Page 80 - Principles of Applied Reservoir Simulation 2E

Part I: Reservoir Engineering Primer 65

as pressure declines during primary reservoir depletion. The natural forces
involved in the displacement of oil during primary production are called
reservoir drives. The most common reservoir drives for oil reservoirs are water
drive, solution or dissolved gas drive, and gas cap drive.
The most efficient drive mechanism is water drive. In this case, water
displaces oil as oil flows to production wells. An effective reservoir management
strategy for a water drive reservoir is to balance oil withdrawal with the rate of
water influx. Water drive recovery typically ranges from 35% to 75% of the
original oil in place (OOIP).
In a solution gas drive, gas dissolved in the oil phase at reservoir
temperature and pressure is liberated as pressure declines. Some oil moves with
the gas to the production wells as the gas expands and moves to the lower
pressure zones in the reservoir. Recovery by solution gas drive ranges from 5%
to 30% OOIP.
A gas cap is a large volume of gas at the top of a reservoir. When
production wells are completed in the oil zone below the gas cap, the drop in
pressure associated with pressure decline causes gas to move from the higher
pressure gas cap down toward the producing wells. The gas movement drives
oil to the wells, and eventually large volumes of gas will be produced with the
oil. Gas cap drive recovery ranges from 20% to 40% OOIP, although recoveries
as high as 60% can occur in steeply dipping reservoirs with enough permeability
to allow oil to drain to downstructure production wells.
Gravity drainage is the least common of the primary production mecha-
nisms. In this case oil flows downstructure to a producing well. This is the result
of a pressure gradient that favors downstructure oil flow to oil movement
upstructure due to gravity segregation. Gravity drainage can be effective when
it works. It is most likely to happen in shallow, highly permeable, steeply
dipping reservoirs.
A schematic comparison of primary production mechanisms on reservoir
pressure and recovery efficiency is sketched in Figure 8-1. In many cases, one
or more drive mechanisms are functioning simultaneously. The behavior of the
field depends on which mechanism is most important at various times during
the life of the field. The best way to predict the behavior of such fields is with