Reservoir Rock Properties Laboratory Manual 3rd stage \ Petroleum engineering at Koya University By Barham Sabir

1. Koya University
Faculty of Engineering
Petroleum Engineering Department
By Barham Sabir
2019
Reservoir Rock Properties Laboratory Manual

2. Experiment #3
Core cleaning

3. Aim / Objective
Cleaning and drying the core samples
Introduction
Prior to most laboratory measurements of porosity and permeability, the original
fluids must be completely removed from the core sample. This is generally
accomplished through flushing, flowing, or contacting with various solvents to
extract hydrocarbons, water, and brine. Various techniques and apparatus for
cleaning a core plug sample such as, direct injection of solvent, centrifuge flushing,
gas driven solvent extraction, soxhlet extraction, dean-stark distillation-extraction,
and vacuum distillation. In below section will briefly describe each of them.
Core clean laboratory methods
1. Direct Injection of Solvent
Extraction of hydrocarbons and salt from reservoir rocks can be achieved by
injecting one or more solvents into the core sample under pressure and
moderate temperature. The pressure used should be dependent on the sample
permeability and may range from 10 to 1,000 psi. The core samples may be
held in a rubber sleeve under overburden pressure or in a suitable core-holding
device that will permit the flow of solvent through the matrix of the sample.

4. The volume of solvent required to completely remove hydrocarbons in the core
sample is dependent on the hydrocarbons present in the sample and the solvent
used. The core is considered clean when the effluent is clean. In some instances,
more than one solvent may be required to remove heavy, asphaltic-type crude
oils.
Figure 1. Direct injection of solvent device
2. Centrifuge flushing
The centrifugal extractor is designed to spray warm, clean solvent from a still
against the core samples. The centrifugal force causes solvent to flow through
samples displacing and extracting the oil (and water). The speed of rotation
should be varied from a few hundred to several thousand revolutions per minute

5. (rpm), depending on the permeability and degree of consolidation of the core.
Most common solvents can be used.
Figure 2. Centrifugal extractor device
3. Gas Driven Solvent Extraction
When a core from an oil-bearing formation is brought to the surface and
depressurized, the gas dissolved in the oil comes out of solution and displaces
some of the oil and water out of the core. This results in some gas-filled pore
space at atmospheric pressure. The gas-filled space in the core can be almost
completely filled with solvent by surrounding the core with a suitable solvent
containing a dissolved gas and applying sufficient hydraulic pressure. Under
this condition, the solvent mixes with oil in the core and subsequent
depressuring to atmospheric pressure removes some of the residual oil.

6. Carbon dioxide gas is excellent for this purpose because of low fire or
explosion hazards and high solubility in most solvents. Some of the solvents
that can be used are naphtha, toluene, or mixtures of solvents. With certain
types of crude oil, cleaning time may be reduced if the core chamber is heated
by a water bath, steam bath, or by electric heaters. One successful application of
this method for routine cleaning of cores uses carbon dioxide and toluene at 200
psig, with a hydraulic pressure of 1,000 psig. Cycles of approximately 30
minutes are used.
Figure 3. CO2/ solvent core cleaner
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 4,
samples to be cleaned are placed in a porous thimble inside the Soxhlet.

7. 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.
Figure 4. Soxhlet extraction device

8. 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 (Fig. 5). 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 5. Dean-Stark Apparatus

9. 6. Vacuum Distillation
The oil and water content of cores may be determined by this method. As
shown in Figure 6, a sample is placed within a leak-proof vacuum system and
heated to a maximum temperature of 230o
C. Liquids within the sample are
vaporized and passed through a condensing column that is cooled by liquid
nitrogen.
Figure 6. Vacuum distillation Apparatus.
Comparison of these methods
The direct-injection method is effective, but it is slow. The method of flushing by
using centrifuge is limited to plug-sized samples. The samples also must have

10. 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 limestones. 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. Vacuum distillation is also 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
 benzes/methol alcohol
 carbon/tetrachloride
 chloroform
 methylene dichloride

11.  mexane, naphtha
 tetra chloroethylene
 toluene
 trichloro ethylene
 xylene
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 wettabilities.
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 105oC. If problems with clay are
expected, drying the samples at 60°C and 40 % relative humidity will not damage
the samples.
Soxhlet principle
The soxhlet distillation extraction method is used to dissolve and extract oil and
brine from rock core sample by using solvents. The cleanliness of the sample is

12. determined from the color of the solvent that siphons periodically from the
extractor which must be clear. The samples are placed in the extractor and cleaned
by refluxing solvent. The solvent is heated and vaporized in boiling flasks and
cooled at the top by condenser. The cooled solvent liquid falls into the sample
chamber. The cleaned solvent fills the chamber and soaks the core sample. When
the chamber is full, the dirty solvent which was used to clean the core siphons back
into the boiling flask and is redistilled again (Fig.7).
Figure 7. Illustrate soxhlet principle
Soxhlet apparatus components
From left to right: stopcock, condenser with connecting tips, adapter, Soxhlet, and
boiling flask. Rod bar and tong is used as frame to hold glassware, heating mantle
and cooling bath.

13. Figure 8. general set of Soxhlet small size
Experiment operation
1. The system is assumed at room temperature.
2. Open the boiling flask. The condenser can remain maintained in place by help
of the tong.
3. Pour solvent (usually toluene) in the boiling flask to raise liquid level up to
green line in illustration below. This line is about 1/3 distance between the
heating mantle and the flask neck. During process, this extra valume will be
transferred into the sample chamber.
Cooling bath
Heating mantle

14. 4. Add some granules of carborundum (SiC) into the flask.
5. Place the sample on the analytical balance, measure the weight of the core
sample and record the result. Then, load the core sample into the sample
chamber inside a cellulose thimble (plug sample) or metallic basket (full size
sample)
6. Slide the condenser into nozzle as per figure 8
7. Maintain the condenser with the tong loosen on glassware.
8. Connect the condenser to a cooling bath with circulating pump (based on figure)
9. Switch on the cooling bath (check bath level). Make sure that flow is driven
through the condenser (check that the bubbles are driven out of the glassware).
10.Turn on the heater and adjust the rate of the boiling so that the reflex from the
condenser is a few drops of solvents (toluene) per second. The water circulation
rate should be adjusted so that excessive cooling does not prevent the condenser
solvent from reaching the core sample.

15. 11.Continue extraction with a minimum of 7 cycles or until the solvent is clear.
12.Monitor the toluene level during the extraction to ensure that the sample
remains completely submerged in addition.
13.When the extraction is complete turn off the heater and water bath. Wait untile
the temperature in the solvent flask is back to room temperature.
14.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.
15.Place the sample on the analytical balance, measure the weight of the dried core
sample and record the result.
Data and calculations:
Sample No:
Formula Unite
Weight of sample befor extraction (W1) Use analytical balance gm
Weight of sample after extraction (W2) Use analytical balance gm
Vol. of water Read from apparatus gm
Water density ----------------- gm/cc
Oil density ------------------ gm/cc
Bulk volume (Vb) Vb=πr2
L cc
Porosity (assume) ----------------- %
Water saturation (Sw) Sw=
𝑉
𝑤
𝑉
𝑝
⁄ 𝑥 100 %
Oil saturation (So) So=
𝑉
𝑤
𝑉
𝑝
⁄ 𝑥 100 %
Gas saturation (Sg) Sg= 1 - (Sw + So) %