Page 32 - Primer on Enhanced Oil Recovery

Hydrocarbon and oil reserves classification 23

for: producing wells grid density; well location on the reservoir structure; current
technical well state; methods and methods of intensifying oil production; implemen-
tation of reservoir pressure maintenance systems; the use of methods of influence
on the reservoir. The list can go on and on.
The Method of Analogies can produce rough estimates which are dependant on
the natural operation modes of the reservoirs (e.g. Reserve Drive Mechanisms).
According to the basic physics laws reservoir liquids will move under existing
(natural) or intentionally generated forces. Such forces include pressure differential,
gravity, capillary and hydrodynamic influences. The lowest pressure in the oil reser-
voir should be near and inside of production well. We assume that the well has per-
forations and oil is actively pumped out. If we have the reservoir pressure higher
than in the production well and capillary forces are lower than the pressure differen-
tial, then the reservoir liquids (we hope that this is mostly oil) will move towards
production well and will be extracted.
Under the oil deposits natural drive regime there are historically existing in the
reservoir forces which move oil (hydrocarbons) in the pore space of reservoir rocks
to the production well perforation holes. The natural reservoir regime has one of
the most important effects on the choice of the most economical reservoir develop-
ment strategy and the best efficiency of reservoir energy utilization.
The driving pressure in the reservoir can be produced by the gravitational force
(Gravity Drive), gas cap (Gas Cap Drive), oil elastic expansion or by Solution Gas
Drive. Water from existing aquafers can “push” oil from the formation side(s) or
from the formation sole (Waterdrive).
On the basis of historical data, modeling and laboratory studies it is accepted
that the natural drive recovery coefficient can have the following values:

Gravity drive $ 0.5
Gas Cap dive 0.2 0.4
Solution Gas Drive 0.05 0.3
Waterdrive 0.35 0.75

Gravity exhibits itself in various roles. If we pump the oil from the bottom of
the formation then oil will keep coming to the production well under gravitational
force until we reach the balance between gravity and the capillary forces. We have
to assume that in place of removed oil the reservoir is filled with some sort of gas.
Gravity force is also important when we have liquids of different density. Let us
assume at this point that we have homogenous oil which has specific gravity lower
that water or, in other terms, API is above 10. In this case water will collect at the
bottom of the reservoir and, having water supply, will continuously push oil
upwards until all reservoir is almost fully filled with water.
On Fig. 2.7 there are examples of combined gravity and water drives. In both
cases there are also some gas caps but it is assumed that the pressure in there is not
driving force.
Water assisted oil displacement also can be observed as related to water proper-
ties itself phenomena. In this case water expands during the initial pressure drop, so