Geothermal Systems
Characterisation of core fractures and diametrical core deformation analysis of a rock core originating from the 5 km deep Basel-1 well
Project framework and goals
Characterisation of the stress state of a rock mass is an important part in the geological assessment of a project site. In the external page Basel-1 petrothermal well about 8 m core was obtained from approximately 4909 m to 4917 m depth. Borehole breakouts of high confidence were identified in this depth range from acoustic televiewer logs. The core pieces show no natural fractures. However, core damage in the form of transverse features that resemble incipient to fully-developed core disking fractures are seen along the entirety of the core. Core disking is a consequence of high borehole-perpendicular stresses, and the disking features can be used as an independent constraint on the directions and relative magnitudes of the principal stresses. Furthermore, the SHmax orientation may also be inferred from the non-circular geometry of a core's circumference caused by the differential lateral expansion of the core upon extraction from a differentially stressed rock mass.
The thesis aims at: 1. investigating the geometry of the cores' circumference and the shape of incipient to fully-developed coring induced fractures (disking) of the Basel-1 core in order to infer horizontal stress directions; 2. comparing the newly-derived datasets with existing stress data from borehole breakout analysis; and 3. assessing the applicability of the applied methods for deducing stress information. In addition, we will characterise rock anisotropy.
Drilling-induced core fracture and core shape analyses are the focus of this thesis and will be investigated by utilising cutting-edge technologies including circumferential optical core scanner, x-ray computed microtomography (μCT), gamma-ray detection, diametrical core scanner, and 3D photogrammetric scans.
Supervisors: Martin Ziegler, Claudio Madonna (SCCER, ETH), Keith F. Evans (Geothermal Energy and Geofluids, ETH), Benoît Valley (Centre for Hydrogeology and Geothermics, Université de Neuchâtel)
Analysis of micro-seismicity during hydraulic fracturing at the Grimsel Test Site
Project framework
Hydraulic stimulations for enhanced geothermal systems involve high pressure fluid injection into low-permeability naturally fractured reservoirs to create an efficient heat exchanger. Fluid injection can cause initiation and propagation of new fractures (i.e. hydro-fracturing) and/or reactivation of existing natural fractures in shear (i.e. hydro-shearing). Numerous investigations towards understanding hydro-mechanically (HM) coupled processes associated with hydraulic stimulation have been performed at different scales using conceptual, analytical and numerical models. Almost all these studies suffer from a lack of HM field data that describe in detail the dominant geomechanical processes during stimulation. A large scale hydraulic shearing experiment that aims to increase our understanding on these processes is currently being planned the Grimsel Test site. In the first phase of this project, which has already been performed, a number of hydraulic fracturing experiments were produced to obtain a direct measurement of the minimum principle stress magnitude. During these experiments, micro-seismicity was recorded with a 32-channel monitoring system in triggered model. Analysis of the microseismic events should give important insight into the orientation of the minimum principle stress orientation.
Project goals
The goal of this Master thesis is to analysis the high-quality seismicity dataset recorded during the hydro-fractures in the Grimsel Test Site, and to provide a mechanical interpretation towards the mechanisms of induced seismicity during fracturing in anisotropic rock. In a first step, events found by the triggered system have to be relocated using an improved seismic velocity models. Then seismicity characteristics such as source mechanisms are discussed and relative magnitudes are computed. The time-dependent structure of relatively relocated seismicity clouds has to be interpreted towards the physical driving mechanisms leading to these seismic events. An important part of the thesis will be running a suite of numerical models that illustrate possible fracture propagation behaviour during fluid injection into an anisotropic medium. Hence, seismicity will not only provide essential information of stress field characteristics, but additionally, the give insights into the physical processes underlying injection-driven fracture initiation and propagation. Within the framework of this thesis, field work in the ongoing project at Grimsel, especially during recording of additional seismicity data during stimulation, is possible.
Supervisors: Dr. Valentin Gischig, Dr. Joseph Doetsch, Dr. Reza Jalali, Dr. Florian Amann
Hydro-mechanical coupled fluid flow in heterogeneous fractures
Project framework
Understanding hydro-mechanical coupled flow in heterogeneous fractures is crucial for many geo-engineering applications such as geothermal and oil reservoirs, groundwater recharge and nuclear waste storage. Numerous hydro-mechanically coupled experiments on fractures have been performed in the past to relate mechanical deformations such as aperture changes or shear dislocations to changes in fracture conduction or hydraulic aperture. These experiments were typically conducted on natural or artificial fractures that exhibit a heterogeneous fracture surface, and the heterogeneous fracture characteristics were often not taken into account for the analysis. Surface heterogeneities may have, however, a substantial influence on the actual fluid flow path that occurs in the pressurized fracture. Further, the flow path may change as the load on the fracture or the size of the fracture will be increased and may be dependent on the utilized flow rate.
Project goals
This study will approach four problems in hydro-mechanical coupled fracture flow: the influence of fracture geometry, the scale dependency of fracture flow, the pressure regime and the flow rate. For the study artificial fractures will be created in a granodiorite stemming from the Grimsel Test Site. The fractures will then be scanned using a high resolution photogrammetric scanner and the initial aperture field will be determined. Further, normal load cycles will be applied incrementally to the fractures (i.e. increments of 1 MPa in a normal stress range between 0.2 and 10 MPa). During each incremental load increase/decrease the fracture conductivity will be tested utilizing a high pressure pump and different flow rates. During the tests the mechanical aperture change will be monitored using high resolution strain gauges.
Experiments will be back-calculated with high fidelity numerical simulations that allow an explicit representation of the fracture heterogeneities. The numerical analysis will be intensively supervised by the project supervisors.
Supervisors: Dr. Florian Amann, Daniel Vogler, Dr. Claudio Madonna
Three-Dimensional Integrative Geological Modelling of In-situ Stimulation and Circulation Experiment at the Grimsel Test Site
Three-Dimensional integrative geological modeling has become a reliable, effective and powerful means to visualize and understand geological structures and their properties in a wide range of fields such as engineering geology, oil & gas exploration, mining, and geothermal. This type of modeling, known as static modeling, tends to collect and integrate various geological related information under a unified platform in order to simulate the geological features and create input values for dynamic models such as pressure, temperature, stress (or deformation), and coupled (combining two or more physical processes) models. This MSc thesis will be focused on three-dimensional geological modeling of the ISC experiment volume at the Grimsel test site using a combination of structural geological analysis and geophysical borehole measurements. Additional different geological data resources such as hydraulic, thermal and tracer tests, cross-hole and cross-tunnel GPR (ground penetration radar) and seismic will be partly acquired in collaboration with the DUG Lab executive team. This model will contain three-dimensional discrete fracture network as well as spatial variation of any fracture and matrix properties which can be measured in terms of real numerical values such as hydraulic (transmissivity, storativity), thermal (thermal conductivity), mechanical (Young’s modulus, fracture stiffness, in-situ stress)and geophysical (P-wave velocities) data.
Supervisors: Dr. Reza Jalali, Hannes Krietsch, Dr. Valentin Gischig, Dr. Joseph Doetsch, Dr. Florian Amann