New Possibilities with the Novel Modelling Approaches developed for WAG and FAWAG

Abstract

Abstract There has been substantial development of modelling tools for viscous instabilities and multi-phase flow in recent years. This has enabled better opportunities of modelling near-miscible WAG (Water-Alternating-Gas), respecting gas fingers and simultaneously representing more correct phase mobilities. The objectives of this paper are to demonstrate advanced near miscible WAG modelling including WAG three-phase hysteresis, and present cases of Foam Assisted WAG (FAWAG) revisited with several novel modelling approaches. The numerical modelling has been performed using commercial reservoir simulators, STARS and GEM from CMG. The methodology of describing viscous fingering, analogue to Sorbie et al. (2020), is a 4-stage approach: (1) selection of fractional flow to maximize total mobility; (2) derivation of the relative permeability; (3) establishing an appropriate random correlated permeability field; and (4) simulating the process with a sufficiently fine grid. Simulations have been performed in 3D models using fine grid and random Gaussian permeability field. Three-phase fluid flow modeling used the GEM implemented version of Larsen and Skauge WAG hysteresis model, and the CMG foam model. We have used two differentrock permeability models, a standard vertical layered model, and a model with heterogeneous permeability within each layer. The fluid flow functions were either a conventional or a WAG hysteresis model respecting three-phase mobilities and phase trapping. The impact on gas finger development was analyzed and was based on simulation production data, but also on the in-situ fluid distribution. WAG hysteresis dampened to some degree the gas fingers but was able to show oil bank formation and enabled interpretation of in-situ fluid diversion. We have expanded the numerical modeling to include foam and specifically the foam assisted WAG (FAWAG) process. This is a revisit of an earlier study (Skauge et al. 2002 on Foam Assisted WAG, a Summary of Field Experience at the Snorre Field), but now updated with the novel modelling approaches. Many factors influence foam strength, with mobility reduction factor (MRF) as the key factor. We used the GEM version of foam description, with MRF as the main factor defining the foam properties. In this approach we were able to describe the reduction in GOR, but also the oil banking and consequently the extra oil production due to FAWAG injection. Simulation studies show that it is possible to include complex modelling in a commercial simulator. The advanced models enable a more correct history match of production and a more systematic analysis of local diversion of fluid flow due to WAG and FAWAG that would not be possible using a conventional approach. With the new approach, improved decisions for field development can be made.