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POROSITY

Reservoir rock properties

Professor Abdulla Ali Al-Dombi

Petroleum Production Engineer

مدونة بالعربية

https://aldambi.blogspot.com/

In English

https://aldambi2.blogspot.com/

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��Porosity��

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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.

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Types of Porosity

Sand grain

Cement material

Effective / connected porosity (25%)

Ineffective Porosity (5%)

Total Porosity (30%)

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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 percent.

Mathematical form is:

φ = V_p/V_b

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Porosity

Porosity is a measure of the void space in

rock, hence, measures how much HC in rock

Porosity φ = V_p/V_b= (V_b-V_m)/V_b; V_b = V_p + V_m

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

Reservoir rock properties

Rock matrix

Water

Oil and/or gas

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��Porosity��

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��Types of Porosity��

As the sediments were deposited and the rocks were being formed during past geological times, some void spaces that developed became isolated from the other void spaces by excessive cementation.
Thus, many of the void spaces are interconnected while some of the pore spaces are completely isolated.
This leads to three different types of porosity, namely:
Total (Absolute) porosity.
Effective porosity.
Ineffective porosity.

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��Types of Porosity(cont.)��

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��Types of Porosity(cont.)��

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Classification of Porosity

Reservoir pores or porosity of rocks are found as two different general types in sedimentary rocks based on their time of formation or according to the mode of origin as original induced. These are:
Original (primary) porosity:

Is defined as porosity in a rock due to sedimentation process or that developed in the deposition of the material.

Induced (secondary) porosity:

Is that developed by some geologic process subsequent to deposition of the rock.

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Classification of Porosity(cont.)

Original (primary) porosity:
Original porosity is illustrated by the intergranular porosity of sandstones and the intercrystalline porosity of some limestones.
Primary porosity is amount of pore space present in the sediment at the time of deposition or formed during sedimentation.
Rocks having original porosity are more uniform in their properties than those rocks in which a large part of the porosity is included.

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Classification of Porosity(cont.)

Induced (secondary) porosity:
It is developed by some geological process subsequent to the deposition of the rock.
It is illustrated by fracture development as found in some shales and limestones and by the vugs or solution cavities in some limestones.
Porosity of this type has been subdivided into three classes based on the mechanism of formation:
Solution porosity
Fracture porosity
Dolomitization porosity

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Solution porosity�

It is called Vugullar porosity - voids formed by the solution

Voids of this type can range in size from small vugs to cavernous masses

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Fractures�

Fractures are an important source of permeability in low porosity carbonate reservoirs.

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Dolomitization:�

This process by which limestone transformed into dolomite

2CaCO₃ + MgCl₂ =CaMg(CO₃)₂+CaCl₂

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Classification of Porosity(cont.)

Intergranular porosity is the porosity due to pore volume between the rock grains.
Intragranular porosity is the porosity due to voids within the rock grains.
Dissolution porosity is the porosity resulting from dissolution of rock grains.
Fracture porosity is the porosity resulting from fractures in the rock.
Vug porosity is the porosity associated with vugs, commonly in carbonate rocks.

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Classification of Porosity(cont.)

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Classification of Porosity(cont.)

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Factors Which Affect Porosity

Packing and size of grains

The absolute sizes of the sand grains which make up a rock do not influence the amount of porosity occurring in the rock.

However variations in the range of sand grains sizes do influence considerably the porosity.

Particle size distribution(sorting)

If spheres of varying sizes are packed together, porosity may be any amount from 48 per cent to a very small amount approaching 0 per cent.

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Factors Which Affect Porosity(cont.)

Particle shape

If the sand grains are elongated or flat and are packed with their flat surfaces together, porosity and permeability may both be low we will discuss further in the context of permeability.

Cement material

Sandstones are compacted and usually cemented together with clays and minerals. The porosity and permeability of a sandstone are both influenced to a marked degree by the amount of cementing material present in the pore space and the way this material occupies the pore space between the sand grains.

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FACTORS THAT AFFECT POROSITY

Reservoir rock properties

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%

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Factors Which Affect Porosity(cont.)

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Factors Which Affect Porosity(cont.)

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Typical Porosity Range(cont.)

A typical value of porosity for clean, consolidated and reasonably uniform sand is 20%.
If φ< 5% then porosity is insignificant
If 5< φ <10% then porosity is low
If 10 < φ < 20 then porosity good
If φ >20 % then porosity very good

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Average Reservoir Porosity

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Average Reservoir Porosity(cont.)

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Average Reservoir Porosity(cont.)

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Porosity Application

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Porosity Application(cont.)

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Porosity Application(cont.)

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Problems(cont.)

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Problems(cont.)

Calculate the arithmetic average, thickness-weighted average and Volumetric-weighted average from the following measurements:

Sample	Thickness, ft		Porosity, %
1	2	5	10
2	3	8	20
3	1	2	15
4	2	6	12
5	1	3	10
6	3	4	15

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Problems(cont.)

Rectangular reservoir has the following
Dimensions and properties:
Reservoir length = 10 miles
Reservoir width = 3 miles
Reservoir thickness = 100 ft
Reservoir average porosity = 22 %

Calculate the reservoir bulk and pore volume:

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Problems(cont.)

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Porosity Measurement

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Porosity Measurement (cont.)

Porosity is measured directly(core samples measurement ) and also indirectly measure(open-hole wire line logs).
The best way of determining porosity is to carry out experiments on core extracted from the well.
It should be noted that core determined porosities have a much higher degree of accuracy than porosities determined from down-hole tools, but suffer from sampling problems.
Taken together core and borehole determined porosities optimize accuracy and high resolution sampling.

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Porosity Measurement (cont.)

Cores obtained from the reservoir formation contain a considerable amount of information about the nature of the rocks themselves and various properties.
They are also a source of material for investigating rock behavior with respect to fluid displacement and its reaction to various fluid types.
An accurate description of rock properties is of a considerable importance in the fields of both applied and theoretical science and especially in the solution of petroleum reservoir engineering problems.

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Core Types

Before examining some of the methods it is important to define the various core types used in examining rock properties and their reaction to the transmission of fluids.
Fresh core

Any newly recovered core material preserved as quickly as possible at the well site to prevent evaporative losses and exposure to oxygen. The fluid type used for coring should be noted, e.g., fresh state ( oil-based drilling fluid), fresh state ( water -based drilling fluid).

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Core Types(cont.)

Preserved core

Similar to fresh core but some period of storage is implied. Preserved core is protected from alteration by a number of techniques such as freezing the core to lock in fluids which would otherwise evaporate.

Cleaned core

Core from which the fluids have been removed by solvents. The cleaning process should be specified. Some solvents could damage the fabric of the rock.

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Porosity Measurements

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Porosity Measurements(cont.)

1- Routine core samples analysis(direct measurement)

Saturation Method
Buoyancy Method
Helium Porisimetry
Mercury Porisimetry

2- Open-hole wire line logs techniques(indirect measurement)

Density Log
Sonic Log
Neutron Log

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Routine Core Samples Analysis

Fraction of volume consisting of pores

Fraction of volume consisting of matrix

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Bulk Volume Measurement

1- Uniformly (regularly) shaped sample

The most common types of samples used in routine core analysis are cylindrical core plugs which allow the determination of bulk volume from the dimensions of the sample (length and diameter).
Use a caliper to obtain several diameter measurements and several length measurements.
Use the average diameter and average length to calculate the bulk volume of the sample.

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Bulk Volume Measurement(cont.)

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Bulk Volume Measurement(cont.)

2-Irregular sample

For irregular sample use Archimedes(Buoyancy) principle to determine bulk volume from fluid displaced by an irregular sample.
To prevent the penetration of the fluid into the pore space by coating the sample with paraffin wax or by saturating the sample with the same fluid used for observing the displacement, or by using mercury.
If the sample is irregular in shape, e.g., a cutting, then bulk volume is estimated by submerging it in fluid and measuring the volume of water it displaces.

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Bulk Volume Measurement(cont.)

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Bulk Volume Measurement(cont.)

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Pore Volume Measurement

All methods used for determining pore volume are based on either extraction of a fluid from the rock sample or introduction of a fluid in the pore spaces of the rock sample.
It is significant that all methods measuring pore volume yield effective porosity because the fluid either extracted from the pore spaces of the rock sample or introduced into the pore spaces of the sample will always be from interconnected pores.

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