|Theme||Visible||Selectable||Appearance||Zoom Range (now: 0)|
ABSTRACT Enhanced geothermal systems (EGS) have been increasingly exploited as an alternative energy source. However, unexpected induced earthquakes bring uncertainty to their wider application. We employ a 2D thermal-hydro-mechanical (THM) coupled hybrid finite-discrete element method (FDEM) to investigate fault slip induced by injecting cold water into a hot fault under geothermal conditions. We show that slip will occur on stressed fault even if the overall stress condition is in the stable regime. This slip is caused by thermal contraction that reduces the fault normal stress locally, which also cause opening of the fault. We also found that convective heat transfer coefficient, which is typically unknown for natural faults, is a controlling factor for induced fault slip. This indicates the importance of understanding fault surface geometry. 1. INTRODUCTION Enhanced geothermal systems (EGS) have been increasingly exploited as they provide an alternative and virtually unlimited energy source. The geothermal energy extraction process requires injection of cold water into hot rock formations. The interaction between cold water and hot rock has been reportedly causing slip on pre-existing rock discontinuities, such as joints and faults. Unexpected earthquakes were induced, for example, in Basel, Switzerland in 2006 and in Pohang, South Korea in 2017 (Mignan et al., 2015; Kim et al., 2018), which caused public backlash to the EGS projects. The triggering mechanism of these earthquakes are not well understood. For example, the recent Pohang earthquake in South Korea occurred at injection volume and pressure smaller than suggested by the widely used empirical model (Kim et al., 2018; McGarr, 2014). Therefore, it is important to understand the triggering mechanisms of induced earthquakes during geothermal energy extraction. It has been speculated that thermal contraction can cause reduction of fault normal stress and trigger fault slip. However, this potential mechanism has never been confirmed in field observations. It is difficulty to conduct in situ measurement due to the high pressure and high temperature conditions. In this case, a properly developed numerical model would provide valuable insights. For example, studies by Ghassemi et al. (2007), Jacquey et al. (2015), and Huang et al. (2019) suggested that thermal stress variation due to cold water injection to hot rock can cause fault slip at field and laboratory scales.
ABSTRACT In 2018, fracture caging was introduced as a concept to control hydraulically stimulated fracture extents and to contain fluid flow within targeted rock volumes in the subsurface. When implemented via stochastic design of well fields, caging has significant potential for bringing a successful outcome for the control of subsurface fracture fluid flow without requiring very much information about the subsurface. Contained high-pressure fluid injection brings the benefit of preventing pore pressure increase away from the intended injection zone. Therefore, caging offers a means to prevent large earthquakes that could otherwise be caused by injection activities. Here we present evidence of this phenomenon by evaluating available field data from a contained enhanced geothermal system in relation to non-caged injection-dominated sites. We also discuss a possible mechanism by which caging could directly control the maximum magnitude of injection induced seismicity. Long term, if caging is ultimately found to be effective for controlling seismicity, we would be able to prevent another Pohang-type damaging induced earthquake event. 1. INTRODUCTION At the ARMA symposium and the Stanford Geothermal Workshop in 2018, the concept of ‘fracture caging’ was introduced (Frash et al., 2018a,b). This concept involved pre-drilling tactical patterns of production wells around injection wells to contain hydraulic fracture extents inside a targeted volume of rock (Fig 1). Laboratory experiments and numerical modelling confirmed that fracture extents could be contained. Next at ARMA in 2019, we proposed a stochastic method to design suitable well layouts that take advantage of this effect to control fractures and fluid flow in the subsurface (Frash et al., 2019). This analysis supported the notion that fracture caged systems could be created in rock systems containing fractures at unknown locations and unknown orientations. Recently, analysis of microseismic data from field and laboratory experiments has revealed that fracture caging could offer a means to limit the number and the magnitude of seismic events (Frash et al., 2020). Here, this evidence is presented and a mechanism is proposed in an attempt to explain how fracture caging could limit the maximum magnitude and number of induced seismic events. If correct, a mechanism such as this could offer a means to directly control induced seismicity and a means to prevent damaging induced seismic events.
Chang, K. W. (Sandia National Laboratories) | Yoon, H. (Sandia National Laboratories) | Kim, Y.-H. (School of Earth and Environmental Sciences, Seoul National University) | Lee, M. Y. (Sandia National Laboratories)
ABSTRACT Recent occurrence of moderate to large seismic events (Mw ≥ 3) after terminating well operations is unlikely to be caused only by pore-pressure diffusion into conductive faults; it is necessary to address additional mechanisms in the earthquake nucleation. Our coupled fluid flow and geomechanical model describes the processes inducing seismicity corresponding to the sequential stimulation operations in Pohang, South Korea. Simulation results show that the combined effect of poroelastic shearing and delayed pore-pressure accumulation can cause slip on a fault, potentially inducing the post shut-in large earthquakes. Alternate injection-extraction operations through multiple wells can enhance the efficacy of pore-pressure diffusion and subsequent stress transfer through rigid and low-permeability basement rocks to the fault. This mechanistic study addresses that comprehensive characterization of the faulting system and optimal injection-extraction strategies are critical to mitigate unexpected seismic hazards associated with the site-specific uncertainty in operational and geological factors. 1. INTRODUCTION Over the past decade a number of induced seismic events have been increasingly observed due to extensive subsurface energy activities such as wastewater injection [e.g., Kim, 2013, Hornbach et al., 2016], geothermal stimulation [e.g., Diehl et al., 2017], or geological carbon storage [e.g., Bauer et al., 2016]. Numerical models provide a critical link between field observations and theory of mechanisms inducing earthquakes by quantifying transient perturbations in pore pressure and stresses throughout a domain of interest. At the Pohang site in South Korea (Fig. 1), the first EGS (Enhanced Geothermal System) stimulation began on 29 January 2016 and total of five phases of injectionproduction operations had taken place at ∼4.3 km of depth through PX-1 and PX-2 wells until September 2017 with a net injected volume of 6,000 m3 (total injected volume of 12,800 m3 and total produced volume of 6,800 m3). The spatial footprint of detected seismic events delineates the geometry of the fault plane (strike/dip = N214°/43°NW), separating PX-1 and PX-2 [GSK, 2019], which was not found prior to the EGS stimulation. The focal mechanisms indicate that the Korean Peninsula is under tectonic compression, and the local stress field reveals that the 2017 Pohang earthquake was induced by the oblique reverse slip of a previously extensional fault at optimal orientation. This fault was critically stressed, implying that a fault slips with a small stress perturbation, and drilling or fluid injection-production initiated seismic activities along the fault.
Lee, Chong-Moo (Korea Research Institute of Ships and Ocean engineering) | Kim, Kihun (Korea Research Institute of Ships and Ocean engineering) | Yoon, Suk-Min (Korea Research Institute of Ships and Ocean engineering)
ABSTRACT The maximum velocity of the URI-L ROV was measured 2.8kts in the water tank. At the design step, the ROV was expected to run at the velocity of over 3.5 knots with 6 thrusters of 5.5kW each. The resistance and thrust of the ROV at different flow velocity were measured in the circulation water channel to analyze the discrepancy of its maximum velocities. It is confirmed that the thrusters arranged at 45° have a greater thrust attenuation than the thrusters arranged in the forward direction. INTRODUCTION The Korea Research Institute of Ships and Ocean engineering (KRISO) has been developing light work ROV as part of underwater construction robot project (Fig. 1). Light work ROV is capable of carrying out various tasks in underwater construction and is compact than other underwater construction robots, and it is designed to be able to reach a maximum speed of 2.5 knots by using 6 horizontal thrusters out of 10 at its own weight of 1.5 tons. The ROV URI-L under development was able to reach speeds of up to 2.8 knots with its own DVL (Doppler Velocity Logger) record in a threedimensional water tank test of the Underwater robotics Test & Evaluation Center at Pohang. This has exceeded the design speed of 2.5 knots, but lower than the initial design expectations. The ROV URI-L uses six thrusters arranged horizontally to achieve a forward speed. Four out of six thrusters are placed at an angle of 45° to the center line at the front, rear, left, and right sides of the ROV, and two are placed at the bottom of the stern (parallel to the centerline). Fig. 2 shows the location of the horizontal thrusters. Most of ROVs with open frame take this type arrangement because of better maneuverability and more room for sub systems than inline arrangement of thrusters. The URI-L ROV is 2.3m long, 1.27m wide and 1.3m tall. In this arrangement, the maximum speed could be 3.5 kts from simple calculation of vector sum of each thrusters as the Eq. 1 where T is the maximum thrust of each propellers.
Farkas, Marton Pal (University of Potsdam) | Hofmann, Hannes (German Research Centre for Geosciences) | Zimmermann, Gunter (German Research Centre for Geosciences) | Zang, Arno (German Research Centre for Geosciences) | Yoon, Jeoung Seok (German Research Centre for Geosciences)
ABSTRACT: In this study we investigate numerically the flow rate controlled cyclic stimulation experiment performed in August 2017 at the Pohang EGS site using the finite element code FracMan. Per definition, a soft stimulation method aims to increase permeability while reducing the risk of inducing larger seismic events. The numerical code enables studying hydro-mechanical processes and investigating main characteristics of induced seismicity such as spatial evolution of events and their moment magnitude in relation to injected fluid volume in three dimensions. The analysis contributes to understanding the fracturing processes and induced seismicity in naturally compartmentalized fractured reservoir. The code can be also used for predicting the relationship between fluid injection volume and spatial extent of generated or reactivated fractures, i.e. the stimulated reservoir volume. The reservoir model will eventually allow different injection strategies to be investigated to design an optimal stimulation procedure ahead of future field application of soft stimulation. Furthermore, it may also serve as a basis for future numerical investigations.
Hofmann, H. (Helmholtz Centre Potsdam GFZ German Research Centre) | Zimmermann, G. (Helmholtz Centre Potsdam GFZ German Research Centre) | Zang, A. (Helmholtz Centre Potsdam GFZ German Research Centre) | Yoon, J. S. (Helmholtz Centre Potsdam GFZ German Research Centre) | Stephansson, O. (Helmholtz Centre Potsdam GFZ German Research Centre) | Kim, K. Y. (Korea Institute of Civil Engineering) | Zhuang, L. (Korea Institute of Civil Engineering) | Diaz, M. (Korea University of Science and Technology) | Min, K.-B. (Seoul National University)
ABSTRACT: Exploitation of unconventional energy resources often requires fluid injection to improve the hydraulic performance of the reservoir. A potential risk associated with these hydraulic stimulation treatments is the unintended development of manmade seismic events. Therefore, mitigation measures for seismic risks associated with fluid injection is of major importance for many applications, such as shale gas, tight oil and enhanced geothermal systems (EGS). As part of a portfolio of options, cyclic injection schemes were recently proposed to reduce the risk of inducing larger seismic events. To prove this concept, a series of experiments with cyclic and constant fluid injection rates were performed in granitic rocks at laboratory scale (Pocheon Granite), at mine scale (Àspo Hard Rock Laboratory, Sweden) and at field scale (Pohang EGS site, Korea). All experiments have in common that the hydraulic performance could be improved with lower magnitude seismic events and lower breakdown pressure, thus showing the potential to mitigate seismic risk of hydraulic stimulation treatments.
Park, Sehyeok (Seoul National University) | Xie, Linmao (Seoul National University) | Kim, Kwang-Il (Seoul National University) | Kwon, Saeha (Seoul National University) | Min, Ki-Bok (Seoul National University) | Choi, Jaiwon (NexGeo Inc.) | Yoon, Woon-Sang (NexGeo Inc.) | Song, Yoonho (Korea Institute of Geoscience and Mineral Resources)
Abstract The first hydraulic stimulation for enhanced geothermal system (EGS) development in Korea had been conducted in the PX-2 well of 4,348 m depth in Pohang EGS site from January 29 to February 20, 2016. Treatment histories of injection rate, wellhead pressure and corresponding induced microseismicity data were obtained from the stimulation test upon 140 m long open hole section at the well bottom. Wellhead pressure was up to 89 MPa and considerable level of flow rate was attempted up to 47 L/sec. Microseismicity observation showed a trend of lager and more frequent seismicity occurrence in shut-in phase than in injection phase. The injectivity index during the stimulation periods had increased as 2.7 times in January 30 at the wellhead pressure of 73 MPa. Postulating the existence of a major fracture zone intersecting the open hole section, the transmissivity and the corresponding equivalent aperture of the fracture were evaluated. Required breakdown pressures by hydrofracturing and hydroshearing mechanisms were estimated based on the various scenarios on the in-situ stress condition, major fracture zone orientation and shear failure criteria. 1. Introduction 1.1. Pohang EGS development site The first enhanced geothermal system (EGS) development project in Korea was launched at the end of 2010 in Pohang. Five boreholes are located within 5 km from the site (Fig. 1): BH-1 of 1.1 km depth, BH-2 of 1.5 km depth, BH-3 of 0.9 km depth, BH-4 of 2.4 km depth, and EXP-1 of 1 km depth. The Pohang EGS site is owned and operated by NexGeo Inc., and it is located at 129°22'46.08″E, 36°06'23.34″N. Drilling of PX-1 and PX-2 wells were finished with final depths of 4,127 m and 4,348 m, respectively, and it is planned to be expanded to a triplet system, i.e., a fluid circulation system with three wells in the target reservoir, after stimulations in PX-2 and PX-1.
Yoo, Hwajung (Seoul National University) | Park, Sehyeok (Seoul National University) | Xie, Linmao (Seoul National University) | Min, Ki-Bok (Seoul National University) | Rutqvist, Jonny (Lawrence Berkeley National Laboratory) | Rinaldi, Antonio P. (Swiss Federal Institute of Technology)
Abstract Numerical modeling of fractured geothermal reservoir is conducted to describe coupled hydromechanical behavior at Pohang Enhanced Geothermal System (EGS) site. A hydraulic stimulation was conducted in the PX-1 well at the depth of 4,362m in Pohang EGS site from Dec 2016. Stress-induced permeability changes are inferred to have occurred from well head pressure and injection rate versus time curves during the stimulation. A numerical model of Pohang EGS reservoir is built to simulate hydromechanical behavior during the hydraulic stimulation at PX-1 well. The well head pressure and injection rate curves are reproduced considering corresponding permeability changes by effective stress changes and hydroshearing. History in hydromechanical property changes such as permeability during the stimulation is estimated from the modeling results. In addition, a relationship between effective stress and permeability is obtained through model calibration against the well head pressure and injection rate data. For the numerical modeling, TOUGH-FLAC, a simulator for coupled thermal-hydraulic-mechanical processes in geological media, is used. 1. Introduction Pohang Enhanced Geothermal System (EGS) project has been operated in Pohang, South Korea since 2010. The geology consists of sedimentary rock from ground surface to 2.4km deep, and of granodiorite below the depth of 2.4km. Two boreholes, PX-1 and PX-2, are drilled up to 4,217m and 4,348m respectively. Hydraulic stimulations were conducted in PX-2 from Jan. 29 to Feb. 10, 2016, and in PX-1 from Dec 15, 2016 to Jan 11, 2017. In PX-2, 1,970m of water was injected, and the maximum wellhead pressure of 89.2MPa was observed. The total amount of injected water to PX-1 was 2,689m, and maximum wellhead pressure reached 27.7MPa. The main flow path is expected to be one or two major fault zones intersecting PX-1 and PX-2. Hydroshearing on the pre-existing faults is highly likely to have happened at wellhead pressure of around 15MPa during the hydraulic stimulation in PX-1 according to a result interpretation (Park, S. et al., 2017).
Park, Sehyeok (Seoul National University) | Kim, Kwang-Il (Seoul National University) | Xie, Linmao (Seoul National University) | Yoo, Hwajung (Seoul National University) | Min, Ki-Bok (Seoul National University) | Choi, Jaiwon (NexGeo Inc.) | Yoon, Woon-Sang (NexGeo Inc.) | Yoon, Kern (NexGeo Inc.) | Song, Yoonho (Korea Institute of Geoscience and Mineral Resources, Daejeon) | Lee, Tae Jong (Korea Institute of Geoscience and Mineral Resources, Daejeon) | Kim, Kwang Yeom (Korea Institute of Civil Engineering and Building Technology, Goyang-si)
Abstract A massive hydraulic stimulation for enhanced geothermal system (EGS) development in Korea had been conducted in the PX-1 well of 4,217 m depth in Pohang EGS site from December 2016 to January 2017. Stimulation operation followed the planned programs composed of test stimulation, main stimulation and post-stimulation test as well as the traffic light system for induced seismicity management. Treatment histories of injection rate, wellhead pressure and corresponding induced seismicity data were obtained during the stimulation operation upon the 313 m of open-hole section at the bottomhole. Compared to the last and the first hydraulic stimulation at Pohang PX-2 well, wellhead pressure was significantly lower in PX-1 well. Injectivity index and reservoir transmissivity were evaluated from the stimulation data of PX-1 well, and those were compared with the case of PX-2 well. The critical fluid pressure for hydroshearing was estimated based on the joint and in-situ stress conditions of Pohang reservoir, and it showed a reasonable agreement with the pressure peaks observed in the first day of stimulation. 1. Introduction The first enhanced geothermal system (EGS) development project in Korea was launched at Pohang in December 2010. Drilling of PX-1 and PX-2 wells were finished with final depths of 4,217 m and 4,348 m as true vertical depths, respectively. The first large-scale hydraulic stimulation in the Pohang EGS site had been conducted in PX-2 well from January to February 2016. After the stimulation in PX-2 well, PX-1 well had been stimulated from December 2016 to January 2017. 3,907 m of water had been injected into the reservoir through 313 m long open hole section located at the bottom of PX-1 well.
Choi, Junhyung (Dong-A University, Busan) | Lee, Kyungbook (Korea Institute of Geoscience and Mineral Resources, Daejeon) | Shinn, Young Jae (Korea Institute of Geoscience and Mineral Resources, Daejeon) | Yasuhara, Hideaki (Ehime University, Matsuyama) | Lee, Dae Sung (Dong-A University, Busan)
Abstract Permeability is one of the important rock properties for CO2 storage capacity and injectivity to investigating potential CO2 geological storage sites. The core drilling of multiple onshore and offshore boreholes was successfully completed for geological reservoir characterization of CO2 storage sites in Southeast Korea. The directional core analysis was designed specially to measure vertical and horizontal permeabilities with directionally plugged rock core samples. The directional hydraulic properties were determined using portable probe permeameter and one-dimensional pressure cell with different confining pressures. The experimental method for measuring permeability was selected both steady state and pressure decay method according to the range of expected permeability measurements. A small diameter portable probe permeameter was applied for rapidly determining gas permeability to select the lab experimental method for estimating permeability. The promising geological formations (739 ~ 779m) were found for prospective CO2 storage reservoirs with high porosity and permeability. Furthermore, flow modelling for CO2 plum migration pathway was conducted to analyses subsequent flow overlying formations with estimated directional hydraulic properties. Assessed correlation and distribution of directional permeabilities for a potential CO2 geological storage site would be utilized for CO2 storage capacity, injectivity, and leakage risk assessment. 1. Introduction The most important properties of a CO2 storage reservoir are porosity and permeability. The overall purpose of this research was to investigate storability of formation in the sea near Pohang. To investigate storability, hydraulic properties are measured though drilled rock samples in Pohang basin. To analyze hydraulic properties of vertical and horizontal direction, plug rock samples are made through vertical and horizontal directional coring using drilled rock sample. The porosity of plug rock samples is measured using helium porosimeter and mercury porosimeter. The permeability of these samples is measured using portable probe permeameter and the broadband rock permeability measurement system.