Capillary pressure and wettability

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Capillary pressure and wettability
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Interfacial tension

• Immiscible fluids when you bring them into
  contact they do not mix
• Two fluids are separated by an interface
• The molecules are attracted more to their own kind

Rock
water
Oil
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Interfacial tension
Interfacial tension is the work required to
create a unit area of new surface
s F / L ( N/m or dynes/cm )
F 2 s L
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Wettability
Young-Laplace equation
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Wettability
Oil
Oil
q
q
water
water
grain surface
grain surface
Water wet
Oil wet
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Types of wettability

• Strongly water wet
• Intermediate wettability
• Fractional wettability
• Neutral
• Mixed wettability
• Strongly oil wet

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Factors affecting wettability

• Oil composition
• No correlation with crude composition
• Water-wet reservoirs have low asphaltene
• Some low-asphaltene crudes have mixed wettability
• Rock mineralogy
• Formation brine and pH
• Pressure and temperature
• Thickness of water layer

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What is capillary pressure?
Pc Poil - Pwater
Capillary pressure is related to curvature High
Pc high curvature Small Pc small curvature
(flat) At typical velocities capillary force is
the strongest force!
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Young-Laplace Equation
s Interfacial tension N/m q Contact
angle R Characteristic radius m
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Capillary pressure in a uniform tube
From Lake, 1989
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Capillary Pressure and Flow
Flow
q
Fluid 1
2R
Fluid 2
PLeft
PRight
0
xL
xxf
From Lake, 1989
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Capillary Pressure and Flow
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Drainage and imbibition
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Hysteresis in capillary pressure
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Hysteresis in Capillary Pressure Curves

• Trapping - Snap off and residual saturations
• Drag- Advancing and receding contact angles
• Surface roughness
• Hydrodynamic effects
• Adsorption of organics

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Hysteresis in contact angle
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Capillary pressure curve

• Physics is determined by
• Fluids Interfacial tension
• Density
• (not viscosity !)
• Rock Pore size distribution
• Mineralogy
• Rock / fluid interaction Wettability,
• Contact angle

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Initial-Residual (IR) Curve
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Initial-Residual (IR) Curve...
Described by Land (1971) equation.

• where C is positive constant
• function of rock type, porosity

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Water Wet
air
water
glass plate
oil
water
height
oil
free water level
water
pressure
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Capillary pressure curves
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(No Transcript)
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Saturation height function relation with
permeability
Depth
0
1
Water saturation
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Reservoir quality
C
h
B
A
0
100
water saturation
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Water-wet rock

• Reservoir originally water-wet
• Migration of oil via cap rock into reservoir

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Mixed wettability

• Drainage process oil displacing water
• Ageing of reservoir at connate water saturation
• Reservoir more oil wet (mixed wet)

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Capillary pressure in pores
where r1, r2 principal radii of curvature
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Water-wet reservoir
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At the end of imbibition, oil is trapped as
isolated blobs in larger pores. HIGH sor, LOW kw
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Oil remains connected and can drain to low
saturations. Water occupies largest pores. LOW
sor, HIGH kw
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Mercury injection apparatus
PHg
Displacement reading
Sample
I I I I I I I I I
I I I I I I I I
Mercury
Mercury pump Up to 4000 bar
Sample chamber
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Laboratory/Reservoir Conversion

• Capillary curves can be converted from lab
  conditions to the reservoir as follows
• ? 0 except Air/Hg where ? 140
• Lab surface tensions have been measured e.g.
• Mercury/air s 480 dynes/cm
• Kerosene/brine s 50 dynes/cm
• Reservoir values are ill-defined - some data is
  available but a range should be used

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Saturation / height function

• Capillary curves can be converted to saturation /
  height functions as follows
• Gravity constant 0.433 psi/ft

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Empirical correlations of Pc

• Drainage capillary pressure

• Imbibition curve

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Estimation of Swi or Swc
Pc Log scale
Sw, log scale
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Dimensionless Pc
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Capillarity wettability summary

• Capillarity rock sucks up liquid against gravity
  and depends on
• - Interfacial tension keeps bubble in shape
• - Wettability fluid that is wetting the rock
• - Pore size distribution small pores suck up
  fluid higher
• Capillary pressure curve describes how much
  wetting fluid can be pulled up, against gravity
• In reservoir capillarity leads to transition
  zone above hydrocarbon/water contact