Butt Cleats Research Articles

An analytical model of coal wellbore stability based on block limit equilibrium considering irregular distribution of cleats

An analytical model of coal wellbore stability is established based on the block limit equilibrium method, taking the irregular distribution of cleats into account. Because coal seam is cut into discontinuous blocks with different shapes and sizes, block slip can be treated as the symbol of coal wellbore instability. According to the geometric characteristic of blocks, blocks surrounding the wellbore can be divided into 3 categorizes: type I, type II and type III. To evaluate the block slip, the stress distribution near the wellbore is determined based on the continuous medium mechanics and the fracture mechanics. Using Mohr–Coulomb criterion and limit equilibrium method, the critical conditions of block slip are derived by defining the failure coefficient, including type I, type II and type III. The proposed model is verified by comparison of the obtained results from the proposed analytical method with the discrete element method as well as the actual drilling. It can be proved that the proposed model can predict accurately block slip. Additionally, the effect of drilling fluid pressures on the wellbore stability is specially investigated. The results show that with the increasing of drilling fluid pressure, the degree of wellbore collapse may be more severe. Meanwhile, the analysis method of reasonable drilling fluid pressure for coal drilling is proposed based on the interval theory, combining the slip area of blocks. Finally, the parametric study is focused on investigating the effects of the internal friction angle of face cleats, the cohesion of face cleats and the tensile strength of butt cleats on the failure coefficients of blocks.

A Calculation Model of the Net Pressure for Forming Map Cracking in Coalbed with Cleats During Hydraulic Fracturing Process

An important prerequisite for achieving efficient exploitation of coalbed methane wells is through forming map cracking by hydraulic fracturing. In order to analyze the mechanical mechanism for forming map cracking of the coal bed with cleats, the mechanical conditions for forming map cracking during hydraulic fracturing process of coal bed was proposed using extensional faulting in elastic mechanics and the shear damage criterion, and the minimum net pressure calculation model for forming map cracking was established when the butt cleat and face cleat in coal opened at the same time. It can be concluded through using the calculation model that the net pressure value that needed for forming map cracking first decreased and then increased with the increasing of the angle between the face cleat and the direction of horizontal minimum principal stress. The cleats and fissures developed along the horizontal maximum principal stress were easy to open and extend under the effect of hydraulic fracturing. The variation of the internal friction coefficient variation of the face cleat had little effect on the minimum net pressure that was needed for forming map fracturing after the angle between the direction of face cleat and horizontal minimum principal stress is determined.

X-Ray Computed Tomography Analysis of Sajau Coal, Berau Basin, Indonesia: 3D Imaging of Cleat and Microcleat Characteristics

The Pliocene Sajau coals of the Berau Basin area have a moderately to highly developed cleat system. Mostly the cleat fractures are well developed in both bright and dull bands, and these cleats are generally inclined or perpendicular to the bedding planes of the seam. The presence of cleat networks/fractures in coal seam is the important point in coalbed methane prospect. The 3D X-ray computed tomography (CT) technique was performed to identify cleats characteristics in the Sajau coal seams, such as the direction of coal cleats, geometry of cleat, and cleats mineralization. By CT scan imaging technique two different types of natural fractures observed in Sajau coals have been identified, that is, face cleats and butt cleats. This technique also identified the direction of face cleats and butt cleats as shown in the resulting 3D images. Based on the images, face cleats show a NNE-SSW direction while butt cleats have a NW-SE direction. The crosscutting relationship indicated that NNE-SSW cleats were formed earlier than NW-SE cleats. The procedure also identified the types of minerals that filled the cleats apertures. Based on their density, the minerals are categorized as follows: very high density minerals (pyrite), high density minerals (anastase), and low density minerals (kaolinite, calcite) were identified filling the cleats aperture.

The impact of cleats on hydraulic fracture initiation and propagation in coal seams

Cleats are systematic, natural fractures in coal seams. They account for most of the permeability and much of the porosity of coalbed methane reservoirs and can have a significant effect on the success of hydraulic fracturing stimulation. Laboratory hydraulic fracturing experiments were conducted on coal blocks under true tri-axial stress to simulate fracturing stimulation of coal seams. Fractures were initiated by injecting a water gel with luminous yellow fluorescent dye into an open hole section of a wellbore. The impact of cleats on initiation and propagation of hydraulic fractures in coal seams is discussed. Three types of hydraulic fracture initiation and propagation pattern were observed in this study: 1) The hydraulic fracture initiated and then grew along the cleat. 2) The hydraulic fracture initiated along a butt cleat or a fracture (natural or induced by drilling) oriented roughly in the minimum horizontal stress direction, then turned to propagate along the first face cleat that it encountered or gradually turned towards the maximum horizontal stress direction. 3) The hydraulic fracture initiated perpendicular to the minimum stress and, when it encountered a face cleat, tended to propagate along the cleats if the extension direction does not deviate greatly (<20° as determined in this paper) from the maximum horizontal stress direction. When a coal seam is hydraulically fractured, the resulting fracture network is controlled by the combined effect of several factors: cleats determine the initiation and extension path of the fracture, the in-situ stress state dominates the main direction of the fracture zone and bedding planes impede fracture height growth.

Identifying sweet spots and moving CBM reservoirs into production

C oalbed methane (CBM) is well recognised as an important natural gas resource. However, producing it commercially has proven to be a challenging process, primarily due to the complex nature of its gas storage and flow mechanisms. Although CBM reservoirs are naturally fractured due to the presence of face and butt cleats, gas stor­age and saturation depend on the adsorption capacity of the coal matrix. The ability to predict permeability variations and its evolu­tion in a CBM reservoir, which is affected by several dynamic CBM properties and operating conditions, also define the design, optimization, and analysis of the gas recovery process. As the permeability is naturally low, interventions are necessary in order to achieve commercially viable gas production rates. CBM are also dual-porosity media where the vast majority of the gas is stored in the low permeability coal matrix by sorp­tion. The flow to production wells, however, occurs through the coal’s natural fracture system, that stores relatively small amounts of gas, because coal matrix practically has no perme­ability. The result is that properties of the coal matrix have the greatest effect on the estimates of gas in place and recovery. It is well understood that the key parameter controlling gas flow in CBM reservoirs are cleat networks, gas storage, desorp­tion and permeability anisotropy (coal shrinkage and swelling). While the role of reservoir simulation is well under­stood for conventional oil and gas reservoirs and its related techniques for prediction and production optimisation, for unconventional reservoirs (CBM, Shale, etc.) current reservoir simulation software needs to be further developed and adapted to represent the performance of these types of fields and gener­ate more acceptable results. For example, the basic governing fluid flow regime in conventional reservoir simulations is limited to Darcy flow. For unconventional reservoirs, however, another flow regime term comes into the equation that reflects the flow of gas in the coal matrix and stands for diffusion. This new flow regime is not very well defined in the current simulators and needs further studies to appropriately address the production performance of a reservoir. Furthermore, the concept of multi-phase flow in reservoirs is well defined by the introduction of relative permeability equations in the flow equations. However, measuring relative permeability in coal is relatively complex (Ham and Kantas, 2008) due to its friability, heterogeneity, stress dependence and porous morphology. Young et al. (1992), for example, noticed that the derived relative permeability from a history match does not resemble the laboratory measurement. This phenomenon could be due to the change of matrix structure on phase flow in addition to wettability and capil­larity. In other words, both phases may flow through different passages over time due to heterogeneity. Thus dynamic rela­tive permeability (phase saturation) is required to accurately predict recovery from a CBM reservoir. As the focus on CBM has increased in the past decades, so has the quest for more effective technologies to help bring the reserves in such complicated conditions into production and to identify sweet spots. Reservoir modelling and reservoir simulation are technologies that are likely to play a key role in achieving this. Firstly, however, it is necessary to look at the challenges of CBM property variations.

Experimental research on coal permeability: The roles of effective stress and gas slippage

The gas permeability of coals varies with effective stress increase, matrix shrinkage/swelling due to gas desorption/adsorption, gas slippage and other factors. In this paper, the nature of helium permeability change was primarily investigated on 14 coal cores under a varying effective stress from 4 to 12 MPa in laboratory. Results show that, for the High-rank Series (five low-volatile bituminous to semi-anthracite cores) and Referee Series (a high volatile bituminous, a moderate volatile bituminous, and a semi-anthracite coal), coal permeability declined exponentially with the rise of effective stress. Under 4–8 MPa confining stress conditions, the permeability declines dramatically; while for 8–12 MPa conditions, the curve of permeability reduces slightly. Gas slippage effect plays a positive role in improving permeability for all the cores, of which the Klinkenberg permeability shows a positive linear relationship with the total permeability, and becomes much more influential for low initial permeability cores. The Heterogeneity Series (samples cut parallel to face and butt cleats, and perpendicular to bedding plane) revealed that the butt cleat has the highest stress sensitivity and the highest cleat compressibility. The permeability rebound happened for all the three direction cores, and the butt cleat direction cores showed the lowest irreversible permeability loss through the 4–12–4 MPa effective stress cycle. Cleat compressibility decreases linearly with the increase of effective stress for all the coal cores, and the higher vitrinite content results in a higher decrease rate.

Lignite cleat studies from the first Middle-Polish (first Lusatian) lignite seam in central Poland

Some Miocene lignite seams are characterized by the presence of natural discontinuities, so-called cleats. Most often they are opening-mode fractures, consisting of two orthogonal sets (face and butt), both almost perpendicular to the bedding. This paper determines distributions of cleat orientation, spacing, and aperture from the first group of the Middle-Polish (first Lusatian) lignite seams (MPLS-1), which has been exploited, inter alia, in the Jóźwin IIB and Kazimierz N opencast mines in central Poland. All observations and measurements were conducted at macroscopic scale.The cleats' mean orientations are NE–SW and NW–SE, dipping at a high angle >75°. The first set of fractures refers to the face cleats and the second one to the butt cleats. The spacing of face cleats is from 10.0 to 17.2cm (averaging 12.65cm) and the spacing of butt cleats is from 9.3 to 24.3cm (averaging 13.75cm). On the other hand, aperture measurements of these cleats range from 0.2 to 1.9cm (averaging 0.68cm) and from 0.1 to 1.5cm (averaging 0.42cm), respectively. The obtained results clearly indicate that face cleat orientations (NW–SE) are strictly parallel to the elongation of the main tectonic structures in the study area. Their origin may be explained in at least two ways, that is, by tectonic folding (the maximum principal stress, σ1, was horizontal) or by salt diapirism (the maximum principal stress, σ1, was vertical). In both cases the intermediate principal stress, σ2, was parallel to the face cleat strikes, namely NW–SE, although the well-developed butt cleats, in the context of regional geology, provide additional evidence for the creation of an orthogonal system of fractures (cleats) in the cover of salt structures. Thus, the NW–SE direction is characteristic of both the extent of the main tectonic units in central Poland and the face cleat orientation in MPLS-1.

Sensitivity analysis in permeability estimation using logging and injection-falloff test data for an anthracite coalbed methane reservoir in Southeast Qinshui Basin, China

This paper presents a sensitivity analysis in permeability estimation by interpreting injection-falloff test and logging data for a producing coalbed methane field in Southeast Qinshui Basin. Injection-falloff test data are interpreted by an infinitely acting, constant compressibility and radial homogeneous model. Log interpretation is based on the Archie equation using data of shallow laterolog resistivity. The cementation factor of coal is calculated using laboratory measurements. Based on the interpreted cementation factors and the assumed fracture geometries, the sensitivity in permeability estimation using logging data is analyzed. Results show that the permeabilities are 0.041, 0.012 and 0.035mD for coal seam #3 in wells #W-1, W-5 and W-6, respectively, from the interpretation of injection-falloff test data; the calculated coal cementation factors range widely from 0.9 to 1.8 with average of 1.3. In order to match the log interpretation results with those from injection-falloff tests, different fracture spacing is used in log interpretation for the three wells. The fracture spacing varies from 0.39 to 0.67mm, and the average fracture aperture varies from 0.229 to 0.398μm for the three wells. The cementation factor has the highest impact on estimation of permeability if the cementation factor, mud filtrate resistivity and fracture spacing are varied with 10% from the base case. Effect of fracture aperture’s heterogeneity on permeability is depending on the standard deviation and average of fracture aperture. Effective permeability has a stronger relationship with geometric mean of face and butt cleats aperture than with total porosity for different face and butt spacing models.

The Breakdown Pressure Calculating Model for Open Hole Completion CBM Well Hydraulic Fracturing

The open hole completion CBM borehole wall intersects with the weak surface such as cleats and fractures. In the process of hydraulic fracturing, the fractures may origin from coal body or cleats, which makes the rupture mechanism and rupture models of the borehole wall being different from the conventional reservoirs. The previous model for calculating breakdown pressure of open hole completion borehole wall considering tension failure has poor applicability for calculating breakdown pressure. Considering the spatial relationship of the intersection of borehole wall and cleats, analyzing the stress state of borehole wall rock and cleats wall, and basing on elastic mechanics and fracture mechanics, the breakdown pressure calculation model for CBM open hole completion hydraulic fracturing was established. In the model, fractures initiate from coal rock body, tensile failure along with face cleats, shear failure along with face cleats, tensile failure along with butt cleats and shear failure along with butt cleats five kinds of damage modes were considered. According to example calculation, the breakdown pressure of HX-L1 well calculated using the model is 14.81 MPa. The actual pressure obtained by bottom hole pressure gage is 15.42 MPa, and the relative error is 3.96%. The calculated result agrees with the actual conditions. It can be concluded that the model can be used to calculate the breakdown pressure for open hole completion CBM well hydraulic fracture.

What are cleats? Preliminary studies from the Konin lignite mine, Miocene of central Poland

Abstract Cleats (fractures, joints) are discontinuities in coals, including lignites. They are important in mining activity because of their gas and water permeability in hard coal, and mainly because of their water permeability in lignites. As opposed to hard-coal cleats, lignite cleats have not been studied in detail before. The present contribution does so, using as an example the 1st Middle-Polish Lignite Seam (MPLS-1) in the Jóźwin IIB opencast mine in central Poland. It should be mentioned here that any remarks in the present contribution concerning MPLS-1 refer exclusively to this lignite seam in the Jóźwin IIB opencast mine. The investigated discontinuities consist of two sets, i.e. the face and butt cleats, which are roughly oriented NW-SE and NE-SW, respectively. The mean spacing of the face cleats is ~12.4 cm, while the mean spacing of the butt cleats is ~12.8 cm. The maximum average aperture is ~4.9 mm for the face cleats and ~4.1 mm for the butt cleats. The cleat spacing and aperture do not depend on the lignite thickness, but the cleat spacing increases with increasing mineral-matter and xylite content, whereas the aperture increases when the contents decrease. The regional folding and local salt diapirism tentatively explain the formation of the orthogonal system of the lignite cleats, partly because of the parallelism of the face cleats and the major tectonic directions in central Poland.

The Numerical Simulation of the Pressure Buildup Process of Cavity Completion for CBM Wells

A numerical model is established to simulate gas injection with high pressure for CBM cave completion. The model is used with the discrete element simulation software UDEC and considers two kinds of fracture morphology exiting in coal bed face cleat and butt cleat. The effective stress of coal matrix, fluid pressure and vector distribution of calculation nodal displacement under the influence of crustal stress is studied as bottom-hole pressure increases. Results show that the model considering cleat system can simulate reservoir fluid migration and matrix deformation more accurately in the process of pressure buildup of cavity completion; the different fluid velocities caused by different directions of face cleat and butt cleat will induce shear fractures for coal bed; the development of coal reservoir extensional fractures can be known by calculating the vector distribution of nodal displacement; the anisotropic formation stresses will make the break and collapse of coal bed more easily.

The Model for Calculating Elastic Modulus and Poisson's Ratio of Coal Body

Coal body is a type of fractured rock mass in which lots of cleat fractures developed. Its mechanical properties vary with the parametric variation of coal rock block, face cleat and butt cleat. Based on the linear elastic theory and displacement equivalent principle and simplifying the face cleat and butt cleat as multi-bank penetrating and intermittent cracks, the model was established to calculate the elastic modulus and Poisson's ratio of coal body combined with cleat. By analyzing the model, it also obtained the influence of the parameter variation of coal rock block, face cleat and butt cleat on the elastic modulus and Poisson's ratio of the coal body. Study results showed that the connectivity rate of butt cleat and the distance between face cleats had a weak influence on elastic modulus of coal body. When the inclination of face cleat was 90°, the elastic modulus of coal body reached the maximal value and it equaled to the elastic modulus of coal rock block. When the inclination of face cleat was 0°, the elastic modulus of coal body was exclusively dependent on the elastic modulus of coal rock block, the normal stiffness of face cleat and the distance between them. When the distance between butt cleats or the connectivity rate of butt cleat was fixed, the Poisson's ratio of the coal body initially increased and then decreased with increasing of the face cleat inclination.

Determining Young's Modulus of Fractured Coal Rock Mass through a Homogenization Method

Three main fractures exist in coal rock mass, including face cleats, butt cleats, and major fractures. The distribution of cleats and beddings in coal rock mass likes a regular reticular. A geological model was established without considering the major fractures. Young's moduli of fractured coal rock mass were gained through a homogenization method monolayer composite micromechanics analysis method. The relations between volume fraction or Young's moduli of cleat and Young's moduli of coal rock mass were investigated by calculation. Results shown that Youngs moduli of coal rock mass have the same change trends with the increase of volume fraction of cleat in the interval of 0.0018mm of width of cleat. And the width of cleat of 0.5mm is a critical point. Youngs moduli of coal rock mass monotonically increase with the increase of Youngs modulus of cleat in the interval of 0.0010.008GPa. Volume fraction and Young's moduli of cleats have notable effects on Young's moduli of coal rock mass.

Stress sensitivity of coal samples in terms of anisotropy

The permeability and porosity of coal seams are anisotropic, and the variation of confining stress may induce deformation in coal samples. In order to study these characteristics, experiments and model analyses were conducted to understand the behaviors of anisotropic stress sensitivity of lean coal samples. The results showed as the closure of cleats and the generation of micro-cracks, the strong stress sensitivity of coal samples and the discrete changes in porosity were caused by confining pressure changes. In the compression period, the anisotropy trend first increased, and then decreased. In the direction perpendicular to the bedding plane, the permeability decrease rate and the irreversible damage rate were the highest. In the direction parallel to the cleats, permeability recovery rate was higher and the irreversible damage rate was lower along butt cleats. Compared to the cube root of permeability to porosity, a 1/6 power relationship was proved to be closer to the experiment results, the new relationship had the highest fit level in the face cleat direction, and the lowest fit level in the vertical direction.

Optimization of hydraulically fractured well configuration in anisotropic coal-bed methane reservoirs

Most coal seams are characterized by an anisotropic network of cleat system in which continuous face cleats have much more permeability than discontinuous butt cleats. In hydraulically fractured and anisotropic CBM reservoirs, pressure propagation and resulting drainage area is in elliptical geometry. The determination of optimal well pattern and well spacing in such a system is the key parameter for higher well productivity and optimized CBM field development plan. In this study, well pattern and well spacing in anisotropic CBM reservoirs with fractured wells are optimized based on the concept of balanced depressurization which is assumed that the reservoir pressure should be declined uniformly in all directions of drainage area. The mathematical relationship for optimal well spacing is first developed and is verified by numerical simulation. It is concluded that, based on the requirement of balanced depressurization, the rectangular or diamond shaped pattern is the best pattern for hydraulically-fractured anisotropic coal seams, and the optimal well spacing is determined by both permeability ratio and hydraulic fracture length. Furthermore, a procedure for optimization of CBM well pattern and spacing is presented for making development strategy.

Characteristic of anisotropic coal permeability and its impact on optimal design of multi-lateral well for coalbed methane production

Coal permeability is usually anisotropic and the permeability anisotropy ratio along the face cleats to the butt cleats can be up to 17:1 for some coals. The characteristic of the anisotropic coal permeability and its impact on the optimal well design for coalbed methane (CBM) production are important, but have not been well studied. This paper investigates this issue through numerical modeling and reservoir simulations. Various case studies are performed on the two commonly used multi-lateral well patterns including the quad-lateral well and the pinnate lateral well to investigate the impact of permeability anisotropy ratio on the layout of the multi-lateral well. The results demonstrate that the optimal well direction of the quad-lateral well is parallel to the butt cleat direction as expected. However, the optimal main well angle of the pinnate lateral well is significantly affected by the permeability anisotropy ratio. The orientations of the branches of the pinnate lateral well are less sensitive than the branch numbers and this indicates that more gas recovery efficiency can be effectively achieved by drilling more branches but not by varying the branch orientations. These conclusions are drawn by using a widely used stress-based permeability model with the fixed permeability anisotropy ratio. In order to investigate the permeability anisotropy change during the CBM production, a strain-based and a stress-based coal permeability models for isotropic condition are improved to incorporate the permeability anisotropy and to quantitatively study the impact of permeability anisotropy ratio change on lateral well pattern. In order to be consistent with the previous permeability model, we implement the improved stress-based model into the reservoir simulation model. The results show that the permeability anisotropy is not only caused by the initial differences in structure and tortuosity of the coal cleats in the two directions, but also induced by the anisotropic mechanical and swelling properties during the CBM production. The permeability anisotropy ratio change during production may also have a significant impact on the optimal design of the multi-lateral well.

Variations in the mechanical behavior of Illinois bituminous coals

Unmineable coal beds are being considered as one of the geological sequestration options for storing carbon dioxide (CO2). The storage mechanisms and potential risks associated with the effects of CO2 on the coal structure are not yet understood and must be evaluated. The mechanical properties of the coal are expected to play an important role in the coal seams’ stability, especially under external perturbations. Typically, the mechanical characteristics of coal are investigated as a bulk material, which averages the effects of various structural inhomogeneities as well as of face and butt cleat fractures present in the coal. In this paper, we attempt to establish baseline mechanical characteristics of Illinois bituminous coals while minimizing the fracture effects. Rectangular coal strips (length <20mm), which showed no visible macro-defects, from two different Illinois bituminous coal seams, were subjected to three-point bending tests. Our results suggest there are significant variations in the flexural modulus (ranging from 0.7GPa to 3.4GPa) of the coal samples even though the coal rectangular strips originated from the same coal chunk. Vibrational spectroscopic analysis on the samples, which underwent mechanical testing, indicates a correlation between the flexural strength and modulus with the intensity of aliphatic groups. However, the mineral content of the coal seems not to influence the mechanical behavior of Illinois bituminous coals.

Fracture and 3D seismic interpretations of the Fruitland Formation and cover strata: Implications for CO2 retention and tracer movement, San Juan Basin Pilot test

Abstract This study presents data on faults and fractures in the Fruitland Formation and its cover strata at a carbon sequestration pilot site in the high-rate Fruitland coal fairway in the northeastern San Juan Basin. The study incorporates satellite imagery, fracture detection and shear wave anisotropy logs and 3D seismic to identify potential issues that could impact long term reservoir integrity, CO2 flow and retention. The properties of fracture networks within the reservoir and sealing strata are important to understand in carbon sequestration and enhanced recovery efforts. Fracture networks exert significant control on the ability of a reservoir to retain CO2 for long (thousand year) time frames with minimal leakage. Local structural complexity at the pilot site resulted in tracer migration patterns that differed from predictions based on regional descriptions of fracture and cleat orientations measured in areas several miles from the site. Perfluorocarbon (PFC) tracer breakthrough in the production stream of nearby producing wells, for example, occurred first in a well to the east, in a direction close to the presumed butt cleat trend rather than in a well at equal distance along what had been assumed to be the more permeable face cleat trend. The study reveals local tectonic complexity not suggested by well log derived structure maps of the area and suggests that fractures and faults disrupt the local Fruitland coal reservoir and the overlying Kirtland Shale primary seal.

Determination of in-situ stress direction from cleat orientation mapping for coal bed methane exploration in south-eastern part of Jharia coalfield, India

Abstract Accurate prediction of in-situ stress directions plays a key role in any Coal Bed Methane (CBM) exploration and exploitation project in order to estimate the production potential of the CBM reservoirs. Permeability is one of the most important factors for determination of CBM productivity. The coal seams in Jharia coalfield generally show low permeability in the range of 0.5 md to 3 md. To estimate the in-situ stress direction in the study area, an attempt has been made to undertake the cleat orientation mapping of four regional coal seams of two underground coal mines located at south-eastern part of Jharia coalfield, India. Cleat orientation mapping is critical to determine the maximum principal compressive horizontal stress (S H ) direction for CBM exploration and exploitation, which in turn controls the direction of maximum gas or water flow though coal beds. From the field study it is found that the average face and butt cleat azimuths are towards N15°W and N75°E respectively. Average permeability of the four above-mentioned major coal seams has been calculated from well logs of nine CBM wells distributing over an area of 7.5 km 2 , adjacent to the underground mines. The cleat orientations are congruous with the regional lineament pattern and fits well with the average permeability contour map of the study area to infer the orientation of in-situ maximum horizontal stress. Goodness of fit for the exponential regressions between vertical stress and permeability for individual coal seams varies between 0.6 and 0.84. The cleat orientation is further validated from the previous fracture analysis using FMI well log in Parbatpur area located southern part of the Jharia coalfield. The major coal seams under the study area exhibit directional permeability, with the maximum permeability, oriented parallel to the direction of face cleat orientation.

High-resolution X-ray computed tomography observations of the thermal drying of lump-sized subbituminous coal

Drying of low-rank coals affects: coal cleaning, combustion, comminution, gasification, liquefaction, and in-seam fluid-flow (water, coalbed methane, and carbon dioxide for sequestration/enhanced coalbed methane). To evaluate the extent of drying-induced transitions, 3 lump-sized (approximately 6 × 2 × 2 cm) Powder River Basin subbituminous coal samples were thermally dried in an air-drying coal oven at 50 °C over two weeks. A high-resolution industrial X-ray computed tomography scanner was utilized to generate (non-destructively) three-dimensional regional volumetric renderings, as-received and over 3-stages of drying. The lumps had cleats, both open and mineral filled, with a degree of fracture diversity along the longitudinal plane. Comparison of the virtual slice surfaces, at identifiable locations, allowed the induced cracking and shrinkage accompanying the transitions during 19% moisture loss to almost dry to be observed. Under these drying conditions, the heat transfer, and thus extent of drying, proceeded radially inward. With increased drying time the fractures extend and become larger in aperture as the coal shrinks. The major fractures mostly followed the existing cleat system. With additional drying, these cleats widened and the aperture increase propagated deeper into the coal extended into the butt cleats. New fractures were located mostly perpendicular to the cleat fracture surface. The external volume of the coal lumps had limited shrinkage. The axial extent of the shrinkage length (lump edge to lump edge) was on the order of 4–6%, the bulk of the shrinkage being accommodated by the internal shrinkage between cleats.