海上特稠油油藏超临界CO_(2)和蒸汽同注提高采收率研究

Study on enhanced oil recovery by co-injection of supercritical CO_(2) and steam in offshore extra-heavy oil reservoirs

稠油油藏开发普遍采用热采工艺,但对于埋深超过800 m的强底水特稠油油藏来说,采用注蒸汽的方式无法达到有效开发。在目标油田进行了SAGD(蒸汽辅助重力泄油)矿场试验,但由于油藏压力高、底水能量强,效果不佳。因此提出了在注汽过程中利用超临界CO_(2)和蒸汽同注的方法来改善SAGD开发效果。该方法的主要机理是注入的气体可以大大降低蒸汽的分压,从而减少注入过程中的热量损失和干度下降。研究结果表明,该方法可以使注蒸汽管柱的热损失降低40%。在超临界条件下,气体在稠油中的溶解度接近100 m^(3)/m^(3),饱和气的稠油在150℃时黏度低于200 mPa·s,比不饱和气时降低了80%,稠油流动性大大提高能够正常采出。SAGD数值模拟表明,当注入气体的摩尔百分比为20%时,一方面气体分压作用使蒸汽能够保持一定干度,另一方面气体能够降低蒸汽与储层之间的热传导,使蒸汽更容易保持蒸汽状态,从而增大蒸汽腔的扩展范围。该方法生产过程表现出了高采收率,与纯蒸汽SAGD方式相当,同时油汽比为0.8~1.2,也远高于现有的纯蒸汽SAGD方式。模拟结果表明,在注采过程中,当储层与底水之间达到平衡后,底水不会流入蒸汽腔,从而降低底水的影响。本文为海上中深层特稠油开发提供了一种有效途径,对降低稠油热采的蒸汽消耗和操作成本具有重要意义。

The thermal recovery process is generally used in the development of heavy oil reservoirs,but for super heavy oil reservoirs with strong bottom water with a burial depth of more than 800 m,steam injection cannot achieve effective development.The SAGD(Steam Assisted Gravity Drainage)field test was carried out in the target oilfield,but the effect was not good due to high reservoir pressure and strong bottom water energy.Therefore,a method of co-injecting supercritical CO_(2) and steam in the steam injection process is proposed to improve the development effect of SAGD.The main mechanism of this method is that the injected gas can greatly reduce the partial pressure of the steam,thereby reducing the heat loss and dryness drop during the injection process.The results of the study show that the method can reduce the heat loss of the steam injection string by 40%.Under supercritical conditions,the solubility of gas in heavy oil is close to 100 m^(3)/m^(3),and the viscosity of heavy oil with saturated gas is lower than 200 mPa·s at 150℃,which is 80%lower than that of unsaturated gas,the fluidity of heavy oil is greatly improved and can be extracted normally.SAGD numerical simulation shows that when the mole percentage of injected gas is 20%,on the one hand,the partial pressure of the gas can keep the steam to a certain dryness,and on the other hand,the gas can reduce the heat conduction between the steam and the reservoir,making the steam easier to maintain steam state,thereby increasing the expansion range of the steam chamber.The production process of this method shows a high recovery rate,which is comparable to the pure steam SAGD method,and the oil-steam ratio is 0.8~1.2,which is also much higher than the existing pure steam SAGD method.The simulation results show that during the injection-production process,when the reservoir and bottom water reach equilibrium,the bottom water will not flow into the steam chamber,thus reducing the influence of bottom water.This paper provides an effective way for the development of offshore medium-deep extra-heavy oil,which is of great significance for reducing the steam consumption and operating cost of heavy oil thermal recovery.