Experimental study on CO2 flooding in tight sandy conglomerate cores: Oil displacement and CO2 storage

Abstract

The unique lithology of tight sandy conglomerate leads to severe microscopic heterogeneity, which significantly affects CO₂ flooding. In this paper, four lithologies were classified according to gravel size and content, then CO₂ flooding experiments were conducted to analyze the impacts of CO₂ injection rate, miscible degree, reservoir temperature, and rock lithology. The results show that during initial stage, dissolved gas flooding governs efficient oil production, CO₂ storage is mainly free storage in pores and dissolved storage in oil. Following breakthrough, CO₂ sweep expands significantly, leading to sustained oil production. Concurrently, dissolved storage decreases while free storage increases. After channeling, oil production rate and storage efficiency decays rapidly. Besides, increasing CO₂ injection rate enhances displacement effect, with the highest oil recovery and CO₂ storage factor of 37.96 % and 10.7 % at 0.1 mL min⁻¹. However, excessive injection rate induces premature breakthrough and exacerbates channeling, diminishing displacement efficiency. Under miscible condition, extraction of light hydrocarbons by CO₂ enhances, increasing oil recovery and CO₂ storage factor by 17.7 % and 3.28 %. Increasing reservoir temperature enhances oil fluidity, improving oil recovery factor by 10.93 %, but CO₂ storage diminishes. The lithology difference of tight sandy conglomerate induces severe microscopic heterogeneity, diminishing displacement efficiency. As gravel size and content increase, core's permeability increases but porosity decreases, leading to limited oil supply. And compounded by dual tortuosity in such cores, fluid migration pathways become increasingly tortuous, further exacerbating channeling. Therefore, compared with gravel-bearing coarse sandstone, oil recovery and CO₂ storage factor in sandy coarse conglomerate decrease by 7.41 % and 4.49 %.