Abstract
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 (Ca) and initial water saturation (Swi) on the residual gas distribution over four magnitudes of injection rates (Q = 0.001, 0.01, 0.1 and 1 mL/min), expressed as Ca (logCa = −8.68, −7.68, −6.68 and −5.68), and three different Swi (Swi = 0%, 39.34% and 62.98%). The NMR amplitude is dependent on pore volumes while the NMR transverse relaxation time (T2) 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 T2 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 Swi, 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 Ca, leading to the difference of residual gas distribution. The flow channels were mainly composed of micropores (pore radius, r < 1 μm) and mesopores (r = 1–10 μm) at logCa = −8.68 and −7.68, while of mesopores and macropores (r > 10 μm) at logCa = −5.68. At both Swi= 0% and 39.34%, residual gas distribution in macropores significantly decreased while that in micropores slightly increased with logCa increasing to −6.68 and −5.68, respectively.
Highlights
Gas reservoir with aquifers is one of the most common type of reservoirs, in which edge-bottom water entering and fingering often occur, leading to residual gas becoming trapped [1]
T2i Ai / Ai i i ri Si / Si i i where T2i is the nuclear magnetic resonance (NMR) transverse relaxation time, Ai is the amplitude of the T2 spectrum at T2i, ri is the sample pore radius obtained by mercury intrusion method, and Si is the mercury injection saturation corresponding to ri on the mercury intrusion curve, and Si is the maximum mercury i injection saturation of sample in mercury intrusion experiment
The T2 spectra of sample #1-1F acquired in spontaneous imbibition experiments at Swi1, Swi2 and
Summary
Gas reservoir with aquifers is one of the most common type of reservoirs, in which edge-bottom water entering and fingering often occur, leading to residual gas becoming trapped [1]. The residual gas distribution in the pore structure of porous media at different Ca and Swi remain ambiguous. These micromodels cannot completely characterize the complex pore structure of reservoir rocks. NMR has been successfully applied to analyze the oil recovery ratio under different displacement volumes [33], the residual oil distribution in rock samples [29] and the effect of micro structures on imbibition [32], etc. Few studies have looked into the microscopic residual gas distribution within rock samples at different Swi and Ca. The interaction between Swi and Ca during imbibition processes is still unknown. Results from this study provide an improved understanding on the processes that affect the development of water-drive gas reservoirs
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