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Reservoir Rock Laboratory Course (1st Ed.)

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1. Without Distillation methods

2. Soxhlet Extraction method

3. Dean-Stark Distillation-Extraction and Vacuum DistillationA. Saturation Determination Experiment

4. Conclusions and Recommendations

1. Porosity definitions

2. Porosity determination

3. Determination of Bulk VolumeA. Determination of Bulk Volume By Mercury Pump

4. Determination of Grain Volume

5. Pore Volume DeterminationA. Pore Volume Determination (Gas Expansion)

6. Effective Porosity Determination by Helium Porosimeter Method

7. Porosity Determination by Liquid Saturating Method

Porosity importance

One of the essential properties of a reservoir rock is that it must be porous.

Porosity is therefore an important property and its accurate determination is relevant to reserve estimates and other petroleum engineering calculations.

The porosity of a material defined as the fraction (or the percentage) of the bulk volume occupied by pores. Thus porosity is a measure of the storage capacity of the rock.

The more porous is the rock, the more is its capacity to store fluids (oil, gas and water) in its pores.

Summer 14 H. AlamiNia Reservoir Rock Laboratory Course (1st Ed.) 5

total or absolute porosity vs. effective porosityTwo types of porosity may be measured:

total or absolute porosity and effective porosity. Total porosity is the ratio of all the pore spaces in a rock to the

bulk volume of the rock.

Effective porosity ϕe is the ratio of interconnected void spaces to the bulk volume. • Thus, only the effective porosity contains fluids that can be

produced from wells.

For granular materials such as sandstone, the effective porosity may approach the total porosity,

however, for shales and for highly cemented or vugular rocks such as some limestones, large variations may exist between effective and total porosity.


Absolute and Effective Porosity

Some of the pores in a rock may be sealed off from other pores by cementing materials. These pores, although present and contribute to the porosity

as defined earlier, do not allow passage or withdrawal of fluids.

If the total pores whether connected or unconnected are considered in determining porosity, the total or absolute porosity is obtained.

On the other hand if only the interconnected pores are considered, the effective porosity will result.

The difference between absolute and effective porosity is known as the dead porosity.


primary vs. secondary porosity

Porosity may be classified according to its origin as either primary or secondary. Primary or original porosity

is developed during deposition of the sediment.

Secondary porosity is caused by some geologic process subsequent to formation of the deposit. These changes in the original pore spaces may be created by

ground stresses, water movement, or various types of geological activities after the original sediments were deposited.

Fracturing or formation of solution cavities often will increase the original porosity of the rock.


Effective parameters on porosity

For a uniform rock grain size, porosity is independent of the size of the grains.

A maximum theoretical porosity of 48% is achieved with cubic packing of spherical grains.

The porosity of the Rhombohedral packing, which is more representative of reservoir conditions, is 26%.

If a second, smaller size of spherical grains is introduced into cubic packing, the porosity decreases from 48% to 14%.

Thus, porosity is dependent on the grain size distribution and the arrangement of the grains, as well as the amount of cementing materials. Not all grains are spherical, and grain shape also influences porosity.


Porosity of different packing types

Cubic packing (a),

rhombohedral (b),

cubic packing with two grain sizes (c),

and typical reservoir sand with irregular grain shape (d).


Effect of Compaction on Porosity

Compaction is the process of volume reduction due to an externally applied pressure. For extreme compaction pressures,

all materials show some irreversible change in porosity. This is due to

distortion and crushing of the grain or matrix elements of the materials, and in some cases, recrystallization.


Formation compressibility

The variation of porosity with change in pressure can be represented by

2 and 1 are porosities at pressure P2 and P1 respectively, and

cf is formation compressibility.

Formation compressibility is defined as summation of

both grain and pore compressibility.

For most petroleum reservoirs, grain compressibility is considered to be negligible. Formation compressibility can be expressed as

• dP is change in reservoir pressure.

• For porous rocks, the compressibility depends explicitly on porosity.


Porosity definition

By definition

It is sometimes convenient to express porosity in percent. So

Since a rock is composed from pores and grains or rock matrix, it is obvious thatBulk volume = grain volume + pore volume

Vb = Vg + Vp and

Vp = Vb – Vg


Porosity calculation

It is clear from the above relations that any two of the three values Vp, Vg and Vb are sufficient to determine the value of porosity.Porosity from pore and bulk volumes

Porosity from pore and grain volumes

Porosity from grain and bulk volumes

It must be noticed that the two volumes used to determine the porosity must

be for the same sample.


Porosity estimation

As indicated before it is necessary to determine two of the three volumes (bulk, grain and pore) to estimate the porosity.

Sometimes the bulk and grain densities may be used instead of bulk and grain volumes.

Depending on the method used, either absolute or effective porosity will result.


Porosity determination techniques

The porosity of reservoir rock may be determined byCore analysis, Well logging technique, Well testing

The question of which source of porosity data is most reliable cannot be answered without reference to a specific interpretation problem. These techniques can all give correct porosity values under

favorable conditions. The core analysis porosity determination has the advantage that

no assumption need to be made as to mineral composition, borehole effects, etc.

However, since the volume of the core is less than the rock volume which is investigated by a logging device, porosity values derived from logs are frequently more accurate in heterogeneous reservoirs.


Bulk Volume Measurement

Although the bulk volume may be computed from measurements of the

dimensions of a uniformly shaped sample,

the usual procedure utilizes the observation of the volume of fluid displaced by the sample.

The fluid displaced by a sample can be observed either volumetrically or gravimetrically. Gravimetric determinations of bulk volume can be

accomplished by observing the loss in weight of the sample when immersed in a fluid or by change in weight of a pycnometer with and without the core sample.


Sample isolation methods

In either procedure it is necessary to prevent the fluid penetration into the pore space of the rock.

This can be accomplished (1) by coating the sample

with paraffin or a similar substance,

(2) by saturating the core with the fluid into which it is to be immersed, or

(3) by using mercury.


Bulk Volume Determination:By Measuring the DimensionsFor a regularly shaped sample, the bulk volume is

found by measuring the dimensions of the sample. For a cylindrical sample with diameter D and length L,

the bulk volume is given by:

For a sample with rectangular cross section

A sliding caliper is used to measure the dimensions. Different reading are usually taken for the diameter and

length and the average values are used.


Bulk Volume Determination:By Russel Volumeter

In this case a sample must by saturated completely with a non-reacative fluid or coated by paraffin wax and then placed in the volumeter. The difference in the fluid level

before and after the sample gives the bulk volume of the sample.

If the sample is coated the volume of the coating material must be found and subtracted from the reading. This obtained by noting the weight of

the coating wax which is the difference between the weight of the sample before and after coating and dividing it by the density of the wax.


Bulk Volume Determination:Gravimetric (Loss of Weight) MethodA coated sample is weighed suspended in air and then

suspended in a liquid (water or kerosene).

The difference in weight is the buoyancy force which is equal to the volume of displaced fluid multiplied by the density of the fluid.

Since the volume of the displaced fluid is the same as the volume of immersed solid, then: volume of coated sample = (W1 – W2) / ρ

W1 = weight in airW2 = weight in liquid ρ = density of liquid

The volume of the coating material must be found and subtracted as explained earlier.


Bulk Volume Determination:By Mercury Pycnometer

A special steel pycnometer is used Figure. It is first filled with mercury. The top is removed and the sample

placed at the mercury surface. The top is then pressed down

allowing excess mercury to overflow into a beaker.

The excess mercury is then collected and its volume determined in a graduated cylinder. For more accuracy, the mercury may

be weighed and the volume determined by dividing the weight of mercury by its density.


Notes about bulk volume determination methods1-In the loss of weight method,

if a saturated sample is used instead of a coated sample, the grain volume of the sample is obtained.

2-The Russel volumeter may be used in the same way described to determine the grain volume of a crushed sample.

3-If the weight of a dry clean sample is determined before coating or saturating the sample, the bulk density of the sample is found from the measured bulk volume.


Bulk Volume Determination:By Mercury PumpWhen a rock has a small fraction of void space, it is

difficult to measure porosity by the mentioned methods. At this case, mercury injection is used. The principle consists of forcing mercury under relatively

high pressure in the rock pores.

A pressure gauge is attached to the cylinder for reading pressure under which measuring fluid is forced into the pores.

The volume of mercury entering the core sample is obtained from the device with accuracy up to 0.01 cm3.


Mercury porometer

Tool designed to measure the gas space and bulk volume of a freshly recovered core sample. The instrument consists of a

hand operated pump, a sample cell equipped with a needle valve mounted on its lid.

The cell can accommodate a sample with a bulk volume of 10 to 15 cm3.


Bulk Volume Determination:Mercury Pump, ProcedureThe pump consists of a core chamber, pump

cylinder with piston and wheel, scales and gauges. First mercury is brought to a fixed mark above the

sample chamber and the pump is brought to zero reading.

The piston is removed withdrawing mercury from the chamber.

The sample is then placed in the chamber and mercury is brought back to the fixed mark.

The reading of the pump scale gives the bulk volume of the sample.


Mercury injection pump (a) and porosity through mercury injection (b)


Methods of Grain Volume MeasurementThe grain volume of pore samples

is sometimes calculated from sample weight and knowledge of average density.

Formations of varying lithology and, hence, grain density limit applicability of this method.

Boyle’s law is often employed with helium as the gas to determine grain volume. The technique is fairly rapid, and

is valid on clean and dry sample.


Methods of Grain Volume Measurement (Cont.)The measurement of the grain volume of

a core sample may also be based on the loss in weight of

a saturated sample plunged in a liquid.

Grain volume may be measured by crushing a dry and clean core sample.

The volume of crushed sample is then determined by (either pycnometer or) immersing in a suitable liquid.


By Russel Volumeter

A part of a clean (extracted) dry sample is crushed into individual grains.

The grains are weighed by analytical balance and the volume is determined by Russel volumeter as in the case of bulk volume determination.


By Pycnometer

ProcedureThe pycnometer is weighed empty and then filled with water

(or kerosene). The crushed sample is weighed then placed in the empty

pycnometer and the weight is determined. Finally the pycnometer with the grains in it is completed with

water until it is completely filled and the total weight is determined.

The grain volume is then calculated as follows:

W1 = weight of pycnometer filled with fluidW0 = weight of empty pycnometerW2 = weight of pycnometer + grain W3 = weight of pycnometer + grain + fluid ρ = density of fluid


Notes about Russel Volumeter and pycnometer (W2 – W0) is the weight of the crushed grains.

This is more accurate than the use of the weight of the grains before placing in pycnometer because some grains may be lost.

The same method can be used to determine the bulk volume of a coated or fully saturated sample.

The grain volume of a sample (uncrushed) can also be obtained by Russel Volumeter or the pycnometer methods provided the sample is unsaturated (dry) and enough

time is allowed for the fluid to penetrate the pores of the sample before the readings are taken.


Loss of Weight Method

The weight of a dry clean sample W1 is determined.

The sample is then fully saturated with a non-reactive liquid. The weight of the sample suspended in the liquid W2 is then

determined.

The difference (loss) of weight is divided by the density of the liquid to find the grain volume of the sample.

The grain volume determined by this method is the effective grain volume which includes any pores that are sealed off. Porosity calculated using this method will be the effective porosity.


Gas Expansion Method

Many porosimeters are designed to use the principle of Boyle’s law of gas expansion to determine the grain volume. The idea is to allow the remaining volume of a chamber

in which a core is placed (V1 – Vg) at pressure P1 to expand by an additional volume V2 and read the final pressure P2.

From Boyle’s Law (at constant temperature).(V1 – Vg) P1 = (V1 – Vg + V2) P2

knowing V1, V2, P1 and P2 allows the calculation of grain volume Vg.Vg = V1 – [(P2 / (P1 – P2)] V2


Calculation of grain density

If we know the weight of the dry clean sample for which the grain volume is determined, the grain density can be calculated by:


The helium porosimeter

The helium porosimeter uses the principle of gas expansion, as described by Boyle’s law. A known volume (reference cell volume) of helium gas,

at a predetermined pressure, is isothermally expanded into a sample chamber.

After expansion, the resultant equilibrium pressure is measured.

This pressure depends on the volume of the sample chamber minus the rock grain volume, and then the porosity can be calculated.


Pore Volume Measurement

All the methods measuring pore volume yield effective porosity. The methods are based on

either the extraction of a fluid from the rock or

the introduction of a fluid into the pore spaces of the rock.

One of the most used methods is the helium technique, which employs Boyle’s law. The helium gas in the reference cell isothermally

expands into a sample cell. After expansion, the resultant equilibrium pressure is measured.


Saturation Method Procedure

A dry clean sample is weighed and placed in a suction flask with two connections to a vacuum pump and a Separatory funnel. First the valve is closed and vacuum is applied.

After sufficient vacuum is reached the vacuum pump is shut off, the valve to the funnel is opened and the liquid is allowed to saturate the sample.

The sample is kept immersed in the liquid for some time to allow complete saturation.

The saturated sample is drained from excess liquid and weighed.


Pore volume calculation by Saturation MethodThe pore volume is then calculated as:

Vp = (W2 – W1) / ρW2 = weight of saturated sample

W1 = weight of dry sample

ρ = density of saturating fluid

Notes:A wetting non-reactive liquid must be used.

Kerosene or tetrachlorethane are usually used.


Mercury Injection Method

The mercury pump described in bulk volume determination is also used for pore volume determination. After a dry sample is placed in the core chamber and the bulk

volume is determined, pressure is applied by moving the piston clockwise allowing mercury to enter the pores of the sample.

Pressure vs. volume of injected mercury is recorded until a pressure of 1000 psia is reached.

The final volume reading gives the pore volume of the sample.

Notes:Macropores and fractures can be detected by a flat curve at the

start where increase in volume is noted without appreciable rise in pressure.

Capillary pressure curves can be calculated from the same experiment.


Washburn Bunting Method Procedure (obsolete and seldom used)This method is based on liberating the air from the

pores of the sample by creating vacuum. This is achieved by first raising the mercury level above

the sample while the valve is open, closing the valve and then lowering the mercury reservoir so that the mercury falls below the sample in the chamber.

The collected air is measured under atmospheric pressure by raising the mercury reservoir until the mercury level is the same in the two sides.

Air is then allowed to escape and the process is repeated until no more air is extruded.

The total volume of air (under atmospheric pressure) is recorded.


Washburn –Bunting type

The experiment is first run without a sample to determine the volume of air adsorbed on the glass surface of the apparatus. This volume is

subtracted from the total air volume obtained before to get the pore volume of the sample.


Gas Expansion Method

The mercury pump (with a vacuum) gauge is used. After the bulk volume is determined and mercury

fills the chamber but does not penetrate the sample, the air in the pores is allowed to expand by withdrawing the mercury from the chamber.

If the volume of mercury withdrawn is V which is read on the pump scale then from Boyle’s Law:Vp P1 = (Vp + V) P2 So: Vp = V[(P2 / (P1 – P2)]

• P2 is the final pressure read on the vacuum gauge and • P1 is initial pressure (atmospheric)

It is clear that if P2 = ½ P1 then Vp = V• So the pore volume would be equal to the volume of mercury

withdrawn from the chamber to reduce the pressure in the chamber to half its original (atmospheric) value.


the helium Gas advantages

Helium has advantages over other gases because: (1) its small molecules rapidly penetrated small pores,

(2) it is inert and does not adsorb on rock surfaces as air may do,

(3) helium can be considered as an ideal gas (i.e., z = 1.0) for pressures and temperatures usually employed in the test,

and

(4) helium has a high diffusivity and therefore affords a useful means

for determining porosity of low permeability rocks.


the helium technique procedure

The helium porosimeter has a reference volume V1, at pressure p1, and a matrix cup with unknown volume V2, and initial pressure p2. The reference cell and the matrix cup are connected

by tubing; the system can be brought to equilibrium when the core

holder valve is opened, allowing determination of the unknown volume V2 by measuring the resultant equilibrium pressure p.

(Pressure p1 and p2 are controlled by the operator; usually p1 = 100 and p2 = 0 psig).

When the core holder valve is opened, the volume of the system will be the equilibrium volume V, which is the sum of the volumes V1 and V2.


the helium technique calculation

Boyle’s law is applicable if the expansion takes place isothermally. Thus the pressure-volume products are equal before and

after opening the core holder valve:P1V1 +P2V2 = P(V1+V2)

Solving the equation for the unknown volume, V2:V2 = (P-P1)V1/(P2-P1)

Since all pressures in the equation must be absolute and it is customary to set p1 = 100 psig and p2 = 0 psig, the Eq. may be simplified as follows:V2 = V1(100-P)/P

• V2 in cm3 is the unknown volume in the matrix cup, and • V1 in cm3 is the known volume of the reference cell. • p in psig is pressure read directly from the gauge.


the helium technique correction factor

Small volume changes occur in the system, including the changes in tubing and fittings

caused by pressure changes during equalization.

A correction factor, G, may be introduced to correct for the composite system expansion. The correction factor G is determined for porosimeters

before they leave the manufacturer, and this correction is built into the gauge calibration in such a way that it is possible to read the volumes directly from the gauge.


Schematic diagram of helium porosimeter apparatus


Conclusions and recommendations

Helium has advantages over other gases because: (1) its small molecules rapidly penetrated small pores,

(2) it is inert and does not adsorb on rock surfaces as air may do,

(3) helium can be considered as an ideal gas (i.e., z = 1.0) for pressures and temperatures usually employed in the test,

(4) helium has a high diffusivity and therefore affords a useful means for determining porosity of low permeability rocks.


Descriptions

The helium porosimeter uses the principle of gas expansion, as described by Boyle’s law. A known volume (reference cell volume) of helium gas,

at a predetermined pressure, is isothermally expanded into a sample chamber.

After expansion, the resultant equilibrium pressure is measured. This pressure depends on the volume of the sample chamber

minus the rock grain volume,

and then the porosity can be calculated.


Procedure:

Measure the diameter and length of the core using caliper.

Give the porosimeter a helium supply, 10 bar.

Determine the volume of the matrix cup with core, V2:Put the cleaned, dried core inside the matrix cup, and

mount the cup in the cup holder.Open “source” and then “supply”.Regulate the needle at 100.Close “source” and then “supply”.Open “core holder”.Take the reading on TOP SCALE, V2 = cm3.


Procedure: (Cont.)

Determine the volume of the matrix cup without core, V1:Take out the core from the matrix cup, and mount the

cup in the cup holder.

Open “source” and then “supply”.

Open “cell 1”.

Regulate the needle at 100.

Close “source and then “supply”.

Open core “holder”.

Take the reading on MIDDLE SCALE, V1 = cm3.


Calculations and report

1. Calculate and fill the data form.

V1 = the volume of the matrix cup without core, cm3.

V2 = the volume of the matrix cup with core, cm3.

Vg = V1-V2, the volume of grain and non-connected pores, cm3.

Vb = the bulk volume of core, cm3.

ϕe = (Vb-Vg)/Vb effective (interconnected) porosity of the core, fraction.


Descriptions

The determination of the effective liquid porosity of a porous plug is the initial part of the measurement of capillary

pressure using porous plate method in core laboratories.

Before the capillary pressure is determined the volume of the saturating liquid (brine or oil)

in the core must be known.

Thus, the effective liquid porosity of the core can be calculated in the beginning of capillary pressure measurement.


Procedure:

Weigh dry Berea plug Wdry, measure its diameter D, and length L, with calliper (1 core for each group).

Put the cores in the beaker inside a vacuum container, run vacuum pump about 1 hour.

Saturate the cores with 36 g/l NaCl brine, ρ brine = 1.02g/cm3.

Weigh the saturated cores, Wsat.


Calculations and report

Calculate the saturated brine weight, Wbrine = Wsat-Wdry.

Calculate the pore volume (saturated brine volume), Vp = Wsat/ ρbrine.

Calculate effective porosity, ϕ e = Vp/Vb.


1. (ABT) Torsæter, O., and M. Abtahi. "Experimental reservoir engineering laboratory work book." Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Science and Technology (NTNU), Trondheim (2003). Chapter 5A. (KSU) M. Kinawy. “Reservoir engineering

laboratory manual" Petroleum and Natural Gas Engineering Department, King Saud University, Riyadh (2009).

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