2. Gas injection and Miscibility
• Gas injection is the most widely applied EOR process for light oils.
• Oil recoveries for gas injection processes are usually greatest when the process is operated
under conditions where the gas can become miscible with the reservoir oil.
• The primary objective of miscible gas injection is to improve local displacement efficiency
and reduce residual oil saturation below the levels typically obtained by water flooding.
• Examples of miscible gas injection are CO2 or N2 at sufficiently high pressure, dry gas
enriched with sufficient quantities of LPG components, and sour or acid gases containing
H2S.
3. • Miscible means that the injected gas mixes with the oil, thereby
reducing its viscosity and interfacial tension of the oil and rock. Miscible
gas flooding also increases oil swelling and localized pressure or drive
within the reservoir.
• "Immiscible "flooding means that the injected-gas does not mix or
go in to solution, but instead provides the energy "push”(drive) by
increased pressure. Immiscible flooding does not produce as much oil
as miscible gas flooding, however there are certain applications and
reservoirs where in immiscible flooding is well-suited
4. What is Miscibility
• Under normal conditions, oil & gas reservoir fluids form distinct, immiscible phases
• Immiscible phases are separated by an interface
– associated with inter-facial tension (IFT)
– when IFT=0, fluids mix => MISCIBILITY
• residual oil saturation to gas (and water) directly proportional to IFT
• miscible displacement characterized by low/zero residual oil saturations
5. Miscible Processes
Three basic types of miscible process
• first-contact miscibility
• condensing-gas drive
• vaporizing-gas drive
6. Miscible Conditions
• Establishment of miscibility depends on
– pressure (MMP)
– fluid system compositions
• Miscibility normally determined by laboratory measurement
7. Compositional Processes
• First Contact Miscible
LPG slugs - designed to achieve first - contact miscibility with oil at leading edge of
slug and with driving gas at trailing edge
• First Contact Miscibility
• Pressure > MMP
• All points between solvent and reservoir oil lie in single phase region
• Need high concentrations of solvent - expensive
8. Multiple Contact Miscibility
• The minimum pressure at which after many contacts the gas and
liquid (oil) become one phase.
Called MCMP - Multiple Contact Miscibility Pressure
• Condensing - Gas Drive
• Pressure < MMP
9. • Injection gas is enriched with
intermediate components such as:
• C2, C3, C4 etc
• Mechanism:
• Phase transfer of intermediate MW
hydrocarbons from the injected gas
into the oil. Some of the gas
“Condenses” into the oil.
• The reservoir oil becomes so
enriched with these materials that
miscibility results between the
injection gas and the enriched oil.
10. Condensing - Gas Drive
• Pressure < MMP
• Solvent and oil not miscible
initially
• Solvent components transfer to
liquid oil phase
• Repeated contact between oil
and solvent moves system
towards plait (critical) point
(dynamic miscibility)
11. Vaporizing Gas Drive Process
• Injection Gas - Lean Gas, C1, CO2, N2
• For vaporizing gas drive - multiple contact miscibility
• Mechanism: Intermediate hydrocarbon components in the oil vaporize
to enrich the gas.
• As the leading edge of the gas slug becomes sufficiently enriched, it
becomes miscible with the reservoir oil.
12. • To experimentally determine the MMP for given [oil, injection gas]
combination in a slim tube, the process and results are similar to the
condensing gas drive discussion
13. HC gas injection
Before introducing the concepts of minimum miscible pressure, hydrocarbon injection
had been used for many years. In necessary pressure part, hydrocarbon located
between nitrogen that needs high pressure and CO2 that needs intermediate pressure
for miscible replacement. In this case, methane is completely correct, anyway in a low
depth reservoir.
which has low pressure it can be done by increase in the percentage of saturated
hydrocarbons (C2 – C4). If it is reasonable economically, in those places in which CO2
is not available (such as Canada) It is a good choice for EOR.
14. Miscible HC Flooding Mechanism
Mechanisms:
• Different methods which are used in
hydrocarbon gas injection include:
• Enriched gas injection, lean gas injection,
gas injection in high pressure, LPG injection
• We can inject some other gases for EOR in
miscible way like: Enriched gas, Lean gas
injection and LPG (liquid petroleum gas).
Mechanisms That Improve Recovery
Viscosity reduction /oil swelling/condensing or
vaporizing gas drive.
15. Screening criteria
Screening criteria for miscible CO2 injection
Reservoir parameters value
API Gravity (°API) > 26
Oil density (kg/m3) < 920
Oil viscosity (cP) < 10
Oil composition C2 – C7
Oil saturation (%) > 20
Net oil thickness (ft) not important
Depth(ft) > 5000
Permeability (mD) wide range
Temperature (°F) wide range
16. Advantages
• Availability (natural or industrial sources)
• Low MMP (minimum miscibility pressure) and high miscibility
with oil
• Decreases oil viscosity and increases oil gravity
18. Viscosity Fingering and Gravity Tonguing
• Viscosity Fingering is caused due the low
viscosity of Injected HC
• Alternating HC and Water injection (WAG)
• Gravity tonguing is caused due to the
differences in mobility between the HC and
residual oil
