An enhanced oil recovery (EOR) technique comprising the alternate injection of gas (CO₂) and water, commonly referred to as water alternating gas (WAG) flooding, is currently ongoing at an oil field in the northeastern Powder River Basin (PRB), where 4 million barrels (MMbbl) of oil were produced between May 2013 and September 2017. During WAG flooding, a large amount of CO₂, which contains some impurities, is produced at the surface with the recovered oil and reinjected into the reservoir to minimize the amount of purchased CO₂. The concentration of these impurities in the CO₂, which are dominated by CH₄, is a key parameter in the design of miscible CO₂ flooding of an oil reservoir and in the quantitative assessment of the associated storage of CO₂ that occurs in the reservoir. For a given oil and reservoir temperature, the CO₂–oil minimum miscibility pressure (MMP) is strongly and adversely affected by the presence of CH₄, the concentration of which varies with time and operational conditions, such as injection pressure, flooding pattern, and injection scheme (i.e., continuous CO₂ injection or WAG). To capture the full range of potential variation in the MMP caused by the presence of CH₄, a series of laboratory experiments were conducted to determine CO₂–oil MMP with different mole percentages of CH₄ in the CO₂. All of the experiments were performed at reservoir temperature (108 °F) using the VIT (vanishing interfacial tension) method. Results showed that MMP increases from 1403 to 4085 psi as the mole percentage of CH₄ in the CO₂ increases from 0% to 100%. To assess the impact of this variation in MMP on the oil recovery performance in the field, a history-matched, compositional reservoir simulation model was used to predict the oil production performance for a WAG flood using CO₂ that contained a range of CH₄ concentrations. The reservoir simulations examined the effects of permeability heterogeneity, fluid crossflow, and phase behavior on the oil displacement performance in three-dimensional space over time. Simulation results indicated that CO₂ floods using a limited range of CO₂–CH₄ mixtures could still maintain multiple-contact miscibility and result in effective EOR. In addition, the ability to reinject produced CO₂–CH₄ mixtures as is, without removal of the CH₄, ensured that this approach to EOR would continue to be cost-effective.

目前，东北粉末河盆地（PRB）的一个油田正在开发一种增强的采收率（EOR）技术，该技术包括交替注入气体（CO ₂）和水（通常称为水交替气（WAG）驱油），在2013年5月至2017年9月期间生产了4百万桶（MMbbl）的石油。在WAG驱油过程中，随着回收的石油在地表产生了大量的CO ₂（其中包含一些杂质），然后将其重新注入油藏以最大程度地减少油藏。购买的CO ₂量。这些杂质中以CH ₄为主的CO ₂中的浓度是可混溶CO ₂设计中的关键参数。油藏的注水，以及对油藏中发生的相关CO ₂储存的定量评估。对于给定的油藏温度，CH ₄的存在会严重不利地影响CO _2-油的最小混溶压力（MMP），CH ₄的浓度会随时间和操作条件（例如注入压力，驱油模式，和注入方案（即连续CO ₂注入或WAG）。为了捕获由于CH ₄的存在而引起的MMP的全部潜在变化，进行了一系列实验室实验以确定具有不同摩尔百分比的CH _4的CO _2-油MMP在CO _2中。所有实验均使用VIT（消失界面张力）方法在储层温度（108°F）下进行。结果显示，随着CH ₄在CO _2中的摩尔百分比从0％增加到100％，MMP从1403 psi增加到4085 psi 。为了评估这种MMP变化对油田采油性能的影响，使用了历史匹配的成分储层模拟模型来预测WAG洪水的采油性能，该WAG洪水使用的CO ₂包含一系列CH ₄浓度。储层模拟研究了渗透率非均质性，流体错流和相行为对三维空间随时间的驱油性能的影响。仿真结果表明，CO ₂用CO洪水的有限范围₂ -CH _4分的混合物仍可保持多触点的混溶性和产生有效的EOR。另外，能够直接注入生产的CO ₂ -CH ₄混合物而无需除去CH _4的能力确保了这种提高采收率的方法将继续具有成本效益。