Former PhD Student of the Geothermal Energy & Geofluids Group
|Dominique Ballarin Dolfin|
|Phone||+41 44 632 3465|
REFEREED PUBLICATIONS IN JOURNALS
Gischig, V.S., D. Giardini, F. Amann, "et al.", Keith F. Evans, "et al.", A. Kittilä, X. Ma, "et al.", M.O. Saar, and "et al.", Hydraulic stimulation and fluid circulation experiments in underground laboratories: Stepping up the scale towards engineered geothermal systems, Geomechanics for Energy and the Environment, 100175, 2020. [Download PDF] [View Abstract]The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS.
Kittilä, A., M.R. Jalali, M.O. Saar, and X.-Z. Kong, Solute tracer test quantification of the effects of hot water injection into hydraulically stimulated crystalline rock, Geothermal Energy, 8/17, 2020. [Download PDF] [View Abstract]When water is injected into a fracture-dominated reservoir that is cooler or hotter than the injected water, the reservoir permeability is expected to be altered by the injection-induced thermo-mechanical effects, resulting in the redistribution of fluid flow in the reservoir. These effects are important to be taken into account when evaluating the performance and lifetime particularly of Enhanced Geothermal Systems (EGS). In this paper, we compare the results from two dye tracer tests, conducted before (at ambient temperature of 13 °C) and during the injection of 45 °C hot water into a fractured crystalline rock at the Grimsel Test Site in Switzerland. Conducting a moment analysis on the recovered tracer residence time distribution (RTD) curves, we observe, after hot water injection, a significant decrease in the total tracer recovery. This recovery decrease strongly suggests that fluid flow was redistributed in the studied rock volume and that the majority of the injected water was lost to the far-field. Furthermore, by using temperature measurements, obtained from the same locations as the tracer RTD curves, we conceptualize an approach to estimate the fracture surface area contributing to the heat exchange between the host rock and the circulating fluid. Our moment analysis and simplified estimation of fracture surface area provide insights into the hydraulic properties of the hydraulically active fracture system and the changes in fluid flow. Such insights are important to assess the heat exchange performance of a geothermal formation during fluid circulation and to estimate the lifetime of the geothermal formation, particularly in EGS.
Kittilä, A., M.R. Jalali, M. Somogyvári, K.F. Evans, M.O. Saar, and X.-Z. Kong, Characterization of the effects of hydraulic stimulation with tracer-based temporal moment analysis and tomographic inversion, Geothermics, 86/101820, 2020. [Download PDF] [View Abstract]Tracer tests were conducted as part of decameter-scale in-situ hydraulic stimulation experiments at the Grimsel Test Site to investigate the hydraulic properties of a stimulated crystalline rock volume and to study the stimulation-induced hydrodynamic changes. Temporal moment analysis yielded an increase in tracer swept pore volume with prominent flow channeling. Post-stimulation tomographic inversion of the hydraulic conductivity, K, distribution indicated an increase in the geometric mean of logK and a decrease in the Dykstra-Parsons heterogeneity index. These results indicate that new flow path connections were created by the stimulation programs, enabling the tracers to sweep larger volumes, while accessing flow paths with larger hydraulic conductivities.
Amann, F., V. Gischig, K.F. Evans, et al., A. Kittilä, S. Wiemer, M.O. Saar, S. Löw, Th. Driesner, H. Maurer, and D. Giardini, The seismo-hydro-mechanical behaviour during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in-situ stimulation experiment, Solid Earth, 9, pp. 115-137, 2018. [Download PDF] [View Abstract]In this contribution, we present a review of scientific research results that address seismo-hydromechanically coupled processes relevant for the development of a sustainable heat exchanger in low-permeability crystalline rock and introduce the design of the In situ Stimulation and Circulation (ISC) experiment at the Grimsel Test Site dedicated to studying such processes under controlled conditions. The review shows that research on reservoir stimulation for deep geothermal energy exploitation has been largely based on laboratory observations, large-scale projects and numerical models. Observations of full-scale reservoir stimulations have yielded important results. However, the limited access to the reservoir and limitations in the control on the experimental conditions during deep reservoir stimulations is insufficient to resolve the details of the hydromechanical processes that would enhance process understanding in a way that aids future stimulation design. Small-scale laboratory experiments provide fundamental insights into various processes relevant for enhanced geothermal energy, but suffer from (1) difficulties and uncertainties in upscaling the results to the field scale and (2) relatively homogeneous material and stress conditions that lead to an oversimplistic fracture flow and/or hydraulic fracture propagation behavior that is not representative of a heterogeneous reservoir. Thus, there is a need for intermediate-scale hydraulic stimulation experiments with high experimental control that bridge the various scales and for which access to the target rock mass with a comprehensive monitoring system is possible. The ISC experiment is designed to address open research questions in a naturally fractured and faulted crystalline rock mass at the Grimsel Test Site (Switzerland). Two hydraulic injection phases were executed to enhance the permeability of the rock mass. During the injection phases the rock mass deformation across fractures and within intact rock, the pore pressure distribution and propagation, and the microseismic response were monitored at a high spatial and temporal resolution.
Mikutis, G., C.A. Deuber, L. Schmid, A. Kittilä, N. Lobsiger, M. Puddu, D.O. Asgeirsson, R.N. Grass, M.O. Saar, and W.J. Stark, Silica-encapsulated DNA-based tracers for aquifer characterization, Environmental Science & Technology, 52, pp. 12142-12152, 2018. [Download PDF] [View Abstract]Environmental tracing is a direct way to characterize aquifers, evaluate the solute transfer parameter in underground reservoirs, and track contamination. By performing multitracer tests, and translating the tracer breakthrough times into tomographic maps, key parameters such as a reservoir’s effective porosity and permeability field may be obtained. DNA, with its modular design, allows the generation of a virtually unlimited number of distinguishable tracers. To overcome the insufficient DNA stability due to microbial activity, heat, and chemical stress, we present a method to encapsulated DNA into silica with control over the particle size. The reliability of DNA quantification is improved by the sample preservation with NaN3 and particle redispersion strategies. In both sand column and unconsolidated aquifer experiments, DNA-based particle tracers exhibited slightly earlier and sharper breakthrough than the traditional solute tracer uranine. The reason behind this observation is the size exclusion effect, whereby larger tracer particles are excluded from small pores, and are therefore transported with higher average velocity, which is pore size-dependent. Identical surface properties, and thus flow behavior, makes the new material an attractive tracer to characterize sandy groundwater reservoirs or to track multiple sources of contaminants with high spatial resolution.
Kong, X.-Z., C. Deuber, A. Kittilä, M. Somogyvari, G. Mikutis, P. Bayer, W.J. Stark, and M.O. Saar, Tomographic reservoir imaging with DNA-labeled silica nanotracers: The first field validation, Environmental Science &Technology, 52/23, pp. 13681-13689, 2018. [Download PDF] [View Abstract]This study presents the first field validation of using DNA-labeled silica nanoparticles as tracers to image subsurface reservoirs by travel time based tomography. During a field campaign in Switzerland, we performed short-pulse tracer tests under a forced hydraulic head gradient to conduct a multisource−multireceiver tracer test and tomographic inversion, determining the two-dimensional hydraulic conductivity field between two vertical wells. Together with three traditional solute dye tracers, we injected spherical silica nanotracers, encoded with synthetic DNA molecules, which are protected by a silica layer against damage due to chemicals, microorganisms, and enzymes. Temporal moment analyses of the recorded tracer concentration breakthrough curves (BTCs) indicate higher mass recovery, less mean residence time, and smaller dispersion of the DNA-labeled nanotracers, compared to solute dye tracers. Importantly, travel time based tomography, using nanotracer BTCs, yields a satisfactory hydraulic conductivity tomogram, validated by the dye tracer results and previous field investigations. These advantages of DNA-labeled nanotracers, in comparison to traditional solute dye tracers, make them well-suited for tomographic reservoir characterizations in fields such as hydrogeology, petroleum engineering, and geothermal energy, particularly with respect to resolving preferential flow paths or the heterogeneity of contact surfaces or by enabling source zone characterizations of dense nonaqueous phase liquids.
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Kittilä, A., Groundwater flow paths in the bedrock fracture zones revealed by using the stable isotopes of oxygen and hydrogen in the Talvivaara mine gypsum pond area, Northeastern Finland, MSc Thesis, University of Helsinki, 68 pp., 2015. [View Abstract]Bedrock fracturing is considerably extensive and distinct in Finland, and the fractures that are open, conductive and interconnected usually control the groundwater flow paths in fractured bedrock. This highlights the importance of knowing the locations and hydraulic connections of water conducting fracture zones particularly in mining areas, because they can transport adverse substances outside the mining area. In this study, it is focused on examining possible hydraulic connections of bedrock groundwater by using the stable isotopes of oxygen (δ18O) and hydrogen (δ2H). The study was carried out in the Talvivaara mining area in Northeastern Finland alongside a project from the Geological Survey of Finland (GTK). After November 2012, when a leakage of acidic, metal-containing waste water occurred in the gypsum ponds, there was an urgent need to study the groundwater transport routes in the bedrock fractures. The aim was to find hydraulic connections between surface water and groundwater, and to study the flow of the groundwater in the fracture zones based on the different isotopic characteristics of waters from different sources and isotopic similarities. Most of the materials used in this study were obtained from the results of the project from the GTK. These materials included geophysical interpretations of the locations and water content of the main fracture zones and the results from the geochemical analyzes. Together with the interpretations of groundwater flow direction based on hydraulic heads these materials formed a frame for this study. The isotope composition of 39 water samples from bedrock wells, shallow wells and surface water was analyzed using cavity ring-down spectroscopy (CRDS) method. The surface waters were clearly distinguished based on their evident evaporation signal, but no significant such a signal was observed in the bedrock and shallow groundwaters. However, similarities between groundwater from different depths of same well were found, in addition to similarities between different wells along same fracture zones. Although the isotopes did not indicate surface water contamination, groundwater contamination with smaller amounts of water is possible, in which case the changes in isotope composition are not yet significant, while certain elements have elevated concentrations. A NE-SW oriented fracture zone passing in the center of the study area was concluded to have the most important role in collecting and transporting groundwater outside the mining area. More detailed interpretations would require regular sampling for a longer period of time to better distinguish naturally and artificially induced changes both in the isotopic but also geochemical compositions. Also the usage of packer tests possibly together with pumping tests would be useful in obtaining more comprehensive image of the groundwater flow in the fracture zones and their hydraulic connections.