Jang, Youngho (Korea Institute of Geoscience and Mineral Resources) | Yoo, Hyunsang (Chonnam National University) | Sung, Wonmo (Hanyang University) | Lee, Jeonghwan (Chonnam National University) | Lee, Won Suk (Korea Institute of Geoscience and Mineral Resources)
Hydraulic fracturing in naturally fractured tight formations causes complex fracture geometry corresponding to the existence of natural fracture as well as geomechanical characteristics. It is extremely important when stimulating fluids such as water, brine, and CO2 are injected into tight carbonate reservoirs for increasing recovery. For fracture propagation modeling, several models have implemented multiple planar fracture with only opening mode, which have a problem in representing realistic fracture behavior.
In this regard, we proposed a hydraulic fracture propagation model implementing multiple planar approach with mixed mode including opening and sliding modes for being able to describe fracture propagation more realistically. When hydraulic fracture encounters natural fracture non-orthogonally, fracture tip slides first along the natural fracture face, and then propagates into rock mass. That is why sliding mode also needs to be considered. With the use of the model, this study analyzed the effects of the brittleness on fracture propagating behavior and gas recovery for tight formations with respect to Poisson's ratio and Young's modulus.
We investigated the modeling results for examining the importance of the sliding mode newly employed in the hydraulic fracture model. In the case of a formation having a high brittleness index, which presents greater deformation longitudinally rather than transversely, called the formation A, the effect of sliding mode was not critical on the fracture propagation. Meanwhile, in a formation having an intermediate brittleness index, named the formation B, the hydraulic fracture less easily crosses natural fracture because Young's modulus of this formation is lower comparing to the formation A, and consequently, implementation of the sliding mode is more dominant. In a formation representing higher Poisson's ratio and Young's modulus compared to formation A, which denotes a low brittleness index, since it shows larger deformation in transverse direction, hydraulic fracture hardly crosses the natural fracture, and thereafter, the propagating direction of the crossed fracture is highly deviated. The larger brittleness index, the greater in stimulated reservoir volumes by up to 17% because fracture crossing occurred easily.
Therefore, the model with the mixed mode proposed in this study was found to be extremely important in the analysis of fracture propagation behavior resulting the stimulated reservoir volumes and gas recovery differently in tight carbonate reservoirs.
Park, Jung-Wook (Korea Institute of Geoscience and Mineral Resources) | Kim, Taehyun (Korea Institute of Geoscience and Mineral Resources) | Park, Eui-Seob (Korea Institute of Geoscience and Mineral Resources)
As part of the DECOVALEX-2019 project Task B-Fault slip modeling, we are developing a numerical model for simulating fault activations induced by water injection. The work of Task B is scheduled to be conducted until 2019 in three research phases. The topic of the first step is developing a numerical method for a benchmark model to simulate the injection test in a single fault zone. We present a numerical model to reproduce the coupled hydro-mechanical process of fault activation using the TOUGH-FLAC simulator. The mechanical behavior of a single fault is represented by the zero-thickness interface element of FLAC3D upon which a slip and/or separation is allowed. The fluid flow along a fault is represented using finite thickness elements in TOUGH2 on the basis of Darcy’s law with the cubic law. The hydro-mechanical coupling between the fracture hydraulic transmissivity and the slip-induced displacement was established for two different fault models (FM1 and FM2). A coupling module was developed in the TOUGH-FLAC simulator to continuously update the changes in geometrical features, as well as hydrological properties induced by mechanical deformation. Then, the transient responses to stepwise pressurization of the fault and host rock were examined during the simulation. The hydro-mechanical behavior, including the injection flowrate, pressure distribution around the borehole, stress conditions, and displacements in normal and shear directions induced by water injection were monitored along the fault and/or surrounding rock. The results of benchmark calculations suggest that the developed model can reasonably represent the hydro-mechanical behavior of a fault and the surrounding rock, including the progressive evolutions of the pathway and fault slip zone. This study will be extended and enhanced through continuing collaboration and interaction with other research teams of Task B.
The DECOVALEX project, which began in 1992, is an international research and model comparison collaboration for thermo-hydro-mechanical-chemical processes in geological systems. Task B of the current DECOVALEX-2019 phase, running from 2016 to 2019, addresses the potential creation of permeable flow paths for contaminant transport in low-permeability host rocks. The objective of the task is to develop numerical models for coupled hydro-mechanical processes of fault activation. The work is planned to be conducted until 2019, through the following three steps of progressively increasing complexity: 1) The benchmark calculation of a simplified single fault plane, 2) the interpretive modeling of an observed activation in a minor fault, and 3) the interpretive modeling of an observed activation in a major fault. The model developed in the benchmark calculation will be modified and verified using the field data from fault activation experiments recently performed at the Mont Terri underground research laboratory in Switzerland.
Cheon, Dae-Sung (Korea Institute of Geoscience and Mineral Resources) | Jin, Kwangmin (Korea Institute of Geoscience and Mineral Resources) | Jung, Yong-Bok (Korea Institute of Geoscience and Mineral Resources)
Microseismicity is an generated elastic wave when a crack is generated due to deformation or damage of a material, and it tends to increase sharply before macro-failure of the material. It can be used to monitor the safety of the rock mass structure such as mine and tunnel etc., and also used to determine the locations of cracks or macro-failures. In order to analyze the source location of cracks, it is important to consider the elastic wave propagation velocity, arrival picking, source location analysis algorithm, and sensor array. However, the location of the sensor may be restricted due to site conditions and economic problems, which may result in inability to interpret the source location or decrease reliability of MS monitoring. In this study, to improve the accuracy of source location analysis, we analyzed source locations according to various arrival picking method and source location algorithm. Among the methods, AIC and Generic algorithm for source location were found to be superior to other methods.
Microseismicity(or Microseismic event) can be defined as a very small earthquake caused by natural(wave, wind etc.) or artificial (hydraulic fracturing, blasting etc.) causes. Generally, it is a small size (< M_w 2.0}) and high frequency (> 50Hz) compared to earthquakes. Earthquakes are primarily caused by nature, but microseismic event is often caused by induced earthquakes. Figure 1 is a brief summary of the frequency domain and the audible domain for earthquakes, microseismic event, and acoustic emissions. Microseismicity is an generated elastic wave when a crack is generated due to deformation or damage of a material, and it tends to increase sharply before macro-failure of the material. Microseismic monitoring can be traced back to 1938 when the U.S. Bureau of Mines attempted to relate seismic wave velocity with pillar load. It is used for geotechnical safety monitoring based on the characteristics of the increase in the number of events before major failures. Thesedays in situ microseismic monitoring of the rock mass fracturing process has been widely used in rock mechanics tests and rock engineering projects throughout the world
A quick, simple and quantitative method for the estimation of surface subsidence susceptibility in mined areas with a lack of detailed geological and geometrical information in underground is presented in this paper. In the method only gangway depth from the surface and the attitude (dip and dip direction) of main geological features are used as input data based on the degree of availability and reliability. Underground gangways are represented as a series of points instead of closed polygons for easy calculation. The core assumption in this method is that the susceptibility to subsidence within a unit area increases both as the depth of the gangway from the surface decreases and as the number of gangways below the unit area increases. In spite of the simplicity of the proposed method, it gave satisfactory results when applied to a virtual excavation model and a closed coal mine where subsidence occurred actually.
Several methods for predicting ground subsidence due to mining excavation such as the profile method and the influence function method have been proposed (Whittaker and Reddish, 1989; Sheory, 2000). The National Coal Board (1975) has presented a basic technique to determine the surface area affected by coal mining based on the height and width of mined areas and the angle of inclination of coal seams. All these methods were developed and verified for conditions involving horizontal coal seams and long wall mining, which are the common mining conditions in Europe. However, coal-associated geological structures in Korea are very complicated, and coal seams have various widths and irregular dip angles. Consequently, the slant chute block caving method has been widely used in Korea, and sinkhole type subsidence is more common than trough type. As a result, the conventional prediction methods must be adapted to the Korean geology and mining conditions, or new subsidence estimation methods must be developed.
The goal of this study is to develop a simple, general, quantitative and reliable method for identifying subsidence susceptibility of the closed or abandoned coal mines, which is proper to be employed in geologically complicated areas. The proposed method in this paper considers only gangway depths and attitude of geological features like dip and dip direction is an optional parameter, because these data are relatively easy to acquire and generally reliable.
2. Estimation of subsidence susceptibility
2.1 Basic assumption
The depth of gangways is selected as an input data of this study after surveying the availability and effectiveness of data because it is reliable and can be easily acquired. In fact, several researchers revealed that the magnitude (volume) and depth of excavation are the principal factors influencing on the subsidence (Whittaker and Reddish, 1989; Singh and Dhar, 1997; MIRECO, 2008).
The method proposed in this study is based on the fact that the excavation volume and shape (or distribution of coal seams) are closely related to the gangway distribution. Two basic assumptions considered in the method are that the susceptibility to subsidence within a unit surface area increases as the depth of a gangway from the surface decreases and the number of gangways below the unit area increases. The first assumption is based on the bulking of failed rock mass which can fill the excavation and prohibit the propagation of roof failure. The second assumption comes from the fact that the rock mass around the excavation is damaged due to blasting and induced stresses.
The susceptibility related to the depth of a gangway is quantified using a negative exponential equation based on the results of numerical analyses (Park et al., 2005) and statistical data of subsidence occurrences in Korean coal mines as shown in Fig. 1 (MIRECO, 2008). Park et al. investigated the influence of the depth and width of excavation and of the spacing and dip of discontinuity on ground subsidence using PFC2D capable of modeling the bulking effect and showed that the overburden remains undamaged as the mining depth increases. Fig. 1 shows that most of subsidence occurred within a depth of 100 m from the surface. The number of subsidence events decreases exponentially as the gangway depth increases.
Kim, Gvan Dek (Seoul National University) | Choi, Jaeho (Seoul National University) | Lee, Kyungbook (Korea Institute of Geoscience and Mineral Resources) | Shin, Hyundon (Inha University) | Choe, Jonggeun (Seoul National University)
In this paper, the selective use of measurement data using Ensemble Smoother is suggested in order to improve its performances by reducing the possibility of misuse of observed data. Key idea is that observed data are selected out on the basis of water breakthrough for better reservoir characterization. We use oil production rates before water breakthrough and water cut rates after water breakthrough for each well because ES cannot interpret the physical characteristic of water breakthrough properly. The consequence is that the proposed method gives us the best reservoir characterization of results with clear channel patterns and connectivity.
Reservoir characterization is one of the most important things for decision making in petroleum engineering. The way to make reliable and proper reservoir models is using static and dynamic data. Prior reservoir models made by using static data only have high geological uncertainties. In order to reduce these uncertainties, history matching is applied to integrate dynamic data, but the uncertainty range might be still high due to modelling error, limited data, or measurement error. Therefore, uncertainty quantification is vital for future performance. To improve performance estimation, many ensemble members are very often utilized to various reservoir characterization methods. The process is called ensemble-based reservoir characterization.
Evensen (1994) offered Ensemble Kalman filter to ocean dynamics. In case of reservoir engineering, EnKF was introduced by Naevdal et al. (2002). EnKF has many advantages such as uncertainty quantification in predicted productions, real-time updating of observed data, easy coupling with any forward simulator, and flexibility for types of model parameters and observed data.
However, EnKF has two critical limitations: overshooting and filter divergence (Aanonsen et al., 2009; Jeong et al., 2010; Oliver and Chen, 2011). These problems occur when model parameters do not follow Gaussian distribution or initial ensembles are not reliable and quite different from the true model. The importance of overcoming EnKF demerits was described by many researchers. However, most of the proposed methods still take high simulation time and have the restart option of a forward simulator due to recursive update of EnKF. Van Leeuwen and Evensen (1996) applied Ensemble Smoother (ES) for meteorology and compared EnKF with ES for history matching. Skjervheim et al. (2011) first proposed ES to reservoir characterization. They suggested that ES showed quite reliable results compare to EnKF. ES is very fast and simple because it assimilates all dynamic data at once, simultaneously. Also it is easier than EnKF for coupling with any reservoir simulator since it doesn't need restart option. However, it is still unstable and exposed to the possible overshooting and filter divergence.
Park, Hyemin (Hanyang University) | Park, Yongjun (Hanyang University) | Yeonkyeong, Yeonkyeong (Hanyang University) | Kim, Joohyung (Hanyang University) | Lee, Wonsuk (Korea Institute of Geoscience and Mineral Resources) | Kwon, Oukwang (Korea National Oil Corporation) | Sung, Wonmo (Hanyang University)
When low-salinity water containing SO42- is injected into carbonate oil reservoirs, rock dissolution and in-situ precipitation occur as chemical equilibria are progressed, altering both permeability and wettability. The Ba2+/Sr2+ ions present in the formation water as impurities chemically react with the SO42- ions, and BaSO4 and SrSO4 are precipitated. In addition, when injected low-salinity water including SO42-, either dissolution of Ca2+-containing minerals or CaSO4 precipitates are occurred. These reactions can cause a serious impact on the efficiency of enhanced oil recovery (EOR). Therefore, the main purpose of this study was to identify EOR efficiency induced by low-salinity waterflooding (LSWF) when Ba2+/Sr2+ were present in a carbonate oil reservoir.
From the results of the effluent analysis and material balance calculation with the produced Ba2+, Sr2+, and Ca2+ concentrations, when Ba2+/Sr2+ concentrations were low, permeability was improved because of rock dissolution predominating over in-situ precipitation. These results concurred to the permeability change which was calculated with the measured pressure data. Also, in the analysis of wettability alteration through the measurements of contact angles and relative permeabilities before and after LSWF, higher concentrations of impurities consumed more SO42- in precipitating BaSO4 and SrSO4, resulting in weaker wettability alteration due to the reduction of sulfate activity. These phenomena ultimately influenced EOR efficiency, i.e., the oil recovery was greater as Ba2+/Sr2+ concentration were lower. Therefore, applying LSWF containing SO42- ion to carbonate oil reservoirs is not always a desirable EOR method when Ba2+ or Sr2+ is present, as an impurity, in the formation water.
Microcracks progressively occur, propagate, coalescence and finally lead to failure when it is subjected to stress. When subjected to loads over a long period, failure may occur below the material strength. It can be considered as a time-dependent behavior of materials. During this behavior, acoustic emissions (AE) are normally generated and using these signals we can evaluate the stability of materials. In this study, we conducted both on static and dynamic long-term strength tests. In the static test, we adopted a subcritical crack growth test for Mode I and Mode II. In case of the dynamic test, cyclic four point bending test and cyclic shear test were used. From the static test, we estimated the characteristics of delayed failure and long-term strength of granite and from the dynamic test, we estimated the fatigue life of concrete and got an S-N curve. We also evaluated the static and dynamic long-term stability using cumulative acoustic emissions curve.
Acoustic emission (AE) is one of elastic waves which is generated when new cracks or cracks propagate in material (Ishida et al., 2017). It occurs when the applied stress exceeds a certain threshold value, it can be related to the long-term strength or fatigue limit. Particularly brittle materials have small displacement or strain before failure, but even in this case, AE tends to occur continuously.
The long-term stability is important in terms of long-term utilization of rock structures such as radioactive waste disposal, CO2 storage, and underground storage of compressed air, etc. The long-term stability can be classified into delayed failure due to static creep and fatigue failure due to dynamic cyclic loading. The related limited researches have been conducted by some researchers due to the time limitation and experimental difficulties (Wilkins, 1980, Backers, 2006, Kim and Kemeny, 2008, Rinnie, 2008, Ko, 2008, Nara et al., 2010, Park and Jeon, 2006).
ABSTRACT: The present study proposes a back analysis method for estimating the geotechnical parameters of the surrounding rock mass in tunneling. This method takes a direct search approach based on a successive response surface method, where the size of the region of interest is adjusted to facilitate a fast convergence of the parameter estimation process. Using a software tool developed in this study, the response surface method is combined with a numerical code, FLAC2D, which has been popularly used in numerical simulations of underground problems. Based on a railway tunnel example, the computational accuracy and efficiency of the proposed method are investigated. The back-calculations between the algorithm, which considers the adjustment of the region of interest, and the non-adjustment algorithm are compared to understand how the adjustment affects the back analysis results. In addition, the effect of different locations of measurement points at the tunnel roof and sidewalls on parameter estimation is investigated. The results of these investigations show that the adjustment algorithm can produce better results in computational accuracy and efficiency than the non-adjustment algorithm. Back-calculated solutions for different locations of measurement points agree well with the exact solution, suggesting there are no significant differences between converged parameter values.
A fundamental element of the observational method in tunneling is the use of monitored data to assess the adequacy of the employed design and to predict the behavior of rock mass during future construction stages. In situ ground movement or convergence of a tunnel is often used as input data to calibrate numerical or analytical models, so that the predicted values of rock mass responses match the corresponding values of measured data. From this calibration, i.e., back analysis, the geotechnical parameters of the surrounding rock mass can be estimated. These estimated parameters can be compared with those determined in the design phase or obtained from previous field tests to improve and modify the material properties of the rock mass.
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)
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 29th to February 20th, 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 30th 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.
1.1. 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.
Jin, Kwangmin (Korea Institute of Geoscience and Mineral Resources) | Ko, Kyoungtae (Korea Institute of Geoscience and Mineral Resources) | Kihm, You Hong (Korea Institute of Geoscience and Mineral Resources)
Spent nuclear fuel is estimated at 10,300 tons in late 2007, and nuclear reactors are expected to be estimated by 2016 when it is stored in a temporary in the nuclear power plant, S. Korea. Therefore, safe nuclear waste disposal is necessary for the use of nuclear energy for the nation’s atomic power. Recently, the deep disposal of spent nuclear fuel is regarded as the most safe disposal method in the world. This study presents the geologic media and geologic structural elements for disposal site selection in South Korea. The basements of South Korea consist of plutonic (25.6 %), metamorphic (29.5 %), volcanic (16.2 %), sedimentary rocks (25.4 %). The plutonic and metamorphic rocks are distributed more than 50 percent among them. These crystalline rocks are considered for their hydrological, mechanical, and geochemical characteristics that are beneficial to repository performance. South Korea divides into Gyeonggi Massif, Ogcheon Belt, Yeongnam Massif, and Gyeongsang Basin based on their geologic characteristics. Also, the major faults distributed in South Korea such as Yangsan Fault System, Ogcheon Fault System, Chugaryeong Fault System. In addition, lineament analysis conducted based on its length, density, and orientation. The dominant orientation of lineament is NNE-SSW and NE-SW. The highest density of lineation is the southeastern part of South Korea. Recently, more than 60 Quaternary faults have been reported along the Yangsan and Ulsan fault systems, major structural features in SE Korea.
Nuclear power plants generate radioactive wastes which do not constitute a health hazard under properly managed. An underground repository for low- and medium-level waste has already constructed. However, the spent fuel is the most hazardous waste as it is very radioactive for long time periods. The radiotoxicity of the spent fuel decays down to the level of the naturally occurring uranium ore after about 100,000 years. Spent fuel in South Korea is estimated around 10,300 tons, which is stored in a temporary storage in the nuclear power plants. These temporary storages, those are managed at each nuclear power plants, will be saturated soon. So, the deep burial of high-level radioactive waste is required for safe disposal. Therefore, deep burial of high-level radioactive waste is currently being studied in South Korea.