Summary
In recent years there has been an increasing interest in water-alternating-gas (WAG) processes, both miscible and immiscible. WAG injection is an oil recovery method initially aimed to improve sweep efficiency during gas injection. In some recent applications produced hydrocarbon gas has been reinjected in water-injection wells with the aim of improving oil recovery and pressure maintenance. Oil recovery by WAG injection has been attributed to contact of unswept zones, especially recovery of attic or cellar oil by exploiting the segregation of gas to the top or the accumulating of water toward the bottom. Because the residual oil after gasflooding is normally lower than the residual oil after waterflooding, and three-phase zones may obtain lower remaining oil saturation, WAG injection has the potential for increased microscopic displacement efficiency. Thus, WAG injection can lead to improved oil recovery by combining better mobility control and contacting unswept zones, and by leading to improved microscopic displacement.
This study is a review of the WAG field experience as it is found in the literature today,1–108 from the first reported WAG injection in 1957 in Canada to the new experience from the North Sea. About 60 fields have been reviewed. Both onshore and offshore projects have been included, as well as WAG injections with hydrocarbon or nonhydrocarbon gases. Well spacing is very different from onshore projects, where fine patterns often are applied, to offshore projects, where well spacing is in the order of 1000 m.
For the fields reviewed, a common trend for the successful injections is an increased oil recovery in the range of 5 to 10% of the oil initially in place (OIIP). Very few field trials have been reported as unsuccessful, but operational problems are often noted. Though the injectivity and production problems are generally not detrimental for the WAG process, special attention has been given to breakthrough of injected phases (water or gas). Improved oil recovery by WAG injection is discussed as influenced by rock type, injection strategy, miscible/immiscible gas, and well spacing.
Introduction
The WAG injection was originally proposed as a method to improve sweep of gas injection, mainly by using the water to control the mobility of the displacement and to stabilize the front. Because the microscopic displacement of the oil by gas is normally better than by water, the WAG injection combines the improved displacement efficiency of the gas flooding with an improved macroscopic sweep by water injection. This has resulted in improved recovery (compared to a pure water injection) for almost all of the field cases reviewed in this work. Although mobility control is an important issue, other advantages of the WAG injection should be noticed as well. Compositional exchanges may give some additional recovery and may influence the fluid densities and viscosities. Reinjection of gas is favorable owing to environmental concerns, enforced restrictions on flaring, and - in some areas - CO2 taxes.
The WAG injection results in a complex saturation pattern because two saturations (gas and water) will increase and decrease alternately. This gives special demands for the relative permeability description for the three phases (oil, gas, and water). There are several correlations for calculating three-phase relative permeability in the literature,95 but only recently has an approach been designed for WAG injection using cycle-dependent relative permeability.95
WAG injection has been applied with success in most field trials. The majority of the fields are located in Canada and the U.S., but there are also some fields in the former USSR. WAG injection has been applied since the early 1960's. Both miscible and immiscible injections have been applied, and many different types of gas have been used. This work gives a review of the WAG injection as it is found in the open literature today. Unfortunately, not all field trials are adequately described, and this overview is limited to the publicly accessible data. We have chosen to use an inclusive definition of WAG injection that covers all cases where both gas and water are injected in the same well. A process where one gas slug is followed by a water slug is, by definition, considered a WAG process. In the literature, WAG injection processes are also referred to as combined water/gas injection (CGW).100
Classification of the WAG Process.
WAG processes can be grouped in many ways. The most common is to distinguish between miscible and immiscible displacements as a first classification.
Miscible WAG Injection.
It is difficult to distinguish between miscible and immiscible WAG injections. In many cases a multicontact gas/oil miscibility may have been obtained, but much uncertainty remains about the actual displacement process. In this paper, we have used only the information from the literature and find that most cases have been defined as miscible. It has not been possible to isolate the degree of compositional effect on oil recovery by WAG injection. Miscible projects are mostly found onshore, and the early cases used expensive solvents like propane, which seem to be a less economically favorable process at present. Most of the miscible projects reviewed are repressurized in order to bring the reservoir pressure above the minimum miscibility pressure (MMP) of the fluids. Because of failure to maintain sufficient pressure, meaning loss of miscibility, real field cases may oscillate between miscible and immiscible gas during the life of the oil production. Most miscible WAG injections have been performed on a close well spacing, but recently miscible processes have also been attempted even at offshore-type well spacing.86–90
Immiscible WAG Injection.
This type of WAG process has been applied with the aim of improving frontal stability or contacting unswept zones. Applications have been in reservoirs where gravity-stable gas injection cannot be applied because of limited gas resources or reservoir properties like low dip or strong heterogeneity. In addition to sweep, the microscopic displacement efficiency may be improved. Residual oil saturations are generally lower for WAG injection than for a waterflood and sometimes even lower than a gasflood, owing to the effect of three-phase and cycle-dependent relative permeability.96,97
Sometimes the first gas slug dissolves to some degree into the oil. This can cause mass exchange (swelling and stripping) and a favorable change in the fluid viscosity/density relations at the displacement front. The displacement can then become near-miscible.
Hybrid WAG Injection.
When a large slug of gas is injected, followed by a number of small slugs of water and gas, the process is referred to as hybrid WAG injection.38–42
Others.
A process where water and gas are injected simultaneously (SWAG injection) has been tested in a few reservoirs.37,106–108 Although this process is not the main scope of the paper, a few comments are given at the end.
A final version of the cyclic injection is in the literature presented as Water Alternating Steam Process (WASP).102 Reviews of field cases will not be included in this paper.
