Evaluation of retrograde condensation damage and research on gas injection for enhanced recovery of condensate gas reservoirs in deep-buried hills

The BZ condensate gas reservoir in the Bohai Sea, China, is a rare fractured buried hill condensate gas reservoir with high saturation and high content of condensate oil. The reservoir features high temperature, high pressure, ultra-low porosity, and ultra-low permeability. Due to the small difference between the fluid dew point and the pressure in the gas reservoir, it is prone to condensate oil precipitation, causing contamination in the near-wellbore zone. In the early development stage, the BZ gas reservoir pilot area was produced using natural energy. When the reservoir pressure drops below the dew point, retrograde condensation intensifies, leading to a rapid increase in the gas-oil ratio and an accelerated decline in production. Therefore, there is an urgent need for the evaluation of retrograde condensation damage and effective remediation methods. Core depletion experiments were conducted under high-temperature and high-pressure conditions using compound condensate gas to simulate retrograde condensate oil contamination. Gas-phase permeability was tested at different depletion pressure points to evaluate the degree of retrograde condensate contamination. Additionally, gas injection experiments were carried out to investigate the mechanisms of damage mitigation. Experimental results showed that as the reservoir pressure decreased, the amount of retrograde condensate in the core increased, and the effective gas-phase permeability decreased significantly. Ultimately, the resulting retrograde condensate damage to the reservoir reached 65.8% to 70.2%. Gas injection could reduce the viscosity of condensate oil, increase the volume expansion coefficient of reservoir fluids, and induce re-vaporization of retrograde condensate oil. This process reduced the amount and saturation of retrograde condensate liquid, relieved retrograde condensate blockage, and improved the effective gas-phase permeability of reservoir cores. The permeability recovery rates for N2, associated gas, and CO2 were 48.1%, 78.6%, and 81.7%, respectively. The final recovery rates for condensate oil reached 43.7%, 66.8%, and 69.2%, respectively. The research results provide technical support for gas injection development in the pilot zone of the BZ buried hill condensate gas reservoir. This approach effectively mitigates production decline and achieves good results, offering important guidance for the efficient large-scale gas injection development in the future.