Mobilization of by-passed oil by viscous crossflow in EOR processes

We believe that the main target oil for all EOR methods is essentially “bypassed oil” at several length scales from the pore scale, to the core scale, to the bed-form scale, to the reservoir layer scale; indeed, at all the scales of heterogeneity present in an oil reservoir. Thus, a waterflood applied as a secondary recovery process will “bypass” oil at all of these scales leaving behind potentially mobile oil, resulting in a lowered recovery factor. The role of most EOR processes - and here we specifically focus on polymer flooding and WAG - is to improve oil recovery by producing as much of this bypassed oil at all scales as is physically possible. Conventional polymer flooding is often described as simply “mobility control” implying that a viscous oil linear displacement efficiency may be improved by viscosifying the injected brine. In fact, this is a secondary effect in most polymer floods in the field, even for viscous oils. Frequently, a more important mechanism is viscous crossflow (VX), not just in layered reservoir systems (where it is indeed an efficient mechanism), but in any heterogeneous reservoir system. Where there is heterogeneity at the pore scale, core scale and upwards, this viscous crossflow mechanism is generally present and is the main, or at least an important, contributor to oil recovery improvement. In this paper, we will use examples from various studies of polymer displacements at the pore, core and field scales to demonstrate the above claims. Furthermore, recent work now shows that the VX mechanism also plays an important role in near-miscible WAG which will also be described briefly here.