Experimental study on the mechanical properties of coal-like materials for hydraulic fracturing simulation
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摘要:
煤层水力压裂模拟实验是研究煤矿井下水力压裂煤岩体致裂增透、弱化机理的有效手段。然而，大尺寸煤岩体原位保真取样技术不成熟，已有大尺寸煤样块多取自卸压区，其在运输和制备过程中会发生二次破坏，导致实验结果失真。因此，使用煤岩体相似材料代替大尺寸原煤执行室内水力压裂模拟实验成为一种可行的选择。煤岩体相似材料试样的力学特性是影响水力压裂效果最重要因素。为精确表征煤岩体相似材料的基础力学特征，选择煤粉、水泥、石膏、砂子为相似材料，设计制作7种配比试样，进行超声波与力学特性耦合响应规律研究。结果表明：煤岩体相似材料试样超声波波速(P波和S波)和强度(单轴抗压和抗拉强度)随着密度的增大而增大，随着孔隙率的增大而减小；相似材料对于超声波波速、强度和密度增大幅度的影响为水泥>砂子>石膏，孔隙率正相反；相似材料水泥和石膏分别在调节试样强度和变形特性方面起主要作用；根据超声波P波波速与强度之间的二次多项式数学模型，通过测定超声波P波波速可提前预测试样的强度；试样力学参数可调整范围大，通过改变相似材料配比可以调整试样的力学性质，精确模拟煤岩体，且试样制作方法简单。此研究可为煤层水力压裂模拟用煤岩体相似材料力学特征相似设计提供依据，促进矿井瓦斯防治技术的发展，具有广泛的应用价值。
Abstract:Simulation experiments on hydraulic fracturing of the coal seam in the laboratory are an effective method to study the mechanism of fracturing and enhancing permeability of coal-rock mass by hydraulic fracturing in coal mines. However, the technology in-situ sampling for large-size coal mass is immature, and the existing large-size coal samples are mostly taken from the stress-relaxation area, which can be further damaged in the transportation and preparation processes, causing large dispersion in test results. Therefore, The use of coal-like materials is a viable option to replace large-size raw coal for hydraulic fracturing simulation experiments. The mechanical properties of coal-like specimens are the most important factor affecting the effectiveness of hydraulic fracturing. In this paper, in order to accurately characterize “the basic mechanical properties of coal-like materials”, coal powder, cement, gypsum, and sand are used to make coal-like samples with seven ratios for examining the coupling response law of ultrasonic and mechanical properties. The experiments results include: The ultrasonic wave velocity (P-and S-waves) and strength (uniaxial compressive and tensile strength) of coal-like samples increase with increasing density and decrease with increasing porosity. The effect of similar materials on ultrasonic wave velocity, strength, and density increments, cement > sand > gypsum, with the opposite porosity. Cement and gypsum play a major role in regulating the strength and deformation characteristics of coal-like samples, respectively. The strength of coal-like samples can be predicted in advance by measuring ultrasonic P-wave velocity based on the quadratic polynomial mathematical model between ultrasonic P-wave velocity and strength. The mechanical properties of the coal-like material can be adjusted in a wide range. Various properties of coal-like samples can be adjusted to simulate coal-rock mass accurately by changing the ratio of similar materials, and the sample can be made simply. This study provides a basis for a similar design of mechanical properties of coal-like materials for hydraulic fracturing simulation, which can promote the development of mine gas prevention technology and has high application value.
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图 8 弹性模量与单轴抗压强度关系
Fig. 8 Relationship of elastic modulus with uniaxial compressive strength
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图 10 峰值应变与水泥:石膏比值的关系
Fig. 10 Relationship of peak strain with different ratio of cement and gypsum
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图 11 超声波波速、密度、孔隙率和强度与材料配比关系
Fig. 11 Relationship of ultrasonic wave velocity, density, porosity and strength with materials ratio
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图 12 超声波波速与孔隙率、密度拟合曲线
Fig. 12 The fitting curve of ultrasonic wave velocity, porosity and density
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图 13 超声波P波波速与抗压和抗拉强度关系
Fig. 13 Relationship of compressive and tensile strength with ultrasonic P-wave velocity
表 1 煤岩体相似材料配比设计
Table 1 Proportion scheme for different samples preparation
试样分组 煤粉 水泥 石膏 砂子 水 CL1 1 1 3 1 3 CL2 1 2 2 1 3 CL3 1 2.5 1.5 1 3 CL4 1 3 1 1 3 CL5 1 2.5 1 1.5 3 CL6 1 2 1 2 3 CL7 1 1 1 3 3 
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表 2 超声波波速、密度和孔隙率测试结果
Table 2 Ultrasonic wave velocity, density, and porosity test results
试样
分组编号 直径×高度/
(mm×mm)P波波速/
(km·s−1)P波波速平均值/
(km·s−1)S波波速/
(km·s−1)S波波速平均值
/
(km·s−1)密度
/
(g·cm−3)孔隙率
/
%CL1 CL1-1 50.02×100.23 1.301 1.218 0.521 0.529 1.276 17.03 CL1-2 49.93×99.95 1.159 0.536 CL1-3 50.02×100.25 1.193 0.531 CL2 CL2-1 49.95×101.23 1.389 1.384 0.853 0.744 1.332 16.51 CL2-2 50.01×99.85 1.453 0.664 CL2-3 50.02×100.72 1.309 0.715 CL3 CL3-1 50.01×100.25 1.506 1.595 0.985 1.145 1.452 15.21 CL3-2 49.99×100.44 1.667 1.239 CL3-3 49.95×99.76 1.613 1.212 CL4 CL4-1 50.01×101.14 1.934 1.932 1.523 1.552 1.732 14.14 CL4-2 49.99×100.78 1.938 1.769 CL4-3 50.02×101.61 1.923 1.663 CL5 CL5-1 49.89×101.04 1.689 1.713 1.329 1.303 1.556 14.68 CL5-2 50.03×100.58 1.748 1.563 CL5-3 50.02×101.05 1.701 1.018 CL6 CL6-1 50.01×100.44 1.543 1.540 0.872 1.062 1.436 15.84 CL6-2 49.99×101.26 1.543 1.126 CL6-3 49.97×101.25 1.534 1.187 CL7 CL7-1 50.01×100.17 1.458 1.432 1.023 0.941 1.380 16.12 CL7-2 50.00×100.06 1.521 0.928 CL7-3 50.02×100.09 1.318 0.871
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表 3 力学参数测定结果
Table 3 Measurement results of mechanical parameters
试样
分组编号 抗压强度
/MPa抗压强度平均值/MPa 峰值应变
/%抗拉强度
/MPa抗拉强度平
均值/MPa弹性模量
/GPa弹性模量平均值/GPa CL1 CL1-1 3.01 2.84 2.15 0.245 0.231 0.263 0.254 CL1-2 2.74 0.213 0.241 CL1-3 2.78 0.235 0.258 CL2 CL2-1 3.31 3.25 2.36 0.246 0.258 0.267 0.265 CL2-2 3.18 0.269 0.272 CL2-3 3.26 0.258 0.257 CL3 CL3-1 4.65 4.74 2.60 0.321 0.313 0.363 0.363 CL3-2 4.84 0.299 0.374 CL3-3 4.72 0.319 0.352 CL4 CL4-1 7.45 7.16 2.89 0.421 0.414 0.519 0.515 CL4-2 6.86 0.416 0.516 CL4-3 7.16 0.406 0.510 CL5 CL5-1 5.05 5.18 4.51 0.352 0.344 0.273 0.286 CL5-2 5.31 0.339 0.296 CL5-3 5.18 0.341 0.289 CL6 CL6-1 4.53 4.36 3.72 0.304 0.312 0.246 0.231 CL6-2 4.35 0.313 0.232 CL6-3 4.21 0.319 0.215 CL7 CL7-1 3.95 3.89 3.58 0.302 0.295 0.163 0.142 CL7-2 3.64 0.291 0.124 CL7-3 4.09 0.293 0.140
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表 4 原煤力学特性数据
Table 4 Mechanical properties data of raw coals
地点 煤样 P波波速
/(km·s−1)S波波速
/(km·s−1)抗压强度
/MPa抗拉强度
/MPa弹性模量
/GPa峰值应变
/%内蒙古 胜利煤田褐煤 1.58 1.15 12.50 — 0.69 2.32 陕西府谷 恒益煤矿烟煤 1.51 — 12.10 2.50 1.08 3.71 陕西榆林 张明沟矿褐煤 1.31 0.93 6.31 0.39 0.32 3.31
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