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The UC Research Repository collects, stores and makes available original research from postgraduate students, researchers and academics based at the University of Canterbury.
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Recent Submissions
Approaches to imaging feedzone diversity with case studied from Sumatra, Indonesia and the Taupō Volcanic Zone, New Zealand
(2020) Wallis I; Rowland, J; Dempsey, David; Allan, G; Sidik, R; Martikno, R; McLean, K; Sihotang, M; Azis, H; Baroek, M
There is a fast-growing inventory of studies on borehole
image logs acquired in geothermal reservoirs as more
operators elect to deploy this technology. Our contribution to
this inventory is to illustrate how judicious use of these data
may reveal the geologic controls on permeability. We also
provide an open source Python library that enables others to
replicate the methods described herein. Our study includes a
discussion of geometric sample bias, as well as those data
integrity and geological factors that influence fracture
frequency. We also demonstrate slip tendency modelling as
an approach to identifying fractures that may be relevant
beyond the borehole wall, which is key for geothermal wells
where thermal stresses have enhanced both the number and
apparent aperture of fractures at the borehole wall. We
illustrate these methods using seven well case studies from a
wide range of lithologies, four reservoirs, and two tectonic
settings—one dominated by a volcano-tectonic rift and the
other a mega-shear zone. The reservoirs are Muara Laboh
and Rantau Dedap in Indonesia and Ngatamariki and
Wairakei in New Zealand.
Heat Extraction Analysis of Multi-Stage Hydraulic Fracturing Doublet Horizontal EGS Wells
(2021) Yu, P; Dempsey, David; Archer R
Enhanced Geothermal System (EGS) are developed using various stimulation technologies to improve the production of heat energy
from hot dry rock (HDR) that has ultra-low permeability. In this study, a horizontal EGS well design with partially bridging multistage
hydraulic fractures is presented. Based on the proposed design, a semi-analytical model for temperature is derived assuming bi-linear
heat transfer in the fractures and stimulated reservoir volume (SRV). The model considers heat conduction and advection in the SRV,
and, depending on the number and spacing of fractures, can be used to optimize EGS design. A 3D numerical model is also developed
to validate the semi-analytical model and test geometry effects. The numerical model is constructed in the COMSOL finite element
solver and compared to a fully bridging hydraulic fracture design. The results show that the partially bridging design can obtain a
longer period of high temperature production, delaying thermal breakthrough by forcing water to traverse the SRV.
A Users Guide to Leak-off Test Procedures and Interpretation for Geothermal Wells
(2021) Wallis I; Pye, DS; Dempsey, David; Rowland, J
Leak-off Tests (LOT) and Extended Leak-off Tests (XLOT) are conducted during drilling to verify the competence of the cement
around a casing shoe and determine the maximum allowable mud weight for the next hole section. (X)LOT interpretation yields the
fracture gradient, which is a key parameter in well control procedures, mud program design and cement operations. These data are
also one of the minimum requirements for any geomechanical study that seeks to understand the role fractures play in reservoir
permeability. Despite its utility for successful well completion and studies investigating reservoir permeability, (X)LOT procedures
are often poorly implemented and the interpretation methods used are not always appropriate for geothermal conditions.
Using a case study, we describe the test procedures and interpretation process for typical geothermal conditions. We address key
issues that set geothermal apart from oil and gas, such as the impact of high temperatures on test results, the high frequency of
naturally occurring fractures, and the variable physical properties of hydrothermally altered volcanic rocks. We illustrate the impact
of quality checking an existing (X)LOT dataset and review methods for estimating the fracture gradient prior to drilling in extensional
environments. While our paper addresses a number of detailed technical issues related to interpretation, it aims to be a pragmatic and
geothermally-relevant guide for those who aspire to improve the quality of these tests.
Assessment of risk on the Kepler Track, Fiordland.
(2002) Ferrick, Bree
With the increase in use of New Zealand's mountain terrain there is a growing need to
identify and manage the hazards that exist. This study investigates the potential for
integrated risk assessment to be undertaken in remote mountainous environments
using methods of aerial photo and contour map interpretation, field studies, climate
data analysis and GIS on the Kepler Track, Fiordland. The study presents an
opportunity to gauge the applicability of GIS in remote regions relatively devoid of
empirical data.
Fieldwork and analysis of previous research identified three main forms of hazard on
the Kepler; that of avalanche, landslide and exposure. The study identified fourteen
avalanche paths in the alpine traverse of Mt Luxmore, characterised by steep, gully type
starting zones and N-NW aspects. The estimated recurrence interval of
avalanching is 0.3-Syrs. Six zones of exposure were mapped on N-NW aspects and
exposed ridgelines. These have the greatest estimated frequency of up to 15 times per
year. Potential landslide failure zones were delineated using GIS. The identified 33
zones were characterised by steep slopes along drainage channels and by frequency of
landsliding estimated at l.6yr⁻¹ in summer and 0.7yr⁻¹ in winter.
Using the estimated frequencies a risk index of overall risk for the individual hazards
was developed. The risk index identified the variation in risk level with season, the
greatest risk being from exposure. GIS then provided a means to assess and integrate
risk to create risk maps for the track in both winter and summer seasons. The
management implications of these findings are outlined. There is potential for GIS to
be employed in risk assessment of remote alpine areas and although the application of
GIS was limited in this study, it now provides a basis on which to assess the use of
GIS in delineation of hazard zones from future events.
Reservoir Microearthquake Modeling Analysis: a Proof-of-concept Study and Its Application to Injection Fluid-Induced Seismicity
(2021) Rivera, J; Dempsey, David
Microearthquakes (MEQs) occur when fluid is reinjected into the reservoir, raising the pressure in the vicinity of the injection well.
The pressure build-up in the reservoir due to fluid injection decreases the rock yield strength, which causes shear failure, thus
triggering a seismic event. This mechanism presents a further opportunity to use microseismicity as a means to calibrate reservoir
parameters, particularly the active faults which tend to be the most conductive fluid flow pathways. The study aims to integrate
MEQ modelling to the reservoir development workflow and to the calibration workflow to estimate the permeability of the
formations and the faults.
The proof-of-concept study considers a synthetic induced seismicity model which represents an area where the fluid is being
injected. Reservoir simulation is conducted to evaluate pressure migration through the reservoir for a given reservoir and fault
parameters. The earthquake model uses the pressure change from the simulation to compute the average seismicity rate of the fault
as well as the spatiotemporal evolution of the seismic events. Synthetic MEQ data is then generated from the earthquake simulation
using the Poisson model, which serves as the data for calibration and inverse modelling. Synthetic inversion is then performed to
estimate the permeability of both the reservoir and the fault using Markov Chain Monte Carlo (MCMC) sampling method. The
study also includes the effects of variation in MEQ data and other uncertainties in the model in parameter estimation. The method
developed in this study is then applied to an injection fluid-induced seismicity from a wastewater injection site.