Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF STABILISING CLAY OR SHALE
The present invention relates to a composition useful for stabilizing a clayey
or
shaley formation surrounding a wellbore. In particular, the present invention
relates to a
clay. or shale swelling inhibitor composition for use during drilling,
completing or
maintaining wellbores.
Drilling fluids are used in the drilling of oil and gas wells. In rotary
drilling
operations drilling fluids are pumped down from the surface through a drill
string to a
drill bit. They emerge through ports in the drill bit and return to the
surface via an
annular space located between the drill string and the walls of the borehole.
The
functions of drilling fluids may be multiple: for example, they serve to cool
and
lubricate both the drill bit and drill string, they transport drill cuttings
to the surface,
they equalize the pressure between the fluids in the wellbore and the
formation fluids,
they prevent "squeezing" of the wellbore or caving of the formation into the
wellbore,
or minimise any potential damage to the "pay zone" of the wellbore.
Drilling fluids are of two basic types, oil-based (hereinafter referred to as
OBMs
1 S - oil based muds), and water-based (hereinafter referred to as WBMs -
water based
muds). OBMs are superior in performance to WBMs in several important respects
including lubricating properties and thermal stability thereby allowing wells
to be
drilled at a faster rate than when using WBMs resulting in considerable cost
savings. In
particular, OBMs are useful where the downhole temperature is high, for
example when
drilling deviated wells through high temperature formations. Furthermore, OBMs
mitigate problems associated with swelling and dispersion of clays or shales,
which are
frequently encountered during drilling with WBMs. Such problems will
hereinafter be
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referred to as "clay or shale destabilization".
Unfortunately, OBMs are less attractive than WBMs from an environmental
perspective. The disposal of spent OBMs, and the associated problems of
cuttings
clean-up and disposal are posing increasing difficulties for the oil and gas
exploration
industry. One proposal has been the use of biodegradable oils to formulate
OBMs, but
such products may not comply with future legislation which is expected to
impose more
stringent limits on disposal of waste materials. The consequence of this is
that much
effort has been expended in the development of WBMs having improved
performance
in terms of reducing clay or shale destabilization. If no attempts are made to
inhibit
hydration and swelling of clays or shales the consequences can be severe.
Thus, clay or
shale destabilization may result in weaknesses developing in the formation,
possibly
leading to erosion of the borehole. Also, as would be well known to the person
skilled
in the art, the phenomenon of "stuck pipe" can occur, and furthermore, logging
operations during drilling can be hampered.
A number of approaches have been proposed for reducing the clay or shale
destabilization characteristics of WBMs. The use of salts such as Group IA
metal salts,
in particular, potassium chloride, to balance the water activity between the
clay or shale
and the drilling fluid, or even to provide an osmotic gradient that leads to a
net flow of
water out of the clay or shale, has been employed to prevent clay or shale
hydration.
This approach may be combined with the use of silicates which are capable of
forming
osmotic membranes on the exposed surface of the clay or shale, as described,
for
example, in US 3640343, and van Oort et. al. in SPE/IADC paper No. 35059,
presented
at the IADC/SPE Drilling Conf. New Orleans, 12-15'h March 1996. The osmotic
membrane facilitates the flow of water out of the clay or shale whilst
inhibiting the
diffusion of ions between the clay or shale and the fluids in the wellbore
thereby
improving the clay or shale stabilization characteristics of the WBM.
The precipitation of silicates on the exposed surface of the clay or shale is
also
believed to produce a physical barrier against invasion of fluids from the
wellbore into
the clay or shale. The use of silicates, however, may be associated with
problems of
high drilling torques and poor lubricity owing to the tendency of silicates to
precipitate
out on metal surfaces, for example, on the drill bit. Also, the use of
silicates may lead to
problems of incompatibility with conventional drilling fluid additives.
Finally, silicate
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solutions are highly alkaline and this can lead to difficulties with safe
storage and
handling.
Other proposals for improving the clay or shale stabilization performance of
WBMs include the addition of glycols or polyols. Suitable glycols or polyols
include,
for example, polyglycerols, glycols, polyalkylene glycols (PAG), e.g.,
polyethylene
glycols (PEG), polypropylene glycols (PPG) and copolymers of ethylene and
propylene
glycols, alcohol ethoxylates and glycol ethers as described in for example, EP
0495579,
US 4830765, US 4172800 and The Society of Petroleum Engineers Reports SPE
25989
and 28818. In The Society of Petroleum Engineers Report SPE 28960 it is
disclosed
that potassium ions work synergistically with glycols in drilling fluids to
improve the
shale stabilization performance of the WBMs.
Other proposals described in the art to improve the clay or shale
stabilization
properties of WBMs include the use of additives such as aluminium complexes,
chemically modified starch, chemically modified cellulosic materials, water
soluble
1 S polyacrylamides and other water soluble polymers, lime or gypsum,
asphaltene derived
products and calcium lignosulphonates.
The use of borates in hydraulic fracturing fluids is described in US 5372732,
US
5445223, GB 2253868 and WO 87/00236 while US 5220960 discloses the use of
borates as cement retardants for completion operations.
CA 1248337 teaches the use of borates in the field for low specific gravity
non-
damaging workover and completion fluids. CA 1303841 describes the use of
borates in
water profile control in oil recovery. E L Bigelow in The Society of Petroleum
Engineers publication SPE 27644 discloses that borates may be used in pulsed
neutron
logging. Borates have also been used in lost circulation treatments (fluid
loss pills)
during drilling operations (as described in US 5372732).
US 6105691 refers to the use of boric acid or glycerol-borate esters in
drilling
fluids for the purposes of improving the lubricity of the drilling fluid and
its clay
dispersion characteristics. However, US 6105691 is silent concerning any
beneficial
effect of borate on the clay or shale stabilizing properties of drilling
fluids.
RU 1699991 relates to a silicate drilling mud having increased mud stability
and
calcium chloride resistance. The drilling mud is prepared by adding boric acid
and
sodium or potassium silicate to water to produce a gel-like mass, which is
then diluted
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until the required viscosity, structural and mechanical properties have been
obtained.
An organic stabilizer may then be added to adjust filtration. There is no
suggestion that
a sugar may be added to the mud to increase its shale or clay stabilization
characteristics.
S An object of the present invention is to provide an aqueous based
composition,
in particular, an aqueous based drilling fluid composition, having improved
clay or
shale stabilization characteristics. A further object of the present invention
is to provide
a method of reducing the swelling of shale or clay encountered in a wellbore,
for
example, during drilling through a formation.
It has now been found that an aqueous based composition which contains a
combination of (a) borate ions and (b) ions selected from the group consisting
of
alkaline earth) metal ions and ammonium ions exhibits markedly improved clay
or
shale stabilization properties compared with aqueous based compositions
containing
components (a) or (b) alone. It has also been found that the addition of a
silicate to the
aqueous based composition containing components (a) and (b) has a detrimental
effect
on its clay or shale stabilization properties. It has further been found that
the addition of
at least one sugar, selected from the group consisting of monosaccharides and
oligosaccharides having from 2 to 4 saccharide groups, to the aqueous based
composition containing components (a) and (b) improves the clay or shale
stabilization
characteristics thereof.
Thus, in a first embodiment of the present invention there is provided a use
of a
combination of (a) a source of borate ions, (b) a source of ions selected from
the group
consisting of alkali metal ions, alkaline earth metal ions and ammonium ions
and (c)
optionally a source of at least one sugar selected from the group consisting
of
monosaccharides and oligosaccharides having from 2 to 4 saccharide groups, in
a
composition comprising a continuous aqueous phase, to improve the clay or
shale
stabilizing properties thereof.
In a second embodiment of the present invention there is provided a method of
reducing the swelling of shale or clay encountered in a wellbore, the method
comprising
introducing into the wellbore a composition comprising:
(a) a continuous aqueous phase;
(b) a source of borate ions;
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(c) a source of ions selected from the group consisting of alkali metal ions,
alkaline
earth metal ions and ammonium ions; and
(d) optionally a source of at least one sugar selected from the group
consisting of
monosaccharides and oligosaccharides having from 2 to 4 saccharide groups.
S The composition that is introduced into the wellbore may be used in
drilling,
completing or maintaining a wellbore. Where the composition is used in
drilling a
wellbore, the composition is preferably circulated in the wellbore thereby
stabilising the
clay or shale.
According to a preferred aspect of the present invention there is provided a
method of reducing the swelling of shale or clay encountered during the
drilling of a
wellbore through a formation using a drill string disposed within the
wellbore, said drill
string having a first end and a second end, the first end of the drill string
being located
at or near the surface of the wellbore and the second end of the drill string
being in
communication with a drill bit having ports therein, wherein:
(A) a drilling fluid composition comprising (a) a continuous aqueous phase,
(b) a source
of borate ions, (c) a source of ions selected from the group consisting of
alkali metal
ions, alkaline earth metal ions and ammonium ions, and (d) optionally a source
of at
least one sugar selected from the group consisting of monosaccharides and
oligosaccharides having from 2 to 4 saccharide groups is introduced into the
first end of
the drill string, is pumped through the drill string from the first end to the
second end
thereof and is discharged into the wellbore through the ports in the drill
bit; and
(B) the drilling fluid composition is recycled to the first end of the drill
string via an
annular space which is provided between the drill string and the walls of the
wellbore.
Suitably, the continuous aqueous phase of the composition may be fresh water,
tap water, sea water, river water or aquifer water.
Preferably, the source of borate ions is an alkali metal borate, an ammonium
borate or mixtures thereof. Preferably, the source of the borate ions is a
borate of
generic formula (I):
n(M120)m(B203).xH20 (I)
wherein M' represents an alkali metal (preferably, sodium or potassium) or
NH4, n is an
integer in the range 0 to 5, m is an integer in the range 1 to 7, the ratio of
n:m is 0 - 1:1
and x is an integer in the range 0 to 10. Regardless of the source of borate
ions, it is
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preferred that the ratio of n:m in the composition is in the range 0.1:1 to
1.2:1. This
ratio may be adjusted by addition of an alkali metal hydroxide (for example,
sodium
hydroxide or potassium hydroxide) or ammonium hydroxide to the composition.
The source of borate ions may be a refined product or a natural mineral.
S Preferably, the source of borate ions may be selected from, but not limited
to, the group
consisting of disodium tetraborate pentahydrate (Na20'2B203'SH20), disodium
tetraborate decahydrate or tincal (NaZO'2BZO3' 1 OHZO), disodium tetraborate
(Na20~2Bz03), sodium metaborate tetrahydrate (Na20'B203'8H20), sodium
metaborate
dehydrate (Na20'B203'4H20), sodium pentaborate pentahydrate
(Na20'SB203'1OH20),
disodium octaborate tetrahydrate (NazO'4B203'4Hz0), boric acid (H3B03),
dipotassium
tetraborate tetrahydrate (Kz0'2BZ03~4Hz0), potassium pentaborate tetrahydrate
(K20'SBz03'8H20), diammonium tetraborate tetrahydrate ((NH4)20'2Bz03'4H20),
ammonium pentaborate tetrahydrate ((NH4)20'SB203'8H20) and caesium pentaborate
tetrahydrate (Cs20'SB203'8H20).
Where the source of borate ions is an alkali metal borate or an ammonium
borate, the borate will also act as a source of alkali metal ions or of
ammonium ions
respectively. If necessary, a source of additional alkali metal ions or
ammonium ions
may be added to the composition, as described above.
The amount of borate, expressed as B203 in lb/bbl (pounds per barrel) present
in
the continuous aqueous phase of the composition is preferably in the range 0.1
to 150
lb/bbl, and more preferably, in the range 0.5 to 50 lb/bbl. ,
It is envisaged that the source of borate ions may be sold as a concentrate
ready
for dilution with the composition. The concentrate may comprise a solution or
.
dispersion of a borate in an aqueous liquid or a non aqueous liquid. Suitable
aqueous
liquids are as described above. Suitable non aqueous liquids include polar
solvents such
as alcohols and glycols. Preferably, the concentrate comprises a solution or
dispersion
of a borate in an aqueous liquid. The amount of borate, expressed as B203 in
lb/bbl
(pounds per barrel) present in the concentrate is preferably in the range 10
to 250 lb/bbl.
Suitably, the source of the ions selected from the group consisting of alkali
metal
ions, alkaline earth metal ions and ammonium ions is a water soluble salt of
an alkaline
earth) metal, a water soluble ammonium salt or mixtures thereof. It is
preferred that the
water soluble salt of the alkaline earth) metal is not a silicate. Preferably
the water
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soluble salt is a potassium salt, caesium salt or ammonium salt including, but
not
limited to, potassium bromide, caesium bromide, ammonium bromide, potassium
chloride, caesium chloride, ammonium chloride, potassium hydroxide,.caesium
hydroxide and ammonium hydroxide. For example, if boric acid is used as the
source
of borate, then the water soluble salt is preferably selected from potassium
hydroxide,
caesium hydroxide and ammonium hydroxide.
Suitably, the alkaline earth) metal salt, ammonium salt or mixtures thereof
may
be present in the continuous aqueous phase of the composition in an amount in
the
range 0.1 to 150 lb/bbl, preferably, in the range 0.5 to 100 lb/bbl.
It is envisaged that the source of the alkaline earth) metal ions, ammonium
ions
or mixtures thereof may be sold as a concentrate ready for dilution with the
composition. Preferably, the concentrate may comprise a solution or dispersion
of an
alkaline earth) metal salt, ammonium salt or mixtures thereof in an aqueous
liquid.
Preferably, the concentration of alkaline earth) metal salt, ammonium salt or
mixtures
thereof in the concentrate is in the range 10 to 1 SO lb/bbl, preferably 10 to
100 lb/bbl.
The sugar may be selected from the group consisting of monosaccharides, and
oligosaccharides having 2 to 4 saccharide groups. Preferred monosaccharides
include
glucose and fructose. Preferred disaccharides include sucrose (for example,
obtained
from cane or beet), maltose and lactose. It is also envisaged that the source
of the sugar
may be a mixture of sugars, for example, glucose syrup, golden syrup, molasses
or
Activ 7 TT'' (a water soluble liquid syrup obtained by partial hydrolysis of
starch).
Suitably, the amount of sugar in the continuous aqueous phase of the
composition is in the range from 0.1 to 150 Ib/bbl, preferably 0.5 to 50
lb/bbl.
It is envisaged that the source of the sugar may be sold as a concentrate
ready for
, dilution with the composition. Preferably, the concentrate may comprise a
solution or
dispersion of the sugar in an aqueous liquid. Preferably, the concentration of
the sugar
in the concentrate is in the range 10 to 300 lb/bbl, preferably 20 to 300
lb/bbl.
It is also envisaged that (a) the source of borate ions, (b) the source of the
alkaline earth) metal ions, ammonium ions or mixtures thereof and (c) the
source of
sugar may be sold as a mixed concentrate. Suitably, the amount of components
(a), (b)
and (c) in the mixed concentrate are as described above for the individual
concentrates.
An advantage of the present invention is that the composition is stable at
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elevated temperature for a prolonged period of time. Typically, the
composition is
stable for a period of at least 12 hours, preferably at least 16 hours at a
temperature of at
least 100°C, preferably at least 120°C.
A further advantage of the present invention is that the source of borate ions
acts
as a pH buffer for the composition thereby controlling the pH at a value which
is
typically above 8, preferably above 9. Accordingly, there is no requirement to
include a
pH control agent such as sodium hydroxide or potassium hydroxide.
The composition may be beneficially used in combination with conventional
additives for improving the clay or shale stabilization properties of a
drilling or
completion fluid. These conventional additives include, but are not limited to
glycols.
The composition may also contain additional ingredients such as weighting
agents, e.g., barite, haematite, or galena; viscosifiers, e.g. xanthan gum;
fluid loss
control agents, e.g., starch or cellulose derivatives (e.g., carboxymethyl
cellulose);
defoamers and lubricants.
Yet a further advantage of the composition is that it is compatible with
conventional fluid loss control agents.
There is no special requirement in relation to the preparation of the
composition.
The source of borate ions, the source of the alkaline earth) metal ions,
ammonium ions
or mixtures thereof, and optionally the source of the sugar may be added in
any order to
the continuous aqueous phase (either as concentrates or as solids). Gentle
heating is
optional to dissolve or disperse the source of borate ions, the source of
alkaline earth)
metal ions, ammonium ions or mixtures thereof, and the optional source of the
sugar in
the continuous aqueous phase of the composition.
As discussed above, the sugar has been found to improve the clay or shale
stabilization
properties of the aqueous based composition used in the present invention.
Thus, in yet
a further embodiment of the present invention there is provided a composition
comprising (a) a continuous aqueous phase, (b) a source of borate ions, (c) a
source of
ions selected from the group consisting of alkali metal ions, alkaline earth
metal ions
and ammonium ions and (d) a source of at least one sugar selected from the
group
consisting of monosaccharides and oligosaccharides having from 2 to 4
saccharide
groups.
The components (a) to (d) of the composition of the present invention have the
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preferred features described above.
The performance of the composition of the present invention in clay or shale
stabilization is now illustrated by reference to the following examples. .
Example 1
Aqueous based compositions, were prepared comprising the components shown
in Table 1. Each composition was made up to a weight of 80g with demineralised
water
and was contained in a 100m1 glass bottle.
A model shale (London Clay; l Og) screened to a particle size range of 2-4 mm,
was added to each bottle. The bottles were then sealed with screw caps and
rolled at
20rpm for 16 hours (bottle rolling test). Each sample mixture was then passed
through a
separate pre-weighed 500 micron sieve (i.e. a sieve which retains particles
having a size
of greater than 500 microns). The clay retained on each sieve was rinsed
gently with
tap water. The sieves plus retained clay were then placed in a drying oven at
a
temperature of 130°C for 24 hours.
Each sieve was re-weighed and the percentage of dry clay retained on the
sieves
was calculated using the following formula, taking into account the original
moisture
content of the clay as 20wt.%:
clay retained = l2.Sx[(weight of sieve + dried retained clay(g)) - (weight of
clean
sieve (g))J
The higher the % clay retained the better the performance of the composition
in
stabilising the model shale. The results obtained are shown in Table ~l .
30
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Table 1: Results of Bottle Rolling Tests
Composition % clay retained
1. 80g demineralised water (comparative) 0.8
2. 251b/bbl KCl (comparative) 9.1
3. 251b/bbl NH4C1 (comparative) 27.5
4. 251b/bbl CsCI (comparative) 79.1
5. 251b/bbl KCl + 3wt.% DCP208' (comparative) 81.9
6. 251b/bbl KCI + 3.47wt.% Activ 7' (comparative) 33.1
7. 251b/bbl LiCI (comparative) -0.9
8. 251b/bbl MgCl2 (comparative) 0.4
9. 251b/bbl CaCl2 (comparative) 0.4
10. 251b/bbl KCl + 2.Swt.% Na20'2B203'SHZO + 2.Swt.%93.6
Na20'B203'8H20
11. 251b/bbl KCl + lOwt.% Na20'B203'8H20 94.0
12. 251b/bbl NH4C1 + 2.Swt.% Na20'2B203~SH20 + 93.6
2.Swt.%
Na2O'B2O3'8H2O
13. 251b/bbl CsCI + 2.Swt.% Na20'2Bz03'SH20 + 2.Swt.%96.0
Na20'B203'8H20
14. 251b/bbl KCl + 3wt.% DCP208' + 2.Swt.% Na20'2B203'SH2097.6
+ ~
2.Swt.% Na20'B203'8Hz0
15. 251b/bbl KCl + 3.47wt.% Activ 7 + l.5lwt.% 97.2
Na20'2B203'SH20 +
1.96wt.% H3B03
16. 251b/bbl LiCI + 2.Swt.% Na2O'2B2O3'SH2O + 2.Swt.%77.4
Na20'B203'8H20
17. 251b/bbl MgCl2 + 2.Swt.% Na20'2B203'SH20 + 64.1
2.Swt.%
Na20'B203'8Hz0
18. 251b/bbl CaCl2 + 2.Swt.% Na20'2B203'SH20 + 74.1
2.Swt.%
Na20'Bz03'8Hz0
19. 4.Owt.% K20'2B203'SH20 (1.826wt.% B203) 80.2
20. 3.4wt.% (NH4)20'2B203'SH20 (1.826wt.% B203) 68.0
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la glycol supplied by BP Chemicals
zwheat starch by-product (glucose syrup) supplied by Roquette Freres
The unexpectedly superior performance of the compositions comprising a source
of borate ions and a source of alkaline earth) metal ions or ammonium ions in
stabilizing shale (examples 10-20) is evident. The effect of adding a source
of sugar
(Activ 7) to a composition containing a source of borate ions and a source of
alkaline
earth) metal ions or ammonium ions to improve the shale stabilization
performance of
the composition still further is evidenced by example 15.
Example 2
The procedure in Example 1 was repeated with further compositions to assess
the effect
of borate ion concentration and the ratio of n:m (i.e. molar ratio of
Na20:Bz03) on shale
stabilization performance. The results are given in Table 2.
Table 2 - Results of Bottle Tests to Determine the Effect of Borate Ion
Concentration and Ratio of n:m on Shale Stabilization Performance
Composition wt.% B203 n:m % retained
21. 10. 251b/bbl KCl + 0.25wt.% 0.183 0.67:159.5
Na20~2B203'SH20 + 0.25wt.% Na2O'B2O3'BHZO
22. 251b/bbl KCl + 1.25wt.% Na20'2B203'SHZO0.913 0.67:196.5
+
1.25wt.% Na20~B203'8H20
23. 251b/bbl KCl + 2.Swt.% Na2O'2B2O3'SH2O1.826 0.67:196.6
+
2.Swt.% Na20'B203'8H20
24. 251b/bbl KCl + 3.24wt.% H3B03 1.826 0.0:1 20.8
25. 251b/bbl KCl + 1.53wt.% NazO'2B203'SH201.826 0.20:176.1
+
1.94wt.% H3B03
26. 251b/bbl KCl + 3.82wt.% Na20'2B203'SH201.826 0.50:197.8
27. 251b/bbl KCl + 7.32wt.% Na20'Bz03'8Hz01.826 1.0:1 97.6
The results indicate that an acceptable shale stabilization performance can be
achieved with compositions having a range of borate ion concentrations and a
range of
n:m ratios.
Example 3
Water based compositions (350 ml), were prepared comprising the components
shown in Table 3 below. Each composition was made up using tap water and was
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prepared in a plastic bottle. To each of the bottles was added 30g of 2-4mm
size range
Oxford clay. The bottles were sealed and rolled at a speed of approximately 30
rpm in
an oven set at a temperature of 65°C for a period of 16 hours. The
compositions were
then repeatedly passed through a 2mm sieve. The clay retained on the sieve was
collected and oven dried for a minimum of 16 hours. The % clay retained in the
test
was calculated taking into account, as in Example 1, the original moisture
content of the
clay. The results are shown in Table 3.
Table 3 - Results of Bottle Rolling Tests using Oxford Clay
Composition pH % clay
retained
28. 251b1bb1 KC1 (comparative) 9.4 11.5
29. 251b/bbl KCl + 3% Glydril MC' (comparative)7.4 49.7
30. 251b/bbl KCI + 2.5% Polyseal silicate'12' 99.1
(comparative)
31. 251b/bbl KCl + 6wt.% Na20~2B203~5H209.3 40.2
32. 251b/bbl KCl + 3.47wt.% Activ 7 + 9' 102.2
1.51wt.%
Na20~2B203'SH20 + 1.96wt% H3B03
'commercial glycol shale inhibitor product
22:1 Si02:Na20
3pH adjusted value
Example 4 '
A so-called Hamster Cage Test for evaluating the shale stabilizing performance
of the compositions according to the present invention was performed. This
test
involved preparing 1500m1 of water based drilling fluid compositions as
presented in
Table 4 (where the aqueous liquid component of the compositions is
demineralised
water):
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Table 4: Compositions for use in Hamster Cage Test
Ingredient Drilling Fluid A Drilling Fluid B
(comparative) Ib/bbl of ingredient
lb/bbl of ingredient
KCl 25 25
Polyanionic cellulose'1.5 1.5
Starch' 4.0 4.0
Xanthan gum' 1.5 1.0
Na20~2B203~5Hz0 0 8.75
Na20~B203~8Hz0 0 8.75
'PAC L - supplied by Baroid
2DEXTRID - supplied by Baroid
3Xanvis - supplied by Schlumberger
The viscosities of the drilling fluid compositions prepared in Table 4 were
measured at a temperature of 20°C using a Fann Viscometer (Model 35SA -
Baroid
Testing Equipment, Texas, USA). The results of the viscosity tests are
presented in
Table 5.
Table 5: Viscosities of Drilling Fluid Compositions
Fann viscometer readingsDrilling Fluid A Drilling Fluid B
(comparative)
600rpm 49 48
300rpm 34.5 31
200rpm 28 . 25
100rpm 19.5 17
6rpm 6 5
3rpm 4.5 4
PV / cP 14.5 17
YP / lb/100sqrft 20 14
PH 10.0 (adjusted with 9.4
NaOH)
These results demonstrate that drilling fluid compositions with desired
rheological properties can be formulated containing borate ions. Furthermore,
the
presence of borate ions enables lower levels of viscosifying polymer to be
employed,
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and ensures that the fluid is well buffered at the correct pH.
Hamster Ca eg Test
The Hamster Cage Test involved removing the lids from pre-weighed cages (of
1 mm mesh size) and adding approximately 80g of London Clay (4-8mm sieved
particle
size range) to each cage. The lids were replaced on the cages which were then
suspended in a first and a second trough previously filled with drilling fluid
compositions A and B respectively. The cages were then rotated automatically
for 4
hours. At the end of this period, the cages were removed from the apparatus,
the lids
opened and the cages and retained cuttings rinsed with tap water. They were
then dried
in an oven for 16 hours at a temperature of 130°C, and finally re-
weighed. The formula
used in Example 1 was employed to calculate the percentage of clay cuttings
retained in
the cage. The results are presented in Table 6:
Table 6: Results of Hamster Cage Tests
Clay Retained
Drilling Fluid A (Comparative) 44.0
Drilling Fluid B 73.1
The improved shale stabihzmg pertbrmance of the drilling fluid composition
containing borate ions and potassium ions (Drilling Fluid B) is evident from
these
results.
Example S
The fluid loss characteristics and viscosity of drilling fluid composition C,
comprising the ingredients given in Table 7, were measured before and after
high
temperature ageing (hot rolling) at a temperature of 120°C for 16
hours. The results are
presented in Table 8.
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Table 7: Drilling Fluid Composition C
Ingredient lb/bbl of Ingredient
KCl 25
Polyanionic cellulose 1.5
Starch 4.0
Xanthan Gum 1.5
Na20-2B203~SH20 8.75
Na20~Bz03~8Hz0 8.75
Barite 100
HMP' 10
1 HMP = Hymod Prima clay obtained from Imerys comprising illite and having a
particle size of about 1 to S microns. HMP is a non-swelling clay and
simulates drilled
solids (cuttings) in the formulation.
Table 8: Viscosity and Fluid Loss Characteristics of Un-aged and Aged (16
hours
at 120°C) Samples of Drilling Fluid Composition C
Fann viscometer readingsUn-aged Aged
600rpm 90 85
300rpm 62 59
200rpm 50 ~ 47
100rpm 36 ' 34
6rpm 12 12
3rpm 9 10
Gel / l Os 11 14
PV / cP 28 26
YP / Ib/100sqrft 34 33
Fluid loss
l min 0.0 0.1
7.5 min 0.6 0.9
30 min 2.9 3.7
pH 9.5 9.3
CA 02469051 2004-06-02
WO 03/052023 PCT/GB02/05635
The fluid loss characteristics of drilling fluid composition C are good and
this
property together with the viscosity of the drilling fluid composition remain
substantially unaltered despite high temperature ageing.
10
20
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