Study on spontaneous imbibition and displacement characteristics of mixed-wet tight sandstone reservoir based on high-precision balance and NMR method

•The spontaneous imbibition (SI) and displacement characteristics of mixed-wet tight sandstone reservoir are studied by high-precision balance and NMR method.•The pores of tight sandstone were divided into adsorption pores, capillary bound fluid pores, and movable fluid pores by NMR T2 spectra.•The SI and displacement behaviors of different pores were studied and compared.•The wettability of different pores in the mixed-wet rock was accurately characterized. Wettability is the key to understanding the complex seepage characteristics of tight sandstone reservoirs, which will affect the distribution of the residual hydrocarbon resources during the reservoir exploitation process. Complex wettability, strong heterogeneity and tiny pore space make it more challenging to study the imbibition characteristics of mixed-wet tight sandstone reservoirs. In this study, the spontaneous imbibition (SI) and displacement characteristics of mixed-wet tight sandstone reservoirs are studied by high precision balance and nuclear magnetic resonance (NMR) T2 experiment. 3 rock plungers (A1, B1, C1) were put into the CaCl2 solution for SI, while their parallel samples (A2, B2, C2) were carried out SI experiments in the experimental oil. Then all the plungers are respectively displaced by the imbibition liquid after SI experiments. The weight and NMR T2 spectra of the plungers in the above different conditions were measured. According to the experiment results, the pores of tight sandstone can be divided into 3 types with different wettability. The adsorption pores with T2 distribution of 0.01–2.5 ms are water-wet, the capillary bound fluid pores with T2 between 2.5 and 50 ms and the movable fluid pore with T2 greater than 50 ms are mainly oil-wet. The SI and displacement behaviors of different pores were studied, the results show the oil absorbed per cc rock is more than 4 times that of brine. The weight gain after SI in the first 24 h can indicate the total SI result. The study results can accurately characterize the wettability of different pores and contribute to a novel wettability evaluation method in mixed wettable tight sandstone.