深部煤-岩组合体力学特征及裂隙扩展规律

doi:10.11743/ogg20240615

Abstract

Abstract:

The rock mechanical properties of coal seams and their roofs and floors are critical factors influencing the initial cracking and fracture propagation of coal reservoirs during fracturing. However， a limited understanding of these properties has constrained the large-scale commercial production of deep coalbed methane （CBM）. This study aims to investigate the mechanical characteristics and fracture propagation patterns of coal-rock assemblages during fracturing. Based on the acoustic emission signals monitored， we perform uniaxial compressive stress tests and finite element analysis （FEA） on coal-rock assemblages from the Wuxiang block in the east-central Qinshui Basin， with the impacts of different coal/rock thickness ratios， lithologies， and coal-rock interfacial angles on the compressive strength， elastic modulus， features of acoustic emission signals， and breakup characteristics of the coal-rock assemblages delved into. Additionally， we explore the fracture propagation behavior under varying coal-rock configurations. The results indicate that the compressive strength and elastic modulus of the coal-rock assemblages decrease with an increase in the coal-rock ratio in thickness and the coal-rock interfacial angle. The coal-rock assemblages with varying lithologies exhibit elastic modulus and compressive strength ranging from 1.75 to 5.44 GPa and from 11.20 to 20.60 MPa， respectively， with the coal-argillaceous shale and coal-sandstone assemblages displaying the weakest and strongest mechanical properties， respectively. A higher coal-rock thickness ratio tends to exacerbate the breakup of the coal-rock assemblages and results in more fractures within. At a given coal-rock ratio in thickness， the energy released from coal fracturing would continue to diffuse to adjacent layers， culminating in the breakup of the whole coal-rock assemblage. The susceptibility to failure among the various coal-rock assemblages decreases in the order of coal-sandstone， coal-limestone， and coal-argillaceous shale assemblages. The coal-rock assemblages are increasingly prone to failure， instability， and fracturing as the interfacial angle increases. As the coal thickness increases， fractures in the coal-rock assemblages evolve from tensile fissures to X-shaped shear fissures while gradually propagating from within coals to the entire assemblages. In contrast， with an increase in the mechanical strength of the coal-rock assemblages of diverse lithologies， fractures in the assemblages tend to evolve from X-shaped shear fissures to tensile fissures， with the propagation range shrinking from the entire assemblages to within coals. Besides， an increase in the interfacial angle between coals and rocks in the coal-rock assemblages leads to larger X-shaped shear fissures within， exacerbating the assemblage failure.

Key words: uniaxial compressive stress test, acoustic emission signal monitoring, mechanical behavior, numerical simulation, fracture propagation, coal-rock assemblage, Wuxiang block, Qinshui Basin

CLC Number:

TE122.2

Yidong CAI, Qian LI, Fan XIAO, Dameng LIU, Feng QIU. Mechanical characteristics and fracture propagation patterns of deep coal-rock assemblages： A case study of the Wuxiang block， Qinshui Basin[J]. Oil & Gas Geology, 2024, 45(6): 1686-1704.

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样品编号	R_{o， max}/%	工业分析组分含量/%				显微组分含量/%
样品编号	R_{o， max}/%	水分	灰分	挥发分	固定碳	镜质组	惰质组	壳质组
WX	1.76	1.32	11.51	14.79	72.38	95.7	4.3	—

组合体类型	煤-岩厚度比	煤-岩交界面接触倾角/（°）
煤岩+砂岩	1∶2	0
	1∶1	0，15，30
	2∶1	0
煤岩+泥页岩	1∶2	0
	1∶1	0，15，30
	2∶1	0
煤岩+灰岩	1∶2	0
	1∶1	0，15，30
	2∶1	0

岩性	轴向应变/%	杨氏模量/GPa	泊松比	抗压强度/MPa
泥页岩	0.994	5.470	0.340	14.980
灰岩	0.840	5.740	0.410	19.700
砂岩	0.500	5.940	0.500	22.200
干燥煤岩	1.700	3.260	0.420	14.500
饱和煤岩	1.450	1.180	0.390	11.200

煤岩/砂岩厚度比	轴向应变/%	杨氏模量/GPa	泊松比	抗压强度/MPa
1∶2	0.590	10.060	0.500	36.000
1∶1	0.620	8.290	0.428	31.300
2∶1	0.890	5.442	0.324	20.200

组合体	轴向应变/%	杨氏模量/GPa	泊松比	抗压强度/MPa
煤岩-泥页岩	0.970	1.754	0.277	11.200
煤岩-灰岩	0.940	4.732	0.438	15.400
煤岩-砂岩	0.890	5.442	0.500	20.600

Mechanical characteristics and fracture propagation patterns of deep coal-rock assemblages： A case study of the Wuxiang block， Qinshui Basin

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Figures/Tables 22

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交界面接触倾角/（°）	轴向应变/%	杨氏模量/GPa	泊松比	抗压强度/MPa
0	0.760	6.927	0.477	24.600
15	0.320	6.405	0.299	22.100
30	0.930	4.732	0.246	15.300

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