Royal Dutch Shell is changing its tune on carbon, saying it will tie executive pay to shorter-term reductions in emissions. Shareholders will vote on the revisions in 2020. Shell's announcement marks a change in stance by Chief Executive Officer Ben van Beurden, who for years rejected investor demands that Shell detail its plans to curtail emissions, saying it would make the company more vulnerable to lawsuits. "Meeting the challenge of tackling climate change requires unprecedented collaboration, and this is demonstrated by our engagements with investors," the Shell chief said in a statement. "This joint statement is the first of its kind, sets a benchmark for the rest of the oil and gas sector, and shows the benefit of engagement—aligning institutional investors' long-term interests with Shell's desire to be at the forefront of the energy transition," Matthews said.
Naka, Ryosuke (Hokkaido University) | Tatekawa, Takuto (Hokkaido University) | Kodama, Jun-ichi (Hokkaido University) | Sugawara, Takayuki (Hokkaido University) | Itakura, Ken-ichi (Muroran Institute of Technology) | Hamanaka, Akihiro (Kyushu University) | Deguchi, Gota (NPO Underground Resources Innovation Network)
Underground Coal Gasification is expected to be efficient technique for coal energy recovery from deep or complex coal seam since directional drilling technique is advancing in these days. Authors have been performing small-scale UCG model tests to clear gasification and combustion process in UCG. Then, we found that radial cracks were initiated from the cavity formed in the artificial coal seam. Understanding mechanism of the crack initiation is important for clarification of the detail process of combustion and gasification and assessment for environmental risks. In this study, thermal stress analysis was performed on the small-scale UCG model tests to consider the initiation mechanism of the cracks by assuming that combustion and gasification of coal were progressing through the following three processes which are often observed in coal carbonization: (A) thermal expansion, (B) softening and melting and (C) thermal contraction. It was found that tensile stress was induced in the vicinity of the cavity in the tangential direction in process C. Direction of principal stress in the coal was almost parallel to tangential or radial direction of the cavity and the magnitude of it exceeded coal tensile strength. It was also found that tensile stress zone was extended into deeper coal seam with increase in temperature and time and compressive stress zone was formed outside of the tensile stress zone. It can be considered that the radial cracks initiated at the surface of the cavity since tangential tensile stress exceeded tensile strength of coal. Then, radial cracks were arrested at the boundary of tensile stress zone and compressive stress zone after they were propagating in coal seam.
Underground Coal Gasification (UCG) is a technique to use coal energy more efficiently and cheaply. In UCG, oxidant is injected into underground through an injection well to gasify coal seam, and syngas is recovered from a production well (Fig. 1). It is expected that UCG increases available amount of coal energy because even low-grade, complex and deep coal can be used by UCG.
It is pointed out that UCG has risks of surface subsidence and groundwater pollution because cracks are likely to initiate in coal seam by combustion and gasification. Therefore, clarification of initiation and growth mechanisms of the cracks is significant for stability assessment of ground as well as assessing environmental risks.
We performed small-scale UCG model tests on massive coal and crushed coal samples to clear gasification and combustion process in UCG. It was found that radical cracks were initiated in an artificial coal seam made by massive coal as well as crushed coal (Fig. 2 (Kodama et al., 2016)). Similar radial cracks were also observed in large-scale UCG model test (NPO Underground Resources Innovation Network, 2016).
Drill and blast is the excavation method adopted to remove overburden material at the open pit coal mine of PT Buma Job Site Lati. Recently, the company applied deep hole drilling for blasting with double rods to reach the depth of 10-18 m. The main explosive was an emulsion explosive with target explosive consumption of 0.23 kg/m3. The blast hole was not fully charged but vertically decoupled using air decks at the middle and bottom of each blasting hole. Blasted rocks were then measured by digital photograph, and the size of P80 was found to range from 200 to 800 mm. The evaluation results indicate there is close relationship between the explosive consumption and the fragment size as well as the digging time. The air deck technique adopted in this study has been giving good results in terms of fragmentation size and explosive consumption.
In mining operations, drilling and blasting with a deep hole is a preferred method to reduce lost production time caused by the delays in blasting, to increase blasting inventory, and to minimize the number of drill pads so the drilling deviation can be minimized as well. Recently, PT BUMA Job Site Lati (called BUMA from now on) applied drilling for blasting operations with double rods to reach the depth of 10-18 m. Consequently, explosive consumption was high in the lower part of the blast hole, then an air deck was used to distribute the explosive along the blast hole. Another benefit of the air deck was that it cut the waiting time of the on-site sensitized explosive expansion to accommodate gassing.
In this study, the performance of blasting operation using the air deck is evaluated in terms of fragmentation. This study aims to check the effectiveness of using an air deck with the productivity target at the particular mine and to develop a blasting-fragmentation model that can be used to predict the size of the fragmented rock.
2. Literature Review
The air deck method is well-known in blasting operations to improve the quality of the blasting results. In the early 1940s, Russian scientists first came up with the idea of using an air gap between explosive columns. This method reduced explosive consumption in blasting activity. Melnikov et al. (1979) mentioned that an air deck can act as an energy accumulator. Marchenko (1982) found that pressure in an air deck would expand micro fractures that were previously generated by the main shock wave during blasting. Pompanna and Chikkareddy (1993) concluded that the presence of an air gap in the blast hole can reduce ground vibration and back break at the Kudrremukh iron mines. Jhanwar et al. (1996) revealed that the mechanism of air deck can reduce 25-30% of explosive consumption. Chiapppeta (2004) conducted experiments in the field and found that the air deck technique could remove the sub-drill which in turn reduced the explosive consumption by 16-25%, decreased vibration due to blasting by 33%, and improved fragmentation by 25%.
Utilization of an air deck will increase the fracture network due to the secondary shock waves formed as the result of wave reflection in the air gap. The fracture degree increases as a result of secondary shock waves as the duration of the shock wave effect on the rock mass around the blast hole becomes longer. The pressure reflections from the upper and lower explosive columns will collide in the middle of the air deck and is expected to interact with the surrounding rock mass to form additional radial fractures (Moxon et al., 1993; Zhang, 2016; see Fig.1). Air deck methods have been used in some open pit mines to reduce the consumption of explosive and to improve fragmentation (Chiapetta, 2004).
Singh, V. K. (CSIR-Central Institute of Mining and Fuel Research) | Singh, J. K. (CSIR-Central Institute of Mining and Fuel Research) | Kumar, A. (CSIR-Central Institute of Mining and Fuel Research) | Roy, S. K. (CSIR-Central Institute of Mining and Fuel Research) | Kumar, R. (CSIR-Central Institute of Mining and Fuel Research) | Singh, R. K. (CSIR-Central Institute of Mining and Fuel Research) | Kumar, M. (CSIR-Central Institute of Mining and Fuel Research)
The paper deals with geotechnical study and slope stability of the Coal and overburden benches in the hanging wall of an up-throw strike fault in the dip side of NMOC-II open cast coal mine, WCL. The opencast mine is located in the Maharashtra state of India. The detailed slope stability analysis is carried out by limit equilibrium method. The overburden slope of the quarry is mainly characterized by sandstone which is fractured and weathered. The mine is mostly covered by black cotton soil. Well fractured rock mass, existing open cast working have made the geo-mining condition of the quarry to drained condition for all practical purposes after implementing an effective drainage system. The relevant strength properties were determined in the soil and rock mechanics laboratory of CIMFR and subsequently used for slope stability analyses. The rock mass rating was also used to estimate the strength properties. The detailed slope stability analysis was carried out by GALENA software based on limit equilibrium method. The optimum design parameters for final pit slopes have been recommended.
The geotechnical study was conducted for slope stability study of the Coal and overburden benches in the hanging wall of an up-throw strike fault in the dip side of NMOC-II open cast coal mine of Majri Area. The mine belongs to Western Coalfields Ltd, a subsidiary of Coal India Ltd. The opencast mine is located in the Maharashtra state of India. Total overburden and coal have been planned to be excavated by Shovel Dumper combination. The OB bench height is kept at 10m. The shovel and dumper up to 5m3 and 60 tonne have been deployed at the project. The coal and overburden waste production are 2 million tones and 12 million m3 respectively. The existing maximum depth of the pit would be 150m. The study was conducted to optimum slope design with special reference to F1 fault without sacrificing the safety of the mine operation (CSIR-CIMFR Report, 2017).
The top soil of about 10m thickness forms the top most lithology in the project area. Soil is underlain by main overburden constituted sandstone with coal seam. Floor of the seam is generally composed of sandstone. The dip of the seam in this area is about 14 degree.
The importance of safe, properly designed and scientifically engineered slope is well known. The benefit of an openpit operation largely depends on the use of the steepest slopes possible, which should not fail during the life of the mine. So, the design engineer is faced with the two opposite requirements, stability and steepness, in designing the deep openpit slopes. Steepening the slopes, thereby reducing the amount of material to be excavated, can save a vast sum of money (Hustrulid et al., 2000). At the same time excessive steepening may result into slope failure leading to loss of production, extra stripping costs to remove failed material, reforming of benches, rerouting of haul roads and production delays. The Directorate of Mines Safety, the highest statutory body, may even close the mine, in case unsafe conditions are created. Therefore, it is necessary that a balance between economics and safety should be achieved.
The mine is mostly covered by black cotton soil. The information regarding geological sequence is therefore available only from the borehole data. Table 1 furnishes summarized statement of lithological formations encountered in the boreholes drilled in the mine quarriable area. The area is structurally disturbed by fault. F1 fault is present in the present mining area.
The failing process of coal sample under loading conditions is investigated by using both laboratory experiment and 3D finite-discrete element method in the present paper. The cohesive zone model was used to characterize nucleation, growth and propagation of cracks, while the potential contact detection and interaction of fractured solids were examined by means of the penalty method in ABAQUS software, where the parallel computation was employed to accelerate the calculations. Uniaxial and Brazilian tests were performed in the laboratory to obtain the mechanical properties of the coal such as Young’s modulus, fracture energy, cohesive strength, friction angle, uniaxial compression and shear strength. Further, these properties were carefully calibrated prior to being taken as input arguments in the continuous-discontinuous modelling. All the simulating results were basically in agreement with that obtained from the tensile tests in laboratory. This study shows that such computational mechanics of discontinua can be employed to gain powerful insight into the failure mechanism of coal, which could also be a useful tool to clarify the collapse mechanism of coal block caving in mining engineering design and rock test scheme optimization.
As a special kind of rock, coal is generally at a complex stress state under mining conditions. Thus, understanding for the mechanical behavior of coal plays a very important role in designing rock structures such as coal mining, underground excavation. In the literature, numerical methods such as continuum and discontinuum are often used to describe the failure mechanism of rock (Bobet et al., 2009; Li et al., 2015; Lisjak and Grasselli, 2014). For example, plastic deformation and damage softening are perhaps the most studied problems in the continuum method while the internal length of geomaterial is usually not considered in its formulation, which is the most serious drawback because of its predictions significantly depended upon mesh size. To bypass the shortcomings mentioned above, an enriched or higher-order continuum formulation for the softening was developed (de Borst and Pamin, 1996; de Borst, 2002) and nonlocal continuum was also introduced (Bazant and Planas, 1998). However, interaction between fragments during the evolution of multi-cracks cannot still be taken into account in the enhanced continuum methods. On the other hand, discontinuous modeling techniques that are known as discrete element methods (DEM) treat the material directly as an assembly of separate blocks or particles, which was originally proposed by Cundell (1971) from the viewpoint of analogous molecular dynamics simulation to better account for and understand the interaction between the blocks. In discontinuous methods, the length scale can be automatically incorporated into the modelling, which naturally accommodates the real size of elements or particles to capture failure zone of the shear process.
Yang, Yiran (Xi’an University of Science and Technology / Ministry of Education of China) | Lai, Xingping (Xi’an University of Science and Technology / Ministry of Education of China) | Shan, Pengfei (Xi’an University of Science and Technology / Ministry of Education of China)
Occurrence and stress setting in steeply inclined and thick coal seams are totally distinct from the ones in gently inclined coal seams. The difference would result in different roof structures when the roof is unstable. Furthermore, it would lead to inappropriate selection of supports without correct understanding of dynamic evolution of roof structure. This paper adopted comprehensive methods including theoretical analysis, 3DEC numerical simulation, field data comparisons and analysis, and aimed at the appropriate selection of supports and safe mining of Urumchi coal field. The results indicated that parameters including length, position of crest section of the asymmetric flat-topped arch structure(AFAS) and its height kept decreasing with increment of coal seam angle, showing extremely asymmetrical characteristic. simultaneously, the total weight of caving coal and rock mass and the average loading of supports were reducing obviously, while the reduced rate tended to be slow. As a result, the safe mining was improved dramatically and it is very crucial to do intense research and master the dynamic evolution.
The appropriate selection of supports has always been a vital ingredient for safe mining (Liu Changyou, 2015, Liu Jinhai, 2012, Zhang Dongsheng, 2013, Wang Jiachen, 2009), while the empirical method that is employed by most coal mines proves to be unreasonable. Unlike the result that is inferred by empirical method, the field monitoring data of ground pressure suggests that the working resistance of supports do not increase obviously with increment of mining depth. For example, both the mean value of measured maximum working resistance and time-weighted average resistance of working face of different depth in Wudong coal mine are less than 8000KN/support. Data and examples indicate that the supports are protected by a temporary roof structure. Many researchers have successively tried to describe the roof structure and determine the average loading of supports with various methods (Lai Xingping, 2010 and 2013, Shao Xiaoping, 2006, 2008 and 2009, Yang Fan, 2006). Shao Xiaoping (2007) proves that the roof structure would change with coal seam angle and supports only bear the weight within roof structure. Niu Shaoqing (2014) believes that roof instability is caused by relative sliding at roof sliding interface and provides support method. In addition, a fitting logarithm function between safety factor of roof and thickness to span ratio and span of roof with strength reduction method has been achieved by Zhao Yanlin (2010). Surely, all of these researches have made great contribution to the study of roof structure. However, seldom study have focused on the dynamic evolution of roof structure. To solve the above mentioned problem, this paper adopted comprehensive methods including theoretical analysis, 3DEC numerical simulation, field data comparisons and analysis. And the results indicated that there do exist a AFAS in the roof of working face. It is precisely the AFAS that allow supports only to bear the weight of caving coal and rock mass within AFAS. It is necessary for appropriate support selection to master the dynamic evolution of roof structure.
Saydam, Serkan (University of New South Wales) | Wu, Saisai (University of New South Wales) | Ramandi, Hamed Lamei (University of New South Wales) | Crosky, Alan (University of New South Wales) | Timms, Wendy (Deakin University) | Hagan, Paul (University of New South Wales) | Hebblewhite, Bruce (University of New South Wales) | Vandermaat, Damon (University of New South Wales) | Craig, Peter (University of New South Wales) | Chen, Honghao (University of New South Wales) | Elias, Elias (University of New South Wales)
Catastrophic failure of rockbolts and cable bolts, due to stress corrosion cracking (SCC), is a major problem in many underground excavations that can compromise both safety of the workers and the economic viability of the operations. This paper reports on development of laboratory instruments and methodologies at UNSW Sydney for simulating SCC in laboratory environments. Both representative coupon testing, and full-scale rockbolt and cable bolt testing methodologies are presented. Coupled with a detailed environment characterisation and field tests, the laboratory methodologies will aid in further understanding of SCC and identifying the potential countermeasures to prevent SCC occurrence in underground excavations.
In underground structures, excavation of rocks reduces the confining pressure on the surrounding rocks, allowing the strata to separate, fold and buckle into the void created (Aydan, 2018). Because rock is weak in tension, this buckling action can lead to fracturing of the strata and a roof failure. To prevent the relative movement and fracturing of the strata, rockbolts and cable bolts are often used to stabilise an excavation (Chen et al., 2016; Hadjigeorgiou and Potvin, 2011; Kilic et al., 2002; Oliveira and Diederichs, 2017; Windsor and Thompson, 1994). Rockbolts used in underground coal mines are usually manufactured from steel rods, typically 22 mm in diameter and 1200-2200 mm long, which are installed by drilling a hole into the rib or roof strata. Cable bolts are an evolution of rockbolting technology which are usually comprised of a number of wires wound together around a central king wire. Cable bolts usually offer a higher flexibility and load capacity than regular rockbolts (Chen et al., 2015; Galvin, 2016; Windsor, 2004). These, together with cable bolts greater length, allow for anchoring to a greater depth where the potential of presence of stable rockmass is high.
With the decline in the global coal reserves accessible for open-cut mining, underground mining at greater mine depths has increased the reliance of coal industry on rock reinforcing techniques. As the mining operations continue in greater depth, rockbolts and cable bolts encounter more challenging geological conditions. In the past few decades, a particular attention has been paid to failure of rock bolts and cable bolts in underground mines. One of the main causes of such failures has been identified to be stress corrosion cracking (SCC), which had been simply overlooked in the past. SCC requires synergistic occurrence of three key elements: stress, an appropriately corrosive medium and a material susceptible to SCC (Gamboa and Atrens, 2003; Jones, 1998). This synergy is described in the schematic shown in Fig. 1. The conditions required to induce SCC vary depending on each of the key element. The stress required to induce SCC is usually below the yield stress of the material. Stress corrosion cracks generally grow at a slow rate until the stress in the remaining section exceeds the fracture strength of the material, at which point the material will fail (Enos and Scully, 2002; Scully, 1975; Wu et al., 2018b). SCC results in a dramatic reduction in mechanical strength with only a very minor removal of material. In most cases, SCC is not noticeable by a casual inspection. Structures affected by SCC generally fail in a fast, sudden, brittle and catastrophic manner (Schweitzer, 2010).
Hamanaka, Akihiro (Kyushu University) | Itakura, Ken-ichi (Muroran Institute of Technology) | Su, Fa-qiang (Henan Polytechnic University) | Deguchi, Gota (Underground Resources Innovation Network) | Kodama, Jun-ichi (Hokkaido University)
Underground coal gasification (UCG) is a process of producing combustible gases by the in-situ conversion of coal into gaseous products. Coal resources abandoned under the ground for either technical or economic reasons can be recovered with economically and less environmental impacts by UCG; therefore, this technology is regarded as a clean coal technology. UCG has several advantages of low investments, high efficiency, and high benefits compared to conventional coal gasification. However, some environmental risks such as gas leakage, surface subsidence, and underground water pollution are difficult to control because the process is invisible. The reactor in UCG is unstable and expands continuously due to fracturing activity caused by coal combustion. It is, therefore, considered that acoustic emission (AE) is an effective tool to monitor the fracturing activities and visualize the inner part of coal. For this study, UCG model experiments were conducted using coal blocks of 0.55 × 0.60 × 2.74 m to discuss the applicability of AE monitoring for the estimation of the crack generations during UCG process and the extent of the gasification area. Temperatures were also monitored because the crack generations were strongly related to thermal stress occurred by coal combustion and heat transfer. The monitoring results of AE agreed with the measured data of temperatures and the gasification area; the source location of AE was detected around the region temperature increased and the gasification area. Additionally, the gasified coal amount can be predicted by using the data of product gas. Therefore, AE monitoring combined with the prediction of reacted coal amount are expected to be a useful tool as monitoring system of the gasifier in the underground.
Underground coal gasification (UCG) is a technique to extract energy from coal in the form of heat energy and combustible gases through the chemical reactions in the underground gasifier. This technique enables to utilize coal resources that remain unrecoverable in underground due to either technological or economic reasons. Most coal mining in Japan was closed by 2001 because of complicated geological conditions for mining development and high prices of domestic coal. However, abundant unused coal resources remain underground, but they are not recoverable because of technical and economic reasons. Such coal resources are estimated to be 30 billion tons. For that reason, UCG has a great potential to recover vast amounts of energy from these coal resources.
Zhao, Jingtao (State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology-Beijing, Beijing 100083, China) | Peng, Suping (State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology-Beijing, Beijing 100083, China) | Cui, Xiaoqin (State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology-Beijing, Beijing 100083, China) | Du, Wenfeng (State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology-Beijing, Beijing 100083, China)
Summary Traditional migration profiles of diffraction imaging may cause confusing to seismic interpretation work, as some really existing weak diffractors may be indistinguishable to migration artifacts. The reflec tivity images of diffractions, including diffracted/scattered waves, depict subsurface discontinuities or inhomogeneities. If a vector is used to represent such a reflectivity distribution pattern, most of its components will tend to zero. Therefore, a sparse least-square diffraction imaging method is formulated using a multi-parameter sparsity model that can simultaneously impose smoothness constraint on reflection images and sparsity constraint on diffraction images. The field data application in coal mining further verifies its significant value in resolving the hidden edges, faults and X-conjugate shear joints for warning potential accidents of water bursting or gas leakage.
Zhu, G. L. (Masdar Institute of Science and Technology) | Sousa, R. L. (Masdar Institute of Science and Technology) | Zhou, P. (China University of Mining and Technology) | Yang, J. (China University of Mining and Technology)
ABSTRACT: An innovative approach to the gob-side entry retaining non-pillar mining is being used to increase the coal seam re cycling rate and productivity in China’s coal mining. The retained entry with a sidewall formed by gob caved-in filling rocks is unique in this method and the stability of caved-in material is critical to ensure efficient and safe mining activities. In this paper a numerical investigation on the stability of the gob-side entry is conducted using a discrete fracture network (DFN) model developed by the Massachusetts Institute of Technology (MIT), GEOFRAC, in combination with discrete element modelling (UDEC). The proposed method is applied to a case study of a gob-side retained entry in an underground coal mine in China. Fracture traces are measured along the gob-side wall of the entry, and statistical methods are used to estimate the fracture intensity and the mean fracture areas, which are the key inputs to GEOFRAC. Fracture networks generated by GEOFRAC estimate the rock blocks in the filling body, and simulations with UDEC are done to evaluate the stability of the gob-side entry. Two models are developed, one considering the generated fractures and the other considering no fractures within the gob-side filling. The results show the effects of considering the fractures in the filling body on the distribution of displacement and field stress in the gob-side entry zone. Also, the stability under the mining impact loading, due to periodic roof caving, is simulated, providing the basis for the optimization of the design of the entry support.
Coal is one of the most significant energy resources, covering about 30% of energy consumption worldwide. China is the largest producer and consumer of coal in the world. Despite, China’s reduction of coal consumption in the past few years, and its target to reduce coal to 58% of total energy consumption by 2020, coal remains an important source of energy, and China is still one of the largest coal producer in the world. Fig.1 shows China’s annual coal production in recent years.