Study on Main Controlling Factors of CO2 Enhanced Gas Recovery and Geological Storage in Tight Gas Reservoirs

 

致密气藏因其低孔、低渗和强非均质性，开发难度大，采收率低。CO₂提高天然气采收率与地质封存（CSEGR）技术将CO₂封存与天然气开发相结合，兼具经济效益与环境效益。本研究基于PR状态方程分析CO₂与天然气的物性差异，建立考虑CO₂溶解与地球化学反应的数值模型，采用正交试验设计方法系统研究注入方式、注入速度、产气速度及关井条件对CSEGR效果的影响。

结果表明，CO₂在压缩因子、密度和黏度方面与天然气差异显著，有利于形成活塞式驱替；间歇注入在提高采收率方面略优于连续注入，但连续注入更有利于CO₂封存；影响采收率的主控因素排序为：注入速度 > 产气速度 > 注入方式 > 关井条件；影响封存效果排序为：注入速度> 注入方式 > 产气速度 > 关井条件。研究为致密气藏CO₂驱提高采收率与封存策略优化提供了理论依据。

CMG软件应用情况

本研究采用 CMG-GEM（v.2022.1） 软件建立数值模拟模型，模拟CO₂在致密气藏中的驱替与封存过程。模型中考虑了：

  • CO₂在天然气中的混相行为；
  • CO₂在地层水中的溶解过程；
  • CO₂-水-岩石之间的地球化学反应（如矿物溶解与沉淀）；
  • 不同注入方式（间歇/连续）、注入速度、产气速度及关井条件对气藏动态的影响。

GEM模块的多组分、多相流体模拟能力与地球化学耦合功能为研究提供了强大支持。模拟结果与文献对比验证，表明模型具有良好的可靠性。

结论

  1. CO₂与天然气在密度、黏度和压缩因子方面差异显著，有利于形成稳定驱替前沿，减少混相损失；
  2. 间歇注入可提高天然气采收率，连续注入则更有利于CO₂封存；
  3. 注入速度是影响采收率和封存效果的最关键因素；
  4. 注入方式、产气速度和关井条件对系统指标有次级影响；
  5. 封存机制以结构封存和残余封存为主，矿物封存贡献较小；
  6. 通过正交试验和多目标优化分析，提出了兼顾采收率与CO₂利用效率的推荐方案（如Case 15），具备良好的经济效益与封存潜力；
  7. 研究为致密气藏实施CSEGR技术提供了理论依据与现场操作建议。

作者单位

  • 中国石油长庆油田公司

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Abstract

Tight gas reservoirs, as important unconventional natural gas resources, face low recovery rates due to low porosity, low permeability, and strong heterogeneity. CO2 Storage with Enhanced Gas Recovery (CSEGR) technology combines CO2 geological storage with natural gas development, providing both economic and environmental benefits. However, the main controlling factors and influence mechanisms remain unclear. This study utilized the PR-EOS to investigate CH4, CO2, and natural gas physical properties, established a numerical simulation model considering CO2 dissolution and geochemical reactions, and explored the influence of injection scheme, injection rate, production rate, and shut-in condition on CO2 enhanced recovery and storage effectiveness through orthogonal design. Results show that CO2 exhibits significant differences in compressibility factor, density, and viscosity compared to natural gas, enabling piston-like displacement. Intermittent injection slightly outperforms continuous injection in recovery enhancement, while continuous injection provides greater CO2 storage capacity. The ranking of the significance of different influencing factors for enhanced oil recovery is as follows: injection rate > production rate > injection scheme > shut-in condition. For the effect of geological storage of CO2, it is as follows: injection rate > injection scheme > production rate > shut-in condition. During gas injection, supercritical, ionic, and dissolved CO2 continuously increase while mineral CO2 decreases, with storage mechanisms dominated by structural and residual trapping. The study provides scientific basis for optimizing CO2 flooding strategies in tight gas reservoirs.

Keywords: tight gas reservoir; CSEGR; numerical simulation; enhanced recovery; CO2 geological storage

 

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