Information
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Patent Grant
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6536526
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Patent Number
6,536,526
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Date Filed
Monday, April 2, 200123 years ago
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Date Issued
Tuesday, March 25, 200321 years ago
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Inventors
-
Original Assignees
-
Examiners
- Bagnell; David
- Gay; Jennifer H
Agents
- Bracewell & Patterson, L.L.P.
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CPC
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US Classifications
Field of Search
US
- 166 248
- 166 57
- 166 60
- 166 651
- 166 302
- 175 17
- 175 16
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International Classifications
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Abstract
A method for retarding temperature loss of fluid being produced in a well employs a fluid of low thermal conductivity in the tubing annulus. The tubing annulus extends between the production casing and the production tubing. It extends from a packer at the lower end of the tubing annulus to a wellhead. The fluid in one case is low density gas created by a partial vacuum. A vacuum is drawn on the tubing annulus to reduce the air density, which in turn reduces the amount of heat that convection currents can carry. In another example, the tubing annulus fluid is viscous hydrocarbon liquid. The hydrocarbon liquid also has a low thermal conductivity. Heat is supplied to the fluids being produced through the tubing annulus by a heater cable that extends into the well.
Description
FIELD OF THE INVENTION
This invention relates in general to a method for decreasing heat transfer from production of a well to the geological formation into which the well bore extends.
BACKGROUND OF THE INVENTION
An oil or gas well normally has one or more strings of casing extending into a well that are cemented in place. The production casing is perforated in an earth formation bearing hydrocarbons. A string of production tubing extends into the production casing. Often, a packer will seal the lower end of the tubing to the production casing at a point above the perforations. Oil and/or gas is produced through the production tubing to the surface.
In arctic regions, a cold permafrost formation layer often extends to depths of 2,000 feet below the surface. Liquids and gases passing through this cold layer may be cooled to the point that viscosity increases and hydrates and condensates begin to form. Water freezing can result, restricting well production.
In temperate zone gas wells, gas expansion through downhole chokes can result in lowering gas temperatures to the level that some of the same problems encountered in arctic wells began to appear. In low pressure, wet gas wells, condensation can form suspended slugs of condensate within the production tubing or casing annulus. This condensate significantly reduces the well's production.
It is known that heating the liquid or gas flowing through the production tubing can retard the undesirable effects mentioned above. One heating device uses resistance type electrical cable suspended within the production tubing or strapped to the outside diameter of the production tubing. While such will retard the cooling of the liquid, much of the heat will be lost through the tubing annulus to the geological formation. This lost heat is not available to increase the temperature of the produced liquid or gas and significantly increases heating costs. It is also known to thermally insulate at least portions of the production tubing in various manners to retard heat loss, however improvements are desired.
SUMMARY OF THE INVENTION
In this invention, temperature loss of fluid being produced in a well is reduced by providing a fluid of low thermal conductivity in the tubing annulus. The tubing annulus extends radially between the casing and the production tubing and axially from a packer just above the perforations to the wellhead. In one method, the low thermal conductivity fluid is provided by drawing at least a partial vacuum on the tubing annulus. This reduces the amount of air left in the tubing annulus, thereby lowering the thermal conductivity. Preferably about 27″ to 29″ of vacuum is drawn on the tubing annulus.
In another aspect of the invention, providing low thermal conductivity fluid in the tubing annulus is accomplished by substantially filling the tubing annulus with a hydrocarbon liquid. The hydrocarbon liquid should be viscous, preferably at least 1,000 centipoise at 100° F. Also, preferably the tubing is centered in the well with a plurality of centralizers that extend between the casing and the tubing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a schematic sectional view of a well constructed in accordance with this invention.
FIG. 2
is an enlarged partial view of the lower end of heater cable employed in FIG.
1
.
FIG. 3
is a sectional view of the well of
FIG. 1
, shown with a liquid hydrocarbon contained in the tubing annulus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to
FIG. 1
, the well has a first set of casing or conductor pipe
11
that extends into the well to a first depth. The well is then drilled deeper and production casing
15
will be installed. Production casing
15
is cemented in place and is suspended in the wellhead
13
by a casing hanger
17
. Casing hanger
17
also seals the annulus surrounding production casing
15
. In deeper wells, there will be at least two strings of casing, with the final string of casing being considered the production casing. The production casing
15
is perforated to form perforations
19
through casing
15
into the earth formation for producing well fluids.
Wellhead
13
includes a tubular head or member
21
, which provides support for a string of production tubing
23
. Tubing
23
is normally made up of sections of conduit secured together and extending into the well, although continuous coiled tubing may also be used. Tubing
23
is supported by a tubing hanger
25
in tubing head
21
. Tubing hanger
25
also seals tubing
23
to tubing head
21
. Wellhead
11
has an outlet
26
for the flow of well fluid from production tubing
23
. In some wells, tubing hanger
25
may be supported by casing hanger
17
, rather than by tubing head
21
.
A packer
27
seals between tubing
23
and casing
15
near the lower end of tubing
23
. Packer
27
will be spaced above perforations
15
. A tubing annulus
28
extends radially from tubing
23
to casing
15
and axially from packer
27
to tubing hanger
25
. Tubing
23
is preferably centered within casing
15
on the longitudinal axis of casing
15
. The centering is accomplished by a plurality of centralizers
29
spaced along the length of tubing
23
. Each centralizer
29
may be an elastomeric annular member that has holes or channels
31
extending through it so as to allow fluid communication above and below each centralizer
29
. Alternately each centralizer
29
maybe a steel bow spring type of conventional design.
A heater cable
33
is used to heat well fluid flowing up production tubing
23
. In this embodiment, heater cable
33
extends alongside tubing
23
and is strapped to it at regular intervals. Alternately, heater cable
33
could be contained in coiled tubing and lowered into production tubing
23
. Heater cable
33
has at least one wire for generating heat when voltage is applied. Preferably, heater cable
33
is constructed as shown in U.S. Pat. No. 5,782,301, Neuroth et al., all of which materials hereby is incorporated by reference. As explained in that patent, heater cable
33
preferably has three conductors
35
of low resistivity. Conductors
35
are coated with insulation layers
37
, which are surrounded by extruded metal sheaths
39
, preferably of lead. A metal armor
41
wraps around the assembly of the three insulated and sheathed conductors. Conductors
35
are connected together at the lower end. A voltage controller
43
located at the surface supplies three phase AC power to heater cable
33
, causing it to generate heat.
Wellhead
13
has a tubing annulus port
45
with a valve
47
for selectively opening and closing communication with tubing annulus
28
. In the embodiment of
FIG. 1
, a vacuum pump
49
is connected by a conduit to tubing annulus port
45
. Vacuum pump
45
is preferably an electrically driven conventional vacuum pump. Tubing annulus
28
will be free of any liquids. Vacuum pump
49
will evacuate the air and/or other gasses within tubing annulus
28
to a desired vacuum level. In one example, the vacuum level is about 27″ to 29″. For a 6,000 ft. well, a vacuum pump driven by a 1 hp electrical motor is able to accomplish a vacuum of this level in about 30 minutes of running time. It is desirable for the vacuum pump
49
to have a sensor that measures the vacuum and periodically turns on vacuum pump
49
should the vacuum decline below a minimum level.
In the operation of the first embodiment, heater cable
33
will be strapped to tubing
23
and lowered into the well while tubing
23
is lowered into the well. Packer
27
will be set, defining the lower end of tubing annulus
28
. Vacuum pump
49
will operate to lower the pressure of the air and/or other gasses within tubing annulus
28
to that less than the atmospheric pressure at wellhead
13
. Three phase power is supplied to heater cable
33
to generate heat. Heat is generated continuously throughout the entire length of heater cable
33
.
The low pressure gas in tubing annulus
28
has less density than if at atmospheric or higher pressure. This reduces the amount of heat that convection currents can carry, reducing convection heat transfer. Low pressure gasses may not be opaque to thermal radiation depending upon the gas and the gas temperature. However, typical electrical heater cable applications in wells operate at temperatures low enough that thermal radiation is a minor factor in heat transfer to the formation. The partial vacuum in tubing annulus
28
retards cooling of well fluid flowing out perforations
19
and up tubing
23
.
In the embodiment of
FIG. 3
, the same numerals are employed for common components. Rather than evacuating tubing annulus
28
, however, a hydrocarbon liquid
51
is placed in tubing annulus
28
. Preferably, liquid
51
substantially fills tubing annulus
28
. It may be filled by opening a sliding sleeve (not shown) in tubing
23
above packer
27
, then circulating hydrocarbon liquid
51
down tubing annulus
28
, with displaced fluid flowing up tubing
23
. The sleeve may then be closed by a wireline tool in a conventional manner. The viscosity of hydrocarbon liquid
51
should be fairly high, although it must not be so high so as to prevent it from being pumped. Preferably the viscosity is at least 1,000 centipoise at 100° F. Hydrocarbon liquid
51
may be a crude oil or a refined petroleum product. Hydrocarbon liquid greatly reduces convection currents and has poor thermal conductivity. Such liquids are also opaque to thermal radiation, blocking heat transfer by that means.
The invention has significant advantages. The low thermal conductivity of the annulus fluid is readily provided, in one case, by low density gasses created by a partial vacuum, and in another case, by a hydrocarbon liquid. This thermal insulation of the tubing annulus reduces the cooling of well fluid being produced through the tubing, avoiding problems that exist in permafrost regions. It also reduces the cooling of flowing wet gas, retarding the creation of slugs of condensate within the production tubing.
While the invention has been shown in only two of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention.
Claims
- 1. A method of retarding temperature loss of fluid being produced in a well having a conduit, a set of perforations in the well into an earth formation, and a string of production tubing extending through the conduit and sealed by a packer to the conduit above the perforations, the method comprising:(a) placing a cable having at least one electrical conductor into the well; (b) providing a fluid of low thermal conductivity throughout a tubing annulus that extends axially from the packer to a wellhead and extends radially from the tubing to the casing; (c) applying electrical power to the cable to cause heat to be generated along at least a substantial portion of the length of the cable for heating the tubing; and (d) flowing well fluid through the perforations and up the production tubing.
- 2. The method according to claim 1, wherein step (b) comprises:removing substantially all liquids from the tubing annulus; and reducing a pressure of gas contained in the tubing annulus to below atmospheric pressure that exists at the wellhead.
- 3. The method according to claim 1, wherein step (b) comprises:placing a hydocarbon liquid in the tubing annulus.
- 4. The method according to claim 1, wherein step (b) comprises:filling the tubing annulus with a hydrocarbon liquid having a viscosity of at least 1000 centipoise at 100 degrees F.
- 5. The method according to claim 1, further comprising:centering the tubing in the well with a plurality of centrilizers extending between the conduit and the tubing.
- 6. A method of producing fluid from a well having a conduit and a set of perforations in the well into an earth formation, the method comprising:(a) lowering a string of production tubing into the conduit and sealing the tubing to the conduit with a packer above the perforations, defining a tubing annulus that extends radially from the tubing to the conduit and axially from the packer to a wellhead; (b) lowering a cable having a plurality of conductors into the well; (c) flowing well fluid through the perforations and up through the tubing; (d) applying electrical power to the conductors to cause heat to be emitted continuously along at least a substantial length of the cable for retarding cooling of the well fluid as the well fluid flows up the tubing; and (e) reducing pressure of gas existing throughout the tubing annulus to less than atmospheric pressure that exists at the wellhead to retard loss of heat through the conduit.
- 7. The method according to claim 6, where step (e) is performed with a vacuum pump placed in communication with the tubing annulus.
- 8. The method according to claim 6, wherein step (a) further comprises centering the tubing in the well with a plurality of centrilizers extending between the conduit and the tubing.
- 9. The method according to claim 6, wherein step (b) is performed by strapping the power cable to the tubing while lowering the tubing into the well.
- 10. A method of producing fluid in a well having a conduit and a set of perforations through the in the well into an earth formation, the method comprising:(a) lowering a string of production tubing into the conduit and sealing the tubing to the conduit with a packer above the perforations, defining a tubing annulus that extends radially from the tubing to the conduit and axially from the packer to a wellhead; (b) lowering a cable having a plurality of conductors into the well; (c) flowing well fluid through the perforations and up through the production tubing; (d) applying electrical power to the conductors to generate heat continuously along at least a substantial portion of the length of the cable for retarding heat loss of the well fluid as the well fluid flows up the tubing; and (e) substantially filling the tubing annulus with a hydocarbon liquid to retard loss of heat through the conduit.
- 11. The method according to claim 10, wherein step (e) comprises providing the hydrocarbon liquid with a viscosity of at least 1000 centipoise at 100 degrees F.
- 12. The method according to claim 10, wherein step (a) further comprises centering the tubing in the well with a plurality of centrilizers extending between the conduit and the tubing.
- 13. The method according to claim 10, wherein step (b) comprises strapping the cable to the tubing and lowering the cable into the conduit while lowering the tubing into the conduit.
US Referenced Citations (19)