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Sun, Zheng (China University of Petroleum at Beijing, Texas A&M University) | Wang, Ke (Southwest Petroleum University) | Yang, Zhaopeng (PetroChina Research Institute of Petroleum Exploration&Development) | Tang, Kang (China University of Petroleum at Beijing) | Xiong, Hao (The University of Oklahoma) | Huang, Liang (University of California, Berkeley) | Miao, Yanan (Shandong University of Science and Technology) | Wang, Zhiqiang (CNPC Chuanqing Drilling Engineering Co., Ltd) | Zhang, Xin (CNPC Liaohe Petroleum Engineering Co., LTd) | Li, Xiangfang (China University of Petroleum at Beijing)
Academic investigations digging into the methane flow mechanisms at the nanoscale have been of high interests in the past decade. On one hand, the prosperity of shale gas/oil industry is supposed to take the responsibility at a certain degree. On the other hand, the complex essence and broad theoretical as well as application value possess great attraction. In this research work, utilizing the molecular dynamics methods nested in LAMMPS softwares, a fundamental framework is established to mimic the nanoconfined fluid flow through realistic organic shale matrix, i.e., composed of specific number of kerogen molecules. Back to the previous literatures related to the technical aspect in this paper, some adopted the graphit material to characterize the organic matter of shale matrix, rather than realistic kerogen molecules. Recently, promotion efforts have been implemented in the academic community with the use of kerogen molecules, the gas flow simulations are still lacking and also the pore shape in current papers is always hypothesized as slit pores. The assumption is regarded as the serious conflicts with the general observation phenomenon according to the advanced laboratory experiments, such as SEM image, AFM technology, and so on, that the organic pores in shale matrix tend to have circular pore geometry. In order to make things right and fill up the knowledge gap between simulation results against realistic cases, the circular nanopore with desirable pore size is constructed with surrounding kerogen molecules, expecting to be more physically and theoretically similar to the realistic organic 2 matter of shale matrix. Then, methane flow simulation is performed by utilization of non-equibrium molecular dynamics (NEMD), the density and velocity distribution under different pressure differences are presented. Furthermore, detailed discussion with respect to the simulation results is given. It suggests that pressure difference acts as a dominant role affecting methane flow velocity, while fails to influence the density distribution, which is considered to be mainly controlled by the strong molecular-wall interactions. In light of the fact that the simulation work is the first attempt for methane flow through circular nanopores inside realistic shale organic matrix, which can provide more accurate evaluation report with regard to the flow behavior and capacity of methane than that of existed documents.
ABSTRACT: In order to experimentally study the evolution features of gas seepage in the micropore structure of coal under the three-dimensional (3D) stress environment as in the mining heading roadway of coal mines, a true triaxial gas-solid coupling coal seepage test device was independently developed. Its 3D stress environment is supplied independently using 2D rigid and 1D soft load ways and controlled by the hydraulic servo system. Such a design can overcome the stuck problem often occurring in the loading processes. Experimental results show that the device can be used to well monitor the changes in structure and permeability of coal body subject to 3D stresses in three directions with good stability and reliability. The true triaxial gas-solid coupling coal body seepage test combined with the CT scanning coal body three-dimensional fracture model construction technology can systematically explore the deformation and seepage of the microscopic pore fracture structure of the coal body.
Wang, Gang (Shandong University of Science and Technology) | Han, Dongyang (Shandong University of Science and Technology / College of Mining and Safety Engineering) | Jiang, Chenghao (Shandong University of Science and Technology / College of Mining and Safety Engineering) | Qin, Xiangjie (Shandong University of Science and Technology / College of Mining and Safety Engineering)
ABSTRACT: In order to investigate the stress distribution characteristics and deformation failure law of micro-coal structure under loading, a digital model that can characterize the true pore structure of coal and rock mass is constructed by combining computed tomography (CT) scanning and 3D reconstruction. The digital model is simulated using the software ABAQUS and the Drucker–Prager constitutive model via uniaxial compression. Results show that stress concentration usually occurs under axial load. The stress distribution law of elliptical pores is related to the inclination of the pores. The tensile stress at the tip of the pore boundary is concentrated in the vertical pores. The stress type in the middle position of the pore boundary changes from tensile to compressive. The distribution law is contrary to that of the stress distribution in the vertical pores. Through the analysis of the stress distribution law of the pores under vertical and horizontal conditions, it is concluded that the inclination degree of the pore and fracture structure will have a certain impact on the cracking. This research reveals the stress of the pore structure of coal and rock mass and the variation law of fracture from a microscopic point of view.
Natural porous media, such as coal and rock bodies, are characterized by large number of pores, cross-distribution of pore size, and complex heterogeneity and anisotropy. and anisotropy. Their mechanical forms also exhibit and anisotropy. Their mechanical forms also exhibit and anisotropy. Their mechanical forms also exhibit complex nonlinearities (Kulenkampff, 2015, Vishal, 2013)[1-2]. Quantitatively characterizing the pore structure is crucial in studying the mechanical failure characteristics of rock mass during compression. It helps monitor and evaluate the stability of rock mass engineering and identify the criterion of rock engineering instability.
Studying the mechanical properties of pore structures is a research process that involves macroscopic and microscopic mechanisms. People often conduct physical experiments to study pores and fractures macroscopically. Shi et al., 2018 studied the short-time creep behavior of rectangular sandstone samples with two pre-existing cracks under uniaxial compression through acoustic emission (AE) experiments and analyzed the mechanical properties of samples with different crack lengths. Kong et al. 2016 and 2017[4-5] analyzed the time-varying AE law during coal rock loading and multifractal characteristics with load and time. Chen et al., 2017 conducted a uniaxial compression test on sandstone samples with double cracks and a single circular pore to investigate the relationship between the crack initiation form of the composite fractured rock sample and the fracture inclination angle (α). However, due to the microscopic nature of the pore structure and the complex disorder of its morphology, the distribution characteristics of the internal pore structure of coal and rock mass cannot be described accurately through physical experiments. Therefore, scholars have used numerical simulation methods to study the microstructure of coal. Jiang et al., 2015 used DEM discrete element to simulate the uniaxial compression of pre-formed fractures of granite, and studied the distribution of fissure stress during compression. Zhang et al., 2017 conducted a numerical analysis of the cracking of coal under sound pressure and surrounding rock stress and analyzed in detail the effect of ultrasonic on the cracks generated in physical experiments. However, due to the complexity and idealization of the model structure, the existence of pores and other structures in the rock mass structure was disregarded in the simulation, which reduced the accuracy of the simulation results.
Entry driven along goaf-side (EDG), which is to develop the entry of the next longwall panel along the goaf-side and isolate the entry from goaf with a small-width yield pillar, has been widely employed in the past few decades in China. The width of such a yield pillar has a crucial effect in EDG layout in terms of ground control, isolation effect and recourse recovery rate. On the basis of a case study, this paper presents a methodology of evaluation, design and optimization of EDG and the yield pillar by considering results from numerical simulation and field practice. In order to analyze the ground stability rigorously, the numerical study begins with the simulation of goaf-side stress and the ground condition. Four global models with identical conditions except the width of the yield pillar are built and the effect of the pillar width on ground stability has been investigated with comparison in aspects of stress distribution, failure propagation, and displacement evolution in the entire service life of entry. On the basis of simulation results, the isolation effect of the pillar acquired from field practice is also taken into consideration. The suggested optimal yield pillar design is validated from a field test in the same mine. Thus the presented methodology provides references and can be utilized for evaluation, design and optimization of EDG and yield pillars under similar geological and geotechnical circumstances.
The stability of roadways is a long-standing issue in underground coal mines, especially for entries that serve and ensure the safe production of longwall panels. The ground stability and failure mechanisms of entries vary depending on stress, geological and geotechnical conditions.
Entry driven along goaf-side (EDG), which is the development of an entry of the next longwall panel along the goaf-side and the isolation of the entry from the goaf with a small-width yield pillar, has been widely employed in China over the past several decades (Li et al. 2015; Wang et al. 2015, Zhang et al 2017). A yield pillar, which is designed to deform progressively during its service life, can transfer its load to adjacent abutments and control the mining-induced stress distribution around the entries (Peng 2008). Hence, it contributes to preventing coal bumps and excessive ground deformation by employing yield pillars, and it has been successfully applied in many coal mines of China and USA (Peng 2008; Li 2015; Chen et al. 2014). For instance, Carr et al. (1985) employed a yield-abutment-yield pillar layout to a four-entry longwall system in the Blue Creek seam in Alabama to control its severe floor deformation. The application of yield pillars in Utah coal fields has achieved a notable effect on preventing coal bumps (Peperakis, 1958; Agapito et al., 1988).
Wang, Gang (Shandong University of Science and Technology) | Xu, Hao (Shandong University of Science and Technology) | Liu, Shimin (The Pennsylvania State University) | Fan, Long (The Pennsylvania State University)
ABSTRACT: Scanning electron microscope (SEM) has been widely used to characterize coal micro-structure. However, SEM images often provide qualitative descriptions which cannot be directly applied to modeling of coal deformation and failure behaviors. In this paper, a PFC2D approach for modeling heterogeneous coal micro-structure is proposed. First, coal samples were scanned by SEM to study micro-structural characteristics and obtain the digital images. Second, Matlab was used to binarize images and filter small zones and scattered spots in images for obtaining gray images that reflect internal micro-structure of coal samples. Finally, gray images processed by Matlab were imported to particle discrete element software PFC2D, to further construct a numerical model. With the proposed PFC2D numerical model, biaxial compressive test was performed on heterogeneous coal samples for the purpose of studying stress-strain curves of coal samples under different confining pressures, and their failure forms. Also, test results between heterogeneous and homogeneous coal samples were compared. The test results indicate that confining pressure can increase coal samples’ strength to some extent. The stress-strain curves derived from simulations were consistent with previous research results, thereby proving that the proposed PFC2D modeling method using SEM images into coal deformation and failure behaviors is reasonable.
Coal is a hugely heterogeneous organic rock that is derived largely from the remains of plants, to have preserved plant cell structures to different extents. Differences of original cell structures in plants and different preservation degrees determined the differences in type, size and structure of pores in coal. Internal heterogeneity is demonstrated by development of various pores, and the universal development of micro-cracks. For heterogeneous coal sample containing some mineral, it mechanical mechanism in deformation is not only dependent on pore compression, particle contact of coal and mineral, and their relative movement, but also closely related to contact and friction between coal and mineral. Rigidity, strength of and contact surface state between coal and mineral influence its mechanical properties, which necessarily lead to difference from homogeneous coal samples in terms of stress-strain relationship, failure mechanism and others. Thus, it is essential to make specific studies on mechanical characteristic of heterogeneous coal samples.
Jiang, Yujing (Nagasaki University) | Wu, Xuezhen (Shandong University of Science and Technology) | Wang, Gang (Nagasaki University) | Li, Bo (Shandong University of Science and Technology) | Iura, Tomomi (Shandong University of Science and Technology)
Yielding bolt has been widely used for rock reinforcement in mining and civil engineering in high stress conditions. However, the interaction mechanism of yielding bolt and matrix mass is still not clear at present, and no analytical model is available to predict its reinforcement effect quantitatively. In this paper, the mainstream yielding rock bolts were reviewed firstly. After that, a coupling model is proposed to account for the interaction between yielding bolt and matrix mass. Based on the plane strain axial symmetry assumption and the incremental theory of plasticity, equilibrium equations and compatibility equations of matrix mass, and also the response of yielding rock bolt are deduced theoretically. The proposed method was programmed in Visual Basic development environment, and a semi-analytical solution for the coupling model is achieved. The reinforcement mechanism of yielding bolt in conventional tunnelling is clearly demonstrated through an illustrative case study. The reinforcement effect of yielding bolt is estimated quantitatively, and the distribution of axial force along the bolt is presented. In addition, the validity of proposed method is verified by numerical simulations.
High stress in surrounding rock mass can cause serious stability problems such as massive squeezing in soft rock and rock burst in hard rock. It is observed that many conventional rock bolts failed when experiencing large displacement, which implies that they are too stiff to sustain large deformation and dynamic conditions (Stillborg, 1994; Hoek et al., 1995). Yielding bolt, which can also be called energy-absorbing bolt, has been widely used for rock reinforcement in mining and civil engineering for these conditions (Neugebauer, E., 2008). This paper focuses on the static reinforcement effects of the yielding bolts with analytical method and numerical simulations.
According to Windsor (1992), the types of rockbolts can be classified as: (1) Continuously Mechanically Coupled (CMC); (2) Continuously Frictionally Coupled (CFC); and (3) Discretely Mechanically or Frictionally Coupled (DMFC). Although several categories of analytical models, represented by Li and Stillborg (1999), Cai et al. (2004) and others, have been proposed to account for the interaction between bolt and rock mass, most of them focused on the CMC and CFC rock bolts. However, most of the existing yielding bolts use two-points anchoring in the borehole, which means that they belong to DMFC bolt.
For the DMFC bolt model, solutions were only obtained by treating the contribution of the bolt as two uniformly compressive distributed loads applied at both ends of the bolts (Carranza-Torres, 2009; Bobet, 2011). However, the assumption of a smeared contribution of the rockbolt is acceptable only under the premise of small bolt spacing. The errors increase as the rockbolt spacing increases. On the other hand, the suddenly jump in radial stress of matrix mass appear at the distal end of the bolt probably will not occur in practice.
Lian, C. J. (Shandong University of Science and Technology) | Hou, J. Z. (Shandong University of Science and Technology) | Gao, G. L. (Taian Taishuo Strata Control Science and Technology Co. Ltd.) | Wang, G. (Taian Taishuo Strata Control Science and Technology Co. Ltd.) | Song, W. T. (Henan Polytechnic University)
ABSTRACT: It has wide distributions and large recoverable reserves of Jurassic period coal seam in China. It is difficult to maintain stability of the development roadways with long service term for Jurassic strata because there are abundant argillaceous rocks and some minerals in the high argillaceous rocks will be expanding while meeting with water. The fractures of the roadways develop well under high stress and are suitable to filling with grouting. But the poor cementing performance of cement with argillaceous rock as well as a heavy water filtration rate of cement slurry had resulted in failures of many engineering cases adopting cement grouting to reinforce this kind of roadways. In this paper, according to characters of the high argillaceous rock in Jurassic strata, a marlaceous inorganic grouting material which possesses the well cementing performance with argillaceous rocks and little filtration rate was introduced; and the grouting reinforcement mechanism, construction technique and engineering application effect about it was clarified. It will be of great significance for reinforcement and maintenance of the development roadways with high argillaceous rocks.
According to statistics, 60% of the proved coal reserves in China distributes in Early-Middle Jurassic period of northern North China, southern Northeast China and Northwest China, along with late Jurassic to early Cretaceous period of Northeast China and east Inner Mongolia. The period of coal forming is short and argillaceous rocks are abundant in Jurassic strata. Moreover, there are quite a few expanded minerals in some strata. So the roadways are easily to be deformed and damaged when affected by mining-induced stress. In addition, this kind of soft rock roadways has an obvious time effect. For the development roadways with long service term, serious deformations are frequently observed and part of the roadways has suffered deformation and maintenance time after time. The stability support of the development roadways really need much cost.
High stress in surrounding rock mass can cause serious stability problems such as squeezing in soft rock and rock burst in hard rock. The support system applied in high in-situ stress condition should be able to carry high load and accommodate large deformation of rockmass. This paper presents a specifically designed rock bolt, called Tension and Compression Coupled Yielding bolt, which can provide support for both squeezing and burst-prone rockmass encountered in mining and tunneling at depth. The new bolt mainly consists of a steel rod and two additional anchors. The steel rod is a round shape bar with varying surface conditions. The inner segment is processed into rough surface, while the middle of the rod has smooth surface. Two additional anchors were welded on both ends of smooth segment. The bolt is fully encapsulated with either cement or resin grout in a borehole. The rough rod and the inner anchor are firmly fixed in the bottom of the borehole, while the smooth segment has no or very weak bonding to the grout, which can stretch to accommodate rock dilatation. Static pull tests show that the load and strain elevations could result in premature failure of conventional rock bolt, as it is strongly bonded to the grout. However, the smooth segment of TCC Yielding bolt can easily detach from the grout under pull loading and provide large deformation to accommodate rock dilations. The coupling action of tension and compression of grout in different position can increase the ultimate bearing capacity of inner anchoring segment greatly. The stress dispersion structure also makes the load of rough rod lower than the smooth rod, preventing the premature failure of steel rod at inner anchoring segment. Finally, a simple method was developed to predict the deformation ability of the new bolt. The bolt elongation will be 386~754mm for 2500~5000mm long bolt at high load level equal to the strength of the material, thereby absorbing a large amount of energy to maintain the stability of surrounding rockmass.
This paper discusses the possibility of using the numerical and the physical methods to face the problem of slope stability. A good program to study the distribution of stress and deformation in rock mass is the three-dimensional distinct element code. But at the site, the ground penetrating radar is accurate to detect the circumstance of a mountain body within the limited depth. By comparing results of numerical method and practical method, we can find out several significant features about slope stability.
In western China, we are planning to construct many roadways and railways among which the typical difficulty of slope stability is met in mountainous regions. What is studied in this paper is exactly a general slope in mountainous regions. That is the designed route of roadway is along the foot of a hill where the horizontal area is 250m*250m=62500m². Vertical distance between road surface and vertex of the hill is about 100 meters and inclinations of its five edges are all 60°. The hill is mainly composed of four rock stratas, which, from above to below, are clay, shale, sandstone and limestone, respectively. Their thickness are 10 m, 20 m, 20 m and 50 m. Inclination of each rock mass in the hill is about 30°. Because properties of these five rock masses are different, features of contact plane adjacent to two media must be different. Therefore, which is the main contact plane or the important rock strata to influence the slope stability will be the concern of engineers and technologists.
2. NUMERICAL SIMULATION
2.1 Numerical simulation method
3DEC is a commercial software developed by Itasca Consulting Group, Inc., and is also a program based on the distinct element method, in which a discontinuous medium can be distinguished from a continuous medium by existing interfaces or contacts between the discrete bodies of the system. A numerical model should embody two kinds of mechanical behaviors, which are behavior of discontinuities and behavior of the solid material. It is a special software used in the geotechnical engineering field, especially to simulate slippage of the slope.
2.2 Model construction
Figure 1 illustrates a three-dimensional model about a slope of the hill. Every rock mass in the slope are divided into many stratas by joints whose inclination is strictly 30°.
2.3 Results of numerical simulation
Slope stability can be assessed clearly with respect to stresses, displacement and deformation etc. which are accepted by numerical calculation. Calculations in this paper includes eleven stages, whose time steps are 100, 1000,2000,3000, , 10,000 respectively. Four stages of 100, 1000, 3000 and 5000 time steps are chosen to interpret the development of slope slippage. And in every chosen stage, shear displacement of joints and distribution of principal stresses of rock masses can be directly used to value the effect of rock stratas and contacts to the slope stability (Liu 2000).
2.3.1 Shear displacement
Figure 2 states that displacement of whole system has a tendency to be larger with the increase of time steps.