Volume 67

Comeplex Fracture Network Fracturing Coupled with CO2-WAG Technology Improves Recovery and Storage Efficiency in Ultra-low Permeability Reservoirs Menghao Zhang, Yu Zhang, Daigang Wang, Kaoping Song, Yuhua Zhao, Chengming Li, Wenhui Gao

Abstract

This paper addresses the challenges of high injection pressures, difficulty in establishing displacement systems, and low recovery rates in ultra-low permeability reservoirs by proposing an integrated development strategy that combines large-scale complex fracture network fracturing with a COâ‚‚ cooperative injection-production process based on the “injection–shut-in–production” cycle. The approach aims to enhance both oil recovery and COâ‚‚ sequestration potential through reservoir modification and crude oil property regulation. A non-intrusive embedded discrete fracture model (EDFM) is used to accurately represent the complex fracture network in three-dimensional space, overcoming the limitations of traditional methods in multi-scale fracture handling. The paper compares the effects of three development methods: conventional water injection, COâ‚‚ huff-and-puff, and COâ‚‚-water alternating injection (COâ‚‚-WAG). Key parameters such as injection timing, total COâ‚‚ volume, injection rate, and shut-in time are optimized, with carbon sequestration efficiency introduced as a core evaluation index. Simulation results show that cyclic COâ‚‚-WAG can increase recovery by 8.9% compared to conventional water injection. Key mechanisms include: COâ‚‚ reducing oil viscosity and expanding its volume during the shut-in phase, water injection controlling COâ‚‚ flow and expanding sweep volume, and periodic injection–shutdown operations restoring fracture conductivity and maintaining reservoir pressure. This method improves fluid exchange between fractured and unfractured zones, reducing recovery discrepancies. Optimized injection gas rates and timing can delay production decline and reduce fracture closure risks. From a carbon sequestration perspective, the optimized COâ‚‚-WAG scheme improves both oil recovery and COâ‚‚ sequestration efficiency, achieving synergistic optimization. This study offers an integrated method for ultra-low permeability tight oil reservoirs, providing a reliable pathway for green and efficient development.

Keywords COâ‚‚-WAG Injection, Complex Fracture Network, Enhanced Oil Recovery, Carbon Sequestration, Ultra-low Permeability Reservoirs