Note: Descriptions are shown in the official language in which they were submitted.
DRILLING FLUID FOR COAL FORMATIONS
FIELD OF THE INVENTION
The present invention relates to drilling fluids and specifically aqueous
brine drilling
fluids that contain one or more fluid loss and wellbore strengthening additive
in addition to
common additives such as corrosion inhibitors, oxygen scavenger, defoamers,
lubricants and
other materials. The drilling fluid may be suitable for drilling through
formations that contain
coal, where strengthening the wellbore and preventing fluid loss to the
formation while drilling is
required.
BACKGROUND OF THE INVENTION
Brines have been commonly used as drilling fluids because they offer a wide
density
range without the need of suspended solids, also known as weighting materials.
Brines are
solutions of dissolved halides, nitrates or carboxylates of mono or divalent
cations such as
sodium, potassium, cesium, calcium and zinc.
Brine drilling has proven successful in increasing bit life and rate of
penetration by 2-3
times because of the minimal amount of solids in the system. Plastic Viscosity
is the restriction to
flow due to the amount of solids in the system. Because of the reduced solids
content, brines have
inherently low Plastic Viscosity.
According to the American Association of Drilling Engineers paper #AADE-05-DF-
HO-
rate of penetration is linked to the Plastic Viscosity. In summary, the paper
states that the
lower the Plastic Viscosity the more effective weight that can be applied to
the bit resulting in
faster drilling.
Besides being cost effective and showing many performance benefits, well bore
stability
issues have been seen when drilling with brines through coal.
Canadian patent CA 2,451,952 suggests the use of cationic polymers to impart
borehole
stability in subterranean coal formations.
1
Date Recue/Date Received 2023-05-17
United States patent no. US 9,102,865 B2 and patent application no. US
2010/0210482
Al teach a method comprising placing a wellbore servicing fluid comprising a
cationic polymer
into wellbore wherein the cationic polymer has a molecular weight of from
about 300,000
Daltons to about 10,000,000 Daltons. A composition comprising a wellbore
servicing fluid, a
cationic polymer, and a brine.
United States patent no. US 6,124,244A teaches a drilling fluid comprising a
blend of
brine and additives for maintaining rheological properties in a neutral or
slightly acidic brine and
a method for using the drilling fluid to drill into a producing formation are
disclosed. The drilling
fluid comprises viscosifying polymer and fluid loss polymer that function in
the brine without
substantially adversely affecting their properties. The brine is substantially
free of insoluble
solids. The viscosifying polymer and/or the fluid loss control polymer can be
acid soluble. Zinc
salts in the form of zinc bromide alone or in a mixture of zinc bromide and
calcium bromide are
=added to increase the density of the brine solution. The viscosifying polymer
has pendant amide
and sulfonic acid or sulfonate groups. Alternatively or additionally, the
viscosifying polymer
comprises a polysaccharide. The fluid loss control polymer can comprise a
cross-linked, cationic
or amphoteric starch. Soluble solids are dispersed in the brine.
United States patent no. US 5,635,458A teaches a water-based drilling fluid
that reduces
water adsorption and hydration of argillaceous rocks. The drilling fluid
comprises a glycol with a
molecular weight of less than about 200, an organic cationic material such as
salts of choline or
an organic salt of potassium, a filtration control agent, a viscosifier and
water.
United States patent no. US 5,620,947A teaches a water-based well completion
and
workover fluid is disclosed which has improved rheological and filtration
control properties at
temperatures up to about 450 F. The composition comprises a saturated brine
solution, a sized-
salt that is insoluble in the saturated brine solution, and a water-soluble
filtration additive
comprising a polymer produced from at least two monomers selected from the
group consisting
of 2-acrylamido-2-methylpropanesulfonate, acrylamide, and 2-vinylpyrrolidone.
Patent Application WO 1995014066 Al discloses that the swelling and migration
of
subterranean clay is inhibited during drilling for and stimulation of the
production of hydrocarbon
fluids, and preparation therefor, by treating said formations with a copolymer
of about 5 % to
about 50 % of an anionic monomer such as acrylic acid, methacrylic acid, or 2-
acrylamido-2-
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methyl propane sulfonic acid and the balance a cationic monomer selected from
dimethyl diallyl
ammonium chloride, or acryloxy or methacryloxy ethyl, propyl or 3-methyl butyl
trimetyl
ammonium chlorides or methosulfates. Permeability damage to the formation is
reduced in the
presence of the copolymer; it is particularly effective in spite of the
presence of a foaming agent.
Despite the existence of the above-mentioned technologies, there still exists
a need for a
composition based on a brine drilling fluid that has minimal viscosity with an
extremely low fluid
loss that can stabilize coal and still give the performance enhancement of the
brine drilling fluids.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is disclosed an
aqueous brine
drilling fluid composition that contains at least one fluid loss and wellbore
strengthening additive.
According to a preferred embodiment, the composition further comprises
additives selected from
the group consisting of: corrosion inhibitors, oxygen scavenger, defoamers,
lubricants and
combinations thereof.
According to another aspect of the present invention, the drilling fluid
composition is
suitable for drilling through formations that contain coal, where
strengthening the wellbore and
preventing fluid loss to the formation while drilling is required.
According to another preferred embodiment of the present invention, the at
least one fluid
loss and wellbore strengthening additives is selected from the group
consisting of: sulphonated
asphalt, wax, oxidized bitumen, resin, crosslinked carboxymethyl starch and
low molecular
weight carboxymethyl cellulose.
According to an aspect of the present invention, there is provided a brine-
based drilling
fluid for use in coal formations, said fluid comprising: a brine component; a
carbohydrate
component and a strengthening component. Preferably, the strengthening
component is selected
from the group consisting of: sulphonated asphalt, oxidized bitumen, resin,
wax and a
combination thereof. More preferably, the strengthening component is
sulphonated asphalt.
According to a preferred embodiment, the carbohydrate component is selected
from the
group consisting of: derivatized starch, low molecular weight carboxymethyl
cellulose and
combinations thereof.
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According to an aspect of the present invention, there is provided a use of a
composition
comprising a brine component; a carbohydrate component and a strengthening
component to
strengthen a borehole in a coal formation.
According to an aspect of the present invention, there is provided a method of
strengthening a borehole in a coal formation, said method comprising applying
to said coal
formation a fluid composition comprising a brine component; a carbohydrate
component and a
strengthening component.
According to yet another aspect of the present invention, there is provided a
fluid
composition for use in drilling comprising:
- a brine-based fluid;
- a carbohydrate component in an amount ranging from 4 to 20 kg/m3; and
- a strengthening component in an amount ranging from 5 to 25 kg/m3.
Preferably, the carbohydrate component is present in an amount ranging from 8
to 15
kg/m3. Also preferably, the strengthening component is present in an amount
ranging from 10 to
15 kg/m'.
According to a preferred embodiment, the brine is selected from the group
consisting of:
calcium chloride brine; potassium formate brine; calcium nitrate brine; and
combinations thereof.
According to an aspect of the present invention, there is provided a fluid
composition for
use in drilling through a coal formation comprising:
- a calcium chloride brine having a specific gravity ranging from 1.100 to
1.355;
- a carbohydrate component in an amount ranging from 4 to 20 kg/m3; and
- a strengthening component in an amount ranging from 5 to 25 kg/m3.
According to yet another aspect of the present invention, there is provided a
fluid
composition for use in drilling through a coal formation comprising:
- a potassium formate brine having a specific gravity ranging from 1.300 to
1.500;
- a carbohydrate component in an amount ranging from 4 to 20 kg/m3; and
- a strengthening component in an amount ranging from 5 to 25 kg/m3.
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According to yet another aspect of the present invention, there is provided a
fluid
composition for use in drilling through a coal formation comprising:
- a brine comprising calcium chloride and calcium nitrate having a specific
gravity
ranging from 1.355-1.600;
- a carbohydrate component in an amount ranging from 4 to 20 kg/m3; and
- a strengthening component in an amount ranging from 5 to 25 kg/m3.
According to yet another aspect of the present invention, there is provided a
method of
reducing borehole instability in a coal formation, said method comprising the
steps of:
- providing fluid composition for use in drilling comprising:
- a brine-based fluid;
- a carbohydrate component in an amount ranging from 4 to 20 kg/m3; and
- a strengthening component in an amount ranging from 5 to 25 kg/m3.
- injecting the fluid composition into the borehole; and
- allowing the fluid composition sufficient contact time onto a surface of the
coal
formation to permit deposition of at least one of the carbohydrate component
and the
strengthening component onto the surface of the coal formation.
According to yet another aspect of the present invention, there is provided a
fluid
composition for use in strengthening a borehole in a coal formation when
drilling through said
coal formation comprising:
- a brine comprising calcium chloride and calcium nitrate having a specific
gravity
ranging from 1.355-1.600;
- a carbohydrate component in an amount ranging from 4 to 20 kg/m3; and
- a strengthening component in an amount ranging from 5 to 25 kg/m3;
wherein, when is use, the PV is less than 15 mPa-s, the YP is less than 2 Pa
and the HTIIP @
80 C is less than 6 int.
BRIEF DESCRIPTION OF THE FIGURES
The invention may be more completely understood in consideration of the
following
description of various embodiments of the invention in connection with the
accompanying
figures, in which:
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Figure la is a picture showing coal pieces broken off from the interior of a
wellbore
drilled through a coal formation and treated with a conventional coal wellbore
treatment
composition; and
Figure lb is a picture showing coal pieces broken off from the interior of a
wellbore
drilled through a coal formation and treated with a coal wellbore treatment
composition according
to a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Unlike most reservoir rocks, coal has low integrity and is friable.
Anthracite, which is
one of the .hardest forms of coal has a hardness of 2.2 on the Mohs hardness
scale. This
contributes to wellbore instability when drilling through coal. As described
by T. Gentzis in his
paper published in Energy Sources, Part A: Recovery, Utilization, and
Environmental Effects,
Volume 33, Issue 24, 2011, the formation of a filter cake or wall coating is
crucial when drilling
horizontally in weak coals at depth. By creating an effective filter cake, the
drilling fluid
maintains a constant positive pressure (AP = BHP ¨ FP; BHP = bottomhole
pressure and FP =
formation pressure) on the walls of the wellbore and prevent it from
collapsing.
The inventors have surprisingly and unexpectedly found that a drilling fluid
can be built
on a brine backbone that has the typical performance benefits of brines while
maintaining
wellbore stability when drilling through coal. It has been hypothesized by the
inventors that the
carbohydrate component when injecting into the borehole and allowed time to
make contact with
the coal formation forms a coat on the coal surface such as a base layer. It
is also thought that the
strengthening component (such as sulphonated asphalt in one preferred
embodiment) acts as a
glue on the coal surface.
According to a preferred embodiment, the brine-based drilling fluid of the
present
invention may comprise one or more of the following: halides, nitrates or
carboxylates of mono
or divalent cations such as sodium, potassium, cesium, calcium and zinc, a
fluid loss additive, a
wellbore strengthening additive, corrosion inhibitors, oxygen scavenger,
defoamers, lubricants
and other materials. It is believed that the fluid loss additive (the
carbohydrate component) and
wellbore strengthening additive work synergistically to provide the desired
wellbore stability in a
coal formation throughout the drilling operation.
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In yet another aspect, the fluid loss additive is selected from the group of
crosslinked and
non-crosslinked derivatized starch and low molecular weight
carboxymethylcellulose. Wellbore
strengthening additives are selected from the group consisting of: sulphonated
asphalt, oxidized
bitumen, resin or wax.
According to a preferred embodiment of the present invention, there is
provided a use of
the additives of the present invention as described herein in a drilling
fluid.
According to a preferred embodiment of the present invention, there is
provided a method
of preparing a drilling fluid, by combining one or more fluid loss and
wellbore strengthening
additives of the present invention as described herein with a brine phase and
optionally one or
more additional drilling fluid components as described herein.
According to a preferred embodiment of the present invention, there is
provided a method
of drilling into a formation that contains coal, by circulating a drilling
fluid in the well that
contains the additive of the present invention as described herein and
optionally one or more
additional drilling fluid components as described herein. The drilling fluid
may be circulated at
any stage during drilling or post-drilling operations.
According to a preferred embodiment of the present invention, there is
provided a drilling
fluid composition that strengthens the wellbore and alleviates wellbore
instability issues to an
acceptable level when used in a drilling operation.
According to a preferred embodiment of the present invention, the additives of
the
present invention that strengthen the wellbore and/or prevent wellbore
instability are selected
from the group of derivatized starch, low molecular weight carboxymethyl
cellulose, sulphonated
asphalt, oxidized bitumen, resin and paraffin wax.
Oil soluble hydrocarbon resins considered within the scope of the present
invention are
selected from the group consisting of: synthetic aliphatic or aromatic homo or
heteropolymers
obtained by the polymerization of at least one of the following monomers:
vinyl toluene,
dicyclopentadiene, indene, methylstyrene, styrene, methylenedienes,
pentadiene, 2-methyl-2-
butene, cyclopentadiene, cyclopentene. The resulting polymer contains many
double bonds that
can be further hydrogenated to obtain fully or partially hydrogenated
hydrocarbon resins.
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Hydrogenation of the polymer generally increases the softening temperature of
the hydrocarbon
resin.
Paraffin wax is a natural product that consists mostly of straight chain and
branched
hydrocarbons that typically contain 20 to 50 carbon atoms.
According to a preferred embodiment of the present invention, the drilling
fluid
composition has a concentration of the fluid loss and wellbore strengthening
additive sufficient to
form a strong filter cake that stabilizes the wellbore. The concentration of
the additive may be up
to about 2% by weight of the drilling fluid. In one aspect, the additive is
present at 0.04% to 2%
w/w.
In the examples evaluating the various preferred embodiments of the present
invention,
brines of various densities were used as the base fluids to which was added
from 8 to 15 kg/m3 of
crosslinked carboxymethyl starch, 10 kg/m3 sulphonated asphalt, 5 L/m3
corrosion inhibitor and 5
L/m3 oxygen scavenger.
The drilling fluid may alternatively comprise essentially a water base and the
one or more
of the additives of the present invention without the addition of a
viscosifier, fluid loss or anti-
accretion additives.
In order to assess the effectiveness of the drilling fluid compositions of the
present
invention to form a strong filter cake that prevents loss of drilling fluid
into the coal formation the
following experiments were conducted.
Example 1: preparation of drilling fluid
Brines of various densities were used as the base fluids to which was added 8
to 15 kg/m3
crosslinked carboxymethyl starch, 10 kg/m3 sulphonated asphalt, 5 L/m3
corrosion inhibitor and 5
L/m3 oxygen scavenger. In addition, 15 kg/m3 coal dust was added to simulate
drilled cuttings.
The mixture was mixed on a Hamilton Beach mixer and aged for 16 h in order to
allow the
chemical additives to hydrate in brine. Rheology and fluid loss properties
were measured at 50
and 80 C, respectively in accordance with API 131 testing procedures. In the
tables below, PV
represents the slope of the shear stress/shear rate line above the yield
point. YP indicates the
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yield stress extrapolated to a shear rate of zero. HPHT is identified as a
test to measure the static
filtration behavior of water mud or oil mud at elevated temperature.
Table 1 reports the results of a series of tests carried out to assess the
Properties of
drilling fluid built with 1.3 S.G. CaCl2 brine with no addition of starch,
addition of 15kg/m3 of
starch and addition of 8kg/m3 of starch. An indication of "no fluid retention"
means that the fluid
penetrates the coal formation rather than coating it at the surface.
Preferably, a YP value of less
than 2 is desirable. Preferably, a PV value of less than 15 is desirable. The
lower the HTHP
value obtained the better the composition rates. Ultimately, the best
combination of the three
values provides the most desirable choice of composition to be used.
Table 1: Properties of drilling fluid built with 1.3 S.G. CaCl2 brine
Rheology (a) 50 CaC12 brine 1.3 S.G.
Unit (No Starch added) 15 kg ni t iic 11
8 kg/m3 Starch
C
=
Dial
600 10.3 17.8 10.9
Reading
Dial
300 5.4 9.7 5.9
Reading
Dial
200 3.5 6.9 3.8
Reading
Dial
100 1.8 3.9 2.1
Reading
Dial
6 0.4 0.2 0.3
Reading
Dial
3 0.3 0.1 0.3
Reading
PV mPa-s 4.9 8.2 5.0
YP Pa 0.25 0.9 0.45
Gel - 10 second Dial 0.3 0 0.3
Reading
Gel - 10 minute Dial 0 0 0.3
Reading
HTHP @ 80 C mL No fluid retention 4.2 4.9
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Table 2 the results of a series of tests carried out to assess the properties
of drilling fluid
built with 1.4 S.G. CaCl2/Ca(NO3)2 brine with no addition of starch, addition
of 15kWm3 of starch
and addition of 8kWm3 of starch.
Table 2: Properties of drilling fluid built with 1.3 S.G. Potassium Formate
brine
KOOCH brine 1.3
Itheology lee 50
Unit S.G. I ku, ni3 Starch 8 kg/m3 Starch
0(
(No Starch added)
Dial
600 3.2 9.0 5.1
Reading
Dial
300 1.8 5.0 2.8
Reading
Dial
200 1.6 3.3 2.3
Reading
Dial
100 1.5 1.7 1.0
Reading
Dial
6 0.3 0.2 0.4
Reading
Dial
3 0.2 0.2 0.4
Reading
PV mPa.s 1.3 4.0 2,1
YP Pa 0.2 0.5 0.35
Dial
Gel - 10 second 0.2 0.2 0.4
Reading
Gel - 10 minute Dial 0.3 0.1 0.2
Reading
F1THP @ 80 C m1_, No fluid retention 8.0 8.6
Table 3 reports the results of a series of tests carried out to assess the
properties of
drilling fluid built with 1.4 S.G. CaC12/Ca(NO3)2 brine with no addition of
starch, addition of
15kg/m3 of starch and addition of 8kg/m3 of starch.
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Table 3: Properties of drilling fluid built with 1.4 S.G. CaCl2/Ca(NO3)2 brine
CaCl2/Ca(NO3)2 brine
Rheology Of 50
Unit 1.4 S.C. 15 kg/m3 Starch 8 kg/m3 Starch
C
(No Starch added)
Dial
600 10.3 33.8 19.7
Reading
Dial
300 5.4 18.2 10.5
Reading
Dial
200 3.5 12.6 7.2
Reading
Dial
100 1.8 6.8 3.8
Reading
Dial
6 0.4 0.5 0.4
Reading
Dial
3 0.3 0.4 0.3
Reading
PV triPa.s 4.9 15.6 9.4
YP Pa 0.25 1.15 0.45
Gel - 10 second Dial 0.3 0.2 0.3
Reading
Gel - 10 minute Dial 0 0.2 0.1
Reading
HTHP @ 80 C mL No fluid retention 2.8 5.0
Table 4 reports the results of a series of tests carried out to assess the
properties of
drilling fluid built with 1.5 S.G. CaCl2/Ca(NO3)2 brine with no addition of
starch, addition of
15kg/m3 of starch and addition of 81(g/m3 of starch.
Table 4: Properties of drilling fluid built with 1.5 S.G. CaCl2/Ca(NO3)2 brine
CaCl2/Ca(Nth)2 brine
Rheology 50 Unit 1.5 S.G. 15 kg m' '4:irch 8 kg/m3 Starch
C
(No Starch added)
Dial
600 19.7 62.9 36.0
Reading
Dial
300 9.9 34.6 18.9
Reading
Dial
200 6.4 24.2 12.7
Reading
Di al
100 3.2 13.2 6.8
Reading
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Dial
6 0.4 1.0 0.5
Reading
Dial
3 0.2 0.7 0.4
Reading
PV mPtrs 9.4 28.7 17.1
YP Pa 0.25 2.9 0.8
Gel ¨ 10 second Dial 0.4 0.4 0.3
Reading
Gel ¨ 10 minute Dial 0.2 0.5 0.2
Reading
HTHP @ 80 C mL No fluid retention 3.6 4.0
Example 2: Field trial
The composition according to a preferred embodiment of the present invention
was
prepared and used in a wellbore drilled through a coal formation. The intent
was to demonstrate
that it provided additional wellbore strength thus maintaining wellbore
integrity to allow for
drilling and other operations to be successfully completed.
Numerous fluids were used during the drilling operation, these included:
JetCorr at a concentration ranging between 8-10L/m3;
JetScav at a concentration of 3L/m3 initially and then 1-2L/m3 monitored at
wellsite;
JetSeal at a concentration of 12kg/m3; and
CoalCure W at a concentration ranging between 8-10kg/m3.
JetCorr is a corrosion inhibiting composition comprising ethanol, 2,2'-oxybis-
, a
reaction product with ammonia). It is a filming amine as it lays a protective
coating on pipe to
prevent corrosions cell from forming. JetScav is an oxygen scavenging
composition comprising
Diethylhydroxylamine. It is designed to react with dissolved oxygen in the
fluid. JETSEAL is a
modified starch composition comprising carboxynethyl starch. It is hydrated in
the brine and
provides an increase in the viscosity of the fluid. COAL CURE W is a
composition comprising
sulphonated asphalt, sodium sulphonated asphalt. It is a malleable composition
whoe function is
mainly to seal and bond coal pleats. JETZAN was also used in an amount
ranging from 3-6
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kg/m3. This composition comprises a xanthan gum and is a biopolymer for
increasing the
viscosity of the fluid.
During the trial, many chemicals were used for pH control, these included:
Brine Buffer
Amine (comprising an alkoxylated amine); MagOX (Magnesium Oxide); lime
(Calcium
Hydroxide); and caustic soda (Sodium Hydroxide).
Many operations were performed during the drilling of the wellbore through a
coal
formation, this ranged from tripping (pulling out the entire drillstring from
the well bore) to
reaming the interior of the wellbore after reaching a distance of 4000 m from
the wellhead. The
use of conventional compositions yielded a more brittle interior wellbore as
exemplified in Figure
la. After treatment with a composition according to a preferred embodiment of
the present
invention, the reaming along a similar distance yielded coal fragments (from
the interior of the
wellbore) as seen in Figure lb. There is a clear difference between the
conventional treatment
and treatment with a composition according to the present invention. Treatment
with a
composition according to the present invention yielded a clearly stronger
inner wellbore through
a coal formation. This stronger inner wellbore provides wells with greater
integrity facilitates
performing successfully various operations when dealing with wellbores drilled
through coal
formations. Such results also leads to shortened durations for the need of a
drill team where the
savings on the need for a drilling crew can be upwards of 100,000$ while the
total savings reach
well beyond 100,000$ compared to the treatment with a conventional coal well
treatment.
Savings can come in various forms, from fewer tripping steps, to fewer
equipment breakdowns,
less downtime and increased rate of penetration.
The terms and descriptions used herein are set forth by way of illustration
only and are
not meant as limitations unless otherwise specifically indicated. Those
skilled in the art will
recognize that many variations are possible within the scope of the invention
as defined in the
following claims, and their equivalents, in which all terms are to be
understood in their broadest
possible sense unless otherwise specifically indicated. While the compositions
shown and
described in detail herein are fully capable of attaining the above-described
aspects of the
invention, the persons skilled in the art will understand that they are but a
preferred embodiment
of the present invention and the invention is not to be limited to those
embodiments.
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