U.S. patent number 3,766,980 [Application Number 05/278,552] was granted by the patent office on 1973-10-23 for permafrost and well protection.
This patent grant is currently assigned to Atlantic Richfield & Company. Invention is credited to Loyd R. Kern.
United States Patent |
3,766,980 |
Kern |
October 23, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
PERMAFROST AND WELL PROTECTION
Abstract
A method for protecting a permafrost zone and a well passing
through that permafrost zone wherein a pipe string is placed in
said well so as to extend through at least part of said permafrost
zone and is heated over its length in said permafrost zone using
electrical energy thereby avoiding alternate freezing and thawing
of the permafrost and sloughing of the permafrost due to thermal
cycling and preventing freeze up of the well itself.
Inventors: |
Kern; Loyd R. (Irving, TX) |
Assignee: |
Atlantic Richfield &
Company (New York, NY)
|
Family
ID: |
23065443 |
Appl.
No.: |
05/278,552 |
Filed: |
August 7, 1972 |
Current U.S.
Class: |
166/248; 166/57;
166/60; 166/65.1; 166/305.1; 166/901; 405/131 |
Current CPC
Class: |
E21B
43/2401 (20130101); E21B 36/00 (20130101); Y10S
166/901 (20130101) |
Current International
Class: |
E21B
36/00 (20060101); E21B 43/24 (20060101); E21B
43/16 (20060101); E21b 043/00 () |
Field of
Search: |
;166/57,60,65,248,302,DIG.1 ;219/277,278 ;65/36A |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Muller, "Permafrost Frozen Ground and Related Eng. Problems," 1947,
pp. 140 and 141. .
Alaska Constr. Oil, "Slip Joint Casing," Petroleum Abstracts, July
11, 1970, No. 130,921 Oil and Gas Journal, June 21, 1971, pp.
115-119. .
McGhee, "Mackenzie Delta Drilling-Making Hole is Least of It," Oil
& Gas Journal, July 3, 1972, pp. 33-39..
|
Primary Examiner: Champion; Marvin A.
Assistant Examiner: Ebel; Jack E.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A method of protecting a permafrost zone around a well and the
well which passes through said permafrost zone comprising emplacing
a pipe string in said well which extends through at least part of
said permafrost zone, and heating said pipe string uniformly over
its length in said permafrost zone using electrical energy.
2. A method according to claim 1 wherein said pipe string is heated
by applying an electrical current directly to an upper portion of
said pipe string, and returning said current from beneath said
permafrost zone.
3. A method according to claim 2 wherein said current is
alternating current.
4. A method according to claim 2 wherein said current is direct
current.
5. A method according to claim 4 wherein said pipe string is
protected from electrolytic corrosion by connecting to said pipe
string a sacrifical anode.
6. A method according to claim 1 wherein said pipe string is heated
by placing at least one electrical conductor in said well along at
least part of the length of said permafrost zone and near said pipe
string, and passing electrical current through said conductor to
heat same and in turn heat said pipe string.
7. A method according to claim 1 wherein said pipe string is heated
by employing at least one electric conductor physically attached to
at least one wall of said pipe string, and passing electrical
current through said conductor to heat same and in turn heat said
pipe string.
Description
BACKGROUND OF THE INVENTION
Heretofore, wells such as oil and gas wells have been successfully
drilled through a permafrost zone without causing substantial
damage to the permafrost itself. During the drilling of a well,
relatively warm fluid is circulated through the well in the
permafrost zone. After drilling a well, relatively warm fluid such
as a mixture of oil and gas is produced through the well to the
earth's surface. Warm fluid, used during drilling or production of
the well, when passing through the permafrost zone establishes a
temperature equilibrium between the well and the permafrost itself.
If after drilling, the well is shut in or if after some production
has taken place the well is shut in or its production rate
substantially reduced, this equilibrium between the well and
surrounding permafrost is altered with the net result that part or
all of the affected permafrost refreezes. If any water is entrained
in the well in the permafrost zone, the water in the well can also
freeze. Thereafter, when production does take place, or resumes, or
increases, thawing can again take place and if this thermal cycling
is carried on often enough and for a long enough period of time
sloughing of permafrost as well as damage to the well itself could
occur.
Thus, once a temperature equilibrium is established between the
permafrost and the well, e.g., during the drilling of the well, it
is preferable to maintain this equilibrium essentially undisturbed
for the life of the well even though the production rate of the
well may vary considerably and for a substantial period of time
even be zero. In this way, a particular well could be shut in for
workover or any other desired purpose without fear of harm to the
well itself or the permafrost surrounding the well by substantially
disturbing the equilibrium previously established between the
producing well and the permafrost.
Also, if a well should become frozen up because it passes through a
permafrost zone and is shut in or produced at too slow a rate or
with too cool a fluid, it must be thawed out before its full
productive capacity can be realized.
Further, natural gas or other hydrocarbon containing gas being
produced through a wellbore or shut into a wellbore which passes
through a permafrost zone can be sufficiently cooled to cause the
formation of a hydrate. The hydrate is a solid ice-like material
containing both water and hydrocarbon which can form at
temperatures substantially above 32.degree. F.
Hydrate can be formed in the produced gas in such quantity that
plugging of the tubing through which the gas is produced and/or
pipes for carrying the gas over the earth's surface can occur. The
plugging is particularly troublesome wherever the flow path of the
gas has to change directions such as when passing through valves,
T's, and L's in the pipe system.
SUMMARY OF THE INVENTION
According to this invention, a pipe string is established in the
wellbore so as to extend through at least part of the permafrost
zone and this pipe string is then uniformly heated over its length
in the permafrost zone using electrical energy.
The electrical energy can be applied directly to the pipe string or
to an electrical conductor associated with the pipe string, but it
is important in obtaining the protection required by this invention
that the heating of the pipe string be substantially uniform along
that length of the pipe which is in the permafrost zone.
Heretofore, downhole heaters have been used in wells to, for
example, melt paraffin, but these heaters have not and cannot
uniformly heat a length of pipe in a wellbore. On the contrary,
these heaters heat only a small localized portion of the wellbore
such as the area immediately adjacent where produced fluid leaves
an underground geologic formation and flows into the wellbore. By
this invention, a substantial length of pipe is heated in a uniform
manner to prevent localized overheating which can damage the pipe
itself and/or the permafrost. This invention also avoids
underheating along the pipe length in the permafrost zone. This
invention also avoids disruption of the thermal equilibrium set up
between the well and the permafrost anywhere along the heated
length of pipe.
By practicing this invention, any length of pipe in the permafrost
zone up to and including, but not limited to, the full depth of the
permafrost zone, is heated in a controlled manner to prevent
freezing of fluids in the wellbore itself, to prevent thawing of
the permafrost, and to prevent hydrate formation in the gas
produced from the well should a gas well be shut in or otherwise
produced at a rate which tends to allow the formation of hydrate
before the gas reaches the earth's surface.
Further, if a well has already frozen up due to a lack of
production or otherwise, the method of this invention can be
employed to thaw out the well without overheating the well piping
or permafrost and to re-establish a desired temperature equilibrium
between the well and the permafrost.
Accordingly, it is an object of this invention to provide a new and
improved method for protecting permafrost. It is another object to
provide a new and improved method for producing a well which is
completed through a permafrost zone. It is another object to
provide a new and improved method for protecting a well which
passes through a permafrost zone. It is another object to provide a
new and improved method for substantially maintaining a temperature
equilibrium set up between a well and surrounding permafrost. It is
another object to provide a new and improved method for avoiding
substantial thermal cycling of permafrost. It is another object to
provide a new and improved method for protecting a well from freeze
up when completed through a permafrost zone. It is another object
to provide a new and improved method for producing a wall through a
permafrost zone at a rate or temperature sufficiently low that
freezing of at least part of the produced fluid could occur. It is
another object to provide a new and improved method for thawing out
a frozen well. It is another object to provide a new and improved
method for preventing hydrate formation in a well.
DETAILED DESCRIPTION OF INVENTION
FIG. 1 shows a cross-section of a well employing one embodiment of
this invention.
FIG. 2 shows a cross-section of a well employing another embodiment
of this invention.
FIG. 3 shows a cross-section of a well employing yet another
embodiment of this invention.
More specifically, FIG. 1 shows the earth's surface 1 below which
is a permafrost zone 2 and through both of which is drilled a
wellbore 3. Emplaced in wellbore 3 is a pipe string 4 which extends
from above the earth's surface to below the lower end 5 of the
permafrost.
An electric current source 6 has one pole connected by way of
electrical wire 7 directly to pipe 4. An electrical return conduit
which can be a metal rod, a pipe string in another wellbore, or any
sort of electrical return is situated at 8 so that its lower end 8'
extends below the lower surface 5 of the permafrost to establish an
electrical return through element 8 and electrical wire 9 to source
6, this return being established beneath the zone 2.
Permafrost zone 2, being substantially completely frozen, will not
conduct substantial amounts of electricity so that the electricity
will not leak off from pipe string 4 to any great extent while
passing through zone 2. As soon as electricity reaches the unfrozen
earth below zone 2, i.e., below lower surface 5, it will migrate
toward the electrical return 8. Thus, it is important to this
embodiment of the invention that the electrical return be
established beneath the permafrost. In this way, uniform heating
over substantially the entire surface of the pipe in zone 2 is
achieved. Although it is not required to achieve the results of
this invention, it is preferred that the electrical return be
another well because by using a pipe string in a second well as a
return conduit, that pipe string is also heated and the permafrost
around both wells protected at the same time.
Alternating current or direct current can be employed in this
embodiment of the invention. If direct current or low frequency
alternating current is employed, it may be desirable to utilize a
sacrificial anode or some other equipment designed to protect pipe
4 for electrolytic corrosion. For example, a sacrificial anode
comprising a zine block 10 can be employed in or below the
permafrost and electrically connected to pipe 4 by way of
electrical conduit 11. Any known electrolytic corrosion device can
be employed.
The voltage employed can be any voltage necessary to obtain a
particular desired result. Thus, substantially any current and
voltage can be employed. The current and voltage used in specific
situations will vary extremely widely since both are affected, not
necessarily in the same manner, by the composition of the pipe, the
heat treatment history of the pipe, the size and/or weight of the
pipe, the resistance and/or impedance of the pipe, the spacing
between the well and its electrical return, the particular result
desired be it thawing or permafrost protection or hydrate
prevention, and the like. However, if voltages greater than about
75 volts, preferably greater than about 50 volts, are employed,
special protective measures should be taken for personnel working
on or about the well.
In the embodiment of FIG. 1, for example, at one point of time,
when using alternating current, the current will pass down pipe 4
through substantially the entire cross section of the pipe thereby
uniformly heating the external and internal surfaces of that pipe
through the full depth of zone 2, leave pipe 4 below zone 2, pass
to conductor 8 and upwardly to the earth's surface and ultimately
through wire 9 to source 6. The path of the electrons from pipe 4
to return 8 will be a field of flow represented by dotted lines
12.
Although substantially no electrical current will leak off in the
permafrost zone, should there be some leak off in this zone, it
would not be detrimental to the overall results of this invention.
This is so because in an area where permafrost exists, the
temperature at the surface of the earth in the winter and near the
top of the permafrost in the summer is always much less than the
temperature at or near the bottom 5 of the permafrost zone. If
there is leak off of electrical current in the permafrost zone, it
most likely would occur at various points along the length of that
zone. Even if there is some slight loss of current in the zone, the
maximum amount of current will be flowing through the upper portion
of the zone where more heating is necessary since the upper portion
is colder. Thus, should there be any leak off of current so that
the magnitude of the current as it reaches the vicinity of bottom 5
of the zone is reduced, this is not critical because the
temperature of the permafrost at bottom 5 is close to the freezing
point of water and sufficient heating can be achieved in this
vicinity with the reduced current, the larger current having
already been used in the upper part of the zone where it is needed
most. Similar reasoning applies for thawing a well or preventing
hydrate formation.
FIG. 2 shows the well of FIG. 1 with an electric source 6 except
that the electrical current is not applied directly to pipe 4 but
electrical energy is still used to obtain uniform heating of pipe 4
at least in the permafrost zone. This is achieved by employing a
loop of electrical conductor 15 which extends in pipe 4 for the
depth of zone 2, one end of conductor 15 being connected to wire 7
and the other end being connected to wire 9 to complete the
electrical circuit from the source 6. In this way an electric
current, be it A.C. or D.C. is used to heat conductor 15 which in
turn heats pipe 4. One or more conductors 15 can be employed in a
single well and can be employed inside or outside of a pipe string
or in an annulus between two pipe strings or both. The loop of
conductor 15 can extend for any length in pipe 4, whether the
length is less than, equal to, or greater than the depth of zone
2.
FIG. 3 shows the well of FIG. 1 wherein electrical conduit 16 is
still connected to wires 7 and 9 but conduit 16, instead of being
hung in an open volume somewhere in the well, is physically
attached to the wall of pipe string 4. Conduit 16 as emplaced on
the wall of the pipe can be insulated or uninsulated. If the wire
is uninsulated it will have some of the heating attributes of FIG.
1 as well as some of the attributes of FIG. 2. If conduit 16 is
insulated it will have the heating attribute of FIG. 2 only except
that it will be in much more intimate contact with the pipe for the
conduction of heat. More than one conduit 16 can be employed in
pipe 4.
To insure continuity of the electrical circuit from one pipe
section to another pipe section in pipe 4, conduit 16 passes from
upper pipe section 18 over coupling 21 to lower pipe section 20 so
that when the two pipe sections are made up at the earth's surface
and joined by way of a conventional coupling 21, electrical
continuity throughout the length of pipe string 4 is insured. The
return loop of the conduit 16 at or near the lower surface 5 of the
permafrost zone merely passes laterally around the wall of the pipe
section as shown at 22 for its return up the outside of pipe 4 to
electrical wire 9. Wire 16 can be physically attached to the pipe
such as by clamps, wire strapping, and the like. More than one wire
16 can be used on pipe 4 on the inside or outside of pipe 4 or
both.
A gas, such as natural gas, other hydrocarbon containing gas,
carbon dioxide, and the like, can be produced from a gas producing
formation (not shown) into, for example, pipe 4 and up the wellbore
through zone 2 to the earth's surface for further processing, use,
and the like.
The gas passing from pipe 4 at the earth's surface normally passes
through a piping system for further processing such as dehydration,
sulfur and sulfur compound removal, and the like. Because of this,
the gas passes through a large number of pipes and changes
direction by way of pipe T's and L's a large number of times
shortly after it leaves pipe 4.
The gas containing hydrate forming water and hydrocarbon can
initially be above the temperature and pressure at which
substantial amounts of hydrate form. By "initially" it is meant the
gas as produced from the formation into the lower portion of
wellbore 3.
The substantially hydrate-free gas is produced upwardly through
wellbore 3 and if the gas is not heated while passing through zone
2, or if the well is shut in and gas trapped in zone 2 of the
wellbore, the gas will be cooled sufficiently to cause substantial
hydrate formation.
The hydrate itself is a complex combination of hydrocarbons and
water. The chemical composition of the hydrate is presently
unknown. The hydrate is formed through the mechanism of water vapor
in the gas condensing and freezing in a manner which ties
hydrocarbon molecules in with the frozen water. The hydrate is
solid like ice but has a substantial hydrocarbon content. The
hydrate forms at temperatures above 32.degree. F. and can form at
temperatures up to about 80.degree. F. and higher in some
situations.
Any water present in the gas produced is a potential hydrate former
so that there is substantially no minimum amount of water in a
hydrocarbon containing gas below which the hydrate formation
potential is nonexistent. Normally, the gas produced from a
formation is saturated with water so that there is a very
substantial potential for the formation of large amounts of
hydrate.
The formation of a hydrate and its pipe plugging propensities are
especially significant in the production of gas wells through a
permafrost zone. The problem of hydrate formation is not presently
considered significant in the production of oil wells. This is so
because there is a smaller amount of gas associated with the liquid
oil and the liquid oil flowing through the conduits and pipes
carries the hydrate out rather than allowing the hydrate to build
up in the piping as was discovered to be the case with gas
wells.
In an exemplary situation, the gas, as initially produced, is above
about 80.degree. F. and above about 100 psig. In this situation the
hydrate normally forms in the gas at less than 80.degree. F. and
less than 10,000 psig. In this situation the gas in pipe 4 is
heated to maintain a temperature of that gas greater than
80.degree. F. while in zone 2. Of course, pressure plays some role
in the determination at which temperature hydrate formation will
occur. Generally, however, there is a temperature such as
80.degree. F. for most natural gases, above which substantially no
hydrate will form at any practical pressure, i.e., less than about
10,000 psig.
Electrical energy can be used in accordance with this invention
(including any of the embodiments of FIGS. 1 through 3) at any time
during the drilling and/or production of a well (or wells if one or
more wells are to serve as a return conduit) or can be used only
when the production rate of one or more affected wells is decreased
or stopped altogether, or combinations thereof.
EXAMPLE
Apparatus is employed substantially as that shown in FIG. 1 wherein
the depth of zone 2 is 2,000 feet and pipe 4 is casing which
extends to 2,600 feet so that casing 4 extends about 600 feet below
the bottom level 5 of zone 2. Casing 4 is conventional 133/8 inch
O.D., 0.514 inch wall thickness, API grade N80 oil well casing.
Direct current is applied to pipe 4. The resistance of 2,000 feet
of pipe 4 is about 0.0071 ohms and about 993 amps of direct current
provide 7 kilowatts in permafrost zone 2. The total input voltage
and power for the well is 11 volts and 11 kilowatts,
respectively.
Reasonable variations and modifications are possible within the
scope of this disclosure without departing from the spirit and
scope of this invention.
* * * * *