13 research outputs found
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
Increasing the permeability of hydrothermally altered andesite by transitory heating
Changes in permeability can impact geological processes, geohazards, and geothermal
energy production. In hydrothermal systems, high-temperature heat sources drive fluid
convection through the pore network of reservoir rocks. Additionally, thermal fluctuations
may induce microfracturing and affect the mineralogical stability of the reservoir rock, thus
modifying the fluid pathways and affecting permeability and strength. This study describes
the results of thermal heating events lasting several hours on a âmoderately alteredâ
plagioclase-clinochlore-calcite-quartz andesite and a âhighly-alteredâ plagioclaseclinozoisite-
quartz-clinochlore andesite from the Rotokawa geothermal field, New Zealand.
We use a low thermal gradient (~ 1.2 °C/min) in an H2O-saturated, 20 MPa pressure
environment to constrain changes in petrophysical properties associated with transitory
thermal phenomena between 350 °C and 739 °C. As the treatment temperature increases, the
mass reduces, while porosity and permeability increase. These effects were greater in the
âmoderately alteredâ andesite than in the âhighly alteredâ andesite. Microfracturing is
responsible for these changes at lower temperatures (e.g. up to 400 °C). At higher
temperatures (e.g. > 400 °C), microfracturing remains partially responsible for these rock
property changes (e.g. higher permeability); however, these changes are also a product of
clinochlore, quartz, and (when present) calcite reacting out of the altered andesite, and
increasing porosity. We propose that at temperatures > 400 °C, volumetric phase changes
associated with heat driven reactions in a wet environment can contribute to micro-cracking
and porosity/permeability changes. Our data support observations where high-temperature
conditions at the margins of magma bodies can be associated with substantial increased
permeability and decreased strength
Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating
info:eu-repo/semantics/publishe
The characterisation of an exhumed high-temperature paleo-geothermal system on Terre-de-Haut Island (the Les Saintes archipelago, Guadeloupe) in terms of clay minerals and petrophysics
Prediction Modeling for the Estimation of Dynamic Elastic Youngâs Modulus of Thermally Treated Sedimentary Rocks Using LinearâNonlinear Regression Analysis, Regularization, and ANFIS
Improved correlation between the static and dynamic elastic modulus of different types of rocks
Thermal deterioration of high-temperature granite after cooling shock: multiple-identification and damage mechanism
UâPb geochronology documents out-of-sequence emplacement of ultramafic layers in the Bushveld Igneous Complex of South Africa
Coda wave interferometry during the heating of deep geothermal reservoir rocks
Abstract Coda wave interferometry (CWI) is a high-resolution technique that aims at tracking small changes in a diffusive medium from the time correlation of seismic waveforms. CWI has been widely used in recent years to monitor the fine-scale evolution of fault zones and more recently of deep reservoirs. However, to provide a quantitative interpretation of the reservoir, direct modeling of physical effects like the influence of temperature on seismic wave scattering is required to investigate temperature effects from measurements of velocity changes. Here, we propose to quantify the impact of thermo-elastic deformation on CWI measurements by comparing experimental results obtained from a previous study on Westerly Granite to a numerical approach based on two combined codes (SPECFEM2D and Code_Aster) for modeling wave propagation in complex media during thermo-elastic deformation. We obtain two major results. First, we show that multiple reflections on the boundaries of our simplified numerical sample reproduce well the wave scattering properties of the experimental granitic sample characterized by a complex mineral assembly and a large set of microcracks. We based our comparison on the wave diffusion model that describes both the experimental and numerical samples (similarity in energy density function and mean free path). We also show that both samples share a similar thermo-elastic behavior, but only after the second heating and cooling cycle. Second, the stretching technique used for CWI measurements on both samples reveals reversible time shifts correlated with the thermo-elastic deformation of the sample. However, the influence of thermo-elastic deformation is different between our numerical proxy and the experimental sample. We discuss the role of irreversible deformation (e.g., microcracking) for the observed discrepancy by introducing temperature dependence of elastic moduli in the model. These results suggest that there are open perspectives to monitor thermal strain in geothermal reservoirs using CWI