Sun Xiaoxiao, Yao Yanbin, Liu Dameng, Elsworth Derek, Pan Zhejun
Coal Reservoir Laboratory of National Engineering Research Center of CBM Development &Utilization, China University of Geosciences, Beijing 100083, PR China.
Department of Energy and Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA.
Sci Rep. 2016 Jan 28;6:19919. doi: 10.1038/srep19919.
The mechanisms by which CO2 and water interact in coal remain unclear and these are key questions for understanding ECBM processes and defining the long-term behaviour of injected CO2. In our experiments, we injected helium/CO2 to displace water in eight water-saturated samples. We used low-field NMR relaxation to investigate CO2 and water interactions in these coals across a variety of time-scales. The injection of helium did not change the T2 spectra of the coals. In contrast, the T2 spectra peaks of micro-capillary water gradually decreased and those of macro-capillary and bulk water increased with time after the injection of CO2. We assume that the CO2 diffuses through and/or dissolves into the capillary water to access the coal matrix interior, which promotes desorption of water molecules from the surfaces of coal micropores and mesopores. The replaced water mass is mainly related to the Langmuir adsorption volume of CO2 and increases as the CO2 adsorption capacity increases. Other factors, such as mineral composition, temperature and pressure, also influence the effective exchange between water and CO2. Finally, we built a quantified model to evaluate the efficiency of water replacement by CO2 injection with respect to temperature and pressure.
二氧化碳与水在煤中的相互作用机制尚不清楚，而这些是理解煤层气强化开采过程和确定注入二氧化碳长期行为的关键问题。在我们的实验中，我们注入氦气/二氧化碳以置换八个水饱和样品中的水。我们使用低场核磁共振弛豫技术，在各种时间尺度上研究这些煤中二氧化碳与水的相互作用。注入氦气并未改变煤的T2谱。相比之下，注入二氧化碳后，微毛细管水的T2谱峰逐渐降低，而大毛细管水和体相水的T2谱峰随时间增加。我们假设二氧化碳扩散穿过和/或溶解到毛细管水中以进入煤基质内部，这促进了水分子从煤微孔和中孔表面的解吸。被置换的水质量主要与二氧化碳的朗缪尔吸附量有关，并随着二氧化碳吸附能力的增加而增加。其他因素，如矿物成分、温度和压力，也会影响水与二氧化碳之间的有效交换。最后，我们建立了一个量化模型，以评估注入二氧化碳置换水在温度和压力方面的效率。
