基于微细观的煤层脉动压裂结构损伤及增透作用机制研究

In order to study on the structural damage and permeation mechanism of pulsating hydraulic fracturing in coal seam based on microcosmic-microscopic view, the research methods of physical experiments and theoretical analysis are used in the project. Pulsating hydraulic fracturing experiment system is built to study the law of superposition and attenuation of pressure propagation in the microscopic structural coal, and establish quantitative mathematical model between pulsation frequency, amplitude, waveform superposition number and coal breaking “threshold value”. And then the pulsating hydraulic fracturing experiment under the double control of true triaxial and gas pressure was carried out. The spatial distribution pattern of meso-fracture was constructed by NMR imaging technique, and the pore size was measured by mercury porosimetry and liquid nitrogen adsorption, to study the characteristics of coal pore-fissure damage from the fractal point under the different pulsation frequency and pressure. The change laws of displacement gas volume, late natural gas desorption and gas diffusion parameters with the pulsation frequency, pressure and time were studied, to reveal the mechanism of gas desorption and diffusion in the damaged coal. At last, wave propagation equation, coal fractal damage equation and gas migration equation are established, as well as the mathematical model of penetration enhanced by pulsating fracture based on microcosmic-microscopic view. This study provide a theoretical basis to reveal improving permeability mechanism and the application of pulsating hydraulic fracturing in the high gas and low permeability coal seam.

为了从微细观角度研究煤层脉动压裂结构损伤与增透作用机制，本项目采用物理实验和理论分析结合的研究方法，首先利用自建的脉动压裂实验装置，研究脉动波在煤体微细观孔隙-裂隙中压力传播的叠加、衰减规律，建立频率、振幅、波形叠加倍频数与破煤压力“门槛值”的定量数学关系。然后开展模拟地应力和瓦斯压力双重控制下的脉动压裂实验，采用NMR成像技术构建损伤煤体细观裂隙空间分布图，利用压汞、液氮吸附测试微观孔隙变化，从分形角度研究不同频率、压力条件下脉动波作用煤体微细观孔隙-裂隙损伤特征。研究损伤煤体中置换-驱替瓦斯量、后期瓦斯自然解吸量及瓦斯扩散参数随脉动频率、压力及时间的变化特性，揭示损伤煤体中瓦斯解吸、扩散微观运移规律。最后，建立波形传播方程、煤体分形损伤方程、瓦斯微观运移方程，构建基于微细观的脉动压裂增透作用数学模型。研究结果为深入揭示高瓦斯、低渗透煤层脉动压裂增透机理及其推广应用提供理论依据。

为了从微细观角度研究煤层脉动压裂结构损伤与增透作用机制，本项目采用物理实验和理论分析结合的研究方法，首先利用自建的脉动压裂实验装置，研究了脉动波在煤体微细观孔隙-裂隙中压力传播的叠加、衰减规律，脉动波形近似等效为正弦波，脉动水以强烈的交变载荷作用于孔隙表面，产生“收缩-膨胀-收缩”的疲劳冲击作用冲击煤体孔隙，对煤体孔隙产生疲劳破坏。然后开展模拟地应力和瓦斯压力双重控制下的脉动压裂实验，采用NMR成像技术构建损伤煤体细观裂隙空间分布图，利用压汞、液氮吸附测试微观孔隙变化，从分形角度研究不同频率、压力条件下脉动波作用煤体微细观孔隙-裂隙损伤特征，得出在脉动载荷下对煤样孔隙度的影响程度大于静压载荷；微孔体积随着脉动波频率和峰值压力的增加而减少，中孔和大孔体积随着脉动波频率和峰值压力的增加而增加。建立基于脉动水力压裂的瓦斯解吸实验系统，探讨脉动压裂过程中瓦斯解吸动力学特性，分析脉动压裂协同控制条件下，前期置换-驱替瓦斯特性、后期瓦斯自然解吸特性，综合分析脉动压裂影响瓦斯解吸效果。低压-高频、高压-低频和高压-高频的脉动压裂对瓦斯综合解吸起促进作用。运用基于第三类边界条件下的瓦斯扩散动力学模型，分析了脉动参量协同控制条件下瓦斯扩散动力学特性。由于煤中瓦斯浓度梯度和瓦斯扩散阻力的综合作用，使脉动压裂后瓦斯在煤中的扩散能力随着脉动频率和峰值压力的增加而增加，且应存在一组临界值，使脉动压裂后煤样的扩散能力大于原煤样。研究结果为深入揭示高瓦斯、低渗透煤层脉动压裂增透机理及其推广应用提供理论依据。

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