Modeling forced versus spontaneous capillary imbibition processes commonly occurring in porous sediments

The extraction of ganglia of non-wetting petroleum fluid from underground reservoirs can occur whenever they are displaced and replaced by an invading wetting (e.g. aqueous) fluid phase. The problem is how to best specify, quantify, observe, and beneficially control such petroleum recovery processes. Cases that are considered are those that result form the fact that there may be two separate but superimposed imbibition mechanisms involved that inherently can reinforce and supplement each other. For example, when the displacing fluid phase more-or-less flows colinearly with the one being replaced, this will happen whenever it is only the imposition of mechanical energy gradients that causes it to happen. These kinds of processes can be thought of as being due to a forced imbibition mechanism. At the same time, however, a parallel spontaneous imbibition mechanism also can come into play whenever the local free interfacial surface energy of the system simultaneously is being diminished with time. The latter happens, of course, whenever the higher energy oil–rock interfacial tensions spontaneously become replaced by lower energy water–rock interfacial tensions. In effect, it is as though the entering wetting fluids push forward in ways somewhat akin to why a ball will roll down an unobstructed hill path by itself! ain a clear picture of what is involved, consideration should be given to the following three self-evident (but easily overlooked) propositions. First, it is easy to show that petroleum recovery due to the spontaneous imbibition effects can occur even when externally imposed forcing effects are totally absent. Second, it turns out to be clearly indicated that even in cases when superficially the magnitudes of the spontaneous imbibition effects appear to be minuscule compared to those caused by prevailing superimposed forced imbibition mechanisms, the influence of the former effects may not themselves prove to be necessarily inconsequential. Third, whenever rationally based algorithms are employed (viz. rather than simplified ones which are intended to mitigate and alleviate some of the computational problems), this will be the best way to guarantee that quantitatively believable reservoir performance simulation outcomes will be achieved. This latter proposition implicitly is affirmed by the work of Rose [Math. Geol. 22 (1990) 641] where reference was made to the apparent way simulation outputs for idealized systems were different depending on either or not spontaneous imbibition effects were incorporated in the analyses.