Nuclear-Magnetic-Resonance Study on Oil Mobilization in Shale Exposed to CO2

Reservoirs in the Qian 3(4) 10 rhythmic layer of the Qianjiang Basin are shale oil reservoirs with intersalt sediments. During the natural depletion and development process, production rate of oil decreases rapidly. Water injection and CO2 injection are potential technologies for enhanced oil recovery (EOR) in shale. Because of high salt content in formations, unsaturated water dissolves salt and damages reservoirs. CO2 does not react with salt, and CO2 injection does not damage reservoirs. Moreover, CO2 could enter the micropores of the reservoir rocks and mobilize oil by diffusion, extraction, and swelling mechanisms. To verify oil mobilization in the shale exposed to CO2, exposure experiments based on nuclear magnetic resonance (NMR) were conducted in this study. NMR T-2 spectrum could reflect the oil in place and be used to calculate the oil content of rock with low permeability. In this study, 10 fresh shale samples (from six depths) were analyzed, and the oil contents were determined using NMR T-2 spectra. Two of the shale samples with high oil contents were selected for the CO2-exposure experiment. At a temperature of 40 degrees C and a pressure of 17.5 MPa, the fresh shale samples were exposed to CO2, and the NMR T-2 spectra obtained were used to continuously determine the oil content of the shale. The oil mobilization in the shale exposed to CO2 was determined. The results of the NMR T-2 spectra showed that the NMR volume fractions of the remaining oil in seven fresh shale samples were above 10%. The recovery of the S5# shale exposed to CO2 was 51.2% after 8 days, whereas that of the S9# shale was 55.8% after 6.1 days. These results indicated that more than half of the shale oil was mobilized during the relatively long exposure time after CO2 injection. NMR T-2 spectroscopy results also showed that oil in all pores could be mobilized as the exposure time increased. This study showed the quantitative results of the CO2-injection method and EOR in a shale oil reservoir of the Qianjiang Basin. All conclusions support starting a CO2-EOR pilot project in the shale oil reservoir with intersalt sediments with ultralow permeability.