弱挥发性碳酸盐岩油藏原油相态特征及注水开发对策

Phase behavior characteristics and water-flooding development technical policy of weakly volatile oil in carbonate reservoirs

以滨里海盆地东缘裂缝-孔隙型碳酸盐岩油藏为例,分析了弱挥发性油藏在地层压力下降过程中的原油相态变化和渗流物理特征变化规律,并提出油藏在不同地层压力水平下注水开发技术对策。实验表明,随地层压力下降,弱挥发性原油脱气,甲烷和中间烃依次析出,原油逐渐转变为普通黑油。随着轻烃组分析出,地层原油饱和度迅速下降,原油黏度增大,油相渗透率迅速降低,从而降低油井产能。裂缝越发育,油井产能下降幅度越大。地层压力保持水平是影响油藏开发效果的主要因素,地层压力保持水平越低,油田最终采收率越低,在低压力保持水平下需要实施注水恢复地层压力。注水时地层压力保持水平越低,合理的压力恢复水平越低,采收率越低。与裂缝不发育区相比,相同地层压力保持水平下实施注水恢复地层压力,裂缝发育区的合理注采比要低,裂缝发育区注水时机对油田采收率的影响更大。因此开发碳酸盐岩弱挥发性油藏应采用早期温和注水开发方式。

A fracture-pore carbonate reservoir in eastem Pre-Caspian basin was taken as an example to analyze the oil phase behavior change and seepage physical characteristics change of weakly volatile oil reservoirs with the decrease of formation pressure. Based on the analysis, the water-flooding development technique policy of the reservoir under different formation pressure was presented. Experiments show that the weakly volatile crude oil degasifies as the formation pressure decreases, with methane and intermediate hydrocarbons separated out successively, and the crude oil gradually transforms into ordinary black oil. With the separation of light hydrocarbons, the saturation of in-place oil drops rapidly, the viscosity increases, and the oil permeability reduces, leading to lower well productivity. Retention of formation pressure is a vital factor controlling the reservoir development effect. The lower the formation pressure, the lower the ultimate recovery. Given low formation pressure, water-flooding is required to recover the formation pressure. In water-flooding, the lower level the formation pressure is retained at, the lower the reasonable pressure to be recovered is, and the lower the ultimate recovery is. Compared with zones without fractures, the zone with fractures provides lower injection-production ratio when water-flooding is conducted under the same formation pressure, and its water-flooding time has more impact on oilfield recovery. Therefore, it is recommended to develop weakly volatile oil reservoir by early water-flooding in a moderate way.