Nitrogen injection to enhance methane and water production: An experimental study using the LF-NMR relaxation method

Abstract

Nitrogen injection is an effective way to improve coalbed methane (CBM) recovery, but there are few studies of the regularity of fluid flow during nitrogen injection. To study the flow of methane and water during nitrogen injection into coal reservoirs, a high-volatile bituminous B coal from the southern margin of the Junggar Basin, Xinjiang and a low-volatility bituminous coal from the central and eastern regions of the Qinshui Basin, Shanxi province were selected, and the real-time flow of methane and water in a coal reservoir under nitrogen injection was monitored by low-field nuclear magnetic resonance (LF-NMR). The results showed that, without nitrogen injection, primarily free methane (bulk methane and methane confined in porous media) and very little adsorbed methane are produced. The rate of production of methane increases logarithmically with time and reaches a balance after a period of time. With nitrogen injection, the production balance is broken, resulting in the flow and production of adsorbed methane, and the production rate is significantly improved. It is suggested that nitrogen injection provides a sustained driving force for the desorption of absorbed methane and flow of methane that causes the production rate to increase linearly with time. The flow of methane is asynchronous during the stages with and without nitrogen injection; that is, the bulk methane is produced first, followed by the methane confined in porous media and finally, the adsorbed methane. This occurs because the resistance to the flow of the methane present in different pore apertures is different, resulting in differences in the flow of methane with time. Nitrogen injection has a significant effect on the production of free water and capillary water but no obvious effect on absorbed water. By comparing two samples with different coal ranks, material composition and fracture development, it is found that the nitrogen injection effect on high-volatile bituminous B coal with more fractures and higher inertinite content is better than the nitrogen injection effect on low-volatility bituminous coal. Nitrogen injection significantly enhanced the recovery of CBM by improving the production of adsorbed methane and free water under in situ reservoir conditions; for reservoirs with different physical properties, the stimulation effect will differ.