Characterization of asphaltene deposition and formation damage during CO2 miscible/immiscible flooding in Low-permeability sandstones using NMR T1-T2

CO₂ flooding in low-permeability oilfields has been widely adopted worldwide as it significantly enhances oil recovery. However, when CO₂ is injected into the reservoir, asphaltene precipitation may occur, damaging the reservoir and resulting in a less pronounced improvement in recovery rate. There is few researchers have directly studied asphaltene deposition during CO₂ displacement at the moment. To shed light on the asphaltene deposition characteristics in CO2 static injection into oil and during CO₂ miscible/immiscible flooding, analyse the degree of damage caused by asphaltene deposition on the porosity and permeability of the core, a few samples were selected from the Tarim Oilfield for four-component experiments and CO₂ core flooding experiments. Through four-component testing experiments, the proportion of asphaltene deposition in oil at different CO₂ injection concentration was determined. When the molar ratio of injected CO₂ to crude oil reached 9:5, an inflection point in the ​relative asphaltene deposition amount was observed. Beyond this threshold, continued CO₂ injection resulted in no significant increase in the relative deposition amount. At a molar injection ratio of 3:1, the ​asphaltene deposition mass fraction peaked at 7.76% with a ​deposition efficiency of 86.7%. At the same time, we conducted CO₂ miscible flooding/immiscible flooding experiment to characterize of asphaltene deposition and reservoir damage degree using NMR technology. The ​asphaltene-associated signatures in NMR T1-T2 relaxation spectra are defined within the ​shared parameter space where: 5 < T₁/T₂ < 14 and T₂ within 1-5 ms. Based on NMR T₁-T₂ of CO₂ flooding, the amount of asphaltene precipitation in the core sample can be visually observed. The asphaltene deposition percentages caused by CO₂ immiscible flooding, near-miscible flooding, and miscible flooding are 11.97%, 16.34%, and 50.64%, respectively. By comparing the T2 spectrum during re-saturation with that of the initial condition, it is evident that asphaltene is mostly deposited in the mesopores and macropores. The pore obstruction caused by asphaltene deposition prevents the resaturated crude oil from entering the pores, which lowers the T2 spectrum curves by 1-10 and > 10 ms. The corresponding percentages of reduction in porosity are 6.71%, 11.8%, and 17.36%, while the percentages of permeability impairment are 23.29%, 60.88%, and 65.25%, respectively. Determining the percentage of asphaltene deposits during CO₂ injection and understanding the extent of damage to the reservoir can significantly improve the effectiveness of CO₂ in EOR.