U.S. patent application number 09/821251 was filed with the patent office on 2002-10-03 for method for preventing critical annular pressure buildup.
This patent application is currently assigned to Hunting OilField Services, Inc.. Invention is credited to Staudt, John J..
Application Number | 20020139536 09/821251 |
Document ID | / |
Family ID | 25232916 |
Filed Date | 2002-10-03 |
United States Patent
Application |
20020139536 |
Kind Code |
A1 |
Staudt, John J. |
October 3, 2002 |
METHOD FOR PREVENTING CRITICAL ANNULAR PRESSURE BUILDUP
Abstract
A modified casing coupling that includes a receptacle, or
receptacles, for a modular burst disk assembly. The burst disk
assembly is retained by threads or a snap ring and is sealed by the
retaining threads, or an integral o-ring seal. The. The disk fails
at pressure specified by the user but before trapped annular
pressure threatens the integrity of the outer casing. The design
allows for the burst disk assembly to be installed on location or
before pipe shipment.
Inventors: |
Staudt, John J.;
(Friendswood, TX) |
Correspondence
Address: |
Charles D. Gunter, Jr.
BRACEWELL & PATTERSON, LLP
201 Main Street, Suite 1600
Fort Worth
TX
76102
US
|
Assignee: |
Hunting OilField Services,
Inc.
|
Family ID: |
25232916 |
Appl. No.: |
09/821251 |
Filed: |
March 29, 2001 |
Current U.S.
Class: |
166/363 ;
166/317; 166/364 |
Current CPC
Class: |
E21B 17/08 20130101;
E21B 34/06 20130101; E21B 41/0021 20130101 |
Class at
Publication: |
166/363 ;
166/364; 166/317 |
International
Class: |
E21B 033/035; E21B
034/08 |
Claims
What is claimed is:
1. A method for the prevention of damage in offshore oil and gas
wells due to trapped annular pressure between successive lengths of
well casing comprising: modifying a casing coupling to include at
least one receptacle for housing a modular bust disk assembly
including a burst disk; installing the modular burst disk assembly
within the receptacle of the modified casing coupling; wherein the
burst disk of the burst disk assembly is exposed to the annular
pressure trapped between successive lengths of well casing; and
wherein the burst disk is selected to fail at a pressure specified
by a user.
2. The method of claim 1 wherein the bust disk assembly has a
threaded exterior which mates with an internally threaded region
within the receptacle.
3. The method of claim 1 wherein the bust disk assembly is secured
by a snap ring within the receptacle of the casing coupling.
4. The method of claim 2 wherein the bust disk assembly is sealed
within the receptacle of the casing coupling by an externally
threaded region.
5. The method of claim 1 wherein the bust disk assembly is sealed
within the casing coupling receptacle by an integral o-ring
seal.
6. The method of claim 1 wherein the selected pressure at which the
bust disk assembly fails is compensated for temperature.
7. The method of claim 1 farther comprising inserting a protective
cap within a bore provided in the burst disk assembly, to protect
the burst disk during handling of the pipe.
8. A burst disk assembly comprising: a cylindrical main body having
an upper region and a lower region with the lower region having an
internal annular recess and the upper region having a hex hole
which communicates with the annular recess for the insertion of a
hex wrench; a disk retainer having an upper surface and a lower
surface and a bore that communicates the upper and lower surfaces;
and a burst disk interposed between the main cylindrical body and
the upper surface of the disk retainer, the burst disk being
constructed of a material which is selected to fail at a pressure
specified by a user.
9. The burst disk assembly of claim 8 further comprising an o-ring
groove in the bottom surface of the disk retainer for the insertion
of an o-ring.
10. The burst disk assembly of claim 8 further comprising an
externally threaded surface on the upper region of the cylindrical
main body.
11. The burst disk assembly of claim 10 wherein the threaded
surface has threads which are UNF threads.
12. The burst disk assembly of claim 10 wherein the threaded
surface has threads which are NPT threads.
13. The burst disk assembly of claim 8 further comprising a ridge
or lip located on the upper region of the main body to act as a
snap ring.
14. The burst disk assembly of claim 8 wherein the disk is made of
INCONEL.TM..
15. The burst disk assembly of claim 8 wherein the main body and
the disk retainer are made of 316 stainless steel.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates generally to a method for the
prevention of damage to oil and gas wells, and, more specifically,
to the prevention of damage to the well casing from critical
annular pressure buildup.
[0003] 2. Description of the Related Art
[0004] The physics of annular pressure buildup (APB) and associated
loads exerted on well casing and tubing strings have been
experienced since the first multi-string completions. APB has drawn
the focus of drilling and completion engineers in recent years. In
modem well completions, all of the factors contributing to APB have
been pushed to the extreme, especially in deep water wells.
[0005] APB can be best understood with reference to a subsea
wellhead installation. In oil and gas wells it is not uncommon that
a section of formation must be isolated from the rest of the well.
This is typically achieved by bringing the top of the cement column
from the subsequent string up inside the annulus above the previous
casing shoe. While this isolates the formation, bringing the cement
up inside the casing shoe effectively blocks the safety valve
provided by nature's fracture gradient. Instead of leaking off at
the shoe, any pressure buildup will be exerted on the casing,
unless it can be bled off at the surface. Most land wells and many
offshore platform wells are equipped with wellheads that provide
access to every casing annulus and an observed pressure increase
can be quickly bled off. Unfortunately, most subsea wellhead
installations do not have access to each casing annulus and often a
sealed annulus is created. Because the annulus is sealed, the
internal pressure can increase significantly in reaction to an
increase in temperature.
[0006] Most casing strings and displaced fluids are installed at
near-static temperatures. On the sea floor the temperature is
around 34.degree. F. The production fluids are drawn from "hot"
formations that dissipate and heat the displaced fluids as the
production fluid is drawn towards the surface. When the displaced
fluid is heated, it expands and a substantial pressure increase may
result. This condition is commonly present in all producing wells,
but is most evident in deep water wells. Deep water wells are
likely to be vulnerable to annular pressure buildup because of the
cold temperature of the displaced fluid, in contrast to elevated
temperature of the production fluid during production. Also, subsea
wellheads do not provide access to all the annulus and any pressure
increase in a sealed annulus cannot be bled off. Sometimes the
pressure can become so great as to collapse the inner string or
even rupture the outer string, thereby destroying the well.
[0007] One previous solution to the problem of APB was to take a
joint in the outer string casing and mill a section off so as to
create a relatively thin wall. However, it was very difficult to
determine the pressure at which the milled wall would fail or
burst. This could create a situation in which an overly weakened
wall would burst when the well was being pressure tested. In other
cases, the milled wall could be too strong, causing the inner
string to collapse before the outer string bursts.
[0008] What is needed is a casing coupling which reliably holds a
sufficient internal pressure to allow for pressure testing of the
casing, but which will collapse or burst at a pressure slightly
less than collapse pressure of the inner string or the burst
pressure of the outer string.
BRIEF SUMMARY OF THE INVENTION
[0009] It is an object of the present invention to provide a casing
coupling that will hold a sufficient internal pressure to allow for
pressure testing of the casing but which will reliably release when
the pressure reaches a predetermined level.
[0010] It is another object of the present invention to provide a
casing coupling that will release at a pressure less than the
collapse pressure of the inner string and less than the burst
pressure of the outer string.
[0011] It is yet another object of the present invention to provide
a casing coupling that is relatively inexpensive to manufacture,
easy to install, and is reliable in a fixed, relatively narrow
range of pressures.
[0012] The above objects are achieved by creating a casing coupling
modified to include at least one receptacle for housing a modular
bust disk assembly wherein the burst disk assembly fails at a
pressure specified by a user. The burst disk assembly is retained
in a suitable manner, as by threads or a snap ring and is sealed by
either the retaining threads, or an integral o-ring seal. The
pressure at which the burst disk fails is specified by the user,
and is compensated for temperature. The disk fails when the trapped
annular pressure threatens the integrity of either the inner or
outer casing. The design allows for the burst disk assembly to be
installed on location or before pipe shipment.
[0013] Additional objects, features and advantages will be apparent
in the written description which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The novel features believed characteristic of the invention
are set forth in the appended claims. The invention itself however,
as well as a preferred mode of use, will best be understood by
reference to the following detailed description of an illustrative
embodiment when read in conjunction with the accompanying drawings,
wherein:
[0015] FIG. 1A is a cross sectional, exploded view of a burst disk
assembly;
[0016] FIG. 1B is a cross sectional view of an assembled burst disk
assembly;
[0017] FIG. 2A is a cross sectional view of burst disk assembly
installed in a casing using threads;
[0018] FIG. 2B is a cross sectional view of burst disk assembly
installed in a casing using thread;
[0019] FIG. 2C is a cross sectional view of burst disk assembly
installed in a casing using a snap ring;
[0020] FIG. 3 is a simplified view of a typical off-shore well rig;
and
[0021] FIG. 4 is a cross sectional view of a bore hole.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0022] FIG. 3 shows a simplified view of a typical offshore well
rig. The derrick 302 stands on top of the deck 304. The deck 304 is
supported by a floating work station 306. Typically, on the deck
304 is a pump 308 and a hoisting apparatus 310 located underneath
the derrick 302. Casing 312 is suspended from the deck 304 and
passes through the subsea conduit 314, the subsea well head
installation 316 and into the borehole 318. The subsea well head
installation 316 rests on the sea floor 320.
[0023] During construction of oil and gas wells, a rotary drill is
typically used to bore through subterranean formations of the earth
to form the borehole 318. As the rotary drill bores through the
earth, a drilling fluid, known in the industry as a "mud," is
circulated through the borehole 318. The mud is usually pumped from
the surface through the interior of the drill pipe. By continuously
pumping the drilling fluid through the drill pipe, the drilling
fluid can be circulated out the bottom of the drill pipe and back
up to the well surface through the annular space between the wall
of the borehole 318 and the drill pipe. The mud is usually returned
to the surface when certain geological information is desired and
when the mud is to be recirculated. The mud is used to help
lubricate and cool the drill bit and facilitates the removal of
cuttings as the borehole 318 is drilled. Also, the hydrostatic
pressure created by the column of mud in the hole prevents blowouts
which would otherwise occur due to the high pressures encountered
within the wellbore. To prevent a blow out caused by the high
pressure, heavy weight is put into the mud so the mud has a
hydrostatic pressure greater than any pressure anticipated in the
drilling.
[0024] Different types of mud must be used at different depths
because the deeper the borehole 318, the higher the pressure. For
example, the pressure at 2,500 ft. is much higher than the pressure
at 1,000 ft. The mud used at 1,000 ft. would not be heavy enough to
use at a depth of 2,500 ft. and a blowout would occur. In subsea
wells the pressure at deep depths is tremendous. Consequently, the
weight of the mud at the extreme depths must be particularly heavy
to counteract the high pressure in the borehole 318. The problem
with using a particularly heavy mud is that if the hydrostatic
pressure of the mud is too heavy, then the mud will start
encroaching or leaking into the formation, creating a loss of
circulation of the mud. Because of this, the same weight of mud
cannot be used at 1,000 feet that is to be used at 2,500 feet. For
this reason, it is impossible to put a single casing string all the
way down to the desired final depth of the borehole 318. The weight
of the mud necessary to reach the great depth would start
encroaching and leaking into the formation at the more shallow
depths, creating a loss of circulation.
[0025] To enable the use of different types of mud, different
strings of casing are employed to eliminate the wide pressure
gradient found in the borehole 318. To start, the borehole 318 is
drilled to a depth where a heavier mud is required and the required
heavier mud has such a high hydrostatic pressure that it would
start encroaching and leaking into the formation at the more
shallow depths. This generally occurs at a little over 1,000 ft.
When this happens, a casing string is inserted into the borehole
318. A cement slurry is pumped into the casing and a plug of fluid,
such as drilling mud or water, is pumped behind the cement slurry
in order to force the cement up into the annulus between the
exterior of the casing and the borehole 318. The amount of water
used in forming the cement slurry will vary over a wide range
depending upon the type of hydraulic cement selected, the required
consistency of the slurry, the strength requirement for a
particular job, and the general job conditions at hand.
[0026] Typically, hydraulic cements, particularly Portland cements,
are used to cement the well casing within the borehole 318.
Hydraulic cements are cements which set and develop compressive
strength due to the occurrence of a hydration reaction which allows
them to set or cure under water. The cement slurry is allowed to
set and harden to hold the casing in place. The cement also
provides zonal isolation of the subsurface formations and helps to
prevent sloughing or erosion of the borehole 318.
[0027] After the first casing is set, the drilling continues until
the borehole 318 is again drilled to a depth where a heavier mud is
required and the required heavier mud would start encroaching and
leaking into the formation. Again, a casing string is inserted into
the borehole 318, generally around 2,500 feet, and a cement slurry
is allowed to set and harden to hold the casing in place as well as
provide zonal isolation of the subsurface formations, and help
prevent sloughing or erosion of the borehole 318.
[0028] Another reason multiple casing strings may be used in a bore
hole is to isolate a section of formation from the rest of the
well. In the earth there are many different layers with each made
of rock, salt, sand, etc. Eventually the borehole 318 is drilled
into a formation that should not communicate with another
formation. For example, a unique feature found in the Gulf of
Mexico is a high pressure fresh water sand that flows at a depth of
about 2,000 feet. Due to the high pressure, an extra casing string
is generally required at that level. Otherwise, the sand would leak
into the mud or production fluid. To avoid such an occurrence, the
borehole 318 is drilled through a formation or section of the
formation that needs to be isolated and a casing string is set by
bringing the top of the cement column from the subsequent string up
inside the annulus above the previous casing shoe to isolate that
formation. This may have to be done as many as six times depending
on how many formations need to be isolated. By bringing the cement
up inside the annulus above the previous casing shoe the fracture
gradient of the shoe is blocked. Because of the blocked casing
shoe, pressure is prevented from leaking off at the shoe and any
pressure buildup will be exerted on the casing. Sometimes this
excessive pressure buildup can be bled off at the surface or a
blowout preventor (BOP) can be attached to the annulus.
[0029] However, a subsea wellhead typically has an outer housing
secured to the sea floor and an inner wellhead housing received
within the outer wellhead housing. During the completion of an
offshore well, the casing and tubing hangers are lowered into
supported positions within the wellhead housing through a BOP stack
installed above the housing. Following completion of the well, the
BOP stack is replaced by a Christmas tree having suitable valves
for controlling the production of well fluids. The casing hanger is
sealed off with respect to the housing bore and the tubing hanger
is sealed off with respect to the casing hanger or the housing
bore, so as to effectively form a fluid barrier in the annulus
between the casing and tubing strings and the bore of the housing
above the tubing hanger. After the casing hanger is positioned and
sealed off, a casing annulus seal is installed for pressure
control. On every well there is a casing annulus seal. If the seal
is on a surface well head, often the seal can have a port that
communicates with the casing annulus. However, in a subsea wellhead
housing, there is a large diameter low pressure housing and a
smaller diameter high pressure housing. Because of the high
pressure, the high pressure housing must be free of any ports for
safety. Once the high pressure housing is sealed it off, there is
no way to have a hole below the casing hanger for blow out
preventor purposes. There are only solid annular members with no
means to relieve excessive pressure buildup.
[0030] FIG. 4 shows a simplified view of a multi string casing in
the borehole 318. The borehole 318 contains casing 430, which has
an inside diameter 432 and an outside diameter 434, casing 436,
which has an inside diameter 438 and an outside diameter 440,
casing 442, which has an inside diameter 444 and an outside
diameter 446, casing 448, which has an inside diameter 450 and an
outside diameter 452. The inside diameter 432 of casing 430 is
larger than the outside diameter 440 of casing 436. The inside
diameter 438 of casing 436 is larger than the outside diameter 446
of casing 442. The inside diameter 444 of casing 442 is larger than
the outside diameter 452 of casing 448. Annular region 402 is
defined by the inside diameter 432 of casing 430 and the outside
diameter 440 of casing 436. Annular region 404 is defined by the
inside diameter 438 of casing 436 and the outside diameter 446 of
casing 442. Annular region 406 is defined by the inside diameter
444 of casing 442 and the outside diameter 452 of casing 448.
Annular regions 402 and 404 are located in the low pressure housing
426 while annular region 406 is located in the high pressure
housing 428. Annular region 402 depicts a typical annular region.
If a pressure increase were to occur in the annular region 402, the
pressure could escape either into formation 412 or be bled off at
the surface through port 414. In the annular region 404 and 406, if
a pressure increase were to occur, the pressure increase could not
escape into the adjacent formation 416 because the formation 416 is
a formation that must be isolated from the well. Because of the
required isolation, the top of the cement 418 from the subsequent
string has been brought up inside the annular regions 404 and 406
above the previous casing shoe 420 to isolate the formation 416. A
pressure build up in the annular region 404 can be bled off because
the annular region 404 is in the low pressure housing 426 and the
port 414 is in communication with the annulus and can be used to
bled off any excessive pressure buildup. In contrast, annular
region 406 is in the high pressure housing 428 and is free of any
ports for safety. As a result, annular region 406 is a sealed
annulus. Any pressure increase in annular region 406 cannot be bled
off at the surface and if the pressure increase gets to great, the
inner casing 448 may collapse or the casing surrounding the annular
region 406 may burst.
[0031] Sometimes a length of fluid is trapped in the solid annular
members between the inside diameter and outside diameter of two
concentric joints of casing. At the time of installation, the
temperature of the trapped annular fluid is the same as the
surrounding environment. If the surrounding environment is a deep
sea bed, then the temperature may be around 34.degree. F. Excessive
pressure buildup is caused when well production is started and the
heat of the produced fluid, 110.degree. F.--300.degree. F., causes
the temperature of the trapped annular fluid to increase. The
heated fluid expands, causing the pressure to increase. Given a
10,000 ft., 31/2-inch tubing inside a 7-inch 35 ppf (0.498-inch
wall) casing, assume the 8.6-ppg water-based completion fluid has a
fluid thermal expansivity of 2.5.times.10.sup.-4 R.sup.-1 and heats
up an average of 70.degree. F. during production.
[0032] When an unconstrained fluid is heated, it will expand to a
larger volume as described by:
V=V.sub.0(1+.alpha..DELTA.T)
[0033] Wherein:
[0034] V=Expanded volume, in..sup.3
[0035] V.sub.0=Initial volume, in..sup.3
[0036] .alpha.=Fluid thermal expansivity, R.sup.-1
[0037] .DELTA.T=Average fluid temperature change, .degree. F.
[0038] The fluid expansion that would result if the fluid were bled
off is:
V.sub.0=10,000(.pi./4)(6.004.sup.2-3.5.sup.2/144=1,298
ft.sup.3=231.2 bbl
V=231.2[1+(2.5.times.10.sup.-4.times.70)]=235.2 bl
.DELTA.V=4.0 bbl
[0039] The resulting pressure increase if the casing and tubing are
assumed to form in a completely rigid container is:
.DELTA.P=(V-V.sub.0)/V.sub.0B.sub.N
[0040] Wherein:
[0041] V=Expanded volume, in..sup.3
[0042] V.sub.o=Initial volume, in..sup.3
[0043] .DELTA.P=Fluid pressure change, psi
[0044] B.sub.N=Fluid compressibility, psi.sup.-1
.DELTA.P=2.5.times.10.sup.-4.times.70/2.8.times.10.sup.-6=6,250
psi.
[0045] The resulting pressure increase of 6,250 psi can easily
exceed the internal burst pressure of the outer casing string, or
the external collapse pressure of the inner casing string.
[0046] The proposed invention is comprised of a modified casing
coupling that includes a receptacle, or receptacles, for a modular
burst disk assembly. Referring first to FIGS. 1A and 1B of the
drawings, the preferred embodiment of a burst disk assembly of the
invention is illustrated generally as 100. The burst disk assembly
100 included a burst disk 102 which is preferably made of
INCONEL.TM., nickel-base alloy containing chromium, molybdenum,
iron, and smaller amounts of other elements. Niobium is often added
to increase the alloy's strength at high temperatures. The nine or
so different commercially avaliable INCONEL.TM. alloys have good
resistance to oxidation, reducing environments, corrosive
environments, high temperature environments, cryogenic
temperatures, relaxation resistance and good mechanical properties.
Similar materials may be used to create the burst disk 102 so long
as the materials can provide a reliable burst range within the
necessary requirements.
[0047] The burst disk 102 is interposed in between a main body 106
and a disk retainer 104 made of 316 stainless steel. The main body
106 is a cylindrical member having an outer diameter of
1.250-inches in the preferred embodiment illustrated. The main body
106 has an upper region R.sub.1 having a height of approximately
0.391-inches and a lower region R.sub.2 having a height of
approximately 0.087-inches which are defined between upper and
lower planar surfaces 116, 118. The upper region also comprises an
externally threaded surface 114 for engaging the mating casing
coupling, as will be described. The upper region R.sub.1 may have a
chamfered edge 130 approximately 0.055-inches long and having a
maximum angle of about 45.degree.. The lower region R.sub.2 also
has a chamfer 131 which forms an approximate 45.degree. angle with
respect to the lower surface 116. The lower region R.sub.2 has an
internal annular recess 120 approximately 0.625-inches in diameter
through the central axis of the body 106. The dimensions of the
internal annular recess 120 can vary depending on the requirements
of a specific use. The upper region R.sub.1 of the main body 106
has a 1/2 inch hex hole 122 for the insertion of a hex wrench. The
internal annular recess 120 and hex hole 122 form an internal
shoulder 129 within the interior of the main body 106.
[0048] The disk retainer 104 is approximately 0.172-inches in
height and has a top surface 124 and a bottom surface 126. The disk
retainer 104 has a continuous bore 148 approximately 0.375-inches
in diameter through the central axis of the disk retainer 104. The
bore 148 communicates the top surface 124 and the bottom surface
126 of disk retainer 104. The bottom surface 126 contains an o-ring
groove 110, approximately 0.139-inches wide, for the insertion of
an o-ring 128.
[0049] The burst disk 102 is interposed between the lower surface
116 of the main body 106 and the top surface 124 of the disk
retainer 104. The main body 106, disk 102, and disk retainer 104
are held together by a weld (108 in FIG. 1B). A protective cap 112
may be inserted into the hex hole 122 to protect the burst disk
102. The protective cap may be made of plastic, metal, or any other
such material that can protect the burst disk 102.
[0050] The burst disk assembly 100 is inserted into a modified
casing coupling 202 shown in FIGS. 2A and 2B. The modified coupling
202 is illustrated in cross section, as viewed from above in FIGS.
2A and 2B and includes an internal diameter 204 and an external
diameter 206. An internal recess 208 is provided for receiving the
burst disk assembly 100. The internal recess 208 has a bottom wall
portion 212 and sidewalls 210. The sidewalls 210 are threaded along
the length thereof for engaging the mating threaded region 114 on
the main body 106 of the burst disk assembly 100. The threaded
region 114 on body 106 may be, for example, 12 UNF threads. The
burst disk assembly 100 is secured in the internal recess 208 by
using an applied force of approximately 200 ft pounds of torque
using a hex torque wrench. The 200 ft pounds of torque is used to
ensure the o-ring 128 is securely seated and sealed on the bottom
wall portion 212 of the internal recess 208.
[0051] It is possible that the o-ring 128 can not be used in
certain casings because of a very thin wall region or diameter 204
of the modified coupling 202, For example, sometimes a 16-inch
casing is used inside a 20-inch casing, leaving very little room
inside the string. Normally a 16-inch coupling has an outside
diameter of 17-inches, however in this instance the coupling would
have to be 161/2-inches in diameter to compensate for the lack of
space. Consequently, the casing wall would be very thin and there
would not be enough room to machine the cylindrical internal recess
208 and leave material at the bottom wall portion 212 for the
o-ring 128 to seat against. In this case, instead of using an
o-ring 128 to seal the burst disk assembly 100, NPT threads can be
used. This version of the coupling and burst disk assembly is
illustrated in FIG. 2B. The assembly is similar to that of FIG. 2A
except that the NPT application has a tapered thread as opposed to
a straight UNF thread when an o-ring 128 is used.
[0052] Snap rings 230 may also provide the securing means. Instead
of providing a threaded region 114 on the body 106, a ridge or lip
232 would extend from the body 106. Also, the threaded sidewalls
210 in the internal recess 208 would be replaced with a mechanism
for securing the burst disk assembly 100 inside the internal recess
208 by engaging the lip or ridge that extends from the body
106.
[0053] The installation and operation of the burst disk assembly of
the invention will now be described. The pressure at which the
burst disk 102 fails is calculated using the temperature of the
formation and the pressure where either the inner string would
collapse or the outer casing would burst, whichever is less. Also,
the burst disk 100 must be able to withstand a certain threshold
pressure. The typical pressure of a well will depend on depth and
can be anywhere from about 1,400 psi to 7,500 psi. Once the outer
string has been set, it must be pressure tested to ensure the
cement permits a good seal and the string is set properly in place.
After the outer casing has been pressure tested, the inner casing
is set. The inner casing has a certain value that it can stand
externally before it collapses in on itself. A pressure range is
determined that is greater than the test pressure of the outer
casing but less than the collapse pressure of the inner casing.
[0054] After allowing for temperature compensation, a suitable
burst disk assembly 100 is chosen based on the pressure range.
Production fluid temperature is generally between 110.degree.
F.-300.degree. F. There is a temperature gradient inside the well
and a temperature loss of 40-50.degree. F. to the outer casing
where the bust disk assembly 100 is located is typical. The
temperature gradient is present because the heat has to be
transferred through the production pipe into the next annulus, then
to the next casing where the bust disk assembly 100 is located.
Also, some heat gets transferred into the formation. At a given
temperature the burst disk 102 has a specific strength. As the
temperature goes up, the strength of the burst disk 102 goes down.
Therefore, as the temperature goes up, the burst pressure of the
burst disk 102 decreases. This loss of strength at elevated
temperatures is overcome by compensating for the loss of strength
at a given temperature.
[0055] Often times the pressure of the well is unknown until just
before the modified coupling 202 is installed and sent down into
the well. The burst disk assembly 100 can be installed on location
at any time before the coupling 202 is sent into the well. Also,
depending on the situation, the modified coupling 202 may need to
be changed or something could happen at the last minute to change
the pressure rating thereby requiring an existing burst disk
assembly 100 to be taken out and replaced. To be prepared, several
bursts disk assemblies 100 could be ordered to cover a range of
pressures. Then when the exact pressure is know, the correct burst
disk assembly 100 could be installed just before the modified
coupling 202 is sent into the well.
[0056] When the burst disk 102 fails, the material of the disk
splits in the center and then radially outward and the comers pop
up. The split disk material remains a solid piece with no loose
parts and looks like a flower that has opened or a banana which has
been peeled with the parts remaining intact. The protective cap 112
is blown out of the way and into the annulus.
[0057] The pressure at which the burst disk 102 fails can be
specified by the user, and is compensated for temperature. The
burst disk 102 fails when the trapped annular pressure threatens
the integrity of either the outer or inner string. The design
allows for the burst disk assembly 100 to be installed in the
factory or in the field. A protective cap 112 is included to
protect the burst disk 102 during shipping and handling of the
pipe.
[0058] An invention has been described with several advantages. The
modified string of casing will hold a sufficient internal pressure
to allow for pressure testing of the casing and will reliably
release or burst when the pressure reaches a predetermined level.
This predetermined level is less than collapse pressure of the
inner string and less than the burst pressure of the outer string.
The burst disk assembly of the invention is relatively inexpensive
to manufacture and is reliable in operation within a fixed, fairly
narrow range of pressure.
[0059] While the invention is shown in only one of its forms, it is
not thus limited but is susceptible to various changes and
modifications without departing from the spirit thereof.
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