Question 1: Definition of fluid saturation

Saturation is the measure of the fluid volume present in the pore volume of a porous medium. By

definition, the saturation of a fluid is the ratio of the fluid volume to the pore volume or the rock.

Hence, considering the fluids typically present in a reservoir rock:

This property is expressed mathematically by the following

relationship:

fluid saturation :total volume of the fluid

pore volume

The average saturation of each reservoir fluid is calculated from the

following equations:

So : volume of oilpore volume

, So: oil saturation

Sg : volume of gaspore volume

,Sg: gas saturation

Sw : volume of waterpore volume

,Sw: water saturation

All the saturation values based on pore volume and not on the gross reservoir volume. The saturation of

each individual phase ranges between zero to 100 percent. By definition, the sum of the saturation is

100% and can be verified with this equation: So +Sg +Sw = 1.0

The fluids in most reservoirs are believed to have reached a state of equilibrium and, therefore,

will have become separated according to their density, i.e., oil overlain by gas and underlain by water. In

addition to the bottom (or edge) water, there will be connate water distributed throughout the oil and

gas zones. The water in these zone will have been reduced to some irreducible minimum. The forces

retaining the water in the oil and gas zones are referred to as capillary forces because they are

important only in pore spaces of capillary size.

Connate (interstitial) water saturation Swc is important primarily because it reduces the amount

of space available between oil and gas. It is generally not uniformly distributed throughout the reservoir

but varies with permeability, lithology, and height above the free water table. Another particular phase

saturation of interest is called the critical saturation and it is associated with each reservoir fluid.

Where Si and Vi, I = w,o,g and are the saturations and volumes of water, oil, and gas. The sum of

saturation of each fluid phase is equal to unity since the pore space is completely filled with fluids (or at

least the effective pore volume). Because the fluids and their saturations in the pore space may vary

from point to point and pore to pore, the values of saturation are meaningful only for samples large

enough for the porous medium to be considered a continuum.

It is important to consider the saturation change occurring in the core from in-situ to surface

conditions. Suppose a core is being recovered while drilling a well with water-based drilling mud. Water

from the drilling mud will enter the rock expulsing oil. As the core is lifted, the reduction in pressure will

cause the oil to release gas and this will expand expulsing oil and water out of the rock.

These Figure presents saturation values at different stages of core extraction showing how

important the change in saturation can be.

Example of Saturation Changes Occurring in the Core from In-situ to Surface Conditions

QUESTION 2: Compare all methods to determine the fluid saturation

1. Direct Injection of Solvent

The solvent is injected into the sample in a continuous process. The sample is held in a rubber sleeve

thus forcing the flow to be uniaxial.

2. Centrifuge Flushing

A centrifuge which has been fitted with a special head sprays warm solvent onto the sample. The

centrifugal force then moves the solvent through the sample. The used solvent can be collected and

recycled.

3. Gas Driven Solvent Extraction

The sample is placed in a pressurized atmosphere of solvent containing dissolved gas. The solvent fills

the pores of sample. When the pressure is decreased, the gas comes out of solution, expands, and

drives fluids out of the rock pore space. This process can be repeated as many times as necessary.

4. Soxhlet Extraction

A Soxhlet extraction apparatus is the most common method for cleaning sample, and is routinely used

by most laboratories. As shown in Figure a, toluene is brought to a slow boil in a Pyrex flask; its vapors

move upwards and the core becomes engulfed in the toluene vapors (at approximately 110°C). Eventual

water within the core sample in the thimble will be vaporized. The toluene and water vapors enter the

inner chamber of the condenser, the cold water circulating about the inner chamber condenses both

vapors to immiscible liquids. Recondensed toluene together with liquid water falls from the base of the

condenser onto the core sample in the thimble; the toluene soaks the core sample and dissolves any oil

with which it come into contact. When the liquid level within the Soxhlet tube reaches the top of the

siphon tube arrangement, the liquids within the Soxhlet tube are automatically emptied by a siphon

effect and flow into the boiling flask. The toluene is then ready to start another cycle. A complete

extraction may take several days to several weeks in the case of low API gravity crude or presence of

heavy residual hydrocarbon deposit within the core. Low permeability rock may also require a long

extraction time.

Electric or gas heaters are used to vaporize the solvent. The hot vapors meet the samples in the

thimble and dissolve the oil and water. Vapors are condensed and cover the sample until over- flown

back to the solvent flask. The extraction process continues for several hours and is terminated when no

more oil remains in the samples. This is recognized when the condensing vapors remain clean because

no oils is left in the cores to be dissolved. After the extraction, samples are dried in an electric oven.

Sometimes vacuum may also be applied to the oven. A complete extraction may take several days to

several weeks in the case of low API gravity crude or presence of heavy residual hydrocarbon deposit

within the core. Low permeability rock may also require a long extraction time. The dried samples are

kept in a desiccator sealed with grease and has some moisture absorbents at its bottom.

5. Dean-Stark Distillation-Extraction

The Dean-Stark distillation provides a direct determination of water content. The oil and water area

extracted by dripping a solvent, usually toluene or a mixture of acetone and chloroform, over the plug

samples. In this method, the water and solvent are vaporized, recondensed in a cooled tube in the top

of the apparatus and the water is collected in a calibrated chamber (Figure b). The solvent overflows and

drips back over the samples. The oil removed from the samples remains in solution in the solvent. Oil

content is calculated by the difference between the weight of water recovered and the total weight loss

after extraction and drying.

Figure: Schematic diagram of Soxhlet (a) and Dean- Stark (b) apparatus

6. Vacuum Distillation

The oil and water content of cores may be determined by this method. As shown in Figure c, a sample is

placed within a leak proof vacuum system and heated to a maximum temperature of 230°C. Liquids

within the sample are vaporized and passed through a condensing column that is cooled by liquid

nitrogen.

Figure C: Vacuum distillation Apparatus.

Summary of method

The direct-injection method is effective, but slow. The method of flushing by using centrifuge is limited

to plug-sized samples. The samples also must have sufficient mechanical strength to withstand the stress

imposed by centrifuging. However, the procedure is fast. The gas driven extraction method is slow. The

disadvantage here is that it is not suitable for poorly consolidated samples or chalky limestone. The

distillation in a Soxhlet apparatus is slow, but is gentle on the samples. The procedure is simple and very

accurate water content determination can be made. Vacuum distillation is often used for full diameter

cores because the process is relatively rapid and frequently used for poorly consolidated cores since the

process does not damage the sample. The oil and water values are measured directly and dependently

of each other.

In each of these methods, the number of cycles or amount of solvent which must be used depends on

the nature of the hydrocarbons being removed and the solvent used. Often, more than one solvent

must be used to clean a sample. The solvents selected must not react with the minerals in the core. The

commonly used solvents are:

Acetone, Benzene, Carbon-tetrachloride, Chloroform, Methylene Dichloride, Toluene, Trichloro

Ethylene, Xylene and etc

Toluene and benzene are most frequently used to remove oil and methanol and water is used to

remove salt from interstitial or filtrate water. The cleaning procedures used are specifically important in

special core analysis tests, as the cleaning itself may change wettability’s.

The core sample is dried for the purpose of removing connate water from the pores, or to remove

solvents used in cleaning the cores. When hydratable minerals are present, the drying procedure is

critical since interstitial water must be removed without mineral alteration. Drying is commonly

performed in a regular oven or a vacuum oven at temperatures between 50°C to 105°C. If problems with

clay are expected, drying the samples at 60°C and 40 % relative humidity will not damage the samples.

Question 3: Assume that you have been asked to carry out an experiment to determine the fluid

saturation in core sample. Choose one method and describe in details the procedure, expected data

and calculations, equations involved and summary of your experiment

Saturation determination: The Dean-Stark distillation

Objective:

- The objective of the experiment is to determine the oil, water and gas saturation of a core sample.

- To study the procedure in cleaning of the core samples from residual fluids. 2.

- To define and determine the oil and gas and water saturation of a core sample using Dean-stark

distillation - extraction method.

Summary:

The objectives of this saturation determination experiment are to study the procedures in

cleaning of the core samples from residual fluids and to define and determine the oil and gas saturation

of a core sample using the Dean-stark distillation-extraction method. But due to technical and laboratory

problem, we were unable to conduct this experiment. However, we managed to do some research and

finding about saturation determination, procedure in cleaning of the core samples from residual fluids

and about the Dean-stark distillation-extraction method in order to achieve the understanding and

objectives of this experiment without conducting the experiment.

We suppose to heat the hydrocarbon solvent which is toluene to its boiling point which is 110

°C. Its vapor will move upward and the rock sample becomes immerse in the toluene vapors that begin

to extract the oil and water present in the rock sample. Then the rising vapor will be condense in

condenser and collected in the graduated tube.

Since toluene is completely miscible with the extracted oil, the condensed liquid in the

graduated tube will consist of two liquid phases which are water and mixed hydrocarbon phase

containing toluene and oil from the rock sample. Due to higher density, the water phase will settles at

the bottom of the graduated tube while the solvent overflow and drips back over the rocks sample. This

process should be continuing until no more water is collect in the receiving tube.

Introduction:

When the core arrives in the laboratory, plugs are usually drilled 20 to 30 cm apart throughout

the reservoir interval. All these plugs are analyzed with respect to porosity, permeability saturation and

lithology. Fluid saturation can be determined by a few methods which include injection of solvent,

centrifuges flushing, gas driven solvent extraction and Dean-stark distillation-extraction. The Dean-stark

distillation-extraction is appropriate for plug samples and for rotary sidewall cores.

This method of determination fluids saturation depends upon the distillation of water fraction

and solvent extraction of oil fraction from the sample. Besides, this method provides a direct

determination of water content. The oil and water are extracted by dripping a solvent, usually toluene

or a mixture of acetone and chloroform aver the plug samples. In this method, water and solvent are

vaporized, re-condensed in cooled tube in on the top of apparatus and water is collected in calibrated

chamber.

The set up basically consist of a longneck round-bottom flask that contains a suitable

hydrocarbon solvent such as toluene, a heating element or electric heater to boil the solvent, a

condenser and a graduated tube receiver to measure the volume of extracted fluids

Theory:

Fluid saturation can be determined by a few methods which includes injection of solvent, centrifuges

flushing, gas driven solvent extraction and Dean-stark distillation-extraction. The Dean-stark Distillation-

extraction method of determining fluids saturation depends upon the distillation of water fraction and

solvent extraction of oil fraction from sample. The Dean-Stark method provides a direct determination

of water content. The oil and water are extracted by dripping a solvent, usually toluene or a mixture of

acetone and chloroforms over the plug samples. In this method, water and solvent are vaporized, re-

condensed in a cooled tube in on the top of apparatus and water is collected in calibrated chamber. The

solvent overflows and drips back over the samples. The oil removed from the samples remains in

solution in the solvent. Oil content is calculated by the difference between the weight of water

recovered and the total weight loss after extraction and drying.

The Dean and Stark procedure can be used to measure water content of a diverse range of

samples, and has been extensively used in industrial laboratories to measure water in petroleum oils.

The oil content is calculated from weight difference and therefore it is important that no sand grains be

lost from the core during the analysis, as this would result in erroneously high calculated residual oil

saturation.

The principle of operation is straightforward. When the core to be analyzed is weighed, the

resulting measurement will consist of the weight of the sand grains, as well as the oil and water present

in the pore space. The sample is then placed within a tear in the apparatus, and this unit is suspended

above a flask containing a solvent such as toluene. Whatever the solvent, it must have a boiling point

higher than water and be both immiscible with and lighter than water. The dripping solvent mixes with

oil from the sample, and both the solvent and oil are returned to the solvent flask. The process

continues until the sample is raised to the boiling point of water. When it does, the water vaporizes,

rises in the condensing tube until it is condensed, and falls back into the calibrated tube. Because it is

heavier than the solvent, it collects at the bottom of the tube, where its volume can be measured. When

successive readings indicate no additional water recovery has occurred, we know all water has been

removed from the sample, and the water volume is recorded for further calculations

Apparatus:

- Dean-stark apparatus

- Rock sample(core plug)

- Solvent

Procedure:

1) Weigh a clean, dry thimble. Use tongs to handle the thimble.

2) Place the cylindrical core plug inside the thimble, then quickly weigh the thimble and sample.

3) Fill the extraction flask two-thirds full with toluene. Place the thimble with sample into the long neck

flask.

4) Tighten the ground joint fittings, but do not apply any lubricant for creating tighter joints. Start

circulating cold water in the condenser.

5) Turn on the heating jacket or plate and adjust the rate of boiling so that the reflux from the

condenser is a few drops of solvent per second. The water circulation rate should be adjusted so

that excessive cooling does not prevent the condenser solvent from reaching the core sample.

6) Continue the extraction until the solvent is clear. Change solvent if necessary

7) Read the volume of collected water in the graduated tube. Turn off the heater and cooling water

and place the sample into the oven (from 105˚C to 120˚C), until the sample weight does not change.

The dried sample should be stored in a desiccater.

8) Obtain the weight of the thimble and the dry core.

9) Calculate the loss in weight WL, of the core sample due to the removal of oil and water.

10) Measure the density of a separate sample of the oil.

11) Calculate the oil, water and gas saturations after the pore volume Vp of the sample is determined.

Data and calculations:

Sample No: Porosity, φ:

Where

Worg: Weight of original saturated sample

Wdry: Weight of desaturated and dry sample

Equations:

Where D and L are diameter and length of the core sample, respectively

Discussion

The main objective of this experiment is to determine the oil, water and gas saturation of a core

sample. Saturation is the measure of how much porosity of porous medium been occupied by fluid. Fluid

saturation is defined as the ratio of the volume of fluid in a given core sample to the pore volume of the

sample.

The experiment need to be carried in such orders as first weighed a clean, dry thimble and then place

the cylindrical core plug inside the thimble, and then quickly weigh the thimble and sample. Next filled

the extraction flask two-thirds full with toluene and placed the thimble with sample into the long neck

flask. After that, start circulating cold water in the condenser “Turn on” the heating jacket or plate and

adjust the rate of boiling so that the reflux from the condenser is a few drops of solvent per second.

Continued the extraction until the solvent is clear. By the time it finish, “Turn off” the heater and cooling

water and place the sample into the oven (from 105 °C to 120°C), until the sample weight does not

change meanwhile the dried sample should be stored in a desiccators. Then, the weight of the thimble

and the dry core is obtained in order to calculate the loss in weight WL, of the core sample due to the

removal of oil and water. Measure the density of a separate sample of the oil and last calculate the oil

and water saturation.

However, due to unavoidable technical problems we are unable to carry out this experiment

thoroughly. For the information, as we know it is important to consider the saturation change occurring

in the core from in-situ to surface conditions. Such as the condition, suppose a core is being recovered

while drilling a well with water-based drilling mud. Water from the drilling mud will enter the rock

expulsing oil. The result as the core is lifted, the reduction in pressure will cause the oil to release gas

and this will expand expulsing oil and water out of the rock.

Conclusion

The main objective of this experiment is to determine the oil, water and gas saturation of a core sample

using the Dean-stark distillation. Through this experiment we can also study the procedures in cleaning

of the core samples from residual fluids. But due to technical and laboratory problem, we were unable

to conduct this experiment. Saturation is the important parameter that we should know to estimate

how much fluids occupied in the pore space. The fluids are oil, water or gas. As we know it is important

to consider the saturation change occurring in the core from in-situ to surface conditions. Such as the

condition, suppose a core is being recovered while drilling a well with water-based drilling mud. Water

from the drilling mud will enter the rock expulsing oil. The result as the core is lifted, the reduction in

pressure will cause the oil to release gas and this will expand expulsing oil and water out of the rock

References

a) Ahmed, Tarek : “Reservoir Engineering Handbook-Ch.4: Fundamentals of Rock Properties”, Second

Edition, Gulf Professional Publishing, 2001.

b) Amyx, James : “Petroleum Reservoir Engineering-Ch.2: Fundamental Properties of Fluid Permeated

Rocks”, 1960

c) Dean-Stark apparatus . (2013, March 15). Retrieved May 19, 2013, from Dean-Stark apparatus:

http://en.wikipedia.org/wiki/Dean-Stark_apparatus 2.

d) O.Torsaeter, M. A. (2000, August). Experimental Reservoir Engineering Laboratory Workbook .

Retrieved May 19, 2013, from Experimental Reservoir Engineering Laboratory Workbook:

http://www.ipt.ntnu.no/~oletor/kompendium4015.pdf 3.

e) http://www.ipt.ntnu.no/~oletor/kompendium4015.pdf

Appendix

Modified Dean and Stark Extraction apparatus for determining Toluene insoluble in phosphorus