Wettability and Capillary pressure

In this module you will learn about

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Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Topic overview

CapillaryPressureCapillaryPressure

1 Introduction

2 Interfacial Tension

3 Rock Wettability

4 The contact angle and interfacial tension

5 Capillary pressure

6 Laboratory Measurments of capillar pressure

The water rises higher in a small pipeThe water rises higher in a small pipe

Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Section 1: Introduction

In a petroleum reservoir, the porous rock is generally saturated with several fluids, e.g., gas, oil, and water. Molecular forces induce a complex system of mutual static interactions between the fluids and the pore walls.

The fluid molecules have different attractions to each other and the rock material, and they may be ordered in a sequence according to the strength of the attractions, implying water-wet, oil-wet and gas-wet material.

The mutual attraction forces between the three fluids and the rock leads to the surface tension acting between fluids and the solid.

Capillary pressure is the pressure difference across a curved fluid interface between two fluid phases. If the fluids are enclosed in a pore channel, a contact angle is defined where the curved fluid interface meets the wall. The contact angle is an expression for the balancing of attraction forces between the fluids and the wall of the pore channel.

Formation rock

Water distribution

Oil distribution

Gas distribution

This is the distribution ofWater, Oil and Gas. As you can see from the other figures,water has an residual saturation throughout the whole column, and the oilzone has an residual saturation in the gas zone.

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Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

The Free Water Level

More about fluid distribution in reservoir

The column to the right shows the composite, actual saturation distribution with height. There is a water-oil contact, a gas-oil contact and a vertical consentration grading of the three fluids. This vertical saturation distribution is determined by the capillary pressure curve. There is a well-defined height, The Free Water Level, where the capillary pressure is zero between oil and water.

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The Free Water Level

Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Section 2: Interfacial TensionThe interfacial tension between two fluids is a measure of how much energy is needed to enlarge the surface by one unit area. That is, the dimension is J/m2, or N/m. If a droplet of oil is deformed in water by external forces, the surface area is increased, and energy is stored as potential energy which is released again if the external forces are removed.

Depending upon the relative magnitude of the intra, and inter fluid cohesive forces ( intermolecular) attractions, the interfacial tension may have different "signs".

• A "positive" interfacial tension ( > 0) means that the molecules of each fluid are most strongly attracted to the molecules of their own kind. Whereby the two fluids are immiscible, and their contact surface is minimized

• A ”neutral” interfacial tension ( = 0) means that the molecules of each fluid are attracted equally to the molecules of their own kind as to those of the other kind, and the two fluids are ”truly” miscible.

• A ”negative” surface tension ( < 0) means that the molecules of one fluid are more strongly attracted to the molecules of the other fluid. This kind of miscibility is called dissolution, wich usually means a chemical reaction between the two fluids, leading to a stable new fluid. Alcohol i water is an example of dissolution.

The natural petroleum reservoir belongs to the immiscible category.

The following types of of interface between immisciblesubstances are relevant to resevoir engineering:

• Liquid-Gas (LG)• Liquid-Liquid (LL)• Solid-Liquid (SL)• Solid-Gas (SG) Read more....

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Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Section 2: Interfacial tension

Because the cohesive force is stronger on the denser fluids side, there is a sharp change in the molecular pressure across the boundary. As a result, the boundary surface - much like the rubber surface of a baloon, is in a state of tangential tension. Called the interfacial tension denoted by (). The magnitude of the interfacial tension, represents the work or energy required to keep the two fluids apart in a pressure equilibrium state.

The stronger the intermolecular attractions within a fluid phase, the greater the work needed to bring its molecules to the surface and the greater the interfacial tension.Typical values of the surface tension range from 10 to 80 mN/m

The table shows some surface tensions of relevant liquids(Liquid-Vapor)(Liquid-Water)

The first column shows the surface tensionBetween a liquid and its own vapor

The molecules at the surface are more attracted to the intrafluid space, and to the neighbouring molecules.Because of these forces, the surface area areminimized, leading to a surface tension

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Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Section 3: Rock WettabilityWettability is a relative concept, expressing the tendency of one fluid to be preferred over the other fluid by the rock walls. If a water-wet rock sample containing mostly oil is submerged in a beaker of water, water will be sucked into the sample, expelling oil, to be seen at the water surface in the beaker.This effect is very important in evaluation of a waterflood. If a fractured reservoir is oil-wet, the injected water wil just be recyled throught the fractures at high pumping costs.The wettablility of a reservoir rock, can be estimated quantitativley by measuring the contact angle between the liquid-liquid, or liquid-gas interface and the solids surface. This angle called the wetting angle () reflects an equilibrium between the interfacial tension of the two fluid phases and their individual adhesive attraction to the solid.

By convention the angle is measured on the denser fluid side. If the measured angle is less than 90, the denser fluid is the wetting phase. Opposite if is greater than 90, the lighter fluid is considered the wetting phase

The table shows some values of the wetting angle and interfacial tension for some typical pairs of fluids relevant to a reservoir engineer

On the picture you can see a core plug,saturated with oil being immersed into water.Because the consolidated, sedimentary plug iswater-wet, the water is attracted into the coreand oil is expelled when the plug is immersed in the water. Since oil is lighterthan water, oil droplets is seen on the water surface.

If the plug had been oil-wet, nothingwould have happened.

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Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Section 4: The contact angle and interfacial tension

The water drop on the rock surface, submerged in oil, will find an equilibrium shape determined by the principle of minimum energy: The sum of the surface tensions (energies) of rock-water, rock-oil, water-oil has to be at a minimum. The tangent to the oil-water surface at the tripple-point defines the contact angle . The cosine of this angle is measure of the wettability. For 100% water-wet material, = 0, for 100% Oil-wet material, = 180, and for indifferent wettability = 90.

Standard symbol for interfacial tension is the greek letter sigma, .

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Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Pulling the dropleta little "delta" out of itsequilibrium position,and expressing the change insurface energies, gives theequilibrium equation abovethat leads to the Young-Dupre equation

Section 4: The contact angle and interfacial tension

For a reservoir system of two immiscible fluids such as oil and water, there are three types of interfacial tensions to be considered: . The three interfacial tensions are not independent. In order to reveal their relationship a simle experiment can be performed on the droplet below. Pulling this droplet slightly out of equilibrium, we set up an equation expressing the change in energy due to the change in area:

Here dA denotes an infinitesimal change in area. Since dAws = - dAos and

dAow = dAws cos, we get the the Young-Dupre equation:

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wowsos ,,

0d)d(d owowososwsws AAA

cos

ow

wsos

The Young-Dupre equationshows that the wetting anglereflects the equilibrium amongthe three interfacial tensions involved.This is a most usefull fact, when it comesto measure the capillary pressure

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Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Section 5: Capillar pressure

Capillary pressure is the pressure diffrence between to fluid phases, caused by thesurface tensions. The Laplace equation shows the relationship between the curvature of the meniscus and the capillary pressure

If the two radii are equal the equation can be reduced to

Topic link w3Have a look on apopular animation

Click toWatch movie

21owwoc

11

RRppp

As you can see here the contact angle determines the capillary pressure

c

cc

cos2

rp

Capillar pressure in reservoir

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The Free Water Level

The animation will have a speed slow enoughso you can read the text

Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overviewWater rise in a sugar lump

The movie shows a sugar lump immersed in coffe-waterto illustrate the effect of surface tensions

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Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview Section 6: Laboratory measurments of capillary pressureLaboratory measurements of capillary pressure generally employ the principal relationship:

With r taken to be the median or modal value of the rock´s pore-size distribution: however, fluids other than the reservoir fluids are normally used for shear convenience.For a particular porous medium and two diffrent pairs of fluids, the following general relationship is valid:

Wich leads to the spesific relationship (for reservoir-lab):

rp

cosc

2

c2

1

c1

coscos pp

lab

pp

cos

cos reslabc,resc,

Instrument for measurement ofcapillary pressure in lab.

Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Developers

Module made by

StudentOdd Egil OverskeidPetroleum Technology Dept.Stavanger University CollegeNorway

Topic author and coordinatorProfessorSvein M. SkjævelandPetroleum Technology Dept.Stavanger University CollegeNorwayhttp://www.ux.his.no/~s-skj/

Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

References

A.B.Zolotukhin and J.-R.UrsinEducation course in Reservoirtechnique 1

Title page

Developers

References

Helpful Facts

1 Introduction

2 Interfacial tension

3 Rock Wettability

4 The wetting angle and interphase tension

5 Capillar pressure

6 Laboratory Measurments Of capillary Pressure

Topic overview

Helpful facts

Capillary pressure is a phenomenon occuring in ALL porous systems, when two phases are present.

You will encounter in the litterature that sometimes the pressure difference between to phases are referred to as “molecular” pressure difference, or in the case of a droplet wetting a surface, just “pressure difference”. And in the case of a capillary pipe: Capillar pressure.They all talk about the same thing: Capillary pressure. In more serious texts, the author usually defines capillary pressure as a general reference to the pressure leap between to phases when their interphase is curved.

The surface forces between a liquid and a solid is usually referred to as adhesion forces. And the forces between to fluids as surface tensionsThe interphase forces acting on a dual fluid system in an porous rock are ALL surface tensions. So we are again talking about the same thing: tensions due to energy difference between the phases. Of course the solid can not "bend" but it´s still surface tension.