Note: Descriptions are shown in the official language in which they were submitted.
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DISPERSIVE RISERLESS DRILLING FLUID
BACKGROUND OF INVENTION
Field of the Invention
[0001] Embodiments relate generally to drilling fluids. More specifically,
embodiments relate to drilling fluids used in a riserless section.
Background Art
[0002] When drilling or completing wells in earth formations, various fluids
typically
are used in the well for a variety of reasons. Common uses for well fluids
include:
lubrication and cooling of drill bit cutting surfaces while drilling generally
or drilling-
in (i.e., drilling in a targeted petroliferous formation), transportation of
"cuttings"
(pieces of formation dislodged by the cutting action of the teeth on a drill
bit) to the
surface, controlling formation fluid pressure to prevent blowouts, maintaining
well
stability, suspending solids in the well, minimizing fluid loss into and
stabilizing the
formation through which the well is being drilled, fracturing the formation in
the
vicinity of the well, displacing the fluid within the well with another fluid,
cleaning
the well, testing the well, transmitting hydraulic horsepower to the drill
bit, fluid used
for emplacing a packer, abandoning the well or preparing the well for
abandonment,
and otherwise treating the well or the formation.
[0003] During traditional drilling practices, mud is pumped down the drill
string,
through the bit and up the casing-drill string annulus to the surface. The
mud's
viscosity is designed to carry drill cuttings back to surface for disposal and
its density
to contain the well's natural pressure.
[0004] Drilling for oil and gas in very deep water presents problems not found
in
terrestrial or shallow water oil and gas exploration. One problem encountered
in deep
water is drilling fluid management. A drilling fluid is a fluid specially
designed to be
circulated through a wellbore as the wellbore is being drilled to facilitate
the drilling
operation. The circulation path of the drilling fluid typically extends from
the drilling
rig down through the drill pipe string to the bit face and back up through the
annular
space between the drill pipe string and wellbore face to the wellhead and/or
riser,
returning to the rig. In addition to the typical functions, the drilling fluid
also
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desirably prevents sloughing and wellbore cave-ins when drilling through water
sensitive formations.
[0005] Offshore drillers have to get mud down to the bottom of the sea, where
the
borehole starts. To do that, they run a steel tube, called a riser, to extend
the borehole
from the bottom of the sea to the rig. One of the basic and most challenging
problems
in deep water operations is the use of the marine riser, which provides a
connection
between the drilling vessel and the wellhead. The riser serves as a guide for
the drill
pipe into the hole and as a mud return path to the vessel and also supports
control
cables and choke and kill lines. Floating drilling operations in deep water
presently
involve the use of a 21 inch outer diameter (OD) marine riser.
[0006] In shallow water drilling, a riser system, which is a separate casing
rising from
the sea floor to the base of a drilling ship or drilling rig, can be used to
return drilling
mud to a drilling ship or platform for reuse. The use of a riser is not
without problems,
and these problems can be exaggerated in deep water drilling projects. One
such
problem is weight. A 6,000-foot riser, 21 inches in diameter, holding drilling
mud has
been estimated to weigh from about 1,000 to 1,500 tons. It is for this reason
that
riserless drilling methods have been disclosed, particularly for deep water
drilling, in
patents such as U.S. Pat. No. 6,102,673 to Mott, et al., and U.S. Pat. No.
4,149,603 to
Arnold.
(00071 Additionally, current drilling technology and required pressure ratings
may
limit the riser diameter to 18 3/ inches when drilling in overpressured
environments.
However, because significant thicknesses of salt ranging from 1,000 to 10,000
feet
may be encountered within a few thousand feet of the mudline, large-diarrmeter
hole
sections are typically needed at shallow depth to accommodate the multiple
casing
strings required to reach the deep reservoir formations. Thus, to drill hole
diameters
greater than 18 3/ inches (e.g., the 28 and 24 inch hole sections) or to
reduce the costs
related to conventional riser drilling, the initial, shallow portion of the
well may be
drilled riserless, with returns (the drilling fluid used and the formation
cuttings) being
discharged to the seabed.
[00081 There are a number of different types of conventional drilling fluids
including
compositions termed "drilling muds." Drilling muds comprise high-density
dispersions of fine solids in an aqueous liquid. Because muds used in
riserless drilling
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are not typically circulated to the rig, the cost of "pumping and dumping"
must be
balanced with the benefits provided by the mud, when muds are pumped at a
minimum of 1,200 gallons per minute into a well. For example, when drilling
riserless, seawater alone, or blends of sea water with muds containing
polymers,
hydrating clays, and salts to improve inhibition, density, viscosity, and
other
rheological properties have been typically used. However, while these
economically
efficient muds may improve some properties, difficulties in drilling have
still resulted
from cuttings accretion and agglomeration, build up of cuttings that cover the
wellhead, bit balling, and hole cleaning issues, such as swabbing, surging,
and
packing off, which may lead to downhole pressure issues. These drilling
difficulties
have been especially problematic when higher density drilling fluids are
required.
[0009] Accordingly, there exists a need for a highly dispersive drilling fluid
that will
reduce potential problems with cuttings accretion and agglomeration, cuttings
build
up, bit balling, and hole cleaning.
SUMMARY OF INVENTION
[0010] In one aspect, embodiments relate to a method for drilling riserless
that
includes providing a drilling fluid to a drilling assembly for drilling a
borehole on a
seafloor, the drilling assembly comprising a drill string and a bottomhole
assembly,
and wherein the drilling fluid includes a brine, and a non-hydratable clay,
wherein the
drilling fluid is substantially free of hydrating clays, and flowing the
drilling fluid and
cuttings through an annulus formed by the drill string and the borehole into
sea water.
10011] In another aspect, embodiments relate to a a method for drilling
riserless that
includes providing a drilling fluid to a drilling assembly for drilling a
borehole on a
seafloor, the drilling assembly comprising a drill string and a bottomhole
assembly,
and wherein the drilling fluid includes a brine, attapulgite clay, and a salt
of an alkali
metal or alkaline earth metal wherein the drilling fluid is substantially fee
of
hydrating clays, and flowing the drilling fluid and cuttings through an
annulus formed
by the drill string and the borehole into sea water.
[0012] In yet another aspect, embodiments relate to a wellbore fluid that
includes an
aqueous fluid, attapulgite clay, and a salt of an alkali metal or alkaline
earth metal,
wherein the wellbore fluid is substantially free of hydrating clays.
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[0013] Other aspects and advantages of the invention will be apparent from the
following description and the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
[0014] FIG. 1 is a schematic view of open hole drilling according to one
embodiment
disclosed herein.
DETAILED DESCRIPTION
[0015] In one aspect, embodiments disclosed herein relate to dispersive
drilling fluids
and methods of drilling with these fluids. In particular, embodiments
disclosed herein
relate to drilling fluids useful in drilling a section of a borehole without a
riser.
[0016] In one embodiment, a drilling fluid may include a brine and a non-
hydratable
clay. As used herein, "brine" is defined as including any aqueous saline
solution and
"non-hydratable clay" is defined as those clays which do not swell appreciably
in
either fresh water or salt water.
[00171 Brine
[00181 In various embodiments of the drilling fluid disclosed herein, the
brine may
include seawater, aqueous solutions wherein the salt concentration is less
than that of
sea water, or aqueous solutions wherein the salt concentration is greater than
that of
sea water. The salinity of seawater may range from about 1 percent to about
4.2
percent salt by weight based on total volume of seawater. Salts that may be
found in
seawater include, but are not limited to, sodium, calcium, sulfur, aluminum,
magnesium, potassium, strontium, silicon, lithium, and phosphorus salts of
chlorides,
bromides, carbonates, iodides, chlorates, bromates, formates, nitrates,
oxides, and
fluorides. Salts that may be incorporated in a given brine include any one or
more of
those present in natural seawater or any other organic or inorganic dissolved
salts.
Additionally, brines that may be used in the drilling fluids disclosed herein
may be
natural or synthetic, with synthetic brines tending to be much simpler in
constitution.
In one embodiment, the density of the drilling fluid may be controlled by
increasing
the salt concentration in the brine (up to saturation). In a particular
embodiment, a
brine may include halide or carboxylate salts of mono- or divalent cations of
metals,
such as cesium, potassium, calcium, zinc, and/or sodium.
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.[0019] Clays
100201 The drilling fluids disclosed herein may also contain a non-hydratable
clay. In
some embodiments, the non-hydratable clay may be a clay having a needle-like
or
chain-like structure. In various other embodiments, the non-hydratable clay
may be
selected from at least one of attapulgite and sepiolite clays. In a particular
embodiment, the non-hydratable clay includes attapulgite clay. While the non-
hydratable clays do not substantially swell in either fresh or salt water,
they may still
operate to thicken salt solutions. This thickening may be attributed to what
is believed
to be a unique orientation of charged colloidal clay particles in the
dispersion
medium, and not actual "hydration."
[0021] As the term "non-hydratable" refers to the clay's characteristic lack
of
swelling, i.e., measurable volume increase, in the presence of salt water, a
given
clay's swellability in sea water may be tested by a procedure described in an
article by
K. Norrish, published as "The swelling of Montmorillonite," Disc. Faraday Soc.
vol.
18, 1954 pp. 120-134. This test involves submersion of the clay for about 2
hours in a
solution of deionized water and about 4 percent sodium chloride by weight per
volume of the salt solution. Similarly, a given clay's swellability in fresh
water may
be tested by an analogous procedure in which the sodium chloride is excluded.
A
"non-hydratable" clay is defined in one embodiment as one that, under this
test,
swells less than 8 times by volume compared with its dry volume. In another
embodiment, a non-hydratable clay exhibits swelling on the order of less than
2 times;
less than 0.3 times in another embodiment; and less than 0.2 times in yet
another
embodiment.
100221 In further embodiments, the drilling fluids disclosed herein may be
substantially free of hydrating clays. As used herein, "hydrating clays" is
defined as
those clays which swell appreciably (i.e., increase their volume by an amount
of at
least about 8 times) in either fresh water or salt water, and "substantially
free" is
defined as an amount that does not significantly affect dispersibility.
Hydrating clays
may include those clays which swell appreciably in contact with fresh water,
but not
when in contact with salt water, include, for example, clays containing sodium
montmorillonite, such as bentonite. Many hydrating clays have a sheet- or
plate-like
structure.
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[0023] Salts
[0024] In various embodiments, the drilling fluid disclosed herein may also
contain at
least one additional salt, including any salt that may be incorporated in
brines, as
disclosed herein. In particular embodiments, at least one of sodium chloride,
calcium
chloride, potassium chloride, and sodium carbonate may be incorporated in the
drilling fluids disclosed here. In one embodiment, the at least one additional
salt may
incorporated into the drilling fluid disclosed herein in an amount ranging
from about
0.5 weight percent to salt saturation.
100251 Additives
[0026] The wellbore fluids disclosed herein may optionally contain various
additives,
depending on the end use of the fluid. For example, weighting agents,
deflocculants,
and combinations thereof may be added to the fluid compositions disclosed
herein for
additional functional properties. The addition of such agents should be well
known to
one of skill in the art of formulating drilling fluids and muds. However, it
should be
noted that the addition of such agents should not adversely interfere with the
properties associated with the mud's ability to disperse cuttings as disclosed
herein.
100271 Weighting agents or density materials suitable for use in the fluids
disclosed
herein include, for example, galena, hematite, magnetite, iron oxides,
illmenite, barite,
siderite, celestite, dolomite, calcite, and the like. The quantity of such
material added,
if any, depends upon the desired density of the final composition. Typically,
weight
material is added to result in a drilling fluid density of up to about 19
pounds per
gallon in one embodiment; and ranging from 9.5 to 14 pounds per gallon in
another
embodiment.
100281 Deflocculants or thinners that may be used in the drilling fluids
disclosed
herein include, for example, lignosulfonates, modified lignosulfonates,
polyphosphates, tannins, and low molecular weight water soluble polymers, such
as
polyacrylates. Deflocculants are typically added to a drilling fluid to reduce
flow
resistance and control gelation tendencies. In a particular embodiment, a
deflocculant
may be desirable when a drilling fluid is formed from a heavier mud diluted
withsea
water. TANNATHIN , an oxidized lignite, is an example of a deflocculant which
is
available from M-I L.L.C. (Houston, Texas).
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[00291 Formulations
[00301 In one embodiment, the drilling fluid may be formulated to have a
density
range from about 9 to 14 pounds per gallon. The drilling fluid may be
initially
formulated to have the desired formulation. Alternatively, the drilling fluid
may be
formed from a concentrated mud, such as a 16 pound per gallon mud, or heavier
which is be blended with a brine prior to use to the desired formulation.
Those having
ordinary skill in the art will appreciate that other densities may be used as
desired.
When blended from a mud and a brine, the mud may optionally contain a salt,
such as
a salt of an alkali metal or alkaline earth metal. In one embodiment, the
drilling fluid
may have a pH greater than about 6. In another embodiment, the drilling fluid
may
have a pH ranging from about 7.5 to 12. The pH of the drilling fluid may be
tailored
with the addition of acidic or basic additives, as recognized by one skilled
in the art.
For example, caustic soda and citric acid may be used to increase or decrease
the pH
of a fluid, respectively.
[0031] Method of Drilling
[00321 When drilling from a vessel or floating platform, the upper portion of
the well
is often drilled by open hole drilling in that no conduit is provided for the
drilling
fluid/cutting returns to flow to the platform. As shown in FIG. 1, to drill
the initial
upper portion of the well 10, a drill string 14 typically extends unsupported
from a
vessel or platform 12 through the water to the seafloor 18 without a riser. In
more
detail, first an outer casing 20, known as "structural casing," typically
having a
diameter of up to 30-inches or 36-inches, is installed in the uppermost
section of the
well, with a low-pressure wellhead housing (not shown separately) connected
thereto.
In soft formations, the structural casing 20 may be jetted into place. In this
process, a
drilling assembly that includes the drill string 14 and a bottom hole assembly
(BHA)
(not shown separately), and casing 20 is lowered to the seafloor via the drill
string 14.
The BHA includes a drill bit 16, and may also include other components such
as, drill
collars and a downhole motor (not shown separately). The bit 16 is positioned
just
below the bottom end of the structural casing 20 and is sized to drill a
borehole 22
with a slightly smaller diameter than the diameter of the casing 20. As the
borehole
22 is drilled, the structural casing 20 moves downwardly with the BHA. The
weight
of the structural casing 20 and BHA drives the casing 20 into the sediments.
The
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structural casing 20, in its final position, may extend downwardly to a depth
of 150 to
400 feet, depending upon the formation conditions and the final well design.
After
the structural casing 20 is in place, it may be released from the drill string
14 and
BHA. The drill string 14 and BHA may be tripped back to the platform, or
alternatively, may be lowered to drill below the structural casing.
100331 Alternatively, the structural casing 20 may be installed in a two-step
process.
First, a borehole larger than the structural casing is drilled. Then the
structural casing
20 is run into the borehole 22 and cemented into place. Typically, the low-
pressure
wellhead housing (not shown separately) is connected to the upper end of the
structural casing 20 and installed at the same time, such that the structural
casing 20
extends below the seafloor with the low-pressure wellhead housing above the
seafloor.
[00341 Once the structural casing 20 and the low-pressure wellhead housing are
installed, the bit 16 on the drill string 14 drills downwardly below the
structural
casing 20 to drill a new borehole section using open hole drilling for an
intermediate
casing 24, known as "conductor casing," which may be, for example, 20-inches
in
diameter. Thus, the structural casing 20 guides the BHA as it begins to drill
the
conductor casing 24 interval. After the borehole section for the conductor
casing 24 is
drilled, the BHA is tripped to the surface. Then conductor casing 24, with a
high-
pressure wellhead housing connected to its upper end, and a float valve
disposed in its
lower end (not shown separately), is run into the drilled conductor borehole
section
extending below the structural casing 20. The conductor casing 24 is cemented
into
place in a well known manner, with the float valve preventing cement from
flowing
upwardly into the conductor casing after cement placement. The conductor
casing 24
generally may extend downwardly to a depth of 1,000 to 3,000 feet below the
seafloor, depending on the formation conditions and the final well design. The
high-
pressure wellhead housing (not shown separately) may engage the low-pressure
wellhead housing (not shown separately) to form the subsea wellhead, thereby
completing the riserless portion of the drilling operations. Installation of a
subsea
blowout preventer (BOP) stack may be conveyed down to the seafloor by a riser
and
latched onto the subsea wellhead housing for subsequent riser drilling.
[00351 During the open hole drilling shown in FIG. 1, drilling fluid flows
through the
drill string 14 and out of the drill bit 16 as shown by downward arrows 26.
The flow
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of the drilling fluid continues through the annulus between the borehole 22
and the
drilling assembly 14, 16. As the drilling fluid flows through this annulus, it
may carry
drilled cuttings through the borehole, indicated by upward arrows 28 and may
exit the
well to be dispersed into the sea, as indicated by arrows 30. Therefore, in
open hole
drilling the returns, i.e. the drilling fluid, cuttings, and well fluids, are
discharged onto
the seafloor and are not conveyed to the surface.
[00361 EXAMPLE
[00371 The following examples were used to test the effectiveness of the
drilling
fluids disclosed herein in dispersing cuttings.
[00381 Drilling muds were formulated having the following components, all of
which
are commercially available, as shown below in Table 1. M-1 GEL' is an example
of a
bentonite clay, SALT GEL is an example of an attapulgite clay, TANNATHIN is
a
lignite, and DUOVIS is a xanthan gum, all of which are commercially available
from M-I L.L.C. (Houston, Texas).
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Table 1: 16 ppg Mud Formulations
A B C D E F G H I J
Tap water (bbl) 0.699 0.697 0.691 0.706 0.691 0.691 0.691 0.691 0.691 0.691
M-I GEL (ppb) 20.0 - - - - - - - - -
NaC1(ppb) - - 7.48 7.64 7.48 7.48 7.48 7.48 7.48 32.43
SALT GEL (ppb) - 20.0 20.0 - 20.0 20.0 20.0 20.0 20.0 20.0
DUOVIS (ppb) - - - 2.0 - - - - - -
Caustic Soda (ppb) 0.50 0.50 0.75 0.5 0.75 0.75 3.0 0.75 0.75 0.75
TANNATHIN (ppb) 2.0 0.10 2.0 - 1.25 1.25 1.25 0.75 1.25 3.0
Barite (ppb) 405.0 407.4 399.97 415.4 399.97 399.97 399.97 399.97 399.97
379.51
Citric Acid (ppb) - - - - - 2.0 - - - -
Soda ash (ppb) - - - - - - - - 3.0 -
[00391 The various mud formulations were then blended with sea water, and
their
rheological properties were determined using a Fann Model 35 Viscometer,
available
from Fann Instrument Company. The muds were then subjected to dispersion
testing,
where 20 grams of shale, dried for 16 hours at 200 F and sized to -6/+20 mesh,
was
added to the mud blends and hot-rolled for 16 hours at 150 F. Seawater was
used as a
blank. After hot-rolling, the samples were cooled. An 80 mesh screen was used
to
recover un-dispersed shale. A percent recovery can be determined by comparing
the
amount of shale recovered to the initial 20 grams of shale used. A lower
percent
recovery indicates greater dispersion of the shale by the fluid. The results
are shown
below in Tables 2a and 2b.
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Table 2a: Dispersion Test
Sample Number
1 2 3 4 5 6 7 8
Mud Formulation - A A A A B C D
16 ppg Mud (bbl) - 0.128 0.262 0.396 0.53 0.53 0.53 0.53
Seawater (bbl) 1.0 0.872 0.738 0.604 0.47 0.47 0.47 0.47
Mud Weight - 9.5 10.5 11.5 12.5 12.5 12.5 12.5
Rheology - 120 120 120 120 120 120 120
Temperature ( F)
600 rpm - 6 13 25 44 33 28 34
300 rpm - 4 10 21 36 29 24 21
200 rpm - 3 9 20 34 23 21 17
100 rpm - 2 8 18 30 20 19 12
6 rpm - 2 5 12 21 15 3 3
3rpm - 1 4 12 21 15 3 2
Plastic Viscosity - 2 3 4 8 4 4 13
(cps)
Yield Point - 2 7 17 28 25 20 8
(lbs/100 ft'-)
Second Gel - 2 6 11 22 17 17 3
10 Minute Gel - 2 11 22 47 22 37 10
pH - 7.7 7.8 8.0 8.1 9.3 9.36 9.69
Recovery (%) 3.4 10.3 85.8 87.8 89.0 69.80 44.65 76.0
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Table 2b: Dispersion Test
Sample Number
9 10 11 12 13 14 15 16
Mud Formulation E E E F G H I
J
16 ppg Mud (bbl) 0.128 0.262 0.396 0.128 0.262 0.396 0.396 0.396
Seawater (bbl) 0.872 0.738 0.604 0.872 0.738 0.604 0.604 0.604
Mud Weight 9.5 10.5 11.5 11.5 11.5 11.5 11.5 11.5
Rheology 120 120 120 120 120 120 120 120
Temperature ( F)
600 rpm 7 12 16 13 22 15 20 17
300 rpm 5 8 12 9 16 10 16 12
200 rpm 4 6 10 8 14 8 14 10
100 rpm 3 5 8 6 12 7 12 9
6rpm 2 2 5 5 9 5 10 6
3rpm 2 2 4 5 9 5 10 6
Plastic Viscosity 2 4 4 4 6 5 4 5
(cps)
Yield Point 3 4 8 5 10 5 12 8
(lbs/100 ft2)
Second Get 3 3 5 6 6 3 8 6
10 Minute Gel 3 5 11 22 12 5 15 10
pH 8.6 8.8 8.9 7.2 12.0 9.3 9.7 8.2
Recovery (%) 6.05 24.75 51.55 50.8 32.7 63.6 59.5 72.4
100401 From the results shown in Tables 2a and 2b, a comparison of Samples 9-
11
(Mud E) to Samples 2-5 (Mud A) shows increased dispersion of the shale into
the
drilling fluid containing attapulgite clay over that containing bentonite
clay. Similarly,
a comparison of Samples 7 (Mud C) to Sample 8 (Mud D) shows increased
dispersion
of the shale into the drilling fluid containing attapulgite over that
containing polymer.
Additionally, dispersion may be further increased at higher pHs, as shown by
comparing Samples 12-16 (Muds F-J) to each other, and upon the addition of a
salt, as
shown by comparing Samples 6 (Mud B) to Sample 7 (Mud Q.
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10041] Advantageously, embodiments disclosed herein may provide for a drilling
fluid that may be used in open hole drilling. The fluids disclosed herein may
provide
the rheological properties needed for drilling without a riser. Additionally,
by
increasing the amount of dispersion of cuttings into the fluids and
subsequently into
the sea water, the fluids may at least reduce cuttings accretion and
agglomeration,
build up of cuttings that cover the wellhead, bit balling, and hole cleaning
issues, such
as swabbing, surging, and packing off, which may lead to pressure issues.
10042] While the invention has been described with respect to a limited number
of
embodiments, those skilled in the art, having benefit of this disclosure, will
appreciate
that other embodiments can be devised which do not depart from the scope of
the
invention as disclosed herein. Accordingly, the scope of the invention should
be
limited only by the attached claims.
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