key strata characteristics controlling the integrity of deep wells in longwall mining areas abstract the damage of vertical oil gas wells in longwall mining areas is mainly a result of strata movement induced by coal extraction strata with contrasting lithology vary dramatically in their movement and potential for well damage with some special combinations of strata in particular having the greatest potential for damage this study investigates the effects of specific combinations of strata transition structures i topsoil bedrock ii a thin weak interlayer sandwiched above and below by two stiff beds and iii the key stratum sandwiched above and below by two soft beds on the magnitude severity and distribution of various anticipated well deformations and explores the optimal drilling path for wells to maximize well integrity results indicate that 1 effects of various combinations of strata on well deformation lie essentially in the mismatch in the mechanical properties of the strata and weak interfaces with the stratum thickness and vertical distance from the coal seam to the stratum interface also exerting a significant influence 2 wells in the upper part of the topsoil are subject to horizontal tension while the lower part is laterally compressed following the extraction of either one panel or both panels large lateral tensile strains normally arise at the upper part of the hard strata which directly underlie the soft strata large lateral compressive strains are concentrated in the strata within 5 m above and below the coal seam and peak in the seam longitudinal well deformation is dominated by compression in soft strata especially in the upper part of the layer and is dominated by tension in stiff strata within the lower portion in particular vertical compression at the interface is larger below the key stratum and peaks at the interface between the coal seam and its immediate roof well distortions in soft strata are 3 to 5 times those of ones in stiff strata 3 well deformations developing both at interfaces and within layers significantly intensify in the vicinity of the seam an integrated consideration of various deformations of five candidate well paths indicates that the optimal position for well stability is the one that deviates from the pillar centerline and is close to the second mined panel

geostatistical modeling of the gas emission zone and its in place gas content for pittsburgh seam mines using sequential gaussian simulation determination of the size of the gas emission zone the locations of gas sources within and especially the amount of gas retained in those zones is one of the most important steps for designing a successful methane control strategy and an efficient ventilation system in longwall coal mining the formation of the gas emission zone and the potential amount of gas in place gip that might be available for migration into a mine are factors of local geology and rock properties that usually show spatial variability in continuity and may also show geometric anisotropy geostatistical methods are used here for modeling and prediction of gas amounts and for assessing their associated uncertainty in gas emission zones of longwall mines for methane control this study used core data obtained from 276 vertical exploration boreholes drilled from the surface to the bottom of the pittsburgh coal seam in a mining district in the northern appalachian basin after identifying important coal and non coal layers for the gas emission zone univariate statistical and semivariogram analyses were conducted for data from different formations to define the distribution and continuity of various attributes sequential simulations performed stochastic assessment of these attributes such as gas content strata thickness and strata displacement these analyses were followed by calculations of gas in place and their uncertainties in the pittsburgh seam caved zone and fractured zone of longwall mines in this mining district grid blanking was used to isolate the volume over the actual panels from the entire modeled district and to calculate gas amounts that were directly related to the emissions in longwall mines results indicated that gas in place in the pittsburgh seam in the caved zone and in the fractured zone as well as displacements in major rock units showed spatial correlations that could be modeled and estimated using geostatistical methods this study showed that gip volumes may change up to 3 mmscf per acre and in a multi panel district may total 9 bcf of methane within the gas emission zone therefore ventilation and gas capture systems should be designed accordingly in addition rock displacements within the gas emission zone are spatially distributed from an engineering and practical point of view spatial distributions of gip and distributions of rock displacements should be correlated with in mine emissions and gob gas venthole productions

an analytical model of coal wellbore stability based on block limit equilibrium considering irregular distribution of cleats abstract an analytical model of coal wellbore stability is established based on the block limit equilibrium method taking the irregular distribution of cleats into account because coal seam is cut into discontinuous blocks with different shapes and sizes block slip can be treated as the symbol of coal wellbore instability according to the geometric characteristic of blocks blocks surrounding the wellbore can be divided into 3 categorizes type i type ii and type iii to evaluate the block slip the stress distribution near the wellbore is determined based on the continuous medium mechanics and the fracture mechanics using mohr coulomb criterion and limit equilibrium method the critical conditions of block slip are derived by defining the failure coefficient including type i type ii and type iii the proposed model is verified by comparison of the obtained results from the proposed analytical method with the discrete element method as well as the actual drilling it can be proved that the proposed model can predict accurately block slip additionally the effect of drilling fluid pressures on the wellbore stability is specially investigated the results show that with the increasing of drilling fluid pressure the degree of wellbore collapse may be more severe meanwhile the analysis method of reasonable drilling fluid pressure for coal drilling is proposed based on the interval theory combining the slip area of blocks finally the parametric study is focused on investigating the effects of the internal friction angle of face cleats the cohesion of face cleats and the tensile strength of butt cleats on the failure coefficients of blocks

seismic anisotropy reveals the long route of the slab through the western central mediterranean mantle abstract in the south eastern corner of the tyrrhenian basin in the central mediterranean sea a tight alignment of earthquakes along a well defined benioff zone marks one of the narrowest active trenches worldwide where one of the last fragment of the former tethys ocean is consumed seismic tomography furnishes snapshot images of the present day position of this slab and seismic anisotropy allows to reconstruct the past kinematics of the subduction process using seismic anisotropy fast directions as a proxy for the present and past mantle flow we look backward for the seismic traces of the slab motion through the western central mediterranean mantle from the starting locus of subduction toward its present day position the result of combining independent data sets provides a coherent pattern of anisotropy that illustrates an example of slab rollback from its initiation point to its present day position

probabilistic modeling using bivariate normal distributions for identification of flow and displacement intervals in longwall overburden abstract gob gas ventholes ggv are used to control methane emissions in longwall mines by capturing it within the overlying fractured strata before it enters the work environment in order for ggvs to effectively capture more methane and less mine air the length of the slotted sections and their proximity to top of the coal bed should be designed based on the potential gas sources and their locations as well as the displacements in the overburden that will create potential flow paths for the gas in this paper an approach to determine the conditional probabilities of depth displacement depth flow percentage depth formation and depth gas content of the formations was developed using bivariate normal distributions the flow percentage displacement and formation data as a function of distance from coal bed used in this study were obtained from a series of borehole experiments contracted by the former us bureau of mines as part of a research project each of these parameters was tested for normality and was modeled using bivariate normal distributions to determine all tail probabilities in addition the probability of coal bed gas content as a function of depth was determined using the same techniques the tail probabilities at various depths were used to calculate conditional probabilities for each of the parameters the conditional probabilities predicted for various values of the critical parameters can be used with the measurements of flow and methane percentage at gob gas ventholes to optimize their performance

the stability of methane capture boreholes around a long wall coal panel this paper describes the application of computational modelling to the prediction of the stability of methane drainage boreholes during the extraction life of an active long wall coal panel within a united kingdom coal mine the modelling was undertaken to investigate the affects that changes in the roof geology and roadway support system may have on the deformation and closure of the gas drainage boreholes drilled from the tailgate across the goaf of an active long wall panel the paper also outlines the development of an analytical model to estimate the bending deformation axial strain and the rupture of borehole casing due to rock shear this model was used in conjunction with the geomechanical strata models to predict the distance behind the face line at which excessive borehole deformation and closure would occur this method was used to evaluate the optimum roadway support system and spacing between boreholes to maintain effective borehole drainage subject to the range of geological conditions encountered during the panel retreat a good correlation was obtained between the model predictions and the geomechanical and gas drainage data measured at a uk colliery

reservoir rock properties of coal measure strata of the lower monongahela group greene county southwestern pennsylvania from methane control and production perspectives abstract the methane emission rate into an underground mine environment from overburden strata during longwall mining is impacted by reservoir and geomechanical characteristics of the coal measure rocks in the overlying strata as well as the presence of any coal seam the reservoir characteristics and how they change during mining potentially affect the performance of gob gas ventholes which consequently impacts the efficiency of methane control in the mining environment this study presents reservoir and elastic properties of coal measure rocks in the lower monongahela group in greene county southwestern pennsylvania of the northern appalachian basin since the source of methane in this region from underground mining is located between the sewickley coal and the pittsburgh coal a specific emphasis was given to this interval core analyses were performed in the laboratory to determine rock porosity and permeability geophysical logging data gamma density sonic obtained from two exploration boreholes were used for evaluating formation boundaries shale contents log porosities and geomechanical properties of formations permeability was also calculated using density log data and empirical equations and compared with laboratory measurements and slug tests performed in isolated intervals of boreholes the results presented in this study can be used as data sources for reservoir studies related to the production and control of methane

effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysis underground borehole drilling usually causes instability in the surrounding coal due to in situ stress redistribution including stress concentration and stress release however the mechanisms of unloading induced coal strength reduction are still poorly understood the primary objective of this study is to investigate the effect of confining pressure unloading on soft coal strength reduction for borehole stability analysis a series of mechanical tests were conducted on both the traditionally and newly reconstituted coal samples under two different experimental stress paths including conventional uniaxial triaxial compression and triaxial compression with confining pressure unloading the unloading stress path was obtained by analyzing the stress redistribution around a borehole to capture a more accurate coal mechanical response according to our experimental results plastic deformation generated before failure under the unloading stress path is smaller than that generated under the conventional loading stress path furthermore the cohesion of the traditionally and newly reconstituted samples diminishes approximately by 44 77 and 29 66 respectively with confining pressure unloading indicating that there is a significant reduction in coal strength due to confining pressure unloading the mechanism for unloading induced coal strength reduction comes from confining pressure unloading induced increase in shear stress on the fracture surface and a decrease in shear strength this effect increases the shear slipping potential whose driving force generates tension fractures at both ends of the preexisting fractures

a numerical study on optimization of multilateral horizontal wellbore patterns for coalbed methane production in southern shanxi province china abstract china s southern shanxi province contains thick gassy coals of anthracite rank these high methane content higher than 500 ft 3 ton coals necessitate pre mining methane drainage to enhance the health and safety of underground coal workers both traditional vertical wells and multilateral horizontal wells which were shown to perform better have been drilled in the area in order to effectively reduce the gas content of coals however optimization of well drilling patterns is crucial to improve the gas drainage by boosting well performance and resulting in significant additional recovered gas in a cost effective manner in this study exploration borehole data from an undeveloped coalbed methane field in china s qinshui basin was obtained where permeability is expected to range from 0 1 to 10 md reservoir modeling conducted in this study concludes that in the presence of low permeability coals less than 1 0 md well patterns can be optimized to maintain high methane production rates in this work relationships to determine combinations of horizontal lateral spacing and permeability that yield similar production curves were developed in order to maximize production pinnate wells with laterals spaced at 600 ft intervals in reservoirs with 2 0 md permeability performed similar to those with 200 ft lateral intervals in 0 5 md reservoirs additional similar relationships were drawn for reservoirs ranging from 0 1 to 8 0 md furthermore because of the qinshui basin s potential high stress regime pitchfork and pinnate horizontal well patterns were analyzed based on their potential for catastrophic wellbore failure at well junctions and producing well branches analyses concluded that pitchfork wells had a significantly lower risk for significant wellbore loss due to borehole collapse while providing the opportunity for higher gas recovery in reservoirs exhibiting directional permeability

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

hydraulic conductivity changes and influencing factors in longwall overburden determined by slug tests in gob gas ventholes abstract this study presents the results of core log analyses from the exploration boreholes the analyses of face advance rates and the results of downhole monitoring studies performed in gob gas ventholes for calculation of changes in hydraulic properties in the longwall overburden at a mine site in southwestern sw pennsylvania section of northern appalachian basin in the first part of the study coal measure rocks in overburden strata were analyzed and the locations where possible fractures and bedding plane separations would occur were evaluated in the second part the hydraulic conductivities were computed by two different slug test analyses methods using the water level changes measured in gob gas ventholes as longwall face approached hydraulic conductivities were analyzed with respect to the changes in overburden depth the locations of the borehole and mine face advance rates these data were used to interpret the potential productivities of the gob gas ventholes as a result of fracturing and changes in hydraulic conductivities the general results showed that the probability of fracturing and bedding plane separations in the overburden increase between strong and weak rock interfaces also the probability of bedding plane separations increases as the interface is close to the extracted coal seam evaluation of slug tests showed that the hydraulic conductivity developments in the boreholes and their potential production performances are affected by the underground strata and the roof materials in situations where the roof material is stiff and thick the development of high permeability fractures around the borehole will be less results also indicated that borehole location with respect to face position affects the fracturing time and permeability evolution as well greater overburden depths generally cause earlier fracturing as longwall face approaches but eventually result in lower hydraulic conductivities and potentially less effective boreholes increasing mining rates also resulted in generally lower hydraulic conductivities in the overburden the results of this study were intended to improve the interpretation of gob gas venthole performance and to provide better siting of these boreholes

an estimation method for cover pressure re establishment distance and pressure distribution in the goaf of longwall coal mines measurement of deformations and stresses in the goaf of longwall panels is difficult due to the inaccessibility of the waste area this paper proposes a method for estimating the distance to return of the cover pressure and the stress distribution in the goaf of flat lying longwall panels where the caving is bulking controlled in the development of the method it has been considered that the final subsidence on the surface is a result of deformation of the caved material and deformation of the strata over the caved zone i e during both the caving and the compaction of the caved material in this respect the stress strain behaviour of the caved rock pile bending of the lowest uncaved strata depending on seam thickness and residual expansion of fractured strata vertically over the caved zone have been described by using previous field and laboratory investigations from the relations established between these parameters and surface subsidence the re establishment of cover pressure distance has been characterised with a three parameter power function in which the independent variables are depth excavation height bulking factor and compressive strength of the rock fragments it has been found that the cover pressure distance increases exponentially by increasing depth unlike previous findings of investigations in which the cover pressure distance increases as a linear function of depth on the other hand the cover pressure distance for a constant depth and mining height increases if the bulking factor of the rock pile increases and or the strength of rock fragments decreases when the mining height increases the bulking factor of the caved rock pile increases and hence this result with increase in cover pressure distance an estimation of the stress distribution curve for panels which are wide enough for the development of cover pressure has been expressed with a six parameter taylor series polynomial function the general form of the stress development curves found from the proposed method has been verified with curves obtained through numerical models

compaction characteristics of the caving zone in a longwall goaf a review broken rock and coal residual coal plus material from the immediate roof and overlying strata fill in the goaf in an area termed the caving zone due to its high porosity and permeability the caving zone contains gas and water which may have originated from the mined coal seam the adjacent unmined coal seam or from any aquifer or surface river thus studying the compaction characteristics of the caving zone can help understand gas and mine water drainage and identify steps to prevent spontaneous combustion of residual coal the stability characteristics of the caving zone after mining are affect surface subsidence as well as water and gas build up and use the caving zone is a potential underground storage of greenhouse gases therefore the time space relationship of caving zone compaction characteristics in the goaf has become an area for research focus in recent years in this study the formation height determination and compaction characteristics of a caving zone are examined reduction in block size and rearrangement of the fill are the main factors affecting the compaction process as re crushing and rearrangement of the broken coal and rock mass affect the secant modulus and pore size of the caving zone causing the secant modulus to gradually increase and pore size to decrease this in turn affects the macroscopic stress strain curve and seepage characteristics of the caving zone the strength and fracturing mode of the caving blocks are the main factors affecting the re crushing and rearrangement of the caving blocks the applicability and reliability of present research results and research methods are analyzed and the focus areas for future studies are identified using a combination of research methods including theoretical analysis laboratory testing numerical simulation and field measurement the compaction characteristics of a caving zone in longwall goaf can be accurately calculated

failure modes of the surface venthole casing during longwall coal extraction a case study abstract the effective control of gas emissions from longwall faces and gob areas in the coal mining industry in china has become increasingly important drilling underground gas drainage holes from a surface is an effective way to release gas once a coal seam is extracted the rock strata above a gob subside and move after which the gas drainage borehole and casing become damaged due to state changes in the rock in this paper we perform a deformation and failure analysis of a surface venthole casing using the numerical simulation method the numerical investigation is carried out on the casing under compression tension and shear conditions the general factors that affect casing failure are discussed and basic failure modes are summarized the experimental casing of the surface venthole at cheng zhuang colliery czc is also analyzed by numerical simulation the actual deformation and failure of the casing are examined using on site monitoring data to which the numerical simulation results are then compared we deduce that the tensile and shear failures occur at the lower part of the casing or at the bedding plane separation monitoring must be focused on these areas and protective measures should be used the numerical simulation model is simplified given various restrictions the integration of numerical simulation and monitoring technology can serve as an efficient method for the quantitative analysis of failure on surface venthole casings the research concepts presented in this paper and the rules deduced are useful references

height of the mining induced fractured zone above a coal face abstract the development height of a gas conducting fracture zone gfz in the gob overlying strata is crucial to the gas drainage and safe production of a coal mine in order to address the issues of excessive gas concentration and uncertain gfz height in no 7435 face overlying strata of kongzhuang coal mine china the caving characteristics of overlying strata were explored using both physical experiments on similar materials and numerical simulations of particle flow code pfc software and verified each other the relationship of cracks development to porosity changing characteristics was introduced to quantitatively determine the height of the local gfz the quantified gfz heights were compared with those measured using the in situ drilling flow method the results showed that 1 pfc software could accurately simulate the overlying strata caving behaviors thus saving manpower materials and financial resources needed for related physical experiments and 2 the temporospatial distribution characteristics of porosity could be used to forecast gfz height and are of significant importance for determination of gfz overall the conclusions are of engineering significance for accurate arrangement of boreholes for gas drainage and reduction of mine gas disasters

reconciling longwall gob gas reservoirs and venthole production performances using multiple rate drawdown well test analysis abstract longwall mining is an underground mining method during which a mechanical shearer progressively mines a large block of coal called a panel in an extensive area during this operation the roof of the coal seam is supported only temporarily with hydraulic supports that protect the workers and the equipment on the coal face as the coal is extracted the supports automatically advance and the roof strata cave behind the supports caving results in fracturing and relaxation of the overlying strata which is called gob due its highly fractured nature gob contains many flow paths for gas migration thus if the overlying strata contain gassy sandstones or sandstone channels gas shales or thinner coal seams which are not suitable for mining then the mining induced changes can cause unexpected or uncontrolled gas migration into the underground workplace vertical gob gas ventholes ggv are drilled into each longwall panel to capture the methane within the overlying fractured strata before it enters the work environment thus it is important first to understand the properties of the gas reservoir created by mining disturbances and second to optimize the well parameters and placement accordingly in this paper the production rate pressure behaviors of six ggvs drilled over three adjacent panels were analyzed by using conventional multi rate drawdown analysis techniques the analyses were performed for infinite acting and pseudo steady state flow models which may be applicable during panel mining dm and after mining am production periods of ggvs these phases were analyzed separately since the reservoir properties due to dynamic subsidence boundary conditions and gas capacity of the gob reservoir may change between these two stages the results suggest that conventional well test analysis techniques can be applicable to highly complex gob reservoirs and ggvs to determine parameters such as skin permeability radius of investigation flow efficiency and damage ratio the insights obtained from well test analyses can be used for a better understanding of the gob and for designing more effective gob gas venthole systems

longwall horizontal gas drainage through goaf pressure control abstract gas drainage in australian longwall mining is increasingly challenging because of complex gassy conditions multi seam environments beneath previously extracted goafs and environments where drilling a large number of conventional surface vertical gas drainage boreholes is not practical this paper presents an alternative approach using horizontal boreholes for longwall goaf gas drainage recent experience shows that this horizontal drainage approach has the potential to capture substantially large and consistent volumes of gas however there are significant risks involved in this approach including inappropriate borehole placement and insufficient goaf gas capture and control recently extensive research has been carried out by commonwealth scientific and industrial research organisation csiro australia to understand longwall induced geomechanical changes in the surrounding strata goaf gas flow dynamics and the mechanisms of horizontal gas drainage the research results show that horizontal drainage boreholes targeted at the bottom of the roof fractured zone above the caved zone are well connected to the goaf gas drainage boreholes located towards the longwall tailgate are able to create low pressure sinks that protect the workings from goaf gas ingresses by changing goaf gas flow directions in this critical area this insight has resulted in a new longwall gas drainage strategy through controlling goaf gas pressure distribution and flow pattern a conceptual model has therefore been developed to provide simple and practical guidance for horizontal gas drainage design

characterization of two phase flow in a single rock joint abstract an experimental study using a triaxial apparatus was used to analyze the two phase flow patterns in jointed rock specimens rock specimens having a single natural fracture were tested for two phase flow of water and air triaxial tests were conducted to characterize the two phase flow through fractured granite specimens at low confining pressures it was found that for a relatively smooth joint jrc 0 25 mpa gas injection pressures the gas occupied the main part of the fracture and the liquid was able to flow as an unstable film forming an annular flow along the joint when the annular flow developed the mixture flow pattern was independent of the air flow velocity this was due to the fact that once the injected air velocity reached a critical value i e 20 m s water velocity inside the joint was negligible for a given confining pressure and injected water pressure further increase in inlet air pressures developed a single phase air flow with no water flow

the application of a coupled artificial neural network and fault tree analysis model to predict coal and gas outbursts this paper proposes the use of a coupled fault tree analysis fta and artificial neural network ann model to improve the prediction of the potential risk of coal and gas outburst events during the underground mining of thick and deep chinese coal seams the model developed has been used to investigate the gas emission characteristics and the geological conditions that exist within the huaibei coal mining region anhui province china the coal seams in this region exhibit a high incidence of coal and gas outbursts an analysis of the results obtained from an initial application of an fta model identified eight dominant model parameters related to the gas content or geological conditions of the coal seams which characterize the potential risk of in situ coal and gas outbursts the eight dominant model parameters identified by the fta method were subsequently used as input variables to an ann model the results produced by the ann model were used to develop a qualitative risk index to characterize the potential risk level of occurrence of coal and gas outburst events four different potential risk alarm levels were defined safe potential high and strong solutions to the prediction model were obtained using a combination of quantitative and qualitative data including the gas content or gas pressure and the geological and geotechnical conditions of coal seams the application of this combined solution method identified more explicit and accurate model relationships between the in situ geological conditions and the potential risk of coal and gas outbursts an analysis of the model solutions concluded that the coupled fta and ann model may offer a reliable alternative method to forecast the potential risk of coal and gas outbursts