The control mechanism of deep coal rock microstructure on in situ stress

doi:10.13745/j.esf.sf.2024.6.26

Abstract

Abstract:

The theory of shallow coalbed methane (CBM) exploration and development cannot be directly applied to deep CBM partly due to the effect of in situ stress. In situ stress in deep coal beds restricts the process of CBM adsorption/desorption and seepage, determines the effectiveness of coalbed fissures, and affects the design of horizontal well trajectory. Thus the pattern of in situ-stress change in deep coal seams is of great significance to the exploration and development of CBM. In this paper, taking the Daji block in Ordos Basin as an example, using data from array acoustic logging, microseismic monitoring, and core testing, considering the microstructure types and attitude, boundary stress conditions, and combination of mechanical properties of top and bottom slabs, we established a three-dimensional geomechanical microstructural model of deep coal beds by using ANSYS finite element software to comprehensively analyze the control mechanism of microstructure types and attitute on in situ stress in deep coal beds. Results show that with smoother microstructure the stress distributes more uniformly; conversely, the stress concentrates more easily. The influence of the attitute of microstructure on in situ stress in coal seams is mainly as follows: as the curvature of the microstructure increases, the differential horizontal stress at the bending point increases, and the minimum principal stress increasingly concentrates around the bending point. According to cross-simulation between microstructure type and mechanical properties of coal seam, lithology of top/bottom slabs, and boundary stress conditions, in situ stress under positive curvature is positively correlated with Poisson’s ratio and negatively correlated with Young’s modulus, whereas the opposite is true under negative curvature. Compared to with sandstone top slab, the magnitute of in situ stress in coal seams with limestone top slab is more significantly affected by the change of microstructure type. The magnitude of the regional stress has relatively small influence on in situ stress in deep coal seams. The research results provide an useful reference for the genetic analysis of in situ stress in deep coal beds, and for the efficient development of coalbed methane and the practice of geoengineering integration.

Key words: microstructure, in situ stress, deep coal rock, Daji Block, finite element simulation

CLC Number:

P618.13

TANG Zhitan, LIU Jingshou, YAN Xia, FENG Yanqing, JIANG Shu, ZHANG Binxin, ZHANG Guanjie, FU Yiming. The control mechanism of deep coal rock microstructure on in situ stress[J]. Earth Science Frontiers, 2024, 31(5): 344-357.

Figures/Tables 20

Fig.1 Location and stratigraphic column diagram of the research area. Modified after [24].

Fig.2 Rockbed microstructure types. Modified after [10].

Fig.3 Three-dimensional models of microstructure types

Fig.4 Stress simulation based on logging curves. (a) Rock mechanical strength logging analysis based on array acoustic logging. (b) Current geostress frequency distribution map of coal seams in the research area. (c) Numerical simulation scheme for boundary stress.

Fig.5 Geomechanical model and stress distribution diagram for the study area. (a) Stress loading in the geomechanical model. (b) 3D stress distribution map of coal seam.

Fig.6 Mechanical property simulation based on logging curve. (a) Rock mechanics parameter logging interpretation based on array acoustic logging. (b) Histograms of coal seam Poisson’s ratios and Young’s modulus. (c) Numerical simulation scheme for rock mechanics experiments.

Fig.7 Distribution of minimum horizontal principal stress (left) and stress difference (right) in reservoirs under different microstructure types

Fig.8 Distribution of minimum horizontal principal stress and stress difference in reservoirs under different attidutes of microstructure

Fig.9 Fitting curves showing the influence of mechanical parameters on stress with same stress type under different microstructure types

Table 1 Formulas and R-squared values relating Poisson’s ratio and minimum horizontal stress of coal seam under different microstructure types

微构造类型	公式	R²
类型1	y=-2.031 9x+7.579 9	0.995 6
类型2	y=-2.027x+3.973 2	0.995 9
类型3	y=-0.837 7x+4.070 7	0.791 4
类型4	y=-1.634 4x+1.240 7	0.994 6
类型5	y=0.425x+1.573	0.994 5

Table 2 Formulas and R-squared values relating Young’s modulu and minimum horizontal stress under different microstructure types

微构造类型	公式	R²
类型1	y=5×10^-11x+0.344 2	0.968 5
类型2	y=1×10^-10x-3.819	0.917 2
类型3	y=-1×10^-10x+2.784 2	0.972 3
类型4	y=2×10^-10x-6.073 7	0.955 7
类型5	y=-2×10^-10x+5.406	0.966

Fig.10 Fitting curves showing the influence of mechanical parameters on stress with variable stress type under different microstructure

Table 3 Formulas and R-squared values relating microstructure type and minimum horizontal stress under different Young’s modulus

杨氏模量	公式	R²
10	y=0.758 9x²-4.193 7x+4.195 8	0.258 3
20	y=0.796 1x²-4.718 6x+5.292 2	0.450 8
25	y=0.646 1x²-2.932 8x+4.323 67	0.616 3
30	y=0.427 8x²-2.932 8x+4.236 7	0.896 5
35	y=0.161 1x²-1.574 1x+3.250 7	0.705 9

Table 4 Formulas and R-squared values relating microstructure type and minimum horizontal stress under different Poisson’s ratios

泊松比	公式	R²
0.275	y=0.390 9x²-3.272 8x+8.171 7	0.525 7
0.300	y=0.763 5x²-5.060 3x+7.978	0.568 3
0.330	y=0.874 3x²-5.081 7x+5.404	0.353
0.350	y=0.966x²-5.111 4x+3.404 5	0.345 2
0.365	y=1.047 9x²-5.143 2x+1.7	0.371 7

Fig.11 Fitting curves showing the effect of top/bottom plate combination and microstructure type on stress

Table 5 The formula and fitting degree of the microstructure type and the minimum horizontal stress of the coal seam under different combinations of roof and bottom plate

顶底板	公式	R²
顶砂底砂	y=-0.469 1x²+1.710 7x+2.057	0.351 6
顶灰底砂	y=-0.850 4x²+4.527 3x-4.158 6	0.344
顶灰底灰	y=-0.862x²+5.645 2x-8.374	0.402 4
顶砂底灰	y=-0.62x²+2.588x+0.086	0.356 4

Table 6 Formulas and R-squared values relating roof/floor combination and minimum horizontal stress under different microstructure types

微构造类型	公式	R²
类型1	y=2.392x²-12.659x+1.966	0.878 4
类型2	y=1.368x²-7.243 2x+10.965	0.860 1
类型3	y=0.413 4x²-2.419 4x+1.431	0.985 2
类型4	y=0.612 5x²-3.301 5x+7.672 5	0.968 9
类型5	y=0.102 5x²-0.811 5x-1.852 5	0.249 1

Table 7 The formula and fitting degree of the maximum horizontal stress of the boundary and the minimum horizontal stress of the coal seam under different microstructural types

微构造类型	公式	R²
类型1	y=-0.005x²-0.007x-1.14	0.994 3
类型2	y=0.084x+2.058	0.966 6
类型3	y=-0.010 8x²-0.013 6x-0.870 4	0.994 7
类型4	y=0.010 7x²-0.027 3x+3.862	0.975 6
类型5	y=-0.027 1x²-0.037 1x-2.346	0.994 7

Fig.12 Fitting curves showing the influence of boundary stress condition and microstructure type on stress under different microstructure types

Table 8 Formulas and R-squared values relating minimum horizontal stresses at boundary and coal seam under different microstructure types

微构造类型	公式	R²
类型1	y=-0.005 7x²-0.013 7x-1.108	0.952 1
类型2	y=0.008 6x²+0.024 6x+2.14	0.947
类型3	y=0.004 3x²-0.001 7x-1.114	0.958 1
类型4	y=0.032 1x²+0.092 1x+3.15	0.947
类型5	y=0.004 3x²+0.020 3x-2.958	0.939 3

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