液氮在煤体微纳米孔隙内运移及致裂机理研究

Aiming at the migration mechanism of liquid nitrogen in micro-nano pores of coal, this is a key scientific problems in coal seam permeability improvement injected with liquid nitrogen. The project intends to combine physical experiment, theoretical analysis and numerical simulation to carry out systematic research. The experimental system of enhanced coal permeability improvement and gas extraction by liquid nitrogen injection is used to study the transformation law of the parameters of porosity, permeability, temperature and strain of coal with the injection of liquid nitrogen, and then the mathematical model of temperature conduction and strain change is established. Three-dimensional pore scanning technology is used to analyze the spatial distribution characteristics of three-dimensional micro-nano pores of coal under the condition of triaxial stress, and then the physical model of three-dimensional micro-nano pores of coal based on spatial positioning is constructed. The governing equations of liquid nitrogen and nitrogen migration in micro-nano pores are established by numerical simulation. The stress, temperature, fluid velocity and gas-liquid distribution characteristics in the pore physical model are studied through the equations, and then the key control factors affecting fluid migration are analyzed. The relationship between liquid nitrogen vaporization by heat and nitrogen driven liquid nitrogen migration is determined to reveal the quantitative law of fluid migration and fracture. The microfracture mechanism of liquid nitrogen is studied from the perspective of liquid nitrogen and nitrogen migration. The results could provide the theoretical basis to the liquid nitrogen fracturing in the field of coals.

针对煤体微纳米孔隙内液氮运移机理，这一煤层液氮增透过程中的关键科学问题。项目拟结合物理实验、理论分析和数值模拟方法进行系统研究。运用液氮注入煤体强化增透与瓦斯抽采实验系统，研究液氮注入变量对煤体的孔隙度、渗透率、温度和应变参数的改造规律，建立温度传导与应变变化数学模型；利用先进的三维孔隙扫描测试技术，分析三轴应力加载煤体三维微纳米孔隙空间分布特征，构建基于空间定位的煤体三维微纳米孔隙物理模型；结合数值模拟方法，建立液氮与氮气在微纳米孔隙内的运移控制方程，研究孔隙物理模型内的应力、温度、流体流速和气液两相分布特征，分析影响流体运移的关键控制因素，明确液氮受热汽化及氮气驱动液氮运移的关系，揭示流体运移与孔隙起裂的定量规律，从液氮与氮气运移角度明晰液氮微观致裂机理。研究成果可为煤层液氮致裂技术应用提供基础理论依据。

液氮致裂技术作为一种无水化煤层增透手段，凭借清洁、无污染等优势近年来迅速成为研究热点。探究煤体微纳米孔隙内液氮运移机理是液氮增透技术的关键科学问题之一。为进一步探究这一问题，项目通过研发液氮注入煤体强化增透-瓦斯抽采试验系统，探究不同注入变量下煤体孔隙度、渗透率、温度和应变等参数的演化规律，并建立冻融过程煤体传热-变形耦合模型。此外，利用三维孔隙扫描测试技术分析了三轴应力加载煤体三维微纳米孔隙空间分布特征，并构建基于空间定位的煤体三维微纳米孔隙物理模型。最后，结合数值模拟方法，建立液氮与氮气在微纳米孔隙内的运移控制方程，研究孔隙物理模型内的应力、温度、流体流速和气液两相分布特征，分析影响流体运移的关键控制因素，明确液氮受热汽化及氮气驱动液氮运移的关系，揭示流体运移与孔隙起裂的定量规律，从液氮与氮气运移角度明晰液氮微观致裂机理。研究成果可为液氮增透技术应用及煤层气增透理论完善提供参考。