Dominant Mechanisms and Field Pilot Test of CO2 Immiscible Flooding for Ultra-Low Permeability Fault-Block Reservoirs

CO2 immiscible flooding emerges as a crucial enhanced oil recovery (EOR) technology for reservoirs with strong heterogeneity, elevated crude oil miscibility pressure, and severe pressure depletion. Focusing on a representative complex fault-block ultra-low permeability reservoir, this study first revealed the degree of crude oil expansion and viscosity reduction with CO2 injection through laboratory experiments. The minimum miscibility pressure (MMP) between CO2 and crude oil was precisely determined using slim-tube experiments. Subsequently, a numerical model calibrated with slim-tube experiments was established to comparatively analyze the distribution characteristics of oil-gas transition zones and interfacial tension under varying miscibility conditions. The spatial distribution patterns of light/heavy hydrocarbon components and CO2 along the cores were systematically summarized, demonstrating the dual mechanisms of immiscible flooding in reservoir pressure increasing and microscopic displacement efficiency enhancement. Taking the M oilfield - a typical ultra-low permeability complex fault-block reservoir - as a case study, key operational parameters were optimized including reservoir pressure, gas injection volume, injection rate, and critical gas-oil ratio (GOR) through 3D high-resolution geological model and numerical simulation. Field pilot test was successfully conducted, demonstrating remarkable performance: 5.05% incremental recovery factor, 0.35 t(Oil)/t (CO2) oil exchange ratio, and significant delay in gas breakthrough. The research results revealed that: (1) Reservoir energy restoration primarily stems from volume expansion induced by gas dissolution; (2) Enhanced displacement efficiency arises from synergistic effects of three mechanisms – dissolution and expansion of oil, interfacial tension reduction, and light component extraction. This methodology provides valuable operational guidance for CO2 flooding and EOR optimization in fault block reservoirs.