The document discusses various primary recovery mechanisms for oil reservoirs, emphasizing their impact on performance metrics like recovery factor and pressure decline. It categorizes reservoirs according to their drive mechanisms, including rock and liquid expansion, depletion, gas cap, water drive, gravity drainage, and combination drives. Key characteristics and expected recovery rates for each mechanism are outlined, highlighting the importance of understanding these systems for optimizing oil production.

1. PART 7: RESERVOIR DRIVE MECHANISMS
Objectives
 Introduce and give a detailed discussion of the various
primary recovery mechanisms and their effects on the overall
performance of oil reservoirs.
 Provide the basic principles of the material balance equation
and other governing relationships that can be used to predict
the volumetric performance of oil reservoirs.
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PET 323– Reservoir Engineering I – D. Abraham and O. Olabode

2. PART 7: RESERVOIR DRIVE MECHANISMS
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3. PART 7: RESERVOIR DRIVE MECHANISMS
Introduction
 All petroleum reservoirs can be grouped according to the
primary recovery mechanism by which they produce.
 Each drive mechanism is characterized by a unique
performance in terms of:
 Ultimate recovery factor
 Pressure decline rate
 Gas-oil ratio evolution
 Water production evolution
Primary Recovery
 This refers to all hydrocarbons produced using the internal
energy of the reservoir without any form of supplements
such as water or gas injection.
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4. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
 In order to be able to predict and optimize the future
performance of the reservoir, it is important to know the
main drive mechanism(s) or energie(s) at play in a reservoir.
 There are six basic reservoir drive mechanisms and they are:
 Rock and liquid expansion drive
 Depletion drive
 Gas cap drive
 Water drive
 Gravity drainage and
 Combination drive
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5. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Rock and Liquid Expansion
 An under saturated reservoir is one which exists above the
bubble point pressure (no gas cap).
 Oil, connate water and rocks are the only materials present in
a reservoir above the bubble point pressure.
 In the course of producing the reservoir fluid (crude oil), the
pressure declines and as the pressure reduces, the rock and
fluids expand due to their compressibilities.
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6. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Rock and Liquid Expansion Above Bubble Point Pressure, Pb
 Two factors influence the reservoir rock compressiblity:
 Expansion of individual rock grains
 Formation compaction due to increase in net overburden pressure
 These two factors act to reduce the pore volume thus forcing
reservoir fluids out of the pore spaces and pushing them
towards the wellbore
 Since the rock and fluids are only slightly compressible, the
reservoir pressure will decline rapidly as production
proceeds.
 This mechanism is the least effective and recovers on average
less than 7 percent of the oil in place.
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7. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
The Depletion Drive Mechanism
 This drive is also known as: Solution Gas Drive, Dissolved
Gas Drive and Internal Gas Drive
 For an initially under saturated reservoir, as the reservoir
pressure decline below the bubble point pressure, dissolved
gas is released from solution
 In this kind of reservoir, the main source of reservoir energy
comes from the expansion of gas bubbles released from
solution as the reservoir pressure declines below the bubble
point
 As production proceeds further, the gas bubbles then expand
and force the crude oil out of the pores and into the well
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8. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
The Depletion Drive Mechanism
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9. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
The Depletion Drive Mechanism
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Characteristics Trend
Reservoir Pressure Declines rapidly and continuously
Gas-oil ratio Increases to maximum and then declines
Water production None
Well behavior Requires pumping at early stage
Oil recovery 5 to 30 %
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10. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Gas Cap Drive
 These are reservoir which have a
gas cap and with little or no water
drive
 As production from the oil zone
proceeds, the reservoir pressure
drops.
 This gives room for the gas in the
gas cap and the solution gas if
liberated, to expand and push oil
from the pores towards the
production wells
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11. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Characteristics of Gas Cap Drive Reservoirs
 Slow and continuous decline of reservoir pressure. The
pressure maintenance tends to be much better, compared to
the other drive mechanisms.
 Absent or negligible water production
 The gas oil ratio (GOR) is seen to rise continuously in well
located high on the structure (well with perforation close to
the gas-oil-contact)
 The expected ultimate recovery (EUR) ranges from 20% to
40% depending on the size of the gas cap compared to the
oil volume.
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12. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Characteristics of Gas Cap Drive Reservoirs
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13. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Characteristics of Gas Cap Drive Reservoirs
 Well Behavior
Factors which Affect the Performance of Gas Cap Drive Reservoirs
 Size of the Original Gas Cap – Impacts on the ultimate recovery
 Vertical Permeability – If high, the evolved gas will migrate
upwards towards the gas cap
 Oil Viscosity – Oil viscosity reduces and evolved gas bypasses oil
and reach the wells
 Degree of Conservation of the Gas - Shut in wells with high
GORs
 Oil Production Rate
 Dip Angle – The higher this is, the more effective the gravity
drainage becomes
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14. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Characteristics of Gas Cap Drive Reservoirs
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15. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
The Water Drive Mechanism
 This results owing to the fact that many reservoirs are
connected to water bearing rocks called “Aquifers”. In other
instances, the reservoir rock simply outcrops and gets
replenished by surface waters
Types of Water Drive
 Bottom Water Drive
 Edge Water Drive
 Artesian Water Drive
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16. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
The Water Drive Mechanism (Artesian Water Drive)
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17. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
The Water Drive Mechanism (Edge and Bottom water drives)
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18. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Characteristics of Water Drive Mechanism
 Reservoir Pressure – Gradual Pressure Decline
 Withdrawal from the reservoir is usually replaced by aquifer water
expanding and flowing into the reservoir
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19. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Characteristics of Water Drive Mechanism
 Water Production – This is usually observed in structurally
low wells. As the aquifer water expands upwards, it moves
into the perforation of the low wells.
 Gas-Oil Ratio – Normally low and close to the intial Rs value
due to the fact that reservoir pressure is maintained.
 Ultimate Recovery – This is largest compared to other
production mechanism. Factors such as K variation have to
be considered. The performances is better for a homogenous
reservoir compared to a heterogeneous one.
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20. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Characteristics of Water Drive Mechanism
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Characteristics Trends
Reservoir Pressure Remains High
Surface GOR Remains low
Water Production
Starts Early and Increases to
appreciable amounts
Well behavior
Flows until water production gets
excessive (liquid loading of wells)
Expected Recovery 35% to 70%
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21. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Characteristics of Water Drive Mechanism
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22. PART 7: RESERVOIR DRIVE MECHANISMS
Primary Drive Mechanisms
Gravity-Drainage-Drive Mechanism
 Results from density difference between fluids in the
reservoir
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