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EXPERIMENTAL STUDY OF INTERACTION SHALE-FLUID THROUGH IMMERSION TESTS

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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.
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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
CIÊNCIA/SCIENCE
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
RABE, C. et al.
Experimental study of interaction shale-fluid...
26 Engenharia Térmica, Edição Especial, 2002, p. 22-28
CIÊNCIA/SCIENCE
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
RABE, C. et al.
Experimental study of interaction shale-fluid...
28 Engenharia Térmica, Edição Especial, 2002, p. 22-28
CIÊNCIA/SCIENCE
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... To avoid or reduce this interaction, new fluids have been developed, forced mainly by environmental legislations that demand that the drilling fluids be biodegradable. One of the new types of fluids developed for this purpose is the formate brines, that was described in Rabe and Fontoura (2003). These fluids have many advantages, such as low corrosion potential (good for drilling and completion fluids), are easily recyclable, inhibit gas hydration and bacterial growth, are compatible with salt containing formations and show good potential for shale stabilization. ...
... A laboratory apparatus was developed in which shale sample is put in contact with fluid, and special sensors measure the electro-chemical properties of the fluid throughout the test. The first version of the equipment, developed at Pontifical Catholic University of Rio de Janeiro (Rabe et al., 2001), allows tests with fluids at atmospheric pressure and at down hole temperature condition. The immersion equipment is presented in Figure 1. ...
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Resumo O presente trabalho tem como objetivo apresentar as mudanças nas propriedades físico-químicas de amostras de folhelhos quando imersos em soluções inorgânicas. Um equipamento de imersão foi desenvolvido no qual os fragmentos de rocha são colocados em contato com os fluidos, enquanto sondas medem as propriedades eletroquímicas do fluido de imersão. Amostras de folhelhos oriundas do Mar do Norte (Noruega) foram usadas neste estudo, assim como soluções a base de cloretos de cálcio, sódio e potássio em elevadas concentrações. Os resultados mostram que a imersão das amostras nas soluções salinas reduz, quando comparada com a imersão em água de-ionizada, as mudanças nas propriedades químicas e eletroquímicas das soluções. Os sais inorgânicos reduzem o teor de umidade das amostras e evitam elevadas variações na capacidade de troca catiônica e na composição do fluido dos poros. A imersão nos sais reduz a desintegração superficial das amostras e a produção de sólidos dispersos nas soluções. Abstract The present work has the objective of studying the changes in the physical-chemical properties of preserved shale samples when immersed in water and inorganic salts. Immersion equipment was developed in which shale samples are put in contact with fluid and special sensors measure the electrochemical properties of the fluid throughout the test. Offshore shale from Norwegian North Sea was used throughout the study. Calcium, potassium and sodium chlorides were used at 20 to 30% w/w. The results show that immersion of shale samples in salt solutions reduce, when compared with de-ionized water, the changes in chemical and electrochemical properties of solutions. The inorganic salts reduce the rock water content, the cation exchange capacity and the chemical composition of interstitial water. The salts avoid or reduce the solid dispersion and the superficial disintegration.
... (1) Para a realização dos ensaios com os folhelhos, foi desenvolvido um equipamento em que um termohigrômetro foi inserido no interior de um erlenmeyer com capacidade de 150 ml. Foram utilizadas cerca de 30 gramas de folhelhos, oriundas de sobras do processo de moldagem dos corpos de prova utilizados nos ensaios de imersão (Rabe et al., 2002). Os resultados do pH e da atividade química do folhelho da Colômbia em função da temperatura estão presentes na Tabela 7. Eles indicam que o folhelho é levemente alcalino e que o folhelho apresenta baixa atividade química à temperatura ambiente. ...
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A instabilidade de poços pode causar sérios problemas durante as operações de perfuração e completação, levando a incidentes que aumentam o tempo de perfuração e os custos de perfuração. Estes problemas tendem a ser mais severos em formações de folhelhos, que são mais suscetíveis devido a mecanismos adicionais de instabilidade que surgem quando os mesmos interagem com os fluidos de perfuração. Para um maior entendimento dos fenômenos de interação entre a rocha e o fluido de perfuração é necessário, primeiramente, que se tenha uma descrição completa destas rochas, tanto do ponto de vista de seus constituintes individuais quanto da sua microestrutura. O presente artigo descreve uma metodologia integrada para caracterizar um folhelho do ponto de vista de engenharia. Um folhelho oriundo de uma bacia sedimentar terrestre da Colômbia foi usado no presente trabalho. A metodologia proposta inclui a realização de ensaios de laboratório para caracterizar os constituintes individuais do folhelho, isto é, partículas sólidas e fluido dos poros, e para descrever a microestrutura da rocha. O artigo sugere que outros elementos, além dos argilominerais, devem ser também analisados para se estimar o potencial de reatividade de folhelhos. O resultados dos ensaios indicam que o folhelho estudado apresenta baixo potencial de reatividade quando em contato com fluidos de perfuração base água. Palavras-Chave: ensaios de laboratório; folhelhos; caracterização integrada.
... Um equipamento de imersão foi desenvolvido no qual, amostras são postas em contato com o fluido, e sensores especiais medem as propriedades eletroquímicas dos fluidos durantes os ensaios. A primeira versão do equipamento (Figura 1), desenvolvida na PUC-Rio (Rabe et al, 2001a), realiza ensaios sob condições de temperatura existente in situ, porém, sob pressão atmosférica. O equipamento é constituído por um balão de fundo redondo que possui cinco entradas: quatro laterais e uma central. ...
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Este artigo apresenta um estudo das variações nas propriedades físico-químicas de amostras preservadas de folhelhos quando imersas em sais orgânicos. As mudanças nas propriedades dos fluidos também são apresentadas. Um folhelho brasileiro de origem offshore foi utilizado para a realização da campanha experimental. Os ensaios laboratoriais foram realizados com três tipos de formiatos. Os resultados obtidos indicam que as amostras sofreram redução do seu teor de umidade e um aumento do pH da rocha. Mudanças também foram observadas na composição do fluido dos poros da rocha e na capacidade de troca catiônica como função dos íons presentes nas soluções salinas. Os ensaios de imersão mostraram um grande potencial para o entendimento dos fenômenos de interação entre o fluido de perfuração e os folhelhos. Palavras-Chave: reatividade; ensaios de imersão; folhelhos; sais orgânicos. Abstract-This paper presents a study of the changes in physico-chemical properties of preserved shale samples when immersed in organic salt solutions. The changes in the immersion fluid are also discussed. An offshore Brazilian shale was used throughout the study. The laboratory tests were conducted with three kinds of formate brines. The obtained results indicated that the samples suffered reductions in water content and increase in the ph of rock. Changes were observed in the chemical composition of rock pore fluid and in the cation exchange capacity as a function of the preponderant ions in the organic salt solutions. This immersion test shows great potential in understanding the interaction between drilling fluids and shales.
... O processo de instabilidade de poços é o resultado de fenômenos físico-químicos ( Rabe et al., 2002a) e mecânicos ( Tan et al., 1997) que ocorrem durante e após a perfuração. Esta interação pode mudar a magnitude das tensões da formação ao redor do poço, gerar excesso de poropressões, hidratar os argilominerais e aumentar o teor de umidade da formação, que podem conduzir a uma perda das ferramentas e até ao fechamento do poço. ...
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Problemas de instabilidade são encontrados durante a perfuração através de folhelhos, onde, tais problemas são o maior fator responsável pelo aumento de custos que ocorrem durante a perfuração de poços de óleo e gás. Tais problemas são creditados, em geral, à interação entre os fluidos de perfuração e as referidas rochas. Para um maior entendimento dos fenômenos de interação entre a rocha e o fluido de perfuração é necessário, primeiramente, que se tenha uma descrição completa destas rochas, tanto do ponto de vista dos seus constituintes individuais quanto da sua microestrutura. A caracterização do folhelho é de vital importância para a análise de seu potencial de reatividade. De um modo geral, esta caracterização é feita em laboratório utilizando amostras provenientes de testemunhos. No presente trabalho foi realizada também a caracterização a partir dos resultados obtidos da perfilagem. O objetivo do presente trabalho é descrever uma metodologia que integre os resultados de campo e de laboratório para a caracterização de folhelhos. Uma análise comparativa é feita utilizando os dados de perfis de Raios Gama (para a identificação de folhelhos), perfis Sônicos e Neutrônicos com resultados de ensaios de caracterização realizados em laboratório. O estudo demonstra que ensaios laboratoriais são importantes para auxiliar na interpretação dos dados obtidos através dos perfis. Palavras-Chave: perfilagem de poços; ensaios de laboratório; caracterização, folhelhos. Abstract-Problems encountered while drilling shale formations are a major factor in the cost of oil and gas wells. In general, these problems are credited to the interaction between the drilling fluid and the rock. In order to understand the complex elements of such an interaction, it is necessary a complete description of shales including both its individual constituents and its microstructure. The shale characterization is essential for the analysis of reactivity potential. In general, this characterization is realized at laboratory using undisturbed samples. In the present paper was conducted a characterization by data obtained too from logs. The objective of the present work is to describe a methodology that integrates the results that came from lab and logs to characterize shales. A comparative analysis was made using the data from gamma ray (to the identification of shales), sonic and neutronic with results of characterization tests realized in laboratory. The study demonstrated that lab tests are important to facilitate the interpretation of well logs.
... These tests give completely different results that are often far from reality. Immersion tests give a visual confirmation of the effect of different types of fluid on rock structure (Rabe et al. 2002;Santos et al. 1997). Immersion tests are used to evaluate the suitability of different drilling fluids for a particular shale formation. ...
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This paper presents qualitative techniques for evaluating shale–fluid interaction. Undesirable shale–fluid interactions lead to wellbore instability, formation damage and other problems that cost the petroleum industry millions of dollars annually. A simple desktop test method, such as immersion testing, can help production engineers choose the appropriate shale inhibitors such as salt, tetramethylammonium chloride (TMAC) and polymers that can effectively reduce the impact of oilfield fluids invading shale and causing it to swell or disperse. The swelling tendency of shale is highly dependent on clay mineralogy and other properties, such as porosity and permeability. A series of immersions tests was performed to study the combined and isolated effects of salt, TMAC, and polyacrylamide on preventing shale from becoming unstable. The merit of each fluid system in shale inhibition is probed for Woodford, Chattanooga and Pride Mountain shale. Rheology of bentonite slurries is studied with different salts and TMAC to probe their efficiency in preventing the swelling of bentonite clay. Additionally, rheology of bentonite with anionic and cationic polyacrylamide and salt is investigated.
... Rabe et al. [97] ont trouvé des résultats semblables de diffusion d'ions et sont allés au-delà en approfondissant ces résultats par des analyses minéralogiques de l'argilite avant et après immersion pendant trois jours dans l'eau déminéralisée. Leurs résultats montrent que la roche n'a subi aucune altération chimique ce qui revientà dire que les ions apparaissant dans l'eau déminéralisée ne sont dus qu'à l'échange ionique entre l'eau des pores et l'eau entre les couches et la solution externe. ...
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Downhole mud/shale interaction can only be properly understood if rock mechanical, shale hydration, and fluid transport phenomena are taken into account. This paper presents a review of Koninklijke Shell E&P Laboratorium's research on borehole stability in shales. Mechanisms relevant to shale stability, including pore pressure penetration (the gradual increase in pore pressure resulting from high mud weight), capillary threshold pressures, compressive and tensile failure, postfailure stabilization, hydration stress, inhibition, and osmotic phenomena are discussed. We attempt to integrate these mechanisms into a comprehensive model for shale behavior.
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Interstitial water from shales and sandstones shows a contrast in concentration and composition. Sidewall cores of shales were taken every 500 ft between 3,000 and 14,000 ft in a well in Calcasieu Parish, Louisiana, which encountered abnormally high fluid pressures just below 10,000 ft. Significant differences between the total dissolved solids concentrations in waters from normally pressured sandstones (600-180,000 mg/l) and highly pressured sandstone (16,000-26,000 mg/l) were noted. Shale pore water has a lower salinity than the water in the adjacent normally pressured sandstones, but the concentrations are more similar in the high pressure zone. Shale water generally has a concentration order of SO4 = > HCO3- > Cl-, whe eas water in normally pressured sandstone has a reversed concentration order. Conversion from predominantly expandable to non-expandable clays accelerates near the top of the high pressure zone, which appears correlative with a major temperature gradient change, an increase in shale porosity (decrease in shale density), a lithology change to a massive shale, an increase in shale conductivity, an increase in fluid pressure, and a decrease in the salinity of the interstitial waters. The data presented suggest that the clays subjected to diagenetic change release two layers of deionized water and that this released water may be responsible for the lower salinity of the water found in the high pressure section.
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SPE Members Abstract A vast majority of drilling problems occur in shales. The traditional explanation is that shales swell when contacted with water. As the use of oil based muds becomes more difficult, recent years have seen a large number of publications dedicated to the study of water/shale interaction. Unfortunately, this large body of evidence contains too many contradictions to be trusted as a sound basis for engineering. A two years long critical review of this material was therefore undertaken revealing that most experimental observations performed in the laboratory were not representative of downhole conditions as air or water vapor had been introduced in the samples, leading to capillary phenomena which have been mistaken for swelling. Further observations revealed that true swelling is unlikely to be an issue downhole. Upon contact with drilling fluids, the occurrence of mineralogical transformations affecting the shale was identified and it was further shown that such transformations will affect the mechanical response of the rock. Background Many sources have evaluated that problems due to wellbore stability add on average between 10 and 15 % to normal drilling costs. Shales are at the origin of most of these problems and it has traditionally been assumed that this is due to the fact that they swell when contacted with water. As a consequence, the industry has turned successfully towards the use of oil based muds to resolve them. This success is clearly illustrated by the statistical analysis of 100 8"1/2 phases drilled in Italy over recent years with both water and oil based muds. For each well, stuck pipe cases were systematically analysed revealing that only 4 wells out of 26 drilled with oil based mud had a stuck pipe whilst 40 out of 74 wells drilled with water based mud had difficulties. Unfortunately, environmental concerns make the use of oil based muds more and more difficult, costly and risky everywhere:–special authorizations need to be obtained,–secure and expensive cuttings disposal procedures need to be divised,–long term legal liability has been imposed in many countries which leaves little room for errors. P. 741
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A new experimentat technique for the investigation of drilling-fluid induced borehole instability in shales is introduced. Pressure transmission transients are used to study pressure penetration in shales, the latter being a main shale-destabilising mechanism. The pressure transmission method provides a powerful tool for the fundamental study of shale-fluid interactions and for development of improved water-based mud formulations to promote borehole stabilisation in shales. The capabilities of the method are illustrated by permeability measurements made in low-permeability shales. In addition, a wide range of conventional and new mud systems is screened and the shale-stabilising performance of these systems is assessed. It is argued that the strategy for promoting borehole stability in shales through improved mud formulation should be based on maintaining pressure isolation between wellbore and formation. 1. INTRODUCTION Borehole instability in shales is a prime technical problem area in oil- and gas-well drilling. Costs for the industry spent on shale stability problems have been estimated at $500 million/year. Shale drilling is notoriously difficult when using water-based drilling fluids, which are now replacing technically superior, but environmentally unacceptable, oil-based muds. Past efforts to improve water-based fluid formulations for shale drilling have been hampered by an imperfect understanding of the fluid properties required to prevent the onset of borehole failure. Recent studies of shale-fluid interactions [1-4], however, have revealed many of the underlying causes of borehole instability and have suggested a new approach to water-based mud design. This development has been aided by a growing awareness that the design of improved water-based muds greatly benefits from laboratory techniques that can realistically simulate downhole shale-fluid interactions. As a result, new downhole simulation techniques are replacing inaccurate and outdated techniques in drilling fluid rerearch. This paper introduces a new technique based on pressure transmission measurements, for monitoring fluid transport and associated pore-pressure effects in shales. 2. DRILLING FLUID INDUCED BOREHOLE INSTABILITY IN SHALES Shales are heterogeneous, low-permeability media of which the matrix consists to a large extent of clays. The forces that act on a shale system can be divided into mechanical and physico-chemical forces. The former include the in-situ stresses, the pore-pressure and mechanical forces in the cementation that may develop in response to tensile or corressive loading. The physico-chemical forces in the clay parts of the shale include the well-known van der Waals force and the double-layer repulsion. Also, at small platelet distances, a variety of short-range forces become important, e.g. oscillatory- and structural forces (for a recent review, see ref. [5]) All physico-chemical forces combined give rise to the ‘hydration stress', which usually is repulsive and has a well-defined direction between two clay platelets. In the inhomogeneously distributed ensemble of clay particles and other shale materials, the preferential direction of the hydration stress is usually averaged out. The hydration effects are then described in terms of a scalar quantity: the ‘hydration pressure'.
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Methylene blue adsorption (MBA) is found to be an effective method of indicating when smectite (group name for the montmorillonite minerals) is present in major amounts in mudrocks or along discontinuities. The basic relationship is that MBA values greater than 15 indicate that smectite is present at 15 is the maximum value found for illite, the next most adsorptive mineral as compared with smectite. MBA requires only a 2-g specimen, and thus clays along discontinuities and clays along thin slip zones of slides can also be analyzed as to type of clay. The method is less expensive than X-ray diffraction analyses but also can supplement X-ray diffraction investigations.
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The current understanding on physico-chemical interactions of shales and drilling fluids is reviewed. The complicated relationship between transport in shales (e.g. hydraulic flow, osmosis, diffusion of ions and pressure) and chemical change (e.g. ion exchange, alteration of water content and swelling pressure) that governs the stability of shales is clarified. This fundamental understanding of shale-fluid interactions is then linked to the latest developments in water-based drilling fluid technology which targets improved cuttings- and wellbore stabilization and reduced bit balling. Different types of water-based shale stabilizers are discussed. In addition, a classification of shale drilling fluids system is given based on their shale-stabilizing action and efficiency. It is shown that oil-based/synthetic-based drilling performance is now achievable with economically and environmentally compatible water-based drilling fluids. Introduction The problem of wellbore stability in shales has frustrated oilfield engineers from the start of oil- and gas well drilling. Wellbore instability is in fact the most significant technical problem area in drilling and one of the largest sources of lost time and trouble cost. A typical example of problems encountered in the field is given in Fig. 1. The 8 1/2" section of this well, drilled with a water-based mud, was enlarged up to 25" despite the presence of additives used especially for shale-stabilization purposes. Operational problems that derive from such instabilities may range from high solids loading of the mud requiring dilution, to hole cleaning problems due to reduced annular velocities in enlarged hole sections, to full-scale stuck pipe as a result of well caving and collapse. Wellbore stability is almost a trivial issue with oil-based and synthetics-based muds. Once mud weight and invert emulsion salinity are properly established, stability can virtually be guaranteed (except for a few cases such as fractured shales). Much more problematic have been the adverse interactions of shales with water-based fluids. These are environmentally attractive alternatives for oil- and synthetic muds, but they are still outmatched by the latter in shale drilling performance. The central issue raised in this paper is : "which chemical means can be exploited to achieve full shale stabilization and reach the desired operational performance with water-based drilling fluids?". The nature of the shale instability problem must be understood first in order to answer these questions. This requires appreciation of transport in shales, the physicochemical changes effected by this transport, and the implications of the former to shale stability. These relationships are clarified in a step-by-step approach. Fundamentals of Shale Behavior A Balance of Forces Fig. 2 gives a simplistic but practical model for the forces acting on a shale system containing clays and other minerals (primarily quartz) at silt size. They can be subdivided into mechanical and physico-chemical forces. The former include:–The in-situ vertical (overburden) and horizontal stresses–The pore pressure–The stress acting at intergranular contact points, e.g. at cementation bonds The latter, acting primarily in the clay fabric, include:–The van der Waals attraction–The electrostatic Born repulsion–Short-range repulsive and attractive forces that derive from hydration/solvation of clay surfaces and the ions that are present in interlayer spacings (adsorbed or free). The latter forces are usually lumped together to form the "hydration stress/pressure" or "swelling stress/pressure", since they are responsible for the characteristic swelling behavior of clays and shales. The term "swelling pressure", well-accepted in oil-field practice, will be used exclusively below. P. 523
Investigation of the effects of sample handling procedures on REFERENCES shale properties
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