Experimental Investigation on the Spontaneous Imbibition Behaviour of Shale Reservoirs

Abstract In a shale reservoir, its pore-throat structure is dictated by the abundant micro/nano-pores and throats, complicated by the intricate connectivity, which leads to significant capillarity effect. Physically, shut-in operations followed a hydraulic fracturing process may enhance oil production/recovery via spontaneous imbibition. In this study, a pragmatic and integrated technique has been developed to experimentally characterize the microscopic imbibition behaviour within pores of various scales in a shale reservoir. More specifically, the petrophysical properties together with pore-throat structure were firstly measured by using core samples collected from a shale oil reservoir. Subsequently, spontaneous imbibition experiments at high-temperature and high-pressure integrated with the nuclear magnetic resonance (NMR) measurement were employed to continuously monitor the oil-water distribution and saturation variation within the pores at different stages. By quantitatively characterizing the imbibition rate and oil recovery factor, the spontaneous imbibition behaviour together with its characteristics of multiscale pores and throats were investigated and analyzed. The imbibition equilibrium time for the core plugs typically ranges from 96 to 120 h. During the initial 24 h of imbibition, the rate of imbibition is higher, resulting in a rapid oil recovery up to 8.0%–9.6%, which accounts for approximately 65.0–70.0% of the total oil recovery. Imbibition behaviour is more likely to occur in pores connected with small throats. The findings presented herein serve as a solid foundation for optimizing the shut-in time after hydraulic fracturing operations in shale reservoirs so as to increase oil production/recovery via spontaneous imbibition.