Abstract:

CO₂-flooding is an important way to improve oil displacement efficiency in extra-/ultra-low permeability reservoirs. However, the microscopic mechanism of CO₂-flooding and oil displacement effect evaluation are hampered by the lack of micro-visualization technology. In this study, we suggested that micro-visualization experiments of CO₂-flooding in extra-/ultra-low permeability reservoirs under high temperature and pressure can satisfactorily reveal the CO₂-flooding mechanism.Ultra-low permeability reservoirs in the Ordos Basin are taken as examples, and micro-visualization experiments of CO₂-flooding were newly and successfully carried out by using the high temperature, high pressure and anti-riot real sandstone model. Dynamic phenomena of the different CO₂ phases (in crude oil displacement) in the complex pore throat of the ultra-low permeability reservoirs were simultaneously observed under the microscope, and the occurrence and seepage of CO₂ in the different pore throat structures were obtained. The pore throat structure, especially the uniformity of pore throat size distribution (critical for the CO₂-flooding efficiency), directly determines whether the CO₂ can enter the reservoir and the subsequent seepage path. The micro-visualization experiment of CO₂-flooding can not only effectively reveal the microscopic mechanism of CO₂-flooding and evaluate the oil displacement effect, but can also provide guidance for later-stage reservoir analyses.

Key words: ultra-low permeability, CO₂-flooding, micro-visualization, pore throat structure, oil displacement efficiency