fractures can be observed as sinusoids on the final imaged data set (Figure 5). Accelerometers and magnetometers within the tool allow accurate structural measurements (strike and dip) to be obtained using processing software RECALL. Further characterization of geological features (e.g. high or low amplitude, fracture density, fracture aperture) is also carried out using this software. The final dataset obtained is a spreadsheet including fracture depth, type, dip, dip direction and aperture. Figure 5: Example of AFIT amplitude response with sinusoidal intersection of fracture plane with wellbore. 4.3 Filters applied for this study Before attempting to correlate fractures with the permeable zones from the completion test a number of filters are applied to the dataset in order to ensure that only fractures potentially open to fluid flow are included. 4.3.1 Confidence filter Fractures in the dataset are classified as low confidence if their shape or existence is unsure. To lend a greater degree of confidence to the conclusions of this study, the low confidence fractures are filtered from the dataset. 4.3.2 Amplitude filter Fractures with a low amplitude signal (dark) on an acoustic image are often interpreted as open. However these dark fractures may also be filled with sulfide minerals and so close attention is paid to alteration geology to distinguish these from other dark fractures. A fracture with a high amplitude signal (bright) is often interpreted as a closed fracture as the high amplitude signal can be attributed to a hydrothermal mineral fill. For the purposes of this study all high amplitude fractures are filtered from the dataset. 4.3.3 Azimuth filter The maximum horizontal stress in the Taupo Volcanic Zone is generally oriented NE-SW as the orientation of the extension in this rift basin (and hence the minimum horizontal stress) is oriented NW-SE. The exact maximum horizontal stress orientation Shmax at the well can be determined directly from measurement of drilling induced tensile fractures (DITF, Figure 6) observed on the AFIT image. The orientation of Shmax ranges between 035 and 045o in the Wairakei wells imaged to date (Table 1). Table 1: Summary of Shmax orientations Well Shmax orientation WK317 045o

Fracture width and spacing distributions from borehole televiewer logs and cores in the Rotokawa Geothermal Field, New Zealand

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, lognormal, 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 (<0.5 5 m) is best modelled by an exponential distribution and higher spacing by lognormal or pareto (power-law) distributions, except for the N – S striking dataset and the NE – SW striking fracture set in well RK32. The change of distribution model at different scales may be linked to the threshold at which fractures start interacting with each other. This work to date underlines the need to combine data spanning a broad range of length scales to conduct a sound statistical analysis of fracture populations and highlights the control on fracture formation by a combination of processes including tectonics, lava cooling and stress perturbations associated with stratigraphic anisotropy. The resulting distributions provide a basis for simulating and calibrating fracture models of geothermal reservoirs beyond those areas directly sampled with BHTV logs or cores and will integrate variations observed over a range of scales between the study wells.

Statistical corrections of fracture sampling bias in boreholes from acoustic televiewer logs

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 undersampled 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.

Quantifying the stress distribution at the Rotokawa Geothermal Field, New Zealand

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 fracturehosted 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.

Statistical methods of fracture characterization using acoustic borehole televiewer log interpretation

Acoustic borehole televiewer (BHTV) logs provide measurements of fracture attributes (orientations, thickness, and spacing) at depth. Orientation, censoring, and truncation sampling biases similar to those described for one-dimensional outcrop scanlines, and other logging or drilling artifacts specific to BHTV logs, can affect the interpretation of fracture attributes from BHTV logs. K-means, fuzzy K-means, and agglomerative clustering methods provide transparent means of separating fracture groups on the basis of their orientation. Fracture spacing is calculated for each of these fracture sets. Maximum likelihood estimation using truncated distributions permits the fitting of several probability distributions to the fracture attribute data sets within truncation limits, which can then be extrapolated over the entire range where they naturally occur. Akaike Information Criterion (AIC) and Schwartz Bayesian Criterion (SBC) statistical information criteria rank the distributions by how well they fit the data. We demonstrate these attribute analysis methods with a data set derived from three BHTV logs acquired from the high-temperature Rotokawa geothermal field, New Zealand. Varying BHTV log quality reduces the number of input data points, but careful selection of the quality levels where fractures are deemed fully sampled increases the reliability of the analysis. Spacing data analysis comprising up to 300 data points and spanning three orders of magnitude can be approximated similarly well (similar AIC rankings) with several distributions. Several clustering configurations and probability distributions can often characterize the data at similar levels of statistical criteria. Thus, several scenarios should be considered when using BHTV log data to constrain numerical fracture models.

Exploring structure and stress from depth to surface in the Wairakei Geothermal Field, New Zealand

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?

Quantitative Analysis of EBSD Data in Rocks and other Crystalline Materials: Investigation of Strain Induced Recrystallisation and Growth of New Phases

Misorientation can be calculated over large datasets and a theme of this paper is the usefulness of examining the results statistically. Comparing the statistics of misorientations calculated from neighbouring pixels (or grains) with those calculated from pairs of pixels (or grains) selected at random helps to indicate deformation and recrystallisation mechanisms. Taking boundary length into account provides a link to grain boundary energy, and boundary length versus misorientation data should be used to examine how boundaries with different misorientations evolve through time. Time lapse misorientation maps indicate how orientation changes through time at particular points in a microstructure during in situ experiments. The size of areas which have changed orientation by particular amounts can be linked to boundary length and boundary migration velocities. When dealing with different phases, the statistics of angular relationships, akin to intraphase misorientation analysis, can indicate orientation relationships in the absence of prior knowledge, which is advantageous in investigating the plethora of minerals that make up the Earth.

The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand

Ultrasonic image logs acquired in the DFDP‐2B borehole yield the first continuous, subsurface description of the transition from schist to mylonite in the hangingwall of the Alpine Fault, New Zealand, to a depth of 818 m below surface. Three feature sets are delineated. One set, comprising foliation and foliation‐parallel veins and fractures, has a constant orientation. The average dip direction of 145° is subparallel to the dip direction of the Alpine Fault, and the average dip magnitude of 60° is similar to nearby outcrop observations of foliation in the Alpine mylonites that occur immediately above the Alpine Fault. We suggest that this foliation orientation is similar to the Alpine Fault plane at ∼1 km depth in the Whataroa valley. The other two auxiliary feature sets are interpreted as joints based on their morphology and orientation. Subvertical joints with NW‐SE (137°) strike occurring dominantly above ∼500 m are interpreted as being formed during the exhumation and unloading of the Alpine Faults hangingwall. Gently dipping joints, predominantly observed below ∼500 m, are interpreted as inherited hydrofractures exhumed from their depth of formation. These three fracture sets, combined with subsidiary brecciated fault zones, define the fluid pathways and anisotropic permeability directions. In addition, high topographic relief, which perturbs the stress tensor, likely enhances the slip potential and thus permeability of subvertical fractures below the ridges, and of gently dipping fractures below the valleys. Thus, DFDP‐2B borehole observations support the inference of a large zone of enhanced permeability in the hangingwall of the Alpine Fault.