EXPERIMENTAL STUDY OF INTERACTION SHALE-FLUID THROUGH IMMERSION TESTS

Abstract

The present work has as objective the study of the behavior of preserved shale samples when immersed in aqueous solutions. The obtained results show that the solutions presented high variations of pH, salinity and electric conductivity, what indicates ionic migration of salts from the rock to the fluid, and small variations of Redox potential. Chemical analysis indicate strong migrations of ions from the rock to the fluid. It can also be observed that the solutions become cloudy, denser and more viscous. Besides, the samples suffer elevations of its water content, small variation of its cation exchange capacity and chemical composition, loss of solid material from superficial disintegration and the development of fractures in the cores. This work contributes to the understanding of the shale behavior in presence of drilling fluid water base.

Full text

22 Engenharia Térmica, Edição Especial, 2002, p. 22-28
CIÊNCIA/SCIENCE
INTRODUCTION
During the drilling of oil wells, the fluid interacts with
the shales layers in the walls of the well, with the cuttings and
at the contact drill bit-rock. The physical-chemical interaction
between shales and the drilling fluids is still a well engineering
unknown, but many problems that happen during the drilling is
credited to this interaction.
The physical-chemical interaction rock/fluids are
consequences of both water and ionic species fluxes from and
to the rock. The mechanisms of transport in shales, like osmosis,
reverse osmosis, diffusion of ions and hydraulic pressure, can
cause chemical changes in the shales and fluids, like cation
exchange capacity, chemical composition of mud filtrate, the
shale matrix and pore fluids, hydration or dehydration of shales
(Chenevert, 1969) and swelling pressure (Santos et al, 1996;
Santarelli and Carminati, 1995). Also, these mechanisms change
the mechanic properties of the fluids, like the weight and
viscosity, that are important to stability of well and the retard
the fluid invasion in shales (van Oort, 1994; Bol et al., 1996).
This paper is a brief extract of a wider project, currently
in development at the Laboratory of Rock-Fluid Interaction of
PUC-Rio, and that aims to study the behavior of preserved
shale samples when immersed in aqueous solutions. Also,
special attention was paid to the fluid properties, which include:
electro-chemical, chemical and mechanical properties of the fluid.
EXPERIMENTAL STUDY OF INTERACTION SHALE-FLUID THROUGH
IMMERSION TESTS
Claudio Rabe*
rabe@civ.puc-rio.br
Sérgio Augusto Barreto da
Fontoura*
fontoura@civ.puc-rio.br
Franklin dos Santos Antunes*
*Grupo de Tecnologia e Engenharia de
Petróleo, Departament of Civil
Enginnering, PUC-Rio
Rua Marquês de São Vicente, 225 –
Edifício Padre Leonel Franca, 6º Andar –
Gávea – Rio de Janeiro RJ – Brazil - CEP
22451-900 - Tel/Fax: +55 21 3114.1458.
ABSTRACT
The present work has as objective the study of the behavior of preserved shale
samples when immersed in aqueous solutions. The obtained results show that the
solutions presented high variations of pH, salinity and electric conductivity, what
indicates ionic migration of salts from the rock to the fluid, and small variations of
Redox potential. Chemical analysis indicate strong migrations of ions from the
rock to the fluid. It can also be observed that the solutions become cloudy, denser
and more viscous. Besides, the samples suffer elevations of its water content,
small variation of its cation exchange capacity and chemical composition, loss of
solid material from superficial disintegration and the development of fractures in
the cores. This work contributes to the understanding of the shale behavior in
presence of drilling fluid water base.
Key words: interaction shale-fluid, immersion tests, physical-chemical.
To do the immersion tests, two kinds of shales were used, and
due to space limitation, the present work presents only the
results corresponding to the deionized water.
IMMERSION TEST
Immersion tests are tests developed to study the shale-
fluid interaction or shale reactivity. An equipment was
developed in which shale samples are put in contact with the
fluid and special sensors measure the electro-chemical
properties of the fluid throughout the test. This first version of
the equipment allows tests with fluids at atmospheric pressure
and at down hole temperature condition. Also, at the end of
test, a photographic camera was used to monitor the visual
qualitative reactions.
The immersion equipment is presented in the figure 1. It is
constituted by one round balloon with five entrance ports: four
laterals and one central. The central one is used to place the shale
pieces inside the balloon and to house the condensator. Three of
lateral ports are used to put inside electro-chemical probes and
last one is used to collect fluids samples during the tests. The
condensator is used to avoid water losses due to vaporization.
Besides this, the equipment possesses a heating blanket to keep
the temperature constant during the test.
The electro-chemical instrumentation is constituted by
a conductimeter/salinometer, a pH meter and a redox meter.

23Engenharia Térmica, Edição Especial, 2002, p. 22-282g
CIÊNCIA/SCIENCE RABE, C. et al.
Experimental study of interaction shale-fluid...
Beside this, the probes monitor the solution temperature.
Fig. 1 - Equipment developed for immersion tests.
SHALES AND SOLUTION PROPERTIES
Two kinds of shales were used: shale originating from
Brazilian submarine platform (shale A-B) and shale from onshore
sedimentary basin of Venezuela (shale V). Table 1 presents some
general characteristics of the tested samples. The specific
gravity of grains was determined using the picnometer and the
clay fraction was evaluated by sedimentation. The analysis of
clay minerals was determined by X-Ray diffraction.
Table 1 – General characteristics of the tested shales.
The results indicate small clay fraction in these shales
with low expansibility argilominerals. Table 2 presents the
physical properties of shale samples.
The determination of water content was obtained by
oven-drying the samples at 105° for a period of 24 hours. The
degree of saturation, voids ratio and porosity values for each
sample were obtained with the use of classical soil mechanics
expressions (Lambe & Whitman, 1979).
Table 2 – Physical properties of shale samples.
These shales present low water content, low porosity
and low void ratio. These results are common in shales. The
sample from Brazil was almost full saturated, and the Venezuela
present low degree of saturation. This is due to the inadequate
process of transport and storage, what generated the drying of
this shale.
Several aqueous solutions have been tested, but due to
space limitation, in the present work only the results
corresponding to the deionized water will be presented.
Chemical analysis by atomic absorption proved the inexistence
of alkaline and earthy alkaline ions in the deionized water. Table
3 presents the properties from deionized water.
Table 3 - Properties from deionized water.
PROCEDURES AND MEASURES
To do each test, 3 rock fragments were used, with an
approximate weight of 70 g. Before the tests, the samples were
handled inside a high humidity room, where the cutting of the
cores was done with the rock immersed in mineral oil and kept
immersed until the beginning of the test, like suggest Santos et
al., (1996). This procedure guarantees that the sample is not
exposed to air for a long time. Pieces cut from core were used to
obtain the original water content from the shale.
Initially, the water was inserted into the balloon and was
heated to 50ºC, while this, the probes start to monitor the fluid
electro-chemical properties. After this, the shale pieces are
inserted into the balloon and the test begins. In these tests, the
relationship shale/fluid was kept in 10%(w/w).
During the test, effluents are collected in a plastic tubes
and sent to a chemistry laboratory to quantify the cations and
anions present in water. At the end of the immersion test, the
effluents are collected to make chemical and physical analysis.
The physical analysis included viscosity and specific density.
ELECTRO-CHEMICAL RESULTS
The fluids electro-chemical properties determine the
behavior of ions during the immersion tests. The pH value is an
indicator of the acidification or alkalization of a solution and
the Redox is an indicator of degree of reduction or oxidation.
The electric conductivity and salinity quantify, respectively,
the amount of free ions and the salts present in the solution.
These properties were used to understand the movements into
or out of fluid in contact with shale A-B and V, during the tests.
Figures 2, 3, 4 and 5 present, respectively, the results of pH,
Redox, electric conductivity and salinity.
The initial values correspond to the deionized water.
The pH results indicated that the solutions become strongly
alkaline. At the same time, the Redox values indicate that the
solutions have become minus reduced. Electric conductivity
of fluids indicate that the ions migrated from the shale samples
to the fluid, that caused in the case of shale A-B, a increase in
the salinity of the fluid.
Shale properties Shale A-B Shale V
Clay fraction (%<2µ)29 12
Specific gravity of grains (g/cm
3
) 2.69 2.71
Clay mineralogy Kaolinite, illite/smectite and chlorite Kaolinite and illite/smectite
Shale properties Shale A-B Shale V
Water content (%) 7.3 3.1
Degree of saturation (%) 95 49
Voids ratio 0.21 0.17
Porosity (%) 17.4 14.5
Water properties Unit Value
pH at 50ºC - 5.6 - 6.2
Redox at 50ºC mV 230-260
Electric conductivity at 50ºC µS/cm 4–5
Salinity at 50ºC mg/l 0
Specific weigh at 20ºC g/ml 0.9984
Viscosity at 50ºC mm
2
/s 0.570

RABE, C. et al.
Experimental study of interaction shale-fluid...
24 Engenharia Térmica, Edição Especial, 2002, p. 22-28
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The results indicate that during the first day of test it
occurs the biggest changes in the pH, Redox, electric
conductivity and salinity, and that suggests that the shale-
fluid interaction occurs more strongly during this period. After
this, the curves tend to become horizontal.
CHEMICAL ANALYSIS
Chemical analysis was performed at the Atomic Emission
Spectrometry Laboratory (cations) and Ionic Chromatography
Laboratory (anions) at PUC-Rio, on the deionized water samples.
In this paper, will just present the following components: Na+,
Si++, Ca++, K+, Cl- and SO-24, since they were the ions that
migrated from the samples to the fluids. Other elements (Mg++,
Al++, Ba++, Sr++ and Fe++) migrated less than 1mg/l. The variation
of Na+, Si++, Ca++, K+, Cl- and SO-24 concentrations on water
during the immersion tests are presented, respectively, in the
figures 6, 7, 8, 9, 10 and 11.
The results show that ions migrated from the shale cores
to the water fluid by chemical diffusion. These elements exit
the rock matrix and go to the fluid, under a chemical gradient.
VARIATIONS IN FLUID DENSITY AND
VISCOSITY
After the immersion tests, the effluent was collected to
quantify the variation on the physical property of the immersion
fluid. The viscosity was performed at the Termo-Science
Laboratory of the Catholic University of Rio de Janeiro (PUC-
Rio). For the accomplishment of the test, the viscosimeter was
immersed in a bath full of glycerin that maintained the
temperature at 50ºC.
The density of water was performed in the volumetric
pipette (10ml) and the weight was controlled in an electronic
balance (0,0001g). Table 4 shows the variations in viscosity
and density of the fluid caused by the interaction shale-fluid
after 3 days of immersion test.
Table 4 – Variations on the physical water properties
caused by 3 days of immersion .
The results indicate that the interaction rock-fluid
generated small increases in the density and the viscosity of
the fluids. This increase in density was due by migration of
ions from the rock to the fluid. As the migration was more
intense in the Brazilian shale, proven by the high variation on
the electric conductivity and in chemical analysis, this shale
generated more elevation on the fluid density.
The fluid in contact with shale V presented a large
increase in its viscosity. This can be attributed to the high
dissolution of the carbonates calcium (Ca++) present in the
shale, since the calcium has elevated viscosity.
V
A-B
5
6
7
8
9
10
0 720 1440 2160 2880 3600 4320
Time (min)
pH
Fig. 2 - pH during the immersion tests.
A-B
V
50
100
150
200
250
0 720 1440 2160 2880 3600 4320
Time (min)
Redox (mV)
Fig. 3 - Redox during the immersion tests.
V
A-B
0
500
1000
1500
2000
0 720 1440 2160 2880 3600 4320
Time (min)
Electric condutivity
(µS/cm)
Fig. 4 - Electric conductivity during the immersion tests.
V
Tendency
line
A-B
Measured
0
50
100
150
200
250
300
350
400
0 720 1440 2160 2880 3600 4320
Time (min)
Salinity (mg/l)
Fig. 5 - Salinity during the immersion tests.
Water proprieties Shale Initial value Final value Variation (%)
A-B 0.9984 0.9995 0.1101Density
(g/ml) V 0.9984 0.9987 0.0300
A-B 0.5708 0.5726 0.3153Viscosity(mm
2
/s)
V 0.5708 0.5737 0.5080

25Engenharia Térmica, Edição Especial, 2002 p. 22-28
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Experimental study of interaction shale-fluid...
V
A-B
0
10
20
30
40
50
60
70
80
90
0 864 1728 2592 3456 4320
Time (min)
Variation of Cl -(mg/l)
Fig. 10 - Variation of Cl- on the deionized water during the
immersion tests.
WATER CONTENT OF SHALES SAMPLES
The water content of the shale samples was obtained
by oven drying at 105 °C for 24 hours as recommended by the
International Society of Rock Mechanics, like was described
before. The same methodology was used at the end of immersion
test, when one core was used to obtain the final free water.
These results show that shales in contact with a
deionized water solution suffer hydration. Is can be cause by
an initial non-saturation of shale samples and by the osmosis
process. The Venezuela shale shows a high hydration process
that can be explained by it low initial saturation value. The
results of variations of free water are present in the table 5.
Table 5 - Water content for the shale samples immersed in
deionized water during 3 days.
Fig. 7 - Variation of Ca++ concentrations on the water fluid
during the immersion tests.
A-B
V
0
1
2
3
4
0 864 1728 2592 3456 4320
Time (min)
Variation of Ca
++
(mg/l)
A-B
V
0
5
10
15
20
0 864 1728 2592 3456 4320
Time (min)
Variation of Si
++
(mg/l)
Fig. 8 - Variation of Si++ on the deionized water during the
immersion tests.
A-B
V
0
5
10
15
20
0 864 1728 2592 3456 4320
Time (min)
Variation of K
+
(mg/l)
Fig. 9 - Variation of K+ on the deionized water during the
immersion tests.
Fig. 11 – Variation of SO4-2 on the deionized water during
the immersion tests.
V
A-B
0
10
20
30
40
50
60
0 864 1728 2592 3456 4320
Time (min)
Variation of SO 4
-2 (mg/l)
Fig. 6 - Variation of Na+ concentrations on the water fluid
during the immersion tests.
A-B
V
0
80
160
240
0 864 1728 2592 3456 4320
Time (min)
Variation of Na
+
(mg/l)
Shale property Shale Original value 3 days Variation (%)
A-B 7.29 11.03 51.30Water content (%)
V 3.09 5.04 63.11

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QUALITATIVE REACTIONS OF SHALE
SAMPLES
A photographic camera was used to take pictures of
shale samples after the immersion tests, monitoring qualitative
reactions. At the end of immersion test, the samples and the
fluids are put in a Becker (500 ml) to see better the cores and the
solutions.
The figure 12 shows the shales immersed in the water
after the test. The photos shows that the fluid used in contact
with shale A-B fluid became cloudy and dark, caused by the
high quantities of solid dispersed in it. This effect was not
observed in the fluid used with shale V. It can be explained by
the strong cementation that precluded particle dispersion. Figure
13 illustrates the presence of fractures generated during the
shale sample hydration.
SOLID DISPERSION
The dispersed solids are presented in the table 6. The
material considered dispersed in the fluid are all the solids
originating from the fragmented shales. The percentage of
dispersed material in the solutions (D%) was calculated by the
following ways, where Wst is the dry weight of the sample before
the immersion, obtained by physical properties and Wsd is the
dry weight of the dispersed material after immersion tests. Wsd
was obtained by oven-drying the dispersed material at 105° for
a period of 24 hours.
D(%) = Wsd/Wst x(100)
Table 6 - Quantification of dispersed solids in the solutions.
The results show that the dispersed solids in the fluids
were no negligeable.The dispersion is a function of changes in
structure of rock matrix and in the bound and crystalline water,
caused by the hydration of the cores.
CATION EXCHANGE CAPACITY
Table 7 presents the results obtained for shales using a
methylene blue test (Higgs, 1988), with concentration of 1 g/l
and control onto the pH value, keeping the solution neutral.
The tests were carried out using 1g from the coarse and clay
fraction of the shales. Also, in the same table are presented the
results obtained by the ammonium acetate method. In this test,4
g were used from coarse fraction of the shale samples. This test
allows the evaluation of the individual cations that are being
interchanged. The Brazilian shale has a high CEC value, and
the main interchangeable cation is Na+ which suggests the
presence of sodic smectite.
The sample from Venezuelan shale presented small CEC
values both in MBT method and in the ammonium acetate test.
This shale showed a large amount of interchangeable cation is
Ca++, which gives, to this shale, a low reactivity profile.
(Eq. 1)
elahS)%(sdilosdesrepsiD
B-A68.5
V23.1
Figure 12 – Shale samples after immersion in water for 3 days. Dispersed solids and superficial disintegration.
Shale A-B Shale V
Figure 13 - Shale samples after immersion in water for 3 days. Creation of fractures in the shale cores.
Shale A-B Shale V

27Engenharia Térmica, Edição Especial, 2002 p. 22-28
CIÊNCIA/SCIENCE RABE, C. et al.
Experimental study of interaction shale-fluid...
Displayed in Table 7, the results indicate a small reduction
of interchangeable cations and the Na+ was the more reduced
in all the shales. We can conclude that, under the test conditions,
the immersion fluid did not cause changes in the structure of
clay minerals during the immersion tests.
Table 7 - Cation Exchange Capacity and Interchangeable
cations.
CHEMICAL ANALYSIS OF TESTED SHALES
The chemical analyses were carried out at the Chemistry
Division of Lakefield Geosol Laboratory by x-ray fluorescence.
The chemical composition of shales from Brazil is rich in the
following oxides: SiO2, Al2O3, CaO and loss on fire and the
shale from Venezuela proved to be rich in SiO2, Al2O3 and loss
on fire. The amount of loss on fire was determined to quantify
the presence of H2O, S and CO2 in the rock matrix (Table 8).
Comparing the values displayed in the table, it can be
observed that the samples from Brazil and Venezuela after 3
days of immersion in deionized water suffered a small percentage
reduction of oxides (minus than 1%), that indicate that the
structure of shales samples suffered small chemical alteration.
The increase of loss on fire results of shale A-B (3.38%) was
larger than shale V (1.02%). These results indicate that the
crystalline water had increase.
Table 8 - Chemical analysis of shale samples before and
after 3 days of immersion test.
CHEMICAL ANALYSIS OF THE PORE FLUID
Table 9 present the results of pore fluid composition for
shales from Brazil and Venezuela, before and after the immersion
tests. The method used to extract the pore fluid is described by
Schmidt (1973), where the sample was dried and the salts
removed by washing. The effluent is used to measure the salt
content. These tests are importants to understand the behavior
of solubility of salts present in the pore fluid during the
immersion tests.
The results indicate that the Brazilian shale has more
cations and anions in its pore fluid than the Shale from
Venezuela. This can be explained by the origin of the shales.
The shale A-B is a rock originated from Brazilian submarine
platform and the shale V came from an onshore sedimentary
basin of Venezuela. The results also indicate that after
immersions tests, the ions migrated by diffusion from the shale
pore fluid to the immersion fluid. This was suggested by the
chemical analysis of water.
CONCLUSIONS
A very simple immersion test is being proposed to study
the shale-fluid interaction, where the equipment is able to
simulate the temperature “in situ” condition. In the present
work, special attention was given to the immersion fluid, by
measuring its electro-chemical, mechanics and chemical
properties.
The electro-chemical results indicate a strong alkalization
and the fluid became less reduced, due to the presence of
sulfates and oxides in the fluid. The electric conductivity
indicated the direction of movement and behavior of the ions.
The results indicated that the ions migrated from the shale
samples to the fluid, and that created an increase in the salinity,
that was noticed during the test carried out with the shale A-B.
The results of chemical analysis indicated that the ions
present on the fluids came from, mainly, the pore fluid, but the
rock matrix suffered a reduction. It was caused by non-ideal
semi-permeable membrane (deionized/shale interface) that, by
chemical diffusion, prevents the transfer of ionic and molecular
from the shale to the fluid.
The increase in the water content indicate that the rock
suffered hydration and that may have caused an increase in
pore pressure. The transfer of water to the shales samples can
be attributed to chemical osmosis and the non-saturation of
shale samples. The alteration in shale structure can be monitored
too, by the reduction of CEC, caused by the lost of
interchangeable cations. The destabilization of shale structure
could be observed by creation of fractures, superficial
disintegration and by dispersed solids in the fluid.
The physicals properties of fluids indicate small
variations. The density became higher as well as the viscosity
of the fluids. This increase was caused by the high of ions and
molecules presents on the water. Some of these elements have
higher viscosity than the water, like the chlorides and sulfates.
ACKNOWLEDGMENTS
The authors wish to thank ANP and CNPq for the
scholarship support. Special thanks go to Dr. Rosana Lomba
by your suggestions and Petrobras for proving the cores used
in this work. This work is dedicated to the Israel’s God, for his
infinite love.
MBT – CTC
(meq/100g) Ammonium Acetate – coarse fraction (meq/100g)
Interchangeable cations
Shale Coarse
fraction
Clay
fraction CTC Na
+
K
+
Ca
++
Mg
++
Sr
++
Ba
+
A-B Natural 24.5 27.0 29.10 14.8 2.9 8.7 2.0 0.63 0.07
A-B H
2
O24.0 25.8 26.90 13.2 2.7 8.4 1.9 0.63 0.07
V Natural 11.4 12.0 16.50 1.52 1.95 12.13 0.77 0.07 0.06
VH
2
O10.6 11.5 16.00 1.09 1.90 12.12 0.76 0.07 0.06
Shale A-B Shale VOxides
(%) Natural After 3 days Natural After 3 days
SiO
2
40.5 39.9 51.2 51.0
Al
2
O
3
13.0 12.3 20.7 20.1
CaO 18.1 17.7 3.7 3.6
Loss on fire 15.43 18.81 11.04 12.16
Fe
2
O
3
5.8 5.5 6.8 6.7
K
2
O2.5 2.3 2.3 2.2
MgO 1.4 1.1 3.1 3.0
Na
2
O2.1 1.3 0.33 0.27
TiO
2
0.80 0.73 0.72 0.69
P
2
O
5
0.32 0.31 0.15 0.15
MnO 0.02 0.02 0.10 0.09
BaO 0.0269 0.0243 0.0433 0.0429
Table 9 - Variations of pore fluid composition for shales
from Brazil and Venezuela, before and after the immersions tests.
Shale A-B Shale VIons
(mg/l) Natural After 3 days Natural After 3 days
Na
+
14,385 14,298 87 83
K
+
354 350 21 19
Ca
++
875 873 32 29
Mg
++
131 129 46 42
Cl
-
12,769 12,707 74 70
2_
4
SO 9,354 9,301 66 64

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