Modelling the Transition between Immiscible and Miscible WAG

Current WAG modelling is either handling three-phase relative permeabilities with hysteresis and phase trapping, but cannot simultaneously include compositional effects. Whereas, conventional, compositional, WAG models lack the correct multi-phase flow functions (3 phase relative permeabilities, 3PRP). Our objective in this paper is to lay out a programme of research which will close the gap between immiscible WAG and miscible WAG by construction a physics based WAG model that includes best choice of fluid flow functions, saturation dependencies, phase trapping, transition to miscibility, and compositional effects. We first lay out a mechanistic phase diagram of the 2 major components of any WAG flood immiscible or miscible (more generally near-miscible), i.e (i) relative permeability effects - flow functiosn and 3PRP and (ii) compositional effects as the gas and oil approach miscibility. At present there is no physically consistent theory which carries over ALL of the various physical effects from the purely immiscible WAG case (with no, or very weak compositional effects) to the near miscible case. As just one example of incorrect physics, the 3 phase wetting order can change as the 3 IFTs change on approach to miscibility; we might think of this change as IFT to gas/oil σgo change towards 0, but in real systems, the IFT usually decreases and then increases again (as light oil components are stripped out). This wetting change should change the saturation dependencies of the 3PRP functions, but we invariably keep the original form of these functions (e.g.as Stone 1 or 2, which strictly only apply for strongly water-wet systems). A full agenda laying out all of the required parts of the physics is laid out in this paper and, where the physics is already known, it is briefly explained, and where not some conjectures are made or the challenges are clarified. The authors hope this paper will stimulate both discussion of the physics of WAG processes and more technical research to address the challenges.