Abstract

Miscible gas flooding and Water-Alternating-Gas (WAG) processes have been widely used to improve oil recovery. Prudhoe Bay Oil Field on the North Slope of Alaska is an example of successful application. Several factors including gravity segregation, solvent types, injection methods, and production/injection well constraints are known to impact the performance of Water-Alternating-Gas (WAG) process and miscible displacement. The presence of varying permeability and heterogeneity in a reservoir affects the displacement of the native fluids by the injected fluid and impacts oil recovery. Channeling of the solvent through high permeability regions reduces the storage and displacement efficiency of the displacing solvent.

In this paper we study the impact of these factors on oil recovery by miscible gas flooding and WAG. We consider various well completions to optimize the design of miscible displacement and WAG processes through numerical simulation of a pattern model with stochastic permeability distribution. The input data used in the simulations are analogous to those found on the North Slope. To study the impact of well placement and completions on performance of miscible displacements in heterogeneous media, we simulated the injection of various solvents and examined the effects of gravity segregation, permeability distribution and anisotropy, horizontal well lengths, orientation of vertical and horizontal wells on oil recovery. Also, we conducted simulations with various WAG ratios and cycle lengths to understand the WAG processes. Performance of miscible gas flooding and WAG process are compared to that of waterflooding. Our study showed that permeability anisotropy, horizontal well length, and gravity segregation were the major factors controlling performance of miscible flooding and WAG processes. The cycle length was found to be critical to WAG process design. The results of this study can be applied to design miscible and WAG displacement in heterogeneous reservoirs.

Introduction

For enhanced oil recovery purpose, miscible gas flooding and water-alternating-gas (WAG) process have been applied successfully in many hydrocarbon reservoirs such as Prudhoe Bay Oil Field on the North Slope of Alaska.1,2 The attractiveness of miscibility is that it can reduce the interfacial tension. The reduction in the interfacial tension has a significant effect on relative permeabilities and residual saturation by increasing the trapping number, which has been formulated mathematically3,4 and tested by experiments5–7. The WAG process is designed to improve the continuous gas injection EOR method, mainly by reducing gas mobility and thereby increasing sweep efficiency in the reservoir.

The main objective of modern reservoir management is to achieve maximum oil recovery when a displacing agent is injected to displace the oil in place in a hydrocarbon reservoir. It is imperative to employ a systematic approach to evaluating and selecting development strategies during the full life cycle of a reservoir. This paper is one of the few quantitative studies of the WAG process, including evaluation of parameters affecting project design, selection of injection solvents, and optimization of production well controls. For example, WAG in particular causes significant mixing of reservoir and injected fluids, depending on the total volume of the gas injected (Slug size), the WAG ratio, and WAG frequency.

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