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.
Boeut, Sophea (Hokkaido University) | Oshima, Teppei (Hokkaido University) | Fujii, Yoshiaki (Hokkaido University) | Kodama, Jun-ichi (Hokkaido University) | Fukuda, Daisuke (Hokkaido University) | Matsumoto, Hiroyuki (Kushiro Coal Mine) | Uchida, Kazumi (Kushiro Coal Mine)
There are several case studies in which vibration produced by earthquakes or underground explosion affected the level and temperature of ground water, petroleum production, etc. These might be due to the change in permeability by transient stress disturbances creating new pathways, clearing particles clogging the pore spaces of the existing pathways. This paper investigated on the permeability change of intact and triaxially fractured Kushiro Cretaceous sandstone by transient axial and pore pressure disturbances. In the room temperature, the specimens dimensioned 30 mm in diameter and 60 mm in length were triaxailly compressed under 1 MPa of pore water pressure, and 3-15 MPa of confining pressures. The permeability was measured before (kI: intact rock) and after (kDI: disturbed intact rock) transient stress disturbances for pre-failure rock and before (kP: post-failure rock) and after (kDP: disturbed post-failure rock) transient stress disturbances for post-failure rock.
Under 0-11 MPa of the axial stress disturbance amplitudes, the permeability of the specimens decreased from kI to kDI and from kP to kDP due to the disturbance, yet it increased from kDI to kP resulted in rock failure. The permeability of pre-failure rock decreased larger with confining pressure and time; however, the decrease amount was almost constant by the disturbance amplitudes. For post-failure, the decrease amount of permeability became higher with the axial stress disturbances. This clarifies that the permeability of fractured Kushiro Cretaceous sandstone decreased by axial stress disturbance.
Under pore pressure disturbance amplitudes of 0.2 to 1.8 MPa, in the pre-failure regime, the permeability decreased at the lower disturbance amplitudes, but increased at higher disturbance amplitude. The permeability continued to increase by rock failure, though, in the post-failures, the permeability decreased by the pore pressure disturbances. The increase might be utilized for the enhanced methane gas recovery.
Transient stress disturbances from either earthquakes or the underground explosion may induce change in the underground properties. The seismic waves, for instance, resulted in the change in level, (Beresnev &; Johnson, 1994; Elkhoury et al., 2006; Wang &; Manga, 2009; Manga et al., 2012) and the temperature (Mogi et al., 1989) of the ground water or the petroleum production (Beresnev &; Johnson, 1994; Pride et al., 2008; Roegiers, 2016). The Union of Soviet Socialist Republic, the US, etc., did the underground nuclear explosion tests between the 1950s and 1960s; and these underground explosion tests may have reduced the number of earthquakes more than M8.0 (Fujii et al., 2017). These occurrences might be due to the change in permeability of rock mass by the transient stress disturbances creating microcracks, clearing the barrier particles clogged the pore spaces of the pathways, etc. (Manga et al., 2012). This paper focused on the change of permeability of intact and triaxially fractured Kushiro Cretaceous sandstone. Increase in permeability, if it is confirmed, may encourage its utilization to enhance gas recovery, to prevent future large earthquakes by inducing small earthquakes, and to de-route underground water flow for various purposes.
Pan, Xiaohua (Nanyang Technological University) | Oliver, Grahame John Henderson (Nanyang Technological University) | Chu, Jian (Nanyang Technological University) | Goh, Kok Hun (Infrastructure Design & Engineering, Land Transport Authority) | Wei, Xiaoqian (Infrastructure Design & Engineering, Land Transport Authority) | Kumarasamy, Jeyatharan (Infrastructure Design & Engineering, Land Transport Authority)
The Sajahat Formation is considered to be the oldest rock unit in Singapore. However, the age of deposition is uncertain. According to the Geological Map of Singapore, the Sajahat Formation has been found on Pulau Tekong, Pulau Sajahat and at Punggol Point. However, the occurrence at Punggol has not been confirmed due to the lack of present day outcrops. As part of a site investigation, two boreholes were drilled at Punggol. Hornfelsed quartzite (very similar to that found on Pulau Sajahat) cut by diorite and granodiorite dykes were logged in the core samples. Zircons from these rocks were radiometrically dated using the Laser Ablation ICPMS U-Pb method. The results of the analysis of the detrital zircons indicate that the quartzite was deposited at or later than 337±3 Ma (Early Carboniferous) and before the intrusion of a diorite dyke at 285±1 Ma (Early Permian). A granodiorite dyke was dated at 260±3 Ma (Late Permian). Therefore, the quartzite at Punggol can be confirmed to be the Sajahat Formation of Carboniferous age and is the oldest dated rock in Singapore. The engineering implication of identifying the types of formations is discussed.
The Sajahat Formation in Singapore is defined as those variably metamorphosed, unfossiliferous, sedimentary rocks comprising quartzite, sandstone, and argillite (Public Works Department 1976, Sharma et al., 1999; Lee and Zhou, 2009; Zhou and Cai, 2011). Previous studies indicate that it is probably the oldest rock unit in Singapore based on outcrops found in Pulau Tekong, Pulau Sajahat and Sajahat Kechil. Lee and Zhou (2009) proposed that the age of deposition of the Sajahat Formation was probably Lower Palaeozoic based on its complex deformation history and multiple intrusion of dykes. However, a Carboniferous to Permian age cannot be ruled out. The Sajahat Formation is very similar to the Mersing Formation in eastern Johor which is assumed to be Carboniferous in age since it is overlain by fossiferous Permian conglomerates with an angular unconformity (Oliver and Gupta, 2017). The deformed Sajahat Formation was considered to predate the undeformed Gombak Norite which has been U-Pb zircon dated by Oliver et al. (2014) at 260±2 Ma (Late Permian). The Sajahat Formation is therefore probably pre-Late Permian in age (Oliver and Gupta, 2017). However, there is no direct evidence of the age of deposition of the Sajahat Formation so far.
To consider the roughness effect on shear strength and deformation of rock joint, this research proposed a joint model for the discrete element method. The background theory of the proposed model is based on Barton’s shear strength criterion which is widely used to describe non-cohesive joint with roughness. To implement Barton’s criterion in DEM software, three calculation modifications were performed, including exceeded force recapture, contact area equalization, and stiffness adjustment. Through the modifications, the force of each joint contact could be calculated, which reasonably reflect the joint mechanical behavior under different normal stress. Afterward, the proposed model was verified by comparing to the theoretical model. The results indicated that the proposed model rationally describes the shear stiffness influenced by mobilized joint roughness coefficient during the shear process. The comparisons showed that the proposed model is versatile in simulating the shear displacement with loading-unloading-reloading cycles, normal closure, and shear dilation of joint.
The strength and deformability of rock mass are heavily influenced by the properties of joints. The joint exhibits highly non-linear behavior under applied stress and is influenced by surface roughness. To describe the joint behavior, Barton proposed a non-linear model for rock joint (Barton, 1973). It not only provided the description of the failure envelope but also considered the evaluation of shear-displacement and dilation relationships. Therefore, it is widely used in the analysis of rock mechanics.
On the other hand, the discrete element method (DEM) has been widely adopted to explore the behavior of rock mass and successfully applied to rock engineering. Lots of models were developed to simulate joint behavior in DEM, including the bond-eliminate method, the smooth-joint method, and so on (Chiu et al., 2013). However, these methods can not reflect the phenomenon in experiments, such as shear-displacement curve and nonlinear failure envelope. To overcome this problem, this study proposed a rock joint model “rough-joint model”. The theory of the proposed model is based on Barton’s model. To implement Barton’s criterion in DEM software, three calculation modifications were needed. After finishing the construction of the joint model, the direct shear test with reverse shearing has been simulated to show the performance. The failure envelope, shear-displacement curve, closure curve and dilation curve fit Barton’s model very well. The above results show that rough-joint model can provide a way to simulated joint behavior with roughness in DEM, which is helpful for researchers to perform numerical analysis for the joint sliding problem.
The in situ stress state is a fundamental parameter in rock mechanics and rock engineering, and is often summarized and represented as the point estimate of the principal stresses associated with the mean of a number of measured stresses. However, to account for the likely large uncertainties associated with stress estimations, it is important to also provide confidence intervals. In this paper, we propose a novel method for constructing confidence intervals for the in situ stress state using a multivariate distribution model and Monte Carlo sampling. We show that this method addresses a particular shortcoming of the customary method for constructing in situ stress confidence intervals, namely that of a misconception of confidence intervals. We also show that the customary method does not yield correct confidence intervals, and this may have a significant impact on designs and analyses in rock engineering that incorporate stress uncertainty.
The state of in situ stress is a fundamental parameter in rock mechanics and rock engineering (Amadei and Stephansson, 1997; Brady and Brown, 2004; Hudson and Harrison, 2000). Often in practice, the in situ stress state within a rock mass volume of interest is summarized and represented using the principal mean stress, which is customarily calculated from the mean of the measured stresses (Amadei and Stephansson, 1997; Gao and Harrison, 2017, 2018a, 2018b; Hudson et al., 2003; Martin and Christiansson, 1991a, b; Walker et al., 1990). Statistically speaking, the principal mean stress of measured stresses is a point estimate of the in situ stress state.
However, it is widely recognized that localized measurements of in situ stress often display significant variability in fractured rock masses (Ask, 2003, 2006; Day-Lewis, 2008; Gao, 2017; Gao and Harrison, 2014, 2015, 2016a, b, 2018a, b; Hudson and Feng, 2010; Harrison et al., 2010), and the estimation of the in situ stress state within a rock mass volume may involve large uncertainty, particularly when, as is often the case in rock engineering, only a limited number of measured stresses are available. Unfortunately, the point estimation provides no information on uncertainty associated with the estimation of the in situ stress state, and ignoring this uncertainty may yield misleading and even erroneous results in rock mechanics and rock engineering analyses. The need to evaluate uncertainty becomes particularly pressing when we consider the continuously increasing application of reliability-based design (RBD) methods in rock engineering, as these explicitly require the uncertainties in design parameters such as stress and material properties to be appropriately characterized and incorporated in analyses (Bozorgzadeh, 2017; Bozorgzadeh et al., 2017). Thus, it is important to also provide confidence interval (CI) as a quantitative measure of uncertainty in estimations of the in situ stress state.
With the advancement of science and technology, humans endeavoured to build massive caverns underground taking the advantage of physico-mechanical properties of the rockmass. The rockmass has inherent discontinuities in it whose properties vary greatly from the host rock aiding in the development of potential failure zones during and after execution of such projects. The change in rockmass behaviour observed in such zones calls for safety controls to alarm the working personnel inside the caverns. There arises the need for placing geotechnical and geodetic instrumentation inside rockmass to capture changes in its behaviour and promptly take up the remedial measures to prevent failures. To acquire correct data for right interpretation, there must be a right procedure to be adopted for planning the type of sensors and its specifications, location inside caverns, mode and frequency of data acquisition, data communication and data analysis.
Similar planning was carried out for the caverns of an underground powerhouse complex of Punatsangchhu-II Hydroelectric Project, Bhutan by the authors. The intrinsic complexities and the problems tackled during planning and execution of such mega project are explained in detail in this paper.
Excavations of underground caverns for storing crude oil, construction of powerhouse, nuclear repositories and mining minerals in recent days have increased tremendously throughout the world, thereby maximizing the utilization of underground space. But since, rock is a discontinuous, inhomogeneous and anisotropic material, the reliability of structural integrity remains uncertain. The act of excavation against nature destabilizes the surrounding rockmass which leads to development of potentially unstable zones which deforms with time and if not properly treated or supported, leads to progressive failure of the structure itself. Based on the scale of excavations, the risk associated with the project to lives and property is assessed. In order to prevent any mishap, underground projects call for geotechnical and geodetic instrumentation, that helps in early detection of such unstable zones and any abnormal behavior of the rockmass. Generally, instrumentation in underground rockmass is implemented to accomplish the needs of diagnosis, prediction, legislation and research i.e. verification of design parameters, suitability of any new construction technique, diagnosing cause of an adverse event or verification of continued satisfaction behavior of the rockmass to different operations (Dunnicliff, 1998).
Micro-seismic and acoustic emission (AE) activities resulting from rock failure are among the main parameters used for understanding the rock burst phenomenon in tunnel excavation. To evaluate the mechanism of AE behavior with rock failure, AE is measured by conducting a rock triaxial compression test. The test results are then used to evaluate the characteristics of AE behavior resulting from rock failure. The laboratory test results are subsequently compared to simulate the AE occurrence behaviors due to rock failure by using the particle flow code (PFC) method. The AE event, AE amplitude, AE frequency, and b-value that are measured by using PFC are able to simulate the actual rock failure. The simulated rock burst signals are closely related to the AE measurements obtained from the rock triaxial compression test.
Rock burst is a type of rock failure that occurs when strain energy is rapidly released by an unstable rock mass, which is usually triggered by deep underground excavation (Hoek and Brown, 1997; Rudajev et al., 2000; Beck and Brady, 2002; Weng et al., 2017). Acoustic emission (AE) is measured by conducting a rock triaxial compression test to predict and evaluate the mechanism and progress of rock burst. The AE occurrence behaviors resulting from rock failure are also simulated by using the particle flow code (PFC) method. The number of AE occurrences, waveform frequency, amplitude value, and b-value obtained from the PFC analysis are then modelled, and the analysis results are subsequently compared with the AE measurements obtained from the rock triaxial compression test. The AE occurrence behaviors obtained from the PFC simulation are discussed below along with the results of the rock triaxial compression test.
2. AE measurements obtained from the rock triaxial compression test
2.1 Testing method and equipment
As shown in Table 1, the rock specimen used for this study was hard granite. The test pieces were prepared by using a boring core with a diameter of 50 mm and height of 100 mm. Figure 1 shows the apparatus for the rock triaxial compression test and AE measurement. AE sensors were placed on the top and bottom pedestals of the stress chamber. The loading pattern was recorded by using the strain control method, and the triaxial compression test was performed at confining pressures of 0.5, 10, 20, 30, and 40 MPa. AE was measured throughout the rock triaxial compression test by using several parameters, including the number of AE events, the frequency of the AE signal wave as obtained by conducting a fast Fourier transform (FFT) analysis and by using the AE signal waveform, the amplitude value of the AE signal waveform, and b-value. All these parameters were obtained by performing calculations based on the amplitude value of the waveform (Mori et al., 2003).
Laser perforating is a new approach to the generation of uniform holes in oil and gas reservoir wells at a selected pitch to improve the permeability of rocks. Laser drilling in rocks is a very complex phenomenon that its performance depends on many factors. Since it is not possible to consider all of these factors in the laboratory, numerical modelling is used. In this study, a finite element code (FEM) has been taken to model the thermal and mechanical stresses induced by ND: YAG laser drilling in the hydrocarbon reservoir rock samples. For this purpose, the software ABAQUS was used to analyze the thermal and mechanical stresses induced by laser. It is found that Numerical models show good agreement with the actual observation of holes drilled by the laser. During laser drilling, the rock temperature quickly increases in a few seconds and immediately reduces, thus instantaneous heating and cooling process cause thermal stresses around the hole. Also, the maximum value of thermo-mechanical stress exceeds the strength of the limestone rock and consequently, the formation of cracks and fractures in the wall of the hole are unavoidable.
Laser perforating is a new scientific way to the creation of uniform holes in petroleum reservoir wells to increase the permeability of rocks (Ahmadi et al., 2011). Thermal stress generated by differential thermal expansion of minerals and high-temperature gradient, cause to break the bonds between the grains. In this range of temperature, physical and chemical changes occur that are associated with the process of spallation. A primary physical change associated with this process is due to the thermal expansion of the grains of the rock. For example, the expansion of quartz and plagioclase grains in sandstone lead to a sudden temperature increase in it (Gahan et al., 2004).
As closely-packed grains in the matrix expand with a rapid rise in temperature, they develop thermal stress fractures and cracks within the grains, as well as break the cementation of adjacent grains. As a result, an affected grain will begin to break free from one another (Salehi et al., 2007). Laser effects appear in two steps in rocks, firstly, the creation of a hole in the rock and secondary include melting, evaporation, laser beam gases and micro fractures.
At present, along with conventional energy sources continually consumed, renewable energy sources are increasingly favored, especially the clean and inexhaustible geothermal resources have been universally valued both at home and abroad. In particular, the Enhanced Geothermal Systems (EGS), which is mainly aimed to exploit the thermal energy of Hot Dry Rock (HDR) at depths of 3 to 10 kilometers underground, has been full of interest to many countries. However, so far there hasn't been an EGS being successfully put into commercial operation because of its shortcomings such as small scale, low efficiency, etc. In this article, in response to the bottleneck of the study on the development of traditional EGS based on drilling technology (EGS-D), a conceptual model of EGS based upon excavation technology (EGS-E) is innovatively proposed and its main components of underground structure are described in this paper. As for ‘High ground stress, High ground temperature and High osmotic pressure’ initial conditions with regards to deep rock mass, the excavation experience, which is worth being learnt from extensive review of previous study as well as practical experience such as the successful excavation of ultra-deep mines in the gold field of South Africa, is summed up. The underground spatial structure that may be reasonable to the so-called EGS-E is being tried establishing. It is expected to provide with a basis for our subsequent numerical modeling.
Currently, seeking and developing clean new energy is the basic energy exploitation strategy, and the clean and inexhaustible geothermal resources have been universally valued both at home and abroad. Geothermal energy is the heat energy mainly generated by the transmutation of radioactive elements in rocks, which is 2.0934×1018 kJ annually. And the geothermal energy stored at depths of less than 10 kilometers underground was estimated to be 170 million times the amount of heat released from all the coals stored in the earth by Pollack and Chapman in 1977 (Wang Ruifeng, 2002). It can be seen that the reserves of geothermal energy are very considerable.
In spite of its advantages of stability, continuity and high utilization coefficient, the scale of the geothermal energy with temperature less than 150 °C at depths of less than 3 kilometers underground is usually too small to maintain the demand for long-term stable electricity production which is mainly hydrothermal and only accounts for 10% of all the geothermal energy stored in the earth (Guo Jian et al., 2014). Therefore, the enhanced geothermal system (EGS) which aims at exploiting the geothermal energy from hot dry rock (HDR) at depths of 3 to 10 kilometers has gradually attracted people's attention.
Wei, Shiming (State Key Laboratory of Petroleum Resources and Prospecting / China University of Petroleum) | Chen, Mian (State Key Laboratory of Petroleum Resources and Prospecting / China University of Petroleum) | Jin, Yan (State Key Laboratory of Petroleum Resources and Prospecting / China University of Petroleum) | Lu, Yunhu (State Key Laboratory of Petroleum Resources and Prospecting / China University of Petroleum) | Xia, Yang (State Key Laboratory of Petroleum Resources and Prospecting / China University of Petroleum)
The carbonate reservoirs in western China have low porosity, low permeability and high heterogeneity, thus multi-stage acid fracturing is usually implemented, and refracturing is also needed. During the procedure of multi-stage acid fracturing, interactions among artificial cracks must be cleared, which are also called stress shadows and related to the success of multi-level fracturing measures. The production of multi-stage fracturing well always reduced rapidly, thus the refracturing procedure is carried out. However, As the well has been produced for some time, the stress field around the wellbore has changed. And we have to design the refracturing scheme according to the new stress field. Using the boundary element method to calculate the crack propagation induced stress field in the multi-stage acid pressure process, we can determine the size of the stress shadow, and whether the subsequent crack can be cracked. Using finite element method based on the poroelastic theory, we can get the production induced stress field. Before that, reservoir’s geometrical model was built with DNF (discrete natural fractures), employing crack propagation simulation results to get the artificial crack parameters, using the logging and seismic data to get characteristics of the natural fracture distribution, and determining the type of natural fractures that artificial cracks link to with the fracturing construction curve. After the construction of reservoir’s geometrical model and before the simulation of production induced stress field, the accuracy of the model is verified using some wells’ production data.
The Carbonate reservoir has been found to dominate the proved reserves in western China, which is an important area for Chinese petroleum production increase. The refracturing technique is an important approach to raise the production of Carbonate reservoirs, which are strongly characterized by low porosity and permeability of matrix wherein a large number of natural fractures is well-developed. Theses carbonate reservoirs gave high outputs after being acid fractured, however, the high flow rate could not last long. Therefore, the refracturing is needed. When we design the fracturing and refracturing, current stress fields are an important reference. And the key technical problem is to calculate the induced stress during fracturing and production process together and accurately.