Petroleum Reservoir Engineering II Lecture 2: Fundamentals

Tishk International UniversityEngineering FacultyPetroleum and Mining Engineering Department

Petroleum Reservoir Engineering II

Third Grade- Spring Semester 2020-2021

Lecture 2: Fundamentals of Rock Properties (Wettability)

Instructor: Sheida Mostafa Sheikheh

Fundamentals of Rock Properties:

■ The material of which a petroleum reservoir rock may be composed can range from

very loose and unconsolidated sand to a very hard and dense sandstone, limestone,

or dolomite.

■ The grains may be bonded together with a number of materials, the most common

of which are silicate, calcite or clay.

■ Knowledge of the physical properties of the rock and the existing interaction

between hydrocarbon system and the formation is essential in understanding and

evaluating the performance of a given reservoir.


■ Rock properties are

determined by performing

laboratory analyses on cores

from the reservoir to be

evaluated.


■ There are basically two main categories of core analysis tests that are preformed on

core samples regarding physical properties of reservoir rocks.

Core Analysis Tests

Routine Core Analysis

Tests

Porosity Permeability Saturation

Special Tests

WettabilitySurface and Interfacial Tension

Capillary Pressure

Relative Permeability

Overburden Pressure

Content (Theory):

■ Wettability

■ Wetting Characteristics

■ Factors Affecting Wettability

Wettability:

❑ Wettability is defined as the tendency of one fluid to spread on or adhere to a solid surface

in the presence of other immiscible fluids.

Wettability:

❑ When a liquid is brought

into contact with a solid

surface, the liquid either

expand over the whole

surface or form small

drops on the surface.

Wettability: ❑ Small drops of three liquids- mercury, oil and water- are

placed on a clean glass plate. Once, the three droplets are

observed from one side, it is noted that the mercury

retains a spherical shape, the oil droplets develop an

approximately hemispherical shape, but the water tends

to spread over the glass surface.

Wettability:

❑ The tendency of a liquid to spread over the surface of a solid is an indication of the

wetting characteristics of the liquid for the solid. This spreading tendency can be

expressed more conveniently by measuring the angle θ of contact at the liquid-solid

surface. This angle, which is always measured through the liquid to the solid, is

called the contact angle θ.

Wettability:

❑ Contact angle and wettability are inversely proportional.

❑ As the contact angle decreases, the wetting characteristics of the liquid increases.

❑ Complete wettability would be evidenced by a zero contact angle.

❑ Complete nonwetting would be evidenced by a contact angle of 180°.

Wettability:

❑ A wetting phase is one which spreads over the solid surface and preferentially wets

the solid. The contact angle approaches zero (and will always be less than 90°).

❑ A non-wetting phase has little or no affinity for a solid and the contact angle will be

greater than 90°.

Wettability:

Wettability:

Which Condition is more favorable for us as

Petroleum Engineers?

Water-wet or Oil-wet Rock?

Which one is preferable for Oil Recovery

Wettability:

❑ The wettability of reservoir rocks to the fluids is important in that the distribution of the

fluids in the porous media is a function of wettability.

• Water-wet: the rock/mineral surface is coated with water, while oil and gas occupy the

central position of the largest pores.

• Oil-wet: the relative positions of oil and water are reversed with respect to the water-wet

state; the rock/mineral surface is coated with oil and the water is in the centre of the

largest pores.

• Intermediate wettability: this term applies to reservoir rocks where there is some

tendency for both oil and water to adhere to the pore surface

❑ Because of the attractive forces, the wetting phase tends to occupy the smaller pores of

the rock and the nonwetting phase occupies the more open channels.

Wettability:

❑ In the oilfield terminology, a rock:

• Is strongly water-wet if the contact angle is 𝜃 = 0 − 70°

• Has intermediate wettability if the contact angle is 𝜃 = 70 − 110°

• Is strongly oil-wet if the contact angle is 𝜃 = 110 − 180

Wettability:

❑ The wettability of a reservoir rock system will depend on many factors:

▪ Reservoir rock material

▪ Geological mechanisms (accumulation and migration)

▪ Composition and amount of oil and water

▪ Physical conditions; pressure and temperature

▪ Mechanisms occurring during production; i.e., change in saturations, pressure and

composition.

Factors affecting Wettability:

❑ Factors related to rock properties:

• Rock composition: Different minerals

have different basic/acidic properties:

✓ Silicate minerals have acidic surfaces:

➢ Repel acidic fluids such as major polar

organic compounds present in some

crude oils

➢ Attract basic compounds


❑ Factors related to rock properties:

• Rock composition: Different minerals

have different basic/acidic properties:

✓ Carbonate minerals have basic surfaces:

➢ Attract acidic compounds of crude oils


❑ Factors related to water properties:

• Water:

✓ Presence of water inhibits oil wetting ability

✓ Low salinity water shifts of the contact angle distribution toward a more

water‐wet state.


❑ Factors related to oil properties:

• Oil composition

- Resins (NSOs)

✓ Nitrogen

✓ Sulphur

✓ Oxygen

- Heavy depositional polar components

✓ Asphaltene, kerogen, bitumen, wax

Increase of Heavy Components= Increase of Oil Wetness


❑ Factors related to water properties:

• Temperate:

✓ Temperature has effect on both oil/water and water/mineral interfaces.

✓ Shift in wettability of mineral surfaces toward water-wet at elevated

temperatures.

✓ Increasing the solubility of adsorbed materials from surfaces and decreasing the

IFT are two different effects of temperature on wettability at elevated

temperature.

Content (Practical):

■ Wettability

■ Imbibition and Drainage Processes

■ Wettability Measurement

Wettability:

Drainage and Imbibition Process:

❑ Drainage: fluid flow process in which the saturation of the nonwetting phase

increases.

❑ Example of drainage is waterflood of an oil reservoir that is oil-wet.

❑ Mobility of nonwetting fluid phase increases as nonwetting phase saturation

increases.

❑ This process of displacing wetting phase by non-wetting phase is called drainage

process.

Drainage and Imbibition Process:

❑ Imbibition: is a fluid flow process in which the saturation of the wetting phase

increases and the nonwetting phase saturation decreases.

❑ Example of imbibition is waterflood of a water-well oil reservoir.

❑ Mobility of wetting phase increases as wetting phase saturation increases.

❑ This process continues to a certain water saturation (maximum value) at which no

more oil can be displaced. This point is called residual oil saturation.

Measurement of Wettability:

❑ No satisfactory method exists for in situ measurement of wettability.

❑ Therefore, it is necessary to estimate the wettability from laboratory measurements.

❑ Imbibition and displacement (Amott – Harvey method) is the most accepted and

widely used test in the oil industry.


❑ The Amott-Harvey Method:

1. Spontaneous Imbibition:

Oil-saturated sample is placed in an

imbibition cell surrounded by water. The

water is allowed to imbibe into the core

sample displacing oil until equilibrium is

reached. The volume of water imbibed is

equal to the oil displaced; 𝑉𝑜1



2. Forced Imbibition:

The core is moved to a core holder and water

is pumped through. The volume of oil

displaced may be measured; 𝑉𝑜2

Water Index:

𝑟𝑤 =𝑉𝑜1

𝑉𝑜1 + 𝑉𝑜2



3. Spontaneous Uptake of Oil:

The core, now saturated with water at

residual oil saturation, is placed in an Amott

cell and surrounded by oil. The oil is

spontaneously taken up and water is

displaced. The volume of water displaced is

measured; 𝑉𝑤1



4. Forced displacement of water

The core is removed from the cell after equilibrium is reached, and remaining water in the core is forced out by displacement in a flooding rig. The volume of water displaced is measured; 𝑉𝑤2

Oil Index:

𝑟𝑜 =𝑉𝑤1

𝑉𝑤1 + 𝑉𝑤2


❑ Amott-Harvey Wettability Index:

𝑊𝐼 =𝑉𝑜1

𝑉𝑜1 + 𝑉𝑜2−

𝑉𝑤1𝑉𝑤1 + 𝑉𝑤2

= 𝑟𝑤 − 𝑟𝑜

𝑉𝑜1= volume of oil produced during water imbibition

𝑉𝑜2= volume of oil produced during water flooding

𝑉𝑤1= volume of water produced during oil “imbibition”

𝑉𝑤2= volume of water produced during oil flooding

𝑟𝑤= water index

𝑟𝑜= oil index


Amott – Harvey Method:


Amott – Harvey Method:



𝑊𝐼 =𝑉𝑜1






𝑊𝐼 =𝑉𝑜1





Example: The following table provides the displacement data for an Amott wettability

test on three cores from an Alaska North Slope reservoir.

Calculate the Amott-Harvey wettability index for each core and determine the wetting

characteristics of each core.


Solution:

Water Index:

𝑟𝑤 =𝑉𝑜1

𝑉𝑜1 + 𝑉𝑜2

𝑟𝑤 =0.81

0.81 + 0.85

𝑟𝑤 = 0.488


Solution:

Oil Index:

𝑟𝑜 =𝑉𝑤1

𝑉𝑤1 + 𝑉𝑤2

𝑟𝑜 =0.05

0.05 + 1.25= 0.038

𝑟𝑜 = 0.038


Solution:

Wettability Index:

𝑊𝐼 =𝑉𝑜1




𝑊𝐼 = 0.488 − 0.038

𝑊𝐼 = 0.488 − 0.038

𝑊𝐼 = 0.45


Homework: The following table provides the displacement data for an Amott wettability test:

Calculate the Amott-Harvey wettability index for each core and determine the wetting

characteristics of each core.


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Petroleum Reservoir Engineering II Lecture 2: Fundamentals