Lab 6 Water Saturation
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1
ABSTRACT
The main objective of this experiment is to measure the fluid saturation of rock sample by
using the Dean-Stark extraction method. The rock sample is saturated with 100% water. Then, a
solvent, usually toluene is dripped into the flask for heating, over the sample. In this method, the
toluene is vaporized and vapor flows through the rock sample; allowing the saturated water to
vaporized and recondensed in a cooled tube in the top of the apparatus and the water is collected
in a calibrated chamber. By applying the formula, fluid saturation can be determined. In
comparison with other methods such as retort method, Dean-Stark extraction method is the best
method compared to the retort method in terms of accuracy. Besides that, the rock sample can be
reused for further experiment. In conclusion, the Dean-Stark extraction method provides a direct
determination of fluid saturation. However, due to lack of experimental data, the actual results
cannot be analyzed.
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INTRODUCTION
A reservoir depends on its critical parameters to produce economically and efficiently. Two of the
parameters are porosity, which can be defined as the measurement of the rock’s ability to hold a
fluid, and permeability, which defines the characteristics of the rock that allows a fluid to flow
through the rock. In this experiment, the final critical parameter is the fluid saturation. Fluid
saturation can be defined as the measurement of the void spaces in the rock that are occupied by
fluids such as gas, oil and water.
In other words, fluid saturation can be identified as the ratio of the total volume of the fluid
to the void spaces volume of the rock. Generally, the critical parameter can be expressed
mathematically by the following relationship:
Fluid Saturation = Total Volume of the fluid
Void spaces volume
Figure 1: Fluid saturation relationship
By applying the above relationship to each reservoir fluids (e.g. gas, oil and water), the
relationships gives the following equations where Sg is the gas saturation, So is the oil saturation
and Sw is the water saturation. The saturation of each reservoir fluid ranges between 0 to 100 %.
By definition, the summation of all the fluid saturations equal to 1.
Sg = Vg
Vp
So = Vo
Vp Sg + So + Sw = 1
Sw = Vw
Vp
Figure 2: Fluid saturation of each fluid
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The amount and availability of each of fluids (gas, oil and water) depend upon gravity and
external hydrodynamic forces such as interfacial or surface tension forces that occur between the
fluids or between the fluids and the rock. Surface tension forces can be described as the forces
acting on the interface while interfacial forces are the one that acting on the same fluids. E.g.
liquid-vapor form. The interfacial forces can be distinguished into various forms such as liquid-
vapor, liquid-liquid, and fluid-liquid forms.
Interfacial/surface tension forces Explanations
Liquid-vapor This force results from the differences of molecular
attractions of gas and liquid molecules
Liquid-liquid This force results from the differences of molecular
attractions of different liquids
Fluid-solid This force results from the preference for the fluid
molecule to be attracted to the solid surface
Table 1: Explanations on interfacial/surface tension forces
The interfacial forces are important to as it gives rise to what known as a capillary pressure.
Capillary pressure is the difference in pressure across the interface between two immiscible fluids.
These fluids can be differentiated into wetting phase and non-wetting phase. For examples, in oil-
water system, the water is the wetting phase while in oil-gas system, the oil is the wetting phase.
The capillary pressure is significant to the fluid saturations. These two parameters can be relate to
each other. For instance, at decreasing water saturation, the capillary pressure increases.
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AIM/ OBJECTIVE
To determine the fluid saturation of a rock sample using retort method
THEORY
The fluid saturation is defined as the ratio of the volume of fluid in a given core sample to the pore
volume of the sample:
Sw = 𝑉𝑤
𝑉𝑝 S0 =
𝑉𝑜
𝑉𝑝 Sg =
𝑉𝑔
𝑉𝑝 (Eqn. 1)
Sw + So + Sg = 1
(Eqn. 2)
Where;
So – Oil saturation Vo – Oil Pore volume
Sg – Gas saturation Vg – Gas pore volume
Sw – Water saturation Vp – Pore volume
Vw – Water pore volume
Fluid saturation can be define either as a fraction of total porosity or as a fraction of
effective porosity. The fluid in void space are not interconnected cannot be produced from a well,
therefore the saturations will be more significant if expressed on the basis of effective porosity.
The weight of water collected from the sample is calculated from the volume of water by the
relationship:
Ww =ρw x Vw (Eqn. -3)
Where;
Ww – weight of water
ρw – water density in g/cm3.
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Oil volume can be calculated as Wo /ρw which is the weight of oil removed from the core and can
be measured by equation below;
Wo = WL – Ww (Eqn. 4)
Where;
Wo – Weight of oil
WL – Weigh of liquid
WL is the weight of liquids removed from the core sample in gram (g). The weight of liquid can
be determined using equation below;
WL = WSat - Wdry
Where;
Wsat – Weight of original saturated sample
Wdry – Weight of desaturated and dry sample
Pore volume Vp is determined by a porosity measurement and bulk volume;
Vp = ф Vb
Where;
Ф – Porosity
Vb – Bulk volume
Bulk volume can be measured by equation below:
Vb = π (D/2)2 L
Where D and L are diameter and length of the core sample, respectively.
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Oil and water saturation may be calculated by Eqn. 1 and gas saturation can be determined using
Eqn. 2.
APPARATUS AND MATERIALS
Apparatus
1. Dean–Stark Apparatus
2. Heating Plate
3. Desiccater
4. Weighing balance
Materials
1. Cylindrical rock sample
2. Thimble
3. Water
4. Toluene
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PROCEDURES
1. A 100% water-saturated rock sample was prepared.
2. A clean and dry thimble was prepared n weighted. Tongs was used to handle the thimble.
3. A cylindrical rock sample was placed inside the thimble, then quickly weigh the thimble
and sample.
4. The extraction flask was filled with two-thirds full with toluene.
5. The thimble with the sample was placed into the long neck flask.
6. The heating plate was turn on and the rate of boiling is adjust so that the reflux from the
condenser is a few drops of solvent per second.
Note: The water circulation rate should be adjusted so that excessive cooling does not
prevent the condenser solvent from reaching the core sample.
7. The volume of collected water in the graduated tube was measured.
8. After the process is complete, place the rock sample was placed into the oven (from 105°C
to 120°C).
9. The dried sample was stored in a desiccater.
10. The weight of the thimble and the dry sample was measured.
11. The loss in weight WL of the core sample due to the removal of oil and water was measured.
12. The density of a separate sample of the oil was measured.
13. The pore volume Vp of the sample was determined.
14. The oil, water and gas saturations was calculated.
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RESULTS
Volume of sample, Vb = 8 cm3
Porosity of rock sample , ø = 0.080
Lose in weight due to removal of oil and water , WL = 0.494 g
Wdry
(g)
Worg
(g)
ρw
(g/cm3)
ρo
(g/cm3)
Vw
(cm3)
Wo
(g)
Vo
(cm3)
Vp
(cm3) So Sw Sg
17.320 17.814 1 0.863 0.331 0.163 0.189 0.64 0.517 0.30 0.19
Table 8.1: Results
SAMPLE OF CALCULATION
Weight of water, Ww = density of water, ρw X Volume of water, Vw
= (1 g/cm3) x (0.331 cm3)
Weight of oil, WO = Lose in weight due to removal of oil and water, WL – weight of water, Ww
= 0.494 g – 0.331 g
= 0.163 g
Pore Volume, Vp = Porosity, ø x Bulk Volume, Vb
= (0.080 x 8cm3)
= 0.64 cm3
Volume of oil, VO = Weight of oil, WO / Pore Volume, Vp
= 0.163 g/ 0.866 gcm-3
= 0.188 cm3
Water saturation, Sw = Water Volume, VW / Pore Volume, Vp
= (0.331 cm3 / 0.64 cm3)
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= 0.517
Oil saturation, SO = Oil Volume, VO / Pore Volume, Vp
= (0.188 cm3 / 0.64 cm3)
= 0.29
Gas saturation,Sg = 1-Sw-So
= 1 – 0.517 – 0.29
= 0.19
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DISCUSSION
The experiment simulates how a rock sample is completely saturated with water (Sw =
100) during the deposition stage when sediments are being deposited (of which usually occurs in
an aqeous environment). This is depicted in the experiment when 100% water saturated cylindrical
rock sample was put inside the long neck flask. It further simulates whereby hydrocarbons entered
the pores due to a nearby active hydrocarbon source rock that pressures the pores thus entering the
pores and occupying the spaces pre-occupied by the water. In the experiment, two-thirds of toluene
was heated to simulate the hydrocarbon entering the porous rock sample and releasing the connate
water (or in this case the pre-water saturated rock sample) that is to be collected and measured in
the graduated tube. The toluene based solvent was further heated until all of it was extracted and
also measured.
Measurement of the toluene is accurate enough as, like oil, it is immiscible and forms a
layer on top of water due to its low density. Measurements done for the volume of accumulated
water can directly determine the water saturation whereas oil saturation and gas saturation can be
determined through an indirect method of calculations based on the stated relationships from
(please refer to the theory section). It was also deemed necessary to dry the sample in an oven and
placed in a desiccater to determine the weight for further calculations to obtain respected fluid
saturations.
Results and calculations from the experiment show that So was higher with 0.517 saturation
as compared, to that of Sw which was 0.3. When compared to the results obtained from the
illustrations for extraction method by New Mexico Tech:
Wdry
(g)
Worg
(g)
ρw
(g/cm3)
ρo
(g/cm3)
Vw
(cm3)
Wo
(g)
Vo
(cm3)
Vp
(cm3)
So Sw Sg
53 57 1 0.88 1.4 4 0.8 5 0.59 0.28 0.13
It could be seen that the saturations for oil, water and gas didn’t vary much for the ones
obtained in this experiment. As it can be seen, oil saturation is half of the pore volume followed
by an average of 30% for water saturation and lastly the balance is occupied by oil. Hydrocarbon
accumulation in the reservoir will reduce water saturation to a minor value, normally 5-40% (for
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this experiment 30%) saturation whereby water can no longer escape the pores. This is such that
water saturation is at the irreducible water saturation where it is immobile or unable to move.
Water saturation in real life reservoirs consists of several more symbols which are Swir, Swc and
Swi. Swir means irreducible water saturation. Swc on the other hand is connate water saturation or
water that existed during the discovery of the reservoir which may or may not be irreducible. Swi
means initial or original water saturation on discovery which may mean irreducible, connate or
interstitial. Saturation of gas (Sg) was determined by subtraction of 100% saturation with Sw and
So giving a balance of Sg. This is applicable because oil and gas reservoirs are always completely
saturated with fluid. Pores of a reservoir will never an occasion or location have void spaces. Pores
are predominantly filled with some combination of fluid.
Presence of hydrocarbons (So and Sg) in a rock sample or real-life reservoir will also mean
higher water saturation after invasion as compared to that during the original reservoir conditions.
This is called imbibition. When oil is produced, water invades and replaces the withdrawn oil as
shown in the diagram. Amount of change is also dependent on the drive mechanism.
Toluene was the preferred solvent used in this experiment due to its immiscibility to water.
This is to ensure that measurement can be obtained due to its lower density compared to water thus
forming a film layer. Apart from that, it is possible to heat the sample until the all the water amount
is extracted from the sample as toluene has a high boiling point. In addition to that, toluene
dissolves all volatiles in the sample whereby accurate measurement can be done.
There are some advantages of using the extraction method to determine water saturation
compared to the retort method. Accuracy wise, oil and water measurements are of the same sample
and the core sample can be used for further analysis whereas the retort method destroys the sample
as a result because of the strong heat applied. Apart from that, contamination and utensils needed
are minimised. The drawback to using the Dean-Stark method is that it requires a long time to
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complete measurements, sometimes even weeks in contrast to the retort method which requires
less than 24 hours. Besides that, drops of water will remain on the walls or pore throats of the
distillate tube thus bypassing a few measurements leading to poor accuracy.
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CONCLUSION
It can be concluded that the experiment is incomplete due to lack of experimental data and
equipment. The experiment is done by researching and reviewing data from previously done
experiment. However, by analyzing the data, it can be said that the Dean-Stark extraction method
provides a direct determination of fluid saturation.
RECOMMENDATIONS
It is recommended to follow the below recommendations and considerations in order to get the
best and accurate saturation results. The recommendations are as followed:
1. Ensure that the rock sample is protected and secured from chemical substances that might
change the rock properties.
2. The procedure must be followed carefully to avoid inaccurate results, thus providing best
saturation results.
3. Before starting the experiment, ensure the equipment and materials are arranged and
assembled accordingly to avoid any mistakes.
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APPENDICES
Figure 1 Thimble
Figure 2 Dean-Stark Distillation
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REFERENCE
Bakkalaurea. (2007). Saturation and Capillary Pressure in Reservoir Rocks. Retrieved on
November 14, 2013 from: https://online.unileoben.ac.at/
Chapter 4: Saturation. (n.d.). Retrieved from New Mexico Tech:
infohost.nmt.edu/~petro/faculty/Engler524/PET524-3a-saturation
Crain, R. (n.d.). Saturation Basics. Retrieved from Crain's Petrophysical Handbook:
http://www.spec2000.net/14-swbasics.htm#b2
Fluid Saturation and Capillary Pressure. Petrophysics MSc Course Notes. Retrieved on November
14, 2013 from: http://www2.ggl.ulaval.ca/
M. Kinawy, M. (n.d.). Reservoir Engineering Laboratory. Retrieved from King Saud University:
faculty.ksu.edu.sa/mkinawy/.../Lab%20Book%20-%20PGE%20363
Tibor Bodi (n.a). Direct And Indirect Connate Water Saturation Determination Method In The
Practice Of Riaes.
Wang, J. (2007, February 15). Saturation and Capillary Pressure in Reservoir Rocks. Retrieved
from Montan University Leoben:
https://online.unileoben.ac.at/mu_online/wbAbs.getDocument?pThesisNr=21329&pAutorNr=&
pOrgNr=15089.
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