Ki-Bok Min

Strength Anisotropy of Berea Sandstone: Results of X-Ray Computed Tomography, Compression Tests, and Discrete Modeling

Berea sandstone in northern Ohio is a transversely isotropic rock. X-ray CT investigations showed that its internal structure is composed of cross-bedded loose layers and relatively thin tightly packed layers called bedding. Uniaxial compression tests were performed on different Berea sandstone specimens. The uniaxial compressive strength (UCS) decreases with increasing porosity, and also decreases with increasing inclination of the bedding plane relative to horizontal line. Two-dimensional discrete modeling was applied to investigate the micromechanical behavior of Berea sandstone. Different microparameters were assigned to loose and tight layers. The UCS simulation results agree well with the experimental results. At the peak stress, cracks almost always develop in loose layers regardless of the bedding plane orientation. In addition, both normal and shear cracks occur earlier for specimens with a higher inclination angle. No correlations were found between the inclination angle of failure planes and the orientation of bedding planes. The bedding planes of Berea sandstone are not weak planes. The strength anisotropy of Berea sandstone is not significant compared with other rocks such as shale, gneiss, and schist.

Understanding coupled stress, flow and transport processes in fractured rocks

This paper presents a review of a systematic research program for understanding the scale and stress effects on the transport behaviors of fractured crystalline rocks, using a hybrid discrete element and particle tracking approach. The motivation is the importance of understanding the stress effects on the behaviors of contaminant transport in fractured crystalline rocks, which is an important issue of rock mechanics for environmental safety assessments of many rock engineering projects. The study is divided into three steps: the first step is a basic study that established the mathematical platform for deriving the conditions, criteria, basic approaches, and test case results for investigating the stress and scale effects on the hydraulic behavior of the fractured rock concerned. At the second step, based on outstanding issues drawn from the first step, the study was extended to consider the effects of the correlation between the fracture aperture and size (represented by trace length) on the permeability of the fractured rock and uncertainties in deriving equivalent continuum properties of fractured rocks. The third step added the particle/solute transport processes to the mathematical platform, including different retardation mechanisms, so that the impact of stress on safety can be directly evaluated, even though it can only be done conceptually. The obtained results show that stress, scale, and inter-parameter correlations of the fracture system geometry are dominant issues for the understanding and characterization of coupled hydro-mechanical processes of fractured rocks and play a significant role for understanding the mass transport behavior in them, with direct impact on geo-environmental safety.

A Study of Locally Changing Pore Characteristics and Hydraulic Anisotropy due to Bedding of Porous Sandstone

Anisotropy observed in sedimentary rock such as sandstone is mainly caused by existence of bedding consequently influencing on its hydraulic characteristics. The aim of this study is to investigate the influence of locally changing pore structure due to bedding on the hydraulic anisotropy of sandstone, in terms of localized porosity. X-ray CT scan is applied to observe the internal pore structures which is hard to be seen by other experimental methods. Permeability test is also conducted for samples cored at every from to with respect to bedding plane. As a result, the permeability anisotropy is manifest having 1.8 of anisotropy ratio () and corresponds with the anisotropy of porosity due to bedding.

EGS field case studies - UK Rosemanowes and Australian Cooper Basin projects

Abstract In order to generate electricity from geothermal energy for non-volcanic region, the concept of enhanced geothermal system (EGS) is introduced which forms an artificial reservoir by injecting high pressure fluid to 5 km deep and circulating geothermal fluid through the reservoir. Demonstration studies have been conducted in various countries and regions for determining the feasibility of EGS. In this technical note, experiences, errors, and implications of EGS demonstration projects in UK Rosemanowes and Australia Cooper Basin which have been carried out since 2002 are introduced to be used for the EGS demonstration project in Korea. Key words Geothermal energy, EGS (Enhanced Geothermal System), Rosemanowes, Cooper basin초 록 심부 5 km 내외에 고압의 유체 주입에 의하여 인공저류층을 형성한 후 지열유체를 순환 생산하여 지열발전을 하는 인공저류층 지열시스템의 실현 가능성을 판단하기 위해 다양한 지역에서 인공저류층 지열시스템 실증연구가 진행되고 있다 . 본 기술보고는 영국 Rosemanowes 에서 진행된 EGS 실증연구와 호주 Cooper Basin에서 2002년 이래 진행되고 있는 EGS 적용 사례를 소개하여 해당 지역에서 진행된 연구의 경험 , 오류, 시사점 등을 정리하고 , 향후 한국에서의 실증연구에 활용하고자 작성되었다 .핵심어 지열에너지, 인공저류층 지열시스템 , Rosemanowes, Cooper basin

Effect of Ground Vibration on Surface Structures and Human Environments -Application of Blasting Vibration to Induced Seismicity in EGS Hydraulic Stimulation-

While microseismicity induced by hydraulic stimulation carried out for EGS is useful means in estimating the range of permeability increase, it also affect surface structures and environments. In order to establish a mitigation plan for microseismicity triggered by hydraulic stimulation, we reviewed world-wide guidelines on the impact of ground vibration on the surface structure and human environment by blasting. Case studies from Europe and USA on the microseismicity by hydraulic stimulation are presented and suggestions are made for the guidelines on ground vibration by hydraulic stimulation for the ongoing Pohang EGS project.

EGS Power Generation and Hydraulic Stimulation

Abstract While geothermal energy provides the only base-load power among renewable energy sources, its development has been carried out predominantly in volcanic area. EGS (Enhanced Geothermal System) is a ubiquitous technology that can allow the geothermal power generation virtually in any area. This manuscript introduces the current state-of-the-art of EGS development in the world and presents the hydraulic stimulation technology and associated microseismicity which are key technical component in EGS. Finally this paper suggests the key research areas required in Korea for further development of EGS. Key words Geothermal energy, EGS (Enhanced Geothermal System), Hydraulic stimulation, Hydraulic fracturing, Hydrauilc shearing초 록 지열에너지는 기저부하를 제공하는 신재생에너지이나 현재까지 화산지대에만 대부분의 지열발전이 이루어져 왔다. 인공저류층 지열시스템 (Enhanced Geothermal System, EGS)는 비화산지대의 지열발전을 가능하게 할 개념으로 알려져 있으며 수리자극 (hydraulic stimulation)이 핵심기술이다. 본 논문은 EGS지열발전의 개발 현황을 소개하고 , 수압파쇄와 수리전단이 주 메커니즘인 수리자극의 핵심원리 , 설계변수 및 수리자극에 수반되어 발생하는 미소진동의 원리 및 관측기술을 소개한다 . 한국에서의 EGS 지열발전을 위하여 필요한 과제를 소개하여 향후 기술개발의 방향을 제시한다 .핵심어 지열에너지, 인공 저류층 지열시스템 , 수리자극, 수압파쇄, 수리전단

Comparison of Two- and Three-dimensional Approaches for the Numerical Determination of Equivalent Mechanical Properties of Fractured Rock Masses

This paper compares the two- and three-dimensional (2D and 3D) approaches for the numerical determination of the equivalent mechanical properties of fractured rock masses. Both orthogonally-fractured model and discrete fracture networks (DFN) were used for the geometry and 2D models were cut in various directions from 3D model to compare their mechanical properties. Geological data were loosely based on the data available from Sellafield, UK. Analytical method based on compliance tensor transformation was used for investigation in orthogonally fractured rock and numerical experiments were conducted on fractured rock mass with DFN geometry. It is shown that 2D approach always overestimates the elastic modulus of fractured rock masses by a factor of up to around two because fractures are assumed to be perpendicular to the model plane in 2D problems. Poisson ratios tend to have larger values in 2D analysis while there is opposite trend in some sections. The study quantitatively demonstrates the limitation of the 2D approach that uses the simplified model from true 3D geometry.

Comparisons of Brittleness Indices of Shale and Correlation Analysis for the Application of Hydraulic Fracturing

Abstract Hydraulic fracturing is the key technology for production of shale gas, which is one of the major unconventional resources. Brittleness index is one of the most important mechanical properties which determine the efficiency of hydraulic fracturing. It was required that the production of shale gas increases with more brittle behavior. Confusingly, there are numerous definitions available for brittleness of rock. This paper summarizes various definitions of brittleness index, and presents correlation analysis of the brittleness indices by using the laboratory experimental results of 48 shale specimens in Korea. Generally, it shows a very weak positive correlation between the brittleness index (B 1 ) which is the ratio of uniaxial compressive strength to tensile strength and the brittleness index (B 3 ) which is calculated by using the Youngs modulus and Poisson’s ratio. In addition, the role of Poissons ratio is not clear in defining brittleness indices. In conclusion, standardization of definition for brittleness index is required to apply it to hydraulic fracturing as a parameter for predicting the efficiency.

An Introduction to the Expansion Plan of the Underground Repository of Low- and Intermediate-level Radioactive Waste In Forsmark, Sweden

The world’s first underground repository for lowand intermediatelevel radioactive waste (SFR1) has been in operation since 1988. SFR1 can accommodate 1,000 m 3 of radioactive waste per year with 4 chambers and 1 silo with a total capacity of 63,000 m 3 of radioactive waste. With extended operation time of 10 of the 12 nuclear power reactors and dismantling of the other 2 nuclear reactors, more nuclear waste need to be disposed in the future. Therefore, Swedish Nuclear Fuel and Waste Management Company (SKB) submitted a license application for a repository extension (SFR3) that consists of 6 additional rock chambers with a capacity of 108,000 m 3 of radioactive waste and for accommodating 9 boiling water reactor tanks. In this study, plans for the extension SFR3 are presented with the geological, geomechanical and hydrogeological issues to be considered.

Case Study on Groß Schönebeck EGS Project Research in Germany

This paper presents a case study of an enhanced geothermal system(EGS) demonstration project conducted in Gros Schonebeck, Northerm Germany, focusing on hydraulic stimulation. The project was conducted with doublet system in sandstone and volcanic formations at 4 - 4.4 km depth. Under normal faulting to strike-slip faulting stress regime, hydraulic stimulations were conducted at injection and production wells by massive waterfrac and gel-proppant fracturing. Injectivity index increased from 0.97 m 3 /(hr*MPa) to 7.5 m 3 /(hr*MPa) and productivity index increased from 2.4 m 3 /(hr*MPa) to 10.1 m 3 /(hr*MPa) by a series of hydraulic stimulations at both wells. After circulation tests through injection and production wells, however, productivity index decreased from 8.9 m 3 /(hr*MPa) to 0.6 m 3 /(hr*MPa) in two years. Slip tendency analysis for the stimulation in volcanic layer estimated the required pressure for shear slip and its preferred orientations and it showed reasonable match with actual stimulation results. Through the microseismicity observation for the stimulation of volcanic formation, only 80 seismic events with its moment magnitudes in -1.8