Abstract: Aiming at the optimization of different injection methods of CO₂ flooding after water flooding, based on the physical model of fracture radial flow, indoor experiment was conducted on CO₂ compound flooding in single fracture and complex fracture in five-spot well pattern (one well in injection and four other wells in production) to study the effect of CO₂ flooding with different injection processes after water flooding in low-permeability fractured reservoir. The results showed that: after the water flooding of the single fracture model, different injection processes, such as continuous CO₂ flooding, modified starch gel injection and ethylenediamine injection, were implemented successively, the recovery percent of the reservoir increased firstly and then decreased, and finally the recovery efficiency could be increased by 65.7% on the basis of the single-fracture water flooding; after water flooding of complex fracture model, different injection processes, different injection processes such as continuous CO₂ flooding, modified starch gel injection, ethylenediamine injection and water alternate gas, were implemented successively; the effect of water alternate gas on improving CO₂ flooding effect was the best and the recovery efficiency could be increased by 28.89% on the basis of single-fracture water flooding. The study results play a guiding role in CO₂ flooding after water flooding in low-permeability fractured reservoirs.

Key words: low permeability, fractures, physical simulation, CO₂ flooding, gas channeling, water and gas alternation, modified starch gel