Water-Alternating-Gas (WAG) injection has become a well established gas EOR method since field applications were first reported in the late 1950s. WAG can be applied by injecting CO2 or hydrocarbon gases as a miscible process where the gas is (usually multi-contact) miscible with the crude oil (MWAG) or where it is immiscible (IWAG). However, we contend that, even in "MWAG" processes, most of the reservoir is actually "near miscible", which we refer to as nMWAG and this case is discussed in detail in this paper. In many cases, WAG has been regarded as a "large scale" EOR method targeting mainly sweep improvement. However, the fluid flow mechanisms are important both for improving macroscopic sweep and also for improved microscopic displacement efficiency.
Improved microscopic efficiency by lowering residual oil saturation in the three-phase zone can greatly add to the benefit of WAG injection. Indeed, we believe that the fluid flow properties at pore scale and core scale when appropriately upscaled lead to significant oil recovery at the reservoir scale. Therefore, the pore/core scale mechanism are key factors for estimating the recovery efficiency of WAG processes. This paper reviews recent development in our understanding of the mechanisms of both immiscible and miscible /near miscible WAG processes. The discussion connects pore scale physics and core scale observations and explains the implications of these smaller scale processes on the field scale oil recovery by IWAG, MWAG and nMWAG.