Face Cleats Research Articles

Calculation of coal cleats spacing for methane degassing by image processing

Coal bed methane is one of the clean energy sources in the world. Methane molecules are confined inside the pores of coals and when the gas drainage wells are drilled into the coal seams, due to the resulted pressure difference. Coal cleats are spread all over the coal seam as face and butt cleats and play an essential role in the methane gas drainage operation from coal mines. Calculating the coal cleats spacing makes great help in modeling the amount of emitted gas followed by calculating the spacing of gas suction wells. Furthermore, the time required for gas drainage will be determined. Computer-based image processing technique has been utilized to identify the cleats spacing. In this study, three-dimensional computed tomography scan images were prepared for coal samples from Tabas Coal Mine in Iran. Computer-based image processing technology was conducted to determine the coal cleats. Their spacing was then calculated. Based on the results obtained using image processing with computer, the average distance between face cleats was 20 to 30 mm and the distance between butt cleats was 15 to 25 mm. The results of this study are used to modeling the amount of methane gas in the coal cleats. Based on this modeling, the amount of released gas and the distance between the wells can be designed.

Effects of orthogonal cleat structures on hydraulic fracture evolution behavior

Hydraulic fracturing operation as an effective enhancing coalbed gas production method is widely used in ultra-low permeability coal seam. However, complex geo-stresses and high heterogeneity between natural cleats structure lead to difficulty predicting hydraulic fracture patterns. Fracture evolution behavior for fracturing operation in coal seams requires a better understanding. In this study, a 2D model of hydraulic fracture propagation was built based on the cohesive zone model of finite element method. The effect of orthogonal cleat system, in-situ stress, dig angle and construction parameters on fracture geometries were main investigated. The main conclusions were as follows: (1) According to the interaction types between hydraulic fracture and cleat system, ladder-shaped fracture and H-shaped fracture geometry was summarized. The difference between them was whether there were continuous and small pressure fluctuation stages. (2) When the horizontal stress difference coefficient was lower and larger than 3/12, fracture geometry was prone to present Η shape and ladder shape respectively. Besides, the dimensionless fracture length and the dimensionless fracture extension aspect ratio of fracture were increasing with larger horizontal stress difference coefficient. (3) The favorable condition for fracture extension was that the dig angle was 45°. Hydraulic fracture tended to propagate along face cleats direction. (4) Larger fracture fluid displacement was beneficial to form more balanced hydraulic fracture geometry and promote large extension scale. As fracture fluid viscosity increased, the fracture geometries transformed from ladder shape to H shape.

Comparison of elastic properties of tectonic coals along the face and butt cleat directions

AbstractCoalbed methane is a hot spot for gas exploration at present, and fractures are widely developed in the coals. However, despite being essential for a number of geophysical applications such as reservoir prediction and hydraulic fracturing, the influence of fractures on the elastic properties and anisotropy of coals is still poorly understood. Therefore, three groups of cylindrical coals were drilled along the face and butt cleat directions to study the effects of pressure and fracture on the elastic properties and anisotropy of tectonic coals. The velocity along the face cleat direction is less sensitive to the pressure than that along the butt cleat direction due to the difference in compliant pore and fracture content. Coals display a strong directional dependence in petrophysical and elastic properties. The permeabilities and velocities of samples along the butt cleat direction are much lower than those along the face cleat direction. The fracture densities are quantitatively characterized by the theoretical model. The values along the face cleat direction are much smaller than those along the butt cleat direction. The influence of fractures on the elastic properties and anisotropy is comprehensively studied by theoretical and laboratory methods. The mineralogy and fracture effects are important factors to the elastic anisotropy of coals, the latter can be a dominant factor to the coal elastic anisotropy. The results can contribute to the understanding of the cleat system of coalbed methane reservoir and can provide a critical rock physics basis for micro‐seismic fracture monitoring and reservoir prediction.

Study on the Complex Electrical Response Characteristics of Loaded Coal and the Control Mechanism of the Main Fracture System Structure.

The structure of coal seam fractures is the main physical property of coalbed methane reservoir evaluation, and the complex resistivity method is a potential geophysical evaluation method for coal seam fractures. In this study, cylindrical coal samples with axial directions perpendicular to the bedding, face cleat, and butt cleat were prepared. The complex electrical parameters of the loaded specimens were tested with test frequencies ranging from 1 Hz to 10 kHz. The complex electrical response characteristics of the loaded coal are summarized, and the control mechanism of the main fracture system structure is analyzed. The results indicated that (1) as the loading pressure increased, the resistance R and the absolute values of reactance X(|X|) gradually decreased, especially in the frequency band where R slowly decreased and the characteristic frequency of X, the decreased amplitude was more significant, and the cutoff frequency of R and the characteristic frequency of X all gradually increased. (2) The complex electrical properties of coal show obvious anisotropic characteristics. Both R and |X| decreased sequentially according to the direction perpendicular to the bedding, face cleat, and butt cleat; the cutoff frequency of R and the characteristic frequency of X all increased sequentially. (3) The dispersion phenomenon of the complex electrical properties of coal is attributed to the induced polarization; the elevated loading stress enhances the polarization effects of the molecular-induced moments of the coal skeleton, and the anisotropic difference of the complex electrical properties is due to the difficulty in the degree of transport of charged particles induced by structural differences of the main fracture system. (4) The resistance R3 and capacitance Xc were selected as the complex electrically sensitive parameters of the loaded coal orthogonal fracture structures. A logarithmic inversion model reflecting the main fracture system structure of coal was constructed. This provides a certain theoretical basis for efficient electrical exploration of coal reservoir fracture structures.

Stereographic Analysis of Cleat Characteristics in Coal and its Controlling Forces

Cleat analysis plays a crucial role in coal mining, although it is often overlooked. Cleat is related to permeability and porosity, serving as a location for the accumulation of gases. In coal seams, cleats control slope stability and serve as pathways for gas and liquid flow. Therefore, the utilization of cleats in coal mining varies significantly, as it can determine mining patterns and serve as a guide for identifying the presence of Coal Bed Methane (CBM). This research was conducted in the village of Jebak and its surroundings, Muara Tembesi District, Batang Hari Regency, Jambi Province. Stereographic methods were used to identify controlling forces in coal structures based on field measurements of face cleats and butt cleats, resulting in the direction of controlling forces in ridge formation. Based on the results and lithological discussions at the research site, the lithology consists of sandstone interbedded with coal, mudstone, and mudstone with coal and fine sandstone inserts. Stereographic analysis of cleat measurements revealed that the resulting forces were compression forces from the Northeast to the Southwest and from the Southwest to the Southeast, indicating that the origin of the cleat falls into the category of Exogenic Cleat.

Anisotropy of fracture and permeability in high-rank coal analyzed using digital rock physics

Rock microstructure analysis and physical property simulation by means of digital rock physics (DRP) can overcome the deficiencies such as poor quantification and visibility in traditional petrophysical experiments. The establishment of a complete set of DRP is the basis of analyzing the microstructure of unconventional oil and gas reservoirs. In this study, high-rank coal, a material with anisotropic fracture and permeability properties, was tested using x-ray computer tomography scanning to reconstruct the digital core, and the fracture direction and structural characteristics were quantified with respect to the main inertia axis and the Feret diameter, respectively. A method for calculating the fractal dimension and tortuosity based on DRP is presented, and the optimal interaction between the lattice Boltzmann method seepage simulation and DRP is identified. The results show that the average length, width, aperture, and volume of fractures in the direction of face cleat (DFC) are 1.13, 1.10, 1.11, and 1.09 times that in the direction of butt cleat (DBC), respectively, and their surface area, count, fracture porosity, fractal dimension, and tortuosity are 1.17, 1.16, 1.26, 1.04, and 1.10 times that of DBC, respectively. The permeability of DFC was found to be 3.46 times that of DBC. This study presents an effective method for determining the dominant direction of fracture structure and fluid migration that is not limited to pores and fractures in rock but can also be used to characterize the physical properties of skeletons or solid particles in other materials.

A review of coal bed methane production techniques and prospects in India

This paper presents a comprehensive analysis of the challenges and opportunities associated with Coalbed Methane (CBM) extraction in the energy industry. It covers various aspects including the extraction process, major challenges faced, water management strategies, and the social and environmental implications of large-scale CBM production. CBM extraction involves the gradual transformation of buried plant materials into coal over millions of years, resulting in the release of methane and other by-product gases. Understanding the adsorption mechanism of CBM molecules onto the coal matrix and the role of the cleat system is crucial for efficient extraction. The cleat system, composed of face cleats and butt cleats, creates interconnected pathways for gas flow within coal seams, facilitating the extraction of CBM. However, water management poses a significant challenge in CBM extraction, as water production within the cleat system needs to be effectively managed to reduce pressure and enable gas desorption. Techniques such as dewatering and gas diffusion are employed to optimize gas production rates while minimizing water production. Large-scale CBM production has social and environmental implications, and it can result in land use changes, community displacement, and conflicts over resource allocation. Additionally, the release of methane during extraction contributes to greenhouse gas emissions and climate change. Mitigation measures and thorough assessment are necessary to minimize these impacts and ensure responsible and sustainable CBM development. India, with its abundant coal reserves and identified CBM producing blocks, holds significant prospects for CBM production. The paper discusses these prospects, considering the country's coal reserves and the current status of CBM exploration and production activities. It informs of future energy planning of CBM development in India. Thus, this comprehensive analysis discusses the multifaceted aspects of CBM production, from extraction challenges to environmental considerations, and details potential of CBM sustainable development in India.

Mechanical response and gas flow characteristics of pre-drilled coal subjected to true triaxial stresses

Many gas pre-drainage boreholes are generally constructed in coal seams to ensure safe underground operation. True triaxial experiments were conducted to study the mechanical response and gas flow characteristics of pre-drilled coal to reveal the in-situ behavior at pre-drilled regions in coal seams. The mechanical parameters, failure modes, and flow rate evolutions of pre-drilled coal samples were collected. The gas flow rate of pre-drilled coal samples showed an S-shaped trend with the increase of strain, and no obvious descending progress was found in the flow rate-strain curves. The typical failure mode for pre-drilled coal samples was shear-type during the true triaxial stress compression. V-shaped macro-fractures around the boreholes were observed when boreholes were perpendicular to bedding planes and parallel to face cleats. In contrast, single shear macro-fractures around the boreholes were observed when boreholes were parallel to butt cleats. The pre-drilled coal samples exhibited lower peak strength values when boreholes were parallel to butt cleats, induced by the relatively low cohesion parameter. Pre-drilled coal's initial flow rates perpendicular to bedding planes and parallel to butt cleats directions were only 33.33% and 41.67% of values measured along face cleats. More complex macro-fractures were formed, and up to a 39.30% decrease in strength was observed in pre-drilled coal samples under true triaxial unloading stress paths. The variations in volumetric strain level and the flow rate evolution were further discussed with an anisotropic conceptual failure model. These mechanical responses and flow characteristics of pre-drilled coal would guide the design of borehole layout and prevention of coal dynamic disasters.

Experimental and numerical model of anisotropic permeability and CO2 injectivity of coal during CO2 enhanced coalbed methane recovery process

Guaranteeing CO2 injectivity has been the precondition for implementing the CO2-enhanced coalbed methane recovery (CO2-ECBM), however, it dramatically decreases during the CO2 injection process because it is influenced by the dynamic change of the anisotropic permeability of coal. To reveal and evaluate the CO2 injectivity of coal, the anisotropic permeability test and the CO2 injectivity simulation test were first conducted, then the corresponding numerical models were established and verified by the experimental data. The results show that the permeability of coal in parallel face cleat direction is the largest, followed by the permeability of coal in parallel butt cleat direction and that in vertical bedding direction is the minimum. The peak value of the instantaneous injectivity rate is enhanced and the injection time is prolonged with the increase of the CO2 injection pressure. The total CO2 injectivity rate is nonlinearly increased from 13.61 to 311.87 cm2/MPa min when the CO2 injection pressure raises from 2 to 10 MPa. The anisotropic permeability model is appropriate to describe the dynamic evolution of permeability under different boundary conditions, the CO2 injectivity prediction model can be used to evaluate the CO2 injectivity during the CO2 injection process. Increasing the CO2 injection pressure may temporarily promote CO2 injectivity, while the CO2 injection increment is limited. The CO2 fracturing by phase transition may be an available reservoir stimulation method for enhancing the CO2 injection and should be focused on in the future.

Prospect Analysis of Paleocene Coalbed Methane: A Case Study of Hangu Formation, Trans-Indus Ranges, Pakistan

The methane trapped in the coal seams has emerged as an unconventional clean energy resource worldwide in this century. The proximate composition, ultimate content, cleat structure, porosity type, and pore structures are the debatable components for its trapping mechanism. Further, the brittle and ductile properties of minerals influence in the extraction of the methane from the coal seams. In this research, Coalbed methane prospect is demonstrated by analysing the Hangu Formation’s coal seam of Trans-Indus Ranges, Pakistan. This case study is helpful to find the occurrence, trapping ability, and methane extraction capacity in its cleat structures and pores. A number of samples were tested from the different coal mines in terms of these debatable components. The results indicate that the rank of studied coal is bituminous to subbituminous in which the carbon ratio, volatile matter, and sulfur contents are increasing with depth towards the south. The transitional connected face and butt cleats are partially filled with minerals and the intergranular, dissolved, and tissue pore types are also identified in it. It can be helpful for the occurrence, migration, and trapping of methane. Furthermore, the higher surface areas and cumulative pore volumes enhance the capacity of gas adsorption with depth in the study area. Moreover, the increasing brittle minerals in the coal composition towards the south can be helpful for the fracking of coal seams for economical gas extraction. It is suggested that this workflow can be implemented in any region with same coal rank and cleat types.

Influence of Coal Cleats on Conical Pick Cutting Process Based on PFC

With the rising mining mechanized level, the conical pick is widely used in mining machines. The structure and mechanical properties of rocks and minerals are important factors affecting the cutting characteristics of the conical pick. In this paper, how the coal cleats affect the cutting process of shearers was researched, which will be of great significance to mining technology design. After analyzing the cutting process of conical pick and the test of coal mechanical properties, a group of coal cutting models of conical pick was established based on the empirical cutting parameters, various cleat spacing, simulation of mechanical performance of coal specimens, profile of conical pick, and SRM (synthetic rock mass) method in PFC (Particle Flow Code). Then, the simulations of the cutting process were carried out. The simulation results show that lump coal chips begin to form and fall off after the conical pick cuts to 50°; the cleats near the conical pick grow to be more obvious fractures; the mean and maximum fluctuations of cutting force increase with the increase of face and butt cleat spacing; the lower the density of cleat is, the more the cracks are formed during the cutting process; the face cleat spacing has greater influence on the service life of conical pick than the butt cleat spacing. This study more vividly reflects the influence of cleats on coal cutting process of the conical pick and plays a guiding role in design of mining technology.

Differential Strain Analysis: An Investigation Over Its Feasibility For Determining Coal’s Cleat Orientation

Gas production from coal bed methane (CBM) has been rising as supplement to conventional gas reservoirs. Effective CBM production is much determined by well placements with regards to orientation of the coal reservoir’s face cleat. This is true since this cleat type provides the main path of fluid migration from storage to wellbore. It is therefore imperative to understand and determine the general orientation of the cleats of concern. This information is usually obtained from visual description on core samples, and whenever available from analogy to outcrop data. This is considered as insufficient and a means for studying core sample’s interior is required in order to ensure consistency between external and internal appearances. This paper presents an investigative study over the possibility of differential strain analysis (DSA) technique to serve the purpose. The technique is normally used for measuring subsurface in situ stress field, and through the use of a similar basic concept it is proposed to be used for determining orientation of face cleats in coal samples. The study includes utilization of DSA data obtained from measurements on samples taken from great depths. Review and re-working over the data has shown that the technique can well indicate the orientation of face cleats. A general orientation over all tested samples is also indicated with reasonable degree of reliability. This leads into conclusion that the DSA technique can be well used to indicate cleat orientation and therefore help in better characterizing coal.

Petrographic and Geochemical Controls on Methane Genesis, Pore Fractal Attributes, and Sorption of Lower Gondwana Coal of Jharia Basin, India.

The Barakar coal seams of Jharia Basin have been evaluated for the geochemical and petrographic control of coalbed methane (CBM) reservoir characteristics. The coal core samples are analyzed for the total gas content, gas chromatography, stable isotopes (δ13C1), and geochemical, petrographic and vitrinite reflectance. The significant face (1.6–7.6%) and butt (0.9–5.3%) cleat intensities specify the brittle characteristics of coal seams and also favor the gas flow mechanism. The thermal cracking position of hydrocarbon compounds was evaluated, which signifies the excellent source rock potential of coal for gas genesis. The inputs of type III and IV organic matter illustrated by the van Krevelan diagram signify thermally matured coal seams. The low values of sorption time (τ) between 2.1 and 5.6 days designate excellent diffusion characteristics that is favored by the cleat intensities. The values of total gas content and sorption capacity (VL) reveal that moderate saturation indicates a higher gas content, attributed to the seam thickness and thermal maturity. Similarly, the CH4 concentrations (89.4–96.6 vol %) display that the genesis pattern is a function of thermal maturity; however, some samples fall under the mixed type substantiated by the stable isotope (δ13C1) (−25.40 to −64.90‰), emphasizing bacterial hold by seasonal influx of freshwater. The ternary facies diagram (Vmmf, Immf, Lmmf) also supports notable generation of methane gas and storage in the coal seams of the Jharia Basin. The volume percentage of each maceral determined from petrographic study was used to estimate the fraction of conversion (f) of the organic content (0.19–0.97). The values of “f” indicate that the Barakar coal has undergone maximum conversion, which may be attributed to the older early Permian coal and placed at a greater depth after deposition due to the basin sink. The high fraction of conversion and thermal maturity may also be explained due to the existence of volcanic intrusion (sills and dykes). The uniformity in the distribution of functional groups is shown by Fourier transform infrared spectra representing moderate to stronger peaks of aromatic carbon (CO and C=C) between 1750 and 1450 cm–1, which indicates that the presence of a larger total organic carbon content likely validates the removal of aliphatic compounds during gas genesis. The variations in the BET curve have been categorized as H1 hysteresis following the type II adsorption pattern, suggesting that cylindrical pores and some of the coal samples have a type IV H4 hysteresis pattern, characterized as the slit type of pores. The average values of the pore diameter indicate the dominance of mesopores suitable for gas storage and release and hence a major part of the pore volume is contributed by the mesopores having a width mainly between 2.98 and 4.48 nm. The significant role of the meso-macropore network (D1 fractals) in methane storage of the coal matrix is represented by a moderate positive relationship of VL with D1, which accentuated that meso-macropores developed due to devolatilization and dehydration of organic matter and also by geochemical alteration of macerals and minerals formed heterogenetic inner surfaces suitable for gas adsorption. The estimated recoverable resource applying Mavor Pratt methods is 8.78 BCM, which is found to be a more realistic resource value for the studied CBM block.

Permeability of Coking Coals and Patterns of Its Change in Leninsky Area, Kuznetsk Coal Basin, Russia

A prediction of the permeability of gas-bearing coking coals in the Leninsky area, Kuznetsk coal basin (Kuzbass), which is promising for the production of coal methane, was performed. The results of laboratory studies of coal permeability and cleat compressibility under hydrostatic stress conditions are presented. As the confining pressure increased by 8 times (from 1 MPa to 8 MPa), the coal permeability perpendicular to the butt cleat direction decreased by 6.7 times (from 60 mD to 9 mD). The coal cleat compressibility was 0.085 MPa−1. On the basis of the results of filtration tests and microstructural analysis of the coking coals, we provide the estimation of the permeability anisotropy along the bedding planes (perpendicular to the face and butt cleat directions). The predicted dependences of gas-bearing coking coal permeability perpendicular to the butt and face cleat directions on depth and on features of coal seam bedding were determined under uniaxial strain conditions. It was found that in the coking coal depth intervals, as the depth increased, their permeability decreased by 61–82%. The obtained results can be used to select facilities and to design industrial works for the extraction of coal methane in the region.

Discrete fracture matrix modelling of fully-coupled CO2 flow – Deformation processes in fractured coal

CO2 interaction causes complex mechanical deformations and flow modifications in coal, depending on the spatial disposition of the fracture-matrix system. Sorption-induced matrix swelling reduces the local fracture aperture and correspondingly the fracture permeability, consequently influencing gas flow throughout the seam. Since these modifications are highly -heterogeneous and -localized, an explicitly-represented geometric model is essential for the accurate modelling of the fully-coupled process. In this study, the CO2 flow – coal deformation process is implemented in a numerical model at the scale of coal constituents (i.e. matrix blocks and cleats), through the inclusion of a spatially distributed 3D – discrete fracture matrix (DFM) network. Fracture geometry is generated from a stochastically simplified 2D fracture network obtained from micro-CT imaging. The approach is initially validated against experimental results from a single-fractured coal specimen and the analysis extended to the complex fracture geometry. The spatial and temporal evolutions of fracture/matrix pressure, adsorbed mass of CO2, adsorption-induced swelling, alterations in local fracture aperture and permeability, and contact modelling at fully fracture closure are specifically analysed with comparison against no-swelling behaviour. Results indicate that the high-permeability fracture pathways provide initial easy access for the CO2 to diffuse into the coal matrix, causing sorption-induced matrix swelling. Although the individual matrix blocks exhibit a slight shrinkage immediately upon injection of high fluid pressures within the fractures, sorption-induced swelling rapidly overcomes this, resulting in an overall volume expansion at full pressure equilibration. This is turn causes a significant reduction in fracture aperture and permeability. The magnitude of the local fracture aperture reduction depends on the swelling behaviour of the bounding matrix, that leads to essentially full-closure of small fractures, causing significant localized flow modifications to further CO2 injection in the vicinity of the particular fractures. The contact modelling approach identifies the timing and locations of fully-closing fractures in the complex geometry, where butt cleats exhibiting initial small apertures are prone to fully-close, compared to larger aperture face cleats that retain flow.

Insights of Cleat Attribute of Barakar Coal Deposits of Son Valley Basin, India: Implication for Coal Bed Methane Exploration

Cleat attributes and coal quality have been evaluated through field observation, SEM images analysis and proximate-ultimate analyses on representative coal samples collected from four different underground coal mines of Son-Valley coal Basin of central India for the characterization of coal bed methane prospective. The coal samples were characterized as durainic-clarodurainic-vitrainic type to durocalrainic-clarainic type based on litho-type composition. Cleats were identified as face cleat and butt cleat range 5-10 cm-1 to 2–3 cm-1 depending on the litho-types whereas, cleat density ranges 7-16 cm-2 revealed that cleat development is influenced by litho-types. The cleat orientation shows two main strikes almost NW-SE to NNW-SSE and NE-SW to ENE-WSW elucidates the face cleat and butt cleats are perpendicular to each other. SEM images for coal samples were analysed to validate the cleat geometry at micro-scale, illustrates that face cleat and butt cleats are orthogonal to each other. The technological properties viz. moisture (Wa), volatile matter yield (VMdaf) ash yield (Ad) of coal samples are placed in the range of 0.66–11.80, 32.0–50.05 and 13.23–43.73 wt.% respectively. The fuel ratios of all the samples varied from 1.05–2.35 indicating that coal samples were of sub-bituminous to bituminous rank. The carbon and hydrogen (Cdaf, Hdaf) content of samples ranges from 68.89–86.08 and 5.22–6.60 wt.% respectively. The atomic ratio H/C and O/C through van-Krevelen diagram demonstrate the predominance of type-III/IV kerogens usually derived from terrestrial source in studied coal belongs to the gas genesis window.

Cleat properties of the Tanjung Agung coalfield, South Sumatra Basin: implications to fluid flow

Observation of the coal cleats of the Muara Enim Formation in Tanjung Agung coalfield aims to characterize the cleat properties and interpret their implications for fluid flow. This study used the scanline method to measure cleat attributes in the form of orientation, aperture, spacing, filler, and cleat connectivity. The face cleat was dominated by NE-SW direction as an indication of the effect of paleostress, while butt cleat with NW-SE direction indicated the effect of stress release. The presence of one major fault and three minor faults trending NW-SE in the mine pit is interpreted as the main controlling factor for cleat development in the study area. These cleats have an aperture ranging from 0.4 - 2.3 mm, good connectivity, and are not filled, so they are considered to be analogous to fluid flow paths with transition and turbulent flow types with relatively NE-SW direction.

Experimental research and numerical simulation: strength and rupture patterns of coal under Brazilian tensile test

Coal seam contains well-developed weak planes known as cleats. The strength and rupture patterns of coal with different cleat directions are significantly different in Brazilian tensile test (BTT). In this paper, a series of Brazilian tensile tests of coal specimens under different inclination angles of face cleat were carried out. Based on the experimental data, the influence of cleat direction on the failure strength (FS) and rupture patterns of coal specimens was analysed. Then the analytical model to calculate the anisotropic failure strength of coal under Brazilian test conditions was derived. The model cannot only predict the rupture patterns of coal specimens with different cleat directions and calculate the corresponding failure strength, but also can reflect the effect of cleat density on the strength. In the simulation, the rupture patterns of numerical model also fall into four types, which are consistent with the experimental results. The cleat density has no influence on the rupture patterns, but will decrease the failure strength. The connectivity of butt cleats is the key to determine whether the coal shear along the face cleats or butt cleats. [Received: August 10, 2019; Accepted: September 29, 2019]

Preliminary rib support requirements for solid coal ribs using a coal pillar rib rating (CPRR)

Researchers from the National Institute for Occupational Safety and Health (NIOSH) are developing a coal pillar rib rating (CPRR) technique to measure the integrity of coal ribs. The CPRR characterizes the rib composition and evaluates its impact on the inherent stability of the coal ribs. The CPRR utilizes four parameters: rib homogeneity, bedding condition, face cleat orientation with respect to entry direction, and rib height. All these parameters are measurable in the field. A rib data collecting procedure and a simple sheet to calculate the CPRR were developed. The developed CPRR can be used as a rib quality mapping tool in underground coal mines and to determine the potential of local rib instabilities and support requirements associated with overburden depth. CPRR calculations were conducted for 22 surveyed solid coal ribs, mainly composed of coal units. Based on this study, the rib performance was classified into four categories. A preliminary minimum primary rib support density (PRSD) line was obtained from these surveyed cases. Two sample cases are presented that illustrate the data collection form and CPRR calculations.

A Stochastic Method to Estimate the Anisotropic Stress-Dependent Coal Permeability by Pore-Volume Distribution and Stress-Strain Measurements

Summary A novel, simple, economical, and time-effective method to estimate the anisotropic permeability of coal is presented in this paper. This method estimates the coal’s anisotropic permeability by avoiding the tedious experimentation using triaxial permeameter or history-matching exercises. This method calculates the absolute magnitude of the permeability of the sample. In this regard, it is unlike other analytical permeability models, such as given by Palmer and Mansoori (1998) and Shi and Durucan (2014), that only calculate the permeability ratio (k/k0). The motivation is to find a method by which the permeability of the coal may be determined with reasonable accuracy by using only two easy measurements: mercury intrusion porosimetry (MIP) and anisotropic stress-strain (σ-ɛ) measurement. The main blocks of the method are based on cleat size that is obtained from MIP and randomly allocated to form flow channels/cleats through the coal; these cleats form parallel paths in the orthogonal face and butt cleat directions that provide the permeability; and the cleat width (b) is stress dependent. This method is further validated by comparing with the experimentally measured stress-dependent permeability of Surat Basin (Australia) coal and German coal in face cleat and butt cleat directions.