Effects of Diffusion, Adsorption, and Hysteresis on Huff-n-Puff Performance in Ultratight Reservoirs with Different Fluid Types and Injection Gases

本文通过大尺度数值模拟研究了超致密页岩油藏中吞吐技术的效果，重点分析了扩散、吸附和相对渗透率滞后效应对不同流体类型（反凝析气、挥发油和黑油）和不同注入气体（CO₂、C₂H₆和C₃H₈）生产效果的影响。研究使用了双孔组分模型，全面评估了多组分扩散、吸附和滞后效应对油藏采收率和注入气体保留能力的影响。结果表明，吞吐技术可提高油藏采收率4.07%至6.2%，且采收率的提升与注入气体和油藏流体类型密切相关。扩散和吸附是页岩油藏中不可忽略的关键机理，而相对渗透率的滞后效应在混相注入中影响较小。扩散效应随着气油比（GOR）的增加而增强，而吸附效应则随着GOR的增加而减弱。研究还发现，延长浸泡期可以进一步降低油密度，从而提高采收率和吞吐效率。

CMG软件应用情况

本文使用了CMG（Computer Modeling Group）的GEM模拟器进行数值模拟。GEM模拟器是一种用于非常规油藏的组分模拟工具，能够解决多相流动和传输现象的空间-时间质量守恒方程。研究中采用了双孔隙模型，结合天然裂缝和基质流动机制，模拟了Eagle Ford页岩油藏的生产效果。模拟器还考虑了扩散、吸附和相对渗透率滞后效应，通过Sigmund相关性预测多组分扩散系数，并使用扩展Langmuir等温线描述多组分竞争吸附行为。模拟结果验证了吞吐技术在不同流体类型和注入气体条件下的效果，为实际油藏开发提供了理论支持。

结论

  1. 吞吐技术可显著提高超致密油藏的采收率，提升范围为4.07%至6.2%。
  2. 注入气体效率与油藏流体类型密切相关，C₂H₆对黑油和挥发油系统效果最佳，CO₂对反凝析气效果最佳。
  3. 扩散和吸附是页岩油藏中不可忽略的关键机理，而相对渗透率滞后效应对生产效果影响较小。
  4. 扩散系数是时空函数，需根据压力、组成和密度的变化动态更新。
  5. 吸附和扩散共同作用降低了油密度，延长浸泡期可进一步提高油的流动性和采收率。
  6. 黑油油藏对注入气体的保留能力最高，而反凝析气和挥发性油油藏的保留能力最低。

 作者单位

美国德克萨斯A&M大学石油工程系

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Abstract

Cyclic solvent injection, known as solvent huff-n-puff, is one of the promising techniques for enhancing oil recovery from shale reservoirs. This study investigates the huff-n-puff performance in ultratight shale reservoirs by conducting large-scale numerical simulations for a wide range of reservoir fluid types (retrograde condensate, volatile oil, and black oil) and different injection gases (CO2, C2H6, and C3H8). A dual-porosity compositional model is utilized to comprehensively evaluate the impact of multicomponent diffusion, adsorption, and hysteresis on the production performance of each reservoir fluid and the retention capacity of the injection gases. The results show that the huff-n-puff process improves oil recovery by 4–6% when injected with 10% PV of gas. Huff-n-puff efficiency increases with decreasing gas-oil ratio (GOR). C2H6 provides the highest recovery for the black oil and volatile oil systems, and CO2 provides the highest recovery for retrograde condensate fluid type. Diffusion and adsorption are essential mechanisms to be considered when modeling gas injection in shale reservoirs. However, the relative permeability hysteresis effect is not significant. Diffusion impact increases with GOR, while adsorption impact decreases with increasing GOR. Oil density reduction caused by diffusion is observed more during the soaking period considering that the diffusion of the injected gas caused a low prediction error, while adsorption for the injected gas showed a noticeable error.

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