水力压裂模拟用煤岩体相似材料基础力学特性实验研究

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。 

Experimental study on the mechanical properties of coal-like materials for hydraulic fracturing simulation

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.