Paсker Sealing–Wellbore Interaction	in Hydraulic Fracturing in Coal Seams

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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Viscoelastic surfactant fracturing fluid for underground hydraulic fracturing in soft coal seams

Journal of Petroleum Science and Engineering ◽

10.1016/j.petrol.2018.06.015 ◽

2018 ◽

Vol 169 ◽

pp. 646-653 ◽

Cited By ~ 8

Author(s):

Yang Feng ◽

Ge Zhaolong ◽

Zheng Jinlong ◽

Tian Zhiyu

Keyword(s):

Hydraulic Fracturing ◽

Coal Seams ◽

Fracturing Fluid ◽

Soft Coal ◽

Viscoelastic Surfactant

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Investigation of Hydraulic Fracturing Crack Propagation Behavior in Multi-Layered Coal Seams

Applied Sciences ◽

10.3390/app10031153 ◽

2020 ◽

Vol 10 (3) ◽

pp. 1153 ◽

Cited By ~ 1

Author(s):

Shirong Cao ◽

Xiyuan Li ◽

Zhe Zhou ◽

Yingwei Wang ◽

Hong Ding

Keyword(s):

Coal Seam ◽

Hydraulic Fracturing ◽

Crack Propagation ◽

Principal Stress ◽

Fracture Propagation ◽

Clean Energy ◽

Burial Depth ◽

Coal Seams ◽

Initiation Pressure ◽

Dip Angle

Coalbed methane is not only a clean energy source, but also a major problem affecting the efficient production of coal mines. Hydraulic fracturing is an effective technology for enhancing the coal seam permeability to achieve the efficient extraction of methane. This study investigated the effect of a coal seam reservoir’s geological factors on the initiation pressure and fracture propagation. Through theoretical analysis, a multi-layered coal seam initiation pressure calculation model was established based on the broken failure criterion of maximum tensile stress theory. Laboratory experiments were carried out to investigate the effects of the coal seam stress and coal seam dip angle on the crack initiation pressure and fracture propagation. The results reveal that the multi-layered coal seam hydraulic fracturing initiation pressure did not change with the coal seam inclination when the burial depth was the same. When the dip angle was the same, the initiation pressure linearly increased with the reservoir depth. A three-dimensional model was established to simulate the actual hydraulic fracturing crack propagation in multi-layered coal seams. The results reveal that the hydraulic crack propagated along the direction of the maximum principal stress and opened in the direction of the minimum principal stress. As the burial depth of the reservoir increased, the width of the hydraulic crack also increased. This study can provide the theoretical foundation for the effective implementation of hydraulic fracturing in multi-layered coal seams.

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Field Investigation of Hydraulic Fracturing in Coal Seams and Its Enhancement for Methane Extraction in the Southeast Sichuan Basin, China

Energies ◽

10.3390/en11123451 ◽

2018 ◽

Vol 11 (12) ◽

pp. 3451 ◽

Cited By ~ 3

Author(s):

Zuxun Zhang ◽

Hongtu Wang ◽

Bozhi Deng ◽

Minghui Li ◽

Dongming Zhang

Keyword(s):

Coal Seam ◽

Hydraulic Fracturing ◽

Sichuan Basin ◽

High Stress ◽

Fluid Pressure ◽

Field Investigation ◽

Stress Sensitivity ◽

Extraction Rate ◽

Coal Seams ◽

Methane Extraction

Hydraulic fracturing is an effective technology for enhancing the extraction of reservoir methane, as proved by field experience and laboratory experiments. However, unlike conventional reservoirs, coal seams had high stress sensitivity and high anisotropy. Therefore, the efficiency of hydraulic fracturing in coal seams needs to be investigated. In this study, hydraulic fracturing was performed at Nantong mine in the southeast Sichuan basin, China. The field investigation indicated that the hydraulic fracturing could significantly enhance the methane extraction rate of boreholes ten times higher than that of normal boreholes in one of the minable coal seams (named #5 coal seam). The performance of hydraulic fracturing in three districts revealed that compared with south flank, the fluid pressure was higher and the injection rate was lower in north flank. The methane extraction rate of south flank was inferior to that of north flank. It indicated hydraulic fracturing had less effect on #5 coal seam in south flank. Moreover, the injection of high-pressure water in coal seams could also drive methane away from boreholes. The methane extraction rate of the test boreholes demonstrated the existence of methane enrichment circles after hydraulic fracturing. It indicated that hydraulic fracturing did act on #5 coal seam in south flank. However, due to the high stress sensitivity of coal seams and the high geo-stress of south flank, the induced artificial fractures in #5 coal seam might close with the decline of the fluid pressure that led to a sharp decline of the methane extraction rate.

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The Calculation Model of Initiation Pressure During Hydraulic Fracturing Process of Perforated Coal Seams

The Open Fuels & Energy Science Journal ◽

10.2174/1876973x20130927003 ◽

2013 ◽

Vol 6 (1) ◽

pp. 30-35 ◽

Cited By ~ 1

Author(s):

Li Yuwei ◽

Ai Chi

Keyword(s):

Hydraulic Fracturing ◽

Pressure Gauge ◽

Calculation Model ◽

Tensile Failure ◽

Coal Seams ◽

Rock Body ◽

Fracture Pressure ◽

Fracturing Process ◽

Pressure Calculation ◽

Initiation Pressure

There are lots of cleats, fractures and many other structure weak planes in coal seams, which make the bullet holes and cleats intersecting. During the hydraulic fracturing process in coal seams, fractures will initiate at coal rock body of borehole wall, and cleats or fractures are different from conventional reservoirs. Thus a new model for initiation pressure calculation during coal seams fracturing should be established. Based on the rock mechanics and elasticity mechanics, and also on network distribution characteristics of coal seam cleats and the space position relationships between the intersected bullet holes and cleats, stress distribution around the bullet holes and at the cleats wall were deducted. The model was established in tensile failure condition. The calculated initiation fracture pressure of Well HX-3 was 10.71MPa. The pressure obtained from bottom hole pressure gauge was 11.24MPa. The relative error was 4.72%. The model could be applied for initiation pressure calculation during hydraulic fracturing process in coal seams. The fractures would initiate at the cleats during fracturing.

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