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.

2

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

3

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.

4

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.

5

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.

6

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

7

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.

8

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)

9

= 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

10

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

11

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

12

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.

13

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.

14

APPENDICES

Figure 1 Thimble

Figure 2 Dean-Stark Distillation

15

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.