NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY
DRILLING AND PRODUCTION RESEARCH PROJECT (PDRP)
The Effect of pH and Salt Concentration on
Barite Sag in Oil Base Drilling Fluids.
Ayoade Dare: Project Investigator
Dare Ayoade
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Contents
Acknowledgment .......................................................................................................................................... 3
Abstract ......................................................................................................................................................... 5
1.1 Introduction ............................................................................................................................................ 6
1.2 Background Study ............................................................................................................................... 7
1.2.1
1.3
2.1
Statement of Problem ................................................................................................................. 11
Experimental Study ......................................................................................................................... 12
2.1.1
3.1
Barite Sag Measurements and Testing Methods .................................................................. 9
Experimental Setup ................................................................................................................. 12
Results & Discussion ....................................................................................................................... 18
3.1.1
Fluid Rheology ......................................................................................................................... 18
3.1.2
Sag Test Result. ....................................................................................................................... 20
3.1.3
Change in pH ........................................................................................................................... 21
3.1.4
Equivalent Circulating Density (ECD) ...................................................................................... 24
3.2
Sag Test Comparison....................................................................................................................... 27
4.0
Applications..................................................................................................................................... 28
5.0
Conclusion ..................................................................................................................................... 30
Works Cited ................................................................................................................................................. 31
APPENDIX .................................................................................................................................................... 33
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Table of Figures
Figure 1: Fluid Sample ................................................................................................................................. 14
Figure 2: Viscometer Sag Shoe Test ........................................................................................................... 16
Figure 3: Schematic of the VSST.................................................................................................................. 17
Figure 4: Fluid Rheology.............................................................................................................................. 19
Figure 5: Sag in drilling fluids samples with varying salt concentration ..................................................... 21
Figure 6: Change in Fluid Rheology with addition of HCL ........................................................................... 23
Figure 7: Sag tendency with change in pH .................................................................................................. 24
Figure 8: Effect of salt concentration on ECD ............................................................................................. 25
Figure 9: Effect of pH & Salt concentration on ECD .................................................................................... 26
Figure 10: Effect of pH on sag ..................................................................................................................... 27
Figure 11: Schematic of a deviated well ..................................................................................................... 29
Figure 12: Effect of flow rate on ECD .......................................................................................................... 34
Figure 13: Fluid Loss in Drilling Sample with reduction in pH..................................................................... 37
Figure 14: Sag in base fluid sample. ............................................................................................................ 37
Figure 15: Sag in sample with reduction in pH ........................................................................................... 38
List of Tables
Table 1: Mud Composition and Mixing Quantities ..................................................................................... 13
Table 2: Mixing Procedure .......................................................................................................................... 13
Table 3: Results of the API recommended tests ......................................................................................... 14
Table 4: Salt Concentration by Weight ....................................................................................................... 15
Table 5: Rheological Data............................................................................................................................ 19
Table 6: VSST Results .................................................................................................................................. 20
Table 7: Rheological data of samples with change in pH............................................................................ 22
Table 8: Effect of pH on sag ........................................................................................................................ 23
Table 9: ECD at 15% salt concentration ...................................................................................................... 35
Table 10: ECD at 20% salt concentration .................................................................................................... 35
Table 11: ECD at 25% salt concentration .................................................................................................... 35
Table 12: ECD at 30% salt concentration .................................................................................................... 36
Table 13: ECD at 35% salt concentration .................................................................................................... 36
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Acknowledgment
I am grateful to Baker Hughes for providing the materials for this experimental work, Jason Maxey, Scott
Paul, Lirio Quintero and other industry professionals that advised and gave critical feedback at various
points in the experimental study.
I am also grateful to my adviser Dr. Tan Nguyen and committee members Dr. Kelly and Dr. M. Riley for
their support, guidance and supervision. Finally, I’d like to appreciate my lab partner Chris Silva.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Abstract
The ability of a drilling fluid to hold particles in suspension is very important. The failure of a
drilling fluid to suspend weighting materials has lead to the problem known as barite sag. Some
of the problems caused by barite sag include well control problems, casing and cementing
problems, formation fracture and borehole instability.
Salt is one of the components of an oil base drilling fluid. Salt is added to an oil base drilling
fluid for a number of reasons. This study describes in detail the effect of salt concentration in the
internal brine phase of an oil base drilling fluid on barite sag. This study varies salt concentration
across various fluid samples while maintaining the same mud weight and simultaneously testing
for sag tendencies. The effect of salinity concentration on barite sag was investigated through a
series of experiments in this study. The 15% CaCl2 drilling fluid sample exhibited the worst sag
tendencies while the 30% CaCl2 drilling fluid sample exhibited the least amount of sag. Sag
progressively increased as salt concentration decreased. These tests demonstrate that sag is a
function of the mass of the weighting material. It is possible to control sag by using smaller
particles or particles with lower density.
The pH values of oil base drilling fluids are not monitored like water base drilling fluids. Oil
base drilling fluids are alkaline solutions and no considerations have been given to the effect of
pH on the fluid properties. Drilling through unexpected zones of acidic formation can change the
pH of the oil base drilling fluid. The free lime in the drilling fluid normally neutralizes this
effect, but with no special attention been paid to the alkalinity of the fluid, the change in pH can
have serious effects on the physical and chemical properties of the fluid. The pH of oil base
drilling fluid samples were varied, and the effect of pH on barite sag was investigated. With
decrease in pH, sag was increased at each concentration. This was due to the physical and
chemical changes that occurred in the drilling fluid samples.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
1.1 Introduction
A drilling fluid is a circulating fluid used while drilling for several purposes. Some of these
functions could include cuttings transport, particle suspension, subsurface pressure control,
wellbore stabilization, bit lubrication, corrosion control, prevention of formation damage,
transmission of hydraulic horse power to the bit, sealing of permeable formations, facilitation of
formation data collection from drilled cuttings, cores and electrical logs. The ability of a drilling
fluid to perform these important functions is dependent on the components that make up the
fluid. A component could perform a specific function, while two or more components could
work together to perform one function. The composition of an oil base drilling fluid could
typically include a base fluid, emulsifier, and viscosifiers, weighting agent, salt, lost circulation
materials, filtration control additives, alkalinity control agents and other additives.
Salt is added as one of the components of an oil base drilling fluid for several reasons. It is added
to the oil base fluid to increase the density of the fluid, to maintain well stability while drilling
water sensitive formations like shale, to reduce the mud resistivity of the fluid, and to react with
the emulsifier for better emulsion.
During the course of a drilling operation, it is not uncommon for the drilling fluid to become
contaminated. These contaminants often change the properties of the drilling fluid and adversely
affect the drilling operation. Oftentimes adjustments have to be made to the drilling fluid on site.
The results of such contamination could be a change in pH of the drilling fluid. This
experimental study will investigate how pH could affect the sag tendency of the drilling fluid.
The ability of a drilling fluid to hold particles in suspension is very important. The failure of a
drilling fluid to suspend weighting materials has lead to the problem known as barite sag. Barite
sag is defined as the slow settling of barite particles or other weighting materials. It is the
dynamic and static settling of weight materials, followed by downward slumping of the fluidized
beds that form on the low side of the wellbore. The formation of these high density beds and
their subsequent recirculation can lead to severe operational problems, including well control
issues, lost circulation, borehole instability and stuck pipe (Nguyen, 2011) . Barite sag results in
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
fluctuations in drilling fluid density leading to well control problems, cementing problems, and
complications during completion operations.
1.2 Background Study
The settling of barite in drilling fluids is not a new phenomenon; however the problem has been
exacerbated by the increase in high angle wells. Sag is a major concern in deviated wellbores.
Sag is reported to be worse at inclination angles of 30⁰ to 75⁰. The most critical region for sag is
from 60⁰ to 75⁰. (Bern, et al. 1998). The increased particle settling rate which occurs in an
inclined fluid column is caused by the Boycott effect.
Boycott settling is the effect of enhanced gravity driven settling in deviated pipes was found by
all the early investigators to be a major contributor to sag in drilling fluids. Boycott reported in
1920 that blood corpuscles settle faster in inclined test tubes than in vertical ones. A thin layer of
clarified fluid appears immediately below the upper wall and another clarified region develops at
the top of the fluid. Particles settle out of the suspension zone and form a sediment bed.
Coincident with a downward slide of the bed, a resulting cross sectional density gradient
generates a pressure imbalance. This causes convection currents which drive the lighter fluid up
and the bed down, accelerating settling in the suspension zone. The combined downward flow
and slide of the bed is called “slumping”. (Nguyen, Miska, et al. 2009).
(Hanson, Rachal and Zamora 1990)Hanson et al focused on practical guidelines to prevent sag.
They also emphasized the importance of dynamic sag: sag while circulating the drilling fluid.
They emphasized that dynamic sag is more difficult to prevent than static sag.
(Bern, et al. 1998) Bern et al, studied dynamic sag and recommended operational guidelines to
minimize sag. Some of the operational guidelines include: (a) barite sag can be minimized by
attention to detail in the areas of well planning, mud properties, and operational practices, (b)
Barite sag and hole cleaning are related in principle, but are distinguished by their bed
characteristics, (c) Barite beds are more responsive to removal by mud velocity and pipe rotation
than most cutting beds,(d) Barite sag can be exacerbated by low annular velocities, eccentric and
stationary drill pipe, (e) Temperature and pressure are critical for drilling fluid design. High
temperatures cause the drilling fluid to thin, which can increase sag tendency. It is important to
ensure that viscosity measurements are taken under HP/HT conditions, (f) broad particle-size
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
distribution can help minimize sag. Very large particles should be eliminated, as these can settle
naturally under static conditions .They discovered the importance of drill pipe rotation to sag.
Drill pipe rotation may induce turbulence sometimes combined with a helical flow pattern that
helps re-suspend the weighting material.
It has been established that the rheological properties other than the viscosity, as measured by the
standard VG-meters used in a drilling fluid laboratory are needed to determine the potential for
sag. Dynamic viscosity parameters contain information on the internal structure of fluids.
(Omland, et al. 2007). The relevance of these parameters for sag became a subject of
investigation in the early 1990s and is still a focus area. They can be measured by generating
small amplitude oscillations that are anticipated not to destroy any internal structure in the fluid.
By measuring dynamic viscosity parameters, it is possible to determine how solid or liquid –like
the fluid is. (Omland, et al. 2007).
(Saasen, et al. 1995) Saasen et al pointed out that sag may occur more rapidly in a fluid that has a
fragile gel structure. A fragile gel is one that may exhibit high yield strength but after the initial
gel breaks, continues to be easily broken. Therefore, the fluid may have relatively high gel
strength and still exhibit severe dynamic sag when exposed to a low shear. They also concluded
that static sag cannot be predicted from VG-meters; Gel formation is important in avoiding static
sag; G’/G’’ can indicate sag potential; Dynamic sag can be large even if static sag is low or
absent.
Measurement of low shear rate viscosity has also been used to evaluate the potential for sag.
Jachnik and Robinson (Jachnik and Robinson 1996) measured viscosity at very low shear rates,
sing a parallel plate geometry. They found that the yield stress in an oil or synthetic based
drilling fluid is usually quite low. It has been suggested that a true yield stress normally does not
exist in these types of fluid.
(Dye, et al. 1999) Dye et al suggested that the viscosity values measured at shear rates below 3-5
1/s can be used as indicators of sag potential. Generally, sag is reduced if the low shear rate
viscosity is increased. However, no correlation has been established which can be used to predict
sag from these data.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
It has been shown that internal phase plays a significant role in the fluid’s sag performance, but
the underlying mechanism is still not known. (Albertsen, et al. 2004) Albertsen investigated the
effect of the internal phase in synthetic and oil base drilling fluid on sag. The use of calcium
nitrate in the internal phase instead of the commonly used calcium chloride, can significantly
improve the fluid’s resistance sag. The choice of both the base fluid and internal phase certainly
affect the efficiency of the emulsifier used to stabilize the emulsion.
1.2.1
Barite Sag Measurements and Testing Methods
There are several testing methods used today to measure sag. They range from simple to
complex methods attempting to simulate real life drilling conditions. A few of these test methods
are applicable both in the laboratory and in the field.
1.2.2 Viscometer Sag Test (VST)
The viscometer sag test was introduced in 1991 as a practical well site test and has since seen
some success in the field and in the laboratory as a direct indicator of sag tendencies. (Zamora
and Bell 2004). The viscometer sag test measures the density increase at the bottom of an API
mud thermo cup after mixing the sample at 100 rpm with a standard field viscometer for 30
minutes. A syringe with a blunt cannula is used to extract samples from the bottom of the thermo
cup. A retort cup, pycnometer, or the syringe itself can be used to provide an accurate volume. A
digital balance or triple beam balance can be used to measure sample weight. (Zamora and Bell
2004).
This test technique was developed at a time when the industry was beginning to fully recognize
the negative effect of sag in directional wells. The VST reinforced the concept that barite sag is
primarily caused by dynamic settling, since sag was generally tested as a static settling problem
prior to that time.
1.2.3 Viscometer Sag Shoe Test.
The viscometer sag shoe test may also be known as the improved viscometer sag test, or the
enhanced viscometer sag test. As the name implies, it is an improvement on the previously
discussed viscometer sag test. The viscometer sag shoe test involves the insertion of a
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
thermoplastic shoe (also known as sag shoe) at the bottom of the thermo cup before running an
otherwise standard VST procedure. (Zamora and Bell 2004)
The sloping surface on the shoe helps to accelerate settling and to concentrate the weight
material into a single collection well at the bottom of the thermo cup. Bed samples are extracted
and replaced in the same location. This means that data can be taken at multiple time intervals to
allow trend comparisons to other tests, such as circulating flow loops. Also it permits
measurements of the relative capability of the test mud to pick up a sag bed formed in thermo
cup. Characterization of the sag bed can suggest how easily it can be removed from a well prior
to tripping out of the hole. (Zamora and Bell 2004)
The VSST procedure is similar to the VST procedure, except that it includes steps to measure
sag pick up. At the conclusion of the normal 30 minute sag test, the bed sample can be replaced
in the shoe collection well using the syringe. The viscometer speed is then increased to 600rpm
for 20 minutes, after which another sample is extracted and weighed. This gives an indication of
how easily the bed can be picked up by an increase in shear. VSST sag is reported as the increase
in mud weight at the bottom of the thermo cup after mixing at 100 rpm for 30 minutes.
1.2.4 Flow loop tests
Flow loop tests can model field conditions such as annular flow, hole angle and eccentricity. The
inclination angle of the test can be varied to suit the drilling condition been simulated. Flow
loops could range from small scale flow loops to large scale flow loops. The design of a flow
loop is normally tailored towards the well type and conditions been simulated.
1.2.5 Nuclear Magnetic Resonance
The nuclear magnetic resonance uses a 2 MHz bench top NMR spectrometer equipped with a
gradient field to characterize the accumulation of solids (barite) at the bottom of a sample tube
filled with oil base drilling fluid.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
1.3
Statement of Problem
Barite sag is a phenomenon that causes many problems during drilling operations. Barite sag
occurs in both flowing and non flowing conditions. Some of the problems caused by barite sag
include well control problems, casing and cementing problems, formation fracture and borehole
instability. Due to the numerous uses of salt in an oil base drilling fluid, it is important to study
the effect of salt concentration on the barite sag in oil base drilling fluids.
Objective
The two main objectives of this study are:
1. Experimentally investigate the effect of salt concentration on barite sag
2. Determine the effects of pH change on barite sag in the drilling fluid samples at various
salt concentrations.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
2.1
Experimental Study
2.1.1 Experimental Setup
The first step in this experimental study was to generate an oil base drilling fluid that meet all the
API recommendations and ensure emulsion stability. Strict industry recommendations on mixing
an oil base fluid were adhered to in mixing the fluid samples. The mixing procedures and mud
composition are given in table 1. The next step was to vary the salt concentration from 15%
CaCl2 by weight to 35% CaCl2 concentration. The oil to water ratio was maintained at 80/20 for
all samples and the density was held constant at 12ppg across all samples. This enabled
comparison of sag tendencies across the various samples. The third step was to test for barite sag
in the samples at various salt concentrations. This was done using the viscometer sag shoe test.
The viscometer sag shoe test involves the insertion of a thermoplastic shoe in the bottom of the
thermo cup before running an otherwise standard VST procedure (Zamora and Bell 2004). The
inclined surface of the sag shoe helps to accelerate the deposition of the weighting material in the
collection well at the bottom of the thermo-cup. The collection well is located at the bottom of
the sag shoe. Bed samples were extracted from the collection well and returned to the collection
well. Figure 2 & 3 shows the setup of the enhanced viscometer sag test.
Methodology/ Approach.
1. Generating the oil base fluid.
The oil base fluid was generated with diesel as the continuous phase and brine as the
internal phase. The salt type of the internal brine phase was CaCl2. Other components
include the viscosifier, primary and secondary emulsifier, lime, filtration control
addictives, lost circulation materials and weighting agent. The weighting agent used in
this study was barite. All mud components were supplied by Baker Hughes. The fluid
was prepared using a Silverson shear mixer in 350ml batches. The quantities of the mud
components to be added were based on a 350ml sample. Total mixing time was 60
minutes and the maximum rpm during the mixing process was 600 rpm. Table 1 and 2
give a detailed mixing procedure and mud composition.
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Table 1: Mud Composition and Mixing Quantities
COMPONENT
FUNCTION
CARBO-MUL
Emulsifier/wetting
HT
agent
CARBO- GEL
COMPOSITION
1400ml mix
350 ml mix
2 lb/bbl
7.98 g/ 1.4 L
1.995g/ 350 ml
Organophilic clay /
2-3 lb/bbl for O/W
7.98g to
1.995g to
Viscosifier
of 75/25 – 80/20
11.98g/1.4 L
2.995g/350 ml
Varies with
Varies with
Varies with
concentration
concentration
concentration
CALCIUM
Salt
CHLORIDE
6.9925g/350
CARBO-TEC
Primary Emulsifier
7 lb/bbl
27.97g/1.4 L
LIME
Alkalinity control
5 lb/bbl
19.98g/1.4 L
4.995g/350ml
MIL- BAR
Weighting agent
Varies
Varies
Varies
OIL- BASE
O/W of 80/20
1120 ml
280ml
WATER
O/W of 80/20
280 ml
70ml
ml
Table 2: Mixing Procedure
Mud Component
Base Oil
Primary
Emulsifier
Secondary
Emulsifier
Viscosifier
Alkalinity
Filtration Control
Brine
Weighting agent
Product Name
Mix Time
Mixer Speed
Diesel
Carbo-Mul HT
0 minute
1 minute
4500 rpm
4500 rpm
Carbo-Tec
1 Minutes
5000 rpm
Carbo-Gel
Lime
Check-Loss, Mil-Carb
CaCl2 & Water
Mil- Bar
3 Minute
4 Minutes
5 Minutes
6 Minutes
22 Minutes
5000 rpm
5000 rpm
5000 rpm
6000 rpm
6000 rpm
Useful Information: 350ml is ¼ lab mix, 1400ml is one lab mix, total mixing time is 42 minutes, mixing speed is
6000rpm, and mixing temperature is 150˚F to ambient temperature
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
2. API Recommended Test.
To ensure that the generated oil base fluid meets the required API standards, the API tests
were carried out. These tests included drilling fluid density, viscosity and gel strength,
oil, water and solids content, low gravity solids and weighting material concentration,
filtration tests and electrical stability test. Results of the API recommended tests are
shown in table 3.
Table 3: Results of the API recommended tests
Density
Base Sample 10.4
PV
22
YP
4
10s/10m
8/15
Retort
Retort
Solids
OWR %
18%
68/14
OWR %
80/20
Figure 1: Fluid Sample
3. Varying Salt Concentration.
The salt concentration of the internal brine phase was varied from 15% to 35%. The exact
quantity of salt to be added was calculated, taking into account the density and purity of
the salt. During the course of the experiment, it was discovered that the salt purity played
a major role in determining the right concentration of salt to be added. Table 4 shows the
salt concentration by weight of each of the samples.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Table 4: Salt Concentration by Weight
Salt %
Weight of Salt
Mass Concentration
(lbs/bbl)
g/1400ml
g/350ml
g/70ml
g/87.5 ml
15
12.34
49.32
12.33
2.47
3.08
20
17.49
69.87
17.47
3.49
4.37
25
23.32
93.16
23.29
4.65
5.82
30
29.98
119.78
29.95
5.99
7.48
35
37.68
150.49
37.62
7.52
9.40
4. Sag Test.
The sag tendencies of the oil base fluids were experimentally determined through the
enhanced viscometer sag test. The viscometer sag shoe test is simply the addition of a
thermoplastic shoe at the bottom of a thermo-cup before running a standard viscometer
sag test. The upper surface of the sag shoe is characterized by two sloped, hemispherical
sections. The sloping surface on the shoe helps to accelerate settling and to concentrate
the weight material into a single collection well at the bottom of the thermo-cup1.the
drilling fluid is rotated for 30 minutes at 600 rpm using a viscometer. A second sag
reading is taken after 20 minutes of rotation at 100 rpm. Bed samples are collected from
the collection well with a 10ml syringe and a cannula. Bed samples are extracted,
weighed and returned to the collection well. This enables multiple data to be taken at
different time intervals. Test equipment setup and procedure are shown below.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
VSST Test Procedure.
1. Pour 140ml of mud into the thermo cup. Mix at 600rpm for 15 minutes, until mud
temperature has stabilized at the set point for at least 5 minutes.
2. Using a 10ml syringe with 6-inch cannula, draw slightly more than 10ml of mud from the
collection well.
3. Weigh and record the mud weight as VST1
4. Gently expel the previously sampled 10ml of mud into the collection well of the shoe.
5. Shift viscometer to 100rpm and run for 30 minutes
6. Repeat steps 2-4 and record the new mud weight as VST2
7. Convert VST1 and VST2 to lb/gal.
8. Subtract VST1 from VST2 and report as VST Sag in lb/gal.
Figure 2: Viscometer Sag Shoe Test
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Figure 3: Schematic of the VSST.
5. Change in pH.
The second part of this experimental study focuses on the influence of pH on barite sag. The pH
values of the previously mixed samples with different salt concentrations are altered. The pH
values were reduced from a value of 8 to 4. Each sample’s pH was altered and new rheological
data were collected for each change in pH value. The chemical used to reduce the pH value of
the fluid samples was hydrochloric acid (HCl). The fluid samples were changed from a basic
solution to an acidic solution. The sag shoe test was then carried out on the new samples. The
sag results from the fluids with varying concentration were compared to the results from the fluid
samples with changes in their pH values.
The pH values of the drilling fluid samples were varied from values of 8 to 4. The chemical used
to reduce the pH of the drilling fluid samples was hydrochloric acid. The pH value of each
sample at each concentration was varied. New rheological data was obtained at each
concentration and pH value. A digital pH meter was used to measure and ensure uniform pH
values across all samples. The viscometer sag test was repeated for the new samples with
reduced pH values at different salt concentrations. Sag results for the new samples was
computed, analyzed and compared with results from the original samples.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
3.1
Results & Discussion
3.1.1 Fluid Rheology
From Figure 4, it can be inferred that the drilling fluid samples can be classified under the
Bingham plastic model. The rheological values of the samples were also a good indication of the
suspension ability of the samples. The 10 seconds and 10 minute gel strength increased as salt
concentration increased. Therefore, the fluid sample with 35% CaCl2 had the highest gel strength
values. The gel strength is the shear stress of a drilling fluid that is measured at low shear rate
after the drilling mud is static for a period of time. It is caused by electrically charged particles
that link together to form a rigid structure in the fluid (Annis and Smith 1996). Barite settling
will occur in a mud with no gel strength. The gel strength is an indication of the suspension
property of the drilling fluid, thus the samples with higher gel strength show a reduction in static
sag tendency.
The yield point for the fluid samples also increased as CaCl2 increased. The yield point is
defined as the resistance to initial flow, or the stress required to initiate fluid movement. At low
shear rates the particles link together, increasing the resistance to flow; at high shear rates the
linking bonds are broken and the fluid becomes more like liquid. These two effects combine to
determine the yield point of a mud (Annis and Smith 1996). Yield point is associated with two
main functions: the hole cleaning capability and pressure control characteristics of the mud
(Annis and Smith 1996). Thus higher yield point will indicate better hole cleaning but higher
pressure loss in the annulus resulting in higher ECD. Therefore the sample with 35% CaCl2
concentration would have a better hole cleaning capability than sample 1 with 15% CaCl2
concentration. High yield point values results in the problem of high circulating pressure drop.
The problem with high circulating pressure drop includes pressure surge and swab from pipe
movement. Details of the rheological data are shown in Table5.
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Shear Stress vs. Shear Rate
Shear Stress (lb/100ft sqr.)
80
70
60
50
15% CaCl2 Conc
40
20% CaCl2 Conc
30
25% CaCl2 Conc.
20
30% CaCl2 Conc
10
35% CaCl2 Conc
0
0
500
1000
Shear Rate (1/s)
Figure 4: Fluid Rheology
Table 5: Rheological Data
Rheology of Sample with Varying CaCl2 Concentration.
600 rpm (lb/100ft )
15%
50.00
20%
51.10
25%
30%
35%
55.70 65.20 69.00
300 rpm (lb/100ft2)
200 rpm (lb/100ft2)
100 rpm (lb/100ft2)
6 rpm (lb/100ft2)
3 rpm (lb/100ft2)
1 rpm (lb/100ft2)
PV(cP)
AV (cP)
10-s Gel (lb/100ft2)
10-min Gel (lb/100ft2)
33.00
24.70
16.00
8.40
8.30
8.20
17.00
25
2.60
0.40
31.80
23.80
16.00
9.80
9.80
9.80
19.30
25.55
3.10
0.90
32.70
24.90
16.50
9.50
9.50
9.40
23.00
27.85
3.20
10
2
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36.20
26.30
16.90
8.20
7.90
8.10
29.00
32.6
4.10
1.10
39.30
28.70
20.10
11.00
11.30
11.50
29.7
33.50
5.0
1.50
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
3.1.2 Sag Test Result.
The data set in table 6 shows the sag test results across samples with varying CaCl2 concentration
from 15% to 35%. From the data, the fluid sample with 35% CaCl2 concentration showed the
least amount of sag, while the sample with 15% CaCl2 concentration showed the greatest amount
of sag. There was a 96% difference in sag between these two samples. The density of all fluid
samples was kept constant at 12ppg. As expected, it took more weighting material to achieve a
density of 12 ppg with a CaCl2 concentration of 15%, versus the fluid sample with a 35% CaCl2
concentration. The Sample with a CaCl2 concentration of 20% had sag of 0.249 ppg, sample
with a CaCl2 concentration of 25% had sag of 0.221 ppg, and sample with 30% CaCl2
concentration had sag of 0.098 ppg. There was 42% difference in sag between fluid samples
with 15% CaCl2 and samples with 20% CaCl2. The difference in sag between samples with 20%
and 25% CaCl2 was 11.2%.
Table 6: VSST Results
VST Results For Mud Samples
15%
20%
25%
30%
35%
Mud Weight
lb/gal
12.00
12.00
12.00
12.00
12.00
Temperature
⁰F
140.00
140.00
140.00
140.00
140.00
PV
YP
10-s Gel
cP
lb/100ft2
lb/100ft2
17.00
13.00
2.60
19.30
14.20
3.10
23.00
14.30
3.20
29.00
15.20
4.10
29.70
15.40
5.00
10-min Gel
lb/100ft2
0.40
0.90
1.00
1.10
1.50
VST Sag
lb/gal
0.44
0.25
0.22
0.01
0.02
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Page 20
Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
VST Sag
0.45
0.40
Sag (ppg)
0.35
0.30
0.25
0.20
15%
0.15
20%
0.10
25%
30%
0.05
35%
0.00
15
20
25
30
35
CaCl2 Concentration (%)
Figure 5: Sag in drilling fluids samples with varying salt concentration
3.1.3 Change in pH
Sag test results for samples with alteration in the pH shows changes in the rheological values
from the original samples. The addition of HCL to the samples increased the values of gel
strength and yield point however, more sag was recorded in the sample. There was fluid loss and
flocculation upon change of pH. The initial pH of the sample was 8. Upon addition of HCL to
the fluid, the rheological data change and fluid loss was observed. Further deterioration in mud
properties occurred as the pH was reduced further to 6 and then to 4. At a pH value of 2, the oil
base fluid had lost its emulsion property and broken down completely. Therefore results for pH
value at 2 were not included in this analysis. As the pH value was reduced, the sag in the drilling
fluid samples was increased. Detailed result of sag and rheological values are shown in Tables 8
and 9 respectively. Figure 6 compares the rheology of the fluid at 15% CaCl2 Concentration and
when the fluid pH value was reduced. Figure 7 shows the effects of pH change on sag tendency
in the drilling fluid samples at various concentrations.
When HCL was added to the mud samples, higher yield point and gel strengths values were
observed. However, flocculation and fluid loss was also observed in the samples. Flocculation is
Dare Ayoade
Page 21
Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
the formation of flocs due to attraction between positive and negative charges of the particles
resulting in the formation of larger particles. Goldberg and Glaubig studied the effect of pH on
flocculation of kaolinite and montmorillonite. Change in pH caused a charge reversal of the
particles, resulting in either dispersion or flocculation for montmorillonite and kaolinite
(Goldberg and Glaubig 1987). However, barite (BaSO4) showed similar results and charge
reversal with change in pH. Figure 7 shows sag in the fluid samples with change in pH. Changes
in the physical properties of the drilling fluid samples as pH is reduced are shown in figures 12,
13, & 14. The fluid samples with reduced pH could be treated with fluid loss additives and
surfactant to prevent fluid loss and the agglomerating of barite into large clusters. Perhaps upon
treatment, the samples with reduction in pH might have better suspension properties and low sag.
Further study needs to be carried out to verify this hypothesis.
Table 7: Rheological data of samples with change in pH
Rheology of Samples at pH of 6
600 rpm (lb/100ft )
15%
77.00
20%
25%
30%
84.50 85.00 86.00
300 rpm (lb/100ft2)
200 rpm (lb/100ft2)
100 rpm (lb/100ft2)
6 rpm (lb/100ft2)
3 rpm (lb/100ft2)
1 rpm (lb/100ft2)
PV(cP)
YP (lb/100ft2)
AV (cP)
10-s Gel (lb/100ft2)
10-min Gel (lb/100ft2)
45.80
36.00
22.80
6.80
6.50
6.30
31.20
14.60
38.50
1.00
0.30
50.50
41.00
24.90
8.80
8.20
8.20
34.00
16.50
42.25
1.40
0.30
2
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51.40
41.30
25.30
9.00
8.40
8.40
33.60
17.80
42.50
1.40
0.50
35%
86.90
52.00 52.7.00
43.00
45.00
26.00
29.00
9.00
11.30
8.50
10.80
8.50
10.80
34.00
34.20
18.00
18.50
43.00
43.45
1.70
2.00
0.70
0.90
Page 22
Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Shear Stress vs. Shear Rate
Shear Stress (lb/100ft sqr.)
90
pH of 6
80
70
60
pH of 8
50
40
ph of 8
30
pH of 6
20
10
0
0
200
400
600
800
1000
1200
Shear Rate (1/S)
Figure 6: Change in Fluid Rheology with addition of HCL
Table 8: Effect of pH on sag
Effect of pH on Sag.
Mud Weight
lb/gal
Temperature ⁰F
15%
12.00
20%
12.00
25%
12.00
30%
12.00
35%
12.00
140.00
140.00
140.00
140.00
140.00
31.20
14.60
1.00
0.30
34.00
16.50
1.40
0.30
33.60
17.80
1.40
0.50
34.00
18.00
1.70
0.70
34.20
18.50
2.00
0.90
PV
YP
10-s Gel
10-min Gel
cP
lb/100ft2
lb/100ft2
lb/100ft2
VST Sag
(pH6)
lb/gal
3.08
3.07
2.92
2.86
2.78
VST Sag
(pH4)
lb/gal
3.70
3.60
3.40
3.17
3.09
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Sag Tendecency of samples with reduced pH
4
3.5
4
6
4
6
6
3
Sag (ppg)
4
4
6
4
6
2.5
2
pH of 6
1.5
pH of 4
1
0.5
0
15
20
25
30
35
Salt Concentration %
Figure 7: Sag tendency with change in pH
3.1.4 Equivalent Circulating Density (ECD)
The equivalent circulating density (ECD) is the effective density exerted by a circulating fluid
against the formation that takes into account the pressure drop in the annulus above the point
been considered (Naganawa and Nomura 2006). Due to friction in the annulus as the mud is
pumped, bottom-hole pressure is slightly higher than when the mud is not being pumped.
The ECD is an important parameter in avoiding kicks and losses, particularly in wells that have a
narrow window between the fracture gradient and pore-pressure gradient. A simulator was used
to predict the annular pressure drop and calculate the ECD. The input data includes flow rate,
hole size, pipe size, yield point, plastic viscosity, and mud weight. The pipe’s outer diameter was
4.5 inches, hole inner diameter was 8.5 inches, and mud weight was maintained at 12 ppg for all
samples. The simulator predicts the flow pattern, wall shear stress, wall shear rate and pressure
gradient.
The ECD is calculated using the formula: 𝐸𝐶𝐷 = 𝑀𝑊 +
Dare Ayoade
𝐴𝑛𝑛𝑢𝑙𝑎𝑟 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑙𝑜𝑠𝑠𝑒𝑠
0.052∗𝑇𝑉𝐷
Page 24
Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Where MW is mud weight and TVD is true vertical depth.
Frictional pressure loss is affected by the following factors:

Length of the annulus

Annular Clearance

Mud Flow rate

Mud Rheology
From figure 8, the ECD changes with increase in salt concentration. The drilling fluid sample
with 15% CaCl2 concentration has the lowest ECD, while the sample with 35% CaCl2
concentration had the highest ECD. An increase in ECD of 0.65% was recorded when the salt
concentration was changed from 15% to 35%. Fluid samples with salt concentrations of 15%,
20%, 25%, 30% and 35% had ECDs of 12.29 lb/gal, 12.32 lb/gal, 12.33 lb/gal, 12.36 lb/gal and
12.37 lb/gal respectively. However, ECD can be managed by reducing the flow rate. The ECD
increased for each salt concentration with increase in flow rate and reduces with increase in
annular clearance. Figure 11 shows the effects of flow rate on ECD at each concentration.
ECD vs. Salt Concentration
12.38
12.36
ECD (PPG)
12.34
12.32
12.3
ECD
12.28
12.26
12.24
15
20
25
30
35
Salt Concentration (%)
Figure 8: Effect of salt concentration on ECD
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Similarly, ECD was also calculated for samples with change in pH at various salt concentrations.
At any given salt concentration, the ECD value for samples with reduction in pH was higher than
the ECD value in the original sample at the same salt concentration. For instance, the ECD value
for 15% CaCl2 concentration was 12.29 ppg in the original sample and 12.36 ppg when the pH
was reduced. The same trend was observed in all the samples at various salt concentrations. ECD
increased with reduction in pH. Figure 9 compares the ECD values in the original samples and
in the samples with reduced pH values.
ECD vs. Salt Concentration & pH
12.5
E CD (PPG)
12.45
12.4
12.35
pH of 8
12.3
pH 0f 6
12.25
12.2
15
20
25
30
35
Salt Concentration (%)
Figure 9: Effect of pH & Salt concentration on ECD
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Page 26
Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
3.2
Sag Test Comparison.
Analysis of the effect of pH change at various CaCl2 concentrations indicates an increase in sag
potential of the drilling fluid at 15% CaCl2 concentration by 85% when the pH value was
changed from 8 to 6. Drilling fluid sample with salt concentration of 35% recorded sag of
0.01ppg at a pH of 8 and a 99% increase in sag (3.09ppg) at a pH of 6. Samples with salt
concentrations of 20% and 25% both recorded 92% increase in sag.
However, further reduction in pH resulted in marginal increase in sag. Sag only increased by
16% when the pH value was reduced from 6 to 4. For concentrations of 20%, 25%, 30% & 35%,
the difference in sag as the pH value of the drilling fluid samples were reduced from 6 to 4
progressively reduced, with only a 14% increase in sag at 20% and 25% concentrations and a 9%
increase in sag potential at 35% CaCl2 concentration. The majority of the increase in sag was
recorded when the initial pH change occurred from 8 to 6. As soon as the fluid sample became
acidic, further decrease in pH had little increase in the sag of the drilling fluid samples. Figure 10
shows the sag of the oil base drilling fluid at different pH values.
Effect of pH on Sag
4
3.5
Sag (ppg)
3
2.5
2
ph of 8
1.5
Ph of 6
1
Phof 4
0.5
0
15
20
25
30
35
Salt Concentration %
Figure 10: Effect of pH on sag
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
4.0
Applications.
Barite sag is a concern in deviated wellbores and extended reach wells with narrow drilling
windows. For the sake of analysis, assume a deviated wellbore with a measured depth of 18,000
ft and a total vertical depth (TVD) of 10,000 ft, having a drilling safety margin of 200 psi
between pore pressure and mud weight pressure. Drilling the above mentioned well with a 12
lb/gal oil-base fluid will generate a bottom-hole well flowing pressure of 6514 psi at 10,000ft
(annular frictional pressure of 273.6psi). If the same well is drilled using the 15% CaCl2 oil-base
fluid sample generated in this experimental study, the bottom-hole pressure at 10,000 ft will be
6722 psi. The 15% CaCl2 sample with a sag of 0.44 lb/gal will generate a fluctuation in mud
density from 11.56 lb/gal to 12.44 lb/gal. This fluctuation in mud density results in a pressure
fluctuation of 228 psi. The drilling safety of 200 psi has already been exceeded with the sag of
0.4 lb/gal at 15% CaCl2 concentration. This will definitely result in well control issues, loss
circulation and other problems associated with barite sag. The 20% CaCl2 drilling fluid sample
with sag of 0.25 lb/gal will result in a pressure fluctuation of 130 psi while the 25% CaCl2
sample will give a pressure fluctuation of 115 psi. Low sags of 0.01 lb/gal and 0.02 lb/gal at 30%
and 35% CaCl2 will generate pressure fluctuation of 52psi and 10.4 psi respectively. Oil base
drilling fluids with 30% and 35% CaCl2 will be suitable for drilling through wells with narrow
drilling windows. This experimental study shows that increase in salt concentration will result in
less sag in the drilling fluid. In other words, the use of a solid free mud would greatly reduce
barite settling. However, it is cheaper to achieve mud weight by using a weighting material
rather than using salt.
Barite sag is a dynamic phenomenon that occurs under low shear in the annulus. Therefore shear
stress at 3rpm and 6 rpm are good indicators of the suspension properties of the drilling fluid.
However rheology alone (yield point, gel strengths and plastic viscosity) is not enough to predict
sag. By analyzing rheological data from the samples with lower pH values, less sag would have
been expected with change in pH. The change in fluid chemistry might have resulted in the
increase in sag recorded. This experimental study shows that drilling through acidic formations
could be problematic. The change in pH results in change of the rheological properties of the
fluid and increase in barite sag.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Pressure (PSI)
Depth
(ft)
10,000 ft
Pore Pressure
Mud Pressure
Measured Depth 18,000 ft
Fracture Pressure
5000 ft
Figure 11: Schematic of a deviated well
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Page 29
Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
5.0
Conclusion
1. These tests demonstrate that sag is a function of the mass of the weighting material. It is
possible to control sag by using smaller particles or particles with lower density (CaCl2).
However, if the formation stability permits the use of higher salt concentration, sag will
be significantly reduced with little to no effects on ECD.
2. The 15% CaCl2 oil-base fluid sample is not recommended for use in deviated wellbores
or wells with narrow windows. Typical salt concentrations used in the industry today
range from 25% to 30%. Salt concentrations of 30% to 35% are highly recommended.
3. Fluid rheology is a good indication of the sag tendency of a drilling fluid, but it cannot be
used as a litmus test for sag. It must be combined with an actual sag test.
4. Change in pH changes the rheology of the drilling fluid sample and causes charge
reversal of the particles resulting in increase in the settling of barite with reduction in pH.
Dare Ayoade
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Works Cited
Albertsen, Tron, Tor Henry Omland, Knut Taugbol, and Arild Saasen. "The Effect of The Synthetic- And
Oil Based Drilling Fluid's Internal Water Phase Composition On Barite Sag." IADC/ SPE Drilling
Conference. Dallas, 2004.
Annis, Max R, and Martin V Smith. "Drilling Fluids Technology." In Drilling Fluids Technology, by Max R
Annis and Martin v Smith, 64-72. Texas: Exxon Company U.S.A., 1996.
Bern, P A, et al. "Barite Sag: Measurement, Modeling and Management." IADC/SPE Asia Pacific Drilling
Conference. Jakarta: SPE, 1998.
Dye, W, M Brandt, T Wesling, T Hemphill, and R Greene. "Rheological Techniques to Minimize the
Potential for Barite Sag." Houston: AADE, 1999.
Goldberg, Sabine, and Robert A Glaubig. "Effect of saturating cation, pH, and aluminium and iron oxide
on the flocculation of kaolinite and montmorillonite." Clays and Clay Minerals 35, no. 3 (1987): 220-227.
Hanson, P M, G Rachal, and M Zamora. "Investigation of Barite Sag in Weighted Driling Fluids in Highly
Deviated Wells." SPE Annual Technical Conference & Exhibition. New Oreans: SPE, 1990.
Jachnik, R, and G Robinson. "Application of Rheological Models to Oil and Synthetic Based Drilling
Fluids." Nordic Rheology Society 4, no. 1 (1996): 12.
Naganawa, S, and T Nomura. "Simulating Transient Behaviour of Cuttings Transport Over Hole Trajectory
of Extended Reach Well." IADC/SPE Asia Pacific Drilling Technology & Exhibition. Bangkok: Society of
Petroleum Engineers, 2006.
Nguyen, Tan. Introduction to Drilling Fluids: PETR 311 Drilling Engineering Class Notes . New Mexico:
New Mexico Tech, 2011.
Nguyen, Tan, et al. "Combined Effects of Eccentricity And Pipe Rotation on Dynamic Barite Sag Analysis
of Different Impacts of Pipe Rotation In A Flow Loop & Rotation In A Modified Rotational Viscometer On
Barite Sag." National Technical Conference & Exhibition. New Orleans: American Association of Drilling
Engineers, 2009.
Omland, T H, A Saasen, C v.d Zwaag, and P A Amundsen. "The Effect of Weighting Material Sag on
Drilling Operation Efficiency." SPE Pacific Oil and Gas Conference & Exhibition. Jakarta: SPE, 2007.
Saasen, A, D Liu, C D Marken, N Sterri, G W Halsey, and P Isambourg. "Prediction of Barite Sag Potential
of Drilling Fluids from Rheological Measurements." SPE/IADC Drilling Conference. Amsterdam: SPE,
1995.
University of Texas at Austin. A Dictionary For The Petroleum Industry. Austin: Petroleum Extension
Services, 2006.
Dare Ayoade
Page 31
Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Zamora, Mario, and Reginald Bell. "Improved Wellsite Test for Monitoring Barite Sag." AADE Driling
Fluids Technical Conference. Houston: AADE, 2004.
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
APPENDIX
API RECOMMENDED TESTS AND RESULTS
Density: the density of the mud was measured at 10.4ppg at 110˚F. 127.5 grams of barite was
added to achieve this mud weight
Viscosity & Gel strength: the apparatus used for this test was the direct indicating viscometer.
The following results were obtained:
R600 = 48˚
R300 = 26˚
10 seconds gel strength = 8(lbs/100ft2)
10 minutes gel strength = 15(lbs/100ft2)
Plastic viscosity = R600 - R300 = 22cp
Yield point YP = 0.48 (R300 – PV) = 1.92 Pascal
YP = R300 – PV = 4 (lbs/100ft2)
Apparent viscosity = R600 /2 = 24 cp
Retort Test
There are two methods to carry out the retort test. They are the volumetric method and the
gravimetric method. Using the volumetric method, the following results were obtained;
Water volume = 1.4 ml
Oil volume = 6.8 ml
% of oil by volume = 68%
% of water by volume = 14 %
% of solids by volume = 100% - (%H2O + % Oil) = 18%
Solids % by weight = 45.19%
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Low gravity solids & weighting material concentration
Grams of oil = 5.44g
Grams of water = 1.4g
Grams of solid = 6.54g
Ml solids = 1.8ml
Average specific gravity of solids = 3.133
Solids % by weight = 45.19%
High gravity solids % by volume = 35.19%
Low gravity solids % by volume = 64.8%
ECD vs. Flowrate
40
35
ECD (PPG)
30
25
15% CaCl2 Conc.
20
20% CaCl2 Conc.
15
25% CaCl2 Conc.
10
30% CaCl2 Conc.
5
35% CaCl2 Conc.
0
0
200
400
600
800
Flow Rate (GPM)
Figure 12: Effect of flow rate on ECD
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Page 34
Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
15%
Average
Velocity
Q
m/s
(GPM)
200
0.4789
300
0.7183
400
0.9578
500
1.1972
600
1.4366
700
1.6761
Pressure
gradient
Reynolds’s wall
no.
shear
stress
lb/100ft2 1/s
Pa/m
342.8366
370.8709
396.4882
420.6358
443.7889
404.1306
wall
shear
rate
454.3898
945.0952
15716.61
2314.673
3159.227
4093.119
8.708
9.4201
10.0708
10.6841
11.2723
10.2649
146.097
187.9837
226.2588
262.3383
296.9331
330.457
Pressure ECD
gradient
psi/ft
0.0152
0.0164
0.0175
0.0186
0.0196
0.0179
ppg
12.29231
12.31538
12.33654
12.35769
12.37692
12.34423
Table 9: ECD at 15% salt concentration
20%
Average
Velocity
Q(GPM) m/s
200
300
400
500
600
700
0.4789
0.7183
0.9578
1.1972
1.4366
1.6761
Pressure
gradient
Reynolds’s wall
no.
shear
stress
lb/100ft2 1/s
Pa/m
377.0317
408.5186
437.3286
464.5144
490.6042
425.2111
wall
shear
rate
413.1787
857.9984
1424.846
2096.026
2857.769
3698.983
9.5766
10.3764
11.1081
11.7987
12.4613
10.8004
143.9193
185.358
233.2739
259.0521
293.388
326.6851
Pressure ECD
gradient
psi/ft
0.0167
0.0181
0.0193
0.0205
0.0217
0.0188
ppg
12.32115
12.34808
12.37115
12.39423
12.41731
12.36154
Table 10: ECD at 20% salt concentration
25%
Average
Velocity
Q(GPM) m/s
200
300
400
500
600
700
0.4789
0.7183
0.9578
1.1972
1.4366
1.6761
Pressure
gradient
Reynolds’s wall
no.
shear
stress
lb/100ft2 1/s
Pa/m
391.7393
427.6039
460.6141
491.909
522.0568
458.6517
wall
shear
rate
397.6662
819.7032
1352.816
1979.298
2685.595
3460.955
9.9502
10.8611
11.6996
12.4945
13.2602
11.6498
134.9276
174.5347
210.9897
245.5499
278.8436
311.2335
Pressure ECD
gradient
psi/ft
ppg
0.0173 12.33269
0.0189 12.36346
0.0204 12.3923
0.0217 12.4173
0.0231 12.4423
0.0203 12.39038
Table 11: ECD at 25% salt concentration
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
30%
Average
Velocity
Q(GPM) m/s
200
300
400
500
600
700
0.4789
0.7183
0.9578
1.1972
1.4366
1.6761
Pressure
gradient
Reynolds’s wall
no.
shear
stress
lb/100ft2 1/s
Pa/m
431.0726
474.3931
514.5149
552.7354
589.6992
625.775
wall
shear
rate
361.0726
738.8562
1211.094
1761.483
2377.54
3049.535
10.9492
12.0496
13.0687
14.0395
14.9784
15.8947
126.6029
164.5458
199.6868
233.1628
265.5378
297.1354
Pressure ECD
gradient
psi/ft
0.0191
0.021
0.0227
0.0244
0.0261
0.0277
ppg
12.36731
12.40385
12.43654
12.46923
12.50192
12.53269
Table 12: ECD at 30% salt concentration
35%
Average
Velocity
Q(GPM) m/s
200
300
400
500
600
700
0.4789
0.7183
0.9578
1.1972
1.4366
1.6761
Pressure
gradient
Reynolds’s wall
no.
shear
stress
lb/100ft2 1/s
Pa/m
437.7471
481.9977
522.9973
562.0659
599.8592
636.7523
wall
shear
rate
355.871
727.1991
1191.452
1732.242
2337.27
2996.963
11.1188
12.2427
13.2841
14.2765
15.2364
16.1735
126.1028
163.9469
199.0104
232.4228
264.7443
296.2959
Pressure ECD
gradient
psi/ft
0.0194
0.0213
0.0231
0.0248
0.0265
0.0281
ppg
12.37308
12.40962
12.44423
12.47692
12.50962
12.54038
Table 13: ECD at 35% salt concentration
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Figure 13: Fluid Loss in Drilling Sample with reduction in pH
Figure 14: Sag in base fluid sample.
Dare Ayoade
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Effect of pH and Salt Concentration on Barite Sag in Oil Base Drilling Fluids
Figure 15: Sag in sample with reduction in pH
Dare Ayoade
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