Effects of pulse wave on the variation of coal pore structure in pulsating hydraulic fracturing process of coal seam

In the process of gas extraction, fracture-pore structure significantly influences the macroscopic permeability of coal seam. However, under the multi-field coupling, the mechanism of coal seam fracture-pore evolution remains to be clarified. In this paper, considering the effect of adsorption expansion, the fractal theory for porous media coupled with the multi-field model for coal seam is considered, and a multi-field coupling mechanical model is constructed by considering the influence of fracture-pore structure. Furthermore, the evolution mechanism of fractal dimension with physical and mechanical parameters of coal seam is studied. It is found that the fractal dimension for coal seam is inversely proportional to mining time and in situ stress, proportional to elastic modulus, Langmuir volume constant and Langmuir volume strain constant, and inversely proportional to Langmuir pressure constant. Compared with other factors, Langmuir pressure constant and Langmuir volume strain constant have the significance influence on the fractal dimension for the fracture length.

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Supercritical CO2 Exposure-Induced Surface Property, Pore Structure, and Adsorption Capacity Alterations in Various Rank Coals

Energies ◽

10.3390/en12173294 ◽

2019 ◽

Vol 12 (17) ◽

pp. 3294 ◽

Cited By ~ 2

Author(s):

Zhenjian Liu ◽

Zhenyu Zhang ◽

Xiaoqian Liu ◽

Tengfei Wu ◽

Xidong Du

Keyword(s):

Coal Seam ◽

Pore Structure ◽

Adsorption Capacity ◽

Surface Property ◽

Supercritical Co2 ◽

Co2 Adsorption ◽

Low Pressure ◽

Coal Seam Gas ◽

Adsorption Sites ◽

Co2 Adsorption Capacity

Carbon dioxide (CO2) has been used to replace coal seam gas for recovery enhancement and carbon sequestration. To better understand the alternations of coal seam in response to CO2 sequestration, the properties of four different coals before and after supercritical CO2 (ScCO2) exposure at 40 °C and 16 MPa were analyzed with Fourier Transform infrared spectroscopy (FTIR), low-pressure nitrogen, and CO2 adsorption methods. Further, high-pressure CO2 adsorption isotherms were performed at 40 °C using a gravimetric method. The results indicate that the density of functional groups and mineral matters on coal surface decreased after ScCO2 exposure, especially for low-rank coal. With ScCO2 exposure, only minimal changes in pore shape were observed for various rank coals. However, the micropore specific surface area (SSA) and pore volume increased while the values for mesopore decreased as determined by low-pressure N2 and CO2 adsorption. The combined effects of surface property and pore structure alterations lead to a higher CO2 adsorption capacity at lower pressures but lower CO2 adsorption capacity at higher pressures. Langmuir model fitting shows a decreasing trend in monolayer capacity after ScCO2 exposure, indicating an elimination of the adsorption sites. The results provide new insights for the long-term safety for the evaluation of CO2-enhanced coal seam gas recovery.

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A Generic Method for Predicting Environmental Concentrations of Hydraulic Fracturing Chemicals in Soil and Shallow Groundwater

Water ◽

10.3390/w12040941 ◽

2020 ◽

Vol 12 (4) ◽

pp. 941 ◽

Cited By ~ 2

Author(s):

Dirk Mallants ◽

Elise Bekele ◽

Wolfgang Schmid ◽

Konrad Miotlinski ◽

Andrew Taylor ◽

...

Keyword(s):

Coal Seam ◽

Hydraulic Fracturing ◽

Shallow Groundwater ◽

Solute Concentration ◽

Coal Seam Gas ◽

Transport Modelling ◽

Environmental Concentrations ◽

Modelling Framework ◽

Recharge Rates ◽

Migration Pathways

Source-pathway-receptor analyses involving solute migration pathways through soil and shallow groundwater are typically undertaken to assess how people and the environment could come into contact with chemicals associated with coal seam gas operations. For the potential short-term and long-term release of coal seam gas fluids from storage ponds, solute concentration and dilution factors have been calculated using a water flow and solute transport modelling framework for an unsaturated zone-shallow groundwater system. Uncertainty about dilution factors was quantified for a range of system parameters: (i) leakage rates from storage ponds combined with recharge rates, (ii) a broad combination of soil and groundwater properties, and (iii) a series of increasing travel distances through soil and groundwater. Calculated dilution factors in the soil increased from sand to loam soil and increased with an increasing recharge rate, while dilution decreased for a decreasing leak rate and leak duration. In groundwater, dilution factors increase with increasing aquifer hydraulic conductivity and riverbed conductance. For a hypothetical leak duration of three years, the combined soil and groundwater dilution factors are larger than 6980 for more than 99.97% of bores that are likely to be farther than 100 m from the source. Dilution factors were more sensitive to uncertainty in leak rates than recharge rates. Based on this dilution factor, a comparison of groundwater predicted environmental concentrations and predicted no-effect concentrations for a subset of hydraulic fracturing chemicals used in Australia revealed that for all but two of the evaluated chemicals the estimated groundwater concentration (for a hypothetical water bore at 100 m from the solute source) is smaller than the no-effect concentration for the protection of aquatic ecosystems.

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Fully Coupled Multi-Scale Model for Gas Extraction from Coal Seam Stimulated by Directional Hydraulic Fracturing

Applied Sciences ◽

10.3390/app9214720 ◽

2019 ◽

Vol 9 (21) ◽

pp. 4720 ◽

Cited By ~ 3

Author(s):

Ge ◽

Zhang ◽

Sun ◽

Hu

Keyword(s):

Coal Seam ◽

Hydraulic Fracturing ◽

Hydraulic Fracture ◽

Scale Model ◽

Gas Extraction ◽

Coal Seams ◽

Directional Hydraulic Fracturing ◽

Coal Mine Methane ◽

Multi Scale ◽

Fully Coupled

Although numerous studies have tried to explain the mechanism of directional hydraulic fracturing in a coal seam, few of them have been conducted on gas migration stimulated by directional hydraulic fracturing during coal mine methane extraction. In this study, a fully coupled multi-scale model to stimulate gas extraction from a coal seam stimulated by directional hydraulic fracturing was developed and calculated by a finite element approach. The model considers gas flow and heat transfer within the hydraulic fractures, the coal matrix, and cleat system, and it accounts for coal deformation. The model was verified using gas amount data from the NO.8 coal seam at Fengchun mine, Chongqing, Southwest China. Model simulation results show that slots and hydraulic fracture can expand the area of gas pressure drop and decrease the time needed to complete the extraction. The evolution of hydraulic fracture apertures and permeability in coal seams is greatly influenced by the effective stress and coal matrix deformation. A series of sensitivity analyses were performed to investigate the impacts of key factors on gas extraction time of completion. The study shows that hydraulic fracture aperture and the cleat permeability of coal seams play crucial roles in gas extraction from a coal seam stimulated by directional hydraulic fracturing. In addition, the reasonable arrangement of directional boreholes could improve the gas extraction efficiency. A large coal seam dip angle and high temperature help to enhance coal mine methane extraction from the coal seam.

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