Waterflooding is the oldest and by far the most important method used by the petroleum industry to increase recovery from both onshore and offshore reservoirs. Waterflood design is a complex problem that must ultimately be handled on an individual reservoir basis. This paper presents factors that should be considered in designing both onshore and offshore waterfloods.
The need for careful examination of the following factors is discussed:
Reservoir geology and method of deposition
Primary production mechanisms and stage of depletion
Reservoir and fluid properties
Well spacing and possible waterfloodpatterns
After these factors are discussed, the effects that pattern selection, timing and injection/producing rates have on project economics arm discussed. A special emphasis is placed on offshore waterflooding since it is now of significant concern.
Waterflooding was first used over 100 years ago, but it was not until the 1950's that it gained popularity when field applications increased at a rapid rate. At the present time, waterflooding is so well regarded as a reliable and economic oil recovery technique that almost every field that does not have a natural water drive, is being or soon will be waterflooded. Waterflood projects from a reservoir engineering viewpoint, are very tedious and require detailed data. There are two basic classifications of water injection projects:
Waterflooding - those which displace oil from semi-depleted and depleted reservoirs, that is, increasing recovery through the more efficient displacement process.
Pressure maintenance - those which maintain a pressure in new or partially depleted reservoirs for sustaining the production rate.
The main difference between secondary recovery (waterflooding) and pressure maintenance operations is the amount of reservoir pressure existing at the time the operations are begun. If the reservoir pressure is fairly high, the operation is called pressure maintenance, but, if the pressure has been substantially depleted, the operation is called secondary recovery. Both operations should increase ultimate recovery from the affected reservoir. Under normal circumstances, pressure maintenance operations will not bring about the rate increase that a waterflood will since it is installed when the reservoir producing rate is at a higher level.
Many factors important to waterflooding are also important to pressure maintenance, so that it is difficult to define a definite point of separation between the two processes. Accordingly, a major portion of the information presented in this paper is applicable to both waterflooding and pressure maintenance by water injection.
In this paper, we have made an attempt to review the reservoir engineering and geological parameters which control waterflood recovery. Also included is a discussion of criteria used in selecting a water injection rate, pattern and the timing of water injection. No attempt has been made to provide details of methods of forecasting waterflood recovery; however, the types of techniques generally used today are mentioned.