Retained water content after nitrogen driving water on flooding saturated high volatile bituminous coal using low-field nuclear magnetic resonance

Retained water in reservoirs can result in water block damage that can lead to low productivity of coalbed methane wells. To research the water saturation increasing process during water flooding and determine the retained water content variation law after nitrogen driving water, considering that high-pressure nitrogen injection is recently regarded as an effective technology to enhance CBM well production, multiple low-field nuclear magnetic resonance methods, including T2 spectrum testing, multi-slice spin-echo sequence imaging, and spatially-resolved T2 distribution measurements, were applied during water flooding experiment and nitrogen driving water for decreasing retained water content of a high volatile bituminous coal sample. Distributions of fractures, pores and minerals were inversed based on micro-CT technology and the corresponding influences on retained water were discussed. The results showed that water saturation increasing process of total apertures, micro- and transition pores, meso-pores, macro-pores and fractures were increased by logarithmic relation with the increase of water flooding time. Nitrogen driving could basically decrease water saturation in macro-pores and fractures completely, and partly in meso-pores with a 50.11% decrease in water saturation. However, it can lead to increased water saturation by 6.89% in micro- and transition-pores. It is relatively easier to decrease retained water content when inlet pressure is increased during nitrogen driving because effective stress is decreased and connections between apertures are improved. The smaller mineral filling porosity, greater total porosity, and better connection of pores and fractures, the greater water content after water flooding. Retained water content of a sample with connected apertures is discharged effectively under nitrogen driving, while a sample that lacks connected apertures remains high in retained water content.