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

Energies ◽  
2018 ◽  
Vol 11 (12) ◽  
pp. 3451 ◽  
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.

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

2019 ◽  
Vol 9 (21) ◽  
pp. 4720 ◽  
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.

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

2020 ◽  
Vol 10 (3) ◽  
pp. 1153 ◽  
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.

Shallow Faults Reactivated by Hydraulic Fracturing: The 2019 Weiyuan Earthquake Sequences in Sichuan, China

2020 ◽  
Vol 91 (6) ◽  
pp. 3171-3181 ◽  
Author(s):  
Maomao Wang ◽  
Hongfeng Yang ◽  
Lihua Fang ◽  
Libo Han ◽  
Dong Jia ◽  
...  
Keyword(s):  
Hydraulic Fracturing ◽  
Sichuan Basin ◽  
Sedimentary Cover ◽  
Fluid Pressure ◽  
Earthquake Hazard ◽  
Induced Earthquakes ◽  
Coupled Flow ◽  
Cascading Effects ◽  
Pressure Diffusion ◽  
Underlying Mechanisms

Abstract Human activity-induced earthquakes are emerging as a global issue, and revealing its underlying mechanisms is essential for earthquake hazard mitigation and energy development. We investigated the relationship between the seismotectonic model and seismic sequences from moderate Mw 4.3 and Mw 5.2 earthquakes that occurred in February and September 2019, respectively, in the Weiyuan anticline of Sichuan basin, China. We found that the Mw 5.2 earthquake ruptured a back thrust of structural wedges and released most aftershocks near the wedge tip. However, the two foreshocks of the Mw 4.3 earthquake sequence occurred in hydrofractured Silurian shale at depth of 2.5–3 km, and the mainshock ruptured the overlying oblique tear fault at a depth of ∼1  km. Hydraulic fracturing in the sedimentary cover of this block may induce earthquakes through fluid pressure diffusion in the Silurian shale and through poroelastic effects on back thrusts within structural wedges, respectively. We assessed the hazard potential of four seismic sources in the Weiyuan block and suggest it is critical to conduct a coupled flow-geomechanics assessment and management on induced seismicity and related cascading effects in the densely inhabited and seismically active Sichuan basin.

The Energy Balance Concept of Hydraulic Fracturing

1968 ◽  
Vol 8 (01) ◽  
pp. 1-12 ◽  
Author(s):  
T.K. Perkins ◽  
W.W. Krech
Keyword(s):  
Energy Balance ◽  
Stress Distribution ◽  
Hydraulic Fracturing ◽  
High Stress ◽  
Fluid Pressure ◽  
Leading Edge ◽  
Energy Balance Equation ◽  
Surface Energies ◽  
New Energy ◽  
Laboratory Models

Abstract This paper explains the concept of a damaged region arising from high stress concentration at the leading edge of a hydraulically created fracture. Approximate stresses near the tip of the crack are calculated, and it is shown that a stable crack shape is possible for which all stresses are finite. A new energy balance is derived incorporating these thoughts, and it is shown that predicted fracturing pressures (using surface energies determined by cleavage) agree with experimental fracturing pressures determined in models. All calculations apply to the case of a nonpenetrating fluid. It is concluded from these studies that in some cases, particularly in small laboratory models, these phenomena significantly affect extension pressures and crack widths. Introduction One of the perplexing questions about hydraulic fracturing that has not been satisfactorily answered is, what pressure is necessary to extend a fracture? For many engineering problems involving failure, it is sufficient to calculate those loading conditions which would bring a stress or elastic strain within the material to a level that could not be tolerated. However, this approach is not useful when considering a sharp-edged crack; calculated stresses and elastic strains always reach infinitely large values near the tip of the crack if fluid pressure is applied all the way to the crack extremity. This difficulty has led to the concept of cohesiveness or absorption of surface energy, implying that behavior near the tip of the crack is not purely elastic. Additional note of the nonideal behavior of rocks will be made in this paper. Then, by simplifying and dealing with an average stress in an inelastic region, the approximate stress distribution around a hydraulic fracture will be calculated and the conditions under which a stable fracture can exist will be shown. A new energy balance equation is then derived incorporating the modified stress picture. Finally, predicted fracture extension pressures are compared with breakdown pressures obtained in laboratory models. This comparison shows that surface energies measured by the cleavage technique are consistent with those values manifested during fracture extension. PROBLEMS OF INDUCED STRESS It will be revealing to consider first the calculated stresses around a penny-shaped line crack, assuming that the rock behaves as a linear, elastic material. Fig. 1 shows the stress distribution in the plane of the crack as calculated with Sneddon's equation. If pressure p is applied uniformly within the crack, then infinitely large tensile stresses would be induced in the rock near the crack tip. Such stresses could not be sustained in a real material. Two approaches have been proposed to explain this dilemma. SPEJ P. 1ˆ

scholarly journals Identification for Abutment Stress by Drilling Cuttings

2021 ◽  
Vol 11 (20) ◽  
pp. 9467
Author(s):  
Jian Tan ◽  
Yunliang Tan ◽  
Zihui Wang ◽  
Yubao Zhang
Keyword(s):  
Stress Concentration ◽  
Coal Seam ◽  
Rock Burst ◽  
Abutment Pressure ◽  
High Stress ◽  
Coal Seams ◽  
Cutting Test ◽  
Key Factor ◽  
Distribution Of Stress ◽  
The Relationship

The concentration of abutment pressure acting on coal seams induced by mining is a key factor to trigger rock burst. Understanding of abutment pressure or stress concentration is fundamental in preventing and controlling rock burst. The influence on abutment pressure fluctuation caused by the inhomogeneity of coal seams needs to be considered, but it is difficult to obtain by the present usual ways such as acoustic transmission, electromagnetic wave transmission, etc. In this article, the relationship between the amount of cuttings drilled in a coal seam and stress level was analyzed by considering the effect of drilling cutting expansion, and the drilling cutting test was carried out in Xinglongzhuang Coal Mine, Shandong Energy Ltd. It is found that the amount of cuttings drilled is positively related to the degree of stress concentration in both the plastic fracture zone and elastic zone. The amount of drilling cuttings is closely related to the roof weighting. In addition, the irregular fluctuation of drilling cuttings is an approximate map of distribution of stress concentration because of the non-uniformity of cracks and other defects in the coal seam. In order to meet the need of rock burst prevention by accurate pressure relief in high-stress zones, enough boreholes are needed.

scholarly journals Case studies of comprehensive gas control method during fully mechanized caving of low permeability ultra-thick coal seams

2020 ◽  
Author(s):  
Bangyou Jiang ◽  
Shitan Gu ◽  
Yunliang Tan ◽  
Guangchao Zhang ◽  
Jihua Zhang
Keyword(s):  
Coal Seam ◽  
Hydraulic Fracturing ◽  
High Efficiency ◽  
Control Method ◽  
Measurement Data ◽  
Low Permeability ◽  
Coal Seams ◽  
Gas Concentration ◽  
High Position ◽  
Gas Control

Abstract Slicing fully mechanized caving mining now is a common high-efficiency mining method for ultra-thick coal seams. However, effective gas control has remained a difficulty in fully mechanized top-coal caving mining of low permeability ultra-thick coal seams. This study focused on mining of the #9-15 coal in Liuhuanggou Coal Mine, Xinjiang Province, China, and combined theoretical analyses and field test results for exploring comprehensive gas control methods for fully mechanized caving of low permeability ultra-thick coal seams. The No. (9-15)06 panel is a top slicing panel of the #9-15 coal with a mining height of 9 m, and the No. (4-5)02 goaf is located on the top of the panel. Through analysis, gas emissions in the No. (9-15)06 panel were mainly sourced from the coal wall, caving of top coal, goaf, and neighboring coal seams. A comprehensive gas control method based on source separation was proposed, which combined gas pre-drainage along the coal seam, high-position drilling on the top, pre-burial of pipes in the goaf, and pressure-balancing ventilation. Considering the poor gas pre-drainage effect for low permeability coal seams, the permeability of the coal seam was enhanced using hydraulic fracturing. According to coal seam and crustal stress distribution characteristics, the arrangement of the boreholes and backward segmented fracturing technology were designed. Field data show that coal underwent remarkable pre-fracturing under hydraulic fracturing. Mean gas pre-drainage from the boreholes was enhanced by nearly 4 times compared to the pre-hydraulic fracturing state. Finally, using the proposed comprehensive control method based on the gas sources, field tests were performed in the No. (9-15)06 panel. Field measurement data demonstrate that gas concentration in the return airflow fluctuated within a range of 0.05%~0.35%, i.e., gas concentration did not exceed the standard. The proposed gas control method can provide insightful reference for the other similar projects.

scholarly journals Case Studies of Comprehensive Gas Control Method during Fully Mechanized Caving of Low-Permeability Ultrathick Coal Seams

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-14
Author(s):  
Bangyou Jiang ◽  
Shitan Gu ◽  
Wenshuai Li ◽  
Guangchao Zhang ◽  
Jihua Zhang
Keyword(s):  
Coal Seam ◽  
Hydraulic Fracturing ◽  
High Efficiency ◽  
Control Method ◽  
Field Tests ◽  
Gas Analysis ◽  
Low Permeability ◽  
Coal Seams ◽  
High Position ◽  
Gas Control

Slicing fully mechanized caving mining is a standard high-efficiency mining method for ultrathick coal seams. However, the effectiveness of gas control has accentuated the difficulty in fully mechanized top coal caving of low-permeability ultrathick coal seams. This study focused on mining the No. 9-15 coal in Liuhuanggou Coal Mine, Xinjiang Province, China. To this aim, the results of theoretical analyses and field tests were combined to explore a comprehensive gas control method for fully mechanized caving of low-permeability ultrathick coal seams. The No. (9-15)06 panel was a top-slicing panel of the No. 9-15 coal with a mining height of 9 m. Gas analysis results revealed that gas emissions in the No. (9-15)06 panel are mainly sourced from the coal wall, caving top coal, goaf, and neighboring coal seams. Based on gas source separation, a comprehensive gas control method was proposed. The proposed method was based on the combination of gas predrainage alongside the coal seam, high-position drilling on the top, preburial of pipes in the goaf, and pressure-balancing ventilation. The permeability and gas predrainage were enhanced by hydraulic fracturing in low-permeability coal seams. According to the characterizations of coal seam and crustal stress distribution, the arrangement of the boreholes and backward-segmented fracturing technology were designed. From the field results, the coal seam presented a remarkable prefracturing under hydraulic fracturing. Besides, the mean gas predrainage from the boreholes was enhanced by four times compared to the prehydraulic fracturing state. Finally, using the proposed comprehensive control method based on the gas sources, field tests were performed in the No. (9-15)06 panel. The measured results demonstrated that gas concentration in the return airflow is fluctuated within a range of 0.05% to 0.35%. The proposed gas control method can provide an insightful reference for other similar projects.

scholarly journals Investigation on the CBM Extraction Techniques in the Broken-Soft and Low-Permeability Coal Seams in Zhaozhuang Coalmine

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-9
Author(s):  
Zhaoying Chen ◽  
Xuehai Fu ◽  
Guofu Li ◽  
Jian Shen ◽  
Qingling Tian ◽  
...  
Keyword(s):  
Coal Seam ◽  
Hydraulic Fracturing ◽  
Coalbed Methane ◽  
Research Result ◽  
Horizontal Stress ◽  
Gas Production ◽  
Vertical Stress ◽  
Low Permeability ◽  
Coal Seams

To enhance the coalbed methane (CBM) extraction in broken-soft coal seams, a method of drilling a horizontal well along the roof to hydraulically fracture the coal seam is studied (i.e., HWR-HFC method). We first tested the physical and mechanical properties of the broken-soft and low-permeability (BSLP) coal resourced from Zhaozhuang coalmine. Afterward, the in situ hydraulic fracturing test was conducted in the No. 3 coal seam of Zhaozhuang coalmine. The results show that (1) the top part of the coal seam is fractured coal, and the bottom is fragmented-mylonitic coal with a firmness coefficient value of less than 1.0. (2) In the hydraulic fracturing test of the layered rock-coal specimens in laboratory, the through-type vertical fractures are usually formed if the applied vertical stress is the maximum principal stress and is greater than 4 MPa compared with the maximum horizontal stress. However, horizontal fractures always developed when horizontal stress is the maximum or it is less than 4 MPa compared with vertical stress. (3) The in situ HWR-HFC hydraulic fracturing tests show that the detected maximum daily gas production is 11,000 m3, and the average gas production is about 7000 m3 per day. This implies that the CBM extraction using this method is increased by 50%~100% compared with traditional hydraulic fracturing in BSLP coal seams. The research result could give an indication of CBM developing in the broken-soft and low-permeability coal seams.

Hydro-splitting of coal seams for their effective degassing preparation through underground wells

2020 ◽  
pp. 15-25
Author(s):  
S. V. Slastunov ◽  
A. V. Ponizov ◽  
A. P. Sadov ◽  
A. M.-B. Khautiev
Keyword(s):  
Coal Seam ◽  
Hydraulic Fracturing ◽  
Underground Mining ◽  
Working Fluid ◽  
Coal Seams ◽  
Natural Systems ◽  
Zone Of Influence ◽  
Complex Technology ◽  
Improved Technology ◽  
Hydrodynamic Effects

The article is devoted to the issues of improving the complex technology of coal seam degassing based on its hydrodynamic treatment. The possible modes of implementation of the introduction of working fluid into the coal seam in the process of its injection through wells drilled from preparatory workings and specially sealed to a certain depth, which ensures minimization of breaks of working fluid into the mine. Studies have been carried out to optimize the main parameters of hydrodynamic effects. Rational values of the effective well length and injection volume are established. The expediency of achieving significant rates of injection of the working agent into the formation for the disclosure of natural systems of cracks in it is shown. The directions of increasing the efficiency of formation degassing are investigated. The effectiveness of the integrated reservoir degassing technology was evaluated based on the factors of increasing methane production from underground reservoir degassing wells drilled into the zone of influence of underground hydraulic fracturing wells, and the total methane removal over the entire period of reservoir degassing. The efficiency of improved technology of hydraulic fracturing in underground mining for factors reducing downtime pollution control equipment, gazoobilnosti stope and the productivity of coal mining in zones of intensive integrated reservoir degassing.

Sign in / Sign up

Export Citation Format

Share Document