It only makes sense to define a capillary number if there are several mechanisms in place, especially Nca usually involves capillary and advective forces.the ratio
For a pure co- or counter-current spontaneous imbibition process Nca is not really relevant since capillary forces per definition are dominating. If you also have an applied injection rate you can define it more uniquely. One very good example is the study of capillary end effects in core flooding. If capillary forces are important you get end effects, but at proper values of Nca they can be ignored since the advective forces are strong enough.