Page 222 - Hydrocarbon

Reservoir Dynamic Behaviour 209

responds by expanding, and water moves into the oil column to replace the voidage
created by production. Since the water compressibility is low, the volume of water
must be large to make this process effective, hence the need for the large connected
aquifer. In this context, ‘large’ would be 10 to 100 the volume of oil in place.
The prediction of the size and permeability of the aquifer is usually difficult,
since there is typically little data collected in the water column; exploration and
appraisal wells are usually targeted at locating oil. Hence the prediction of aquifer
response often remains a major uncertainty during reservoir development planning.
In order to see the reaction of an aquifer, it is necessary to produce from the oil
column, and measure the response in terms of reservoir pressure and fluid contact
movement; use is made of the material balance technique to determine the
contribution to pressure support made by the aquifer. Typically 5% of the STOIIP
must be produced to measure the response; this may take a number of years.
Water drive may be imposed by water injection into the reservoir, preferably by
injecting into the water column to avoid by-passing down-dip oil (Figure 9.7).
If the permeability in the water leg is significantly reduced due to compaction or
diagenesis, it may be necessary to inject into the oil column. Once water injection is
adopted, the potential effect of any natural aquifer is usually negated. Clearly if it
were possible to predict the natural aquifer response at the development planning
stage, the decision to install water injection facilities would be made easier. A
common solution is to initially produce the reservoir using natural depletion, and to
install water injection facilities in the event of little aquifer support.
The aquifer response (or impact of the water injection wells) may maintain the
reservoir pressure close to the initial pressure, providing a long plateau period and
slow decline of oil production (Figure 9.8). The producing GOR may remain
approximately at the solution GOR if the reservoir pressure is maintained above
the bubble point. The outstanding feature of the production profile is the large
increase in water cut over the life of the field, which is usually the main reason for

oil producers

water injectors
oil producers

water injectors

OWC
Steep Dip
Shallow Dip
Figure 9.7 Location of wells for water drive.