Gas Seepage Characteristics Research Articles

Model development and analysis of coal permeability based on the equivalent characteristics of dual-porosity structure

Abstract Mining is a dynamic fracture process of coal and/or rock. The structural failure of coal bodies will change the coal matrix-fracture characteristics and then affect the distribution characteristics of the coalbed methane (CBM). Because of the structural complexity of coal, the coal matrices and fractures will be assumed to the geometries with rule shapes when the gas seepage characteristics in coals are analyzed. The size of the simplified geometries is the equivalent scale of dual-porosity coal structures (i.e. the equivalent fracture width and equivalent matrix scale). In this paper, according to the reasonable assumptions with regarding to dual-porosity coal structures, a new coal permeability evolution model based on the equivalent characteristics of dual-porosity structure (ECDP model) was built and the effect of the equivalent characteristics of dual-porosity structure on the coal permeability evolution law was analyzed. It is observed that if the initial fracture porosity is constant and the equivalent matrix scale increases, the range in which the permeability of coal rises with rising gas pressure increases; if the equivalent fracture width decreases and the equivalent matrix scale is constant, the range in which the permeability of coal rises with rising gas pressure decreases. The ECDP model is more suitable for revealing the evolution law of the coal permeability when large deformations occur in the coal bodies and/or the coal structure is damaged irreversibly, especially during enhancing CBM recovery.

Quantitative analysis of microscopic structure and gas seepage characteristics of low-rank coal based on CT three-dimensional reconstruction of CT images and fractal theory

The pores/fractures are places for gas migration and storage in coal. Their structure complexity will directly influence the pore structure parameters and gas seepage properties of coal. Therefore, we selected 6 low-rank coal samples and quantitatively studied these influences utilizing the high-resolution micro computed tomography (μCT) and fractal theory. The total pore structure and connected pore structure parameters of 6 samples were acquired from three-dimensional (3D) reconstruction of CT images. The fractal dimensions of total pore structures (Df), solid structures (Ds) and connected pore structures (Dc) were calculated using 3D box-counting method. The relationship between Df and porosity was analyzed through the 3D total pore model, and the relationships between Dc and connectivity as well as permeability were analyzed through the 3D connected pore model. The results show that Df, which is between 2.10 and 2.43, could accurately describe the fractal characteristics of coal and the solid structure of coal is not fractal. The porosities of the 6 samples are between 3.33% and 12.18% and their relationship with Df can be defined by a monotonically increasing power function. The larger the Df is, the larger the porosity of the tested coal sample. Dc is between 2.28 and 2.32, the connectivity is between 0.54 and 0.83, and the permeability is in the range of 1.76–11.63 × 10−8 m2 (except samples 1 and 6). Dc is negatively correlated with connectivity and permeability and its relationship with connectivity and permeability can be represented by the quadratic function and monotone decreasing power function, respectively.

Experimental Study on Damage and Gas Migration Characteristics of Gas-Bearing Coal with Different Pore Structures under Sorption-Sudden Unloading of Methane

Coal and gas outburst is one of the most serious hazards in underground mine operations. In this paper, we explored the damage and gas seepage characteristics of gas-bearing coal samples with different pore structures under methane sorption-sudden unloading conditions using a self-developed experimental apparatus. The results show that (1) the deformation of coal samples can be divided into three stages, namely, pressure-increase-induced compression, sorption-induced expansion, and sudden-unloading-induced deformation or failure stage; (2) with the increase of the number of diffusion pores in the coal sample, the amount of gas sorption gradually increases and the expansion and failure of coal samples become more obvious; (3) in the experiment, the damage of coal samples is mainly along the axial direction and rarely along the radial direction; (4) the radial strain is always greater than the axial strain.

Experimental study of gas pressure and effective stress influencing on gas seepage characteristics of bituminous coal in both axial and radial directions

Experimental study of gas pressure and effective stress influencing on gas seepage characteristics of bituminous coal in both axial and radial directions

Experimental study of gas pressure and effective stress influencing on gas seepage characteristics of bituminous coal in both axial and radial directions

The effects of factors including effective stress, gas pressure, and Klinkenberg effect on the axial and radial gas seepage characteristics of bituminous coal, and the strain of the coal in the radial seepage process, are investigated with self-built GFDTA system. The concept of effective stress sensitivity coefficient is proposed, and the sensitivity of the permeability to the effective stress is negligible when the effective stress sensitivity coefficient is 0.01. It is concluded that the effective stress sensitivity coefficient decreases with increasing effective stress, and increases with increasing gas pressure. With the confining pressure and gas pressure increasing, the effect of unloading on the decrease degree of coal axial permeability decreases. In the axial seepage process, the sensitivity of permeability to confining stress is higher than that of axial pressure. In the radial seepage process, the influence of gas pressure on permeability is larger than that of axial pressure. [Received: September 21, 2017; Accepted: December 1, 2017]

Experimental study on the effects of the moisture content of bituminous coal on its gas seepage characteristics

Moisture content is the main factor affecting the occurrence and flow of gas in bituminous coal and restricts the gas permeability of the coal seam, which affects the effectiveness of gas extraction from the coal seam directly. In order to study the influence of moisture content on the gas seepage characteristics of bituminous coal, this paper focused on bituminous coal from the Xutuan coal mine and used a bespoke laboratory unit called a Gas Flow and Displacement Testing Apparatus (GFDTA). The moisture content of bituminous coal was measured, and the axial and radial gas seepage experiments of bituminous coal under different moisture content conditions were carried out. The average original moisture content (1.3%) and the average saturated moisture content (2.4%) of the bituminous coal sample were obtained. It was observed that, with the increase of time, the original moisture content of the coal decreased with a negative exponential function and the wetting moisture content increased with an Exponential Association function. The gas axial seepage experimental results showed that when the moisture content was lower, the coal adsorbed CH4; when the moisture content was higher than Mad, the two fields of gas and liquid are coupled and affect the axial flow of the CH4, decreasing the moisture content. With higher moisture content, the interaction between the two is more readily evident and the diffusion behavior of the CH4 has a greater impact on the moisture content. Axial and radial gas seepage experiments, under the same gas pressure, axial pressure, and confining pressure, revealed that with the increase of moisture content, the axial and radial permeability of bituminous coal first increased and then decreased. This phenomenon is analyzed by the water lock effect, the effects of sorption on gas seepage and moisture content on gas adsorption effects. Among them, critical moisture content of approximately 1% exists when the axial and confining pressures are loaded or unloaded at the same time as well as the axial pressure loading or unloading alone. However, the critical moisture content is about 0.5% under confining pressure loading or unloading alone. In addition, in radial seepage experiments, it was observed that the volumetric strain of the coal decreased with the increase in the moisture content.

Estimation of activity the methane seepage from the Black Sea floor using MODIS images and geosciences data

The publication presents the results of a certain stage of the study: the features of methane gas seepage characteristics in the transition zone of shelf-continental slope in the region of the Dnieper Paleodelts in the northwestern part of the Black Sea are identified, taking into account the data set: information of satellite images and of geological-geophysical (acoustic) data. About 2000 of the MODIS satellite images (product MOD021KM) are acquired in stages over one year period. The image processing was carried out in the GIS environment. It was established that the volume of methane gas seepage on the investigated site is fixed on satellite images and is most pronounced during increased seismotectonic activity (activation of earthquakes in the Black Sea region) - this is due to the tectonic structure of the study area, which is in the regional plan on the edge of the Eastern European platform, and in the local area, is the Dnepr paleo-delta in the zone of decomposition, which is an additional factor for gas outlets here. On satellite images small clouds are observed in cloudy weather or absence of clouds on the background of continuous clouds. This is due to the physical properties of methane, which is (according to geochemical studies) about 99% of the released gas. Methane rising from the bottom of the sea to the surface, and then to the troposphere is much colder than the surface water and warmer than the atmosphere layer, which provokes the formation of condensate over the site with methane gas seepage, and when the weather conditions change, forming a rainy cloud with a sharp drop in temperature, the volume of methane gas of seepage up are much warmer than clouds, forming a cloudy area on a background of continuous clouds. This phenomenon is recorded in various spectral (thermal and infrared) bands in satellite images (NASA – MODIS, NOAA, etc.), methane rising to the lower layers of the atmosphere intensively absorbs thermal radiation of the Earth in the infrared spectral region at a wavelength of 7.66 µm. It enables to determine the methane gas seepage in satellite images for future studies of gas outlets from the bottom of the sea, taking into account the geological information and additional hydrological and weather data. The conducted monitoring of the satellite images showed that during the period of increased seismic activity in this region (in particular under the influence of the zone Vranch), it directly affects the tectonic situation in the northwestern part of the Black Sea and increases the emissions of methane gases from the bottom of the sea, which are then recorded on the MODIS sensor.

Experimental Investigation of Stress Rate and Grain Size on Gas Seepage Characteristics of Granular Coal

Coal seam gas, held within the inner pores of unmineable coal, is an important energy resource. Gas release largely depends on the gas seepage characteristics and their evolution within granular coal. To monitor this evolution, a series of experiments were conducted to study the effects of applied compressive stress and original grain size distribution (GSD) on the variations in the gas seepage characteristics of granular coal samples. Grain crushing under higher stress rates was observed to be more intense. Isolated fractures in the larger diameter fractions transformed from self–extending to inter-connecting pathways at a critical compressive stress. Grain crushing was mainly caused by compression and high-speed impact. Based on the test results of the original GSD effect, the overall process of porosity and permeability evolution during compression can be divided into three different phases: (1) rapid reduction in the void ratio; (2) continued reduction in the void ratio and large particle crushing; and (3) continued crushing of large particles. Void size reduction and particle crushing were mainly attributed to the porosity and permeability decreases that occurred. The performance of an empirical model, for porosity and permeability evolution, was also investigated. The predictive results indicate that grain crushing caused permeability increases during compression, and that this appeared to be the main cause for the predictive values being lower than those obtained from the experimental tests. The predictive accuracy would be the same for samples under different stress rates and the lowest for the sample with the highest proportion of large grain diameters.

Impacts of long-term water inrush on characteristics of coal gas adsorption and seepage

Frequent coal mine flooding is bound to impact not only the stope stress effects of working face, but also the absorption and seepage characteristics of coal mass. In this paper, we examined the effects of long-term water inrush on coal pore structure as well as gas adsorption and seepage characteristics using water-flooded Taoyuan Coal Mine, Anhui Province, China, as an example. Results showed that: 1) after water inrush, the volume, area, and porosity of coal samples, as well as the proportion of micro-pores decreased significantly with the distance from the water inrush site increasing; 2) the amount of gas adsorption of coal samples near the water inrush site was higher than that away from the site; 3) within 13 m from the site, the seepage capacity of coal gas increased; 4) the water seeped into the coal dissolves a mass of inorganic minerals and organic matter and carries them away. [Received: December 26, 2014; Accepted: June 13, 2015]

Impacts of long-term water inrush on characteristics of coal gas adsorption and seepage

Frequent coal mine flooding is bound to impact not only the stope stress effects of working face, but also the absorption and seepage characteristics of coal mass. In this paper, we examined the effects of long-term water inrush on coal pore structure as well as gas adsorption and seepage characteristics using water-flooded Taoyuan Coal Mine, Anhui Province, China, as an example. Results showed that: 1) after water inrush, the volume, area, and porosity of coal samples, as well as the proportion of micro-pores decreased significantly with the distance from the water inrush site increasing; 2) the amount of gas adsorption of coal samples near the water inrush site was higher than that away from the site; 3) within 13 m from the site, the seepage capacity of coal gas increased; 4) the water seeped into the coal dissolves a mass of inorganic minerals and organic matter and carries them away. [Received: December 26, 2014; Accepted: June 13, 2015]

Research on mechanical and gas seepage characteristics of coal-containing gas in the process of unloading confining pressure before and after peak strength

This research studied the effects of a triaxial stress thermal–hydromechanical coal-containing-gas permeameter on mechanical and gas seepage characteristics under different confining pressures, different initial axial stress and different unloading speeds of the confining pressure. The results showed that in the process of unloading the confining pressure before peak strength, the strain (permeability) could be divided into three stages. As the unloading speed was increased the duration of the initial axial stress time would decrease, along with the duration of the axial stress platform. The permeability was closely related to the unloading of the confining pressure and the initial axial stress, which, when increased, would lead to accelerated strain growth. Additionally, the results of the experiment that unloaded the confining pressure after peak strength showed that strain (permeability) could be divided into two stages, and when compared to the experiment done before peak strength, the duration of the axial stress platform was greatly shortened.

Numerical Simulation of Abandoned Gob Methane Drainage through Surface Vertical Wells.

The influence of the ventilation system on the abandoned gob weakens, so the gas seepage characteristics in the abandoned gob are significantly different from those in a normal mining gob. In connection with this, this study physically simulated the movement of overlying rock strata. A spatial distribution function for gob permeability was derived. A numerical model using FLUENT for abandoned gob methane drainage through surface wells was established, and the derived spatial distribution function for gob permeability was imported into the numerical model. The control range of surface wells, flow patterns and distribution rules for static pressure in the abandoned gob under different well locations were determined using the calculated results from the numerical model.

Experimental Simulation of Gas Seepage Characteristics of a Low-Permeability Volcanic Rock Gas Reservoir Under Different Water Saturations

During the gas production process, the permeability of a low-permeability volcanic gas reservoir increases with decreasing water saturation. Additionally, at constant water saturation, the permeability generally increases with decreasing pore pressure if the water saturation is lower than 40%, and the permeability increases slightly with increasing pore pressure if the water saturation is higher than 40%. We undertook this research work in the Kalamay volcanic gas reservoir in the Junggar Basin of China because this gas reservoir has typical seepage characteristics and there are unknown low-permeability volcanic gas reservoirs. Therefore, to study the seepage characteristics of gas in a low-permeability volcanic gas reservoir at different water saturations, we designed experimental methods of measuring the permeability, saturation, and flow rate in the course of core gas displacement to examine the effects of water saturation on gas seepage during the gas production process. In the study, we performed simulation experiments on eight volcanic cores sampled from three wells in the research area and analyzed their gas seepage characteristics at different levels of water saturation. The results reveal that the gas permeability of water-bearing volcanic reservoirs with low permeability values depends mainly on water saturation and pore pressure. Regardless of the slippage effect on the gas, the permeability of the reservoirs depends largely on the degree of original water saturation. In other words, the degree of water saturation directly determines the seepage capability of gas in reservoirs. The critical value of water saturation in the low-permeability volcanic gas reservoirs may be defined as 40%. When the water saturation is higher than 40%, the permeability increases with increasing pore pressure but does not increase substantially. Under these conditions, the gas seepage shows the characteristics of liquid-phase seepage, the gas-water seepage relation is complex, and the resistance to gas seepage is large. All of these make development very difficult and ineffective. When the water saturation is lower than 40%, the permeability increases with decreasing pore pressure, and the seepage characteristics of the gas follow those of a single-phase gas. Under these conditions, the degree of water saturation only influences the permeability of the reservoir and has little impact on the seepage characteristics of the gas.

Experimental Study of Mechanical and Seepage Characteristics of Two Kinds of Coal Samples

Mechanical and seepage characteristics of raw coal samples are the most important research contents of enhancing coal bed methane recovery, but the produce of raw coal sample is very difficult. Based on gas seepage instrument, mechanical and seepage characteristics were performed on normal briquette coal samples and coal samples containing gypsum. The results show that it can increase the cohesion and intensity level by adding gypsum in briquette coal. Porosity and permeability of coal containing gypsum is less than ordinary briquette coal sample. Permeability with axial strain curves of two kinds of coal samples can be fitted with a quadratic function well. In expansion stage, volumetric strain of ordinary coal sample is much smaller than the coal samples containing gypsum. The research has an important guiding significance on enhancing coal bed methane recovery and gateway supporting.

Gas Fractal Seepage Properties Research in Soft-Slicing Coal with Triaxial Pressure System

Pore structure of soft-slicing coal (SSC) is analyzed with physical adsorption method. Several experiment schemes are designed to carry out the tests of gas seepage characteristics of soft-slicing coal at the different confining pressures, gas pressures and axial pressures, with the self-developed gas seepage triaxial pressure experimental system. The typical coal from TianFu coal mine is especially sampled with different designed surface area parameters. The experiment results demonstrate the relations between the seepage velocity, the effective axial feed, and gas pressure. Under the conditions of the constant axial and confining pressure, the seepage velocity increases in an exponent way with gas pressure increasing, the permeability coefficient decreases in negative exponent, and this seepage process shows capacity fractal properties.

Experimental Investigation of the Influence of the Adsorption Gas on the Permeability Characteristics of the Coal Containing Gas

A large number of laboratory experiments on the gas seepage characteristics by the self-developed gas-bearing coal triaxial compression experimental system and conducts the comparative analysis of the similarities and differences of the permeability among CO2, CH4 and N2. The results show that given the condition of constant gas pressure, the permeability of the coal sample decreases with the increase of the confining pressure; under the constant confining pressure, the permeability of the coal sample decreases with the increase of the gas pressure; gases of different adsorbabilities have different permeabilities. The stronger the gas adsorption is, the worse its permeability will be; in the axial loading case, the permeabilities of different gases all reduce firstly and increase afterward, showing the generally V-shaped variation law. The results are of certain theoretical values on the in-depth understanding of the migration law of the gas in coal seams.

Experiment Study on the Relationship between the Height of Capping Layer and the Porosity of Broken Rocks

The gas seepage characteristics of four kinds of broken rocks of different gradations and the relationship between the capping height and the porosity were studied by means of the simulation experiment and the theoretical analysis. The results show that the air flow in the capping layer composed of broken rocks is in the non-Darcy state which can be described as Forchheimer equation and Ergun equation. Moreover, on the basis of the experimental data, the relational expressions between the permeability, the non-Darcy β factor and the porosity were determined. Through the numerical fitting, the equation of the minimum capping height and the porosity was ultimately derived, which provided the basis and the reference for the determination of capping height and the air leakage control of mine.

Experiment Study of Mechanical and Gas Seepage Characteristics of Containing-Gas Coal in the Process of Unloading Confining Pressure

By using triaxial stress thermal-hydro-mechanical coal containing gas permeameter，mechanical and gas seepage characteristics is researched under different confining pressure、different initial axial stress and different unloading speed of confining pressure，the results shows that the increasing of unloading speed of confining pressure and the initial axial stress will shorten the time of the time of axial stress platform，From the aspect of gas permeation，the increasing of unloading speed of confining pressure and intial axial stress will accelerate the increasing of permeability，therefore， reasonable arrangement of roadway and method which can reduce the supporting stress should be proceed to preventing the outburst accident.