Questions and Answers
Which measurement techniques are commonly used for wettability measurement? (Select all that apply)
What does the symbol \$\theta$ represent?
Contact angle between the oil/water/solid interface
Wettability affects capillary pressure.
True
Match the following interfacial tensions with their definitions:
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What defines how a fluid adheres to a surface when two fluids are present?
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If the contact angle is greater than 90°, the rock is considered water wet.
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What is the irreducible water saturation denoted as?
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Capillary pressure is defined as _____ between two immiscible fluids across the interface.
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At what contact angle is a rock considered to be strongly water wet?
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What is the formula for capillary pressure?
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What impact does wettability have on oil recovery during water flooding?
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The tension between two different liquids is known as _____ tension.
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What are the typical irreducible water saturation values for water-wet reservoirs?
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Capillary pressure is always positive in oil-wet reservoirs.
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A universal capillary pressure curve exists for all rock types.
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What does the J-function help in averaging?
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The J-function is __________ for a particular rock type.
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What are the conditions under which reservoir rock is considered water-wet?
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What is imbibition in the context of fluid flow?
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What is the relationship between adhesion tension and wettability?
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Which phase fluid does NOT preferentially wet the solid rock surface?
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Natural gas is considered the wetting phase in hydrocarbon reservoirs.
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Match the following wettability characteristics with their descriptions:
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What is the effect of wettability on oil recovery during waterflooding?
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Imbibition involves the fluid flow process where the __________ phase saturation increases.
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Study Notes
Wettability
- Wettability defines how a fluid adheres to a surface (or rock in a reservoir) when there are two fluids present, e.g., water and air.
- The angle measured through the water is the contact angle (θ).
- If θ < 90°, the rock is water-wet; if θ > 90°, the rock is oil-wet.
- Most reservoir rocks are water-wet.
Irreducible Water Saturation
- In a formation, the minimum saturation induced by displacement is where the wetting phase becomes discontinuous.
- In normal water-wet rocks, this is the irreducible water saturation (Swirr).
- Large-grained rocks have a low irreducible water saturation compared to small-grained formations because the capillary pressure is smaller.
Capillary Pressure
- It is the pressure difference between two immiscible fluids, the non-wetting and wetting phases, across the interface, when the two fluids are at equilibrium in a capillary tube or a porous medium: Pc = Pnwt – Pwt.
- Capillary pressure can be positive or negative depending on the wettability preference.
- For water-wet reservoirs, the capillary pressure is positive.
Capillary Forces
- In a simple water and air system, the wettability gives rise to a curved interface between the two fluids.
- The experiment has a glass tube attached to a reservoir of water, where the water "wets" the glass, causing the pressure on the concave side (water) to exceed that on the convex side (air).
- This excess pressure is the capillary pressure.
Capillary Forces and Rocks
- In a reservoir, the two fluids are oil and water, which are immiscible, hence they exhibit capillary pressure phenomena.
- This is seen by the rise in the water above the point where the capillary pressure is zero.
- The height depends on the density difference and the radius of the capillaries.
Effect of Wettability on Oil Recovery
- Oil recovery is strongly affected by wettability.
- For oil wet reservoirs:
- Water breakthrough occurs much earlier.
- The recovery at water breakthrough is small.
- Significant amounts of oil can be produced following water breakthrough.
Interfacial Tensions and a Solid Surface
- When a drop of one immiscible fluid is immersed in another and comes to rest on a solid surface, the shape of the resulting interface is governed by the balance of adhesive and cohesive forces.
Contact Angle
- The angle between the fluid and solid phases is called the contact angle.
- Contact angles are always measured in the denser fluid phase.
- If θ < 90°, the fluid is said to "wet" the surface. If θ > 90°, the fluid is said to be "non-wetting".
Wettability and Oil Recovery
- The wettability of a rock refers to the contact angle for the oil-brine interface.
- If θ < 90°, the reservoir is said to be "water-wet". If θ > 90°, the reservoir is said to be "oil-wet".
- Wettability is affected by many factors, including fluid compositions, mineral surface properties, microbial activity, and temperature and pressure.
Implications of Wettability
- Oil recovery under waterflooding is affected by the wettability of the system.
- A water-wet system will exhibit greater oil recovery under waterflooding.
Capillary Pressure and Interfacial Tension
- The effect of interfacial tension is to create a finite pressure difference between immiscible fluids called the capillary pressure: Pc = Pnw - Pw.
- Capillary pressure depends on the properties of the fluids and solid surfaces, σwa and cosθwa, and the tube (i.e., pore throat) radius, r.### Capillary Pressure Curve
- Capillary pressure curves are affected by the pore size distribution of the rock
- The Pc intercept is called the displacement pressure
- The process of decreasing the wetting phase saturation is called drainage
- The process of decreasing the non-wetting phase saturation is called imbibition
- The imbibition and drainage paths are different, and this effect is called hysteresis
Asymptotic (Irreducible) Saturations
- The asymptotic minimum value of water saturation, Swi, is known as the irreducible water saturation or connate water saturation
- The asymptotic minimum oil saturation on the imbibition curve, Soi, is known as the irreducible oil saturation
- For water-wet reservoirs, Swi is typically 20-25% and Soi is typically 15-20%
- For oil-wet reservoirs, Swi is usually 10-15% and Soi is usually 15-20%
Oil-Water Transition Zone
- At elevations greater than the capillary head, hc, the oil saturation is (1 - Swi)
- At the OWC, the water saturation is 1
- Between ho and hc, the saturations vary continuously through the capillary transition zone
Leverett J-Function
- The Leverett J-function is a dimensionless, universal capillary pressure curve
- J(sw) = (C Pc) / (σ cosθ φ1/2 k1/2)
- C is a unit conversion factor, Pc is capillary pressure, σ is interfacial tension, cosθ is the contact angle, φ is porosity, and k is permeability
- The J-function is useful for averaging capillary pressure data from a given rock type from a given reservoir
Interfacial Tension and Wettability
- Interfacial (boundary) tension is the energy per unit area (force per unit distance) at the surface between phases
- Wettability is the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids
- Wettability refers to the interaction between fluid and solid phases
- Adhesion tension is expressed as the difference between two solid-fluid interfacial tensions: AT = σ(os) - σ(ws) = -σ(ow) cosθ
Reservoir Seal
- The seal for a reservoir is usually provided by a water-wet zone with low (but finite) permeability
- Typically, the seal is a shale
- Darcy's Law would indicate that with a finite permeability, gravity effect alone would cause petroleum to pass upward through the seal due to density difference, over a long (geologic) time period
- For multiple phases flowing, flow is controlled by pressure, gravity, and capillary pressure
- The effect of displacement pressure of the seal halts upward migration of petroleum in the trap### Contact Angle and Wettability
- Contact angle (θ) is measured through the denser liquid phase and defines which fluid wets the solid surface.
- θ < 90° indicates water-wet, θ > 90° indicates oil-wet, and θ = 90° indicates neutral wettability.
Wettability Phases
- Wetting phase fluid preferentially wets the solid rock surface, often with low mobility.
- Attractive forces between rock and fluid draw the wetting phase into small pores.
- Wetting phase fluid often has low irreducible saturation.
- Nonwetting phase does not preferentially wet the solid rock surface.
- Repulsive forces between rock and fluid cause nonwetting phase to occupy larger pores.
- Nonwetting phase fluid is often highly mobile, especially at large nonwetting phase saturations.
Water-Wet and Oil-Wet Reservoir Rock
- Water-wet reservoir rock: water preferentially wets the rock surface (θ < 90°).
- Conditions for water-wet reservoir rock: σws > σos, AT < 0, and 0° < θ < 90°.
- Oil-wet reservoir rock: oil preferentially wets the rock surface (θ > 90°).
- Conditions for oil-wet reservoir rock: σos > σws, AT > 0, and 90° < θ < 180°.
Interfacial Contact Angles
- Silicate minerals have acidic surfaces, repel acidic fluids, and attract basic compounds.
- Carbonate minerals have basic surfaces, attract acidic compounds, and are neutral to oil-wet surfaces.
Wettability Classification
- Strongly oil- or water-wetting, neutral wettability, fractional wettability, and mixed wettability.
Imbibition and Drainage
- Imbibition: fluid flow process where the saturation of the wetting phase increases, and the nonwetting phase saturation decreases.
- Drainage: fluid flow process where the saturation of the nonwetting phase increases, and the wetting phase saturation decreases.
Implications of Wettability
- Primary oil recovery is affected by wettability, with water-wet systems exhibiting greater primary oil recovery.
- Oil recovery under waterflooding is affected by wettability, with water-wet systems exhibiting greater oil recovery.
- Wettability affects the shape of relative permeability curves, with oil moving more easily in water-wet rocks than oil-wet rocks.
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Description
This quiz covers the concept of wettability in reservoir rocks, including contact angle and its significance in determining whether a rock is water-wet or oil-wet.
