海上特殊凝析气藏流体相态及混相特征

Fluid phase and miscibility characteristics of offshore special condensate gas reservoirs

随着海上油气勘探不断向深层拓展,以近临界态近饱和、高含凝析油为特征的特殊凝析气藏越来越多,但目前对海上该类凝析气藏的高压物性及注气混相机理仍认识不清,有待进一步深入研究。以渤海特殊凝析气藏流体高压物性测试(PVT)为基础,通过开展室内注气膨胀、多次接触及长细管最低混相压力实验分析,结合注气相态数值模拟方法,模拟从油气层上、中、下部不同深度进行伴生气注入,明确了海上特殊凝析气藏的相态特征,并探索了伴生气与地层剩余油气体系的配伍性及混相机理。结果表明:海上特殊凝析气藏一旦发生压降,地层中会产生大量的反凝析液,形成带气顶的挥发油藏,恒质膨胀过程中会发生乳光效应及轻烃抽提分层现象;伴生气对降低顶部凝析气体系的反凝析伤害作用不大,降低反凝析液量小于1%,中部轻烃凝析油体系与伴生气的多次接触混相压力为26.80 MPa,伴生气与底部凝析油体系通过溶解作用存在凝析气驱混相机理的同时,通过抽提作用存在蒸发气驱混相机理,最低混相压力为41.58 MPa,亦低于地层压力,能够达到很好的混相效果。研究成果对海上特殊凝析气藏注气提高采收率意义重大,为中后期气藏高效开发设计提供了坚实可靠的理论依据。

With deepening development of offshore oil and gas exploration,there are more and more special condensate gas reservoirs characterized by near-critical,near-saturation and high condensate oil content.At present,there is little understanding of high-pressure properties and gas injection miscibility mechanism of this kind of condensate gas reservoirs,which is of great significance for further research.Based on Bohai Sea special condensate gas reservoir fluid high pressure properties tests(PVT),through lab analysis of gas injection expansion,multiple contacts and long slim tube minimum miscible pressure,and combined with gas injection phase numerical simulation method to simulate associated gas injection from different depth in upper,middle and lower parts of reservoir,the phase behavior characteristics of special offshore condensate gas reservoir are determined,and the compatibility and miscibility of associated gas and residual oil and gas system are explored.The results show that once the pressure drop occurs in special offshore condensate reservoir,a large amount of retrograde condensate may generate in the formation,forming volatile oil reservoir with gas cap,and the opalescence effect and light hydrocarbon extraction stratification may occur in the constant mass expansion process.Associated gas has little effect of reducing retrograde condensation damage of the top condensate gas system,with reduced amount of retrograde condensate<1%.The multiple-contact miscible pressure of central light hydrocarbon condensate oil system and associated gas is 26.80 MPa.Condensate gas miscible displacement mechanism by dissolving effect of and evaporation miscible gas displacement mechanism by extraction effect exist simultaneously in the bottom of associated gas and gas condensate oil system,with the lowest miscible pressure of 41.58 MPa,also lower than formation pressure,and can achieve good miscible effect.This study is of great significance to EOR of special offshore condensate gas reservoirs by gas injection and provides theoretical basis for efficient development planning of gas reservoirs in mid-to-late stage.