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Reservoir Rock Laboratory Course (1st Ed.)

1. Without Distillation methods
2. Soxhlet Extraction method
3. Dean-Stark Distillation-Extraction and
Vacuum Distillation
A. Saturation Determination Experiment
4. Conclusions and Recommendations

1. Porosity definitions
2. Porosity determination
3. Determination of Bulk Volume
A. Determination of Bulk Volume By Mercury Pump
4. Determination of Grain Volume
5. Pore Volume Determination
A. 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 porosity
Two 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 that
Bulk 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 by
Core 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 Dimensions
For 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.

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.

Gravimetric (Loss of Weight) Method
A 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 air
W2 = weight in liquid
 ρ = density of liquid
The volume of the coating material must be found and
subtracted as explained earlier.

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 methods
1-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.

By Mercury Pump
When 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.

Mercury Pump, Procedure
The 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 Measurement
The 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
Procedure
The 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 fluid
W0 = weight of empty pycnometer
W2 = 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
Method
The pore volume is then calculated as:
Vp = (W2 – W1) / ρ
W2 = 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 5
A. (KSU) M. Kinawy. “Reservoir engineering
laboratory manual" Petroleum and Natural Gas
Engineering Department, King Saud University,
Riyadh (2009).

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