WAG Optimization for Gas Floods above the MME

Water-alternating-gas floods (WAG) are commonly used to improve sweep efficiency in heterogeneous reservoirs. There has been little reported in the literature, however, on the effectiveness of WAG processes where the gas is enriched above the minimum miscibility enrichment composition (MME). This paper examines how to optimize WAG processes for enriched gas floods above the MME, particularly as a primary recovery method. Compositional simulations of x-z cross-sections are used to quantify the effects of WAG parameters, numerical dispersion, level of enrichment, and heterogeneity on local displacement efficiency and sweep efficiency. The main conclusions of this research show that the richer the gas above the MME, the fewer the number of WAG cycles required for maximum oil recovery at a given WAG ratio. Another significant observation is that over-enrichment above the MME improves recovery the most when the largest permeability layers are at the bottom of the reservoir, Continuous slug injection performs better than WAG when the largest permeability layers are at the bottom of the aquifer, richer gases are used, and the vertical to horizontal permeability ratio is small.