Research on the Differential Oil Producing in the Various Scale Pores under Different CO2 Flooding Modes with a Fluid Distribution Pore Classification Method

CO2 injection is an effective way to improve oil recovery for tight sandstone reservoirs. In this study, the tight sandstone cores collected from Chang 6 Member of Yanchang Formation in Ordos Basin underwent continuous CO2 flooding, and CO2 huff and puff oil displacement measurements were collected under five various CO2 injection pressures, 4, 8, 12, 16, and 20 MPa respectively; combined with the classification of pores in the tight sandstone cores, the differences in oil producing degree in varying scale of pores were discussed in detail. Under continuous CO2 flooding, the oil producing degree reaches up to 73.29%, while the CO2 huff and puff features a higher oil producing degree, and it reaches almost 82.03%. The oil producing degree for these two modes shows a triple-stage with increasing CO2 injection pressures. There are five types of pores in tight sandstone pores, including I-1, I-2, I-3, I-4, and II, and the I-3 and I-4 pores feature not only a significantly high oil producing degree but also the smallest differences in oil producing degree under the two different CO2 flooding modes. The pore size effect is more significant in the tight sandstone cores with the increase in CO2 injection pressures; in comparison with the continuous CO2 flooding, the difference of oil producing degree in the pores with smaller apertures is significantly higher for the CO2 huff and puff, while it is not obvious in the pores with larger pore apertures, especially when the CO2 injection pressure exceeds the minimum miscible pressure (MMP). In addition, the CO2 injection pressure decreases to obtain the optimal oil producing degree in pores with larger pore aperture. This study can help in selecting appropriate CO2 injection modes to achieve the oil enhanced recovery for various types of tight sandstone reservoirs.