This paper presents a numerical study of co-current/counter-current spontaneous imbibition (SI). Spontaneous imbibition is an essential mechanism for oil recovery in naturally fractured reservoirs, where the wetting phase enters the matrix by capillary forces to spontaneously displace the non-wetting phase. When matrix blocks in the reservoir are simultaneously exposed to water and oil in the surrounding fractures, oil recovery by SI mainly occurs co-currently with oil leaving from the oil-exposed surfaces. Counter-current SI, with oil production from water-exposed surfaces occurs to less extent, because the non-wetting oil must overcome a capillary back pressure (CBP) associated with the formation of droplets from the porous structure to be produced into the surrounding water phase. The CBP represents an additional resistance for non-wetting phase to be produced counter-currently, and can as such affect the interpretation of experimental core scale measurements, used to predict field scale behavior.
In this study, laboratory SI imbibition tests were performed with combined co-/counter-current flows in high permeability sandpacks in glass tubes using a boundary condition termed TEOFSI (Two Ends Open Free Spontaneous Imbibition) defined by one end face in contact with wetting phase and the other with non-wetting phase. This setup favors co-current SI behavior, with minor counter-current production. Front-like displacement with a sharp saturation gradient was observed visually for most experiments. The non-wetting to wetting viscosity ratio was adjusted between experiments, and ranged from 0.017 to 68. The results were interpreted using the core scale simulation software IORCoreSim which, in particular, incorporates the CBP mechanism.
Four of the five tests presented in this work were reasonably matched using the same parameters, while variation in experimental conditions couldexplain the last. The simulations revealed that the measurements were influenced by experimental artifacts associated with the inlet and outlet properties, due to the low capillarity of the sand packs. A flow restriction at the outlet end was required to reasonably model the tests. Thin paper filters used to keep the sand in place affected the rate of imbibition and counter-current imbibition. Further, a hydrostatic column of up to 5 cm was estimated to affect the front breakthrough time by 15 %. The history matching was quality checked by comparing trends with previously published data. The simulations then correctly captured strong variations in time scale and end recovery with viscosity ratio, and showed that this variation, often observed during co-current SI,can be explained as a macro scale phenomena, not necessarily related to changes in residual saturation. Based on the findings in this work, several procedures have been adjusted in the experimental design.