Note: Descriptions are shown in the official language in which they were submitted.
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E~CAPSULATED WATER ABSORBENT POLYMERS AS LOST
CIRCVLATION ADDITIVES POR AQUEOUS DRILLING FLUIDS
(D#78,441-F)
BACKGROUND OF THE INVENTION
This invention is related to U.S. Patents 4,635,726 .
issued on January 13, l987 and 4,633,950 issued on January 6,
1987. .:
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The invention concerns novel additives for reducing
lost circulation when aqueous drilling fluids are used and a .. -
method for reducing such lost circulation. More particularly,
the lost circulation additives are encapsulated water absorbent
polymers which will be unable to absorb water and expand to plug
fissures and thief zones until absorption is desired. ',
Drilling fluids, or drilling muds as they are sometimes ,.
called, are slurries of clay solids used in the drilling of wells -,~
in the earth for the purpose of recovering hydrocarbons and other .`
fluid materials. Drilling fluids have a number of functions, the
most important of which are: lubricating the drilling tool and
drill pipe which carries the tool, removing formation cuttings
from the well, counterbalancing formation pressures to prevent ,'
the inflow of gas, oil or water from permeable rocks which may be
encountered at var~ous lèvels as drilling continues, and holding .'~
the cuttings in suqpension in the event of a shutdown in the .,
drilling an~ pwnping of the drilling fluid.
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For a drilling fluid to perform these functions and
allow drilling to continue, the drilling fluid must stay in the
borehole. Frequently, undesirable formation conditions are en-
countered in which substantial amounts or, in some cases, practi-
cally all of the drilling fluid may be lost to the formation.
Drilling fluid can leave the borehole through large or small fis-
sures or fractures in the formation or through pores in the rock
matrix surrounding the borehole.
Most wells are drilled with the intent of forming a
filter cake of varying thickness on the sides of the borehole.
The primary purpose of the filter cake is to reduce the large
losses of drilling fluid to the surrounding formation. Unfortu-
nately, formation conditions are frequently encountered which may
result in unacceptable losses of drilling fluid to the surround-
ing formation despite the type of drilling fluid employed and
filter cake created.
A variety of different substances are now pumped down
well bores in attempts to reduce ~he large losses of drilling
fluid to fractures and the like in the surrounding formation.
Different forms of cellulose are the preferred materials em-
ployed. Some substances which have been pumped into well bores
to control lost circulation are: almond hulls, walnut hulls,
bagasse, dried tumbleweed, paper, coarse and fine mica, and even
pieces of rubber tires. These and other prior art additives are
described in U. S. Patent No. 4,498,995.
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Another process that is employed to close off large
lost circulation problems is referred to in the art as gunk
squeeze. In the gunk squeeze process, a quantity of a powdered
bentonite is mixed in diesel oil and pumped down the well bore.
Water injection follows the bentonite and diesel oil. If mixed
well, the water and bentonite will harden to form a gunky
semi-solid mess, which will reduce lost circulation. Problems
frequently occur in trying to adequately mix the bentonite and
- water in the well. The bentonite must also be kept dry until it
reaches the desired point in the well. This method is disclosed
in U. S. Patent No. 3,082,823.
Many of the methods devised to control lost circulation
involve the use of a water expandable clay such as bentonite
which may be mixed with another ingredient to form a viscous
paste or cement. U. S. Patent No. 2,890,169 discloses a lost
circulation fluid made by forming a slurry of bentonite and ce-
ment in oil. The slurry is mixed with a surfactant and water to
form a composition comprising a water-in-oil emulsion having
bentonite and cement dispersed in the continuous oil phase. As
this composition is pumped down the wellbore, the oil expands and
flocculates the bentonite which, under the right conditions,
forms a filter cake on the wellbore surface in the lost circu-
lation area. Hopefully, the filter cake will break the emulsion
causing the emulsified water to react with the cement to form a
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solid coating on the filter cake. But such a complex process can
easily go wrong.
U. S. Patent No. 3,448,800 discloses another los-t cir-
culation method wherein a water soluble polymer is slurried in a
nonaqueous medium and injected into a well. An aqueous slurry of
a mineral material such as barite, cement or plaster of paris is
subsequently injected into the well to mix with the first slurry
to form a cement-like plug in the wellbore.
U. S. Patent No. 4,261 t 422 describes the use of an
expandable clay such as bentonite or montmorillonite which is
dispersed in a liquid hydrocarbon for injection into the well.
After injection, the bentonite or montmorillonite will expand
upon contact with water in the formation. Thus, it is hoped that
the expanding clay will close off water producing intervals but
not harm oil producing intervals.
; A similar method is disclosed in U. S. Patent
No. 3,078,920 which uses a solution of polymerized methacrylate
dissolved in a nonaqueous solvent such as acetic acid, acetic
anhydride, propionic acid and liquid aliphatic ketones such as
acetone and methyl-ethyl ketone. The methacrylate will expand
upon contact with formation water in the water producing
intervals of the well.
It has also been proposed to mix bentonite with water
in the presence of a water soluble polymer which will flocculate
and congeal the clay to form a much stronger and stiffer cement-
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like plug than will form i~ bentonite is mixed with water. U. S.
Patent No. 3,909,421 discloses such a fluid made by blending a
dry powdered polyacrylamide with bentonite followed by mixing the
powder blend with water. U. S. Patent No. 4,128,528 claims a
powdered bentonite/polyacrylamide thickening composition prepared
by mixing a water-in-oil emulsio;n with bentonite to form a pow-
dered composition which rapidiy becomes a viscous stiff material
when mixed with water. U. S. Patent Nos. 4,503,170; 4,475,594;
4,445,576; 4,442,241; and 4,391,925 teach the use of a water
expandable clay dispersed in the oily phase of a water-in-oil
emulsion containing a surfactant to stabilize the emulsion and a
polymer dispersed in the aqueous phase. When the emulsion is
sheared, it breaks and a bentonite paste is formed which hardens
into a cement-like plug. The patent discloses the use of such
polymers as polyacrylamide, polyethylene oxide and copolymers of
acrylamide and acrylic or methacrylic acid.
U. S. Patent No. 4,124,748 discloses a cross-linked
copolymer of a vinyl ester and an ethylenically unsaturated
carboxylic acid or derivative thereof that can absorb about 200
to 800~ of its weight in water and expand substantially in volume
when doing so. No. 4,124,748 also discloses the water absorbing
capability of saponified products of starch-acrylonitrile graft
copolymers.
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Another highly water absorbent copolymer which expands
upon water absorption is described in U. S. Patent No. 4,320,040.
The described compound is derivecl by polymerizing acrylic acid
and/or methacrylic acid in the presence o~ polyvinyl alcohol
followed by neutralization and a heat treatment.
Highly absorbent spongy polymer materials which may
absorb large quantities of water and hydrocarbons causing an in-
crease in volume are disclosed in U. S. Patent No. 3,878,175.
These are copolymers of an alkyl acrylate and a heterocyclic
N-vinyl monomer containing a carbonyl functionality and a
cross-linking agent in the presence of a hydrophobic liquid
diluent. U. S. Patent No. 4,182,677 discloses that natural and
synthetic rubbers also swell in size upon absorbing water.
The last few years have witnessed a drastic increase in
research on encapsulated products and methods to produce such
products. This is particularly so in the pharmaceutical field.
And it is now becoming recognized that encapsulation technology
may be useful in many other fields.
U.S. Patent No. 3,971,852 describes a process for
encapsulating various fragrance oils such as oils with citrus and
spice odors. The oils are encapsulated in a matrix comprised of
polysaccharide and polyhydroxy compounds by converting an
emulsion of the fragrance oil droplets in a solution of the
matrix ingredients to an encapsulated solid state during a spray
drying process. The patent also mentions that miscellaneous
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chemicals can be encapsulated by the invention method such as
drilling fluids and waxes.
U.S. Patent No. 4,269,279 discloses the use of plastic
coated magnetic particles in a bead form to increase lubrication
for drilling fluids. The encapsulated ferromagnetic particles
can be recovered for reuse with a magnetic separator.
An encapsulated invention which has been disclosed for
use in boreholes is described in U.S. Patent No. 4,078,612. The
patent describes an explodable material encapsulated in natural
gums slurried in a liquid vehicle. The material is pumped into
the formation around the wellbore and exploded -to increase per-
meability.
The use of bentonite encapsulated within a
water-insoluble polymeric coating has been disclosed for lost
circulation control. U. S. Patent No. 2,836,555 describes bentonite
encapsulated within a polymeric coating having a tiny hole drilled
therethrough. When the encapsulated bentonite is pumped down the
wellbore, water will seep through the hole in the encapsulation
causing the bentonite to swell and ultimately rupture the coat-
ng .
Another U.S. Patent, No. 4,036,301 describes anencapsulated material useful in cementing a well, wherein a
cement accelerator is encapsulated in a waxy material and placed
within a highly retarded cement slurry. The cement slurry is
pumped into the well with the encapsulated accelerator. After
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proper placement of the cement, circulation is decreased so that
the temperature of the cement fluid approaches the bottom hole
temperature of the well and melts the encapsulated material,
freeing the accelerator which sets the cement.
U.S. Patent No. 4,362,566 discloses an additional use
of encapsulated materials. The patent suggests encapsulating one
component of a two or more component adhesive or cement mixture
so that the adhesive or cement will not set until the
encapsulated component is freed from its reaction-preventive cas-
ing.
SUMMARY OF THE INVENTION
The invention is a novel drilling fluid additive for
reducing lost circulation in wellbores. The lost circulation
additive is a highly water absorbent polymer encapsulated by a
reaction-preventive protective casing to prevent the polymer from
absorbing water and substantially increasing in size in the
formation and borehole until it is desired to breach the casing.
The reaction-preventive protective casing may be a film
which will dissolve after a desired time of residence in the
borehole or a film or waxy substance which will dissolve or melt
at a desired temperature in the borehole. Once the casing around
the polymer is dissolved, melted or breached in some manner, the
polymer will be released to absorb water in the formation or the
borehole, causing a drastic increase in polymer size. Thus,
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fissures and pores through which drilling fluid was being lost
will be substantially closed by the now expanded polymer. If the
polymer is released in a clay based drilling fluid environment, a
highly viscous, non-flowing mass will be formed with the expanded
polymer and clay.
A method for employing the invention drilling fluid
additive and reducing lost circulation is also disclosed. The
encapsulated water absorbent pol~ymer is placed in the drilling
fluid and circulated in the wellbore. Fractures and large pores
which are responsible for lost circulation of drilling fluids
will claim additional drilling fluid including the encapsulated
polymer. Once the encapsulated polymer has been circulated
throughout the well, circulation of the drilling fluid is
stopped. Once circulation has ceased, the temperature of the
drilling fluid will rise and approach the formation temperature.
The film or waxy substance encapsulating the highly water absor-
bent polymer will melt, releasing the polymer to absorb water and
producing an expanded material which will seal off fractures and
large pores. At this time, circulation of the drilling fluid i5
resumed to flush all residue from the wellbore. The highly
expanded polymer in the pores and fractures will remain in the
formation, blocking these avenues of lost circulation.
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DETAILED DESCRIPTION
Drilling fluids are formulated to intentionally plug
porous formations durin~ drillinq in order to stabilize the
borehole and to control fluid loss. However, formations are fre-
quently encountered that are so porous as to increase the loss of
drilling fluids beyond an acceptable limit despite the use of
lost circulation additives. Furt:hermore, a borehole may pene-
trate a fracture in the formation through which most of the
drilling fluid may be lost.
In order to close off large pores and fractures which
drain drilling fluid from the borehole, it is necessary to accu-
rately place the lost circulation material at the correct
locations and be able to clean up the wellbore after treatment is
completed. The beauty of the present invention is that the
encapsulated polymer can be placed at just the right spots to
close off the pores and fractures which drain drilling fluid from
the borehole. Because of the mechanics of the process, the
unneeded polymer can be easily circulated out of the hole.
Any polymer which will significantly increase in size
after water absorption may be encapsulated and used as the lost
circulation additive of the present invention. Polymers which
absorb hydrocarbons may also be encapsulated for use in the
invention provided that the polymers also absorb water and
increase in size upon water absorption. A class of polymers
known as superabsorbent polymers perform very well.
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Superabsorbent polymers absorb many times their own
weight in water, causing the polymer volume to drastically
expand. Several of these highly water absorbent polymers
preferred for encapsulation are: alkali metal polyacrylates
including J-500 and J-550, trademarked sodium polyacrylate
polymers sold by Grain Processing Co.; A-100, a trademarked
starch graft copolymer of polyacrylic acid and polyacrylamide
sold by Grain Processing Co.; A-~00, a trademarked
polyacrylamidecosodium acrylate sold by Grain Processing Co.; and
B-200, a trademarked potassium salt of A-400 sold by Grain Pro-
cessing Co.
The amount of water these superabsorbent polymer will
absorb is astounding. The J-500 polymer will absorb 375 ml of
water per gram of J-500 polymer. The A-100 polymer will suck up
140 ml of water per gram of polymer. However, salt water has an
adverse effect on water absorption. The addition of 0.4% NaCl to
water will decrease the absorption of A-100 to 55 ml of water per
gram of A-100 and decrease absorption of J-500 from 375 ml to
100 ml of water per gram of J-500.
Another group of water absorbent polymers which perform
well in the invention are prepared by polymerizing one or more of
the acids from the group consisting of acrylic acid and
methacrylate acid in the presence of polyvinyl alcohol, neu-
tralizing the polymer, and heat treating the polymer at about
50C to about 150C. These polymers may also be cross-linked by
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carrying out the polymerization in the presence of a
cross-linking agent. The hydrophilic gel polymers prepared
according to this method are disclosled in U. S. Patent
No. 4,320,0~0. The same patent also discloses the use of
saponified starch-acrylonitrile graft copolymers. All of these
polymers expand substantially in size upon water absorption and
absorb from two to eight times their weight in water.
Water absorbent polymers which may also be encapsulated
are saponified copolymers of a vinyl ester and a compound
selected from the group consisting of ethylenically unsaturated
carboxylic acids and derivatives of ethylenically unsaturated
carboxylic acids. U. S. Patent No. 4,124,748, the disclosure of
which is incorporated herein by reference, states that these
copolymers may be cross-linked by polymerizing in the presence of
a cross-linking agent. The cross-linking agent may include
polyallyl compounds such as diallyl phthalate, diallyl maleate,
diallyl terephthalate, triallyl cyanurate or triallyl phosphate;
polyvinyl compounds such as divinyl benzene,
N,N'-methylene-bis-acrylamide, ethylene glycol diacrylate,
ethylene glycol dimethacrylate, or glycerine trimethacrylate;
allyl acrylate and allyl methacrylate. As the degree of
cross-linking is increased with an increase in the amount of
cross-linking agent, the water absorbing ability decreases.
Thus, only a moderate amount of cross-linking is desired. These
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polymers increase significantly in size when absorbing as much as
ten times their own weight in water. Furthermore, their gel
formation ability is stable in a hydrated state for a long period
of time.
U. S. Patent No. 3,878,17~ describes highly water
absorbent and oil absorbent spongy polymers which can be
encapsulated for use in the present invention. The highly
absorbent copolymers are prepared by heating a solution in a
hydrophobic liquid diluent of a mixture consisting essentially of
from about 30 to about 90% by weight of a heterocyclic N-vinyl
monomer containing a carbonyl functionality adjacent to the
nitrogen in its heterocyclic moiety, about 10 to about 70% by
weight of a comonomer selected from the class of vinyl esters and
acrylate esters, about 0 to about 30% by weight of acrylamide or
methacrylamide, and from about 0.5 to about 12~ by weight, based
on the total weight of the comonomer mixture of said heterocyclic
N-vinyl monomer and said ester of a polyethylene glycol
dimethacrylate as cross-linking agent, said heating conducted in
the absence of atmospheric oxygen at a temperature ranging from
ambient to about 60C for a period of time sufficient to yield a
slightly gelled mass, followed by continued heating in an inert
atmosphere to a temperature of about 100C for a pariod of
about l to about 3 hours. Such spongy polymers readily swell
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upon immersion in water or organic substances to a water content
of about 30 to 95~.
Natural and synthetic rubbers, which also swell upon
water absorption may be employed in the present invention. Of
course, other compounds which absorb water and expand in size
which are not mentioned herein, may also be encapsulated to
control lost circulation in aqueous drilling fluids. Another
possible option is to use two or more polymers, each encapsulated
separately, which will react with each other to form a stronger
block of the lost circulation ~one. If the polymer is
structurally weak, a substrate may be used to help support the
polymer.
The invention is applicable to aqueous drilling fluids
which contain clays and is not applicable to oil based drilling
fluids. The mud should have a pH of about 7 to about 8.5. Most
clay-wate~ systems with no supplemental treatment fall within
this pH range because of the natural buffering effect of the
clays.
Different types of reaction-preventive protective cas-
ings can be employed to encapsulate the highly water absorbent
polymer and prevent it from reacting with clays in the borehole
until the desired time. The casing may be a film which will dis-
solve after a desired time of residence in the borehole, releas-
ing the polymer. An example is a partially water soluble casing
which will take a measured time to dissolve, said time be
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sufficien-tly long to permit the encapsulated polymer to be
circulated in the borehole and lost to the formation pores and
fractures desired to be sealed off.
A second possibility is a film casing which will dis-
solve at desired temperatures, releasing the polymer so it may
absorb the water in the mud. It is quite easy to raise the tem-
perature of the mud environment :in the drilling hole by slowing
down or stopping mud clrculation,. Once circulation has ceased,
the temperature of the formation will heat the drilling fluid
constantly until the drilling fluid reaches formation tempera-
ture.
A third type of reaction-preventive protective casing
is a waxy substance such as a petroleum derived wax which is
selected so that its melting point is below the temperature of
the underground formation, but above the bottom hole circulating
temperature of the drilling fluid within the borehole. With a
wax casing around the polymer, the encapsulated polymer can be
circulated in the borehole until sufficient fluid has been lost
to fractures and pores that it is desired to seal off. At that
time, circulation can be stopped, permitting the temperature of
the drilling fluid to rise to formation temperature. The in-
crease in temperature of the drilling fluid will melt the waxy
casing, releasing the polymer to expand by absorbing water.
Petroleum derived paraffinic waxes having a melting point between
about 100F and about ~50F are preferred. The wax casings can
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be selected so that the invention additive will work at different
formation temperatures in different wellbores.
~ axes normally melt over a range of temperatures.
Thus, it is necessary to select a wax casing for the specific
range of temperatures which will be encountered when practicing
the invention in a particular wellbore. A hydrocarbon formation
along the Gulf Coast may have a formation temperature of 248F
which will heat up a 90F drilling fluid to a temperature of
about 210F at the bottom of a 16,000 foot hole while circulating
mud. This assumes a temperature gradent of 1.1F per hundred
feet of well depth. Thus, it is necessary to have a film casing
or a wax casing which will dissolve somewhere between the temper-
atures of 210F and 248F.
Since it is preferred to have some margin of safety
over the bottom hole circulation temperature of 210F, it is pre-
ferred to select a wax or film casing which will dissolve within
the temperature range of about 220F to about 247F. With such a
reaction-preventive protective casing, the water absorbent
polymer will not be released for water absorption until mud cir-
culation is ceased and the formation has had some time to heat up
the drilling fluid to near the formation temperature of 248~F.
Since it is undesirable to have the protective casing melt or
dissolve before the encapsulated polymer is spotted at the right
location, the protective casing should be designed with a melting
point close to the formation temperature. It is also preferred
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to have the polymer released for reaction in the well within
about 4 to 6 hours to allow sufficient time for the polymer to be
placed within the loss zone.
The reaction-preventivle casing must be substantially
chemically inert to the encapsulated polymer and to the external
medium around it. The casing should be resistant to diffusion in
either direction, somewhat resistant to breakage from mechanical
forces, and generally stable through temperature variations.
Additionally, the casing must dissolve or melt at the proper
time.
The above described properties are general properties
and are subject to exceptions. If a reaction-preventive protec-
tive casing is employed which will dissolve after a given period
of time in a medium to release the water absorbent polymer, pref-
erably at least several hours, the protective casing will not be
chemically inert to the external medium containing it. Protec-
tive casings are also employed in the pharmaceutical industry
which permit limited diffusion of the encased material through
the protective encapsulation. Another alternate embodiment
relies on designing the casing to allow water diffusion through
the casing to the polymer at a carefully controlled rate. After
a sufficient time, the polymer will have absorbed enough water
through the casing to expand and rupture the casing. Stability
of temperature variation is also important since it may be
necessary for the encapsulated polymer to go from an 80F
temperature to about a 200F-350F temperature without releasing
the polymer.
The capsules must also be able to resist substantial
physical and mechanical forces p:Laced on the protective casings
without breaking. The encapsulation must remain sufficiently
intact during the pumping and circulation process so that a sig-
nificant amount of water absorbent polymer is not released into
the drilling fluid before the desired time. The shearing forces
p~aced on the casings may be substantial during mud circulation.
Additionally, the protective casings must be able to survive
storage, wherein capsule breakage may occur as a result of the
weight of the encapsulated polymer stored in barrels and tanks~
For ease of encapsulation, spherical shaped particles
are generally preferred for most encapsulation processes.
However, a spherical shape is not crucial for some coating
processes. Furthermore, a spherical shape is stronger
structurally than other shapes, and is more likely to survive
unbroken in storage and mud circulation.
The preferred encapsulating materials of the invention
are organic waxes, especially petroleum derived paraffinic waxes.
The organic waxes include organic esters, higher fatty acids and
alcohols, and their mixtures, petroleum paraffinic waxes, syn-
thetic waxes such as acrylic and vinyl polymers, polyolefins, and
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acrylic, olefinic or vinyl modified natural waxes and their
mixtures. Preferred encapsulation materials are
petroleum-derived waxes alone or in combination with vinyl
resins, such as copolymers of ethyllene and vinyl acetate, which
provide superior strength and abrasion resistance.
The encapsulation process can be one of many methods
well known in the art, such as spray coating, condensation, elec-
trostatic coating, and solvent depo~sition. If a wax casing is
employed which will melt at a desired temperature within the
borehole, it is not even necessary that the casing be of a uni-
form thickness. The casing need not completely encapsulate the
polymer, provided that polymer outsida of the casing is removed
or neutralized by water absorption or some other method prior to
placing the encapsulated polymer within the drilling fluid. U.S.
Patent Nos. 3,971,852; 4,0~6,301; and 4,362,566 describe several
methods of encapsulating materials.
One method of encapsulating a highly water absorbent
polymer with a waxy substance to form the invention lost circu-
lation additive is to mix the polymer with a melted waxy sub-
stance until a uniform mixture is obtained. The mixture is then
solidified and sheared to form relatively small particles. After
shearing, the particles are placed in a bath to remove and neu-
tralize any exposed polymer. Some encapsulation methods such as
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spray drying may permit simultaneous cooling and shearing of the
polymer and wa~ mixture to form relatively small particles.
The encapsulated particles may be sized over a wide
range. The size of the passages through the circulating jets in
the drill bit is the absolute maximum particle size. However,
the encapsulated particles should be of a small enough size so as
to be able to enter the formation through fissures, small
fractures and large pores. A preferred range of particle size is
about 0.1 microns to 5 millimeters. It is believed that the
smaller the encapsulated particle size, the more expensive the
encapsulated additive would be, unless the additive was
encapsulated by wax as noted above. With such a wax
encapsulation, there should be no significant cost difference
between large and small particles. It is believed that a larger
particle size would result in a stronger blockage of a lost cir-
culation zone such as a large fracture. The particles should be
sized according to the properties of the formation and the lost
circulation zone.
The invention also includes a method for reducing the
lost circulation of drilling fluids in a borehole by employing
the encapsulated water absorbent polymer. The method steps are,
of course, dependent upon the particular type of encapsulation
employed. If a wax with the desired melting point range
encapsulates the polymer, the encapsulated polymer is circulated
through the borehole in the drilling fluid until the encapsulated
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polymer is properly spotted. The circulation of the drilling
fluid is stopped for a sufficient time to allow the temperature
of the drilling fluid to be raised above the melting point of the
wax casings. The wax melts, releasing the polymer to absorb
water and expand, reducing lost circula-tion of drilling fluid to
thieving zones and fissures. Circulation is then resumed to
clear the borehole of undesired compounds.
This embodiment of the invention is particularly
effective for closing off fractures and large pores without
clogging the borehole. This is because the underground formation
heats up the drilling fluid in the borehole starting at the
outside of the borehole and working towards the center of the
borehole. Thus, the wax casings will dissolve on the outside of
the borehole releasing the polymer to absorb water at a
substantially faster rate than the wax casings will melt in the
middle of the borehole. Substantial expansion of the polymer
will occur in the formation outside of the borehole and on the
edges of the borehole before significant polymer is released from
its protective casing at the center of the borehole, permitting
resumed circulation to clean out the borehole of undesired
compounds. This is particularly true when the invention method
is used to seal off fractures which the borehole has penetrated.
Normally, the fractures are smaller than the wellbore so that the
water absoxbent polymer will be released in the fractures long
before it is released in the borehole.
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Even if substantial polymer is released in the bore-
hole, no damage will occur. The drilling fluid containing the
polymer can still be circulated out of the hole~ leaving behind
the highly expanded, trapped polymer in the formation. The drill
stem can be raised after the treatment and then brought back down
to flush and clean the expanded ;polymer from the hole. If an
aqueous drilling mud is used to expand the polymer instead of a
clay free water, the clay in the drilling fluid can set up with
the expanded polymer to form an extremely viscous, non-flowing
mass. This mass, however, can be washed out of the borehole.
The use of a sma l pill or slug of drilling fluid
containing the lost circulation additive is the preferred means
of delivery to a lost circulation zone. A slug of drilling
fluid, containing as little as 100 gallons of drilling fluid with
the encapsulated polymer, can be introduced into the wellbore.
Once the encapsulated polymer slug is properly spotted at the
lost circulation zone, the rams of the blowout preventer are
closed and additional fluid is pumped into the well, forcing the
encapsulated polymer slug into the lost circulation zone.
The encapsulated polymer should be added to the
drilling fluid in an amount sufficient to seal off the lost
circulation zone. Depending on the type of polymer, the
encapsulation used, and the formation and lost circulation zone
drilled through, the drilling fluid should contain about two to
about 250 pounds of encapsulated polymer per barrel of drilling
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fluid, preferably located only in the small slug of drilling
fluid targeted for -the lost circulation zone.
If the polymer is set with a clay free water and it is
desired to reverse the treatment, it is only necessary to pump
salt water into the borehole. Upon ccntact with salt water the
expanded polymer will break up and release most of its absorbed
water. The formerly expanded polymer can then be washed out of
the formation. The preferred superabsorbent polymers
encapsulated for this invention absorb only one-fourth to
one-third as much salt water as fresh water when the salt water
~- concentration is 0.4~ NaCl. Higher salt concentrations result in
even less salt water absorption.
Usually, it is immediately apparent when a fracture is
penetrated by the wellbore. The mud pressure will drop and less
drilling fluid will be circulated back to the top of the hole.
Large fractures can be responsible for draining off almost all of
the drilling fluid. When this occurs, the encapsulated polymer
should be placed in the drilling fluid and pumped down the hole.
Sufficient drilling fluid containing the encapsulated polymer is
circulated to insure that the fracture contains substantial
amounts of the treated drilling fluid. Of course, the
encapsulated polymer will accompany the drilling fluid into the
fracture. Circulation is then stopped. After a period of time
necessary for the wax casings to melt and release the water
absorbent polymer, circulation is resumed. Drilling fluid
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pressure will shortly begin increasing and more mud will be
circulated back to the surface of the well. The increase in
pressure and mud returned to the surface is a strong indication
that the invention has worked and the fracture is sealed.
Circulation is then increased to clear the borehole of undesired
compounds.
The following example will further illustrate the novel
lost circulation additive and invention method of the present
invention. This example is given by way of illustration and not
as a limitation of a scope of the invention. Thus, it should be
clearly understood that the invention additive and method may be
varied to achieve similar results within the scope of the inven-
tion.
EXAMPLE
A C27- C28 petroleum based wax with a melting point of
about 135-150F was employed to encapsulate the trademarked
A-400 polymer, a polyacrylamidecosodium acrylate, sold by Grain
Processing Co. lO grams of A-400 were added to 90 cc of melted
wax. The melted wax and polymer was mixed thoroughly until a
uniform mixture was obtained. This hot suspension was then
poured into a blender containing cold tap water and the blender
turned on. The mixture was solidified and thoroughly sheared at
the same time to form small granules.
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The solid wax/polymer mixture was removed and liquid
was filtered off. The mixture was then washed with water to
remove excess polymer. No expansion of the small granules was
noted. Thus, only polymer outside of the wax coating absorbed
polymer, and that polymer was washed off.
A drilling fluid was prepared by adding 25 grams of
Aquagel to 350 cc of tap water. Aquagel is a trademarked clay,
primarily montmorillonite, sold ]by N. L. Baroid, Inc. This gave
the equivalent of a 25 pounds per barrel drilling fluid.
25 grams of the encapsulated polymer was then added to
350 ml of the gel/water mixture to yield the equivalent of
25 pounds of polymer per barrel of drilling fluid. No signifi-
cant changes in the mud properties were noted after the addition
of the encapsulated polymer.
The encapsulated polymer and drilling fluid mixture
were then placed in an oven at 150F. After two hours, the wax
had melted and the polymer/bentonite/water mix had formed a
semi-solid mass. The mass was extremely viscous and would not
flow. This semi-solid mass, when formed in a zone of lost circu-
lation, would be extremely effective in shutting off the zone.
Many other variations and modifications may be made in
the concepts described above by those skilled in the art without
departing from the concepts of the present invention.
Accordingly, it should be clearly understood that the concepts in
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the description are illustrative only and are not intended as
limitations on the scope of the invention.
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