Physical and Chemical Analysis of Drilling Fluid Properties
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Transcript of Physical and Chemical Analysis of Drilling Fluid Properties
Bachelors of Applied Science in Petroleum Engineering
2015
Year 3
Physical and chemical analysis of drilling fluid properties
Course Title: Drilling Engineering
Course Code: DRLG3001
Submitted to: Jasmine Medina
Submitted by:
Andrew Grant
ID#:65188
Lab day: 13th October 2015
Due date: 22nd October 2015
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Executive Summary
This laboratory experiment was mainly evaluating the physical and chemical properties of
drilling fluids. Six test were conducted to ascertain and correlate drilling fluid properties to
their performance. As such, identifying the types of contaminants present in water based
drilling fluids were of paramount importance for recommending the relevant treatments that
were applicable for right type of contaminant. Contaminants identified were: calcium
carbonate, oil, sodium chloride and the recommended chemical treatments were soda ash,
caustic soda, gypsum and flocculation. Additionally for removal of other contaminants by
mechanical means, the following treatments were subscribed: Screen, forced settling and
dilution. These treatments are available and are widely used in the hydrocarbon industry in
order to optimize drilling operations while simultaneously reducing operational cost without
adversely affecting the environment.
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Objective/Aim(s):
To determine the density and the rheological properties of original sample A and
contaminated mud samples: B, C, D, and O, using mud balance and viscometer
apparatus.
To separate and measure the volumes of water, oil, and solids contained in both
original and contaminated (samples as stated above) via retort analysis.
To ascertain the percentage of sand content of water based drilling fluids (both
original and contaminated samples) by utilizing the sand content funnel, tube, sieve-
mesh @ 75µm and 15% hydrochloric acid solution (HCl).
To determine the filtration behaviour and wall-cake-building characteristics of the
drilling fluid samples given, at low temperature and pressure using an API LPLT filter
press.
To perform chemical analysis of water based drilling samples, for determination of
the following:
o Filtration pH – using pH strips.
o Whole mud alkalinity – titrating with N/50 Sulfuric acid and using
phenolphthalein solution as indicator.
o MBT and, bentonite equivalent – using 0.5 mL of methylene blue
solution.
o Calcium carbonate concentration (CaCO3) –using 2mL of 1.0N
Versenate Hardness Buffer Solution, Calver 11 solution as indicator
and 20 Epm versanate hardness titrating solution.
o Calcium concentration –using Versenate hardness buffer solution,
Versenate hardness indicator solution and 0.02N EDTA Versenate
hardness titrant solution.
o Chloride ion content –using 0.02N (N/50) sulphuric acid, potassium
chromate indicator solution (K2CrO4), and 0.0282N Silver Nitrate
Solution (AgNO3).
Sodium Chloride and potassium chloride content using phenolphthalein indicator
solution, 0.02N Sulphuric acid solution, potassium chromate indicator solution, 0.282N
silver nitrate solution, standard sodium perchlorate solution and a hand crank centrifuge.
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Theory:
Background information:The successful completion of an oil well and its cost depends on considerably on the
extent of the properties of the drilling fluid. Many requirements are placed on the drilling
fluid. In the past, main purpose of the drilling fluid was to serve as a vehicle to remove
cutting from the well bore, however in recent times, the applications of drilling fluids has
been more diversified (Gray, Caenn and Darley 1983). Hence in rotary drilling, the
principal functions performed by the drilling fluid includes the following:
Carry cuttings from beneath the bit, transport them up the annulus, and permit their
separation at the surface.
Cool and clean the bit.
Reduce friction between the drilling string and the sides of the hole.
Maintain stability of uncased sections of the borehole.
Prevent the inflow of fluids – oil, gas, or water – from permeable rocks that were
penetrated.
Form a thin and relatively impermeable filter cake which seals pores and other
openings in formations penetrated by the bit.
Assist in the collection and interpretation of information available from drill cuttings,
cores, and electrical logs.
Drilling fluids are categorized in accordance to their base .i.e. water based and oil based
muds. Water based muds are consist of solid particles suspended in water or brine. In
some cases, oil may be emulsified in water, in these cases water is considered as the
continuous phase. Whereas, oil based muds comprise of solid particles suspended in oil.
If water or brine is emulsified in oil then the oil is considered to be the continuous phase.
Another type of drilling fluid is gas. This is where drill cuttings are removed by a high
velocity stream of air or natural gas. Foaming agents are added to remove minor inflows
of water.
In water based muds, the solids consist of clays and organic colloids added to provide
the required viscous and filtration characteristics, heavy minerals (generally barite are
added to increase density when needed) and solids from the formation that become
dispersed in the mud in the course of drilling. The water contains dissolved salts either
from contamination with formation waters or purposely added for any number of reasons.
The following sections are brief introductions of the six diagnostic tests that will be
performed on the drilling fluid samples.
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Density and Rheological properties:
The density Drilling fluid must be maintained to provide the required hydrostatic head to
prevent flux of formation fluids, but not so high as to cause loss of circulation or
unfavourably affect the rate of drilling and formation damage. Consequently, one of the
first test to be performed on a drilling rig is mud weight or density.
Figure 1 showing typical diagram of mud balance (Bourgoyne Jr., et al. 1986)
In this experiment, the density and rheological properties of the original and
contaminated samples is being performed. The apparatus used to conduct this test was
the mud balance (shown in figure 1 above). The test consists of essentially of filling the
cup with a mud sample and determining the rider position required for balance. The
balance is calibrated by adding lead shot to a calibration chamber at the end of the
scale. Water usually is used for the calibration fluid. The density of fresh water is 8.33
lbm/gal. The drilling fluid is normally degassed before being placed in the mud balance to
ensure an accurate measurement (Bourgoyne Jr., et al. 1986).
In this section of the experiment, rheological properties of the drilling samples will be
measured using the rotational viscometer. Viscometer, measures viscosity quantitatively,
whereas the marsh funnel measures qualitatively in terms of determining drilling mud
consistency. Mud is sheared at a constant rate between the inner bob and an outer
rotating speed sleeve. Six standard speeds and a variable speed setting are available on
the viscometer. The dimensions of the bob and rotor are chosen so that the dial reading
is equal to the apparent Newtonian viscosity in centipoise at a rotor speed of 300 rpm. At
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other rotator speeds, the apparent viscosity is given by μa=
300θN
N , where θN, is the
dial reading in degrees and N is the rotor speed in revolutions per minutes. The
viscometer could also determine rheological parameters that exhibit non-Newtonian fluid
behaviour for example, the flow parameters of Bingham plastic model as shown in figure
2 below.
Figure 2 showing Newtonian and Non-Newtonian curves (King Fahd University of Petroleum
and Minerals, 2003)
Retort Analysis:
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Figure 3 above shows the retort distillation apparatus consisting of three principal
components: a heating unit, a condenser and a receiver. The heating unit, is used to
bombard the reservoir rock sample with extreme heat. Rock samples can either be
crushed or small cylindrical core plugs in dimensions... These rock samples, either
consolidated or non-consolidated, are generally weighed before placing them in the
retort. Heat is dispensed at either in stages or directly to temperatures as high as 650 0 C
resulting in the vaporization of oil, and water. This vaporized oil and water, is then
condensed in the condenser and collected in a small receiving graduated cylinder, where
the volumes of oil and water can be measured directly. No further extraction of pore
fluids (K, 2006) are indicated by the presence of a horizontal plateau in the plot of
collected oil and water volume vs. the heating times.
Sand content of water based drilling fluids:
According to Baroid Incorporated (2015), measurement of the sand content of mud
should be made regularly, because excessive sand makes a thicker filter cake, this in
turn causes abrasive wear of pump parts, bit and pipe, may also settle when circulation
is stopped and interfere with pipe movement or settling of casing. Sand content (API)
method is defined as the percentage by volume of solids in the mud that are retained on
a 200-mesh sieve. Below shows a table that defines and characterize sieve sizes for
different types of sand.
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Figure 4 below shows the standard API sand sieve that will be used for determination of
sand content in water based drilling fluids (Gray, Caenn and Darley 1983).
API fluid loss:
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Fluid loss is usually termed as the loss of a mud filtrate (liquid phase) into a permeable
formation that is being drilled. Because of positive differential pressure (i.e. the pressure
difference between the mud pressure in the wellbore and the formation pore pressure),
the mud filtrate tends to flow into the formation; Consequently, this creates a an
accumulation of mud solids deposited on the wellbore walls, thus forming what is
generally referred to as mud cake (filter cake). Furthermore, initial loss of filtrate to the
formation at time zero is termed as initial spurt loss. After a mud cake is formed, the
presence of any loss of filtrate is categorized as the continuous loss (Azar & Samuel,
2007).
In the hydrocarbon industry, there are two types of filtration involved in drilling an oil well:
static filtration and dynamic filtration. Static filtration occurs when the mud is being not
being circulated and filter cake growth is undisturbed. However, dynamic filtration occurs
when the mud is circulated and growth of the filter cake is limited by the erosive action of
mud stream. The filtration of properties of drilling fluids are generally evaluated and
controlled by the API filter loss test which is a static test. However, because a static test
being performed, this is not a reliable guide to the downhole filtration which is usually
dynamic (Gray, Caenn and Darley 1983).
Chemical Analysis
In order to determine the concentration of various ions present in drilling fluids, a wide
array of chemical analyses will be performed. These tests include determination for OH -,
Cl-, and Ca2+, which are required to complete the API drilling mud report form.
Furthermore, a titration apparatus is used to conduct these type of tests. Titration
involves the reaction of a known volume and concentration. The concentration of ion to
be tested will be determined from knowledge of the chemical taking place (Bourgoyne
Jr., et al. 1986).
Experience has shown that certain chemical analyses are useful in the control of mud
performance, for example, an increase in chloride content may adversely affect the mud
properties unless the mud has been designed to withstand contamination by salt. Those
analyses that have been found to be adaptable to use in the field have been included in
API RB 13 B (Gray, Caenn and Darley 1983).
Salt Analysis- determination of sodium and potassium chloride content.
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A sample of mud filtrate (neutralized, if alkaline) is titrated silver nitrate solution, using
potassium chromate as indicator. The results are usually reported in parts per million
chloride ion, although actually measured in terms of mg Cl- ion per 1000cm3 of filtrate. In
order to determine the chloride content of an oil mud, the sample will be diluted with a
mixture of Exosol and isopropyl alcohol (3:1) and diluted with water, neutralized to the
phenolphthalein end point and then titrated the usual way (Gray, Caenn and Darley
1983). However, in this laboratory session only the salinity of water based mud samples
will be examined.
Procedure:
As per lab manual
Results:
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SampleDensity (lb/bbl)
Viscometer Speed/RPM 75°F 120°F 75°F 120°F 75°F 120°F 75°F 120°F 75°F 120°F 75°F 120°F600 40 33 61 58 148 135 45 50 43 35 40 33300 25 22 52 51 102 95 40 49 27 22 25 22200 20 18 48 48 82 75 37 49 22 20 20 18100 13 12.5 44 45 54.5 50 31 48 13 13 13 12.5
6 4 5 35 35 18 13 31 42 5 6 4 53 4.6 5 31 26 16 18 31 42 3 8 4.5 5
Pv 15 11 9 7 46 40 5 1 16 13 15 11Yp 10 11 43 44 56 55 35 48 11 9 10 11
10 second gel 3 3.5 5 34 27 11 17 20 74 410 minute gel 4.5 7 35 25 25 30 30 66 6 10
5 min7.5 min10 min15 min20 min30 min
FC Properties
Filtrate pHPmPf
Chlorides, mg/lCalcium, mg/l
MBTBentonite Eq., lb/bbl
CaCO3, lb/bbl
% solids b/f acidization% solids after acidization
% sand
% Oil% Water% Solids
% CST% NaCl
Contaminant
Sand Content
Retort Analysis
Salts
5.26.27.1
0.2004
28.50
8.610.212.4
Soft and Pliable
80.6
35.632.1
9
Orignal9
A7.7
B9.25
Rheological Properties
Chemical Analysis
34.714
C8
D9.2
O9.25
17.419.822.427.730.837
8/32
4.2
10
55.86.67.88.810.6
4.85.56.67.69.2
5.46.27.28.69.611.7
0.8 1
3/32Soft and Pliable '4/32
32.0612.95
1.10.150.30.65
0.6
0.81.5
-
991
1.78020
960.2
4
2/32
3.74.5
28.53.5
100.81.20
0.27
49.90.7
100.50.2
4
107416
0991
8020
960
7416
0982
982
0 2NaCl
0.3Bentonite
019901.5
Pure0
Oil1.5
CaCO3
2321.50
141.42
6193
2276.2
Table 1 above showing results for six experiments performed on drilling fluid samples (A-D).
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0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 00
5
10
15
20
25
30
35
40
f(x) = 2.0869695760317 x + 0.679211299935657
f(x) = 1.74326822765552 x + 0.454079029029487
f(x) = 1.58280146243316 x + 0.125606921247678
f(x) = 1.27002245409114 x + 0.312899083410306
f(x) = 1.12762362029637 x + 0.178339151665386
Graph 1 showing spurt loss V vs √tSample B Linear (Sample B) Sample CLinear (Sample C) Original Sample Linear (Original Sample)Sample D Linear (Sample D) Sample ALinear (Sample A)
√t
Volu
me
filtr
ate
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Calculations:
The following are sample calculations for each experiment conducted.
Using data from original sample for all sample calculations, except where specified.
Density and Rheological Properties
PV (Plastic viscosity, (lbs/100ft2)/300rpm) = θ600-θ300
YP (yield point in lbs/100ft2) = θ300 – PV
PV=θ600−θ300=40−25=15 lbs /100 ft 2 /300rpmYP=θ300−PV=25−15=10lbs /100 ft2
Retort Analysis
Volume Percent (%) Oil = Vo =
100 (Oil volume collected, mL)Sample Volume, mL
Volume Percent (%) water=V w=
100 (Water volume collected, mL )Sample Volume, mL
Volume Percent (%) Solids =Vs=100−(Vo+Vw )
⇒Vo=100(0 )10
=0 %
⇒Vw=100(0 . 99)10
=99 %
⇒Vs=100−(0+99)=1%
Chemical Analysis
Methylene Blue Capacity (MBT) = methylene Blue, mL/Drilling fluid, mL
Bentonite equivalent, lb/bbl = 5 (Methylene Blue, mL)/Drilling Fluid, mL
⇒MBT=8ml2. 0ml
=4
⇒5 (4 )/0 .71=28 lb /bbl
Volume (mL) titrating solution) * (3.5) = lb/bbl calcium carbonate
For sample B, titrating solution = 1.8 mL
13The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
1 .8∗3 .5=6 .3lb /bbl
Total hardness for sample B as calcium, mg/I= 400 x (VEDTA/VS)
Where VEDTA = volume of EDTA solution, mg/I
Vs = Volume of sample, mL
mg / I=400∗1 cm3
1cm3 =400
NaCl- Determination
c[Cl] = 10000 x (Vsn/Vf)
Where Vsn = the volume of silver nitrate solution, ml
Vf = the volume of filtrate sample, ml
c[Cl] : 10000 x {12.1/1} = 121000 mg/I
∴NaCl=1. 65∗12100=199 ,650mg /I
Conversion of Mg/I to weight percent and PPM at 68 0 F gives ≈180,000 ppm.
Using the graph of NaCl weight% against NaCl mg/I and 180,000ppm gives 15 wt% of NaCl.
KCl determination
c[KCl], ppb = (7/Vf) x (x-axis value from standard curve, ppb)
c[KCl], ppb = (7/0.65) x 11.6 = 124.92 ppb
c[K+]+ = 1500 x c[KCl], ppb
c[K+]+ = 1500 x 124.92 = 187,384.61
y ml KCl ppt = 0.0393 x (lb/bbl ppt ) + 0.2042
y ml KCl ppt = 0.393 x 11.6 + 0.2042 = 4.763 lb/bbl
14The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
Discussion:
Density and Rheological properties:
Density or mud weight, was determined by weighing a precise volume of mud and dividing it
by the volume. The mud balance was the instrument utilized to obtain the density for both
original and contaminated mud samples. To date in the petroleum industry, the mud balance
provides the most convenient way of obtaining a precise volume. The procedure that is
normal used on a drilling rig, is to fill the cup with mud, put on the lid, wipe off the excess
mud from the lid, move the rider along the arm until a balance is achieved and the density
was read at the side of the rider towards the knife edge (Gray, Caenn and Darley 1983).
Density could be expressed in pounds per gallon (lb/gal), pounds per cubic foot (lb/ft3), and
grams per cubic centimetre (g/cm3) or as a gradient exerted per unit depth.
As shown in table one, the density for the original sample was recorded at 9.0 ppg, whereas
samples A to O densities were recorded at: 7.7, 9.25, 8, 9.2 and 9.25 lb/bbl respectively. The
disparity in densities between the original mud sample and the contaminated mud samples
could be attributed to the following:
Contaminated samples may have excess API barite. API barite is a dense, inert
mineral having a specific gravity of approximately 4.2 and this could be added to any
clay / water mixture to increase density (Bourgoyne Jr., et al. 1986).
Furthermore, the contaminated samples could also contain inert solids. These solids
are termed inert, because they do not hydrate with other components of the mud.
Inert solids are generally classified as sand, silt, limestone, feldspar and also API
barite. In this experiment mud samples B, D and O have higher densities than the
original mud sample. It was observed from table one above, that these samples have
relatively more percentage sand content than the original sample. This observation
was supported by the presence of calcium carbonate in samples B, D and O. When
these samples were in the same mixture containing hydrogen chloride, they
effervescence and their sand content after acidification was reduced. This reaction
was not observed in the original mud sample.
When inert solids such as sand are present in drilling fluids, they adversely affect the
functionality of the drilling fluid; such as, they may increase the frictional pressure drop, in
the fluid system, but they do not greatly increase the ability to carry the rock cuttings to the
surface. The filter cake formed from these solids is thick and permeable rather than thin and
relatively impermeable. Consequently, delay in drilling activities arises dud to stuck pipe,
15The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
excessive pipe torque and drag, loss of circulation and poor cement bonding to the formation
(Bourgoyne Jr., et al. 1986).
In addition excessive mud density due to inert solids could possibility increase the
hydrostatic pressure on the borehole walls so much so that the hole fails in tension. This
failure is known as induced fracturing. This phenomenon is where mud is lost into the facture
that formed and the level of the annulus falls until equilibrium conditions are obtained.
Another disadvantage of excessive mud densities is their adverse influence on rate of
penetration. This occurrence have been proven by laboratory experiments and field
experience that in the event of mud overbalance especially drilling in very low permeability
rocks, the rate of penetration is significantly reduced. Also, a high overbalance pressure
increases the probable risk of sticking the drill pipe. Finally, these aforementioned problems
that could arise from the presence of inert solids in drilling fluids causes unnecessary drilling
costs and overruns. To date, excess concentration of inert solids in drilling muds can be
reduced to a desirable levels by: screening, forced settling, chemical flocculation and
dilution.
Rheological properties
The rotational viscometer was used to measure the rheological characteristics of the mud
samples prepared. The mud was sheared at a constant rate between an inner bob and an
outer rotating sleeve. The viscometer, was also used to determine rheological parameters
that described Non-Newtonian fluid behaviour. Two flow parameters that were required to
characterize the mud samples that follow the Bingham plastic model were plastic viscosity
and yield point. The plastic viscosity, cP, in centipoise was computed using: cP = θ600-θ300
where θ600 was the dial reading with the viscometer operating at 600 rpm and θ300 was equal
to the dial reading with the viscometer operating at 300 rpm (Bourgoyne Jr., et al. 1986). The
shear stress divided by the shear rate (at any given rate of shear) is known as the effective
or apparent viscosity. Effective viscosity decreases with the increase of shear rate, and was
therefore a valid parameter for hydraulic calculations only at the shear rate at which it was
measured.
Moreover, the decrease in effective viscosity with increase in shear rate is known as shear
thinning, and normally this is a desirable property, because of the effective viscosity would
be relatively low at the high shear rates prevailing in the drill pipe, thereby reducing pumping
pressures, and relatively high at low share rates prevailing the annulus, thereby increasing
cutting carrying capacity.
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The fact that the consistency curves (illustrated in graph 1) of clay muds intercept at the
stress axis (i.e. y-axis), at a greater value that zero was indicative of gel structure
development.
Clay particles in drilling fluids are highly anisodimensional and can build a structure at very
low solid concentrations, because of interaction between attractive and repulsive forces. At
low shear rates the behaviour of clay particles was influenced by these forces and as a
result, the particles viscosity were relatively high, but as shear rate increases, the particles
gradually align themselves in the direction of flow and the viscosity then becomes largely
dependent on the concentration of all solids present in the mud. This phenomena was
observed for all drilling sample fluids; because of these occurrences, the degree of deviation
from linearity in the Bingham plastic consistency curves (as shown in graph one above) of
drilling muds differs from mud to mud in the rotary viscometer and this was depended on
particle size and shape, and concentration of bentonite (Gray, Caenn and Darley 1983).
This phenomenon directly affected the filter cake properties developed by the samples in the
lab as seen in table1. This type of behaviour was observed with samples with low solid muds
containing a high proportion of clay particles and high solid muds such as barite.
Unfortunately, it is highly challenging to determine the linearity of the consistency curves,
other than by measurement in a multispeed rotary viscometer. In practice the most widely
use of the PV and YP quantities is for the evaluation of drilling mud performance and is used
as a guide for drilling mud treatments. Thus PV is sensitive to the concentration of solids and
this is indicative of dilution requirements; YP is sensitive to the electrochemical environment,
and hence indicates the need for chemical treatment (Gray, Caenn and Darley 1983).
Usually, the consistency curve of a Bingham plastic in a rotary viscometer should be linear at
rotor speeds above that required to keep all the fluid in the annulus in laminar flow. In reality
however, drilling fluids are not ideal Bingham plastics and as such they deviate from linearity
at low shear rates.
A third non-Newtonian rheological parameter called gel strength, in units of lbf/100 sq ft2 was
obtained by noting the maximum deflection when the rotational viscometer was turned on at
a low rotator speed of 3 rpm. Gel strength was termed as observing the maximum deflection
before the gel breaks. Gel strength for all the samples were measured after allowing the mud
to stand quiescent for 10 seconds, the maximum dial deflection obtained when the
viscometer was turned on was the initial gel strength. The gel strength of fresh water clay
muds, increases with time after agitation has ceased, a phenomenon called thixotropy.
Furthermore, after standing quiescent the mud was subjected to a constant rate of shear, its
viscosity decreases with time as its gel structure was broken up, until an equilibrium viscosity
17The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
was reached. Thus the effective viscosity of a thixotropic mud was time dependent as well
as shear-dependent.
Retort Analysis
The mud samples were placed in a steel container and were heated (approximately 516 +/-
22 0 C) until the liquid was vaporized. The vapours passed through the condenser and were
collected in a graduated cylinder. The volumes of the respected samples were measured
and then converted to a percentage based on the volume of whole mud in the retort cup.
Volume percent of solids (Vs) = 100- (V0 + Vw)............................................... 1
From equation one above, the solids both suspended and dissolved, were equal to 100%
minus the liquid percent. This procedure also gave the percent of oil in the mud sample.
Consequently, it was found that sample C was the only sample contaminated with oil. The
mud samples were then subjected to further tests to elucidate the nature of contaminated oil
content.
The retort procedure was a very rapid and simple technique, however, the retort distillation
method has notable disadvantages. Firstly, the rock samples were completely destroyed and
secondly, high temperatures were required. However, the application of extreme heat was
unavoidable because, oil in the reservoir rock samples contained very high molecular weight
or high boiling point substituents. Consequently, the application of very high temperatures
was essential to ensure that all the oil was completely extracted from the rock samples (K,
2006). Using elevated temperatures of this magnitude resulted in the following errors:
At such high temperatures, the water of crystallization within the rock was driven off,
causing the water recovery values to be greater than the pore water (K, 2006).
High temperatures also may fracture and the coke in the oil causing the collected oil volume
not to correspond to the volume of oil initially in the rock sample. The cracking and coking of
the hydrocarbon molecules, may likely to reduce the liquid volume and also in some cases
may also coat the internal walls of the rock sample itself. The water of crystallization and the
cracking and coking of hydrocarbons was quantified in Emdahl based on the core analysis of
Wilcox sands in which fluid saturations were measured by the retort distillation method,
indicating an error of around 33% in the water saturation with the volume of oil recovered
and the volume of oil in the sample varied due to V oil actually in the sample = 1.2198 (V0.859
oil collected in receiver)..........................2.18
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Equation 2 indicates that the volume of oil recovered or collected in the receiver was
decreased due to cracking and coking of the hydrocarbon molecules (K, 2006).
In addition to these errors, other practical errors could also occur in the retort distillation
method, such as formation of oil-water emulsions that do not allow accurate volume
measurements and the absence of clear demarcation between the plateaus of pore space
water and the water of crystallization which could introduce uncertain measurement of water
volume.
Sand content and water based drilling fluids
The sand content test was a measure of the amount of particles larger than 200 mesh
present in mud samples. Effectively this test defines the size and not the composition of the
particles (Gray, Caenn and Darley 1983). The mud samples were first subject to dilution by
adding mud and water to the respective marks inscribed in the glass tube. The mixture was
then shaken and poured through the screen in the upper of cylinder, and then washed with
tap water until clean. The substance that remain on the screen was then backwashed
through the funnel into the glass tube and allowed to settle and finally the gross volume was
read from the gradulations on the bottom of the tube.
As mentioned earlier, the presence of excessive sands in drilling fluids have direct
catastrophic problems on drilling operations of a well and as such there were four methods
that could be employed to prevent a high concentration of inert solids. These were:
Screening. This method is usually applied first in processing the annular mud stream.
This allows the removal of most of the solids before their size has been reduced to
the size of the API barite particles. API specifications for commercial barium sulphate
require that 97% of the particles pass through a 200-mesh screen. Particles less than
approximately 74µm in diameter will normally pass through the 200-mesh screen.
Forced Settling. When natural settling failed to screen out inert particles, devices
such as hydroclones and centrifuges are utilized to increase the gravitational force
acting on the particles. At present, both devices are used as forced settling
instruments with unweighted muds (Bourgoyne Jr., et al. 1986).
Chemical flocculation. The removal of fine active clay particles could also be used by
adding chemicals that cause the clay particles to flocculate or agglomerate into larger
units. Once agglomeration of fine clay particles have been achieved, separation can
be facilitated more easily.
Dilution. This method requires discarding a portion of additives used in previous mud
treatments.
19The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
API fluid loss
The API fluid loss test was used to determine the static filtration characteristics of the mud
and the need for treatment with fluid loss additives (only used for water based muds). The
filter press was used to determine, the filtration rate through a standard filter paper and the
rate at which the mud-cake thickness increases on the standard filter paper under standard
test conditions. This test was indicative of the rate at which permeable formations were
sealed by the deposition of a mud-cake after being penetrated by the bit.
If unit volume of a stable suspension of solids was filtered against a permeable substrate,
and x-volumes of filtrate were expressed, then 1-x volumes of cake (solids plus liquid) would
be deposited on the substrate. Therefore if Qc be the volume of the cake and Qw the volume
of the filtrate:
QcQw
= 1−xx
and the cake thickness (h) per unit area of cake in unit time would be
h=1−xx
∗Qw
However, Darcy’s law stated that:
dqdt
= KPμh
Therefore
dqdt
= KPμQw
∗ x1−x
Integrating
Qw2=2 KPμ
∗ x1−x
∗t
Then substituting:
20The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
Qw2=2 KPμ
∗QwQc
∗t If the area of the filter cake was A, then
Qw2=2 KPA2
μ∗QwQc
∗t This is the fundamental equation governing filtration under static
environment.
According to Bourgoyne et al (1986) the filtrate volume should be proportional to the square
root of the time period used. Thus, the filtrate collected after 7.5 minutes should
approximately be half the filtrate collected in 30 minutes. This phenomenon was observed
for all drilling fluid mud samples. . In order to determine if a significant spurt loss of volume of
filtrate was observed for each of the mud samples the volume of filtrate collected vs square
root of time (√t) was plotted on a graph (see graph 1 above). The spurt loss was determined
by extrapolation and the following equation was utilized: V30 = 2(V7.5-Vsp) + Vsp . Spurt loss of
volume of filtrate Vsp was often observed before the porosity and permeability of the filter
cake stabilizes (Bourgoyne Jr, Chenevert, Millheim, & Young Jr, 1984). As of consequence,
the API cake thickness differ for each mud sample.
Cake permeability: The higher the cake permeability the higher the fluid loss (Azar &
Samuel, 2007). That is, the more interconnected pore space a mud has, the higher its
effective porosity, as of consequence the higher its fluid loss. It was inferred from
observation that both original sample and sample B were more permeable and thus a higher
fluid loss relatively to the other mud samples.
Chemical Analysis
pH paper strip method
The pH, or hydrogen ion concentration, was a measure of the relative acidity or alkalinity.
The pH values ranges from 0 to 14, with 0-6 being acid, 7 being neutral and 8-14 being
alkaline. For the purpose of this experiment, pH strips were used. These strips change
colour in accordance with the acidity or alkalinity of the filtrate or mud. The pH determined
for the original sample and contaminated samples A to D were: 8, 10,10,9, and 10
respectively. The pH of mud plays a major role in controlling the solubility of calcium. At high
pH values –as shown above-, calcium solubility was very limited; this makes high pH mud
suitable for use in the drilling of carbonate formations, which normally were susceptible to
erosion and dissolution by freshwater mud. The pH value was also an important indicator for
the control of corrosion. According to Azar and Samuel (1984), a minimum of 9.5 should
21The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
always be maintained to prevent oxygen corrosion of casting, drill pipe, etc. A high pH tends
to disperse the active clays in the mud.
Whole mud alkalinity (Pm)
Alkalinity refers to the ability of a solution or mixture to react with an acid. The
phenolphthalein alkalinity refers to the amount of acid required to reduce the pH to 8.3, the
phenolphthalein end point. The phenolphthalein alkalinity of the mud and mud filtrate is
called the Pm and Pf, respectively. The Pf test includes the effect of only dissolved bases and
salts while the Pm test included the effect of both dissolved and suspended bases and salts.
The methyl orange alkalinity refers to the amount of acid required to reduce the pH to 4.3,
the methyl orange endpoint. The methyl orange alkalinity of the mud and mud filtrate is
called the Mm and Mf, respectively. The API diagnostic test include the determination of Pm,
Pf and Mf. The Pf and Mf test were designed to establish the concentration of hydroxyl,
bicarbonate, and carbonate ions in the aqueous phase of the mud. At a pH of 8.3, the
conversion of hydroxides to water and carbonates to bicarbonates was essentially complete
(Bourgoyne Jr, Chenevert, Millheim, & Young Jr, 1984). The bicarbonates originally present
in solution do not enter the reactions. Thus, at a pH of 8.3,
OH- + H+ HOH, and CO32- + H+ HCO-3
-.
As the pH was further reduced to 4.3, the acid then reacts with the bicarbonate ions to form
carbon dioxide and water:
HCO3- + H + CO2 + HOH.
However, one disadvantage of this type of test is that in many mud filtrates, other ions and
organic acids are normally present that can adversely affect the Mf test.
The Pf and Pm test results indicate the reserve alkalinity of the suspended solids. As the
[OH-] solution was reduced, the lime and limestone suspended in the mud would go into
solution and tend to stabilize the pH. This reserve alkalinity generally was expressed as an
equivalent lime concentration. Converting the Ca(OH)2 concentration from 0.02N to field
units of lbm/bbl yields
0.02 gew/1L x 37.05/g/L = 0.26 lbm/bbl. Thus, free lime was by 0.26 (Pm –fw * Pf), where
fw was the volume fraction of water in mud which was reported to be 0.0098.
MBT and Bentonite equivalent
22The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
This test gives an estimate of the cation exchange capacity of mud solids as well as to
indicate the amount of active clays in the mud system. Also this test could be used to
determine colloidal characteristics of clay minerals. A standardized solution of methylene
blue dye was added to 1 ml of mud that has been treated with hydrogen peroxide and
sulfuric acid and was then gently boiled to decompose the polymers and organics (which
have a very high exchange capacity and would otherwise interfere with the test). The
methylene blue was added in 0.5 ml increments until the mud solids no longer absorb the
dye. This endpoint was determined by putting a drop of the solution on a standard Whatman
filter paper. When the dye was in excess, a halo of free dye formed around the blue dot. The
halo that formed was turquoise blue in colour and was very distinct form the blue colour of
the dye. This was reported as equivalent lbs/bbl bentonite.
Calcium Carbonate Determination
After retort and sand analysis were performed on both original and contaminated samples,
determination of calcium carbonate content of the water based drilling samples were then
carried out. In order to determine the calcium content of both samples, the total hardness of
the samples were estimated using the versanate method. The hardness of water or drilling
fluid was due to mainly the presence of calcium and magnesium ions. When EDTA was
added to water, it combined with calcium ions and the endpoint was determined in the
presence of calver 11 indicator. When all the calcium ions was complexed with the EDTA
solution, it gave a colour change at a pH of 12-13. The colour change observed in the
solution was from a wine colour to blue black. It was observed that sample D showed
presence of calcium carbonate contaminant.
In the petroleum industry, the practice of chemical removal of contaminants are utilized. The
addition of chemical contaminants to the drilling fluid, either at the surface or through the
wellbore, produces an imbalance in the chemical equilibrium of the fluid, which can cause
serious rheological or drilling problems to develop. For example, when calcium enters the
mud, sodium montmorillonite will convert to calcium montmorillonite, which first produces
flocculation and eventually aggregation of the montmorillonite. This is often desirable to
remove the calcium by chemical treatment. In most cases, calcium is removed from the mud
system by adding soda ash (Na2C03) which forms in soluble calcium carbonate:
Ca2+ + 2OH- + Na2O3 ⇒CaCO3 ↓+ 2Na+ + 2OH-
Furthermore, if cement or lime get into the mud, the pH usually increases to unacceptable
levels because of the hydroxyl ions as well as calcium have been added. In these
23The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
circumstances, either sodium acid pyrophosphate SAPP-Na2H2P2O7 or sodium bicarbonate
is usually added. When SAPP is added the following occurs:
Ca2+ + 4OH- + Na2H2P2O7 →Ca2P2O7 ↓+ 2Na+ +2OH- + 2H2O. In this reaction, calcium
was removed and the four hydroxyl ions on the left side of the equation are reduced to two
hydroxyl ions on the right side (Bourgoyne Jr, et al. 1984).
Calcium test/Water Hardness
The mud hardness indicates the amount of calcium suspended in the mud as well as the
calcium in solution. This test usually is made on Gypsum-treated muds to indicate the
amount of excess CaSO4 present in suspension. A small contaminated sample of mud was
first diluted to 50 times its original volume with deionized water so that any undissolved
calcium or magnesium compounds can go into solution. Since the mud samples were diluted
50 times their original volume, a 50 cm3 sample was titrated to determine the calcium and
magnesium present in 1 cm3 of mud. Water containing large amounts of Ca 2+ and Mg 2+
ions is known as hard water. These contaminants were often present in the water available
for use in the drilling fluid. In addition, Ca 2+ can enter the mud when anhydrite (CaSO4) or
Gypsum (CaSO4.2H2O) formations are drilled. Cement also contains calcium and can
contaminate the mud. The total Ca 2+ and Mg 2+ concentration was determined by titrating
with a standard (0.02 N) Versenate (EDTA) solution. The standard Versenate solution
contains sodium Versenate, an organic compound capable of forming a chelate with Ca 2+
and Mg 2+ . The chelate ring structure very stable and essentially removes the Ca2+ and Mg2+
from solution. Disodium ethylenediaminetetraacetic acid (EDTA) plus calcium yields the
EDTA chelate ring: See chemical reaction below (Bourgoyne Jr., Chenevert, Millheim, &
Young Jr., 1984).
24The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
Magnesium ion forms a wine red complex with the dye Eriochrome Black T. Since the
solution containing both Ca 2+ and Mg 2+ was titrated in the presence of this dye, the
Versenate first forms a calcium complex. After the [Ca2+] has been reduce to a very low
level, the Versenate then forms a complex with the magnesium ions. The depletion of the
available Mg2+ ions from the dye Eriochrome Black T causes the colour of the solution to
change from wine-red to blue.
These unwanted ions could be removed by chemical treatment. Magnesium could be
removed by the addition sodium hydroxide as seen in the chemical reaction below:
Mg 2+ + 2 NaOH →Mg(OH)2+ 2Na +
Chloride Ion content
Salt can enter and contaminate the mud system when salt formations were and when saline
formation water enters the well bore. The chloride concentration was determined by titration
with silver nitrate solution. This caused the chloride to be removed from the solution as AgCl,
a white precipitate:
Ag + + Cl- AgCl
The endpoint was detected using a potassium chromate indicator. The excess Ag+ present
after all Cl- has been removed from the solution reacts with the chromate to form Ag2CrO4,
and orange-red precipitate:
2 Ag+ + CrO4 Ag2CrO4
Since AgCl was less soluble than Ag2CrO4 , the latter cannot form permanently in the
mixture until the precipitation of AgCl has reduced the [Cl-] to a very small value. For
titration, .02 N AgNO3 concentration was used (Bourgoyne Jr., Chenevert, Millheim, &
Young Jr., 1984)
Salt analysis
7ml of filtrate was measured and 3ml of standard sodium perchlorate solution was added to
this. The resultant mixture was then centrifuged at 1800 rpm for one minute and the
precipitate volume were recorded which was 0.65 and this was extrapolated on the
calibration curve to obtain 11 lb/bbl of KCl.
The maximum density of a solids-free fluid depends on the type of salt used. Each salt has a
maximum concentration before it reaches saturation. The table below indicates the
maximum densities of various brines. Thermal expansion of the water affects the density of
clear brine. At elevated temperatures the density decreases. Densities were reported at a 25
The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
specific temperature such as 70 0F. Combinations of salts can be used to economically
achieve densities from 8.34 to 19.2 ppg (Geo Drilling Fluids Inc, 2014).
Questions
1. Both original sample and sample B had relatively the same mud cake thickness i.e.
‘4/32 and they were both thin soft and pliable. However, sample A’s mud thickness
was recorded to be 8/32 which was twice the mud thickness of the original mud
sample. This could be attributed to contaminants such as NaCl and hardness of
water. Hardness of water means that there were calcium and magnesium ions present
in the mud system thereby reducing sodium montmorillonite to expand and hydrate in
water. Furthermore, high concentration of salts in water could greatly affect the ability
of some clays to hydrate in water.
2. Percent solids in original sample was found to be 1%; in contrast to the contaminated
samples B and C contained 4% bentonite and 16% oil.
3. Sample A has sand and Sample D has carbonates.
API Fluid loss Questions
1. The original sample has more concentration of bentonite which acts as a
viscosifier and readily hydrates in water thus increasing viscosity of the mud and
decreasing fluid loss.
2. Removal of contaminants and adding other types of high yield clays such as
smectite or attapulgite as well as CMC’s and other polymers.
3. Some factors are: Filtrate viscosity, cake permeability, pressure differential.
4. See graph1. Spurt loss is calculated by the following:
a. The spurt loss of the cell can be obtained by extrapolation to zero time and
finding the gradient.
For the original sample:
Time Filtrate Volume
1 4.472 cm3
7.5 10
4 . 472−(10−4 . 472√7 .5−√1 )∗√1=1 . 29cm3
26The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.
5.Qw2= 2 KPA2
μ∗QwQc
∗t
Vf=√2kΔp( fscfsm−1)∗A √ √t
√μ This equation indicates that the filtrate volume is
proportional to the square root of the time period used. Thus, the filtrate collected
after 7.5 minutes should be half the filtrate volume in collected after 30 minutes. It
was concluded that filtration rate increases with temperature because the viscosity
of the filtrate is reduced.
Determination of KCl concentration questions
1. Sample A had NaCl.
Conclusion:
Drilling fluid densities for water based Lignosulfonate mud was obtained using a non-
pressured mud balance. One of the product formulation was Bentonite, which was added for
mud viscosity, gel strength and even fluid loss control. Within an industry setting, the
presence of bentonite is beneficial for cuttings-carrying-capacity and filter cake
characteristics. Rheological properties were investigated using a rotating viscometer (which
is a type of diagnostic test for mud properties and thus performance). It was determined that
the mud samples were non-Newtonian in character. Meaning that the apparent viscosity for
the mud samples did not exhibit a direct proportionality between shear stress and shear rate.
27The University of Trinidad and Tobago, Point Lisas Campus Esperanza Road, Brechin Castle, Couva.