Experimental Study on Improving Condensate Oil Recovery by CO₂ Huff and Puff in Condensate Gas Reservoirs

The application of CO₂ injection to improve the recovery of natural gas and condensate oil is still in the development stage. The CO₂ injection mode, injection time, and the interaction mechanism between CO₂ and reservoir fluid are not clear. Through the constant volume depletion experiment of CO₂ injection into condensate gas reservoirs and the measurement experiment of oil-gas interfacial tension, a stepwise incremental CO₂ huff and puff method was proposed on the basis of determining the maximum retrograde condensate pressure, liquid volume, and CO₂-condensate oil interaction mode. In addition, the huff and puff effect and CO₂ storage ratio were evaluated by full diameter core experiment. The results show that after the CO₂ is injected into the condensate gas reservoir, the dew point pressure, the maximum retrograde condensate pressure, and the condensate oil volume are continuously reduced, and the critical point moves to the lower left corner. The two-phase envelope area shrinks inward, and the fluid components become lighter. The phase mixing between CO₂ and condensate oil can be achieved gradually only after multiple contacts. A higher CO₂ pressure indicates fewer contacts required to achieve the phase mixing. The condensate oil recovery after four rounds of incremental CO₂ huff and puff is 1.2%, 14.4%, 25.8%, and 3.6%, respectively, which is 45.0 percentage points higher than that after natural depletion. The proportion of CO₂ storage decreases with the increase in huff and puff times, which are 82.2%, 72.1%, 46.4%, and 9.2%, respectively. CO₂ huff and puff times should be controlled within 3 to achieve the optimal effect. The CO₂ huff and puff effect is mainly affected by CO₂ injection pressure and core system pressure. When the CO₂ injection pressure is lower than the minimum miscible pressure (MMP), it is not conducive to CO₂ huff and puff and storage. The research results provide a reference for the efficient development of condensate gas reservoirs and the improvement of retrograde condensate damage.