2. Reservoir Drive Mechanisms
Definition
A reservoir drive mechanism is a source of
energy for driving the fluids out through the
wellbore
It is not necessarily the energy lifting the fluids
to the surface, although in many cases, the
same energy is capable of lifting the fluids to
the surface
3. Reservoir Drive Mechanisms
The various drive mechanisms come from the
impact of a number of phenomena:
Expansion of reservoir fluids, oil, gas & water
Liberation, expansion of solution gas
Expansion of reservoir rock and reduction of
pore volume.
Gravity forces.
4. Reservoir Drive Mechanisms
There are a number of drive mechanisms:
Depletion drive
Solution gas drive
Gas Cap drive
Water drive
Compaction drive
Gravity drainage drive
Combination drive
5. Depletion Drive Reservoirs
A depletion drive reservoir is one in which the
hydrocarbons are NOT in contact with a
supporting aquifer.
6. A water drive reservoir is a reservoir in
contact with a supporting aquifer.
Two types:
due to expansion of water as reservoir is
produced
due to artesian flow from an outcrop.
Water Drive Reservoirs
7. Water Drive Reservoirs
Expansion of an active aquifer
Active aquifer
Key issue-relative
size of aquifer to
hydrocarbon
accumulation
1 V
c
V P
9. Compaction Drive
Not a common
drive mechanism.
Characteristics
can be dramatic
as a result of
increase in net
overburden stress
as pore pressure
reduced.
Nature of the rock
or its consolidation
determines extent
of mechanism
10. Gravity Drainage Drive
Due to the relative density of the fluids and high
vertical permeabilities.
Fractured reservoirs
11. Depletion Type Reservoirs
Energy comes from the expansion of fluids in
the reservoir and its associated pore space
Two types:
Solution gas drive
Gas cap drive
12. Solution Gas Drive
Two stages of drive:
Above the bubble point.
Fluid production comes from the effective
compressibility of the system:
Oil
Water
Pore space
Compressibility drive
Below the bubble point
13. Solution Gas Drive
Above bubble point:
Low compressibility of
oil, connate water and
pore space.
Rapid decline in
pressure.
Impact of water and
pore space should not
be neglected
14. Solution Gas Drive
When bubble point reached:
Expanding gas provides force to drive oil.
Solution gas drive
Dissolved gas drive
15. Liberated gas can also migrate vertically and
form secondary gas cap
Solution Gas Drive
Wellbore
Liberated solution gas
Secondary gas cap
16. Depletion Drive - Gas Cap Drive
Where there is already
free gas in the reservoir.
Compared to the initial
undersaturated condition
for solution gas drive.
Energy from very high
compressibility of gas
cap.
Some energy from
solution gas drive which
is also present.
Oil expansion is very low
18. Combination Drive
Rare for reservoirs to fit into the simple pure
drive classification
Many have a combination of drives during
production period.
19. Reservoir Performance
The production characteristics of reservoirs.
Oil-gas ratio
pressure decline
water production
can give indications of drive mechanism
20. Reservoir Performance-Solution Gas Drive
When bubble point reached gas comes out of solution.
Initially no flow of gas since critical gas saturation not achieved
21. Solution Gas Drive
Initial pressure drop rapid due to low compressibility of system
Pressure continues to decline and solution gas drive becomes
effective
Reduced oil
production due to
decreasing pressure
and reducing relative
permeability to oil
Gas production
increases as gas comes
out of solution and
moves ahead of
associated oil due to
favourable relative
permeability
22. Solution Gas Drive
Distinctive feature of solution gas drive is the
producing gas to oil ratio- Rp
Above bubble point all gas in solution Rp =Rsi
At bubble point initial
gas produced below
critical gas
saturation. Rp<Rsi
Gas becomes
mobile and moves
ahead of its oil
Rp>Rsi
Maximum GOR as
oil produced with a
low GOR
23. Solution Gas Drive
By definition should be
no water production.
Due to rapid pressure
drop artificial lift
required in early years.
Expected oil recovery,
low, 5-30% STOIIP.
Well locations low to
encourage vertical gas
migration
24. Gas Cap Drive
Initial condition free gas in gas cap.
Gas contact will be at bubble point.
Gas has considerable compressibility.
To get flow gas comes out of solution at producing
interval. Some degree of solution gas drive.
25. Gas Cap Drive
Oil Production
Has a significant decline but less than
solution gas drive.
Decline due to reducing pressure and
solution gas drive
Pressure
Production of fluids
largely due to high
compressibility of gas
cap.
Pressure declines
slowly depending on
gas cap size.
Gas-Oil Ratio
Early stages GOR
steady.
Slowly impact of
solution gas drive
increases Rp
Low gas viscosity>high
gas mobility
Gas by-passing oil
26. Gas Cap Drive Water Production
Like solution gas
drive negligible
water production.
Well behaviour
Longer, depends on
gas cap size
Recovery
20-40% STOIIP
Well Locations
Away from gas oil
contact
Not too close to
water oil contact
27. Water Drive
Majority of water drive reservoirs energy from
compressibility of aquifer.
Effectiveness depends on ability of water to replace
volume of oil produced.
Key issue- size and permeability of aquifer.
For compressibility to be effective the relative size
needs to be very large.
Challenge to reservoir engineer is to predict
behaviour prior to production.
Difficult to justify exploration costs to determine the
size of a water accumulation
28. Water Drive - Rate Sensitivity
The features of a natural water drive are strongly
influenced by the rate sensitivity of these
reservoirs.
Can the water replace the rate of voidage loss
due to oil production?
If not the pressure will drop and another drive
mechanism will also be effective. e.g. solution
gas drive,
29. Water Drive - Artesian flow
Oil flowrate is less than
potential flow of water from
aquifer
Producing GOR Rp
Remains constant since
reservoir
undersaturated.
Pressure
at oil water contact
constant
Plateau phase possible.
Decline due to water
production
Water cut, determines when abandonment of well occurs
30. Water Drive - Compressibility of aquifer .
Oil flowrate is less than potential flow
of water from aquifer
Producing GOR
Remains constant since reservoir
undersaturated.
Pressure
Declines as
aquifer
decompresses
Productivity
remains high.
Reduces as
water production
increases
31. Solution Gas Drive
Characteristics
Reservoir pressure
Gas-Oil Ratio
Production rate
Water production
Well behaviour
Expected recovery
Trend
Declines rapidly
First low then rises through a maximum
First high, then declines rapidly
None
Requires artificial lift early
5-30% STOIIP
32. Gas Cap Drive
Characteristics
Reservoir pressure
Gas-Oil Ratio
Production rate
Water production
Well behaviour
Expected recovery
Trend
falls slowly and continuously
Rises continuously
First high, then declines gradually
Absent or negligible
Long flowing life depending on gas cap
20-40% STOIIP
33. Water Drive
Characteristics
Reservoir pressure
Gas-Oil Ratio
Water production
Well behaviour
Expected recovery
Trend
Remains high
Steady
Early and increases to large amount
Flow until water production excessive
up to 60% STOIIP
