Effect of displacement rates on fluid distributions and dynamics during water flooding in tight oil sandstone cores from nuclear magnetic resonance (NMR)

Water flooding is widely used as an economical and recyclable method to improve oil recovery in unconventional reservoirs. In this study, the effects of injection rates on oil distribution, the seepage dynamics and the contributions of increasing injection rates on enhancing oil recovery during water flooding are investigated. Typical tight oil sandstone cores with different permeabilities are obtained from Junggar Basin, Northwest China, for experimental studies that incorporate nuclear magnetic resonance analyses. Synthetic kerosene (viscosity = 11.2 mPa s) and heavy water (D2O) are used to simulate crude oil and formation water under formation conditions. The samples are first saturated with simulation water and thereafter displaced by synthetic oil until no water is produced to simulate the fully oil-saturated condition. To simulate the water flooding process, the samples are stably displaced by simulation water at injection rates increasing from 0.01 mL/min until no oil is produced. The T-2 spectra, injection pressures, and produced oil at different injection rates and water injection volumes are measured simultaneously. The transverse relaxation time (T-2) is converted for the corresponding pore-throat radius (r) by combining pore-throat size distributions (obtained from constant-rate mercury injection) and the fully oil saturated T-2 spectra. It indicates that the amplitude of T-2 spectra for macropore (>100 mu m) and mesopore (10-100 mu m) spaces decreases more rapidly than those for the micropore spaces (0-10 mu m) as injection rate increases. The classical capillary number theory is introduced to represent the effect of injection rate and characterize the dynamics of seepage processes. Results show that the oil recovery factor can be considerably improved by increasing the injection rate in tight oil cores, and this significant improvement is primarily attributed to mesopores and micropores.