Abstract:

Ultra-deep fault-controlled reservoirs exhibit tabular shapes， extreme thickness， and significant gravitational differentiation in the components of multistage fluids. Their post-injection fluid phases differ greatly from those of conventional sandstone reservoirs. A lack of clear understanding of the mechanisms underlying gas injections in these reservoirs has severely hindered their gas injection performance. Using high-temperature and high-pressure full-field visualized experiments on oil-gas miscibility to clarify the static contact and miscible characteristics of oil and gas， we use a vertically placed empty， long sand-pack tube to simulate fault-controlled cavity-type reservoirs. For condensate gas with a high liquid content after retrograde condensation， phase behavior experiments are conducted with top and bottom gas injections. Based on the analysis of fluid samples collected at multiple stages at different positions along the long sand-pack tube， we investigate the contact modes and phase transition characteristics of oil and gas under both gas injection ways， and then clarify vertical compositional gradients formed under the action of gravity and diffusion. The results indicate that top gas injection leads to a limited oil-gas contact area under miscibility pressure， resulting in a low exchange rate of oil and gas components， insufficient volumetric expansion of oil， and ultimately the failure to form oil-gas miscible phase. Under the combined effects of gravity and diffusion， the dynamic cycle of “heavy drop and light rise” of components under the action of gravity and diffusion occurs， forming a vertical component gradient. Notably， the C₂₀₊ content in the gas condensate at the bottom of the tube is 3.4 times that at the top. In contrast， bottom gas injection enhances both gas injection volume and the area and frequency of oil-gas contacts， facilitating multiple oil-gas contacts to develop local miscible phases， while expanding the range of recoverable gas condensate components， reducing the vertical difference in C₂₀₊ content in gas condensate to 2.1 times. The cumulative recovery of gas condensate from three rounds of bottom gas injections is enhanced to 32.66 %， more than double that achieved through three rounds of top gas injections （14.13 %）.

Key words: phase behavior, component gradient, gas injection method, enhanced oil recovery （EOR）, condensate gas, ultra-deep stratum, Shunbei area, Tarim Basin