Parametric Investigation of WAG Floods Above the MME
- Leonardo Bermudez (University of Texas at Austin) | Russell Taylor Johns (University of Texas at Austin) | Harshad Champaklal Parakh (University of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- June 2007
- Document Type
- Journal Paper
- 224 - 234
- 2007. Society of Petroleum Engineers
- 5.3.1 Flow in Porous Media, 5.4 Enhanced Recovery, 5.4.1 Waterflooding, 5.6.4 Drillstem/Well Testing, 5.5.1 Simulator Development, 5.3.2 Multiphase Flow, 5.2.1 Phase Behavior and PVT Measurements, 5.4.2 Gas Injection Methods, 4.6 Natural Gas, 5.7.2 Recovery Factors, 4.3.4 Scale, 5.4.3 Gas Cycling, 5.5 Reservoir Simulation, 5.2 Reservoir Fluid Dynamics, 5.4.9 Miscible Methods
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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 gasfloods 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 overenrichment 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.
Gas enrichment is one of the important optimization variables in WAG enriched-gas floods.Recoveries from slimtube experiments with continuous gas injection often give a sharp bend at the minimum enrichment for miscibility (MME). Above the MME, slimtube recoveries do not increase significantly with enrichment. The optimum enrichment required to maximize recovery on a pattern scale in the field, however, is likely different from the MME. The difference in the optimum enrichment may be largely the result of greater mixing in the reservoir than exists in slimtubes. In addition, enrichment may impact sweep efficiency in 2D displacements. Oil and gas mixing in a reservoir can include mechanisms such as molecular diffusion, mechanical dispersion, gravity crossflow, viscous crossflow, and capillary crossflow. WAG in particular causes significant mixing of reservoir and injected fluids, depending on the total volume of the gas injected (slug volume), the WAG ratio, and the number of gas cycles or WAG frequency.
There are several reasons why recovery could increase for gas enrichments above the MME. First, the density and viscosity of the gas will increase with enrichment, which may improve sweep efficiency. Second, mixing can cause an otherwise multicontact miscible flood (MCM) to develop some two-phase flow (Johns et al. 1993; Walsh and Orr 1990; Pande and Orr 1989; Lake 1989). Richer gases mix closer to the critical locus in the two-phase zone, which causes a smaller and slower lean gas bank. A smaller lean gas bank could improve sweep efficiency. Last, richer gases, which mix near the critical locus, decrease "miscible residual oil?? by increasing the velocity of the trailing evaporation fronts.
|File Size||2 MB||Number of Pages||11|
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