FUNDAMENTALS OF ROCK PROPERTIES

AKHILOME .E. CHRISTIAN

SIWES/PED/81002

Introduction Porosity Permeability Saturation Wettability Surface and Interfacial tension Capillary pressure Rock Compressibility

PRESENTATION OVERVIEW

Reservoir rock properties

Oil created by the source rock won’t be useful unless it winds up being stored in an easily accessible container, a rock that has room to “suck it up”

A reservoir rock is a place that oil migrates to and is held underground.

Reservoir rocks are dominantly sedimentary (sandstones, carbonates); however, highly fractured igneous and metamorphic rocks have been known to produce hydrocarbons, but on a much smaller scale.

WHAT IS A RESERVOIR ROCK

Reservoir rock properties

Reservoir rock properties

Properties of Reservoir Rocks

A good reservoir rock must have porosity in which petroleum can exist.

Another characteristics of reservoir rock is that it must be permeable.

Other properties include: Fluid Saturation, Saturation, Wettability, Surface and Interfacial tension, Capillary pressureRock Compressibility, Overburden pressure etc.

Reservoir rock properties

Reservoir rock properties

POROSITY

Even though a reservoir rock looks solid to the naked eye, a microscopic examination reveals the existence of tiny openings in the rock, called pores. These spaces or voids are the one in which reservoir fluids are present.

Porosity(φ) : Porosity is the capability of a rock to hold fluids in pore. It is the ratio of the pore volume in a rock

to the bulk volume of that rock. Expressed in per cent.

Mathematical form is:φ = Vp/Vb

Porosity

Reservoir rock properties

Porosity is a measure of the void space in rock, hence, measures how much HC in rock Porosity φ = Vp/Vb = (Vb-Vm)/Vb; Vb = Vp + Vm

◦ theoretically, φ varies from 0% - 47.6%◦ In practice, φ varies between 3% and 37%

Porosity is a function of particle size distribution:◦ Framework materials (sandstone) – high φ◦ Interstitial materials (shaly-sand) – low φ

Porosity

Rock matrix

Water

Oil and/or gas

Reservoir rock properties

Porosity can be classified into;

1. Original porosity2. Induced porosity

Original porosity (primary) is formed during the deposition of rock materials, e.g. porosity between granular in sandstone, porosity among crystal and oolitic in limestone Induced porosity (secondary) is developed by some geological process on the

deposited rock material. E.g; Fractures, or vugs cavity usual occur in limestone (chemical reaction b/w CaCO3 and MgCl2)

Classification of Porosity

Reservoir rock properties

Types of Porosity

Sand grain

Cement material

Effective / connected porosity (25%)Ineffective Porosity (5%)

Total Porosity (30%)

Reservoir rock properties

PRIMARY• Particle sphericity and angularity• Packing• Sorting (variable grain sizes)

• Cementing materials• Overburden stress (compaction)• Vugs, dissolution, and fractures

FACTORS THAT AFFECT POROSITY

SECONDARY (DIAGENETIC)

Reservoir rock properties

FACTORS THAT AFFECT POROSITY

cubic packing of spheres resulting in a least-compact arrangement with a porosity of 47.64%

Rhombohedral packing of spheres resulting in a most-compact arrangement with a porosity of 26%

Spherical size variation influences type &

volume of solid porosity

Effect of cement material

Porosity 36%

Porosity 20%

Reservoir rock properties

PERMEABILITY

Reservoir rock properties

Permeability is a property of the porous medium that measures the ability of the formation to transmit fluids. It’s the a measure of the ease with which the rock will permit the passage of fluids.

Unlike porosity, permeability cannot be defined apart from fluid flow. For a rock to be Permeable, it must contain inter-connected pores

Reservoir permeability is usually quoted in millidarcies, (md).

Permeability

Reservoir rock properties

Absolute permeability - the permeability of a porous medium with only one fluid present (single-phase flow).

When two or more fluids are present permeability of the rock to a flowing fluid is called effective permeability (ko, kg, kw).

Relative permeability is the ratio of absolute and effective permeabilities kro=ko/k, krg=kg/k, krw=kw/k.

Three types of permeability

Reservoir rock properties

Darcy’s law helps us to measure the degree of permeability.

Darcy’s “K” was determined to be a combination of◦ k, permeability of the sand pack (porous medium, e.g. reservoir rock)◦ K is a constant of proportionality◦ , viscosity of the liquid◦ dp, Pressure gradient

K constant may be written as;

Permeability

dLdpKAQ

μk

K

Reservoir rock properties

The unit of permeability in the empirical equation is the Darcy and the dimension is (L2). It is usually too large to be convenientin hydrocarbon reservoirs. The millidarcy (10-3 Darcy) is therefore used. Generally the permeability is termed as : Poor if; k<1, Fair if; 1<k<10, Moderate if; 10<k<50, Good if; 50<k<250, Very good if; k>250.

Permeability

Reservoir rock properties

Permeability is a very important rock property because it controls the

directional movement and flowrate of the reservoir fluid in the formation.

The factors affecting the magnitude of Permeability are:

Shape and size of grain sizes, cementation overburden pressure fracturing and Dissolution

Permeability

Reservoir rock properties

Fluid SaturationSaturation is defined as that fraction, or percent, of the pore volumeoccupied by a particular fluid (oil, gas, or water). This property isexpressed mathematically by the following relationship:

Applying the above mathematical concept of saturation to each reservoir fluid gives

Reservoir rock properties

Where So = Oil saturation Sg = Gas saturation Sw = Water saturation

The saturation of each individual phase range from 0-100%.

By definition, the sum of saturation is 100% therefore,

Sg+So+Sw=1.0

Fluid Saturation

Reservoir rock properties

The major saturation types of interest in a Reservoir are; Critical Oil saturation, Soc Movable oil saturation, Som Residual Oil Saturation, Sor Connate water Saturation, Swc

Saturation types

Reservoir rock properties

WETTABILITY This is the tendency of a fluid to spread on or adhere to a solid

surface in the presence of other immiscible fluid. The angles made by the fluid with the surface with which it is in contact is known as the “contact angle”.

Depending on the type of fluid in contact with a solid surface, a reservoir could be; water-wet or oil-wet. Because of the attractive force, the wetting phase tends to occupy the smaller pores of the rock and the nonwetting phase occupies the more channels.

Reservoir rock properties

Knowledge of the wettability of reservoir rocks is essential in determining the appropriate drive mechanism for a particular reservoir. It is an important control on the amount of recovery.

Hydrocarbon wet system retard hydrocarbon mobility while water wet systems promotes hydrocarbon mobility.

Wettability

Reservoir rock properties

Petroleum reservoirs commonly have 2 – 3 fluids (multiphase systems)

It is necessary to consider the effect of the forces at the interface when two immiscible fluids are in contact.

When these two fluids are liquid and gas, the term surface tension is used to describe the forces acting at the interface. When the interface is between two liquids, the acting forces are called interfacial tension.

When 2 or more fluids are present, there are at least 3 sets of forces acting on the fluids and affecting HC recovery

Surface and Interfacial Tension

Reservoir rock properties

Surface and 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

Oil

Rock

water

Reservoir rock properties

When two immiscible fluids are in contact, a discontinuity in pressure exits between the two fluids, which depends upon the curvature of the interface separating the fluids. We call this pressure difference the capillary pressure, Pc.

Similarly, it can be defined as the pressure differential between two immiscible fluid phases occupying the same pores caused by interfacial tension between the two phases that must be overcome to initiate flow.

Capillary Pressure

Reservoir rock properties

Reservoir rocks are subjected to the internal stress exerted by fluids contained in the pores, and to external stress which is in part exerted by the overlying rocks.

The weight of the overburden simply applies a compressive force to the reservoir rock. Compressibility typically decreases with increasing porosity and effective overburden pressure.

Porosity is a function of compaction. It is generally reduced by increase in compaction. Compaction is a function of depth of burial.

Rock Compressibility

Reservoir rock properties

Rock compressibility is the fractional change in Volume per unit change in pressure

Expressed as

Three types of compressibilityRock- matrix (grain) compressibility, Cs

Rock-bulk compressibility, Cb

Pore-volume compressibility, Cp

Formation Compressibility is important to Reservoir engineers. It plays a role in depletion of fluid from pore spaces, internal rock stress changes which results in change in Vp, Vm, Vb

Pv

Vc

pp

1=

Reservoir rock properties

Knowledge of reservoir rock properties is very essential to evaluating Reservoir Performance and understanding reservoir behavior.

The aforementioned rock properties are essential for reservoir engineering calculations as they directly affect both the quantity and the distribution of hydrocarbons and, when combined with fluid properties, control the flow of the existing phases (i.e., gas, oil and water) within the reservoir.

CONCLUSION

Reservoir rock properties

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