TY - JOUR A1 - Farkas, Márton Pál A1 - Yoon, Jeoung Seok A1 - Zang, Arno A1 - Zimmermann, Günter A1 - Stephansson, Ove A1 - Lemon, Michael A1 - Danko, Gyula T1 - Effect of foliation and fluid viscosity on hydraulic fracturing tests in mica schists investigated using distinct element modeling and field data JF - Rock Mechanics and Rock Engineering N2 - Several hydraulic fracturing tests were performed in boreholes located in central Hungary in order to determine the in-situ stress for a geological site investigation. At a depth of about 540m, the observed pressure versus time curves in mica schist with low dip angle foliation shows atypical pressure versus time results. After each pressurization cycle, the fracture breakdown pressure in the first fracturing cycle is lower than the refracturing or reopening pressure in the subsequent pressurizations. It is assumed that the viscosity of the drilling mud and observed foliation of the mica schist have a significant influence on the pressure values. In order to study this problem, numerical modeling was performed using the distinct element code particle flow code, which has been proven to be a valuable tool to investigate rock engineering problems such as hydraulic fracturing. The two-dimensional version of the code applied in this study can simulate hydro-mechanically coupled fluid flow in crystalline rock with low porosity and pre-existing fractures. In this study, the effect of foliation angle and fluid viscosity on the peak pressure is tested. The atypical characteristics of the pressure behaviour are interpreted so that mud with higher viscosity penetrates the sub-horizontal foliation plane, blocks the plane of weakness and makes the partly opened fracture tight and increase the pore pressure which decreases slowly with time. We see this viscous blocking effect as one explanation for the observed increase in fracture reopening pressure in subsequent pressurization cycles. KW - Hydraulic fracturing KW - Stress measurement KW - Particle flow code KW - Hydro-mechanical coupling KW - Microcrack KW - Viscous blocking Y1 - 2019 U6 - https://doi.org/10.1007/s00603-018-1598-7 SN - 0723-2632 SN - 1434-453X VL - 52 IS - 2 SP - 555 EP - 574 PB - Springer CY - Wien ER - TY - THES A1 - Farkas, Marton Pal T1 - Hydraulic fracturing in hard rock – numerical studies from laboratory to reservoir scale T1 - Hydraulische Brüche in Hartgestein - Numerische Studien vom Labor- bis zum Reservoirmaßstab N2 - Hydraulic-driven fractures play a key role in subsurface energy technologies across several scales. By injecting fluid at high hydraulic pressure into rock with intrinsic low permeability, in-situ stress field and fracture development pattern can be characterised as well as rock permeability can be enhanced. Hydraulic fracturing is a commercial standard procedure for enhanced oil and gas production of rock reservoirs with low permeability in petroleum industry. However, in EGS utilization, a major geological concern is the unsolicited generation of earthquakes due to fault reactivation, referred to as induced seismicity, with a magnitude large enough to be felt on the surface or to damage facilities and buildings. Furthermore, reliable interpretation of hydraulic fracturing tests for stress measurement is a great challenge for the energy technologies. Therefore, in this cumulative doctoral thesis the following research questions are investigated. (1): How do hydraulic fractures grow in hard rock at various scales?; (2): Which parameters control hydraulic fracturing and hydro-mechanical coupling?; and (3): How can hydraulic fracturing in hard rock be modelled? In the laboratory scale study, several laboratory hydraulic fracturing experiments are investigated numerically using Irazu2D that were performed on intact cubic Pocheon granite samples from South Korea applying different injection protocols. The goal of the laboratory experiments is to test the concept of cyclic soft stimulation which may enable sustainable permeability enhancement (Publication 1). In the borehole scale study, hydraulic fracturing tests are reported that were performed in boreholes located in central Hungary to determine the in-situ stress for a geological site investigation. At depth of about 540 m, the recorded pressure versus time curves in mica schist with low dip angle foliation show atypical evolution. In order to provide explanation for this observation, a series of discrete element computations using Particle Flow Code 2D are performed (Publication 2). In the reservoir scale study, the hydro-mechanical behaviour of fractured crystalline rock due to one of the five hydraulic stimulations at the Pohang Enhanced Geothermal site in South Korea is studied. Fluid pressure perturbation at faults of several hundred-meter lengths during hydraulic stimulation is simulated using FracMan (Publication 3). The doctoral research shows that the resulting hydraulic fracturing geometry will depend “locally”, i.e. at the length scale of representative elementary volume (REV) and below that (sub-REV), on the geometry and strength of natural fractures, and “globally”, i.e. at super-REV domain volume, on far-field stresses. Regarding hydro-mechanical coupling, it is suggested to define separate coupling relationship for intact rock mass and natural fractures. Furthermore, the relative importance of parameters affecting the magnitude of formation breakdown pressure, a parameter characterising hydro-mechanical coupling, is defined. It can be also concluded that there is a clear gap between the capacity of the simulation software and the complexity of the studied problems. Therefore, the computational time of the simulation of complex hydraulic fracture geometries must be reduced while maintaining high fidelity simulation results. This can be achieved either by extending the computational resources via parallelization techniques or using time scaling techniques. The ongoing development of used numerical models focuses on tackling these methodological challenges. N2 - Hydraulische Risserzeugung (aus dem Englischen „Hydraulic Fracturing“; auch hydraulische Stimulation genannt) spielt eine Schlüsselrolle in unterirdischen Energietechnologien auf verschiedenen Skalen. Durch Injektion von Flüssigkeit mit hohem hydraulischem Druck im Gestein mit geringer Permeabilität können das Spannungsfeld und das Bruchentwicklungsmuster in-situ charakterisiert sowie die Gesteinspermeabilität erhöht werden. Hydraulic Fracturing ist ein kommerzielles Standardverfahren zur verbesserten Öl- und Gasförderung aus geringpermeablen Gesteinsformationen in der Erdölindustrie. Ein großes geologisches Problem bei der geothermischen Nutzung ist die ungewollte Erzeugung von Erdbeben aufgrund einer Verwerfungsreaktivierung, die als induzierte Seismizität bezeichnet wird und eine Größenordnung hat, die groß genug ist, dass sie an der Oberfläche zu spüren ist und sogar Gebäude beschädigen kann. Darüber hinaus ist die zuverlässige Interpretation von Hydraulic-Fracturing-Tests zur Spannungsmessung eine große Herausforderung für die Energietechnologien. Daher werden in dieser kumulativen Dissertation folgende Forschungsfragen untersucht: (1): Wie wachsen hydraulische Risse in Hartgestein in verschiedenen Skalen? (2): Welche Parameter steuern das hydraulische Versagen und die hydromechanische Kopplung? und (3): Wie kann hydraulische Risserzeugung in Hartgestein modelliert werden? In der Studie im Labormaßstab werden mehrere Hydrofracturing-Laborexperimente numerisch mit Irazu2D untersucht, die an intakten kubischen Pocheon-Granitproben aus Südkorea unter Anwendung verschiedener Injektionsprotokolle durchgeführt wurden. Das Ziel der Laborexperimente ist es, das Konzept der zyklischen sanften Stimulation zu testen, die eine nachhaltige Permeabilitätserhöhung ermöglichen kann (Veröffentlichung 1). Die Studie im Bohrlochmaßstab untersucht Hydraulic Fracturing Tests, die in Bohrlöchern in Mittel-Ungarn durchgeführt wurden, um das in-situ Spannungsfeld für eine geologische Standortuntersuchung zu bestimmen. In einer Tiefe von etwa 540 m zeigen die aufgezeichneten Druck-Zeit-Kurven im Glimmerschiefer mit einer Schieferung mit geringem Neigungswinkel eine atypische Entwicklung. Um diese Beobachtung zu erklären, wird eine Reihe von diskreten Elementberechnungen unter Verwendung von Particle Flow Code 2D durchgeführt (Veröffentlichung 2). In der Studie im Reservoirmaßstab wird das hydromechanische Verhalten des aufgebrochenen kristallinen Gesteins an einer der fünf hydraulischen Stimulationen am Pohang Enhanced Geothermal System (EGS) Standort in Südkorea untersucht. Mit FracMan wird die Fluiddruckstörung an Verwerfungen von mehreren hundert Metern Länge während der hydraulischen Stimulation simuliert (Veröffentlichung 3). Die Ergebnisse dieser Dissertation zeigen, dass die resultierende hydraulische Bruchgeometrie „lokal“, d. h. auf der Längenskala des repräsentativen Elementarvolumens (REV) und darunter (sub-REV) von der Geometrie und Stärke natürlicher Risse und „global“, d.h. bei Super-REV-Domänenvolumen, vom Spannungsfeld abhängt. In Bezug auf die hydromechanische Kopplung wird vorgeschlagen, separate Kopplungsbeziehungen für intakte Gesteinsmassen und natürliche Risse zu definieren. Darüber hinaus wird die relative Bedeutung von Parametern definiert, die die Größe des Formationsbruchdrucks beeinflussen, ein Parameter, der die hydromechanische Kopplung charakterisiert. Es kann auch festgestellt werden, dass es eine klare Lücke zwischen der Leistungsfähigkeit der Simulationssoftware und der Komplexität der untersuchten Probleme gibt. Daher muss die Rechenzeit der Simulation komplexer hydraulischer Rissgeometrien reduziert werden, währenddessen die Simulationsergebnisse mit hoher Genauigkeit beibehalten werden. Dies kann entweder durch Erweiterung der Rechenressourcen über Parallelisierungstechniken oder durch Verwendung von Zeitskalierungstechniken erreicht werden. Die Weiterentwicklung der verwendeten numerischen Modelle konzentriert sich auf die Bewältigung dieser methodischen Herausforderungen. KW - hydraulic fracturing KW - enhanced geothermal system KW - stress measurement KW - numerical modelling KW - hydraulische Risserzeugung KW - petrothermales System (EGS) KW - Spannungsmessung KW - numerische Modellierung Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-549343 ER - TY - JOUR A1 - Farkas, Márton Pál A1 - Hofmann, Hannes A1 - Zimmermann, Günter A1 - Zang, Arno A1 - Bethmann, Falko A1 - Meier, Peter A1 - Cottrell, Mark A1 - Josephson, Neal T1 - Hydromechanical analysis of the second hydraulic stimulation in well PX-1 at the Pohang fractured geothermal reservoir, South Korea JF - Geothermics : an international journal of geothermal research and its applications N2 - In this study, we investigate numerically the hydro-mechanical behavior of fractured crystalline rock due to one of the five hydraulic stimulations at the Pohang Enhanced Geothermal site in South Korea. We use the commercial code FracMan (Golder Associates) that enables studying hydro-mechanical coupled processes in fractured media in three dimensions combining the finite element method with a discrete fracture network. The software is used to simulate fluid pressure perturbation at fractures during hydraulic stimulation. Our numerical simulation shows that pressure history matching can be obtained by partitioning the treatment into separate phases. This results in adjusted stress-aperture relationships. The evolution of aperture adjustment implies that the stimulation mechanism could be a combination of hydraulic fracturing and shearing. The simulated extent of the 0.01 MPa overpressure contour at the end of the treatment equals to similar to 180 m around the injection point. KW - Enhanced Geothermal System KW - Pohang geothermal reservoir KW - hydraulic KW - stimulation KW - PX-1 KW - FracMan Y1 - 2021 U6 - https://doi.org/10.1016/j.geothermics.2020.101990 SN - 0375-6505 SN - 1879-3576 VL - 89 PB - Elsevier CY - Amsterdam [u.a.] ER - TY - JOUR A1 - Hofmann, Hannes A1 - Zimmermann, Günter A1 - Farkas, Márton Pál A1 - Huenges, Ernst A1 - Zang, Arno A1 - Leonhardt, Maria A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Bohnhoff, Marco A1 - Min, Ki-Bok A1 - Fokker, Peter A1 - Westaway, Rob A1 - Bethmann, Falko A1 - Meier, Peter A1 - Yoon, Kern Shin A1 - Choi, JaiWon A1 - Lee, Tae Jong A1 - Kim, Kwang Yeom T1 - First field application of cyclic soft stimulation at the Pohang Enhanced Geothermal System site in Korea JF - Geophysical journal international N2 - Large-magnitude fluid-injection induced seismic events are a potential risk for geothermal energy developments worldwide. One potential risk mitigation measure is the application of cyclic injection schemes. After validation at small (laboratory) and meso (mine) scale, the concept has now been applied for the first time at field scale at the Pohang Enhanced Geothermal System (EGS) site in Korea. From 7 August until 14 August 2017 a total of 1756 m(3) of surface water was injected into Pohang well PX-1 at flow rates between 1 and 10 l s(-1), with a maximum wellhead pressure (WHP) of 22.8 MPa, according to a site-specific cyclic soft stimulation schedule and traffic light system. A total of 52 induced microearthquakes were detected in real-time during and shortly after the injection, the largest of M-w 1.9. After that event a total of 1771 m(3) of water was produced back from the well over roughly 1 month, during which time no larger-magnitude seismic event was observed. The hydraulic data set exhibits pressure-dependent injectivity increase with fracture opening between 15 and 17 MPa WHP, but no significant permanent transmissivity increase was observed. The maximum magnitude of the induced seismicity during the stimulation period was below the target threshold of M-w 2.0 and additional knowledge about the stimulated reservoir was gained. Additionally, the technical feasibility of cyclic injection at field scale was evaluated. The major factors that limited the maximum earthquake magnitude are believed to be: limiting the injected net fluid volume, flowback after the occurrence of the largest induced seismic event, using a cyclic injection scheme, the application of a traffic light system, and including a priori information from previous investigations and operations in the treatment design. KW - Cyclic soft stimulation (CSS) KW - induced seismicity KW - risk mitigation KW - enhanced geothermal systems (EGS) KW - granite KW - Pohang (Korea) Y1 - 2019 U6 - https://doi.org/10.1093/gji/ggz058 SN - 0956-540X SN - 1365-246X VL - 217 IS - 2 SP - 926 EP - 949 PB - Oxford Univ. Press CY - Oxford ER -