Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ) hosts 23 geothermal fields, seven of which are currently utilised for power generation. Ngatamariki geothermal field (NGF) is one of the latest geothermal power generation developments in New Zealand (commissioned in 2013), located approximately 15 km north of Taupo. Samples of reservoir rocks were taken from the Tahorakuri Formation and Ngatamariki Intrusive Complex, from five wells at the NGF at depths ranging from 1354 to 3284 m. The samples were categorised according to whether their microstructure was pore or microfracture dominated. Image analysis of thin sections impregnated with an epoxy fluorescent dye was used to characterise and quantify the porosity structures and their physical properties were measured in the laboratory. Our results show that the physical properties of the samples correspond to the relative dominance of microfractures compared to pores. Microfracture-dominated samples have low connected porosity and permeability, and the permeability decreases sharply in response to increasing confining pressure. The pore-dominated samples have high connected porosity and permeability, and lower permeability decrease in response to increasing confining pressure. Samples with both microfractures and pores have a wide range of porosity and relatively high permeability that is moderately sensitive to confining pressure. A general trend of decreasing connected porosity and permeability associated with increasing dry bulk density and sonic velocity occurs with depth; however, variations in these parameters are more closely related to changes in lithology and processes such as dissolution and secondary veining and re-crystallisation. This study provides the first broad matrix permeability characterisation of rocks from depth at Ngatamariki, providing inputs for modelling of the geothermal system. We conclude that the complex response of permeability to confining pressure is in part due to the intricate dissolution, veining, and recrystallization textures of many of these rocks that lead to a wide variety of pore shapes and sizes. While the laboratory results are relevant only to similar rocks in the Taupo Volcanic Zone, the relationships they highlight are applicable to other geothermal fields, as well as rock mechanic applications to, for example, aspects of volcanology, landslide stabilisation, mining, and tunnelling at depth

Comparative field study of shallow rhyolite intrusions in Iceland:emplacement mechanisms and impact on country rocks

Shallow silicic intrusions are known to exist in many active volcanoes and can fuel both eruptions and hydrothermal fields. However, our knowledge of magma intrusions remains far from complete, and processes occurring at intrusion margins are poorly understood. In this field-based study, we characterise four shallows, dissected rhyolitic intrusions at three sites in Iceland (Njarðvík-Dyrfjöll, Krafla and Húsafell central volcanoes). We focus on the relationship between intrusion emplacement mechanisms and country rock response, employing scanline mapping of fractures and in-situ rock property measurements (hardness and permeability) along transects from the intrusion margins to damaged and undamaged country rocks.We identify various scenarios of shallow intrusion emplacement style, based upon their diverse geometry and lithofacies architecture. Additional information from rock properties and characteristics of fractures and vesicles, indicates that initial country rock properties strongly influence the emplacement style. We identify two discrete types of country rock response to magma injection. The matrix permeability of weak, porous and permeable lithologies (conglomerate and hyaloclastite) is reduced by >1 order of magnitude adjacent to intrusions due to pore occlusion. Stronger and denser, low-permeability lithologies (basalt and welded ignimbrite) undergo a decrease in hardness by a factor >2 related to an up to fivefold increase in fracture density, with no significant change in matrix permeability.Our observations highlight the importance of robust characterisation of the mechanical properties of caldera-filling or geothermal reservoir formations, for appropriate forecasting of magma mobility, geophysical data interpretation, and geothermal resources characterisation

Thermal Stimulation of the Rotokawa Andesite: A Laboratory Approach

Thermal stimulation of geothermal wells is a production enhancement technique that is an attractive option to operators of geothermal fields as a way to enhance and revitalize well performance capabilities through injection of cold water into the geothermal reservoir. This thesis presents a review of thermal stimulation procedures that have been carried out at various geothermal fields worldwide, and then sets out to demonstrate through laboratory experiments the effects of thermal stimulation on typical reservoir rocks. Thermal damage to crustal rocks is important in many fields of practical engineering applications. Thermal fractures have been discussed in many studies, however their formation under fully water saturated conditions as a result of rapid quenching is not fully understood. In this study, a new methodology is designed to replicate thermal stimulation in such an environment, using an apparatus that allows rocks to be heated to 350°C at up to 22 MPa confining pressure and rapidly quenched with cold water to ambient temperature while maintaining system pressure. The results indicate that through thermal cycling in the apparatus, porosity was increased, density decreased, acoustic velocities attenuated and mechanical properties significantly altered. Maximum damage occurred during the first thermal cycle, a product of the thermo-mechanical Kaiser effect such that rocks should not experience additional damage unless a previous maximum stress is surpassed. The thesis details a comprehensive evaluation of the Rotokawa Andesite sourced from the Rotokawa Geothermal field located in the Taupo Volcanic Zone, New Zealand. The importance of microstructural fabrics on the physical properties of this reservoir lithology is demonstrated. The mineralogical and petrological fabrics of the rocks are coupled with detailed studies of the microstructural fracture networks, including measurements of porosity, density and permeability. Acoustic wave velocities and dynamic elastic moduli were determined. Uniaxial compressive strength testing coupled with acoustic emission have helped to determine the behavior of the rock under deformation and provided data to characterize the static elastic moduli of the rocks. These data are then utilized to build empirical, micromechanical and geometric relationships. To better constrain important engineering concerns such as wellbore stability, reservoir forecasting and stimulation procedures, thermal property measurements were carried out on samples recovered from the Rotokawa Andesite. In particular, measurements of linear thermal expansion, thermogravimetric analysis, and differential scanning calorimetry were measured utilizing varied experimental heating rates of 2, 5 and 20 K/min. The property analyses were carried out to determine if heating rates influenced the measurement of thermal properties, specifically thermal expansion coefficients and strain rate in the samples. Results indicate that thermal expansion is not heating rate dependent within the range investigated though the strain rate is significantly dependent on heating rate, with higher strain rates observed in conjunction with higher heating rates. By using a one dimensional stress model, a failure criterion can be established for the Rotokawa Andesite when subject to thermal stressing. The importance of this study is to further understand the critical heating and cooling rates at which thermal stress causes cracking within the Rotokawa reservoir. This can enhance permeability but can also affect wellbore stability, so constraining these conditions can be beneficial to resource utilization. To test effects of thermal stimulation in the laboratory, Rotokawa Andesite core was heated to 325ºC at pressure of 20 MPa and quenched rapidly to 20ºC while maintaining a pressure of 20 MPa. Permeability increased by an order of magnitude over original pre-treatment values. Ultrasonic velocities also reflected a significant change after stimulation testing. Scanning electron microscopy showed significant microstructural change to samples and supplemented physical property investigations. The results imply that thermal stimulation can be successfully repeated in the laboratory and is coupled with both thermal and chemical components. The results of these investigations are of profound importance for effective utilization and maintenance of the Rotokawa Geothermal field and the results also have implications for geothermal fields worldwide

Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand

Background Geothermal systems are commonly hosted in highly altered and fractured rock. As a result, the relationships between physical properties such as strength and permeability can be complex. Understanding such properties can assist in the optimal utilization of geothermal reservoirs. To resolve this issue, detailed laboratory studies on core samples from active geothermal reservoirs are required. This study details the results of the physical property investigations on Rotokawa Andesite which hosts a significant geothermal reservoir. Methods We have characterized the microstructure (microfracture density), porosity, density, permeability, elastic wave velocities, and strength of core from the high-enthalpy Rotokawa Andesite geothermal reservoir under controlled laboratory conditions. We have built empirical relationships from our observations and also used a classical micromechanical model for brittle failure. Further, we compare our results to a Kozeny-Carman permeability model to better constrain the fluid flow behavior of the rocks. Results We show that the strength, porosity, elastic moduli, and permeability are greatly influenced by pre-existing fracture occurrence within the andesite. Increasing porosity (or microfracture density) correlates well to a decreasing uniaxial compressive strength, increasing permeability, and a decreasing compressional wave velocity. Conclusions Our results indicate that properties readily measurable by borehole geophysical logging (such as porosity and acoustic velocities) can be used to constrain more complex and pertinent properties such as strength and permeability. The relationships that we have provided can then be applied to further understand processes in the Rotokawa reservoir and other reservoirs worldwide

The development and application of the alteration strength index equation

We have developed an Alteration Strength Index (ASI) equation to address the effect of hydrothermal alteration on mechanical rock properties. This equation can be used to estimate a range of rock strengths, comparable to uniaxial compressive strength (UCS), based on rapid analysis of mineralogy and microstructure. We used rock samples from three geothermal fields in the Taupo Volcanic Zone (TVZ) to represent a range of alteration types. These are sedimentary, intrusive and extrusive rocks, typical of geothermal systems, from shallow and deep boreholes (72 measured Depth (mD) to 3280 mD). The parameters used in ASI were selected based on literature relating these aspects of mineralogy and microstructure to rock strength. The parameters in ASI define the geological characteristics of the rock, such as proportions of primary and secondary mineralogy, individual mineral hardness, porosity and fracture number. We calibrated the ASI against measured UCS for our samples from the TVZ to produce a strong correlation (R2 of 0.86), and from this correlation we were able to derive an equation to convert ASI to UCS. Because the ASI–UCS relationship is based on an empirical fit, the UCS value that is obtained from conversion of the ASI includes an error of 7 MPa for the 50th percentile and 25 MPa for the 90th percentile with a mean error of 11 MPa. A sensitivity analysis showed that the mineralogy parameter is the dominant characteristic in this equation, and the ASI equation using only mineralogy can be used to provide an estimated UCS range, although the error (or uncertainty) becomes greater. This provides the ability to estimate strength even when either fracture or porosity information are not available, for example in the case of logging drill cuttings. This research has also allowed us to provide ranges of rock strengths based solely on the alteration zones, mineralogy, and depth of lithologies found in a typical geothermal field that can be used to update conceptual models of geothermal fields

ACID SOLUBILITY TESTING OF GREYWACKE CORE AND IMPLICATIONS FOR WELL PERMEABILITY ENHANCEMENT

Acidizing of geothermal wells can be a cost-effective and attractive option to recover well permeability and productivity compared to drilling new wells. Understanding how the reservoir rock may react to acid injection is important to ensure that damage to the reservoir formation does not occur and also to help understand if permeability can be improved. In an effort to investigate the effect of acid stimulation on Greywacke core, a series of core plugs were extracted from core taken from an injection well hosted in Greywacke. These plugs were measured for porosity, density, seismic wave velocity, and permeability. The samples were then selected to have a first suite exposed to HCl acid, a second to both an initial HCl treatment followed by HCl/HF treatment and a third untreated to act as control. The samples were re-measured for changes in their physical properties following acid exposure. The initial HCl testing results show changes in physical properties across the samples. The HCl/HF testing also resulted in changes to physical properties. Permeability increased in all samples exposed to acid treatment. Further, after physical characterization, the samples were mechanically tested to determine Uniaxial Compressive Strength (UCS). Strength decreased in all samples treated with acid. The results of the testing are discussed here and the implications for changes to reservoir permeability and strength are explored with respect to acid treatments on both injection and production wells hosted in Greywacke

Experimental Thermal Stimulation of the Rotokawa Andesite

Thermal cycling of rock by heating and rapid quenching in water significantly affects its physical, mechanical and elastic properties. In this study we present a novel technique where specially designed equipment simulates the cyclic thermal stimulation processes employed by the conventional geothermal industry. To enhance productivity and injectivity of geothermal wells, geothermal operators commonly inject fluids cooler than reservoirs into wells at pressures less than natural fracture gradients which can result in enhanced fluid handling capacities. In an attempt to better understand this process, the investigation of thermal stimulation at a laboratory scale has been conceived and implemented. We have designed and built an apparatus that allows the heating and quenching of representative samples by thermal stimulation in a pressure vessel capable of attaining 350°C and 24 MPa and sustaining pressure during quenching cycles. Core sourced from production wells in an active commercial geothermal field has been tested in the apparatus. Our studies have characterized specimens prior to and subsequent to thermal stimulation for density, porosity, permeability, micro-structural texture, mineralogical fabrics, acoustic velocities, dynamic and static elastic moduli. Our results indicate that our stimulation apparatus is capable of enhancing both microscopic and macroscopic permeability, increasing porosity, reducing bulk density and attenuating seismic velocities. We have enhanced porosity in our specimens by up to 1.0 (vol% ) over original values, attenuated compressional wave velocities by up to 15% and enhanced permeabilities by nearly an order of magnitude over initially observed values. We utilize scanning electron microscopy to evaluate the microstructural change to samples, supplementing physical property investigations. The results imply that thermal stimulation can be successfully replicated in the laboratory and is coupled with both thermal and chemical components. The implications of this study for future laboratory and field scale stimulation testing are then considered

Deformation, Strength, and Failure Mode of Deep Geothermal Reservoir Rocks

Rocks sourced from active geothermal systems can have unique responses to deformation, due to unique alteration mineralogy and complex microstructure. The current state of understanding of mechanical behaviour of rocks under varying stress conditions is well established on suites of rocks with simple mineralogy and microstructure. Brittle failure can increase porosity and permeability and generate seismicity, whereas inelastic deformation in the ductile regime will decrease porosity and will likely decrease permeability, and generate no or distinct low frequency seismicity. Many studies have focused on the behaviour of siliclastic and carbonate rocks to establish the transition form brittle to ductile behaviour. The geothermal systems in New Zealand, and many other areas, are hosted in mainly volcanic rocks, limiting the applicability of current data and knowledge to these systems. We present results from laboratory triaxial deformation and strength testing of drill core sampled from a deep geothermal reservoir. We have used our findings to construct failure criteria based on our investigations and compared them to the in-situ and induced stress conditions that may lead to macroscopically brittle or ductile deformation of the host rock. Our results show that under the current stress conditions at the Rotokawa geothermal field the host rock behaves in a brittle, rather than compactive, fashion. Under these in-situ stress conditions brittle fracture generation dominates over cataclastic pore collapse, resulting in a rock mass with suitable macroscale permeability for fluid extraction. Our results also show that the rock strength is typically too high for the induced stresses during drilling to initiate borehole breakout. This is supported by borehole observations revealing very little borehole damage in the host rock