注气驱替煤层瓦斯过程气体双向扩散机制的实验研究

Using gas injection to enhance coal seam gas drainage or coal bed methane production is a promising technology from the experiences of past field trials, however, its mechanism still needs to be further understood. The previous studies were focusing on the permeability change, the competitive sorption and the displacement ratio during gas injection, while the counter-diffusion process and its mechanism have been overlooked and related investigations are rare. Based on the self-designed ‘gas counter-diffusion test system’, this program will use experimental and theoretical study methods to investigate the following topics: CO2/N2 and methane counter-diffusion process during gas injection to coal sample with pre-adsorbed methane; the suitable diffusion model to represent and predict the counter-diffusion process and the corresponding diffusion coefficients; the difference and its mechanism between binary gas counter-diffusion and unary gas diffusion . Since the counter-diffusion process decides the exchange ratio of the injected gas and methane between coal cleat and matrix, the prospective results are of great importance to understand the gas flow mechanism in coal seam during gas injection. Besides, the acquired diffusion model will provide theoretical foundations to improve the numerical model of gas injection, predict the application prospect in different coal seam conditions, and optimize the key parameters of gas injection design.

注气驱替煤层瓦斯技术是一项具有推广使用前景，值得深入研究的煤层瓦斯强化抽采与煤层气开发技术。以往的研究集中在驱替过程的渗透率变化、竞争吸附和驱替比例等问题上，而对驱替过程中注入气体与瓦斯的“双向扩散机制”的研究很少。本项目拟利用自行设计的“气体双向扩散规律测试装置”，通过实验研究与理论分析相结合的研究方法，考察二氧化碳和氮气驱替煤体瓦斯过程中的气体双向扩散规律，研究适宜表征双向扩散过程的扩散模型并计算不同气体的扩散因子，分析二元气体双向扩散过程与单一气体单向扩散过程的差异性并探讨其产生原因。由于双向扩散过程决定了注入气体和瓦斯在煤层裂隙与基质煤体之间的交换速率，预期研究成果对于深入理解注气驱替过程的气体流动机制具有非常重要的意义，同时所获得的扩散模型可以为未来建立更为完善的注气驱替数学模型，评估不同煤储层条件下的注气驱替技术应用前景和工程设计中的关键参数优化等工作提供理论基础。

注气驱替煤层瓦斯技术是一项具有推广使用前景，值得深入研究的煤层瓦斯强化抽采与煤层气开发技术。以往的研究集中在驱替过程的渗透率变化、竞争吸附和驱替比例等问题上，而对驱替过程中注入气体与瓦斯的“双向扩散机制”的研究很少。针对该问题，本项目研发了注气驱替过程气体双向扩散实验装置，装置能够对高压条件下的注入气体向内扩散以及煤层气体向外扩散的规律进行测试。实验结果表明，与单向扩散实验结果相比较，在同等吸附平衡压力下，氮气驱替二氧化碳过程中，二氧化碳总排放量提高了1.31~1.66倍，解吸量提高了1.69~2.1倍；氮气驱替甲烷过程中，甲烷总排放量提高了1.42~1.59倍，解吸量提高了2.22~2.46倍。推导了气体双向扩散的数值解法和三种扩散模型，计算了气体双向扩散与单向扩散的扩散因子。无论是单孔扩散模型、双孔扩散模型I、还是双孔扩散模型II，对应的扩散系数均随着驱替压力的增大而增大。双孔扩散模型I与双孔扩散模型II的拟合效果要优于单孔扩散模型，双孔扩散模型I与双孔扩散模型II的拟合效果基本接近；函数y=ax/(b+x)能很好的描述压力与扩散速度因子 的关系。同一压力条件下，二氧化碳、甲烷单向扩散的扩散系数均大于双向扩散。注入的氮气在向煤体内部“扩散-吸附”的过程中一定程度上降低了煤体预吸附的二氧化碳或甲烷向外“解吸-扩散”的速率，但只要保证足够的驱替时间，驱替效果就能得到很大提高。