Enhanced oil-recovery

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GATE 2016
PETROELUM ENGINEERING
Enhanced Oil Recovery

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Basics: Crude oil development and production in oil reservoir can include three different
phases: Primary, Secondary and tertiary.
Primary recovery: Reservoir energy is sufficient to lift up the oil to the surface. Artificial
lift technology is part of primary recovery.
Secondary recovery: reservoir pressure maintenance is part of secondary recovery.
Reservoir pressure is maintained by injection of water or gas, which drives oil to the
wellbore.
Tertiary recovery: tertiary recovery is set of multiple processes out of which most
suitable process is implemented on reservoir in order to increase hydrocarbons
production.
Processes under tertiary recovery-
- Thermal recovery
- Gas injection (Miscible and Immiscible)
Miscible flooding is where injected gas is mixed (forms homogeneous mixture) with
oil and drive oil to the wellbore by reducing its viscosity. In Immiscible flooding gas
doesn’t form homogeneous mixture with oil. Hydrocarbons are displaced (piston
like displacement) towards producing wellbore.
- Chemical injection
EOR processes involves injection of fluid or some type of fluid into a reservoir. The
injected fluid maintains the reservoir pressure thus it drives the hydrocarbons into wellbore.
Injected fluids also interact with hydrocarbons which changes its physical properties and
makes recovery possible. These interactions include lowering of interfacial tension,
reducing viscosity and alteration of reservoir wettability etc.

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Terms use in EOR recoveries:
Microscopic and macroscopic displacement:
Displacement efficiency of any EOR process is the fraction of oil that been recovered from
a zone swept by a displacement process. Mathematically displacement efficiency is given
as,
𝐄 =
𝐕𝐨𝐢 − 𝐕𝐨𝐫
𝐕𝐨𝐢
Where Voi is volume of oil at start of flood
Vor is volume of oil after the flood
On other hand ‘displacement efficiency’ can be given as product of microscopic and
macroscopic efficiency. Now what is microscopic and macroscopic efficiency?
In simple terms ‘microscopic efficiency’ deals with the displacement of oil from reservoir
at pore level while ‘macroscopic efficiency’ deals with the displacement of oil at reservoir
level or the area contacted by flood front.
(First diagram shows pore is completely filled with oil, second diagram shows the some
volume of oil in pore is displaced by displacing fluid which can water or gas. From this
diagram, microscopic efficiency is can be defined as ratio of volume of oil displaced from

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pore to original oil volume in pore scale. Generally instead of volume, microscopic
efficiency mathematically described in terms of saturations)
Now since reservoir is three-dimensional geometry, it has a volume. Volume is nothing
but the product of area and height. When looking at the reservoir scale, fluid injected into
reservoir doesn’t contact the reservoir equally due to presence of heterogeneities. In high
permeable areas flood is advancing at faster velocity than low permeable areas. So again
coming back to ‘macroscopic efficiency’ which is efficiency of EOR process at reservoir
level, it can also be defined as the product of ‘areal efficiency’ and ‘vertical efficiency’.
If you look at the above diagram from top view, it gives you fair idea about ‘areal
efficiency’ and ‘front view’ gives an idea of vertical efficiency’. So basically
macroscopic displacement efficiency is volumetric efficiency of the process which
measures how effectively the displacing fluid sweeps out the reservoir ‘areally’ and
‘vertically’.
So basically in ideal EOR, displacing fluid (injected fluid) should completely displace the
oil from pores thus reducing it saturation equal to zero.

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Summary:
1. 𝐄 = 𝐄𝐝 𝐗 𝐄𝐯
E = Efficiency
ED = Microscopic efficiency
Ev = Macroscopic or volumetric efficiency
2. 𝐄𝐝 =
𝐒𝐨𝐢−𝐒𝐨𝐫
𝐒𝐨𝐢
Soi initial oil saturation before flooding
Sor residual oil saturation after flooding
Ed microscopic efficiency
3. 𝐄𝐯 = 𝐄𝐚 𝐗 𝐄𝐡
Ev volumetric efficiency
Ea areal efficiency
Eh vertical efficiency

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Fluid displacement from pores:
Effective displacement of from pores is known as microscopic displacement. Effect of
capillary forces, gravity forces dominates the procedure of displacement hence its
understanding is very important.
Capillary forces: whenever two immiscible fluids co-exist, capillary forces arise. to
understand capillary forces, we should have an idea about surface tension and interfacial
tension. Surface tension is force exist on air-liquid surface as a result of cohesive forces
between molecules (Cohesive force is attraction between same molecules. Attraction
between water-water molecules is cohesion. Adhesive force is attraction between different
molecules. Attraction between water-air is adhesion). It’s surface property. Coming to
interfacial tension, it’s a surface forces comes to exist when two immiscible liquids comes
together. Surface tension is measured with capillary tube.

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Surface tension can be calculated using capillary tube experiment. If capillary tube is
inserted into water, rise of water in tube can be seen as result of wettability of solid and
fluid.
From diagram general force balance:
𝜎 ∗ (2𝜋𝑟) ∗ 𝐶𝑜𝑠𝜃 = (𝜌𝑜 − 𝜌𝑔) ∗ 𝑔 ∗ ℎ ∗ 𝜋𝑟2
Here 𝜎 𝑖𝑠 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑡𝑒𝑛𝑠𝑖𝑜𝑛.
𝜎 =
(𝜌𝑜 − 𝜌𝑔) ∗ 𝑔 ∗ ℎ
2 ∗ cosθ
Interfacial tension is measured with ‘tensiometer’ method.
Solid wettability: solid wettability is tendency of fluid to spread over the surface of solid.
If there are two immiscible liquids such as water and oil, one preferentially get attracted
towards solid (which is generally water, not always), the fluid is known as ‘wetting phase’
or ‘wetting fluid’. Another one (In this case oil) is known as ‘Non-wetting’ phase.

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So basically what is capillary pressure?
Whenever interaction between two fluids, fluid-rock occurs, interfaces are intension.
Pressure difference is exists across these interfaces. This pressure difference is called as
‘Capillary Pressure’.
𝐏𝐧𝐨𝐧 𝐰𝐞𝐭𝐭𝐢𝐧𝐠 − 𝐏𝐰𝐞𝐭𝐭𝐢𝐧𝐠 = (𝛒𝐰 − 𝛒𝐨)𝐗 𝐠 𝐗 𝐡 = 𝐏𝐜𝐚𝐩𝐢𝐥𝐥𝐚𝐫𝐲
Capillary pressure is result of wettability of fluid and the interfacial tension between
fluids.
Viscous forces:
Flow of hydrocarbons through porous media causes additional pressure drop due to
frictional pressure. Frictional pressure is given by poiseulle’s equation;
∆𝑷 =
(𝟔. 𝟐𝟐 ∗ 𝟏𝟎−
𝟖) ∗ 𝝁 ∗ 𝑳 ∗ 𝑽
𝒓 𝟐 ∗ 𝒈𝒄
Where 𝜇 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦, 𝑐𝑒𝑛𝑡𝑖𝑝𝑜𝑖𝑠𝑒
L length of medium, feet
V velocity, feet/day
r radius in inches
gc gravity constant; ∆P psi

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Phase trapping:
Different condition of trapping droplet:
a. Phase trapped when radius is changing
𝑷𝒃 − 𝑷𝒂 = 𝟐 ∗ 𝝈𝒐𝒘 ∗ 𝒄𝒐𝒔𝜽 ∗ ((
𝟏
𝒓𝒂
) − (
𝟏
𝒓𝒃
))
b. Phase trapped with different angles
𝑷𝒃 − 𝑷𝒂 =
𝟐 ∗ 𝝈𝒐𝒘
𝒓
∗ (𝒄𝒐𝒔𝜽𝒂 − 𝒄𝒐𝒔𝜽𝒃)
c. Phase trapped with variation 𝝈
𝑷𝒃 − 𝑷𝒂 =
𝟐 ∗ (𝝈𝒈𝒘𝒂 − 𝝈𝒈𝒐𝒃)
𝒓
∗ 𝒄𝒐𝒔𝜽

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Capillary number: capillary number is ratio of viscous forces to capillary forces.
𝑵𝒄𝒂 =
𝒗 ∗ 𝝁
𝝈𝒐𝒘
V is interstitial velocity
𝜇 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦
𝜎𝑜𝑤 𝑖𝑛𝑡𝑒𝑟𝑓𝑐𝑖𝑎𝑙 𝑡𝑒𝑛𝑠𝑖𝑜𝑛
As capillary number increases, magnitude of residual oil decreases. General water flood
operations operate at capillary number < 10-6
. To increase the capillary number,
1. Increase flow rate of displacing fluid
2. Increase viscosity of displacing fluid
3. Decrease interfacial tension between two phases
Concept of ternary diagrams:
(Article from http://csmres.jmu.edu/geollab/Fichter/SedRx/readternary.html)
A ternary diagram is triangle with each apex reading composition such component A, B &
C.
The drawing to the left has only the skeleton of the triangle present as we concentrate on
point A. Point A is at the top of the heavy vertical red line (arrow). Along this line is
indicated percent of A. A point plotted at the top of the vertical line nearest A indicates
100% A. A horizontal bar at the bottom of the line (farthest from A) represents 0% of A.

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Any other percentage can be indicated by a line appropriately located along the line
between 0% and 100%, as shown by the numbers off to the right.
The horizontal lines that represent various percents of A can be of any length since they
run parallel to the base line and remain the same distance from the bottom and top of the
triangle. The lines are projected out to the right of the red arrow line just as far as where
the imaginary side of the triangle will be, and their percentage abundances written along
the right side of the triangle. By doing this the right side of the triangle becomes the scale
for percent abundance of A.
To be complete the horizontal lines also extend to the left until they contact the left side of
the imaginary triangle, but no percent abundances are written there. In the final ternary
diagram the red vertical arrow is removed.
Similarly process is applicable for B and C at other apexes.
Ternary diagram in EOR is used to describe phase behavior. The vertexes on diagram are
pure components.

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Macroscopic displacement of fluids in reservoir
Macroscopic displacement in oil reservoir is related to how efficiently the injected fluid
contacted the reservoir area. Production from reservoir increases with the increase in
contact area. Macroscopic efficiency of reservoir is fraction of reservoir invaded by the
injected fluid.
Volumetric efficiency of process largely depends on following parameters;
- Properties of displacing (injected) fluid
- Properties of displaced fluid
- Geological properties of reservoir
- Well spacing
Total oil production from EOR process can be easily obtained using material balance.
Np = ((
soi
Boi
) − (
Sor
Bor
)) ∗ A ∗ h ∗ ∅ ∗ Ev
Or simply,
Np = ((
soi
Boi
) − (
Sor
Bor
)) ∗ Vp ∗ Ev
Dividing both sides by initial oil in place (N barrels),
N =
Vp ∗ Soi
Boi
(
Np
N
) = (
(
soi
Boi) − (
Sor
Bor)
Soi
Boi
) ∗ Ev
We know that,
Ed =
Soi − Sor
Soi

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So basically,
(
𝐍𝐩
𝐍
) = 𝐄𝐝 𝐗 𝐄𝐯
Mobility ratio:
Mobility is of fluid is ratio of effective permeability of fluid to its viscosity.
𝜆𝑖 =
𝑘𝑖
𝜇𝑖
Mobility ratio is the ratio of mobility of displacing fluid (injected fluid) to displaced
fluid.
𝑀 =
𝞴𝒅𝒊𝒔𝒑𝒂𝒄𝒊𝒏𝒈𝒇𝒍𝒖𝒊𝒅
𝞴𝒅𝒊𝒔𝒑𝒂𝒍𝒄𝒆𝒅 𝒇𝒍𝒖𝒊𝒅
M >1 The flow process is unstable, which
leads to viscous fingering.
M = 1 Stable process
M < 1 Stable process. Favorable mobility ratio.