Suez University

Faculty of Petroleum & Mining Engineering

Reservoir Mechanics

Student

Belal Farouk El-saied Ibrahim

Class / III

Section / Engineering Geology and Geophysics

The Reference / Geology of Petroleum

(A.J.Leversen)

Presented to

Prof. Dr. / Shouhdi E. Shalaby

Main Topics

Phase Relationships.

Interface Phenomena.

Surface Tension.

Interfacial Tension.

Surface Free energy.

Pressure-Temperature Diagram

Figure 1 shows a typical pressure-temperature diagram of a

multicomponent system with a specific overall composition.

Although a different hydrocarbon system would have a

different phase diagram, the general configuration is similar.

These multicomponent pressure-temperature diagrams are

essentially

used to:

• Classify reservoirs

• Classify the naturally occurring hydrocarbon systems

• Describe the phase behavior of the reservoir fluid

Phase Relationships.

• Critical point—The critical point for a

multicomponent mixture is referred to as

the state of pressure and temperature at

which all intensive properties of the gas

and liquid phases are equal (point C). At

the critical point, the corresponding

pressure and temperature are called the

critical pressure pc and critical temperature

Tc of the mixture.

Pressure-Temperature Diagram

• Bubble-point curve—The bubble-point

curve (line BC) is defined as the line

separating the liquid-phase region from the

two-phase region.

• Dew-point curve—The dew-point curve

(line AC) is defined as the line separating the

vapor-phase region from the two-phase

region.

Pressure-Temperature Diagram

• Oil reservoirs—If the reservoir temperature

T is less than the critical temperature Tc of

the reservoir fluid, the reservoir is classified

as an oil reservoir.

• Gas reservoirs—If the reservoir

temperature is greater than the critical

temperature of the hydrocarbon fluid, the

reservoir is considered a gas reservoir.

Pressure-Temperature Diagram

When phases exist together, the

boundary between two of them is

termed an interface.

The properties of the molecules

forming the interface are often

sufficiently from those in the bulk of

each phase that they are referred to

as forming an interfacial phase.

Interfacial Phenomena

Several types of interface can exist, depending on whether the two adjacent phases are in the solid, liquid or gaseous state.

For convenience, we shall divide these various combinations into two groups, namely liquid interfaces and solid interfaces.

Interfacial Phenomena

Interfacial Phenomena

Classification of Interfaces

Solid-solid interface, powder particles in contact.

ySSSolid - solid

Liquid-solid interface, suspensionyLSLiquid - solid

Liquid-liquid interface, emulsionyLLLiquid - liquid

Solid surface, table topySVGas - solid

Liquid surface, body of water exposed to atmosphere

уLVGas - liquid

No interface possible-Gas - gas

Types & Examples of InterfaceInterfacial Tension

Phase

Liquid InterfacesSurface and Interfacial Tension

Surface

The term surface is customarily used when referring to either a gas-solid or a gas-liquid interface.

“Every surface is an interface.”

Liquid Interfaces

Surface tension-

a force pulling

the molecules of

the interface

together resulting

in a contracted

surface.

- Force per unit

area applied

parallel to the

surface.Unit in

dynes/cm or N/m

Liquid Interfaces

Interfacial

tension

Is the force per

unit length

existing at the

interface

between two

immiscible liquid

phases and like

surface tension,

has the units of

dyne/cm..

Liquid Interfaces Surface Free

energy – increase

in energy of the

liquid and the

surface of the

liquid increase.

-work must be done

to increase liquid

surface.

γ – surface tension or surface free energy per unit surface.

Liquid Interface

Surface Free energy

W = γ ∆ A

where W is work done or surface free energy increase

express in ergs(dyne.cm); γ is surface tension in

dynes/cm and ∆ A is increase in area in cm sq.

What in the work required to increase area of a liquid

droplet by 10 cm sq if the surface tension is 49

dynes/cm?

W = 49 dynes/cm x 10 cm sq = 490 ergs

Liquid Interfaces

When oleic acid is

placed on the

surface of a water ,

a film will be

formed if the force

of adhesion b/n

oleic accid

molecules and

water molecules is

greater than the

cohesive forces b/n

the oleic acid

molecules

themselves.

Liquid Interfaces

Work of adhesion(Wa), which is the energy

required to break the attraction between the unlike

molecules.(water to oil)

Work of cohesion(Wc), required to separate the

molecules of the spreading liquid so that it can flow

over the sublayer.(oil to oil and water to water)

Spreading of oil to water occurs if the work of adhesion

is greater than the work of cohesion.

Spreading coefficient(S) – difference between Wa

and Wc.

Positive S – if oil spreads over a water surface.

Liquid InterfacesSurface and Interfacial Tension

When a drop of oil is added on the surface of water, three things may happen:

1. The drop may spread as a thin film on the surface of water.(positve S)

2. It may form a liquid lens if the oil cannot spread on the surface of water.(negative S)

3. The drop may spread as a monolayer film with areas that are identified as lenses.

Liquid Interfaces

50.4

45.8

45.5

45.2

42.4

32 (250)

24.6

13

8.9

3.4

0.22

-3.19

-13.4

Ethyl alcohol

Propionic acid

Ethyl ether

Acetic acid

Acetone

Undecyclenic acid

Oleic acid

Chloroform

Benzene

Hexane

Octane

Ethylene dibromide

Liquid petrolatum

S (dynes/cm)Substance

Initial Spreading Coefficients, S, at 20◦C

• Water and oil (or gas) in reservoirs coexist in an

immiscible state (i.e., the water phase does not

mix miscibly with the hydrocarbon phase). There

is a natural and strong interfacial tension between

the two fluids that keeps them separate, regardless

of how small the individual droplets may be. A

common example of this immiscible nature is a

household salad dressing made of oil and vinegar.

Wettability

• In all reservoirs connate water is immiscible with

the oil or gas, but chemicals can be injected into

the reservoir to reduce interfacial tension and make

the water phase miscible with the oil. There are

advantages in doing this, and it is a form of

enhanced oil recovery.

• The oil and gas phases in reservoirs also generally

behave immiscibly. However, at certain pressures,

temperatures, and compositions, they may become

miscible.

• Wettability can be defined as the ability of a fluid

phase to preferentially wet a solid surface in the

presence of a second immiscible phase. In the

reservoir context, it refers to the state of the rock

and fluid system; i.e., whether the reservoir is

water or oil wet. Three possible states of

wettability in oil reservoirs exist as shown in

Figure 2. The arrows represent the tangent to the

angle between the water droplet and the rock

surface. The water droplet is surrounded by the oil

phase.

• Wettability is generally classified into three

categories: (1) The reservoir is said to be

water wet; that is, water preferentially wets

the reservoir rock, when the contact angle

between the rock and water is less than 90,

(2) neutral wettability case would exist at a

contact angle of 90, and (3) oil wet occurs

at a contact angle greater than 90.

• Other lesser known types of wettability are:

• Neutral or intermediate wettability – no preference is shown by the rock to either fluid; i.e., equally wet.

Figure 2 Three possible states of wettability in oil reservoirs.

• Fractional wettability – heterogeneous wetting; i.e., portions of the rock are strongly oil wet, whereas other portions are strongly water wet. Occurs due to variation in minerals with different surface chemical properties. Silicate water interface is acidic, therefore basic constituents in oils will readily be absorbed resulting in an oil-wet surface. In contrast, the carbonate water interface is basic and will attract and absorb acid compounds. Since crude oils generally contain acidic polar compounds, there is a tendency for silicate rocks to be neutral to water-wet and carbonates to be neutral to oil-wet.

• Mixed wettability – refers to small pores occupied by water and are water-wet, while larger pores are oil-wet and continuous. Subsequently, oil displacement occurs at very low oil saturations resulting in unusually low residual oil saturation.

• Figures 3and 4 represent microscopic views of

water-wet and oil-wet systems, respectively.

Figure 3 Microscopic fluid saturation distribution in a water-wet rock [Pirson, 1963]

Figure 4 Microscopic fluid saturation distribution in a oil-wet rock [Pirson, 1963]

• The contact angle is a measure of the wettability of the

rock-fluid system, and is related to the interfacial

energies by Young’s equation,

• os - ws = ow cos (1)

• where:

• os = interfacial energy between oil and solid, dyne/cm;

• ws = interfacial energy between water and solid,

dyne/cm;

• ow = interfacial energy, or interfacial tension, between

oil and water, dyne/cm;

• contact angle at oil-water-solid interface measured

through the water phase,

• deg.

• Figure 5 identifies the variables in Equation (1)

Figure 5 Relationship of oil-water-solid interfacial tensions and contact angle