Water mixing during spontaneous imbibition at different boundary and wettability conditions

The displacement of connate water by spontaneously imbibing water was determined for chalk core plugs at different wettability conditions. The development and distribution of both aqueous phases were determined by nuclear tracer imaging (NTI) to obtain 1D fluid saturation profiles. Two boundary geometries were applied to investigate the displacement and water mixing during strictly counter-current imbibition (One-End-Open: OEO) and during both co- and counter-current imbibition (Two-Ends-Open: TEO). The connate water was displaced from the open end face exposed to the invading water phase for both boundary geometries and all wettabilities, but the rate and displacement efficiency were highly influenced by the matrix wettability. The average connate water saturation did not change during spontaneous imbibition, but its distribution in the core plug changed as the invading water phase saturation increased in all tests. A connate water bank formed at strongly water-wet conditions for both boundary conditions. In both cases the connate water accumulation constituted the entire fraction of water saturation at these positions, with little or no mixing between the connate and invading water phase during the displacement. At the end of the test the connate water accumulated in the middle of the core at TEO imbibition and at the closed end face during OEO. At weakly water-wet conditions the connate water banking was reduced and the mixing of waters was increased. The invading water displacement of the connate water was also reduced, and the aqueous phases mixed by diffusion when redistribution of fluids stopped.

► Invading water displace connate water during spontaneous imbibition.
► Connate water banking occur at strongly, moderately and less water-wet conditions.
► The invading water progressed with √time during counter-current imbibition.
► Water mixing and connate water banking were reduced at reduced wettabilities.