Abstract:

Under the influence of the ultra-deep, ultra-high temperature, and ultra-high pressure reservoir formation and control mechanism, stratum fluid in the Shunbei ultra-deep fault-controlled condensate gas reservoir exhibits complex phase characteristics of supercritical condensate gas. The impact of the complex phase evolution behavior on the development dynamics is an urgent issue to be addressed in the development process of the Shunbei ultra-deep fault-controlled condensate gas reservoir. Using the supercritical condensate gas with high condensate oil content from the Shunbei No. 4 fault zone as the subject of study, high-temperature and high-pressure visual experimental observation methods were first used to study the near-critical condensate oil and gas fluid’s critical opalescence, non-equilibrium phase transition, mist retrograde condensation sedimentation, critical slow-down phenomenon, and special phase evolution behavior such as fluid stratification in the retrograde condensation process. Subsequently, the non-equilibrium phase evolution characteristics of near-critical condensate gas fluid under high-temperature and ultra-high pressure conditions were analyzed, revealing the impact of non-equilibrium phase transition and seepage on early-stage mist retrograde condensation, sedimentation and relaxation slow-down phenomenon during gas well depletion, ultimately affecting the recovery degree of condensate oil. The relaxation time for mist condensate oil gravitational settling from the dew point pressure to the maximum retrograde condensation pressure in the PVT (pressure-volume-temperature) apparatus could reach 1 193 s, which when scaled to the deep fracture-cavity flow space of the Shunbei condensate gas reservoir, could correspond to 7 026 s. By reasonably controlling the production rate, the recovery degree of mist retrograde condensate oil could be increased by 4.99%. Based on long-core non-equilibrium seepage experiments, it was found that reasonably controlling the production rate could increase the recovery degree of retrograde condensate oil by 7.14%. Combining the the production curve patterns of typical gas wells in the Shunbei No. 4 zone, the influence of non-equilibrium phase transition and mist retrograde condensation sedimentation relaxation effect on condensate oil recovery degree during the retrograde condensation stage was explored. Actual production data showed that the current condensate oil recovery degree was more than 5% higher than that of natural gas. The developed high-temperature and high-pressure in-situ phase observation system has been successfully applied to ultra-deep gas reservoirs in Tarim Basin and Sichuan Basin, and can provide a reference for the analysis of the development dynamics and the adjustment of development strategies for the Shunbei ultra-deep condensate gas reservoir.

Key words: ultra-deep fault-controlled condensate gas reservoir, supercritical condensate gas, non-equilibrium phase transition, retrograde condensation sedimentation, relaxation slow-down phenomenon, non-equilibrium seepage