U.S. patent application number 11/932729 was filed with the patent office on 2009-04-30 for applications of degradable polymers for delayed mechanical changes in wells.
Invention is credited to Claude E. Cooke, JR..
Application Number | 20090107684 11/932729 |
Document ID | / |
Family ID | 40581342 |
Filed Date | 2009-04-30 |
United States Patent
Application |
20090107684 |
Kind Code |
A1 |
Cooke, JR.; Claude E. |
April 30, 2009 |
Applications of degradable polymers for delayed mechanical changes
in wells
Abstract
Degradable polylactic or polyhydroxyalkanoate polymers may be
used to viscosify aqueous fluids for use in wells, Sand control
screen or liner can be coated with a solid degradable polymer
during placement in a well. Mechanical changes or flow changes in a
well can be caused by solid degradable polymer that changes
physical properties after it is placed in a well. Parts of devices
or entire devices can be made of solid degradable polymer that
converts to a fluid after selected times in an aqueous environment
in a well.
Inventors: |
Cooke, JR.; Claude E.;
(Montgomery, TX) |
Correspondence
Address: |
BURLESON COOKE L.L.P.
2040 NORTH LOOP 336 WEST, SUITE 123
CONROE
TX
77304
US
|
Family ID: |
40581342 |
Appl. No.: |
11/932729 |
Filed: |
October 31, 2007 |
Current U.S.
Class: |
166/376 ;
166/243 |
Current CPC
Class: |
E21B 43/08 20130101;
E21B 34/00 20130101; E21B 23/00 20130101; E21B 2200/05 20200501;
E21B 34/063 20130101; E21B 33/134 20130101; E21B 33/12
20130101 |
Class at
Publication: |
166/376 ;
166/243 |
International
Class: |
E21B 23/00 20060101
E21B023/00 |
Claims
1. A method for delaying a mechanical action in a well comprising
placing a solid degradable polymer, the solid degradable polymer
comprising a polyester, in a device in an aqueous environment such
that degradation of a mechanical property of the degradable polymer
after a selected range of time allows operation of the mechanical
action.
2. The method of claim 1 wherein the mechanical action is the
release of a spring-loaded mechanism.
3. The method of claim 1 wherein the device is attached to a
tubular.
4. The method of claim 1 wherein the operation of the mechanical
action is a change of position, status or operation of the
device.
5. The method of claim 1 wherein the solid degradable polymer
comprises PLA or PHA.
6. The method of claim 1 wherein the device includes a support.
7. The method of claim 1 wherein the mechanical property is
compressive strength, tensile strength, bending strength or a
combination thereof.
8. A device or part of the device for use in a well comprising a
solid degradable polymer, the solid degradable polymer comprising a
polyester, selected to decrease in strength a selected amount in a
selected range of time in an aqueous environment in the well.
9. The device or part of the device of claim 8 wherein the device
is a packer, bridge plug or cement retainer.
10. The device or part of the device of claim 9 wherein the
decrease in strength releases the device in the well.
11. The device or part of the device of claim 8 wherein the device
is a flotation container.
12. The device or part of the device of claim 8 wherein the solid
degradable polymer is PLA or PHA.
13. A device to be placed in a well in an aqueous environment, the
device being made of a solid degradable polymer, the solid
degradable polymer comprising a polyester.
14. The device of claim 13 wherein the degradable polymer is PLA or
PHA.
15. The device of claim 13 wherein the degradable polymer includes
solid particles so as to form composite with the solid degradable
polymer.
16. The device of claim 13 wherein the device is packer, bridge
plug or cement retainer.
17. The device of claim 13 wherein the device is a nipple or pipe.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/470,738, filed May 15, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention pertains to mechanical and chemical
operations in wells. In one embodiment, polymer and method of
preparing viscous fluid for use in wells is provided. In another
embodiment, material and methods are provided for coating of sand
control screens used in wellbores. In another embodiment, this
invention pertains to delayed mechanical or flow changes in a
wellbore after equipment is placed in the well. In another
embodiment, this invention pertains to equipment that is placed in
a wellbore and is degraded in the wellbore by contact with aqueous
fluid.
[0004] 2. Discussion of Related Art
[0005] A large number of mechanical and chemical operations are
carried out in and around wells. Most of these wells are used for
producing hydrocarbons from the earth. They are located at depths
ranging from a few hundred feet below the surface of the earth to
more than 30,000 feet. The temperature at the bottom of the wells
likewise varies over a wide range--from about 100.degree. F. to
more than 400.degree. F.
[0006] After the hole is drilled in the earth in the process of
constructing a well, the process of placing casing in the well and
cementing it in place is commenced. Mechanical devices to aid in
the cementing process may be placed on the outside of the casing
before it is placed in the hole. Instruments and communication
cables may be placed on the casing. Multiple lateral holes may be
drilled from a single hole and casing may be placed in each. When
casing has been cemented, the process of "completing" the well may
begin. This involves forming holes ("perforating") the casing
opposite an interval of a formation where fluids are to be produced
or injected and, in most cases, placing tubing in the well. Various
types of mechanical equipment may be placed in the wellbore, for
safety, flow control and other purposes. Viscous, non-damaging
fluids having a selected specific gravity are needed in wells
during completion operations. In many wells various types of
treatment fluids are then injected into the well to provide greater
capacity of the well to produce hydrocarbons, in processes such as
hydraulic fracturing and acidizing, called "stimulation" processes.
The use of a degradable polymer in the form of ball sealers or
particulates to divert fluid or control fluid loss from a well
during completion or stimulation operations has been disclosed.
(U.S. Pat. No. 4,716,964)
[0007] In some wells, the formation where hydrocarbons are found
has low mechanical strength, which can result in "sand" being
produced into the well along with hydrocarbons. The well then
requires application of a "sand control" process. One of these
processes requires placing a "screen" in the well. The solid
particles (cuttings) and drilling fluid in the well may plug or
partially plug the screen as it is placed in a well. This problem
can be particularly severe in directional or horizontal wells. A
recent U.S. Patent Application Publication (US2002/0142919 A1)
discloses screen coatings that melt or dissolve within a wellbore
and release reactive materials effective in degrading or dissolving
materials that could plug a screen. The problem of screen plugging
during placement was recognized many years ago ("Downhole
Protection of Sand Control Screens," Society of Petroleum Engineers
Paper No. 8803, 1980).
[0008] In well operations used for completing or stimulating a
well, viscous fluids may be used. In most cases, it is desirable
that the fluid become lower viscosity with time after it is placed
in a well or formation around a well. When the fluid becomes low
viscosity it should contain no significant amount of solid or gel
material. One example application of such fluids is hydraulic
fracturing of wells. U.S. Patent Application Publication
2003/0060374A1, which is hereby incorporated by reference herein,
discloses the use of highly concentrated degradable polymers in an
aqueous liquid in such application. As explained in that
Publication, there is a need for fracturing fluids that degrade to
low viscosity without leaving a residue.
[0009] Other applications where a viscous fluid may be injected
into a well or used in a well include completion fluids,
perforating fluids and fluids for carrying gravel (sand) into a
well. These fluids are preferably solids-free and degradable to low
viscosity fluid having low solid or gel content that could degrade
permeability of a porous rock. Other applications where a viscous
liquid in a wellbore may be advantageous include a completion or
workover fluid that is placed in a well during running of a
mechanical device into the well or other mechanical operation in
the well. These fluids may contain high concentrations of compounds
soluble in water that increase the density of the fluid, such as
sodium bromide or zinc bromide, or solid weighting materials. The
viscosifying material in the fluids should degrade with time and
leave little or no residue of solid or gel that could damage the
permeability of a formation around the well.
[0010] A wide variety of mechanical devices are placed in wells
during completion and workover operations. These devices are used
to control fluid flow, to seal around tubulars in the well, to
perform measurements of physical or chemical parameters and various
other purposes. These devices may be needed for only a limited time
and then an operator may wish to have them no longer effective or
to no longer have mechanical strength. For example, packers, bridge
plugs and cement retainers may be needed for a limited time in a
well. There may be a need to release a mechanical device or open a
port after a selected time in an inaccessible portion of a
wellbore, such as in an annulus between tubular strings, where an
aqueous fluid is located.
[0011] What are needed in a variety of well operations or processes
are viscous liquids that degrade to low viscosity liquid at a
predictable rate and leave low amounts of solid or gel residue, a
degradable coating for screens, and solids that lose mechanical
strength at a predictable rate in the presence of an aqueous liquid
to allow delayed flow or mechanical changes in inaccessible
locations in wellbores or degradation of mechanical equipment that
is no longer needed in a wellbore.
SUMMARY OF THE INVENTION
[0012] Degradable polymers and methods for using in wells are
provided. In one embodiment, the degradable polymer is used to
viscosify fluids used in wellbore operations. In another
embodiment, the degradable polymer is used to protect a sand
control screen from plugging as it is placed in a well. In yet
another embodiment, the degradable polymer is used to delay to a
selected range of time a change in mechanical or flow conditions in
a well. In yet another embodiment, the solid degradable polymer is
used to form equipment that is temporarily used in well
operations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows a sketch of a cased well having tubing and the
surrounding formation.
[0014] FIG. 2 shows a cross-section of a wire-wrapped sand control
screen protected by a degradable polymer.
[0015] FIG. 3 shows spring-loaded apparatus in the annulus between
tubing and casing in a well that is released by degradation of a
degradable polymer.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Referring to FIG. 1, wellbore 10 penetrates formation 20
where fluid is to be produced or injected. Wellbore 10 has casing
12 extending through formation 20, casing 12 being cemented in
place by cement sheath 17. Perforations 14 have been formed through
the wall of casing 12 and cement sheath 17 into formation 20.
Perforations 14 may extend over the entire thickness of formation
20 or may extend only over a selected interval of formation 20 less
than the total thickness surrounding wellbore 10. In some wells,
hydraulic fracture 30 may have been formed around wellbore 10 by a
previous treatment employing conventional fracturing fluid and
proppant, using techniques well-known in industry. Alternatively,
fracture 30 may not be present. Tubing 16 may have been suspended
inside casing 12 and packer 18 may have been set near the bottom of
tubing 16 to seal the annulus between tubing 16 and casing 12.
Packer 18 may not be present in some wells, tubing 16 may not be
present in some wells, and even casing 12 may not be present in
some wells, although most wells in which the methods disclosed here
will be applied contain casing and tubing with a packer near the
bottom of the tubing. Packer 18 may have a controllable port for
circulating fluids in the annulus of the well (not shown) or tubing
16 may be releasable from packer 18 to allow circulation of fluids
down the tubing and up the tubing-casing annulus. Alternatively,
tubing 16 may contain a sliding sleeve above and near packer 18,
which is well known in industry.
[0017] In an embodiment for damage removal near wellbore 10, the
materials and methods disclosed in U.S. Patent Application
Publication 2003/0060374A1, which is incorporated by reference, may
be used to form short hydraulic fracture 32 around wellbore 10 by
injecting the degradable fracturing fluid at a pressure above the
fracturing pressure of formation 20. The fracturing fluid disclosed
herein is similar to fracturing fluids normally used, in which a
polymer is dispersed in a liquid to increase viscosity of the
liquid, and has Theological properties similar to the conventional
fracturing fluids. The fracturing fluid disclosed herein is a more
dilute mixture of the degradable polymer contained in the "polymer
phase" disclosed in the cited '374 Publication, and it may be used
to form hydraulic fracture 30 or hydraulic fracture 32, as shown in
FIG. 1. The preferred degradable polymer is a polymer that is
polymerized to a preferred range of molecular weight or is degraded
(decreased in molecular weight) by reaction with water (herein
"water-degradable") to a desirable range of molecular weight for
use in a wellbore fluid. The polymer is dispersed or dissolved in
an aqueous liquid and then degrades to mostly water-soluble
monomers or oligomers over a period of time in the presence of
water.
[0018] The use of solid water-degradable polymers in wells is
known. Their use in wellbores for diverting fluids between
perforations or decreasing fluid loss from a hydraulic fracture
when particles of the polymer are dispersed in fracturing fluid has
been disclosed. U.S. Pat. No. 4,716,964 discloses use of such
polymers in "ball sealers" and as a fluid loss material in well
treating fluids. Ball sealers are rigid spheres added to a well
treatment fluid to seal on perforations and divert flow of the
treatment fluid to other perforations. Fluid loss additives are
finely divided solid polymer particles that are dispersed in the
fracturing fluid or other well treatment fluid and injected into a
well. The polymers disclosed in the '964 patent include poly
(D,L-lactide) and copolymers of lactide and glycolide.
[0019] A significant amount of research and development has been
performed in recent years to commercialize polymers that degrade to
water-soluble chemicals. In addition to the polylactic acid (PLA)
polymers commercialized by Cargill Dow Polymers LLC, other
degradable polymers, including other polyesters (based on
polyethylene terephthalate, for example), starches,
polycaprolactone, polyhydroxybutyrates and blends of these
materials have been developed. Properties of lactide polymers are
reviewed in the article "Properties of lactic acid based polymers
and their correlation with composition," A. Sodergard and M. Stolt,
Prog. in Pol. Sci., July, 2002. Further development is underway for
other degradable or biodegradable polymers. Metabolix, Inc. of
Cambridge, Mass., for example, is developing a family of degradable
polymers known as PHAs (polyhydroxyalkanoates). PHA polymers (also
polyesters) are produced by photosynthesis, either indirectly using
highly efficient fermentation processes, or directly in plant
crops. The price of these polymers is expected to decrease to about
the cost of oil-derived polymers within a few years. The properties
of such polymers can be adjusted by molecular weight distribution,
crystallinity, co-polymers and additives to control physical
properties and degradation time under selected environments.
Polymers such as PLAs and selected PHAs, such as
polyhydroxybutyrate, can be optimized for the applications
disclosed herein by varying manufacturing methods and conditions.
Polyydroxybutyrate will be, in general, more stable to degradation
than PLA. Different polymerization variables can be controlled
during manufacture and/or compounding to provide desirable
degradation times under a broad range of environmental conditions
that exist in underground formations. The PHAs can also be
optimized by varying microbes used in the fermentation
processes.
[0020] Degradation of solid polyesters occurs first by water
penetrating the bulk of the polymer, preferentially attacking the
chemical bonds in the amorphous polymer and converting long chains
into shorter water-soluble fragments. Degradation rates can be
controlled by incorporation of various additives. The control of
properties of thermoplastic polymers by addition of plasticizers
and other additives is well known. Of course, exposure of the
plastics to moisture before their use can be controlled to prevent
premature degradation. Biodegradable polymers may also be degraded
by enzymes, which may be used to contact the polymers, as is known
in the art. If there is need to increase the degradation rate of
polymers left in a wellbore, for example, heating of the polymers
in the wellbore can be used to increase degradation rate or the
polymer may be contacted by a solution containing enzymes. The
Sodergard and Stolt article, cited above, discusses biodegradation
of degradable polymers, including polyesters, and polylactic acid
in particular. The degradation rate (hydrolysis) of polylactic
acids may be increased significantly by enzymes pronase, proteinase
K and bromelain.
[0021] Since water is always present in hydrocarbon reservoirs and
aqueous liquids are usually used in wellbore operations, there is
nearly always a mechanism to cause polymer degradation of
water-degradable polymers in a wellbore or in a reservoir. Rate of
polymer degradation will depend primarily on polymer composition,
polymer structure and temperature. For any degradable polymer
selected, degradation time can be determined by heating a sample of
the polymer to be injected. A water-degradable polymer can be
exposed to an aqueous liquid and subjected to a thermal history
simulating the conditions the polymer would experience in a well
where it is to be used. The thermal history of the polymer as it is
placed in a wellbore or injected down a wellbore and resides in the
wellbore or the subsurface formation while degrading may be
simulated in laboratory tests to select the polymer or co-polymers
and any additives used with the polymer.
[0022] A fracturing fluid, completion or workover wellbore fluid,
fluid for carrying gravel into a fracpack or gravel pack or fluid
for other well operations may be formed by polymerizing lactic acid
to PLA or forming PHA or other biopolymer having a range of
molecular weight that can be dissolved in an aqueous liquid to be
used in the well operation and adding the resulting polymer to
aqueous liquid. If the molecular weight of the manufactured PLA or
PHA is too high to allow solubility in the aqueous liquid, the
molecular weight of the polymer can be decreased by applying heat
to the polymer in the presence of water. For example, steam or hot
water may be applied to solid or liquid polymer for a selected time
to obtain a molecular weight range of the polymer such that it can
be dissolved in the aqueous liquid to be used in a well operation.
Polymer having a desired range of molecular weight may be
stabilized or partially stabilized against further decrease of
molecular weight until it is used in a well operation by removing
water from the polymer (drying) or by lowering the temperature of
the polymer in an aqueous fluid.
[0023] The well treatment fluid disclosed herein may be placed in
wellbore 10 (FIG. 1) by pumping the viscous polymer down the well
from the surface as fluids of the prior art are pumped. The polymer
is added to the aqueous well treatment fluid to a concentration
selected to achieve the desired range of viscosity of the treatment
fluid. The polymer may be cross-linked to increase the effective
viscosity of the solution using well known cross-linking
agents.
[0024] The properties of polylactide are affected by the isomeric
content of the polymer. In addition to the D, L-polylactide
disclosed in U.S. Pat. No. 4,716,964, discussed above, which is a
racemic mixture, a polylactide formed from 13 per cent D-isomer and
87 percent L-isomer, available from Cargill-Dow, is primarily
amorphous in the solid state and degrades to form a viscous liquid
in the presence of water. Preferably, a polymer that is amorphous
or not highly crystalline in the solid state will be used to form
the well treatment fluid of this invention. At the boiling point of
water, viscous liquid formed from solid pellets of the 13 per cent
D-isomer material, whereas a polylactide containing only about 6
per cent D-isomer did not degrade to a viscous liquid under the
same conditions but degraded to a crystalline polymer. Therefore,
the relative amount of D- and L-isomer should be selected in the
range from about 10 per cent to about 90 per cent of an isomer or
in a range to form an amorphous or not highly crystalline polymer.
It is believed that isomer compositions in this range form an
amorphous polymer and the lower molecular weight polymers and the
oligomers formed during degradation form less crystalline material,
allowing formation of the viscous liquid during degradation of the
polymer. The viscous liquid can be diluted to form a solution
having desired Theological properties. Amorphous forms of other
polyesters are preferred for the same reasons.
[0025] In addition to the application of degradable polymers to
form viscous aqueous liquid for use in wells, the polymers may be
applied in the solid form in a variety of processes or methods. The
primary characteristic of the polymer in some of these applications
is that the mechanical properties of the polymer change in a
predictable manner with time in the presence of water or an aqueous
liquid. If desired, an initial solid polymer may finally go in
solution in the aqueous phase. In some applications, only a
decrease in mechanical properties (modulus, bending strength,
tensile or compressive strength, for example) in a predictable time
range may be necessary for application of the polymer. In other
applications, the polymer may maintain its mechanical properties
until it is employed, then decrease in mechanical properties and
become a low-strength gel or low-strength crystalline solid or
become soluble in an aqueous phase in a wellbore.
[0026] In one embodiment of the invention disclosed herein,
degradable polymer is used to coat a sand control screen or slotted
liner before it is placed in a well. Such an application is
described in a recently published U.S. Patent Application (No.
2002/0142919A1), which is hereby incorporated by reference. The
material used to coat the screen is called a "binder." In the '919
Publication, it is disclosed that the binder may contain components
that "react with potentially plugging materials in the near
wellbore area" when the components are released as the binder melts
or dissolves. Such components are well known (scale, paraffin and
clays, for example). The use of wax and soluble solids as a binder
is disclosed. The use of a water-degradable solid polymer that
decreases in molecular weight with time is not disclosed.
[0027] FIG. 2 shows a cross-section of a wire-wrapped sand control
screen. The screen includes basepipe 130, stand-offs 134 and wire
136. Washpipe or tail pipe 132 is shown inside the screen. The
protective coating on the screen is designated 122. It should be
understood that a screen is illustrated, but a perforated liner or
permeable sintered medium may be protected by a protective coating
such as coating 122.
[0028] The use of PLA, PHA and other polyester polymers makes
possible a timed degradation of the coating, rather than the
employment of temperature alone or dissolution in a fluid as
disclosed in the '919 Publication. The properties of the polyester
may be selected to maintain sufficient mechanical strength to
prevent displacement of the polymer from the screen as it is placed
in a well. This time may be from several hours to days, depending
on the time required to place the coated screen in a well. An
example of the decrease in molecular weight of poly (DL-lactide)
with time is provided in the paper "Further investigations on the
hydrolytic degradation of poly (DL-lactide)," Biomaterials 20
(1999) 35-44. The data in the paper were obtained at 37.degree. C.
and at 60.degree. C. As can be noted in the U.S. Pat. No.
4,716,964, referenced above, the rate of degradation is much more
rapid at temperatures more typical of the temperature in wells. The
polymer coating initially should have a melting point higher than
the temperature expected in the well. The polymer should degrade to
form a material that can be displaced from the well. If the polymer
should flow outwardly from the screen, the polymer should not
permanently damage permeability of the gravel placed in the well.
Some or the entire polymer may be produced from the well as a
viscous liquid. The initial strength of the solid polymer should be
sufficient to prevent flow across the screen, in the area where the
polymer is applied, under pressure differentials across the screen
as it is placed in the well. The polymer coating may be used, for
example, to prevent flow through only selected areas of the screen
as it is put in a well. To increase initial strength of the
polymer, a composite may be formed with the polymer by
incorporating particles of a rigid solid, which may be a soluble
crystalline material, for example, in the polymer before it is
placed on the screen. Polymers having varying degradation rates may
be used on different areas of a screen. For example, a more rapidly
degrading polymer may be used over the lower portion of a
screen.
[0029] Degradable polymer, such as PLA, may be applied to the
screen, for example, by heating the polymer to allow flow or
extrusion and coating the polymer on the finished screen. The
screen may be heated before application of the polymer to allow
more uniform flow of polymer into the screen. Alternatively, the
polymer may be applied from solution in a solvent and the solvent
removed to form a solid polymer. Alternatively, the base pipe or
mandrel of the screen may be coated and the holes plugged with hot
PLA or other water-degradable polymer before the wire of a screen
is applied. Alternatively, blank pipe to be run into a well may be
coated with the degradable polymer. The degradable polymer may be
formulated to contain any or all the additives taught by the '919
Publication. The additives would then be released to enter the
fluids around the screen or blank pipe as the polymer degrades.
[0030] There are reasons to attach various mechanical devices to
the outside of tubulars as they are placed in a well. The devices
may be used to measure physical or chemical variables or to modify
flow conditions in the well, for example. A change in the position,
status or operation of the device after a selected time may be
desirable. A degradable polymer, such as PLA or a PHA, may be used
to form a mechanical part of the device or a support for the
device. The degradation rate of the degradable polymer may be
selected to allow the desired change to occur in a selected range
of time after placement of the device in a well. For example, FIG.
3 is adapted from U.S. Pat. No. 5,509,474. In this example, tubing
106 has been placed in a well inside casing 12. The annulus between
tubing and casing will ordinarily be filled by an aqueous fluid.
Sensors 111 are designed to be released from the vicinity of the
outside surface of tubing 106 and then to spring against the inside
wall of casing 12. An electromechanical device could be used to
release the spring-loaded sensors. Alternatively, groove 200 may be
formed in insulating material collar 114 and the spring-loaded
sensors may be held in groove 200 by placing a selected solid
water-degradable polymer over the sensor, shown at 111(a), in the
groove, using techniques of placement such as described above for a
screen. After tubing 106 is placed in a well in an aqueous fluid
environment, polymer in groove 200 degrades to a range of
mechanical properties (determined by the decrease in molecular
weight of the degradable polymer) that allows sensor 111 to be
released and to spring into the position shown at 111, which is in
contact with the inside surface of casing 12.
[0031] The applications of degradable polymers disclosed herein to
allow a timed change in location of a part or parts of mechanical
devices can be readily seen by one of skill in the art of each
device. The degradable polymer may easily be configured to allow
the change to occur as compressive strength of the polymer
degrades, as tensile strength degrades, as bending strength
degrades, or as a combination of properties changes. The time of
change can be determined by selecting a degradable polymer that
changes in properties at a rate to allow the change to occur in a
desired time range. This range may be hours, days or months,
depending on the mechanical configuration and the polymer
selected.
[0032] In other applications, flow configuration or pressure
changes may be desired in a well after a selected time. For
example, a port may preferably be opened after a selected time, in
the range of hours, days or months. The port may be inaccessible or
require expensive operations to open. For example, the port may be
used to co-mingle fluid streams being produced from a well and be
in an aqueous environment. A plug may be formed from a degradable
polymer as disclosed herein. Tests can be performed with different
polymer compositions to select the polymer providing the opening of
the port in the desired time range and at the pressure differential
existing across the port when in the well. Measurements of physical
properties of a selected degradable polymer as a function of time
and at selected temperatures in an aqueous environment may also be
used to predict the time of opening of a selected port under
selected conditions. Alternative, the polymer can be made in the
form of a seal or gasket that degrades in time to allow flow. Such
measurements and tests should take into account the dimensions of
the degradable polymer body that is degrading, since such changes
in properties are known to be affected by dimensions of the body,
which affect the length of the diffusion path of water molecules
into the degradable material and the diffusion path of reaction
products from the polymer.
[0033] In another embodiment, mechanical devices or selected parts
of mechanical devices that are placed in a well may be formed from
solid degradable polymer such as PLA or PHA. For example, parts of
a packer, a bridge plug or a cement retainer may be formed of
water-degradable polymer. After a selected range of time, from
hours, to days or months, the device or selected parts of the
device may be designed to decrease in properties so as to release
the device and facilitate retrieval. Alternatively, the entire
device may be formed of a degradable polymer where strength of the
polymer is adequate. For example, a nipple or pipe section may be
formed of degradable polymer. The nipple or pipe may degrade and
later be produced from a well. The pipe may be the "tail pipe" used
in a sand control screen, for example.
[0034] Whereas the PLA used in fluids is preferably amorphous, as
described above, the PLA used in mechanical or flow control devices
may be amorphous or crystalline. The bending strength of rods of
poly (D-lactide) (PLA) (which would be crystalline) when made by
routine injection molding has been measured to be in the range of
40-140 MPA. Rods formed by solid state extrusion had bending
strengths up to 200 MPA ("Enhancement of the mechanical properties
of polylactides by solid-state extrusion," Biomaterials 17, (March,
1996, 529-535). Further information about PLA and its properties is
provided in a chapter entitled "Present and Future of PLA Polymers"
in the book Degradable Polymers, Recycling, and Plastics Waste
Management, Ed. by Ann-Christine Albertsson and S. J. Huang, Marcel
Dekker, Inc. It is well known that strength may be increased by the
use of composites made of the thermoplastic polymer. Where added
strength is desired, composite formed from a degradable polymer may
be used. PLA, for example, can be molded as other thermoplastic
materials are formed or it may be formed by extrusion other
processing steps known in industry.
[0035] An example of a simple mechanical device that may be formed
or partially formed from PLA or other water-degradable polymer is a
flotation container to be attached to casing being run into a
horizontal well. Such flotation devices made of metal are well
known. The walls and ends of such a container may be formed from
degradable polymer, with adequate supports of degradable polymer
between the ends to prevent collapse, or the ends and supports may
be formed of degradable polymer and designed to allow walls to
collapse after a selected time in the well (and before
cementing).
[0036] Other mechanical parts that may be more easily left in a
well than retrieved may also be formed from degradable polymer such
as PLA. For example, the case or container of perforating devices
may be formed of degradable polymer. After a selected time, the
device may then be easily flowed from the well, if desired.
[0037] Although the present invention has been described with
reference to specific details, it is not intended that such details
should be regarded as limitations on the scope of the invention,
except as and to the extent that they are included in the
accompanying claims.
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