Week 2 - Drive Mechanisms
Week 2 - Drive Mechanisms
Week 2 - Drive Mechanisms
Heriot-Watt
DEPARTMENT OF PETROLEUM ENGINEERING
Drive Mechanisms
Adrian C Todd
Reservoir Drive Mechanisms
z Definition
ÊA reservoir drive mechanism is a source of
energy for driving the fluids out through the
wellbore
ÊIt 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
Reservoir Drive Mechanisms
Active aquifer
Key issue-relative
size of aquifer to
hydrocarbon
accumulation
1 ∆V
c=−
V ∆P
Water Drive Reservoirs
z Artesian Flow
Key issues:
Mobility of
water in
aquifer
Barriers to flow
Compaction Drive
z Not a common
drive mechanism.
z Characteristics
can be dramatic
as a result of
increase in net
overburden stress
as pore pressure
reduced.
z Nature of the rock
or its consolidation
determines extent
of mechanism
Compaction Drive
z Ekofisk Field a dramatic example
Gravity Drainage Drive
z Due to the relative density of the fluids and high
vertical permeabilities.
z Fractured reservoirs
Gravity Drainage Drive
z Examples - Lake Maracaibo - Venezuala
Depletion Type Reservoirs
Wellbore
z Two types
z Edge water drive
z Bottom water drive
Combination Drive
Oil relative
permeability
decreases.
Reservoir Performance-Solution Gas Drive
Gas moves ahead of the associated oil
Depending on vertical relative permeability
secondary gas cap formed
Production Phases
z Production build up - may exist depends on drilling strategy.
z Plateau phase- production maintained at design capacity.-
duration depends on economics of project.
z Decline phase - reservoir not able to deliver design capacity
z Abandonment- rate depends on size of project and op. costs.
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
Solution Gas Drive
z 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
Solution Gas Drive
z By definition should be
no water production.
z Due to rapid pressure
drop artificial lift
required in early years.
z Expected oil recovery,
low, 5-30% STOIIP.
z Well locations low to
encourage vertical gas
migration
Gas Cap Drive
z Initial condition free gas in gas cap.
z Gas contact will be at bubble point.
z Gas has considerable compressibility.
z To get flow gas comes out of solution at producing
interval. Some degree of solution gas drive.
Gas Cap Drive Pressure
Oil Production Production of fluids
Has a significant decline but less than largely due to high
solution gas drive. compressibility of gas
Decline due to reducing pressure and cap.
solution gas drive 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
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
Water Drive
z Majority of water drive reservoirs energy from
compressibility of aquifer.
z Effectiveness depends on ability of water to replace
volume of oil produced.
z Key issue- size and permeability of aquifer.
z For compressibility to be effective the relative size
needs to be very large.
z Challenge to reservoir engineer is to predict
behaviour prior to production.
z Difficult to justify exploration costs to determine the
size of a water accumulation
Water Drive - Rate Sensitivity
z The features of a natural water drive are strongly
influenced by the rate sensitivity of these
reservoirs.
z Can the water replace the rate of voidage loss
due to oil production?
z If not the pressure will drop and another drive
mechanism will also be effective. e.g. solution
gas drive,
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
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
Water Drive- Oil flowrate is more than potential
Rate Sensitivity flow of water from aquifer
GOR increases
Pressure drops below bubble point
Solution gas drive-combination drive
Cutting back oil production enable aquifer to support production-water drive
Water Drive
z Characteristics z Trend
Reservoir pressure Ê Declines rapidly
Gas-Oil Ratio Ê First low then rises through a maximum
Production rate Ê First high, then declines rapidly
Water production Ê None
Well behaviour Ê Requires artificial lift early
Expected recovery Ê 5-30% STOIIP
Gas Cap Drive
z Characteristics z Trend
Reservoir pressure Ê falls slowly and continuously
Gas-Oil Ratio Ê Rises continuously
Production rate Ê First high, then declines gradually
Water production Ê Absent or negligible
Well behaviour Ê Long flowing life depending on gas cap
Expected recovery Ê 20-40% STOIIP
Water Drive
z Characteristics z Trend
Reservoir pressure Ê Remains high
Gas-Oil Ratio Ê Steady
Water production Ê Early and increases to large amount
Well behaviour Ê Flow until water production excessive
Expected recovery Ê up to 60% STOIIP
Recovery
GOR