Heping Xie

Direct Estimation of the Fractal Dimensions of a Fracture Surface of Rock

A direct determination of the fractal dimensions of a fracture surface is essential for a better understanding of its complete topographic characteristics. In this paper, a laser profilometer is employed to measure the topography of a rocks fracture surface. With the use of the triangular prism surface area and projective covering methods, the resultant data set enables us to directly determine the fractal dimensions of a rocks fracture surface. Moreover, a new method, referred to as the cubic covering method, is proposed. The theoretical issues of fractal dimension estimation are also discussed.

Direct fractal measurement of fracture surfaces

Abstract To overcome the difficulties in estimation of fractal dimension for fracture surfaces, a new method of fractal measurement—the projective covering method (PCM) is proposed in this paper. Based on the measurements using a laser scanner, the fractal dimension D s ∈ [2, 3) of fracture surface is directly estimated. The research results agree with the theory of fractal geometry and measurement data.

Study on generation of rock fracture surfaces by using fractal interpolation

Abstract The mathematical model of a fractal interpolation surface on a rectangular field is proposed in this paper. The theory of a fractal interpolation surface is applied to the generation of rock fracture surfaces. The methods of the improved self-affine fractal interpolation surface are proposed. Based on the statistical fractal character of fracture surfaces, the new ideas and methods of the partition of the local field and the determination of a vertical scaling factor are given by using the principles of geostatistics and trend surface analyses, so that the local stochastic irregular roughness on the fracture surface can be simulated. Fractal interpolated surfaces based on 5.5–25.62% of information data on 12 measured rock fracture surfaces are in good agreement with the measured rock fracture surfaces, and the relationship of the information point number used in the interpolation and the interpolation precision is obtained.

Energy Dissipation and Release During Coal Failure Under Conventional Triaxial Compression

Theoretical and experimental studies have revealed that energy dissipation and release play an important role in the deformation and failure of coal rocks. To determine the relationship between energy transformation and coal failure, the mechanical behaviors of coal specimens taken from a 600-m deep mine were investigated by conventional triaxial compression tests using five different confining pressures. Each coal specimen was scanned by microfocus computed tomography before and after testing to examine the crack patterns. Sieve analysis was used to measure the post-failure coal fragments, and a fractal model was developed for describing the size distribution of the fragments. Based on the test results, a damage evolution model of the rigidity degeneration of coal before the peak strength was also developed and used to determine the initial damage and critical damage variables. It was found that the peak strength increased with increasing confining pressure, but the critical damage variable was almost invariant. More new cracks were initiated in the coal specimens when there was no confining pressure or the pressure was too high. The parameters of failure energy ratio β and stress drop coefficient α are further proposed to describe the failure mode of coal under different confining pressures. The test results revealed that β was approximately linearly related to the fractal dimension of the coal fragments and that a higher failure energy ratio corresponded to a larger fractal dimension and more severe failure. The stress drop coefficient α decreased approximately exponentially with increasing confining pressure, and could be used to appropriately describe the evolution of the coal failure mode from brittle to ductile with increasing confining pressure. A large β and small α under a high confining pressure were noticed during the tests, which implied that the failure of the coal was a kind of pseudo-ductile failure. Brittle failure occurred when the confining pressure was unloaded—an observation that is important for the safety assessment of deep mines, where a high in situ stress might result in brittle failure of the coal seam, or sudden outburst.

Energy analysis and criteria for structural failure of rocks

Abstract: The intrinsic relationships between energy dissipation, energy release, strength and abrupt structural failure are key to understanding the evolution of deformational processes in rocks. Theoretical and experimental studies confirm that energy plays an important role in rock deformation and failure. Dissipated energy from external forces produces damage and irreversible deformation within rock and decreases rock strength over time. Structural failure of rocks is caused by an abrupt release of strain energy that manifests as a catastrophic breakdown of the rock under certain conditions. The strain energy released in the rock volume plays a pivotal role in generating this abrupt structural failure in the rocks. In this paper, we propose criteria governing (1) the deterioration of rock strength based on energy dissipation and (2) the abrupt structural failure of rocks based on energy release. The critical stresses at the time of abrupt structural failure under various stress states can be determined by these criteria. As an example, the criteria have been used to analyze the failure conditions of surrounding rock of a circular tunnel.

Visualization and Transparentization of the Structure and Stress Field of Aggregated Geomaterials Through 3D Printing and Photoelastic Techniques

Natural resource reservoirs usually consist of heterogeneous aggregated geomaterials containing a large number of randomly distributed particles with irregular geometry. As a result, the accurate characterization of the stress field, which essentially governs the mechanical behaviour of such geomaterials, through analytical and experimental methods, is considerably difficult. Physical visualization of the stress field is a promising method to quantitatively characterize and reveal the evolution and distribution of stress in aggregated geomaterials subjected to excavation loads. This paper presents a novel integration of X-ray computed tomography (CT) imaging, three-dimensional (3D) printing, and photoelastic testing for the transparentization and visualization of the aggregated structure and stress field of heterogeneous geomaterials. In this study, a glutenite rock sample was analysed by CT to acquire the 3D aggregate structure, following which 3D printing was adopted to produce transparent models with the same aggregate structure as that of the glutenite sample. Uniaxial compression tests incorporated with photoelastic techniques were performed on the transparent models to acquire and visualize the stress distribution of the aggregated models at various loading stages. The effect of randomly distributed aggregates on the stress field characteristics of the models, occurrence of plastic zones, and fracture initiation was analysed. The stress field characteristics of the aggregated models were analysed using the finite element method (FEM). The failure process was simulated using the distinct element method (DEM). Both FEM and DEM results were compared with the experimental observations. The results showed that the proposed method can very well visualize the stress field of aggregated solids during uniaxial loading. The results of the visualization tests were in good agreement with those of the numerical simulations.

Differences in the acoustic emission characteristics of rock salt compared with granite and marble during the damage evolution process

Rock salt cavities for energy storage are important elements of energy supply management and sustainability. The mechanical properties of other common rocks have already been adequately investigated, but a systematic characterization of the differences in acoustic emission (AE) characteristics among rock salt and other common rocks is needed. In this study, the AE characteristics of the full-regime uniaxial compression of rock salt and common rocks such as granite and marble were determined. A damage variable based on AE parameters and a methodology for its determination were established, including an analysis of the characteristics of the AE time–space evolution, the AE amplitude distribution and the damage evolution based on AE parameters. Clear differences were observed in the AE characteristics of rock salt compared with granite and marble during the damage evolution process. The AE activity and energy release of rock salt with coarse grains and nonuniform structures decreased gradually with increasing stress level. The AE activity of rock salt appeared in a continuous, group-occurring form. The AE spatial distribution of rock salt was relatively uniform, without a definite rupture surface. The AE amplitude distribution varied slowly, and the proportion of AE events with small amplitudes increased gradually during compression in rock salt. Moreover, the damage in rock salt predominantly occured during the pre-peak period, resulting in a large damage variable of 0.9 at peak stress. By contrast, the damage variables of granite and marble are only 0.5 and 0.1, respectively, at peak stress. Rock salt exhibited less damage when approaching failure, resulting in a gentler and steadier process. And the plastic characteristics of rock salt are relatively obvious. These properties, to some extent, ensure the safety of underground storage engineering when rock salt is chosen as the medium.

The relationships among stress, effective porosity and permeability of coal considering the distribution of natural fractures: theoretical and experimental analyses

Although the relationships among stress, effective porosity and permeability of coal are a fundamental research topic that has been studied for decades and are widely used in analyzing the mechanical behavior of coal seams and predicting coalbed methane production, most relevant studies are based on idealized models and do not consider the influence of natural fracture distributions. To obtain a comprehensive understanding of the interrelationships among stress, effective porosity and permeability of coal, a series of effective porosity and permeability determinations have been conducted under different overburden stresses using an automated permeameter–porosimeter considering the directional distribution of natural fractures in coal. The experimental results show that the directional distribution of natural fractures provides a substantial contribution to the anisotropy of the effective porosity and absolute permeability of coal, which exponentially decrease with increasing overburden stress. An existing permeability model was modified to reflect the influence of the natural fracture distribution on the power law relationship between effective porosity and permeability, i.e., the exponent is not constant, but a variable related to the natural fracture distribution. The anisotropic effective porosity sensitivity and stress sensitivity of coal are also discussed, and the coal mass is shown to have the highest effective porosity sensitivity and lowest stress sensitivity in the direction perpendicular to the bedding planes compared to those in other directions.

Unconventional gas resources in China

Coalbed methane, tight and shale gas are three important unconventional gas resources in China. ‘‘According to data from the Ministry of Land and Resources, at the end of 2013, China had 11, 12 and 25 trillion cubic meters, respectively of remaining technically recoverable resources of coalbed methane, tight gas and shale gas which are still in the early stage of development’’. (Source CNPC and ARA International Limited). China is developing its shale gas exploration and exploitation strategies for economic reasons and social aspects (so-called ‘‘shale gas revolution’’). Huge shale gas reserves provide a reliable source for sustaining China’s economic development and contributing to China’s ‘‘Energiewende’’. Zhao et al. (2015a) analyze the strategic measures and advantages for a safe and vigorous development of shale gas in China and review the developments of hydraulic fracturing technologies in China. Economic exploitation for shale gas is highly dependent on the complexity of the fracture network caused by hydraulic fracturing technology, so it is necessary to accurately assess the effect of the fracture network on gas flow behavior and productivity (Li et al. 2015c). This thematic issue is particularly dedicated to recent researches in unconventional gas resources in China related to the reservoirs indicated in Fig. 1 in order to substitute coal by ‘‘clean’’ gas energy resources as a transitional technology towards renewable energy resources. The thematic issue compiles theoretical, experimental as well as field studies in China. Different reservoir types are under investigation such as tight and shale gas reservoirs as well as coal mines with coalbed methane resources. Fundamental aspects on process understanding in unconventional gas reservoirs as well as the international context of research and technology deployment are discussed in this volume. Tight gas reservoirs have become an important resource for the world’s gas supply. Such reservoirs have very low permeability (usually below 0.1 mD) and show a strong stress sensitivity to fluid transport properties and a considerable productivity decline during the production process due to decreasing reservoir pressure as well as increasing effective stress. In an experimental study by Albrecht and Reitenbach (2015) several measurement series were performed on plugs from the North-German Rotliegend tight gas reservoirs to determine the effects of changing stress and pore pressure conditions on reservoir & Olaf Kolditz olaf.kolditz@ufz.de

Scientific and Engineering Progress in CO 2 Mineralization Using Industrial Waste and Natural Minerals

ABSTRACT The issues of reducing CO 2 levels in the atmosphere, sustainably utilizing natural mineral resources, and dealing with industrial waste offer challenging opportunities for sustainable development in energy and the environment. The latest advances in CO 2 mineralization technology involving natural minerals and industrial waste are summarized in this paper, with great emphasis on the advancement of fundamental science, economic evaluation, and engineering applications. We discuss several leading large-scale CO 2 mineralization methodologies from a technical and engineering-science perspective. For each technology option, we give an overview of the technical parameters, reaction pathway, reactivity, procedural scheme, and laboratorial and pilot devices. Furthermore, we present a discussion of each technology based on experimental results and the literature. Finally, current gaps in knowledge are identified in the conclusion, and an overview of the challenges and opportunities for future research in this field is provided.