Evaluation of tight waterflooding reservoirs with complex wettability by NMR data: A case study from Chang 6 and 8 members, Ordos Basin, NW China

After long-term water injection, many developed oilfields in the Ordos Basin, China, have entered a period of the high water-cut stage. In waterflooded layers, formation water is a mixture of injected water and primary formation water. The variation of formation water salinity will significantly affect the evaluation of reservoirs using resistivity logging curves while having little effect on the NMR response. It lays a foundation for identifying fluid type and evaluation of waterflooding grade by NMR logging. However, in waterflooding layers with mixed wettability, the NMR T-2 responses are still complicated and unclear. In this paper, the tight sandstone samples with different wettability of Chang 6 and Chang 8 members of Triassic Yanchang Formation in the Ordos Basin are used to investigate the NMR experiments under three saturation states: water-saturated state, oil displacing to bound water state (abbreviated as oil flooding) and water flooding to the residual oil state (abbreviated as water flooding). Results show that the distributions of standard T-2 spectra, without an external gradient magnetic field, of the water-saturated state in mix-wetted samples is very close to that of after oil flooding. Thus, the standard T-2 spectra will be hard to distinguish the oil layer, water layer, and waterflooded layer of mix-wetted reservoirs. The diffusion coefficients of water and oil in pores differ, resulting in different shift T-2 spectra results under gradient magnetic field. Therefore, the shift T-2 spectra with the external gradient magnetic field and different echo time TEs were calculated and measured. When the echo time TE = 3.6 ms is adopted, the T-2 signal of water-saturated tight rock samples will be nearly distributed less than 100 ms with magnetic field gradient G = 18 G/cm, rather the T-2 signal greater than 100 ms is the oil signal. This characteristic makes the shift T-2 spectra outstanding in the fluid identification of tight reservoirs with complex wettability. Finally, based on this experiment, a residual oil index based on the shift T-2 spectrum is constructed to evaluate the flooded grades of tight waterflooding reservoirs. The evaluation results are consistent with the core description and oil test results in the study infilled wells.

Automated summary

After long-term water injection, many developed oilfields in the Ordos Basin, China, have entered a high water-cut stage. The authors conducted NMR experiments on tight sandstone samples with different wettability to investigate the water and oil signal characteristics. They proposed a residual oil index based on the shift T-2 spectrum to evaluate the flooded grades of tight waterflooding reservoirs.