Change of phase state during multi-cycle injection and production process of condensate gas reservoir based underground gas storage

Abstract Based on the differences in production mode and operation process between gas storage and gas reservoir, we established a phase balance test procedure and a theoretical simulation model of phase balance during multi-cycles of injection and production of underground gas storage (UGS) rebuilt from condensate gas reservoir to study the phase characteristics of produced and remaining fluids during multi-cycles of injection and production. Take condensate reservoir gas storage as example, the composition of produced fluid and remaining fluid, phase state of remaining fluid, retrograde condensate saturation and condensate recovery degree in the process of multi-cycles of injection-production were studied through multi-cycle injection-production experiment and phase equilibrium theory simulation. The injected gas could greatly improve the recovery of condensate oil in the gas reservoir, and the condensate oil recovery increased by 42% after 5 cycles of injection and production; the injected gas had significant evaporative and extraction effects on the condensate, especially during the first two cycles; the condensate oil saturation of the formation decreased with the increase of injection-production cycles, and the condensate oil saturation after multi-cycles of injection-production was almost 0; the storage capacity increased by about 7.5% after multi-cycles of injection and production, and the cumulative gas injection volume in the 5th cycle increased by about 25% compared with that in the 1st cycle.