Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Drill Fluid and Method for Tunneling
Field
This invention relates to drilling fluids and methods boring a subterranean
tunnel
through a formation, such as for pipelines.
Background
In the tunnel boring process, a drill bit, mud motor and other drilling
assembly
components including reamers are used to bore a tunnel below a river, a lake,
an
ocean strait, a road or other obstacle. A drilling fluid with specific
physical
properties is circulated down through a drill pipe, out through the bit and up
the
annulus in order to clean the tunnel. In some situations the fluid is pumped
out the
drill pipe and suspends drilled cuttings that must be pushed out the other end
of
the tunnel.
Pipeline tunnel boring is an operation that is required to safely place
pipelines,
such as those used for transmitting oil, gas, water, sewage, below rivers,
ocean
straits, roads, lakes and other such natural and man-made obstacles in an
efficient
and environmentally safe manner. Pipeline boring involves the use of a rotary
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drilling rig or sometimes two rigs with one rig on either side of a river for
example.
The drilling process typically involves a drill bit with a directional motor
assembly
that is capable of steering the drilling down, then to horizontal and then up
to the
surface on the other side of the obstacle (for example the opposite side of a
river).
The process of tunnel boring has a number of unique technical challenges.
The size of the tunnels varies but the most challenging tunnels have diameters
in
the range of 30 to 80 inches, for example about 50 to 60 inches with one
common
tunnel size being 1371.6 mm (54"). The length of the tunnel from its surface
entry
point to its exit can be in the order of kilometers. They are often drilled in
very
young formations that can contain a very large fraction of easily hydratable
clays
or loose tills. In addition, the true vertical depths are often 100m or less.
The
fracture gradients of the formations drilled can be low and it is possible to
induce
fractures to surface (underneath a river for example) with the result of whole
losses
of the circulating drilling fluid. It is possible that such fractures will
communicate
with the surface with the result that the surface becomes contaminated with
the
drilling fluid. For example, it is a significant concern that an
environmentally
sensitive area such as a river becomes contaminated with drilling fluid. It is
possible that such contamination results in the problematic accumulation of
the
fluid in the environment.
Other formation types encountered while drilling pipeline tunnels can include
carbonates which may be naturally fractured and which can also be the source
of
losses, sandstones that can be very permeable, can be the source of losses.
Such
losses may either dissipate into the subterranean formation or communicate
with
the surface as an outcrop. Gravel deposits can cause losses and also sometimes
require the fluid conveyed removal of very large rocks out of the tunnel. Coal
and
lignite can also be encountered and these can result in losses to friable and
fractured formations or result in tunnel borehole instability due to the
weakness of
the formations. There can also be problems with the interaction of coal or
lignite
with the components used to viscosify the drilling fluid (usually evidenced by
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thinning) and this can result in increased difficulties cleaning the tunnel
borehole
of drilled cuttings.
For larger diameter tunnel bores, the process of drilling can include drilling
a
smaller diameter pilot hole and then deploying an expandable reamer that
scrapes
the tunnel borehole walls and increases the diameter on one or more subsequent
passes to the required size.
The engineering parameters coupled with the equipment and techniques deployed
can present challenges. For example, current equipment used have limited fluid
pump capacity and, for very large diameter tunnel boreholes, the fluid annular
velocity can be very low. It is possible that drilled cuttings accumulate and
if not
remedied will result in the drilled cuttings plugging the tunnel borehole and
restricting the fluid flow to the point that the tunnel borehole pressures up.
If this
occurs, there can be induced fracturing of the formation being bored with the
subsequent loss of fluid to either subterranean formations or, in the worst
cases,
to surface.
Summary
In one aspect of the invention, there is provided a method for drilling a
tunnel
through a formation, the method comprising: preparing a mixed metal-
viscosified
drilling fluid including bentonite, a mixed metal viscosifier and controlling
pH to 8.5
to 9.5 to permit a reaction between the bentonite and the mixed metal
viscosifier;
adding at least one of: (i) calcium sulfate and (ii) a potassium salt; and
pumping
the drilling fluid while drilling the tunnel with the pH lowered to 7 ¨ 9.
In accordance with another aspect of the present invention, there is provided
a
method for drilling a tunnel through a formation, the method comprising:
providing
a mixed metal-viscosified drilling fluid; circulating the drilling fluid
through the
tunnel while drilling into the formation; monitoring the drilling fluid for a
condition of
drilling indicative of a problematic increase in viscosity of the drilling
fluid; adding
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to the drilling fluid at least 1`)/0 w/v potassium salt; and adding a non-
toxic anionic
thinner to the drilling fluid to adjust the viscosity of the drilling fluid.
In accordance with another aspect of the present invention, there is provided
a
method for drilling a tunnel through a formation, the method comprising:
providing
a mixed metal-viscosified drilling fluid; circulating the drilling fluid
through the
tunnel while drilling into the formation; monitoring the drilling fluid for a
condition of
drilling indicative of a problematic increase in viscosity of the drilling
fluid; adding
calcium sulfate to the drilling fluid to bring the concentration to at least
0.05% w/v
calcium sulfate in the drilling fluid; and adding a non-toxic anionic thinner
to the
drilling fluid to adjust the viscosity of the drilling fluid.
In accordance with another aspect of the present invention, there is provided
a
mixed metal-viscosified drilling fluid comprising: water; 15 to 45 kg/m3
bentonite;
a mixed metal viscosifier at a weight ratio of 1:3 to 1:30, viscosifier to
bentonite; at
least 0.05% w/v calcium sulfate and/or at least 1% w/v potassium salt; a
polyacrylate anionic thinner; and a base to maintain the pH above 7Ø
In accordance with another aspect of the present invention, there is provided
a
method for boring a tunnel including circulating an MMH-based drilling fluid
with a
non-toxic anionic additive. The fluid is able to incorporate a large amount of
reactive clays and a non-toxic anionic thinner reduces the viscosity in a
controlled
manner and is environmentally acceptable.
It is to be understood that other aspects of the present invention will become
readily
apparent to those skilled in the art from the following detailed description,
wherein
various embodiments of the invention are shown and described by way of
example. As will be realized, the invention is capable for other and different
embodiments and its several details are capable of modification in various
other
respects, all without departing from the spirit and scope of the present
invention.
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Accordingly the detailed description and examples are to be regarded as
illustrative in nature and not as restrictive.
Description of Various Embodiments
The detailed description and examples set forth below are intended as a
description of various embodiments of the present invention and are not
intended
to represent the only embodiments contemplated by the inventor. The detailed
description includes specific details for the purpose of providing a
comprehensive
understanding of the present invention. However, it will be apparent to those
skilled in the art that the present invention may be practiced without these
specific
details.
While other drilling systems by the present applicant can be useful for
drilling
boreholes of all kinds including those for tunnels, a number of environmental
and
other issues have been considered for the drilling fluids used in this process
due
to the possibility of exposure to the ecosystems in surface soil, ground
water,
rivers, oceans and wildlife habitats. These include:
1) Toxicity ¨ there are restrictions on the toxicity of drilling fluid
components:
all components must be non-toxic. Many components used in
conventional oil and gas drilling fluids cannot be used;
2) pH ¨ usually restricted to 6.0 ¨ 9.5 range;
3) Density ¨ high fluid density can result in induced fracturing of
formations;
and
4) Aqueous - All fluids must be water based -- For boring under ocean straits
the base fluid may be sea water.
Mixed metal-viscosified drilling fluids include a mixed metal viscosifier,
which is an
inorganic particle based on magnesium/aluminum oxides and/or hydroxides. They
are commonly known as mixed metal hydroxides and sometimes referred to as
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mixed metal oxide (MMO), mixed metal hydroxide (MMH) and combinations of
mixed metal oxide and hydroxide (MMOH). Mixed metal viscosifier, sometimes
collectively referred to as MMH, is a mixed metal layered hydroxide compound
of
the following empirical formula:
MImM"n(OH)(2m+3n+qa+br)(A9a(Br)b XH20,
where M' represents at least one divalent metal cation and m is an amount of
from
greater than zero to about 8; where M" represents at least one trivalent metal
cation and n is an amount of from greater than zero to about 6; where A is an
anion
or negative-valence radical that is monovalent or polyvalent, and a is an
amount
of A ions of valence q, provided that if A is monovalent, a is from greater
than zero
to about 8, and if A is polyvalent, a is from greater than zero to about 4;
where B
is a second anion or negative-valence radical that is monovalent or
polyvalent, and
where b is an amount of B ions of valence r and b is from zero to about 4;
provided
(m+n) is greater than or equal to 1; further provided qa+br cannot be greater
than
2m+3n; provided that qa cannot equal 2m+3n; and still further provided that
(2m+3n+qa+br) is less than 3; and where xH20 represents excess waters of
hydration, with x being zero or more. In
certain preferred embodiments
(2m+3n+qa+br) is less than 2, more preferably less than 1, and most preferably
less than 0.5.
While M' can represent any divalent metal cation of the Groups IA, IIA, VIIB,
VIII,
IB or IIB of the Periodic Table, preferred divalent cations are Mg, Ca, Mn,
Fe, Co,
Ni, Cu, and Zn, and more preferred are Mg and Ca. M" is a trivalent metal
cation
selected from Groups IA or VIII, but preferred are Al, Ga and Fe, and more
preferred is Al.
There must also be present at least one anion or negative-valence radical, A,
and
in some cases one (or more) additional anions or negative-valence radicals, B,
may also be present. Examples of these anions and negative-valence radicals
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include carbonates, amines, amides, chlorides, oxides, and the like. Preferred
therefor are carbonates, oxides, chlorides and amides.
Alternatively, a combination of materials which can contribute the proportions
of
constituents of the above empirical formula can be employed.
One mixed metal viscosifier of interest is the mixed metal hydroxide of the
formula
[Mg0.7A10.3(OH)2](OH)0.3. Another mixed metal viscosifier of interest is
Al/Mg(OH)4.7C10.3. Mixed metal viscosifiers are commercially available such as
from BASF Oilfield Polymers Inc. under the trademark PolyvisTM. For example,
Polyvis IITM is a mixed metal hydroxide viscosifier.
Mixed metal viscosified drilling fluids have become more popular recently due
to
the present applicant's efforts to employ salts in the fluids that reduce or
prevent
the thinning effect from drilling coals with MMH viscosified fluids. Calcium
sulfate
and/or potassium salts including one or more of potassium sulfate, potassium
chloride, potassium acetate and potassium formate may substantially maintain
the
rheology of mixed metal-viscosified drilling fluids when drilling with coal
contaminants. Such salts may add a benefit of shale swelling inhibition,
possibly
as a result of the presence of the potassium ion or calcium ion from the salt.
The
salts are believed to protect the electrostatic relationship between the clay
and the
viscosifier.
These MMH-based fluids are excellent hole cleaning fluids and in many ways are
well suited for applications such as tunneling. However, the fluids are pH
dependent and require a basic pH to maintain appropriate rheology. Also, the
MMH additive interacts strongly with the young high reactive clays that can be
incorporated through a tunneling process. Clay incorporation can result in a
dramatic and sometimes uncontrollable increase in fluid viscosity.
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Surprisingly, however, methods have been found for tunneling with MMH-based
fluids and, in fact, these fluids have proven to be valuable to address holes
where
there is the risk of significant fluid loss such as is encountered in
tunneling for
pipeline installs. In one embodiment, a method for tunneling employs a
drilling fluid
that is water-based, pH controlled and includes bentonite, a mixed metal
viscosifier
and at least one of: (i) calcium sulfate and/or (ii) potassium salts including
one or
more of potassium sulfate, potassium chloride, potassium acetate and potassium
formate. Calcium sulfate (gypsum) has proven to be quite useful.
In an MMH-based fluid including calcium sulfate, concentrations of calcium
sulfate
greater than 0.05% (weight by volume) may be effective in the mixed metal-
viscosified drilling fluid. While amounts of up to 5% or more may be used,
generally
concentrations of 0.05% ¨ 1.0% (weight by volume) calcium sulfate and, for
example, 0.05 - 0.5% salt (weight by volume) or 0.1 ¨ 0.5% concentrations have
been found to be both effective for stabilizing the drilling fluid against
adverse
rheological changes and advantageous in terms of economics. These fluids have
been employed for drilling coals. For example, in younger coals or where
significant coal deposits must be drilled, higher concentrations (for example
greater than 0.3% and for example 0.3 ¨ 1.0%) of calcium sulfate in the
drilling
fluid may be useful. It is believed that the calcium sulfate reaches
saturation at
about 2 to 3 kg/m3, (0.2 to 0.3% (w/v)), but excess amounts may be added
without
an adverse effect and in fact may create a buffer of salt to maintain
activity,
provided the fluid remains a liquid which can be circulated through the tunnel
borehole. Generally, based on a cost/benefit analysis, an upper limit of 1.0%
or
more likely 0.5% is considered sound.
With respect to potassium salts, potassium sulfate and/or potassium chloride
have
shown the best results with potassium sulfate being particularly preferred. A
wide
range of potassium salt concentrations, such as concentrations greater than 1
(:)/0
(weight by volume), may be effective in the mixed metal-viscosified drilling
fluid.
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Generally concentrations of 1 ¨ 10% (weight by volume) salt and, for example,
1 -
5% salt (weight by volume) concentrations have been found to be both effective
for stabilizing the drilling fluid against adverse rheological changes due to
coal
contamination and advantageous in terms of economics. The amount of salt
added to the drilling fluid may be determined by the amount of coal to be
drilled
and/or by the shale reactivity. For example, younger coals, more so than older
coals, tend to create greater rheological instability for mixed metal-
viscosified
drilling fluids and, thus, higher concentrations (for example greater than 3%
and
for example 3 ¨ 10%) of potassium salts in the drilling fluid may be useful.
Also, if
it is determined that there are significant coal deposits through which the
tunnel
must be drilled, again higher concentrations of potassium salts may be useful.
Bentonite, which is a form of clay, is commonly used in drilling fluids and
its use
will be well understood by those skilled in the art. In this method, various
types of
bentonite are useful such as polymer-treated bentonite, untreated bentonite.
An
untreated bentonite may be particularly useful. Such a bentonite may be known
commercially as untreated bentonite with a high content of sodium
montmorillonite,
natural bentonite or untreated Wyoming bentonite.
Generally, mixed metal-viscosified drilling fluids are made up with low
concentrations of bentonite such as, for example, about 15 to 45 kg/m3 or 20
to
40 kg/m3 bentonite in fresh water. Sea water-based mixed metal-viscosified
drilling fluids can accommodate more bentonite, as will be appreciated.
Considering that many bentonite based (non-mixed metal) drilling fluids can
contain many multiples more (i.e. two to four times) bentonite than in a mixed
metal-viscosified drilling fluid, it can be appreciated that the viscosity
generated
using such low concentrations of bentonite for mixed metal-viscosified
drilling fluids
might be insufficient for hole cleaning.
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The addition of mixed metal oxide, mixed metal hydroxide or mixed metal oxide
and hydroxide at a weight ratio of 1:3 to 1:20, 1:7 to 1:12 or 1:8 to 1:10.5
to the
added bentonite produces a stable fluid. When tunneling, contact with clays in
the
formation may drive the actual clay concentration up considerably. As
mentioned,
the rheology of mixed metal-viscosified drilling fluids is sensitive to
increases in
clay content, as may occur when drilling young, sedimentary formations, which
are
the common formations bored during tunnel boring. As noted above, mixed metal-
viscosified drilling fluid systems can only operate within a relatively narrow
range
of active clay concentrations. If such a system does incorporate a significant
amount of water-reactive clays, it will develop a problematic rheological
profile for
example a large increase in viscosity. While, as noted above, MMH-viscosified
fluids had been shown to react adversely to high clay loading, in the current
method, drilling at depths of less than 100m proceeds successfully even with
high
clay to MMH ratios. After drilling is initiated with the initial drilling
fluid wherein
MMH is at a weight ratio of 1:3 to 1:20 to the added bentonite, drilling can
proceed
with only limited addition of, or without adding, further MMH viscosifier to
maintain
that ratio. It is noted that a methylene blue test (MBT) procedure can be
employed
to quantitatively analyze the drilling fluid clay content (both added and
entrained
by drilling). In one embodiment, the method includes preparing a drilling
fluid
wherein MMH is at a weight ratio of 1:3 to 1:20 to the added bentonite. This
may
include making up the drilling fluid with slightly less bentonite than
previously used,
such as a weight ratio of 1:3 to 1:12 or 1:7 to 1:9:5 MMH : added bentonite.
The
drilling fluid will also include the salt: calcium sulfate or potassium salts.
Then,
after commencing drilling, the method can include continuing to pump the
drilling
fluid with the MMH : total clay (for example as determined by MBT) being up to
1:30, for example, in the range of 1:20 ¨ 1:30. This method includes drilling
into
young reactive clays and a portion of the total clay arising from the
entrainment of
young reactive clays. In some methods, the drilling fluid's total clay is in
excess of
100kg/m3. As drilling proceeds, further amounts of the salt may be added to
maintain the salt concentrations noted above. Possibly, MMH may be added in
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limited quantities. However, it may not be necessary to add further bentonite,
as
its concentration will be maintained by entrained clays. This offers a
considerable
cost savings on chemicals and is environmentally advantageous, since any
chemical load on the environment is significantly reduced.
As noted above, the MMH-viscosified fluids are considered pH sensitive. In
order
to create a stable fluid, prior methods raised the pH to greater than 10.
However,
the current method includes mixing the bentonite, MMH and calcium sulfate or
potassium salt and, in order to trigger the reaction between the MMH and the
bentonite, bringing the pH to 8.5 to 9.5 by addition of caustic soda, caustic
potash,
potassium carbonate, lime and/or soda ash. Then, once the bentonite/mixed
metal
viscosifier reaction is complete and a gel is formed, the drilling fluid can
be pumped
with the pH lowered to between pH 7 and 9 without a problematic loss in
viscosity.
Caustic soda, caustic potash, potassium carbonate, lime and/or soda ash may be
added to control the pH in the range of 7-9. If environmental toxicity is a
concern,
pH control may use lime.
In the fluid with lower pH, the higher MBTs noted above can be more readily
achieved. It is believed that the pH 7-9 reduces the density of anionic charge
around the clay platelets and attenuates the interaction between cationic clay
platelet faces and the overall anionic edges.
In one embodiment, a mixed metal-viscosified drilling fluid may include an
aqueous
mixture of about 20 to 30 kg/m3 bentonite, a mixed metal moiety in a quantity
of
about 1:7 to 1:10 MMO, MMH or MMOH to bentonite, pH controlled to pH 8.5-9.5
and 1 to 5% potassium salt and/or 0.05 to 1.0% calcium sulphate. After the gel
forms, the pH may be reduced to pH 7-9.
Additives for fluid loss control, lost circulation, etc. may be added to the
drilling fluid
mixture, as desired. Non or minor-ionic additives may be most useful. Some
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examples may include starch for fluid loss reduction, organophillic lost
circulation
materials (LCM), etc. Simple testing may verify the compatibility of any
particular
additive with the drilling fluid.
To produce the drilling fluid, the bentonite may first be hydrated in water.
Then the
mixed metal moiety is added and pH is adjusted. The potassium/calcium salt can
be added to the aqueous mixture of bentonite and mixed metal at any time.
Additives such as LCM, fluid loss control agents, etc. can also be added when
appropriate, as will be appreciated.
As will be appreciated, the drilling fluid may be circulated through the drill
string,
drill bit and tunnel bore annulus while drilling. Circulation of the drilling
fluid may
continue even when drilling is stopped in order to condition the well, prevent
string
sticking, etc.
During the drilling and circulation, the yield point of the drilling fluid may
be
maintained above 10Pa to provide advantageous effects.
The mixed metal-viscosified drilling fluids described herein are useful for
boring
into various types of formations. Even when contacting coal or clay, such
fluids
retain their advantageous properties such as relatively high yield points,
high low
end rheology and high and fragile gel strengths. Such
properties are
advantageous for use in boring tunnels, whether vertical, directional or
horizontal
due to superior hole cleaning capabilities and because these fluids mitigate
against
whole mud fluid losses to formations whether via formation fractures or high
permeability sections.
While the tunnel will generally be drilled to a depth of less than 100m, the
tunnel
can be at any depth, any orientation and through any rock type, such as for
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example, through gravels, clay, carbonates, sandstones, shales, coal, oil
shales,
etc. The formation can be one known to contain clay and/or coal or otherwise.
The addition of salts, such as for example calcium sulfate, in sufficient
amounts as
noted above, prevents the collapse of the unique visco-elastic properties of
the
MMH ¨ bentonite fluids when exposed to coal or lignite almost completely and
it is
possible to drill through coal seams, even horizontally where significant coal
contact may be encountered. The use of such a fluid mitigates against whole
fluid
loss into the coal formation, which are typically highly fractured due to the
unique
rheological properties of the fluid.
The present method is suitable for drilling with clay loading in excess of
what was
previously thought possible.
However, even with the present method,
incorporating very high concentrations of clay may risk problematic rheology
and
may stress pumping systems. In fact, drilling with the present drilling fluid
through
active, young clay zones, with unavoidable incorporation of clay to excessive
MBTs, may increase fluid rheology, such that the drilling fluid may become
substantially un-usable (i.e. unpumpable).
Thus, in one embodiment, a method for boring a tunnel through a formation
includes: adding a non-toxic anionic thinner to the drilling fluid to adjust
the
viscosity of the drilling fluid. One method for example, includes: providing a
mixed
metal-viscosified drilling fluid; circulating the drilling fluid through the
bore hole
while boring into the formation; adding calcium sulfate to the drilling fluid
to bring
the concentration to 0.05 to 1.0% w/v calcium sulfate; and adding a non-toxic
anionic thinner to the drilling fluid to adjust the viscosity of the drilling
fluid.
Alternately, at least 1`)/0 w/v potassium salt, as described above, may be
employed
in the method in place of or in addition to the calcium salt.
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Without the addition of a potassium salt or the calcium sulfate, the use of
anionic
thinners would reduce the viscosity of the mixed metal-viscosified drilling
fluid to
nearly that of water. Without the use of an anionic thinner, the fluid may
become
unworkably viscous when drilling into clay.
In this embodiment, the mixed metal-viscosified drilling fluid can be
according to
that described above: an aqueous mixture of a mixed metal viscosifier, as
described above, and bentonite, as described above, with pH control between 7-
9.5, possibly with the method as described above, and a non-toxic anionic
thinner.
The drilling fluid including the anionic thinner can be employed for the
entire tunnel
boring project or in response to identifying a condition of problematic
rheological
change such as one indicative of a problematic increase in the clay content of
the
drilling fluid. For example, the step of identifying may consider the location
of the
hole being drilling, for example using drilling measurements, relative to the
location
of known clay deposits, for example using formation logs or offset coring. If
it is
determined that the hole being drilled may, or is going to, pass through
problematic
clay deposits, then this can be noted according to the method and the step of
adding an anionic thinner may be initiated when or before the drilling process
begins in the clay deposit.
The salt (potassium salt and/or calcium sulfate) may be added to the drilling
fluid
at any time. For example, the salt may be added during the initial production
of
the drilling fluid, such that the salt is present in the system throughout the
drilling
operation or the salt may be added only after identifying a condition of
drilling
indicative of an increase in the clay content of the drilling fluid. Although
the salt
may be added after problematic clay contamination occurs, it is recommended to
pre-treat the system for best results. In one embodiment, for example, the
initial
hole can be drilled down to approximately the level of the first clay deposit
using
any drilling fluid of interest, including for example, prior art mixed metal-
viscosified
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drilling fluids. When it is determined that a clay deposit is close below
bottom hole
or when the clay deposit has been reached, the drilling fluid may be changed
over
to one including a mixed metal-viscosified drilling fluid containing an amount
of the
salt and the anionic thinner.
Alternately, the borehole may be drilled down to and into a clay deposit using
a
mixed metal-viscosified drilling fluid containing greater than 1% w/v
potassium salt
and/or greater than 0.05% calcium sulfate and then the anionic thinner may be
added.
As another example, the entire tunnel substantially from surface, which may
include drilling from surface or from below the overburden, may be drilled
using a
drilling fluid including a mixed metal viscosifier, bentonite, the appropriate
amount
of the salt and the non-toxic anionic thinner.
The anionic thinner could be added at any time. In one embodiment, the entire
tunnel is bored using the fluid with the non-toxic anionic thinner. In another
embodiment, the anionic thinner may be added to the drilling fluid after
identifying
a condition indicating a problem with the fluid such as a risk of fluid loss
or that the
drilling fluid has an increased clay concentration. For example, the anionic
thinner
may be added when it is expected that the tunnel borehole is to be drilled
into a
clay deposit.
Alternately or in addition, fluid rheology can be monitored, the viscosity of
the fluid
can be measured or the concentration of clay in the drilling fluid can be
monitored
directly to identify a condition indicative of an increase in the clay
concentration.
In one embodiment, for example, the MBT procedure can be employed to
quantitatively analyze the clay content of the drilling fluid. In another
embodiment,
the fluid viscosity may be monitored as by determining the funnel viscosity or
more
accurately with a device such as a rheometer, such as a Fann 35 rheometer.
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When the viscosity increases beyond an acceptable level, a condition
indicative of
an increase in clay content is identified. Once the clay concentration or the
viscosity indicates a problematic condition, the thinner may be added. In one
embodiment, for example, thinner is added to address problematic rheological
profiles.
The condition indicating that there is a problematic condition may vary
depending
on the equipment and operator's preferences. The fluid must be pumpable and
the anionic thinner may be added to ensure that the drilling fluid remains
pumpable. In some example embodiments, the thinner may be added as follows:
a) when funnel viscosity reaches 70 seconds/litre;
b) when the Fann 35 YP reaches 35 Pa to 45 Pa or possibly when the yield
point reaches 30 Pa (at yield point=60 Pa pumping generally becomes
problematic
for most rigs); or
c) using MBT, when the test indicates clay at 60kg/m3.
Anionic thinners of interest are anionic chemicals or minerals including coal
fines,
lignite, lignite resin, humalite, poly-anionic cellulose, tetra potassium
pyrophosphate (TKPP), sodium acid pyrophosphate, tetra sodium pyrophosphate,
polyacrylic acid, polyacrylates, polyacrylate co-polymers or xanthan gum that
are
non-toxic to environmental ecosystems, considered by regulators to be
compatible
with inclusion in ground water at the concentrations used. Polyacrylate-based
thinners such as polyacrylic acid, polyacrylates, polyacrylate co-polymers are
of
particular interest as they are considered environmentally safe and have a
high
activity. Suitable polyacrylate polymers typically have a molecular weight of
less
than 10,000 and in most cases less than 1,000 g/mol. Some anionic thinners
such
as caustic and tannin including sulfonated tannin (which is for example,
available
as DescoTM) are not appropriate, as they are considered toxic to some
ecosystems.
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The non-toxic anionic thinner may be added to the circulating drilling fluid.
If the
thinner is free flowing liquid or powder, it may be added directly.
The thinner is added in an amount sufficient to bring the fluid parameters to
acceptable levels. For example, thinner may be added and the fluid viscosity
monitored until the fluid has a viscosity reduced to less than YP=60 Pa or
possibly
less than 45 Pa or in some embodiments below YP=30 Pa (for example measured
using a Fann 35 rheometer). In any event, during the drilling and circulation,
the
yield point of the drilling fluid should be maintained above 10Pa to provide
advantageous effects.
The actual amounts of thinner used to achieve this above-noted rheological
profile
will vary depending on the activity of the thinner, the amount of clay
contamination,
etc. A useful concentration range for polyacrylate-based thinners is 0.1 to 10
L/m3
in the drilling fluid. Concentrations are dependent on desired final
viscosity.
After drilling through the one or more clay deposits in the path of the
borehole, the
present drilling fluid may continue to be used for the remainder of the tunnel
boring
operation or other drilling fluids may be used. However, if clay can continue
to
become entrained in the drilling fluid, for example where a clay deposit
remains
open to contact by the drilling fluid, it may be useful to continue using the
present
drilling fluid until drilling is complete or the possibility of clay
contamination is
eliminated. If desired, the drilling fluid returning to the mud tanks at
surface may
be monitored to determine the concentration of salt and thinner therein,
and/or
other parameters indicative of problematic clay content, to ensure that fluid
characteristics are maintained. For example, any one or more of the bentonite,
mixed metal viscosifier, base, salt or anion thinner may be added during
drilling to
adjust the drilling fluid parameters. In one embodiment, for example, an
amount
of mixed metal viscosifier may be added to the fluid during the course of a
drilling
operation where reactive formations are drilled and drill cuttings become
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incorporated into, and change the rheology of, the drilling fluid. In such a
case, the
addition of an amount of mixed metal viscosifier can cause the viscosity of
the fluid
to increase. In another embodiment, for example, an initial amount of an
anionic
thinner and further amounts of that or another anionic thinner may be added to
the
fluid during the course of a drilling operation where reactive clay formations
are
drilled and clay becomes incorporated into, and changes the rheology of, the
drilling fluid. In such a case, the addition of an amount of thinner can cause
the
viscosity of the fluid to decrease.
As noted above, other additives may be employed in the drilling fluid such as
starch
for fluid loss reduction, organophillic lost circulation materials (LCM), etc.
Simple
testing may verify the compatibility of any particular additive with the
drilling fluid.
To produce the drilling fluid, the bentonite may first be hydrated in water.
Then the
mixed metal moiety is added and pH is adjusted. The salt can be added to the
aqueous mixture of bentonite and mixed metal with or before the thinner.
Additives
such as LCM, fluid loss control agents, etc. can also be added when
appropriate,
as will be appreciated.
The following examples are included for the purposes of illustration only, and
are
not intended to limit the scope of the invention or claims.
Examples:
Example I:
A representative MMH viscosified base fluid was tested for controlled thinning
with
the test thinning agent short chain polyacrylate
Base fluid comprised of:
Natural Bentonite = 39.1 kg/m3
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MMH = 10.66 kg/m3
Gypsum = 12.25 kg/m3
Attapulgite = 17.4 kg/m3
Lime to control pH
Simulated Sea Water = 13 % v/v
The Base fluid was mixed and measured to which was added a concentrated
aqueous (40 % w/w) low molecular weight (LMW) polyacrylate polymer. The
polyacrylate solution comprises of polymer that typically has a molecular
weight of
less than 1000 g/mol.
Results are shown in Table I
Table I:
Dial Base Base + Base +
0.25L/m3 Base + 0.5L/m3
reading 3Kg/m3 (40% w/w) LMW (40%
w/w) LMW
Lignite Polyacrylate
Polyacrylate Sol'n
solution
R600 100 97 67 47
R300 92 89 62 42
R200 87 85 59 39
R100 81 80 56 35
R6 52 52 42 26
R3 36 36 37 25
PV 8 8 5 5
(mPa.sec)
YP (Pa) 42 40.5 28.5 18.5
The addition of 3 kg/m3 of lignite thinned the Base MMH viscosified fluid only
marginally as expected. The addition of 40% w/v low molecular weight
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polyacrylate solution (a concentration in the drilling fluid of approx. 0.1
kg/m3) thins
the fluid measurably but controllably. This compares with MMH viscosified
fluids
without the addition of calcium sulphate where the addition of polyacrylate
reduces
the viscosity of the Base fluid to nearly that of water.
Example II
Background: Nr Fox Creek Alberta (Crossing #26). Boring a 315m tunnel for
pipeline installation. The tunnel boring operations started with a 251mm pilot
hole
using the present invention as the drilling fluid. The pilot hole was reamed
to 508
mm and then to 762 mm.
Drilling Fluid: The circulating drilling fluid system for the 762mm ream
consisted of
a mixture of added natural bentonite pre-hydrated in fresh water, a mixed
metal
hydroxide viscosifier (MMH), gypsum and lime (pH = 8.0 ¨ 8.5) carried over
from
the 508 mm ream operation and that had incorporated a significant amount of
hydratable clays.
762 mm tunnel reaming operations: Made up the bottom hole assembly (BHA) with
reamer and commenced reaming operations with 1.17 m3/min pump rates. Large
amounts of sand were drilled and removed. The funnel viscosity was increased
from 46 to 65 s/L with the addition of pre-hydrated natural bentonite and MMH.
The
rheology and chemical properties were monitored throughout this operation.
Fluid
properties are shown in Table II.
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Table II:
Day 1 Day 3
Distance from Entry (m) 45 315
Density (kg/m3) 1150 1170
Funnel Viscosity (s/L) 46 49
pH 8.0 7.5
600 rpm 45 55
300 rpm 39 49
6 rpm 30 38
3 rpm 26 37
s - Gel strength (Pa) 13 18
10 min - Gel strength (Pa) 14 19
PV (mPa.$) 6 6
YP (Pa) 16.5 21.5
MBT (kg/m3 bentonite 64 64
equiv.)
MBT is a measure of the amount of reactive bentonitic clays in the drilling
fluid and
is the sum of the intentionally added natural bentonite plus a contribution of
reactive drilled clay cuttings incorporated into the drilling fluid.
The drilling fluid was successfully used to ream the tunnel from 508mm to
762mm
through sand and clay formations. The pH was allowed to drop from 8.0 to 7.5
while maintaining advantageous rheology properties. Successfully pulled 508 mm
diameter pipe through the tunnel
At the end of the tunneling project:
Total volume of fluid used: 350 m3
Total weight of MMH added: 911 kg
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Total weight of natural bentonite added: 7620 kg
MBT (kg/m3 bentonite equivalent): 64 kg/m3
Concentration of Bentonite added: 21.8 kg/m3
Concentration of MMH added: 2.6 kg/m3
Ratio of MBT: MMH = 24.6: 1
Example III:
Background: Nr Fox Creek Alberta (Crossing #9). Boring a 555m tunnel for
pipeline
installation. The tunnel boring operations started with a 311mm pilot hole
using the
present invention as the drilling fluid. The pilot hole was reamed to 457 mm
and
then to 762 mm.
Drilling Fluid: The circulating drilling fluid system for the 762mm ream
consisted of
a mixture of added natural bentonite pre-hydrated in fresh water, a mixed
metal
hydroxide viscosifier (MMH), gypsum and lime (pH = 8.5) carried over from the
457
mm ream operation and that had incorporated a significant amount of hydratable
clays (MBT = 93 kg/m3).
762 mm tunnel reaming operations: Made up the bottom hole assembly (BHA) with
reamer and commenced pull reaming operations from the exit side with 1.28
m3/min pump rates. A problematic increase in viscosity occurred once 762 mm
reaming operations commenced. The pull ream operations commenced using a
drilling fluid with a funnel viscosity of 125 s/L. The viscosity had been
raised to this
level on the previous reamed section to deal with hole cleaning and lost
circulation
issues. Upon commencing reaming to 762 mm the funnel viscosity increased by
39m to 175 s/L. 3 kg/m3 of lignite and some fresh water (10 m3) was added to
the
system and the funnel viscosity reduced to 80 s/L. By 64m the funnel viscosity
had
reduced to 64 s/L. The MBT had reduced to 71 kg/m3 equivalent through the
combination of fresh water additions, possible influx of ground (aquifer)
water and
increased efficiency of the solids removal equipment.
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The results are shown in Table III.
Table III:
Day 1 (9:30 pm) Day 2 (4 am) Day 2 (5:30 am)
Distance from Entry (94) 26 64
(m)
Density (kg/m3) 1175 1195 1140
Funnel Viscosity (s/L) 125 175 64
pH 8.5 8.5 8.0
600 rpm 109 130 62
300 rpm 97 112 57
6 rpm 80 87 45
3 rpm 74 76 44
s - Gel strength 37 38 22
(Pa)
10 min - Gel strength 37 38 22
(Pa)
PV (mPa.$) 12 18 5
YP (Pa) 42.5 47 26
MBT (kg/m3 93 100 71
bentonite equiv.)
The drilling fluid was successfully used to ream the 555 m tunnel to 762mm
through
sand and clay formations with full control of the rheology properties.
Successfully
pulled 508 mm diameter pipe through the tunnel.
The previous description of the disclosed embodiments is provided to enable
any
person skilled in the art to make or use the present invention. Various
modifications to those embodiments will be readily apparent to those skilled
in the
art, and the generic principles defined herein may be applied to other
embodiments
without departing from the spirit or scope of the invention. Thus, the present
invention is not intended to be limited to the embodiments shown herein, but
is to
be accorded the full scope consistent with the claims, wherein reference to an
element in the singular, such as by use of the article "a" or "an" is not
intended to
mean "one and only one" unless specifically so stated, but rather "one or
more".
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All structural and functional equivalents to the elements of the various
embodiments described throughout the disclosure that are known or later come
to
be known to those of ordinary skill in the art are intended to be encompassed
by
the elements of the claims. Moreover, nothing disclosed herein is intended to
be
dedicated to the public regardless of whether such disclosure is explicitly
recited
in the claims. No claim element is to be construed under the provisions of 35
USC
112, sixth paragraph, unless the element is expressly recited using the phrase
"means for" or "step for".
