Pore Structure in Coal: Pore Evolution after Cryogenic Freezing with Cyclic Liquid Nitrogen Injection and Its Implication on Coalbed Methane Extraction
Pore Structure in Coal: Pore Evolution after Cryogenic Freezing with Cyclic Liquid Nitrogen Injection and Its Implication on Coalbed Methane Extraction
复制标题
煤的孔隙结构：循环液氮注入深冷冷冻后孔隙演化及其对煤层气抽采的影响
DOI：
10.1021/acs.energyfuels.6b00920
复制
发表时间：
2016-07-01
期刊：
影响因子：
5.3
通讯作者：
Wu, Shiliang
中科院分区：
文献类型：
--
作者：
Zhai, Cheng;Qin, Lei;Wu, Shiliang
煤中循环注入液氮（液氮）会发生冻融循环。冻融处理破坏了煤的孔隙结构，从而增加了煤的渗透性。在这项研究中，核磁共振和应变监测研究煤结构的变化时，煤样在低温处理下使用液氮。将冻融过程分为四个阶段，第一、三阶段以渗透孔隙发育为主，第二、四阶段以吸附孔隙发育为主。结果发现，液氮冻融循环可以引起煤的结构恶化，从而提高裂隙密度和整体渗透性。结果表明，煤的有效孔隙度和总孔隙度的增加速率与液氮冻结时间和冻结循环次数呈正相关，而与残余孔隙度呈负相关。在相同的液氮冻结时间下，循环冻融处理对煤中孔隙的增长速率和纵波速度的降低速率的影响大于单一冻融处理。研究还发现，冻融循环次数是形成较大孔隙（可连接裂缝网络的孔隙）的一个非常重要的因素。结果表明，适当控制冻融循环次数可以有效地实现煤的压裂。
Freezing and thawing cycles occur with cyclic liquid nitrogen (LN2) injection in coal. The freeze-thaw treatment damages the pore structure of coal and thus increases its permeability. In this study, NMR and strain monitoring were employed to investigate the changes in coal structure when the coal specimens were under cryogenic treatment using LN2. We classified freeze thaw process into four stages; stages I and III are dominated by seepage pore development, and stages II and IV are dominated by adsorption pore development. It was found that LN2 freeze thaw cycles can cause structural deterioration in the coal so as to improve both fracture density and overall permeability. The results demonstrate that the rate of increase of both the effective porosity and total porosity of the coal are positively correlated with the LN2 freezing time and the number of freezing cycles but negatively correlated with the residual porosity. For the same absolute LN2 freezing time, cyclic freeze thawing has a greater effect on the rate of growth of pore spaces and reduction of P-wave velocity in the coal compared with single freeze thaw treatment. It was also found that the number of freeze thaw cycles is a very important factor for the creation of larger pores, pores that can connect the fracture network. The results suggest that appropriate control of the number of freeze thaw cycles can result in effective fracturing of coal.