Effects of Temperature and Pressure on Spontaneous Counter-Current Imbibition in Unsaturated Porous Media

Capillary spontaneous imbibition mainly occurs in fractured reservoirs, low permeability reservoirs, and unconventional reservoirs, simultaneously accompanied by high temperature and pressure. In this paper, we present computations of spontaneous imbibition based on the classical fractional flow theory and proposed temperature- and pressure-dependent IFT relationships. Our work emphasizes that there are some discrepancies if we evaluate the spontaneous imbibition characteristics under geological conditions based on the traditional air/water/rock system experiments in an atmospheric environment. In detail, both the increasing temperature and pressure can decrease the IFT (capillary driving force), and the pressure effect is more significant. The increasing temperature will facilitate the water intake, as the results of competition between the positive role of enhanced wetting fluidity and negative role of reduced IFT, while the increasing pressure will slow down the water propagation due to the cooperation of reduced capillary driving force and increased nonwetting flow resistance. The nonwetting (gas) phase type will also influence the imbibition process, and the distinction between the methane/water/rock system and air/water/rock system is controlled by the nonwetting phase viscosity difference at low pressure and by the IFT difference at relatively high pressure. Furthermore, for the given porous media and fluid properties, faster water saturation profile propagation for the initial water saturation range (0 < S-i < 0.8) and slower movement for the initial water saturation range (0.8 < S-i < 1) are observed under the reservoir conditions (P = 25 MPa and T = 358 K). The former stage is essentially controlled by the enhancing water fluidity and decreasing capillary pressure, while the latter stage will be significantly affected by the increasing viscosity of the gas phase. Overall, a correction is needed to obtain the imbibition characteristics under geological conditions based on the traditional laboratory experiment for the air/water/rock system in an atmosphere environment.