Imaging Geothermal Reservoirs Using Vertical Seismic Profiling

• Main Author: Reiser, Fabienne
• Format: Thesis
• Language: English
• Published: ETH Zurich 2018
• Subjects: Geophysics; FOS Earth and related environmental sciences; Geothermal energy; Vertical seismic profile; infoeu-repo/classification/ddc/550; Earth sciences; Krafla; Iceland
• Online Access: https://dx.doi.org/10.3929/ethz-b-000299570; http://hdl.handle.net/20.500.11850/299570

Renewable resources play an important role in the present energy policies of most nations because of the worldwide rising energy demand as well as the limited availability of oil and gas reserves. Increased public concerns regarding negative environmental impacts of fossil fuels and nuclear power plants require alternative energy resources to produce electricity in a sustainable manner. Geothermal resources provide a promising energy supply and can be used for direct heating systems, space heating, bathing, cooling, aquaculture, greenhouses or electricity production. However, they still play a minor role on the global scale compared to other renewable energy resources like hydropower, solar energy, wind power or biomass energy. Establishing geothermal power plants is still risky and requires high costs. Further research and development is necessary to build economically viable geothermal power plants. It is challenging to identifying suitable geothermal reservoirs with high enough temperatures and sufficient permeability to enable fluid flow. Seismic exploration can contribute to improved exploration of geothermal sites, better well siting and monitoring during production. However, seismic exploration over geothermal reservoirs is often challenging and, hence, improved exploration techniques are needed. Vertical seismic profiling (VSP) is a promising method to image the subsurface in complex environments. In this thesis, the benefits and limitations of VSP techniques in geothermal environments are critically examined. Advanced seismic imaging methods are tested and seismic acquisition and processing workflows are established for an improved and cost-optimized exploration of geothermal reservoirs. In the first part of the thesis, an experimental design study on synthetic VSP data was performed for imaging fracture zones within a crystalline basement. 2D and 3D acoustic data are simulated and processed up to the pre-stack depth migration stage to optimize survey layouts for mapping fracture zones of different dip and location. The optimal survey layout strongly depends on the position and dip of the fracture zone and hence stresses the importance of à priori information. We have found that fracture zones located at the same depth range as the receivers but with 300m lateral offset can be imaged reliably for various dips. More source positions and larger offsets are required for steeper dipping fracture zones. A useful image of the fracture zone can be already obtained using a relatively small number of suitably placed source positions. Adding sources outside the optimal spread did not improve the results, but rather deteriorated the quality of the migrated image in some cases. Hence, the optimization procedure established does not only help to optimally plan future field surveys with a favorable benefit-cost ratio, but also allows the selection of useful data subsets for optimal target-oriented processing. The second part of the thesis study comprises the analysis of a multi-offset VSP field dataset from a high-temperature geothermal area in Krafla, Iceland. Seismic imaging in volcanic environments is generally very difficult due to intense scattering and absorption of the seismic waves caused by the pronounced heterogeneity. The design of the VSP survey was not optimal due to limited financial resources and entailed only a few source positions. A workflow was established for processing this sparse VSP dataset with a focus on first-arrival traveltime tomography and seismic reflection processing. A multicomponent (vectorial) pre-stack Kirchhoff migration algorithm, which combines wavefield separation and depth imaging, was applied to obtain separate PP, PS and SS migrated images. This helped to assess the reliability of reflections and to demonstrate the benefits of using 3C geophones. By considering à priori information, such as geological information and hypocenter locations from microseismicity studies, we were able to constrain the processing results which enabled a more reliable interpretation. We conclude that VSP surveying is able to image key lithological boundaries, volcanic stratigraphy, fracture zones, dykes and potentially magma chambers. However, à priori knowledge was essential to constrain the seismic processing and to interpret the migrated images which, due to the paucity of shots, contained significant migration artefacts. Based on this field study we formulated strategies for future VSP acquisition in complex magmatic environments. In the last part of the thesis, elastic full waveform inversion (FWI) was applied to a multi-offset VSP data set recorded in the geothermal area of Thonex, Switzerland. It turned out that the data acquisition was unsuitable for the application of FWI due to very limited receiver coverage. Consequently, the inversion problem was highly underdetermined and was hampered by a strong trade-off between the velocity model and the source wavelet estimation. Including a far-offset source position led to improved results due the longer receiver array. However, the generally sparse dataset prevented a reliable interpretation of the velocity models. Further field data sets need to be analyzed to assess the potential of FWI in geothermal exploration.