由于液氮致裂技术是一种重要的煤层增渗技术, 且低温下的煤孔隙水相变严重影响煤孔隙结构, 因此, 研究液氮循环冻结导致的煤孔隙水相变和孔隙演化对评价液氮致裂增渗效果, 以及提高煤层气抽采效率具有重要意义。该文对饱水烟煤进行不同次数的液氮冻融循环处理, 基于低场核磁共振测试技术, 测试煤样的横向弛豫时间曲线、 累计孔隙度和累计孔喉分布。结果表明： 煤样在-196 ℃冻结条件下, 煤孔隙内仍存在微量未冻水, 随温度升高, 小孔隙冰先开始融化, 产生小孔隙未冻水, 孔径越小, 孔隙冰的熔点越低。随着孔隙冰逐渐融化, 累计孔隙度不断增大, 增长过程可分为指数函数式快速增长、 一次函数式匀速增长和二次函数式增长3个阶段。对横向弛豫时间曲线进行积分, 可获得煤样融化过程中的孔隙未冻水变化, 并进行拟合, 可观察到, 在负温区间, 未冻水含量随温度升高呈指数函数式变化, 越接近0 ℃, 未冻水含量增速越快。-196～-34 ℃, 微孔未冻水占比接近100%; -20～0 ℃, 以中、 大孔隙未冻水为主。循环冻结可有效提高煤样的有效孔隙度和渗透率。该文基于SDR(Schlumberger-Doll Research)渗透率模型, 计算了煤样冻融循环后的渗透率变化。煤样的有效孔隙度和渗透率的增长率随循环次数的增加而增加, 当循环次数由5次增至30 次时, 有效孔隙度增长率由20.68%增至24.15%, 渗透率增长率由109.73%增至122.20%, 但随着循环次数不断增加, 冻融循环次数对煤样增渗作用的边际效应愈发明显。该文研究结果可为低温介质循环冻融煤体渗流孔隙结构演变研究和与液氮致裂增渗相关的现场作业提供参考。
Abstract
[Objective] Coalbed methane (CBM) outbursts pose a threat to coal mining safety, particularly in China, where many coal seams exhibit low permeability, making CBM extraction inefficient. Liquid nitrogen fracturing technology is an important anhydrous method for enhancing coal seam permeability. The phase transition of coal pore water under low temperature conditions profoundly affects the coal pore structure. Studying the pore-water phase transition and pore changes during low-temperature liquid nitrogen freeze-thaw cycles is crucial for evaluating the effectiveness of this technology and improving CBM extraction efficiency. [Methods] This experiment involves four groups of water-saturated bituminous coal samples, each subjected to different numbers of liquid nitrogen freeze-thaw cycles. The transverse relaxation time T2 curve, cumulative porosity, and cumulative pore throat distribution of coal samples in the initial, frozen and thawed states were evaluated by using low-field nuclear magnetic resonance (NMR). [Results] These findings showed the following: (1) Even at -196 ℃, trace amounts of unfrozen water remained in the coal pores. As the temperature increased, the ice in the smaller pores first melted, producing micropore unfrozen water. Smaller pore diameters corresponded to lower pore melting pints of pore ice. (2) As the pore ice gradually melted, the cumulative porosity increased in three stages: exponential function type rapid growth, primary function type uniform growth, and quadratic function type growth. (3) By integrating the T2 curve, it is observed that the unfrozen water content changed exponentially with increasing temperature in the negative temperature intervals. Closer to 0 ℃, the unfrozen water content growth rate increased. At temperatures between -196 ℃ and -34 ℃, micropore unfrozen water constituted nearly 100%, whereas temperatures between -20 ℃ and 0 ℃ saw mesopore and macropore unfrozen water dominating. (4) NMR tests on raw and fully melted coal samples revealed that the permeability of coal samples increased with the number of freeze-thaw cycles. The effective porosity and permeability growth rates increased from 20.68% to 24.15% and 109.73% to 122.20% respectively, as the number of cycles increased from 5 to 30. However, the marginal utility of permeability enhancement of coal samples became increasingly pronounced. [Conclusions] Liquid nitrogen cyclic freezing of coal effectively increases the effective porosity and permeability of water-saturated coal. Liquid nitrogen fracturing can be used as an anhydrous fracturing technology to increase the coal seam permeability in water-scarce coal mining areas, thus improving the CBM extraction efficiency. The results of this study can provide valuable insights into the evolution of seepage pore structure during low-temperature medium cyclic freezing and thawing of coal and guide field operations related to liquid nitrogen fracturing and permeability enhancement.
关键词
液氮致裂 /
未冻水 /
孔隙结构 /
低场核磁共振
Key words
liquid nitrogen fracturing /
unfrozen water /
pore structure /
low-field nuclear magnetic resonance
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基金
中国科协青年人才托举工程项目(2022QNRC001);陕西省青年人才托举计划项目(20220437);陕西省青年科技新星项目(2022KJXX-59);国家自然科学基金项目(51904237)
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