Face Cleats Research Articles

The propagation research of hydraulically created fractures in coal seams based on discrete element method

Coalbed methane (CBM) is clean unconventional energy that can be exploited with stimulation treatment to realize commercial value, and hydraulic fracturing is the key technology for increasing CBM production. The simulation of hydraulic fractures is an important research content that can guide the engineering practice to achieve the purpose of increasing production. Based on the distribution characteristic of cleats in coal, the research on the propagation of hydraulic fractures is carried out via Discrete element method (DEM). The simulated results show that: hydraulic fractures mainly propagate along the cleats towards the maximum principal stress. The fracture network can be formed due to the intersection of face cleats and butt cleats that can propagate at certain pressure. During the injection process of fracturing fluid, the pressure at injection point decreases with the fracture propagation until the pressure tends to be stable. With the increase of injection rate and fracturing fluid viscosity at the same condition, the maximum aperture of hydraulic fracture increases, while the length of principal hydraulic fracture shortens. Therefore, to achieve the purpose of forming hydraulic fracture network in coal seams with cleats, low viscosity fracturing fluid and low injection rate need be applied to the fracturing technology. As the cleat density increases, the number of branch fractures increases, but the length of principal hydraulic fracture becomes short. Long and narrow hydraulic fractures are easily formed in coal seams with lower cleat density.

Permeability of coal associated with cleat and bedding structure: measurement and modeling

ABSTRACT The permeability of coal shows anisotropic features due to its cleat and bedding structures. This work measured methane permeability of three cylindrical coal specimens, which represents the three orthogonal axes of the coal seam about bedding plane and cleat (face cleat and butt cleat). Twenty-seven permeability measurement tests were conducted under the different confining pressures and axial compression (4, 6, and 8 MPa) at different gas pressures (0.5, 1, and 1.5 MPa). Results clearly revealed the anisotropy in permeability of the tested coal specimens under test conditions. The specimen parallel to the bedding plane in face cleat direction shows the highest permeability while the specimen perpendicular to the bedding plane shows the lowest permeability. Within gas pressure range from 0.5 to 1.5 MPa, the lower the gas pressure the higher the measured coal permeability. Spatial permeability distribution around coal mine methane drainage borehole is obtained through a three-dimensional permeability model. The spatial permeability around drainage borehole parallel to butt cleat direction is much higher than that parallel to face cleat direction. In-mine methane drainage boreholes parallel to bedding plane in the butt cleat direction are recommended to use in engineering application.

Observing the effects of transition zone properties on fracture vertical propagation behavior for coal measure strata

The transition zone is a typical interface with enough thickness and high heterogeneity between the sandstone and coal and has a great effect on the fracture propagation in coal measure strata. In the study, the true tri-axial experiments were conducted with the combination of sandstone, coal and limestone outcrops. The effect of cementing strength and thickness of the transition zone was mainly studied. Three types of hydraulic fracture initiation positions were used to simulate three fracturing situations in the field. The results showed that the effect of the transition zone on the fracture vertical extension could be concluded: the fracture was easy to cross or slip with low cementing strength and turned inside the transition zone with high cementing strength; the fracture tended to branch or be blocked by the thick transition zone. The horizontal flow through face cleats and natural fracture activation in coal contributed to a more developed fracture network. The characters of pumping curve pressure “increasing – fluctuation – dropping”, and the “fluctuation pressure more than initiation pressure” indicated the interface crossing. The results of experiments agreed with the previous numerical simulation and helped to optimize the fracturing design in layered coal strata.

Mechanical anisotropy of coal with considerations of realistic microstructures and external loading directions

This paper investigates the anisotropy of coal under uniaxial compression conditions using both experimental and numerical methods. Primary wave (P-wave) velocity measurements, X-ray micro-computed tomography (CT) scanning, and uniaxial compression tests were conducted to assess the anisotropic characteristics of selected coal samples. Based on the experimental results, comprehensive analyses were carried out to study the influences of different contributing factors on the anisotropic properties of coal. A three-dimensional (3D) finite difference model was then built to further investigate the effects of failure initiation, stress redistribution and the presence of microstructures on the anisotropy of coal. The experimental and numerical results of uniaxial compression tests indicate that the coal strength anisotropy is affected by the directional distribution of the microstructures. Bedding planes directly control the uniaxial compressive strength of coal, and face cleats have a greater influence on the mechanical properties than butter cleats. The presence of mineral inclusions increases the heterogeneity of coal because of the significant differences in strength and deformation in the mineral inclusion zones. The mechanical and seismic anisotropies are both directly related to the directional distribution of the microstructures, which is demonstrated by the variations in the anisotropic strength and P-wave velocity of the coal. The correlations between the uniaxial compressive strength (UCS) and P-wave velocity are different when the coals are loaded in different directions, and an exponential correlation was suitable for defining this relationship.

Anisotropic coal permeability estimation by determining cleat compressibility using mercury intrusion porosimetry and stress–strain measurements

This paper presents a novel method to calculate the anisotropic and stress-dependent coal permeability by determining cleat compressibility using the Mercury Intrusion Porosimetry (MIP) and stress–strain measurements. Cleat compressibility is often assumed isotropic and constant in the literature and is usually obtained by numerical fitting to a matchstick permeability model e.g. the model by Seidle et al. (1992) despite the gross simplification of this representation of the coal pore network. This paper provides a method to calculate anisotropic cleat compressibility using only MIP and stress–strain measurements which are easy to conduct, and without permeability information, which is harder to come by, requiring laboratory experiments on the core or through fitting field data. We report the measured stress–strain behaviour of a coal sample, with hydrodynamic loading/unloading over the range 0.5–4.0 MPa, and the permeability in face cleat (kF) and butt cleat (kB) directions using the Triaxial Stress Permeameter (TSR). The stress–strain measurement is used to calculate the anisotropic modulus of elasticity (EF, EB, and EV) in face cleat, butt cleat and bedding plane directions, and cleat compressibility in the face cleat (CfF) and butt cleat (CfB) directions using the fractal dimension analysis with MIP measurement. Finally, the cleat compressibilities are used to calculate the anisotropic coal permeability by Seidle et al. (1992) permeability model and compared with the measured permeability of the coal sample.

Anisotropic Mechanical Properties and the Permeability Evolution of Cubic Coal Under True Triaxial Stress Paths

The geological sequestration of CO2, underground coal mining, and coalbed methane production in deep coal reservoirs is executed under high levels of 3-D geo-stress, and it is accompanied by significant variations in both vertical and horizontal stresses. In addition, coal is a highly fractured porous medium characterized by complex natural fracture systems. This exterior and interior anisotropy complicates the replication of in situ conditions in the laboratory. In this study, we divided pre-existing fracture systems of cubic coal into three flow planes: a bedding plane, face cleat plane, and butt cleat plane. Gas flowed through cubic coal samples along each flow plane under differently designed true triaxial stress paths. We then further analyzed anisotropic mechanical and flow property responses of cubic coal after failure by model fitting, CT scan reconstruction, and fractal representation. The experimental results indicate that pre-existing flow planes play significant roles in the strength levels, failure modes, and permeability levels. Low strength levels, typical shear failure patterns, and low initial permeability levels were observed in the butt cleat plane direction. Anisotropic strength data can be effectively fit by applying a linear relationship between octahedral shear stress and mean effective normal stress. After coal failure, the peak permeability observed in the face and butt cleat plane directions also presents a strong linear relationship with the fractal dimension. An anisotropic conceptual failure process model was established for the description of internal fracture development during stress loading. Horizontal stress unloading decreased the strength and formed a more complex fracture system in cubic coal regardless of the different flow planes involved, producing the increments of associated peak permeability.

Tectonic Control on the Formation of Cleats in the Coal Beds of the Sajau Formation, Berau Basin, Northeast Kalimantan

DOI: 10.17014/ijog.5.3.235-250Coal seams of lignite- to subbituminous rank of Pliocene-age from the Sajau Formation of the Berau Basin are characterized by the presence of a natural fracture system, including cleats. Often, these opening-mode fractures consist of two orthogonal sets (face and butt cleats), both almost perpendicular to the bedding. This paper describes the distribution of orientation, spacing, and aperture of cleats formed in the seams of the Sajau Formation. All observations and measurements were conducted at macroscopic, mesoscopic, and microscopic scales. The butt cleat mean orientations are NE-SW and NW-SE, whereas the face cleat mean orientations are NNE-SSW and NE-SE, and the cleat dip is at a high angle of 75° to the north. The angles between the orientations of these cleat sets are nearly 90°; that is, they are orthogonal. The spacing of the macroface cleats is between 9.52 and 14.46 cm (averaging 11.61 cm), while the spacing of the butt cleats is between 2.3 and 11.3 cm (averaging 5.35 cm). The mean aperture is 0.54 mm. In contrast, the mean spacing of the mesoface cleats is 3.09 mm, and the aperture measurements of these cleats range from 0.0478 to 0.0741 mm (averaging 0.0549 mm). The results obtained from outcrops and X-ray CT (computed tomography) scan clearly indicate that the NNE-SSW face cleat orientations are strictly parallel to the elongation of the main tectonic structures in the studied area. Their origin may be explained by their relationship with local tectonic stress (the maximum principal stress, σ1, was horizontal). Partial least square analysis of data collected from cleats and faults in the area indicates that a power law distribution exists between the cleat characteristics (spacing, density, and aperture), and the distance of the faults (R2 = 0.56). Cleat formation in the Sajau Formation was mainly controlled by the mechanical response to tectonic stresses, which generated a unique cleat network in the coal. Based on the SEM photography, it is concluded that the development of microcleats in the Sajau Formation resulted from both endogenic processes and tectonic activities as indicated by the change in the shape of the cleats, from straight line to curved shape and branching (intersection of multiple cleats that form a conical network at the end).

Changes in the anisotropic permeability of low-rank coal under varying effective stress in Fukang mining area, China

Fractures are coalbed methane (CBM) migration pathways and key factors in determining coal reservoir permeability. During the depletion of CBM wells, the increase of effective stress with water pumping and gas desorption will affect gas production by changing reservoir permeability. However, the current understanding of the impacts of fracturing in different directions on CBM recovery is inadequate. In this work, we collected 4 types of cubic coal samples (samples cut parallel to face and butt cleats and perpendicular to the bedding plane) from the Fukang mining area in China to investigate the anisotropic characteristics of P-wave velocity, fractures and permeability under varying effective stresses from 3.5 to 8.5 MPa in the laboratory. The results indicate that the permeability exponentially decreased with increasing P-wave velocity. The acoustic wave velocities significantly decreased from direction perpendicular to the bedding plane to the butt cleat direction to the face cleat direction. However, the fracture aperture exhibited the opposite trend. The maximum acoustic wave velocities ratios of the perpendicular bedding plane direction to the parallel bedding plane direction are 1.12, 1.23, 1.05 and 1.15 for sample LY, DH, XG and QM, respectively. However, the ratios of butt cleat direction to face cleat direction are 1.23, 1.21, 1.01 and 1.07. A good coupling effect was detected between the aperture, porosity, connectivity, and permeability. The permeability decreased exponentially as the effective stress increased in the selected coal samples. Under the 3.5–6.5 MPa effective stress conditions, the permeability decreased rapidly, and from 6.5 to 8.5 MPa, the permeability decreased more slowly. Contrary to the permeability loss rate (PLR), the stress sensitivity coefficient and dimensionless permeability parallel to the bedding plane were greater than perpendicular to the bedding plane. The effective stress returned from 8.5 MPa to 3.5 MPa, and the permeability rebounded but did not fully recover. The irreversible permeability loss rate (IPLR) showed that the permeability was restored the least in the direction perpendicular to the bedding plane. The maximum IPLR ratios of the perpendicular to the bedding plane direction to the parallel bedding plane direction are 3.39, 5.34, 1.86 and 23.00 in sample LY, DH, XG and QM, respectively. According to the study of anisotropic fractures and permeability, multi-lateral well is recommended as the appropriate well type to obtain the optimal CBM recovery effect.

Experimental investigation of propagation mechanisms and fracture morphology for coalbed methane reservoirs

Fracture propagation mechanisms in coalbed methane (CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fractures in coal specimens at different angles between the face cleat and the maximum horizontal principal stress were investigated with hydraulic fracturing tests. The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. “Step-like” fractures were formed in most experiments due to the existence of discontinuous butt cleats. The hydraulic fractures were more likely to divert or propagate along the butt cleat with an increase in the angles and a decrease in the horizontal principal stress difference. An increase in the injection rate and a decrease in the fracturing fluid viscosity were more conducive to fracture networks. In addition, the influence on fracture propagation of the residual coal fines in the wellbore was also studied. The existence of coal fines was an obstacle in fracturing, and no effective connection can be formed between fractures. The experimental investigation revealed the fracture propagation mechanisms and can provide guidance for hydraulic fracturing design of CBM reservoirs.

Multi-well synchronous hydraulic conformance fracturing technology used for deep coal beds and its field application in the Southern Qinshui Basin

For deep CBM productivity improvement, a technical concept of conducting multi-well synchronous hydraulic conformance fracturing was proposed based on its geological characteristics. First, a mathematical model for multi-fracture induced stress was established by using the boundary element displacement discontinuity method, to simulate the distribution of induced stress field in deep coal beds and analyze the possibility of the formation of complex fracture networks induced by the hydraulic conformance fracturing. Then, the propagation situation of fracture networks interfered by stress and its influencing factors were studied by using the discrete element method. And finally, the feasibility of synchronous hydraulic conformance fracturing technology was verified through tri-axial fracturing experiment and field application. It is shown that by virtue of synchronous hydraulic conformance fracturing technology, stress interference area and strength are increased, so horizontal major stress difference is decreased and even the direction of earth stress is changed regionally, which is conducive to the connection of developed face cleats and butt cleats in coal rocks, so as to form large, efficient and complex fracture networks. Furthermore, the favorable conditions for the formation of complex fracture networks by hydraulic conformance fracturing include lower initial horizontal major stress difference, low Poisson's ratio, short well spacing and low fracturing fluid viscosity and high net pressure inside the fractures. Finally, it is shown from the 3D true physical simulation experiments that by virtue of this synchronous hydraulic conformance fracturing technology, natural fractures in coal rocks can be connected sufficiently, and consequently complex fracture networks composed of hydraulic fractures, face cleats and butt cleats are created. Based on these research results, a set of optimization design method for the synchronous hydraulic conformance fracturing of deep coal beds was proposed. Five vertical wells located in the deep coal beds of North Shizhuang Block in the Southern Qinshui Basin were chosen for the pilot test. It is indicated from fracture monitoring and drainage/production data that the stimulated reservoir volume (SRV) of synchronous hydraulic fractured wells is large and its fracture network is complex; and that compared with the conventionally fractured wells, the synchronous hydraulic fractured well is earlier in gas breakthrough, and higher and more stable in production rates and casing pressure and its regional pressure drop even spreads to the adjacent wells, so that their production rates are remarkably raised.

Influence of bedding and cleats on the mechanical properties of a hard coal

The extensive bedding and cleats have a great influence on the mechanical properties of coal. The previous research mainly focused on the bedding effect, and few studies took the influence of both bedding and cleats into consideration. The scanning electron microscopy (SEM), uniaxial compression experiments, and acoustic emission (AE) monitoring were conducted on one hard coal specimen with different bedding and cleat angles. The experimental results show that for the bedding dip of 0° and face and butt cleat dip of 90°, the properties of coal specimens were characterized by the highest peak strength, the drastic failure process, the post-peak brittle behavior, the long initial quiet stage, and the short violent booming stage of AE hits number. For the bedding and face cleat dip of 45°, and the butt cleat dip of 90°, the properties of coal specimens were characterized by the lowest peak strength, the slip failure mode, the step-like drops of the post-peak stress-strain curve, and the several peaks of AE hits number. On the other hand, for the bedding and butt cleat dip of 90°, and the face cleat dip of 0°, the properties of coal specimens were presented as tensile failure mode with plate cracks, the short non-linear stage and the slight step-like drops of stress-strain curve, the short initial quiet stage of AE hits number, and the large value of elastic modulus. Then, a theoretical model of coal containing bedding and cleats was established in this paper. The simulation results of peak strength and failure mode verified the proposed model. The simulation of coal strength shows that bedding had the decisive influence on the uniaxial compressive strength of coal, while the weakening effect of cleats was non-negligible. When bedding and cleats were taken into consideration together, the minimum peak strength was reduced and the bedding dip related to the minimum peak strength changed.

Permeability evolution of anthracite coal considering true triaxial stress conditions and structural anisotropy

It is critical to understand the gas flow behavior in coal under a reservoir stress condition for coal bed methane production, underground coal mining, and CO2-sequestration in deep coal seams. With respect to coal seams, the in-situ stress is anisotropic and generally exists under true triaxial stress (σ1 > σ2 > σ3) conditions. Additionally, the flow channels determining the permeability of coal are also anisotropic. This dual anisotropy produces difficulties in replicating the gas transport characteristics of coal at the laboratory scale, and there is a paucity of relevant studies. In this study, we performed a series of permeability measurements using cubic anthracite coal samples and changing the principal stresses and flow directions under various true triaxial stress conditions. The coal permeability exhibited greater anisotropy in the vertical direction as a result of the presence of minerals in cleats across the bedding plane. After each principal stress compression at a differential stress of 20 MPa, the permeability in each direction decreased by an order of magnitude. With an increase in the intermediate stress parameter, the permeability values of two horizontal cleats experienced higher decreasing rates compared with the vertical bedding permeability. This increased the significance of the horizontal permeability anisotropy. With respect to the true triaxial stress condition with a higher horizontal principal stress (σH > σh > σv), a higher permeability reduction was observed during the principal stress loading period. The butt cleat plane was more sensitive to changes in the principal stress because of the lower connectivity of the flow channels induced by the closure of the face cleat that acted as a cross-linked pathway. The anisotropic permeability data measured under true triaxial stress conditions were well expressed by an exponential equation containing different mean cleat compressibility and stress terms. The cleat compressibility values in different directions were obtained by data fitting.

Brazilian Tensile Strength of Anisotropic Rocks: Review and New Insights

Strength anisotropy is one of the most distinct features of anisotropic rocks, and it also normally reveals strong anisotropy in Brazilian test Strength (“BtS”). Theoretical research on the “BtS” of anisotropic rocks is seldom performed, and in particular some significant factors, such as the anisotropic tensile strength of anisotropic rocks, the initial Brazilian disc fracture points, and the stress distribution on the Brazilian disc, are often ignored. The aim of the present paper is to review the state of the art in the experimental studies on the “BtS” of anisotropic rocks since the pioneering work was introduced in 1964, and to propose a novel theoretical method to underpin the failure mechanisms and predict the “BtS” of anisotropic rocks under Brazilian test conditions. The experimental data of Longmaxi Shale-I and Jixi Coal were utilized to verify the proposed method. The results show the predicted “BtS” results show strong agreement with experimental data, the maximum error is only ~6.55% for Longmaxi Shale-I and ~7.50% for Jixi Coal, and the simulated failure patterns of the Longmaxi Shale-I are also consistent with the test results. For the Longmaxi Shale-I, the Brazilian disc experiences tensile failure of the intact rock when 0° ≤ βw ≤ 24°, shear failure along the weakness planes when 24° ≤ βw ≤ 76°, and tensile failure along the weakness planes when 76° ≤ βw ≤ 90°. For the Jixi Coal, the Brazilian disc experiences tensile failure when 0° ≤ βw ≤ 23° or 76° ≤ βw ≤ 90°, shear failure along the butt cleats when 23° ≤ βw ≤ 32°, and shear failure along the face cleats when 32° ≤ βw ≤ 76°. The proposed method can not only be used to predict the “BtS” and underpin the failure mechanisms of anisotropic rocks containing a single group of weakness planes, but can also be generalized for fractured rocks containing multi-groups of weakness planes.

Anisotropic Adsorption Swelling and Permeability Characteristics with Injecting CO2 in Coal

The changes of anisotropic adsorption–swelling and permeability with injecting CO2 in coal influence the CO2 injectivity during CO2-ECBM or CGS (ECBM = enhancing coal bed methane; CGS = CO2 geological sequestration). To strengthen the understanding of this issue, two special-made cubic coal samples were adopted to test the porosity, swelling, and permeability in parallel face cleat and bedding plane direction, parallel butt cleat and bedding plane direction, and vertical bedding plane direction. To quantitatively characterize the anisotropic porosity, anisotropic swelling, and anisotropic permeability, an anisotropy index was introduced in this work. The results show that porosity anisotropy reflects the pore connectivity in different directions, which fall in the order of parallel face cleat and bedding plane direction > parallel butt cleat and bedding plane direction > vertical bedding plane direction. The porosity varieties can be owed to the compaction effect, thermal evolution effect, banded structur...

Effects of gas sorption-induced swelling/shrinkage on the cleat compressibility of coal under different bedding directions

The cleat compressibility of coal is a key parameter that is extensively used in modeling the coal reservoir permeability for Coal Bed Methane (CBM) recovery. Cleat compressibility is often determined from the permeability measurement made at different confining pressures but with a constant pore pressure. Hence, this parameter ignores the sorption strain effects on the cleat compressibility. By using the transient pulse decay (TPD) technique, this study presents the results from a laboratory characterization program using coal core drilled from different bedding directions to estimate gas permeability and coal cleat compressibility under different pore pressures while maintaining effective stress constant. Cleat compressibility was determined from permeability and sorption strain measurements that are made at different pore pressures under an effective stress constant. Results show that the cleat compressibility of coal increases slightly with the increase of pore pressure. Moreover, the cleat compressibility of Sample P (representing the face cleats in coal) is larger than that of Sample C (representing the butt cleats in coal). This result suggests that cleat compressibility should not be regarded as constant in the modeling of the CBM recovery. Furthermore, the compressibility of face cleats is considerably sensitive to the sorption-induced swelling/shrinkage and offers significant effects on the coal permeability.

Tectonics and Geological Factors Controlling Cleat Development in the Barito Basin, Indonesia

Cleats are natural fractures within coal seams. This paper presents the results of regional cleat mapping and characterization in relation to coalbed methane (CBM) exploration and development in the Barito Basin, South Kalimantan, Indonesia involving two major coal layers, namely the Late Eocene Tanjung Formation and Early to Middle Miocene Warukin Formation with thicknesses ranging from 2 to 50 m. The coal is classified as lignite to subbituminous with calorific values ranging from 6000-8000 Cal/gr with ash content 0.68-17.42%. We applied two methods of cleat measurement, i.e. scanline and window sampling using a 100 x 100 cm grid. More than 20,000 cleats were measured from 65 scanline and 37 window sampling locations. The results revealed that face and butt cleats are predominantly oriented in the WNW-ESE and NNE-SSW directions, respectively. The results showed that cleat density increases related to structural position such as fold hinge and fault zone. The formation of the cleats may be influenced by several geological processes, where the cleats, which form during coalification, are superimposed by later processes such as fluid pressure and tectonic stresses and are seemingly also affected by the composition of the coal.

Cleat Orientation from Ground Mapping and Image Log Studies for In Situ Stress Analysis: Coal Bed Methane Exploration in South Karanpura Coalfield, India

Conventional well logs from three wells in South Karanpura coalfield are analyzed to correlate 10 major coal seams belonging to the Barakar formation for coalbed methane (CBM) exploration. Face cleat orientations are studied for the Simana and Argada coal seams from the Sirka and Argada mines. It varies from 130 to 140°N for a total 349 face cleats observed from Simana and Argada coal seams. Cleat identification and its orientation have also been analyzed from simultaneous acoustic and resistivity (STAR) log from two CBM wells located 20–22 km away from the Sirka and Argada mines. Total number of fractures is found to be 1118 from two wells. Face cleats are not distinguishable from STAR image. Fracture density in coal is varying from 2.48 fractures per meter to 23.8 fractures per meter with a maximum density in the Banasgarah seam. Fracture orientations observed in Saunda, Sayal, Balkudra, Kurse, Hathidari, Banasgarah, and Argada vary from 10°N to 115°N. The in situ maximum horizontal stress (SH) is compu...

Simulation of the crack development in coal without confining stress under ultrasonic wave treatment

Ultrasonic waves can potentially be used to improve the permeability of low-permeability coalbed methane (CBM) reservoirs by enhancing cleats propagation and new cracks generation. Two polished blocks of semi-anthracite coal were subjected to ultrasonic waves for 360h without confining stress. The variations in cracks development due to the effects of original cracks (especially face cleats) and of coal bedding are investigated by an optical microscope, a scanning electron microscope, and an ultrasonic analyzer. Ultrasonic waves induce crack propagation along face cleats, butt cleats, and coal bedding planes. The cracks are generated especially in the bedding planes and ultimately break coal into two parts with a rugged crack surface. Cracks primarily propagate in desmocollinite along cleats in the raw coal in the earlier stage of ultrasonic vibration. In the latter stage, crack propagation primarily occurs in fusinite or at the interfaces along the direction of the coal bedding. The surface crack ratio and acoustic attenuation measured by an ultrasonic analyzer indicate that cracks propagate along face cleats and bedding in the first 36h of vibration more rapidly than in the latter stage. Ultrasonic waves are loaded at the crack tip, where concentrated stress is generated, promoting crack propagation along face and butt cleats under periodic tensile and compressive stresses. Cracks and the interfaces between different compositional features in coal are mechanically weak planes. The macerals and minerals in bedding planes have large contact areas and various degrees of microhardness. Owing to the effects of different wave impedances of the components, the reflected stress fields and transmitted stress fields differ on both sides of the component interface. This periodic stress difference at a mechanically weak interface generates microcracks in bedding planes and the cracks ultimately develop into the uneven crack surface through the entire coal.

CO2 gas fracturing: A novel reservoir stimulation technology in low permeability gassy coal seams

Coal mine methane (CMM) is one of the major safety hazards in China’s gassy coal mines. To prevent the gas related mining hazards, effective gas drainage is a pivot technology to reduce the gas content in coal seams. For low permeability coal seams, permeability enhancement is one of the top challenges for enhancing gas drainage efficiency. In the study, a novel methane drainage technology for coal permeability enhancement, namely CO2 gas fracturing (CO2-Frac), has been proposed and investigated in Luan coal mine field. CO2-Frac is achieved by multiple stage directional CO2 gas fracturing with a newly renovated CARDOX system. Through optimization of the CO2-Frac boreholes parallel to face cleats, a complex and high permeability fracture network was generated in CO2-Frac borehole and hence the improvement of methane gas drainage effectiveness. Mine-scale tests of CO2-Frac have been conducted in 5 coal mines in Luan mine field from December 2011 to December 2015. Methane concentrations and flow rates from both CO2-Frac boreholes and adjacent boreholes without CO2-Frac were continuously monitored for evaluating the stimulation efficiency. It was observed that permeability and methane drainage effectiveness in the CO2-Frac boreholes was significantly improved: (1) gas permeability increases up to 20–50 folds; (2) extracted methane concentration increases ∼10 folds ranging from 50 to 90%, compared the concentration of 5–30% in normal boreholes without CO2-Frac; (3) methane rate in a single CO2-Frac hole varies from 140 to 280m3/d with an increase of 10–20 folds, compared the rate of 14–20m3/d in normal boreholes; (4) methane content of the coal seam in drainage area reduced from original 11.7m3/t to 6.5m3/t after 210days of drainage; (5) upgraded methane from CO2-Frac boreholes can be better utilized in air ventilation methane (AVM) power plant. CO2-Frac is confirmed as the best technology to drainage methane in this low permeability coal seams in Luan mine field, and it is not only beneficial on coal mine gas control and safety management but the CMM economic utilization by upgrading drained methane gas concentration.

Image processing based characterisation of coal cleat networks

Characterisation of the cleat network serves as the basis for estimating the hydraulic and mechanical seam properties which in turn are fundamental for flow and geomechanical modelling in the context of underground coal mining. Cleat and cleat network geometry can be described as a function of frequency, aperture, size, orientation relative to in situ stresses, connectivity and porosity, with mineralised and un-mineralised cleats occurring. To describe these properties, CT-scans of core samples of a Bowen Basin coal in central Queensland, Australia, are analysed.A unique image processing workflow method is introduced to extract the key statistical parameters of perpendicular butt and face cleats present in a two-dimensional image. As face and butt cleats have different characteristics, the presented method distinguishes face cleats and butt cleats by direction and present detailed data for both cleat types. The results comprise cleat length, apertures, sizes, intensities, densities, shape parameter, spacing, orientation and connectivity and are therefore more comprehensive than previous cleat descriptions. Three generally different cleat geometries are considered within this study, one sample shows perpendicular face and butt cleats, the second two sets of face cleats intersected by butt cleats and the third parallel face cleats only.