| Summary: | The determination of microscopic residual gas distribution is beneficial for exploiting reservoirs to their maximum potential. In this work, both forced and spontaneous imbibition (waterflooding) experiments were performed on a high-pressure displacement experimental setup, which was integrated with nuclear magnetic resonance (NMR) to reveal the impacts of capillary number (<i>Ca</i>) and initial water saturation (<i>S</i><sub>wi</sub>) on the residual gas distribution over four magnitudes of injection rates (<i>Q</i> = 0.001, 0.01, 0.1 and 1 mL/min), expressed as <i>Ca</i> (log<i>Ca</i> = −8.68, −7.68, −6.68 and −5.68), and three different <i>S</i><sub>wi</sub> (<i>S</i><sub>wi</sub> = 0%, 39.34% and 62.98%). The NMR amplitude is dependent on pore volumes while the NMR transverse relaxation time (<i>T</i><sub>2</sub>) spectrum reflects the characteristics of pore size distribution, which is determined based on a mercury injection (MI) experiment. Using this method, the residual gas distribution was quantified by comparing the <i>T</i><sub>2</sub> spectrum of the sample measured after imbibition with the sample fully saturated by brine before imbibition. The results showed that capillary trapping efficiency increased with increasing <i>S</i><sub>wi</sub>, and above 90% of residual gas existed in pores larger than 1 μm in the spontaneous imbibition experiments. The residual gas was trapped in pores by different capillary trapping mechanisms under different <i>Ca</i>, leading to the difference of residual gas distribution. The flow channels were mainly composed of micropores (pore radius, <i>r</i> < 1 μm) and mesopores (<i>r</i> = 1−10 μm) at log<i>Ca</i> = −8.68 and −7.68, while of mesopores and macropores (<i>r</i> > 10 μm) at log<i>Ca</i> = −5.68. At both <i>S</i><sub>wi</sub>= 0% and 39.34%, residual gas distribution in macropores significantly decreased while that in micropores slightly increased with log<i>Ca</i> increasing to −6.68 and −5.68, respectively.
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