Spontaneous imbibition in coal: Experimental and model analysis

Abstract

The understanding of the mechanisms that govern spontaneous water imbibition in gas-water systems plays a significant role in operating the hydraulic fracturing and the development of unconventional gas reservoirs. The main purpose of this paper is addressing how the spontaneous imbibition in coal reservoir affects reservoir permeability, and what is the microscopic mechanism of water transport during this process. To solve this problem, air-water co-current imbibition experiments, contact-angle tests and permeability tests were carried out on eight coal plugs originated from three Chinese coalbed methane hot spots. Moreover, NMR spectrometer is introduced as a method for monitoring the volume of water imbibed in pores with different sizes. The results reveal that the water imbibition rate of bituminous coal follows microfracture > micropores > meso/macropores, whereas for anthracite, the imbibition rate is in the order of microfracure > meso/macroproes > micropores. Permeability damage ratio due to the spontaneous imbibition follows a positive exponential relationship with increasing contact angle. Considering the multi-scale pore system and complex pore tortuosity, we established a model to estimate the spontaneous imbibition rate, which agrees well with experimental results at the early stage of imbibition process when the capillary force is dominant. Finally, four successive stages of moisture migration during spontaneous imbibition and spontaneous evaporation were divided based on the movement of the NMR T2 peak. The implications from this work is important for better understanding the mechanism controlling fluid loss and gas production in unconventional reservoir, which is significant for optimizing the recovery program and minimizing reservoir damage.