Differences in the Fluid Characteristics between Spontaneous Imbibition and Drainage in Tight Sandstone Cores from Nuclear Magnetic Resonance

As the water injected during hydraulic fracturing is imbibed and then displaced in the matrix of tight reservoirs, it is meaningful to characterize the fluid distributions and percolation dynamics during these two processes as they significantly affect production. In this study, 6 tight sandstone cores from the Chinese Ordos Basin were selected to experimentally study the fluid seepage and distributions during spontaneous imbibition and drainage using a low-field nuclear magnetic resonance (NMR) core analysis unit. Dry cores were first tested via cocurrent imbibition and then centrifuged from fully saturated to irreducible water saturation to simulate the imbibition and drainage processes, respectively. The weights and T₂ spectra were measured concurrently for each step. The results showed that the volume of imbibed water increased rapidly during the first 3000 min and reached a constant value at the end of the experiment; the final imbibition water saturation Swᵢₘ ranged from 48.52% to 89.20%. The irreducible water saturation Swᵢᵣ was reached after a centrifuge speed of 10 000 r/min and varied from 40.46% to 53.28%. The gas and water relative permeabilities were estimated by combining the T₂ spectra for different water saturations, which indicated the effect of the capillary force and pore–throat structure on the different fluid seepage characteristics during spontaneous imbibition and drainage. The T₂ time was converted to the corresponding pore–throat radius by combining the fully water-saturated T₂ spectra and pore distributions from a constant-rate mercury injection. Micropores of 0–2 μm were identified as the dominant pore spaces for imbibed water during spontaneous imbibition and the water remained after centrifugation, whereas a small amount of water was imbibed or remained in pores larger than 20 μm. The physical properties, microstructure, and dispersed clay minerals were considered to be the three dominant factors of the fluid dynamics and distribution differences, and the effects were stronger during spontaneous imbibition.