Geothermal potential and opportunities in Vietnam
Hung Vuong University, Vietnam
Located between the Pacific Ring of Fire and Alpide Belt, Vietnam possesses significant potential geothermal resources, with more than 300 hot spring sites with temperatures from 40oC to 100oC have been detected in the territory. Based on available data, Vietnam is estimated to have a geothermal potential of up to 400 MW power generation. A pilot 25MW power plant was constructed in 2013 in Đakrông - Quảng Tri Province, central of Vietnam.
From geothermal measurements in oil and gas exploration boreholes, heat flow anomalies have been identified that are higher than the average heat flow of the Earth 100 mW/m2 in the Southeast of the Red River Delta (at a depth of 3,000 m temperature reaches more than 140oC) and coastal Binh Thuan (volcanic activity in Tro island in 1923) has an area of hundreds of square kilometers.
High heat flow anomalies have been identified in some places like Phu Tho, Hue, Quang Ngai, Kon Tum, associated with active tectonic geological structures and have many hot water eruptions on the surface. Therefore, geothermal resources in Vietnam are prosperous, belonging to low to medium potential heat sources, with conditions for small capacity power generation.
In Politburo's resolutions regarding the national electricity development plan, Vietnam will develop breakthrough mechanisms and policies to encourage and promote renewable energy sources, with the capacity of renewable energy plants reaching 30% by 2030 and 40% by 2045. Overall assessment of geothermal energy's potential and development orientation will be established; then deploy several application models and conduct experimental exploits to evaluate the effectiveness. Geothermal energy can be as base-load electricity and offers an opportunity for a country with naturally free-resource and less dependence on fossil fuel. To jump-start the geothermal exploitation rather than solely relying on knowledge, introducing the techniques outside Vietnam is needed.
Hydro-mechanical parameters of Cornubian and Odenwald reservoir granitoids with focus on fracture stiffness testing
1Geothermal Science and Technology, Technical University of Darmstadt; 2Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Section: Geoenergy
For the resource development, geothermal systems need to be improved to increase the profitability of the investment. One aspect to support this aim is the reservoir productivity, a key parameter, which depends on the hydraulic and mechanical properties of the reservoir formation. In order to develop possible improvement strategies for the profitability enhancement of geothermal reservoirs and/or nuclear waste repositories, hydraulic and mechanical properties of artificial generated fractures were investigated. The importance of the fracture geometry yields the fracture network of the geothermal system. As a result, the influence of stress exerted on single fractures was exploited. Hereby, the fracture aperture represents a key parameter for several other parameters such as the fracture permeability and fracture stiffness. Therefore, experiments of progressive and constant cyclic loading were performed to analyze the fluid flow inside of fractured rock samples from geothermal reservoirs. The specimens analyzed in this research project are the Odenwald Granodiorite which was extracted from the Bergstrasse in Heppenheim, Germany, and the Cornwall Granite from the St. Austell pluton in Cornwall, England. The progressive cyclic loading test (PCL) was performed with confining pressure maxima of 15 MPa, 30 MPa, 45 MPa, and 60 MPa. Within the constant cyclic loading test (CCL), the maximum pressure was raised up to 60 MPa to ensure reproducibility. Axial and lateral strain deformation were measured with LVDT extensometers to calculate the fracture and matrix deformation. Fracture stiffness, -permeability, and -closure were evaluated from the collected dataset. Moreover, the fracture geometry was taken into account by 3D surface scans to display fracture aperture distribution and to model the change in surface structure and its impact on the fracture behavior. The fracture stiffness visualized for both granitoids is similar in terms of values and behavior, despite their different origin and, respectively, their petrographical composition. Moreover, the PCL displayed a linear trend of the fracture stiffness in the 1st cycle and before exceeding the previous stress maximum. This feature transformed into a non-linear trend when exceeding the previous stress level. The transition seems to be related to a stress-memory effect and the behavior of the ‘Kaiser effect’ for acoustic emissions. Both features were additionally detected in the fracture permeability results. The outcome of a similar research by Kluge et al. (underreview) with the Flechtingen Sandstone shows the same characteristics in a different domain for the fracture stiffness, -permeability, and –closure values. Last but not least, the fracture permeability reduction turned out very similar in the PCL and the CCL test, a result that contrasts the outcome of Kluge et al. (under review). Experimental and theoretical results on single fractured rock specimens are discussed and display the importance of fracture stiffness on geothermal systems.
Large hydraulic diffusivity of a single fault
1Universite de Strasbourg, France; 2Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences
We provided an approach to estimate hydraulic diffusivity of a single fault by solving the linear diffusion equation in a partly open rough fracture under drained conditions when applying small pressure drop fluctuations (10^-5 Pa) along the fault. In contrast to the traditional calculation for the fracture hydraulic diffusivity using parameters such as hydraulic aperture, fluid compressibility, fluid viscosity, we here directly used time-dependent pressure profile p(x, y, t) to match the analytical solution for an equivalent parallel plate model, which contains hydraulic diffusivity as unknown. The method considered transient pressure diffusion process, which might give a more accurate value for hydraulic diffusivity compared to traditionally calculated one. Our results under large closure (hydraulic diffusivities are of orders 10^2 m2/s – 10^4 m^2/s) are consistent with the values derived from analysis of some earthquake sequences (Noir et al., 1997; Antonioli et al., 2005; Malagnini et al., 2012; Dempsey and Riffault, 2019; Schmittbuhl et al, 2021). Those earthquakes were assumed to be triggered by the diffusion of pore pressure perturbation in a fractured medium, and the seismicity migration was then evidenced to be compatible with pore pressure relaxation. The hydraulic diffusivity estimated by Noir et al. (1997) for the 1989 Dobi earthquake sequence of Central Afar ranges between 10^3 – 10^4 m^2/s, which corresponds to the characteristic width (i.e., effective aperture) 1 mm - 3 cm. The consistency with our results indicates that our model might be used to predict potential earthquake migration, in particular, when a single fault path dominates the fluid flow. Compared to diffusivities estimated from direct hydraulic tests, the values obtained from our simulations are somehow large. The discrepancy could be attributed to several aspects: the diffusivity from direct hydraulic test is commonly affected by fracture networks instead of a single fault, and combines matrix diffusivity and fracture diffusivity (Ortiz R et al., 2013; Sayler et al., 2018), which lower the value. In addition, it also depends on temperature, mineral sealing, fault movement and tested methods, e.g., lower hydraulic diffusivities were observed in constant rate tests than periodic tests (Guiltinan and Becker, 2015). In cases that fluid flow was dominated by a constrained planar fracture., e.g., Sayler et al. (2018), it was evidenced that flow between an interval with large diffusivities (up to 10^3 m^2/s). We also compare our results to the hydraulic diffusivity assessment for the recent Strasbourg earthquake sequence: 25 m^2/s (Schmittbuhl et al, 2021). Our approach can be extended to estimate hydraulic diffusivity for fracture networks when considering roughness (varied aperture distribution) for each fracture (Haagenson and Rajaram, 2021).
Parametric optimization and comparative study of an organic Rankine cycle power plant for two-phase geothermal sources
1Helmholtz Centre for Environmental Research GmbH – UFZ, Germany; 2Applied Environmental Systems Analysis, Dresden University of Technology; 3Flensburg University of Applied Sciences
For two-phase geothermal resource, Organic Rankine Cycle (ORC) based binary plant is often applied for power production. In this work, a network topology was built with the Thermal Engineering Systems in Python (TESPy) software to simulate the stationary operation of the ORC plant. With this topology, the performance of nine different working fluids are compared. From the thermodynamic perspective, the gross and net power output is optimized respectively. Results show that R600 has the highest gross power output of 17.55MW, while R245fa has the highest net power output of 12.93MW. However, the turbine inlet temperatures for these two working fluids need to be designed at the upper limit of 131℃. It is also found that R245ca and R601a (Isopentane) require the heat exchange rates of IHE to be larger than 1.51MW and 0.99MW to satisfy the re-injection temperature limit, which are smaller than the R600 (6.7MW) and R245fa (6.0MW) cases. Besides, in order to establish a stable ORC plant, the lower geo-steam fraction, the working fluid with lower critical state is preferred. The workflow for the ORC design and optimization in this work is generic, and can be further applied to thermo-economic investigation.
Seismic Monitoring of DeepStor: Using low-cost sensors for ambient noise correlation methods and Citizen Science
Karlsruhe Institute of Technology, Germany
DeepStor is an experimental facility with the goal to investigate High Temperature Aquifer Thermal Energy Storage (HT-ATES) systems at KIT Campus North. The operational seismic monitoring of DeepStor includes a network of five broadband and one borehole seismometer. In addition, we plan to install a scientific monitoring network with low-cost seismometers (such as the Raspberry Shake and the Quakesaver Hidra) to test innovative monitoring methods and for a Citizen Science project.
Ambient noise tomography and coda wave interferometry are being used increasingly to image and monitor geothermal reservoirs. Especially in locations with a high anthropogenic noise level, such as the Oberrheingraben, these methods could potentially provide valuable insights in the evolution of the storage acquifer during injection/production cycles. Our monitoring approach focuses on the use of a larger number of these low-cost sensors instead of fewer and more expensive broadband instruments (Large-N approach). With the broadband network and established monitoring methods as a benchmark, DeepStor provides the ideal testing ground to explore the benefits of a dense network of low-cost sensors.
The Citizen Science project will build on the successful Gecko project, which involved the public in the conceptualization of geothermal energy usage. A major conclusion from the Gecko workshops was the importance of transparent monitoring processes for the acceptance of geothermal energy usage. Consequently, we plan to involve the communities around KIT Campus North in the monitoring of DeepStor by distributing sensors for radon and seismicity. To ensure trust in the monitoring process, we will follow open data practice and investigate options to make the data easily accessible and understandable.