基于低场核磁共振的煤润湿性分析
Determination of Coal Wettability by Using Low-field Nuclear Magnetic Resonance

摘要： 煤润湿性在煤化工、煤矿安全生产及煤层气开发利用中有重要意义。目前煤的润湿性测定方法过程繁琐、操作困难、现场实用性差、测量结果可对比性差，不能快速有效地应用于煤层气储层的润湿性评价研究。基于系列低场核磁共振(LFNMR)实验，探讨了LFNMR应用于煤润湿性研究的可行性与研究方法。研究发现，LFNMR可应用于识别煤粉中不同状态水分：对于强水湿性煤粉，其T2弛豫谱仅存在吸附态水峰与颗粒间水峰，而弱水湿性煤粉则存在吸附态水峰、颗粒间水峰和自由态水峰3个峰值；煤的水湿性越强，其颗粒间水峰和自由态水峰弛豫时间越短，并随着润湿时间的增加弛豫逐渐变快。当水对煤的润湿达到稳定状态时，煤样的自由态水峰和颗粒间水峰的加权平均值与型煤接触角法测得的接触角呈相关性较好的对数关系。该对数关系可应用于煤水润湿性的定量、半定量评价，这为采用该方法进一步研究煤储层的气、水相互作用提供了理论基础。
Abstract: The research on coal wettability is significant in coal chemical industry, safety of coal mining, and coalbed methane (CBM) production. The conventional methods for coal wettability determination are difficult to be performed and unsuitable for an in-situ measurement, which make it difficult to be used as in-situ analysis method for determining wettability of CBM reservoirs. Based on a series of low-field nuclear magnetic resonance (LFNMR) experiments, this paper discussed the practicality of the application of LFNMR in evaluation of the wettability of CBM reservoirs. It was found that moistures in the coals have different phases including the free water, interparticle capillary confined water, and adsorption water that can be distinguished by the water transverse relaxation time (T2) spectra in a LFNMR measurement. For the hydrophilic powdered coal, there are adsorbed water and interparticle water that correspond to two distinct peaks in the T2 spectra. For hydrophobic powdered coal, there exist adsorbed, interparticle and free waters, corresponding to three distinct peaks in the T2 spectra. The stronger the hydrophilicity is, the faster the relaxation time of capillary water and free water is. In an equilibrium system of the coal-water wettability, the geometric mean value of T2 for free and capillary waters is found to be logarithmically correlated with the contact angle between coal and water that is determined from a conventional method. We suggest that the low-field NMR is quantifiably or semiquantifiably applicable for estimation of coal wettability, which provides an important theoretical foundation for evaluating the water-gas interactions in a CBM reservoir by using LFNMR