derivation of a global land elevation data set from satellite radar altimeter data for topographic mapping radar altimeter data from the 336 day cycle of the geodetic phase of ers 1 has been processed over all land surfaces to produce a consistent homogenous data set for topographic mapping purposes several processing steps were required to achieve this the most important of which were waveform retracking and data filtering no slope correction was applied to the data as this was found not to improve the accuracy the original radar altimeter data set was 145 gb in size and a more compact higher level product was required for land topography applications three higher level products were created 1 an unaveraged data set comprising elevation position backscatter and a small number ofauxiliary parameters 2 an averaged data set of quasi regular mean elevations position and standard deviation at 5 arc min resolution 3 a 5 arc min digital elevation model interpolated and extrapolated from the second data set the accuracy of the radar altimeter data was investigated using six point data sets of both global and regional coverage including airport runway elevations national geodetic data gravity station heights and navigation beacon heights a small number of erroneous elevations in both the radar and the point data sets were identified after removing outliers with a difference of greater than 300 m the mean difference between the radar and point data was 0 5 m 44 m full width at half maximum

experimental study of fluid transport processes in the matrix system of the european organic rich shales i scandinavian alum shale abstract this contribution presents results from a laboratory study investigating the fluid gas water transport properties in the matrix system of the scandinavian alum shale the maturity of the organic matter of the shale samples ranged between 0 5 and 2 4 vitrinite reflectance vr r gas he ar ch 4 and water flow properties were determined at effective stresses ranging between 5 and 30 mpa and a temperature of 45 c the effects of different controlling factors parameters on the fluid conductivity including permeating fluid moisture content anisotropy heterogeneity effective stress pore pressure and load cycling were analyzed and discussed pore volume measurements by helium expansion were conducted under controlled in situ effective stress conditions on a limited number of plugs drilled parallel and perpendicular to bedding for alum shale the intrinsic permeability coefficients measured parallel and perpendicular to bedding 6 10 22 8 10 18 m 2 were within the range previously reported for other shales and mudstones permeability coefficients were strongly dependent on permeating fluid moisture content anisotropy effective stress and other sample to sample variations the intrinsic absolute permeabilities measured with helium were consistently higher up to five times than those measured with argon and methane permeability coefficients he ch 4 measured on a dry sample were up to six times higher than those measured on an as received sample depending on effective stress the effect of moisture on measured permeability coefficients became more significant as effective stress increased permeability coefficients he ch 4 measured parallel to bedding were up to more than one order of magnitude higher than those measured perpendicular to bedding parallel to bedding all samples showed a nonlinear reduction in permeability with increasing effective stress 5 30 mpa the stress dependence of permeability could be well described by an exponential relationship

permeability evolution of shale under anisotropic true triaxial stress conditions abstract although permeability data for shale have been reported previously and permeability models have been proposed studies on permeability for shale under anisotropic true triaxial stress conditions are lacking in this work the permeability of shale under true triaxial stress conditions was measured using a newly developed true triaxial geophysical apparatus the permeability always decreased with increasing principal stress the permeability variations induced by varying each principal stress were distinct the greatest permeability reductions were observed with increasing stress normal to the bedding planes based on a consideration of shale anisotropy a permeability model was established to be applicable to shale under anisotropic true triaxial stress conditions ttp model this model was verified by matching the permeability data in our experiments it was found that a more accurate match could be obtained using the ttp model which showed advantages in reflecting the permeability changes related to increments in each principal stress

anisotropy of gas permeability associated with cleat pattern in a coal seam of the kushiro coalfield in japan an underground investigation has been performed in a subbituminous coal seam exhibiting a particular cleat pattern in the kushiro coalfield japan the coal cleat pattern shows some analogy to isolated straight joints and is believed to have been formed during the late tertiary period by a compressive tectonic stress roughly in the east west direction three cylindrical coal specimens representing the three orthogonal axes of the coal seam with respect to the bedding plane and its associated cleat were cored from a large block of coal gas permeabilities of the three coal specimens were measured under the same hydrostatic pressure conditions results clearly revealed anisotropy in permeability of the coal seam under relatively low confining pressures of less than about 12 mpa the specimen cored parallel to both the bedding plane and cleat strike showed the highest permeability even though the cleats were partly filled with calcite and clay minerals the permeability in this direction was 2 5 times higher than perpendicular to the bedding plane and 3 times higher than in the direction parallel to the bedding plane but perpendicular to the cleat strike this suggests that the cleats play a greater role than bedding planes in controlling fluid flow in the coal seam the permeability in the three orientations however converged to the same value at confining pressures above about 16 mpa this may suggest that both cleats and bedding planes in a coal seam can close due to earth pressure if the coal seam is located below a certain depth it further indicates that the traditional view that gas permeability is always greater parallel to the coal bedding than perpendicular to it should be reconsidered

comparisons between geosat and seasat tracking over non ocean surfaces comparisons between geosat and seasat altimeter tracking over non ocean surfaces are presented the geosat tracker shows more agility over the rougher surfaces and therefore is able to maintain lock better than seasat for repeat groundtracks over the greenland ice sheet geosat provided twice as many usable height measurements as seasat large off nadir attitude excursions caused differences in geosat tracking between repeat cycles the increased geosat pulse width caused an acquisition problem mostly over the quickly varying land surfaces the geosat altimeter was sometimes able to track over surfaces seasat could not but at times the acquisition problem was worse the effect of all the modifications is that the geosat measurements give more detailed along track information of the small scale topography over the ice sheets geosat yields denser coverage but over land seasat measurements give a better overall picture of the general topography these results suggest that the geosat elevation data set over land can best be used to augment the information obtained from the seasat data and is most valuable when investigating topography at scales less than 100 km

a three parameter permeability model for the cracking process of fractured rocks under temperature change and external loading abstract a three parameter permeability model is proposed to describe the cracking effects of fractured rocks induced by changes in effective stress temperature or both first a fractured rock is a composite of fractures soft part and matrix hard part in the proposed model the natural strain based hooke s law and engineering strain based hooke s law are used for the deformation of the soft and hard parts respectively second the compaction of fractured rocks and generation of new fractures in the matrix are analyzed based on the energy principle and weibull distribution third a three parameter permeability model is established for the entire deformation process the model has three parameters the proportion of fracture aperture in the soft part the evolution of mean fracture aperture and the effect of temperature change and external loading on fracture density finally the proposed model is applied to fit five sets of experimental data available in the public literature these include a china shale china coal france granite china red sandstone and usa coal datasets the fitting results show that this three parameter permeability model can effectively describe the permeability behaviors of fractured rock throughout the deformation process under effective stress temperature change or both it can also describe the heterogeneity and interaction of deformations in the fractures and matrix

an analytical coal permeability model for tri axial strain and stress conditions coal permeability is sensitive to the effective stress and is therefore coupled to the geomechanical behaviour of the seam during gas migration as coal shrinks with gas desorption and swells with adsorption understanding this coupling to geomechanical behaviour is central to interpreting coal permeability existing coal permeability models such as those proposed by shi and durucan 2004 and palmer and mansoori 1996 simplify the geomechanical processes by assuming uni axial strain and constant vertical stress however it is difficult to replicate these conditions in laboratory tri axial permeability testing and during laboratory core flooding tests for enhanced coal bed methane often laboratory tests involve a hydrostatic stress state where the pressure in the confining fluid within the tri axial cell is uniformly applied to the sample exterior in this experimental arrangement the sample is allowed to undergo tri axial strain this paper presents two new analytical permeability model representations derived from the general linear poroelastic constitutive law that include the effects of tri axial strain and stress for coal undergoing gas adsorption induced swelling a novel approach is presented to the representation of the effect of coal sorption strain on cleat porosity and thus permeability this involves distinguishing between the sorption strain of the coal matrix the pores or cleats and the bulk coal the developed model representations are applied to the results from a series of laboratory tests and it is shown that the models can predict the laboratory permeability data as part of this characterisation the various sorption strains are identified and it is shown that the pore strain is significantly larger than approximately 50 times the bulk sorption strain the models also provide further insight into how coal permeability varies with coal shrinkage and swelling and how the permeability rebound pressure depends upon the effective stress applied

surface elevation contours of greenland and antarctic ice sheets surface elevations of the ice sheets are contoured at 50 m intervals for the region of greenland covered by seasat radar altimetry south of 72 n and at 100 m intervals for a region of east antarctica north of 72 s the surface elevations were obtained from computer retracking of the radar altimeter waveforms which were recorded at 0 1 s intervals corresponding to 662 m spacings on the surface the precision of the elevation measurements before adjustment for radial orbit errors is 1 9 m as shown by analysis of elevation differences at orbital crossover points this precision is partly determined by radial errors of approximately 1 0 m in orbit determination and partly by noise due to ice surface irregularities adjustment of the radial components of the orbits to minimize the differences in elevations at crossovers over a small relatively flat region reduces the rms difference to 0 25 m which is indicative of the optimum precision obtainable over the ice sheets however the precision degrades as the slope of the surface or amplitude of the undulations increases yielding an overall precision of 1 6 m the preliminary contour maps are not corrected for slope induced displacements a 2 m contour map in a region of highest data density illustrates the three dimensional characteristics of some surface undulations

permeability distribution characteristics of protected coal seams during unloading of the coal body abstract based on the engineering practices used in mining the protective coal seam at huainan coalfield in china this study investigates the temporal evolution of three dimensional stress of the protected coal seam by numerical simulation the coal body of the protected coal seam should first be loaded and then unloaded with the three dimensional stress being reduced simultaneously in accordance with the forcing process around the coal body of the protected coal seam permeability experiments were performed under fixed axial displacement with unloading confining pressure faducp and fixed deviatoric stress with unloading confining pressure fdsucp stress paths during the loading stage before the unloading point the permeability and axial strain have a negative exponential relationship however during the unloading stage after the unloading point the permeability and axial strain have a positive exponential relationship using the relationship between permeability and axial strain along with the swelling and deformation of the protected coal seam the permeability distribution characteristics for the protected coal seam was obtained according to the permeability distribution characteristics of the protected coal seam drilling designs for gas drainage can be optimized finally the rationality of the drilling design was verified using gas data measured during mining of the protected coal seam

slope induced errors in radar altimetry over continental ice sheets ranges obtained by radar altimetry from a satellite to a sloping surface on the earth are not measurements of the surface elevation at the subsatellite point instead the reflecting point is displaced upslope from the subsatellite point causing a slope induced error between the true range to the subsatellite point and the indicated range for the seasat 1 altimeter this range error is nearly 80 m for a 0 8 degree slope ice sheet surface slopes are frequently large enough to cause errors of 10 m or more in addition undulations in the surface about a mean slope cause pronounced variations in these errors altimeter measurements of ranges to modeled irregular surfaces are simulated and two correction schemes are used to reconstruct the modeled surfaces from the simulated data the results illustrate the fundamental limitations inherent in single beam radar altimetry for mapping irregular surfaces in the simpler two dimensional case for which cross track slopes are neglected a relocation scheme which constructs a surface consistent with the altimeter ranges removes 85 of the rms error the mean error along profiles of about 75 km or longer is usually reduced more than the rms error an alternate slope correction scheme which uses the local slope to calculate the expected error is less effective in the two dimensional case however over a simulated three dimensional surface where groundtracks are widely spaced and cross track slopes are significant the slope correction method must be used in at least the cross track direction the effectiveness of the three dimensional correction depends on the relative size of the errors caused by the local slope on surface undulations in comparison to the regional slope

co2 ecbm field tests in the ishikari coal basin of japan abstract the feasibility of extracting gas from coal seam while storing carbon dioxide underground was evaluated in japan a co2 ecbm project had begun near the town of yubari on the island of hokkaido in northern japan the primary coal seam of interest was a 5 6 m thick yubari coal seam located at the depth of 900 m a micro pilot test with a single well and multi well co2 injection tests involving an injection and production wells were carried out in the period between may 2004 and october 2007 there were a variety of tests conducted in the injection well including an initial water injection fall off test and a series of co2 injection and fall off tests although gas production rate was obviously enhanced by co2 injection water production rate was not clearly affected by co2 injection several injection tests suggested that injectivity of co2 into the virgin coal seam saturated with water was eventually increased as the water saturation near the injector was decreased by the injected co2 it was estimated that low injectivity of co2 was caused by the reduction in permeability induced by coal swelling n2 flooding test was performed in 2006 to evaluate the effectiveness of n2 injection on improving well injectivity the n2 flooding test showed that daily co2 injection rate was boosted but only temporarily moreover the permeability did not return to the initial value after co2 and n2 were repeatedly injected it was also indicated that the coal matrix swelling might create a high stress zone near to the injection well

swelling induced volumetric strains internal to a stressed coal associated with co2 sorption abstract it is generally accepted that typical coalbed gases methane and carbon dioxide are sorbed both adsorbed and absorbed in the coal matrix causing it to swell and resulting in local stress and strain variations in a coalbed confined under overburden pressure the swelling interactions of gases within the coal matrix and the resultant changes in the permeability sorption gas flow mechanics in the reservoir and stress state of the coal can impact a number of reservoir related factors these include effective production of coalbed methane degasification of future mining areas by drilling horizontal and vertical degasification wells injection of co2 as an enhanced coalbed methane recovery technique and concurrent co2 sequestration such information can also provide an understanding of the mechanisms behind gas outbursts in underground coal mines the spatio temporal volumetric strains in a consolidated pittsburgh seam coal sample were evaluated while both confining pressure and carbon dioxide co2 pore pressure were increased to keep a constant positive effective stress on the sample the changes internal to the sample were evaluated by maps of density and atomic number determined by dual energy x ray computed tomography x ray ct early time images as soon as co2 was introduced were also used to calculate the macroporosity in the coal sample scanning electron microscopy sem and photographic images of the polished section of the coal sample at x ray ct image location were used to identify the microlithotypes and microstructures the co2 sorption associated swelling and volumetric strains in consolidated coal under constant effective stress are heterogeneous processes depending on the lithotypes present in the time scale of the experiment vitrite showed the highest degree of swelling due to dissolution of co2 while the clay kaolinite and inertite region was compressed in response the volumetric strains associated with swelling and compression were between 15 depending on the location although the effective stress on the sample was constant it varied within the sample as a result of the internal stresses created by gas sorption related structural changes sem images and porosity calculations revealed that the kaolinite and inertite bearing layer was highly porous which enabled the fastest co2 uptake and the highest degree of compression

an accurate method for permeability evaluation of undersaturated coalbed methane reservoirs using early dewatering data abstract accurate determination of permeability is critical for the efficient development of coalbed methane cbm reservoirs permeability determination is mainly based on the feature straight line analysis method and the plate fitting method requiring pressure build up data although few analytical methods only require dynamic production data they usually have limitations in application due to complicated solution processes or insufficient consideration of cbm reservoir characteristics this study improves the previous method for determining the permeability of undersaturated cbm reservoirs firstly mathematical models are established for calculating permeability of undersaturated cbm reservoirs with constant permeability and variable permeability due to pulverized coal output blockage matrix shrinkage and stress dependence during the production process secondly the proposed models are validated by numerical simulation analytical and pressure build up test methods finally the models are applied to the muai cbm reservoir in sichuan basin china high middle and low permeability zones of the muai reservoir are classified the results show that when bottom hole pressure and water production data of cbm wells during single phase water flow period are processed by the proposed method and plotted in a rectangular coordinate system a strong linear relationship can be obtained for cbm reservoirs with constant permeability and variable permeability the slope of the fitted equations can be used to determine permeability efficiently and accurately compared with previous methods the new method provides better interpretation results for cbm wells with strongly fluctuating production data since the use of the integration of fluctuating data and cumulative water production reduce the effect of production fluctuation on the results due to its conciseness this method can rapidly calculate reservoir permeability around a large number of production wells and obtain permeability distribution in the reservoir providing guidance for cbm productivity forecasting and development policy adjustment

carbon dioxide gas permeability of coal core samples and estimation of fracture aperture width abstract the structure of cleats or natural fractures in coal seams is an important factor affecting gas permeability coal seam permeabilities have been classified into fracture and matrix flows however fractures in coal seams are generally not regular and usually are discontinuous in addition it is difficult to definitively distinguish the fracture and matrix permeabilities and to measure the fracture widths or other necessary properties of fractures for permeability estimation typically gas permeability has been determined by using the darcy equation for incompressible fluid flow and homogeneous porous media even if the gas flow through fractures has different characteristics with respect to effective stress which is given by the confined stress minus the gas pressure in this study permeability measurements were carried out with a new procedure and methodology to estimate the fracture width in coal core samples this measurement method is an improvement on conventional permeability measuring methods in this research it was identified that fracture permeability can be estimated from measurements of gas flow rates in a limited small area using a pipe attachment on the end of the surface of the core finally a linear equation composed of fracture permeability and matrix permeability is presented

permeability of coals and characteristics of desorption tests implications for coalbed methane production abstract desorption tests conducted before development drilling can predict early production rates of coalbed methane wells in this study results from 136 desorption tests from 30 coreholes are compared to subsequent production histories of surrounding wells coals studied are from the carboniferous pottsville formation in the black warrior basin of alabama usa the investigation indicates that the best predictor for gas production is gas yield at 120 days divided by sample depth this quotient has a higher correlation with actual gas production than other variables examined including original reserves in place gas content of coal generally increases with depth however coal permeability and hence the recovery factor tends to decrease with depth therefore coalbed methane exploration should focus on finding the optimal balance between gas yield and depth commercially successful wells are characterized by coals with high gas yields at shallow depths desorption tests especially of slowly desorbing coals should be run for a minimum of 120 days or else should be extrapolated to that time if tests are terminated earlier

coal bed reservoir simulation with geostatistical property realizations for simultaneous multi well production history matching a case study from illinois basin indiana usa abstract coal seam degasification is a means to recover energy from the methane gas retained in coal and is also a supplementary measure to ventilation which is proven to be one of the most effective ways to reduce methane emissions to a safe level in coal mines reservoir simulation is probably the most effective way to assess the coal seam as a gas reservoir and thereby its fluid storage and flow related properties this objective is achieved by taking advantage of history matching of wellbore production reservoir simulation with multi well history matching is a tedious process as important coal properties that affect wells production characteristics are spatially variable across the seam the common practice is to change various properties at the well blocks during the history matching process and assume that they are uniform across the domain of interest this process however often does not produce realistic and effective results for well or coal reservoir management in this work a multi level approach to coal bed reservoir simulation is demonstrated for a group of coalbed methane wells in the illinois basin producing from the seelyville coal member of the linton formation of the carbondale group pennsylvanian in indiana this approach includes in order gas and water deliverability analyses of wells geostatistical simulation and co simulation and coal bed reservoir simulation it is shown that a reservoir model which utilizes the geostatistical maps of important coal properties is effective for simultaneous history matching of all wells and eliminates the need for guessing and changing values of coal properties at and around individual well blocks this methodology also provides realistic distributions of reservoir parameters and how they change during gas depletion and thus aids in coal seam and coal gas management

implications of the pore pressure and in situ stress for the coalbed methane exploration in the southern junggar basin china abstract the southern junggar basin hosts abundant coalbed methane cbm resources and is expected to be an important cbm production base in china the pore pressure and in situ stress are key factors that influence the exploration and development of cbm however these factors are poorly understood in this area based on extended leak off test data mathematical models were established for pore pressure and in situ stress prediction using regression analysis subsequently the correlations between the pore pressure in situ stress and permeability were analyzed and the stress ratios were investigated finally the forming mechanism of stress zonation in the study area was discussed the results show that cbm reservoirs are generally underpressured and partly overpressured at depths greater than 800 m in the study area the study area is characterized by notable stress zoning the stress regime varies from reverse to strike slip to normal faulting with increasing depth significantly positive correlations exist between the pore pressure maximum principal stress minimum principal stress vertical stress and principal stress difference the minimum horizontal stress is approximately 70 of the vertical stress in the study area a mud weight window varying from the pore pressure to the most likely breakdown pressure is suggested at depths greater than 600 m a relatively high principal stress difference is observed which indicates that deep coals have a relatively high mechanical strength and their permeability can be enhanced by hydraulic fracturing the depth dependent stress zonation is mainly due to the influence of thrusting on the in situ stress which diminishes with increasing depth the compressional stress regime in the shallow area benefits the sealing of faults and the preservation of cbm resources in the study area the deep cbm resources have a development potential the outcomes of this study contribute to the cbm exploration and development in the study area

stress path with depletion in coalbed methane reservoirs and stress based permeability modeling abstract that permeability is a critical parameter dictating the performance of naturally fractured reservoirs like coalbed methane cbm is evident form the available field experimental and permeability modeling information in the literature although modeling is often achieved at the expense of several input parameters the exercise is typically unable to match sudden increases in coal permeability encountered in deep coals after significant depletion this paper is aimed at coupling stress and permeability in order to reduce the number of parameters required for modeling the permeability variation stresses in the reservoir are translated to invariants and stress path of coal is established in octahedral effective stress plane based on a detailed analysis of the stress path of three different coal types a permeability model is presented in terms of stresses alone that is applicable for elastic as well as inelastic deformations of coal the model is validated using pressure dependent permeability experimental data for three coal types along with the geomechanical testing data used to develop the failure envelope the primary implication of the study is improved capability to predict permeability of deep coal deposits given that they are likely to undergo inelastic deformation or shear failure with continued depletion using one parameter only finally realistic constraints on the values of the parameter are provided to enable operators with the necessary tools to use the model for field applications particularly in the new and upcoming cbm fields

a novel approach for modelling coal permeability during transition from elastic to post failure state using a modified logistic growth function abstract although many coal permeability models have been developed in the past decades to describe the coal permeability behaviour under elastic state few of them address the coal permeability change under plastic and post failure state which is often the case within the plastic region adjacent to the excavation face in underground coal mining in this study a methodology to model permeability change from elastic to post failure state is developed by using a modified logistic growth function in conjunction with the classic exponential coal permeability correlation the proposed coal permeability model is a function of mean effective stress which controls the coal compaction and deviatoric effective stress which controls coal fracturing the coal permeability may increase by up to several orders of magnitude after failure and then reaches a plateau during triaxial tests the new model is able to capture this behaviour by matching a set of permeability data in transition from elastic to post failure state under triaxial stress conditions this modelling approach may be used to better understand coal permeability changes associated with mining activities which have applications in the prediction of gas emission risk assessment of coal and gas outburst and analysis of gas drainage near mining openings it is anticipated that the current work may attract more attentions on coal permeability modelling under plastic condition a critical issue for mining safety

petrophysical characteristics of shales with different lithofacies in jiaoshiba area sichuan basin china implications for shale gas accumulation mechanism abstract the petrophysical characteristics of unconventional reservoirs have significant impacts on hydrocarbon storage and transport in this study 26 shale samples with various lithofacies from lower silurian longmaxi shales in the jiaoshiba area of sichuan basin are investigated the pore network morphology porosity and pore structure parameters is comprehensively characterized using scanning electron microscopy helium pycnometry and low pressure gas adsorption techniques permeability coefficients are derived from pulse decay measurements on parallel to bedding samples geological controls on the pore shape porosity pore size pore size distribution and permeability of the shales are analyzed and quantitative relationships with which to estimate the permeabilities and pore throat apertures controlling gas flow are developed three general lithofacies are summarized in view of their mineralogy siliceous argillaceous siliceous mixed and argillaceous shales pore structure parameters porosity specific surface area and pore volume show positive correlations with total organic carbon toc content organic matter hosted pores contribute to approximate 67 of the porosity of siliceous shales while inorganic matter contributes to 55 of the porosity of argillaceous shales klinkenberg corrected permeabilities range between 0 01 and 46 4 d and are controlled by the alignment of clay flakes negative relationships exist between toc content porosity and permeability values siliceous shales generally have the lowest permeabilities whereas the argillaceous shales with developed oriented clay flakes have relatively high permeabilities the seepage pore apertures estimated using the winland correlation range from 9 to 476 nm these values are much higher than those obtained from nitrogen adsorption analysis adsorption pore apertures a power correlation exists between seepage pore apertures and total clay minerals content of shales studied a fracture model sheet is used to predict absolute permeability based on seepage pore apertures the predictions are reasonable and justify the application of the sheet model and seepage pore apertures the siliceous shales are featured by a high toc content large porosity and specific surface area and low permeability which are favorable to gas preservation

analysis and retracking of continental ice sheet radar altimeter waveforms the seasat i radar altimeter data set acquired over both the antarctic and greenland continental ice sheets is analyzed to obtain corrected ranges to the ice surface the radar altimeter functional response over the continental ice sheets is considerably more complex than over the oceans causal factors identified in this complicated response include sloping surfaces undulating ice surfaces with characteristic wavelengths on the same spatial scale as the altimeter beam limited footprint off track reflections and dynamic lag of the altimeter tracking circuit retracking methods using the altimeter return pulse waveforms give range corrections that are typically several meters the entire set of seasat i altimetry over the continental ice sheets is being retracked by fitting a multi parameter function to each waveform many waveforms have double ramps indicating near normal reflections from two distinct portions of the ice surface within the altimeter beam two independent range measurements differing by less than 25 m are obtained from retracking the double ramp waveforms

characteristics and origins of coal cleat a review abstract cleats are natural opening mode fractures in coal beds they account for most of the permeability and much of the porosity of coalbed gas reservoirs and can have a significant effect on the success of engineering procedures such as cavity stimulations because permeability and stimulation success are commonly limiting factors in gas well performance knowledge of cleat characteristics and origins is essential for successful exploration and production although the coal cleat literature spans at least 160 years mining issues have been the principal focus and quantitative data are almost exclusively limited to orientation and spacing information few data are available on apertures heights lengths connectivity and the relation of cleat formation to diagenesis characteristics that are critical to permeability moreover recent studies of cleat orientation patterns and fracture style suggest that new investigations of even these well studied parameters can yield insight into coal permeability more effective predictions of cleat patterns will come from advances in understanding cleat origins although cleat formation has been speculatively attributed to diagenetic and or tectonic processes a viable mechanical process for creating cleats has yet to be demonstrated progress in this area may come from recent developments in fracture mechanics and in coal geochemistry

three dimensional effects of hydraulic conductivity enhancement and desaturation around mined panels changes in the three dimensional hydraulic conductivity field that accompany undermining by a longwall panel are evaluated to define the potential for changes in flow patterns and desaturation relations linking changes in conductivity with the mining induced strain field that develops around the advancing face are defined through the assumption of an equivalent porous medium with the strain partition between fractures and matrix apportioned relative to respective deformation constants model results are validated against behavior recorded at a well instrumented field site replicating the complex steady flow pattern that develops around an advancing longwall face significant hydraulic conductivity increases are restricted to strata at shallow depth ahead of the advancing face and to deep zones behind the face particularly in the caving zone and abutment shear zones these very large increases in conductivity at depth that include the out of section conductivity enhancement that develops along the panel rib exercise primary control on induced water level changes and the potential for saturation zones of desaturation are readily defined with these enabling locations of well vulnerability to water loss to be identified these zones of vulnerability are directly above the panel with their extent potentially modulated by the presence and the location of flatlying layers of low conductivity the complex response recorded in the near face region mandates use of three dimensional models to adequately represent observed behavior

a formalism to compute permeability changes in anisotropic fractured rocks due to arbitrary deformations abstract the liu elsworth 1997 formalism that predicts permeability in deformed fractured rock has been highly useful and successful in numerous engineering situations involving mining coal bed methane and shale gas extraction the formalism assumes the rock s fractures are aligned with three orthogonal axes and that the rock is subjected to strains along these three directions only here we relax these assumptions our proposed formula yields the permeability tensor for an anisotropic arbitrarily fractured rock subjected to arbitrary deformations unlike the original liu elsworth approach our formula may therefore be used confidently in situations involving any stress and strain states including those that are non diagonal and with rocks with any distribution of fracture orientations including those that are non orthogonal this is advantageous in numerical simulation and in mining situations that frequently involve such situations our formula adds no discernible computational load to numerical simulations our proposal agrees with lab and in situ direct and indirect tests of permeability

discrimination of water inrush source and evolution analysis of hydrochemical environment under mining in renlou coal mine anhui province china with the development of mining the hydrochemical environment has changed in the renlou coal mine anhui province china it will be of great significance to coal mine safety production by exploring the temporal and spatial evolution law of hydrochemistry of main water inrush aquifers in renlou coal mine under the effect of mining groundwater samples from three main water inrush aquifers in different periods were collected to analyze their principal components and establish the discrimination model of water inrush source the first and second principal components with eigenvalues greater than 1 were selected in which the first principal component represents sulfate dissolution and pyrite oxidation and the second principal component represents desulfurization and cation alternating adsorption the evolution law of hydrochemical environment was analyzed by combining the distribution characteristics of principal component scores with the spatial location of misjudged groundwater samples the results show that 1 quaternary aquifer is greatly affected by mining and the hydraulic connections with carboniferous limestone aquifer in the central region leads to two misjudged groundwater samples in the discriminant model 2 permian sandstone aquifer is to a certain extent affected by mining which shows a relatively closed hydrogeological environment in most areas of coal mine and an opened environment in some areas with time there are five misjudged samples in the discriminant model due to hydraulic connections with the quaternary aquifer and carboniferous aquifer in some areas 3 carboniferous limestone aquifer is controlled by typical faults and collapse columns and mining has a slight influence on it in the central area there are hydraulic connections with the quaternary aquifer and the permian sandstone aquifer due to the collapse columns which lead to four misjudged groundwater samples in the discriminant model

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

permeability enhancements of borehole outburst cavitation in outburst prone coal seams abstract borehole outbursts eject coal power create cavities and relieve in situ stress while drilling cross measure boreholes into outburst coal seams for pre drainage this paper highlights the borehole outburst cavitation to enhance the permeability of coal for the first time borehole outburst is a mechanical behavior and the outburst trigger and development depend on the status of gas pressure gradient near the opening coal wall it was testified that the high pressure water with 10 15 mpa in pressure and 20 30 m3 h in flow was feasible to remove the failure coal near the opening wall and stimulate a new set of outbursts thus creating bigger cavities and better permeability enhancement furthermore an outburst control system was designed for borehole outburst cavitation the system contains an outburst stimulation subsystem a gas drainage subsystem and a separation subsystem thus ensuring the flow and the separation of outburst ejections within a relatively closed environment the outburst cavity reconstitutes the in situ stress and permeability of the surrounding coal mass leading to a failure zone a plastic zone and a visco elastic zone the failure zone and the plastic zone due to borehole outburst cavitation are both effective in permeability enhancement finally a field test in the no 10 coal seam at the luling mine proved that the sufficient permeability enhancement scope is approximately twelve times of the cavity radius the permeability enhancement promotes the increase of borehole distance from 5 m to 12 m thus reducing 78 7 of boreholes in number for the pre drainage of the no 10 coal seam

preliminary study on the feasibility of co exploitation of coal and uranium abstract both coal and uranium resources are found in the dongsheng area which is located in the northern part of ordos basin china with the coal seam below the uranium deposit in this paper we attempt to study the feasibility of co exploitation of the coal seam and the uranium deposit the permeability evolution in strata around the coal seam are selected as the key parameter indexing the separate and safe concurrent recovery of two deposits and a novel strain dependent permeability model is proposed covering the full deformation range of rock the proposed model is first verified with experimental data with good fitting of the results then an upscaled numerical model is established considering the geology conditions of dongsheng area this is implemented using comsol multiphysics and the interface of comsol with matlab is used to study the damage to the rock mass we show that a compared with the stress dependent permeability model the strain dependent permeability model proposed in this work has the advantage that each strain value is associated with a unique permeability value b the feasibility of co exploitation of the coal seam and the uranium deposit depends on the relative distance between two resources when the caved and fractured zone encroaches on the uranium deposit the coal seam should be abandoned conversely when the uranium deposit lays in the constrained zone the recovery of the coal seam would benefit recovery of the uranium deposit c the feasibility of the co recovery of the two resources can be optimized by the choice of mining approach the top caving mining approach and room and pillar mining approach can reduce the height of the caved and fractured zones and make the co exploitation of the uranium and coal feasible this work provides a new approach to investigate the feasibility of co exploitation of uranium and coal

co2 sequestration potential of charqueadas coal field in brazil abstract although coal is not the primary source of energy in brazil there is growing interest to evaluate the potential of coal from the south of the country for various activities the i2b coal seam in the charqueadas coal field has been considered a target for enhanced coal bed methane production and co 2 sequestration a detailed experimental study of the samples from this seam was conducted at the netl with assistance from the pontificia universidade catolica do rio grande do sul such properties as sorption capacity internal structure of the samples porosity and permeability were of primary interest in this characterization study the samples used were low rank coals high volatile bituminous and sub bituminous obtained from the i2b seam it was observed that the temperature effect on adsorption capacity correlates negatively with as received water and mineral content langmuir co 2 adsorption capacity of the coal samples ranged 0 61 2 09 mmol g the upper i2b seam appears to be overall more heterogeneous and less permeable than the lower i2b seam the lower seam coal appears to have a large amount of micro fractures that do not close even at 11 mpa of confining pressure

laboratory characterisation of fracture compressibility for coal and shale gas reservoir rocks a review abstract unconventional natural gas including coalbed methane and shale gas has become important natural gas resources coal and shale reservoirs are characterised by low porosity and low permeability and difficult for gas production these reservoirs are also considered as fractured reservoirs i e the natural fracture cleat system in coals and bedding direction microfractures in shales permeabilities of these reservoirs are sensitive to stress change during gas production the pressure drawdown significantly increases effective stress and thus decreases the absolute permeability the relationship between permeability and stress is characterised by fracture compressibility which is difficult and costly to be obtained from the field but can be acquired easily from laboratory measurement in this review article the laboratory methods to obtain fracture compressibility were reviewed literature data on fracture compressibility for coals and shales were collated and the relationships between fracture compressibility and pressure stress and rock properties were discussed it is found that fracture compressibility is higher for coals than for shales and the fracture compressibility for proppant supported fracture is even lower than that for the same shale or coal moreover fracture compressibility is variable depending on gas type gas pressure and stress fracture compressibility has no correlation with absolute permeability in general but has a weak positive correlation for the same sample

investigation of increasing hydraulic fracture conductivity within producing ultra deep coal seams using time lapse rate transient analysis a long term pilot experiment in the cooper basin australia abstract ultra low permeability inertinite rich poorly cleated dehydrated overpressured coal seams deeper than 9000 ft 2740 m in the cooper basin of central australia represent a fundamentally different play type to shallow and deep coal seam gas reservoirs proof of concept gas flow was achieved in 2007 four patchawarra formation coal seams were subjected to low proppant concentration slick water hydraulic fracture stimulation within a dedicated vertical wellbore at a depth of 9500 ft 2900 m gas was produced for 81 2 years at a slowly increasing base flow rate averaging 0 1 mmscfd 2 8 mscmd experimental data were gathered for characterising dynamic reservoir behaviour this included multiple extended pressure build up tests the authors have previously investigated the three largest of these using time lapse pressure transient analysis the published results reveal a dominant bilinear flow regime associated with an isolated domain of increasing coal fabric permeability surrounding the hydraulic fracture this paper builds upon the time lapse pressure transient analysis study by specifically investigating conductivity of the hydraulic fracture which the authors postulate to have also increased the hypothesis is tested by applying time lapse rate transient analysis to the two specially designed pressure drawdown tests to atmospheric pressure that immediately follow the first two pressure build up tests of the time lapse pressure transient analysis between the two pressure drawdown tests spaced 586 days apart the wellbore flowed gas continuously for 327 days at an average flowing bottom hole pressure of 580 psig 4 0 mpag hence there was ample opportunity for hydraulic fracture conductivity to change between the tests both pressure drawdown tests were monitored for 24 h using high resolution surface pressure gauges each was initiated from the same surface shut in pressure of 2500 psig 17 2 mpag the flow pressure data are initially used to construct diagnostic plots that clearly identify the hydraulic fracture linear flow regime early in each test immediately after the dissipation of wellbore storage hydraulic fracture properties are then extracted using specialty plots that display rate normalised pseudo pressure difference versus linear superposition time comparing the slopes of the two speciality plot trends indicates that the hydraulic fracture flow property b f k f f increased during the 327 day gas flow period by a factor of 4 this is supported by a 60 increase in the initial gas flow rate from hydraulic fracture storage from 7 5 to 12 0 mmscfd 212 4 to 340 0 mscmd additionally despite a significantly larger volume of hydraulic fracture storage gas being produced to surface during the second test the duration of the hydraulic fracture linear flow regime is less than for the first these observations are consistent with an increasingly more conductive hydraulic fracture over flowback time that allows compressed gas within it to discharge more rapidly

an improved relative permeability model for coal reservoirs abstract in this work the conventional relative permeability model for two phase flow in porous media is improved to describe the relative permeability for coal the fracture geometry is considered through applying the matchstick model instead of the bundle of capillary tubes model which is often used as the conceptual model for conventional porous media to derive the relative permeability model the effect of porosity change on relative permeability for coal is taken into account by introducing a residual phase saturation model and a shape factor as functions of permeability ratio in the improved model the relative permeability is dependent on both the phase saturation and the porosity or permeability change this improved model shows a strong capability to match the experimental data for different coal relative permeability measurements furthermore we evaluate the relative permeability models as a unary function of wetting phase saturation and as a binary function of wetting phase saturation and permeability ratio in a coupled numerical model for water gas flow in coal seams the results illustrate that the relative permeability change due to the porosity change can significantly affect the evolution of wetting phase saturation and the gas production rate

characterization of pore systems in seal rocks using nitrogen gas adsorption combined with mercury injection capillary pressure techniques abstract porous microstructure parameters of seal rock samples originating from different depths within brazilian geological formations were correlated to empirical models which predict the intrinsic permeability mercury injection capillary pressure micp and nitrogen gas adsorption n 2 ga were applied in combination as complementary techniques micp to obtain the porosity values and the size distribution of meso and macropores and n 2 ga associated with the brunauer emmett and teller bet theory to determine the specific surface area s o the barret joyner and hallenda bjh theory was applied to find the size distribution of the micro and mesopores the combination of the micp and n 2 ga curves showed that the samples analyzed present a polymodal pore size distribution psd and a total porosity ranging from 0 33 to 10 44 the s o values measured by n 2 ga were higher than those calculated by micp due to the majority of the samples having a mean pore size of 20 1000 a the intrinsic permeability could also be predicted applying the measured parameters s o psd curves and total porosity in the carman kozeny and series parallel models the ranges of permeability values obtained were 4 09 10 24 4 96 10 21 m 2 and 9 48 10 27 9 14 10 22 m 2 respectively these results were compared with values reported in the related literature and those obtained for four samples submitted to pressure pulse decay permeability pdp tests

coupled flow and geomechanical processes during gas production from coal seams abstract permeability in coal seams is sensitive to stress and pore pressure changes an additional influence on this behaviour is the nature of the coal matrix to shrink with gas desorption and swell with adsorption concise relationships for permeability have been developed which incorporate these mechanisms some of which are derived from a geomechanical basis such as the shi durucan model and the palmer mansoori model while these expressions are attractive approaches for defining coal permeability during gas migration the geomechanical behaviour has been simplified by assuming uni axial strain and constant vertical stress in this paper a coupled numerical model is developed and used to investigate the applicability of these geomechanical assumptions for gas drainage from coal seams the modelling approach involved coupling the existing coal seam gas reservoir simulator simed ii with the geomechanical simulator flac3d while simed ii was used to simulate gas migration in a hypothetical coal seam and a series of production scenarios flac3d simulated the geomechanical response of the coal and the adjacent non coal geological formations to fluid pressure and gas content changes imported from simed ii the simulations which considered a range of property values relevant to the san juan basin found that while the assumption of uni axial strain introduced negligible discrepancy the assumption of constant vertical stress leads to significant differences between the shi durucan permeability estimate and that calculated from the simed flac3d simulation especially at early times during production these differences were a result of the pressure and sorption strain changes induced by production leading to arching of stresses in the vicinity of the production well the mechanism was shown to produce significant differences in the calculated gas rate particularly at early times during production

permeability changes in coal analytical modeling abstract permeability changes can be very large during depletion of coalbed methane wells up to 100 times in the san juan basin analytic models of permeability increase during depletion are accessible easy to use and practical they are more transparent and less complicated than coupled numerical models however there are minor differences between the analytic models and these are reviewed here there are also discrepancies in attempting to match field data which is the final test of a model and these are discussed two different sets of san juan data have been modeled after suppressing the pressure dependent permeability with reasonably consistent reservoir parameters the matching parameters appear to be justifiable the small porosities do not seem to be ruled out by reservoir experts and the changes of elastic parameters including pore volume compressibility with depletion do not seem unreasonable the analytic models are used in reservoir simulators to model and predict injection of greenhouse gases in coal seams only when we can match the permeability increase of coalbed methane wells during depletion will we have the confidence to use the analytic models in reservoir simulators to match or predict sequestration of greenhouse gases such as co2

coal mining impacts on water environs around the barapukuria coal mining area dinajpur bangladesh the present research makes an effort towards awareness of the impact of underground coal mining on water environment around the barapukuria coal mining area dinajpur by direct field investigation questionnaire survey and laboratory analysis for this research the three foremost errands have been mulled over which are the water level data analysis for 10 years from 2001 to 2011 ground water major parametric analysis and the questionnaire survey on the availability of ground water before and after coal mining operation the results of field and laboratory analysis show that the characteristics and concentrations of all the major physical and chemical parameters such as ph ec temperature hco3 no3 so4 2 cl na k mg2 ca2 and fe total are still tolerable for all purposes and also within the standard limit on the other hand the questionnaire survey and water level data analysis confirm almost similar results regarding the depletion of water level the water level has depleted more than 5 m from 2001 to 2011 therefore currently the availability of ground water is normal in the rainy and winter seasons but is slight problematic in the dry season where ground water was available at all times prior to coal mining in the area from these scenarios it is comprehensible that the ground water level moves downwards than earlier because of the excess pumping of water from the mine area besides the natural recharge condition is not enough and somewhere breaks off while some of the mines out areas are subsided consequently the upper part of the water bearing formations aquifer loses its porous and permeable properties resulting water recharging problem which is directly responsible for depleting the ground water level over the area moreover the water levels will also decline relative to the location depth recharge and discharge conditions of the mine both aerially and vertically while there is a typical relationship between the depth of mining and static water level which is water levels will decline more as the mine goes deeper therefore taking into account the current ground water condition and the depth of barapukuria coal mine this research implied that the water level will deplete more in day coming and the water crisis will be more for future thus this research recommends a sustainable guideline for long term planning and also suggests that regular monitoring with time to time more detail qualitative and quantitative assessments of water bodies in the area

the seasat altimeter mean sea surface model gridding techniques are used to combine an 18 day set of seasat altimeter data and two precisely computed seasat ephemerides in order to arrive at global contour maps of the mean sea surface topography the altimeter data have an rms agreement of 111 cm with the ss3 mean sea surface computed by means of the pgs s3 ephemerides and of 70 cm with the ss4 mean surface derived from the pgs s4 ephemerides while comparisons with the gem 10b 1 x 1 deg gravimetric geoid have yielded rms differences of 2 8 m those with a global mean sea surface derived from geos 3 altimeter data show rms differences of 1 3 m and 1 1 m for the cases of the ss3 and ss4 surfaces respectively an ss4 mean sea surface topograph is featured among the study findings presented further improvements in the representation of mean sea surface topography are expected with the development of more accurate gravity models for orbit computation

characterization of underground tunnel water hydrochemical system and uses through multivariate statistical methods a case study from maddhapara granite mine dinajpur bangladesh a quality study of the drained water from maddhapara granite mine underground tunnel was undertaken to study their hydrochemical variations and suitability for various uses employing chemical analysis basic statistics correlation matrix r cluster analysis principal component factor analyses and anova as the multivariate statistical methods the results of chemical analysis of water show the modest variation in their ionic assemblage among different sampling points of the tunnel where ca hco3 type of hydrochemical facies is principally dominated the correlation matrix shows a very strong to very weak positive even negative correlation relationship suggesting the influence of different processes such as geochemical biochemical processes and multiple anthropogenic sources on controlling the hydrochemical evolution and variations of water in the mine area cluster analysis confirms that cluster 1 contains 68 75 of total samples whereas cluster 2 contains 31 25 on the whole the dominated chemical ions of first cluster groups are ca and hco3 suggesting a natural process similar to dissolution of carbonate minerals the second cluster group consisted of cl and so4 2 ions representing natural and anthropogenic hydrochemical process the results of pca fa analysis illustrate that different processes are involved in controlling the chemical composition of groundwater in the mine area the factor 1 loadings showed that ph ec tds na mg chloride and sulfate which have high loading in this factor are expected to come from carbonate dissolution to oxidation conditions one way anova describes the significance of dependent variables with respect to independent variables anova gives us the idea that ec k fetotal so 4 2 as and pb are the most important factors in controlling spatial differences in water quality in this tunnel but different results have been encountered for different independent variables which might be due to dissimilar sources of water from the qualitative analysis it is clear that water quality is not very favorable for aquatic creatures as well as for drinking purposes the water can be used for irrigation purposes without any doubt as sar and rsc analysis provides good results moreover the results of this research confirmed that the application of multivariate statistical analysis methods is apposite to inferring complex water quality data sets with its possible pollution sources at the end this research recommends 1 as water becomes more and more important water treatment plants should be built before the water being used 2 a detailed water step utilization plan should be set beforehand to guarantee tunnel water being used effectively and 3 after the water being used for agriculture elements in crops should be monitored continuously to ensure that ions and compounds that come from the tunnel water are lower than guideline values for human beings health

analytical models for coal permeability changes during coalbed methane recovery model comparison and performance evaluation abstract an in depth comparison of four permeability models palmer and mansoori p m model shi and durucan s d model cui and bustin c b model and the improved p m model developed under uniaxial strain conditions prevailing in coalbed reservoirs was carried out focusing on the relative influence of the matrix shrinkage term over the compaction term in each model the ratio of the coefficients of the two terms is shown to have a direct impact on the magnitude of permeability rebound pressure which is a key parameter controlling the modelled response of coalbed permeability to reservoir drawdown p m model and c b model are found to yield essentially the same permeability rebound pressure which is significantly lower than that given by s d model cleat porosity change in p m model has been shown to be controlled by the effective mean stress indicating that it is essentially a mean stress model as c b model with the introduction of an empirical parameter g

multi branched horizontal wells for coalbed methane production field performance and well structure analysis abstract horizontal wells such as multi lateral and multi branched horizontal wells mbhws have been effectively used in the development of coalbed methane cbm fields especially for coal beds with very low permeability and low compressive strength in which the performance of conventional fracture stimulated vertical wells is ineffective in this study the performance of mbhws in the liulin block of the ordos basin in central north china is analyzed and compared to that of hydraulically fractured vertical wells the field pilot data show that the gas production rate of most fractured vertical wells decreased rapidly after a short period of time far below expectation while the performance of mbhws is satisfactory and relatively stable during 3 years of production a numerical simulation model was established based on the coal reservoir characteristics the productivities of different well types are predicted and compared to the field data the poor performance of the fractured vertical wells is thought to be caused by the early closure of the fractures and proppant embedded in the coal matrix or by a poor proppant delivery inside the fractures the high and stable productivity of the mbhws is attributed to their large drainage volume and to the stability of the wellbores simulation results show that the parameters of a mbhw such as the branch angle length and spacing can be optimized to maximize its productivity though the drilling cost of a mbhw is relatively high in comparison to vertical wells its high and stable productivity can compensate for the drilling cost therefore mbhws are thought to be more appropriate than vertical wells for the successful exploitation of the cbm resource potential in the liulin block and surrounding area

the effects of stress and fracturing on permeability of coal abstract this paper examines the changes in structure and permeability of coal under the stress conditions experienced around longwall faces the effect of fracturing on permeability of coal is discussed and the stress permeability relationship for fractured coal is compared with that of non fractured coal an empirical relationship between stress and permeability for coal is derived from the experimental data which relates the stress permeability behaviour of coal to its rank through the compressibility of the coal itself

the role of spatial variability in coal seam parameters on gas outburst behaviour during coal mining abstract gas outburst is recognized as a potentially fatal hazard to be managed during the mining of gassy coal seams gas outbursts in australian coal mines have been associated with the presence of geological structures in the coal and surrounding rocks having a range of spatial scales from millimetres to metres the contributing mechanisms are influenced by coal gas reservoir variables including gas composition fluid pressure desorption pressure and rate porosity intrinsic permeability and relative permeability they are also influenced by geomechanical variables such as strength in situ stress and mining induced stress the work described here concerns the influence of variability of permeability and strength on outburst behaviour key coal properties are characterised for a field site in an operational underground coal mine and then used in a series of hypothetical simulations of outburst initiation at the field site an array of horizontal holes was cored in seam well tests for permeability and stress were performed in the holes and recovered core was tested in the laboratory for permeability strength and sorption properties for permeability and strength sufficient numbers of measurements were obtained to describe these properties statistically assuming that the measurements are uncorrelated spatially analysis of the cumulative probability distributions of the field and laboratory permeability data shows a strong size effect that cannot be linearly up scaled using monte carlo techniques these statistical descriptions are used to generate realisations of property values across the model grid used in the simulation analyses the stochastic model approach demonstrates that the variability of the permeability and strength fields can lead to both outburst and non outburst outcomes from the same measured input data depending on the corresponding spatial distribution of permeability and strength at the face these results suggest the potential application of this approach as a tool for outburst risk analysis and assessment

deformation and permeability evolution of coals considering the effect of beddings abstract coal belongs to sedimentary rock with obvious bedding planes the existence of bedding weakens the continuity and integrity of coal deformation and permeability anisotropy of coals to a certain extent is attributed to the bedding planes we performed experiments of deformation and permeability evolution of coals under true triaxial stress conditions the experimental results indicate that the volumetric strain and bedding jointly affect permeability an analytical model for stress strain relationship of coals containing beddings is developed to quantify the bedding effects under true triaxial stress conditions in the model the deformation and permeability of the bedding and non bedding are separated through bedding ratio and bedding elastic modulus the bedding ratio is defined as the ratio of the total length of bedding to the total length of coal normal to bedding planes which is a parameter closely related to the porosity the elastic modulus is divided into four parts a bedding elastic modulus and a non bedding elastic modulus in the direction normal to bedding planes and two elastic moduli in the direction parallel to bedding planes the four elastic moduli are calculated by the analytical model the bedding elastic modulus is relatively small which shows that the bedding effect cannot be ignored the permeability is highly sensitive to the bedding of coal with low stress in the direction normal to the bedding the bedding ratio always affects the volumetric strain the permeability of the coal is divided into bedding and non bedding permeability the ratio of the bedding permeability to the total permeability is different due to different stresses in the direction normal to bedding planes which is linking with the compression of bedding a permeability model which can reflect the different effects of beddings and non beddings to the total permeability is proposed and verified

laboratory based mathematical modelling of graded proppant injection in cbm reservoirs abstract the graded proppant injection into a connected cleat system of coal bed methane cbm reservoir allows deeper particle penetration and straining in the remote open cleats yielding higher well productivity index the analytical model for axisymmetric flow has been derived for exponential stress permeability relationship and accounting for permeability variation outside the stimulated zone laboratory proppant injections into coal cores have been performed for different proppant sizes and water salinities it is shown that the proppant suspension based on low salinity water prevents the particle particle and particle coal attraction with the consequent core inlet plugging and external cake formation however low salinity of the injected water may cause mobilisation migration and straining of the natural reservoir fines resulting in high formation damage the interval where salinity is low enough for the rock inlet not to be plugged by the injected proppant and is high enough for large formation damage due to fine migration not to occur is proposed for the cores under investigation the analytical model is tuned from the laboratory data and used for well index prediction ignoring the model matching by the laboratory results causes the overestimation of the incremental productivity index as achieved by the graded proppant injection into coal beds below the fracturing pressure

coupled flow and geomechanical processes during enhanced coal seam methane recovery through co2 sequestration abstract the sensitivity of coal permeability to the effective stress means that changes in stress as well as pore pressure within a coal seam lead to changes in permeability in addition coal swells with gas adsorption and shrinks with desorption these sorption strains impact on the coal stress state and thus the permeability therefore the consideration of gas migration in coal requires an appreciation of the coupled geomechanical behaviour a number of approaches to representing coal permeability incorporate the geomechanical response and have found widespread use in reservoir simulation however these approaches are based on two simplifying assumptions uniaxial strain i e zero strain in the horizontal plane and constant vertical stress this paper investigates the accuracy of these assumptions for reservoir simulation of enhanced coalbed methane through co 2 sequestration a coupled simulation approach is used where the coalbed methane simulator simed ii is coupled with the geomechanical model flac3d this model is applied to three simulation case studies assembled from information presented in the literature two of these are for 100 co 2 injection while the final example is where a flue gas 12 5 co 2 and 87 5 n 2 is injected it was found that the horizontal contrast in sorption strain within the coal seam caused by spatial differences in the total gas content leads to vertical stress variation thus the permeability calculated from the coupled simulation and that using an existing coal permeability model the shi durucan model are significantly different for the region in the vicinity of the production well the coupled permeability is greater than the shi durucan model in the vicinity of the injection well the permeability is less than that calculated using the shi durucan model this response is a function of the magnitude of the strain contrast within the seam and dissipates as these contrasts diminish

modelling permeability for coal reservoirs a review of analytical models and testing data abstract as with other reservoir types permeability is a key controlling factor for gas migration in coalbed methane reservoirs the absolute permeability of coal reservoirs changes significantly during gas production often initially decreasing but then increasing as the reservoir pressure and gas content is drawn down it has also been observed to decrease markedly during co2 injection to enhance coalbed methane recovery in order to predict gas migration models for coal permeability must represent the mechanisms leading to these observed behaviours the permeability of coal reservoirs behaves in a similar fashion to other fractured reservoirs with respect to effective stress decreasing exponentially as the effective stress increases however a unique effect of coal is that it shrinks with gas desorption and swells with adsorption within the reservoir this swelling shrinkage strain leads to a geomechanical response changing the effective stress and thus the permeability modelling coal permeability incorporating the impacts from both effective stress and coal swelling shrinkage dates back about 25 years since then a number of permeability models have been developed in recent years this topic has seen a great deal of activity with a growing body of research on coal permeability behaviour and model development this article presents a review of coal permeability and the approaches to modelling its behaviour as an important part of this the field and laboratory data used to test the models are reviewed in detail this article also aims to identify some potential areas for future work

enhancement of gas drainage efficiency in a special thick coal seam through hydraulic flushing abstract coal seam gas is a kind of precious energy but it also will be a terrible disaster in coal mining if not well controlled underground coal seam gas drainage is one of the most effective methods for lowering the risk of gas disaster however it is difficult to extract gas in many coal seams with low permeability especially in tectonic coal seams which were severely crushed or even pulverized by tectonism the key to improving the gas drainage efficiency is stress relief and permeability increase there are many conventional technologies to improve the gas drainage efficiency of a single coal seam but they are inoperative on the special thick coal seam with a soft mylonitic tectonic coal sub layer at the lower part in this paper to enhance gas drainage in this coal seam we presented a hydraulic flushing gas drainage technology numerical simulation was conducted to analyze the stress evolution plastic damage permeability changes and gas drainage of the coal after hydraulic flushing further abundant field experiments were carried out to investigate and verify the mechanism of stress relief and permeability increase the results show that after adopting the hydraulic flushing gas drainage technology more tectonic coal in the thick coal seam was flushed out and the big hole in the tectonic coal sub layer enlarged the range of stress relief and permeability increase and the gas drainage efficiency increased significantly

hydrodynamics of coalbed methane reservoirs in the black warrior basin key to understanding reservoir performance and environmental issues abstract the black warrior basin of the southeastern united states hosts one of the world s most prolific and long lived coalbed methane plays and the wealth of experience in this basin provides insight into the relationships among basin hydrology production performance and environmental issues along the southeast margin of the basin meteoric recharge of reservoir coal beds exposed in an upturned fold limb exerts a strong control on water chemistry reservoir pressure and production performance fresh water plumes containing na hco3 waters with low tds content extend from the structurally upturned basin margin into the interior of the basin northwest of the plumes coal beds contain na cl waters with moderate to high tds content carbon isotope data from produced gas and mineral cements suggest that the fresh water plumes have been the site of significant bacterial activity and that the coalbed methane reservoirs contain a mixture of thermogenic and late stage biogenic gases water produced from the fresh water plumes may be disposed safely at the surface whereas underground injection has been used locally to dispose of highly saline water wells in areas that had normal hydrostatic reservoir pressure prior to development tend to produce large volumes of water and may take up to 4 a to reach peak gas production in contrast wells drilled in naturally underpressured areas distal to the fresh water plumes typically produce little water and achieve peak gas rates during the first year of production environmental debate has focused largely on issues associated with hydrologic communication between deep reservoir coal beds and shallow aquifers in the coalbed methane fields of the black warrior basin a broad range of geologic evidence suggests that flow is effectively confined within coal and that the thick intervals of marine shale separating coal zones limit cross formational flow

variation in permeability with lithotype and maceral composition of cretaceous coals of the canadian cordillera abstract the permeability of high and medium volatile bituminous cretaceous coal at the megascopic and microscopic scale is investigated utilizing a pressure decay permeameter the order of decreasing permeability with lithotype is bright average 4 1 md banded 0 79 md fibrous 0 50 md banded dull 0 14 md and dull 0 016 md bright coals are the most permeable because of associated macro fracturing cleat banded dull coal permeability generally increases with increasing vitrinite content the lowest permeabilities occur in dull coals with a high mineral and inertinite content fibrous coal has a higher permeability than dull coal of the same rank due to the abundance of macroporous fusinite in fibrous coal dull coal permeability decreases with an increase in rank but these results are overshadowed by compositional variability between samples pressure decay measurements are more reliable for dull lithotypes as these lithotypes do not fracture as easily during sample preparation although measured permeabilities are optimistic due to the relaxation of stress upon exposure of coal to atmospheric pressure the results obtained are representative of the range of permeabilities of coal lithotypes and provide insight into the variability in permeability that may exist at high effective stress in the subsurface

australian topography from seasat overland altimetry retracking of overland returns from the seasat altimeter using algorithms originally developed for recovering elevations over ice has led to the successful recovery of high quality continental topography over australia and other continents cross over analysis both before and after orbit adjustment shows the altimetric data over land to have a 2 3 m quality direct comparison of gridded seasat data with surface data re averaged in the same way shows excellent agreement except where seasat data are sparse due either to poor track spacing or to dropouts caused by loss of tracker lock over steeply sloping ground these results suggest that useful topographic data can be derived from seasat and the more recent geosat altimeters for parts of the world where surface data are few or of poor quality

coupled fluid flow and geomechanics modeling of stress sensitive production behavior in fractured shale gas reservoirs abstract compared with conventional reservoirs gas flow in shale formation is affected by additional nonlinear coupled processes such as matrix fracture deformations we present a fully coupled fluid flow and geomechanics model to accurately characterize the complex production behaviors of fractured shale gas reservoirs the flow equations are discretized using a mimetic finite difference method and poro elasticity equations by a galerkin finite element approximation a unified apparent permeability model is implemented to quantify the combined impacts of the non darcy flow adsorbed layer and pore structure alterations on matrix permeability the discrete fracture matrix dfm model based on a conformal unstructured grid is employed to explicitly represent fractures the nonlinear contact problem between the two fracture surfaces is introduced to describe the fracture mechanics behavior a splitting node technique is used to deal with the discontinuities in the displacement field across the fracture interface under the effects of pressure decline and high confining stresses on the fracture faces proppant compaction and embedment may occur causing fracture closure and thus substantial production loss hence we also develop a comprehensive proppant fracture model which is based on the theories of elasto plastic contact mechanics to capture the complex interactions between proppant and fracture the multiphysics numerical model enables us to investigate which factors have the most influential effect on the gas recovery of shale formations high fidelity numerical solutions are provided to characterize the rate transient signatures in the presence of the different flow and geomechanical mechanisms

volumetric strain associated with methane desorption and its impact on coalbed gas production from deep coal seams the permeability of deep 1000 m 3300 ft coal seams is commonly low for deep coal seams significant reservoir pressure drawdown is required to promote gas desorption because of the langmuir type isotherm that typifies coals hence a large permeability decline may occur because of pressure drawdown and the resulting increase in effective stress depending on coal properties and the stress field during production however the permeability decline can potentially be offset by the permeability enhancement caused by the matrix shrinkage associated with methane desorption the predictability of varying permeability is critical for coalbed gas exploration and production well management we have investigated quantitatively the effects of reservoir pressure and sorption induced volumetric strain on coal seam permeability with constraints from the adsorption isotherm and associated volumetric strain measured on a cretaceous mesaverde group coal piceance basin and derived a stress dependent permeability model our results suggest that the favorable coal properties that can result in less permeability reduction during earlier production and an earlier strong permeability rebound increase in permeability caused by coal shrinkage with methane desorption include 1 large bulk or young s modulus 2 large adsorption or langmuir volume 3 high langmuir pressure 4 high initial permeability and dense cleat spacing and 5 low initial reservoir pressure and high in situ gas content permeability variation with gas production is further dependent on the orientation of the coal seam the reservoir stress field and the cleat structure well completion with injection of n2 and displacement of ch4 only results in short term enhancement of permeability and does not promote the overall gas production for the coal studied

the effect of sedimentary redbeds on coalbed methane occurrence in the xutuan and zhaoji coal mines huaibei coalfield china abstract the lithologies and permeabilities of caprocks have significant effects on coalbed methane cbm preservation to study the controlling effects of redbeds deposits on cbm occurrence in the xutuan and zhaoji coal mines of the huaibei coalfield in china the variations in mineralogical composition microstructure pore structure permeability and diffusion among the redbeds caprock samples neogene clay rock and ordinary coal measure strata sandstone mudstone and siltstone have been investigated the results indicate that redbeds are porous rocks with weaker sealing capabilities than coal strata caprocks allowing cbm to escape from the coal seam the formation of the lithological properties of the redbeds is closely related to their mineralogical composition and diagenesis redbeds are mainly composed of illite 57 64 calcite 25 33 quartz 6 12 muscovite 3 8 and pyrite

permeability variation associated with fines production from anthracite coal during water injection abstract produced water during coal seam gas csg production carries fines a few laboratory studies reported in open media suggest that fines migration can cause variation in coal permeability during water flow however a detailed laboratory study has not been reported so far to explain this variation in coal permeability the characterization of coal and produced fines and water quality may shed light on the root causes of variation in coal permeability this paper presents an experimental study on an anthracite coal sample from a csg field in china to investigate the impact of fines migration on coal permeability proximate petrographic and xrd tests are conducted for a robust characterization of the coal sample the coal sample is first covered with araldite epoxy to minimize any confining stress effect during the flow test then the coal sample is saturated with filtered distilled water which is also injected to the coal sample the injection pressure is kept constant during the flow and the production rate is measured continuously to calculate permeability once the measured permeability stabilizes the injection pressure is increased to see the effect of pressure gradient on fines migration and permeability variation effluent water is collected frequently and analyzed by a laser particle counter to determine the concentration and size of the produced fines the fines are then separated from the water samples using membrane filters and analyzed under a scanning electron microscope and electron dispersive x ray sem edx to investigate their composition and morphology the proximate test shows 9 6 ash air dried basis in the coal sample while the low temperature ashing xrd shows kaolinite 38 5 illite 26 2 and chloride 2 8 clay in the mineral matters production of fines and permeability increases and decreases are observed during water injection the permeability decrease is attributed to the blockage of cleats by fines whereas the permeability increase indicates the mobilization of trapped fines the characterization of produced fines shows that the majority of fines are clay particles with some coal fines also observed

determination of sealing depth of in seam boreholes for seam gas drainage based on drilling process of a drifter abstract in order to determine a reasonable sealing depth for seam gas drainage boreholes an innovative method is proposed to study the geological conditions in the coal seam by monitoring the response of the drift while drilling the boreholes based on the investigation of the in situ tests on the drift we found that the idle pressure of the hydraulic motor which features a rapid increase at the depth of approximately 14 to 16 m from the roadway wall appeared to be suitable for characterizing the coal mass condition then a numerical simulation was performed to investigate the reason for the response of the idle pressure it reveals that high stress and gas pressure are the main factors accounting for the rapid increase of the idle pressure accordingly the rapid increase of idle pressure becomes an indication for high stress and gas pressure next for determining the sealing depth the gas drainage process with different sealing depths was studied via numerical simulation and a simplified theoretical analysis was conducted to comprehend the flow of air from the roadway to the borehole the analysis reveals that the reasonable sealing depth should be several meters larger than the stress concentration area which is indicated in practice by the idle pressure then a sealing depth of 16 to 18 m is determined for the iii1011 mining face in the haizi coal mine this research provide a real time method to determine the sealing depth for in seam boreholes

experimental study of fluid transport processes in the matrix system of the european organic rich shales ii posidonia shale lower toarcian northern germany abstract a laboratory study has been conducted to investigate the transport of gases he ar ch 4 and water in the matrix of lower toarcian posidonia shale samples from three shallow boreholes in northern germany the maturity of the organic matter of the shale samples ranged between 0 53 and 1 45 vitrinite reflectance vr r the measurements were performed at effective stresses ranging between 6 and 37 mpa and a temperature of 45 c the effects of different controlling factors including permeating fluid maturity anisotropy moisture content and effective stress on the fluid conductivity were analyzed and discussed permeability coefficients measured perpendicular and parallel to bedding 3 10 22 to 9 7 10 17 m 2 were within the range previously reported for other shales and mudstones they exhibited a strong dependence on permeating fluid maturity anisotropy moisture content and effective stress the permeability coefficients measured with helium as permeate were consistently higher up to two times than those measured with argon and methane permeability coefficients measured with water were up to three orders of magnitude lower than klinkenberg corrected gas permeability coefficients measured with helium and methane under similar experimental conditions among the samples studied the lowest porosity and permeability coefficients were measured on samples of intermediate thermal maturity 0 88 vr r oil window permeability coefficients he ch 4 measured parallel to bedding were up to more than one order of magnitude higher than those measured perpendicular to bedding permeability coefficients measured with he and ch 4 on a dry sample were up to two times higher than those measured on a sample with the as received moisture content all samples showed a nonlinear reduction in permeability parallel to bedding with increasing effective stress 6 37 mpa the permeability anisotropy and stress dependence of permeability were controlled by the mineralogy

nature and origin of fractures in permian coals from the bowen basin queensland australia a field investigation into the development of cleat and other fractures within permian coal measures was undertaken in the bowen basin of eastern queensland australia to provide predictive information on coal permeability for use in coalbed methane exploration data were collected from five open cut and three underground mines in the southern and central bowen basin covering the upper permian german creek formation and rangal coal measures fractures noted in the coals fall readily into the four categories of faults and shear zones extension and compression related joint sets mining induced fractures and coal cleats four geometric varieties of coal cleats were identified class a orthogonal sets class b sinusoidal sets class c polygonal sets and class d chaotic sets superimposed or overprinted type a sets were also observed locally on a regional scale the cleats typically parallel dip face cleats and strike butt cleats but local departures from this pattern occur in proximity to faults where face and butt cleat directions were often reversed rotated or multimodal field data suggest that coal cleats of classes a b and c were formed by brittle fracturing of the coal during burial wheras class d and superimposed cleats were formed during later compressional events constraints on the timing of deformation events affecting the bowen basin and geochemical data provided by other workers indicate that the cleats were formed within 5 ma of formation of precursor peats but were in many instances modified by later structural events

coal seam porosity and fracture heterogeneity of macrolithotypes in the hancheng block eastern margin ordos basin china abstract heterogeneity in cleats and pore size distribution within coal macrolithotypes bright semi bright semi dull and dull coals determined from the overall relative luster and percentage of bright components impact coalbed methane cbm extraction however few studies have investigated the characteristics of macrolithotypes across multiple length scales here the macrolithotype description was performed for the no 3 5 and 11 coal seams in the hancheng block of ordos basin the cleats were characterized by optical microscopy and x ray computed tomography ct while pores and their connectivity were determined from mercury intrusion porosimetry n 2 and co 2 adsorption and low field nuclear magnetic resonance nmr of water saturated cores evaluated both cleats and pores the ch 4 adsorption capacity of coal samples scraped from the surface of the macrolithotype layers was also determined dull lithotypes semi dull and dull were dominant followed by bright lithotypes bright and semi bright in these seams the bright lithotype coal has a higher microfracture density averaging 339 microcleats per 9 cm 2 m mm better connectivity and a larger fractal dimension averaging 1 7 from optical microscopy the dull lithotype has a lower average microfracture density 272 microcleats per 9 cm 2 and a smaller fractal dimension 1 6 this higher connectivity observation was consistent with x ray ct observations and low field nmr of the water saturated cores and the transition upon water removal from the fractures and larger pores also the ease of mercury injection from cores also supported higher connectivity in the bright lithotypes both the semi bright and semi dull lithotypes have equivalent pore proportions micro to macropores and good apparent connectivity at the nm m scale despite mineral occlusions based on the ch 4 adsorption capacities of the macrolithotypes the bright coal has the greatest gas adsorption capacity due to the largest smaller micropores

modeling anisotropic permeability of coal and its effects on co2 sequestration and enhanced coalbed methane recovery abstract this paper presents an experimental and numerical investigation on anisotropic permeability of coal in order to evaluate the effects of the anisotropic permeability variation on co 2 sequestration in coal and co 2 sequestration enhanced coalbed methane co 2 ecbm recovery from coal seams an alternative permeability model has been developed to improve reservoir simulation for co 2 sequestration in coal and coalbed methane cbm recovery and provide better understanding of the co 2 ecbm process combined with experimental observations the permeability model adopts a discontinuum medium approach by which coal is treated as a discontinuum medium containing anisotropic matrixes and cleats the permeability variations and anisotropic permeability ratios under isotropic net stresses were tested with relatively large coal samples to provide experimental information for model parameterization and validation the model was further incorporated into a 3d coal reservoir model for reservoir simulations to investigate the impact of the anisotropic permeability of coal on coalbed methane cbm recovery the simulations show good agreements with the experimental data revealing that the developed model is superior for describing stress and sorption induced permeability variations in coals compared with commonly used permeability models that treat coal as isotropic media and use constant values for stress dependent parameters the results suggest that anisotropic permeability has significant effects on gas production and co 2 breakthrough time implying that it is a critical parameter in determining well pattern and orientation of horizontal wells

geomechanical properties and permeability of coals from the foothills and mountain regions of western canada abstract static and dynamic geomechanical properties and hydraulic permeability were determined for six large blocks of bituminous coal sampled from active mines in the foothills and mountain regions of western canada testing showed a nonlinear shear strength envelope and brittle failure during triaxial loading the dominant failure mode was along distinct shear planes effective compressive strengths increased from 8 6 mpa to 80 8 mpa with increasing confining pressure the mohr coulomb failure criterion showed that the coals had friction angle of 29 8 to 39 8 and cohesion ranging between 3 4 mpa and 8 0 mpa with increasing confining stress the non linear hoek brown failure envelope was also fit to the data which provided a better estimation of the strength values for static young s modulus ranged from 1119 mpa to 5070 mpa and poisson s ratio ranged from 0 26 to 0 48 also varying with the confining stress concurrent ultrasonic measurements indicate that the values for dynamic moduli are consistently higher than those obtained from quasi static measurements permeability of the coals tested at simulated in situ stress conditions and parallel to bedding surfaces was highly variable ranging from 2 09 md in seam 3 greenhills mine to less than 0 001 md in seam 10 elkview mine this variation may be rationalized based on variations in maceral composition stress history degree of shearing and the mode of deformation brittle or ductile of the coal seams in these tectonically complex regions

stochastic modeling of coal fracture network by direct use of micro computed tomography images abstract characterization of coalbed methane reservoirs is a challenging task because of complex petrophysical properties of coal coal cleat system has a key role in permeability of gas through coalbed previous computational methods for characterization and modeling in coal formations do not account for the actual complexities in cleat systems as they commonly rely on simple statistical properties for describing the fractures in this study unlike the previous methods that try to extract some of the spatial statistical properties the 2d 3d micro computed tomography images are used directly without any simplifications and assumptions the generated models are compared to discrete fracture networks as one of the widely used method for the modeling of such complex systems of coal cleats results show that the utilized algorithm produces visually satisfactory realizations of both coal matrix and cleat system to quantify such similarities autocorrelation functions connectivity with two distinct indices average fracture length and orientation are computed moreover permeabilities and porosities of the reconstructed samples are calculated and compared with the original sample it is demonstrated that the proposed reconstruction method reproduces samples with similar statistical and petrophysical properties but with different patterns of both coal porous region and fracture system finally the proposed method and the dfn realizations are also compared extensively the results of this study can be used for characterization of coal samples with any degree of complexity and heterogeneity by producing several realistic stochastic models consequently petrophysical properties and their corresponding uncertainties can be evaluated more accurately

experimental research on the permeability of high rank coal under a varying stress and its influencing factors abstract in order to investigate the permeability behavior of high rank coal during early depletion of cbm and its influencing factors we collected 14 coal samples from ordos basin in northwest china and determined their air permeability under a varying effective stress of 2 5 20 mpa in laboratory we used effective confining pressure to simulate effective stress it turns out that high rank coal permeability is susceptible to effective stress permeability of coal samples declines exponentially with the rise of effective stress on the whole when effective confining pressure variation ranges from 2 5 mpa to 10 mpa permeability varies dramatically when effective confining pressure rises to above 10 mpa the curve of permeability decline gets gentle and the stress sensitivity becomes relatively weak coal permeability is also affected by coal moisture maceral fracturing and metamorphism degree permeability rises with the increase of vitrinite content and fracturing degree and decreases with the increase of moisture content permeability of wet and fractured coal samples is more sensitive to effective stress than dry intact ones and the irreversible permeability loss rate of fractured coal cores is significant over 80 comparison of tests results of medium and low rank coal samples and high rank ones shows that the stress sensitivity coefficient decreases with the increase of the maximum reflectance of vitrinite if experiencing the same load relief process the irreversible permeability loss rate of low rank coal is less than that of medium and high rank coals

desorption induced shear failure of coal bed seams during gas depletion the recovery of natural gas from coal bed seams is usually accompanied by a significant increase of permeability induced by coal matrix shrinkage and stress relaxation upon gas desorption this advantageous increase in permeability may be impaired sometimes by mechanical failure of the reservoir rock and ensuing production of coal fines near wellbore stress concentration and reduction of lateral stresses are known to promote shear failure during depletion in oil and gas reservoir formations yet conventional analyses have shown limited success in predicting coal failure since other chemo physical mechanisms may be responsible in enhancing the conditions towards mechanical failure in the coal bed reservoir rock we show a set of triaxial experiments involving gas desorption from coal cores under zero lateral strain condition radial stress measured and controlled and constant total vertical stress meant to simulate the stress path during production far from the wellbore co2 is used as surrogate fluid for ch4 the experimental data indicates that desorption can significantly help reduce lateral stress and increasing deviatoric stress until shear failure occurs the results suggest that depletion induced shear failure is much more likely to occur in coal seams than in conventional non sorbing reservoir rocks the adsorptive mechanical coupling turns out to be a key phenomenon in the process numerical simulations at the representative elementary volume scale adopting a double porosity poromechanical model support the experimental findings and permit calculating a critical gas pressure for shear failure to happen this emergent phenomenon is comparable to the outcome of other situations such as mineral dissolution or thermal contraction where shrinkage relaxes lateral stress and acts as an intensifying driver for promoting shear failure within the reservoir rock coupled numerical simulation is needed to include near wellbore effects and validate our findings with actual field observations a thorough understanding of the coupled response of coal seams is necessary to enhance reservoir management and mitigate the effects of coal failure on fines production