Note: Descriptions are shown in the official language in which they were submitted.
CA 02246504 1998-08-12
WO 97133070 PCT/EP97/01145
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DOWNHOLE FLOW STIMULATION IN A NATURAL GAS WELL
'" The invention relates to a method and apparatus for
downhole flow stimulation in a natural gas well.
When natural gas is produced from an ageing gas field
in which the formation pressure is declining the velocity
of the produced gas will decline as well. This often
leads to a situation where the velocity of the gas
becomes insufficient to move liquids existing in or
condensing from the produced gas upwardly through the
production tubing towards the surface.
At that stage the problem of liquid loading occurs.
At the onset of liquid loading the gas production rate
typically begins to drop at a fairly rapid rate which is
a manifestation of the weight of the condensed water and
other condensates that accumulate in the production
tubing. After the liquid loading has continued for some
time the gradually increasing weight of the liquid column
in the production tubing may eventually balance the
formation pressure whereupon fluid production is halted
and the well dies.
US patent specification No. 3,887,008 discloses that
the upward fluid velocity in a conventionally sized
production tubing should be maintained at about 1.5-3 m/s
in order to propel liquid droplets through the tubing
against the effect of gravity. This prior art reference
further discloses that the fluid velocity in a production
tubing may be increased by recirculating dried gas into
the well and using the recirculated gas to drive a
downhole jet pump. A disadvantage of this known technique
is that a jet pump has a low efficiency so that a
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rel-ativc~ly large proportion of the produced gas has to be
recirculated.
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CA 02246504 1998-08-12 " .
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US patent specification No. 5,105,889 discloses
another jet pump design whereby condensed liquids may be
lifted from a gas well. US patent specification
No. 5,211,242 discloses that a downhole liquid collection
chamber may be used when liquid loading occurs in a gas
well and that the liquid may be intermittently lifted out
of the well by intermittently injecting high pressure gas
into the chamber via a gas injection tubing which is
parallel to the production tubing. This prior art
reference further discloses that the production tubing
may be heated to minimize condensation and liquid
fallout.
Disadvantages of the techniques disclosed in this
prior art reference are that heating of the production
tubing is costly and that the intermittent injection of
high pressure gas to remove liquid from the collection
chamber becomes costly if it has to be done on a frequent
basis if the gas has a high liquid content.
The apparatus according to the preamble of claim 1
and the method according to the preamble of claim 5 are
known from EP-A-48050I~. The downhole electrically driven
rotary compressor disclosed in this prior art reference
comprises a helical screw blade and is believed
unsuitable for pumping of large volumes of gas at such
high velocity through the production tubing that the
problem of liquid loading would be effectively reduced.
It is an object of the present invention to provide a
method and apparatus for downhole flow stimulation in a
gas well which enable a continuous reduction of liquid
loading without the requirement of intermittent or
continuous re-injection of produced gas and which can be
used efficiently even if the produced gas has a high
liquid content and the formation pressure is low.
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The apparatus according to the invention comprises a
multi-stage rotary compressor having a shaft equipped
with gas bearings, which compressor is provided with a
separate gas compression unit for supplying a fraction of
the produced gas to the gas bearings for the creation of
a gas film between a stator and a rotor part of each
bearing when the compressor is in use.
It is preferred that the gas compressor is driven by
a brushless electric motor with a rotor comprising
rotating permanent magnets that produce a first magnetic
field and a stator comprising an armature winding which
is connected to a source of electrical current to produce
a second magnetic field, said first and second magnetic
field being capable of interacting to create an
electromagnetic torque that induces the rotor to rotate
relative to the stator.
Motors of this type are known for use in various
applications are disclosed, for example, in US patent
specifications Nos. 4,125,792; 4,276,490; 4,443,906 and
5,428,522 and in European patent specification
No. 533,359. ~ -
The known motors, however, are not designed for
downhole use in hydrocarbon fluid production operations
and a surprising benefit of these motors for use downhole
is that they can be designed as a small diameter drive
unit which operates at much higher rotational speeds than
other electric motors so that the motor shaft can be
connected directly to the compressor shaft and the
presence of a gear box between these shafts may be
eliminated.
The method according to the invention comprises
boosting the gas velocity downhole in a production tubing
within a gas well by means of a multistage rotary
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compressor having a shaft which is equipped with gas
bearings wherein a fraction of the produced gas is
compressed by a separate gas compression unit and
supplied to the gas bearings in order to create a gas
film between stator and rotor parts of these gas
- bearings, which gas compressor is driven by an electric
motor which is controlled by power control means which
limit power exerted by the motor shaft to the compressor
such that the discharge temperature of the gas compressed
by the compressor is maintained below 250 °C.
The compressor will usually be installed at such a
depth within the well that condensation is insignificant
at the relevant depth.
It is preferred that the production tubing is
provided along part of its length with a thermal
insulation and that this thermal insulation is provided
by filling an annular space surrounding the production
tubing along at least part of its length with a gaseous
fluid and by at least partly evacuating said space.
Since the reservoir temperature of subsurface gas
bearing formations is'ab~out 100 °C and the temperature of
formation layers surrounding the gas well will gradually
decline towards the atmospheric temperature at the
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surface, the produced gas will gradually cool off. By
insulating the production tubing the reduction of the
temperature of. the produced fluid is reduced thereby
* reducing the velocity decrease resulting from thermal
compaction and also delaying the onset of liquid loading.
r These and other features, objects and advantages of
the method and apparatus according to the invention will
become apparent from the accompanying claims, abstract
and drawing which shows a schematical, partially
longitudinal sectional view of the apparatus according to
the invention.
Referring now to the drawing, there is shown a
pumping apparatus according to the invention which is
suspended at the Lower end of a production tubing 1
within a gas production well. A casing or production
liner 2 is arranged at the inner circumference-of the
wellbore to prevent caving in of the surrounding
formation 3. The casing or production liner 2 contains
perforations 4 to permit inflow of fluids from the gas
bearing formation 3 into the wellbore.
The apparatus according to the invention comprises a
cylindrical housing 5, an electrical motor 6 having a
stator part 6A and a rotor part 6B which is mounted on a
motor shaft 7, a gear box 8 for transmitting power from
the motor shaft 7 to a compressor shaft 9, and a multi-
stage rotary compressor 10 which is mounted onthe
compressor shaft 9.
Electrical power is supplied to the motor 6 via an
umbilical 11. The motor compartment and-gear box
compartment of the housing 5 are filled with oil and the
umbilical 11 may comprise oil supply conduits for the
supply of lubricant during operation of the apparatus.
Due to the limited width of the wellbore the multi-
stage r-otary compressor 10 has an elongate shape and
contains a large amount of stages. Consequently, the
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compressor shaft 9 is also so long that it has to be
supported by a series of journal bearings 12 which are
mounted between at least some of the compressor stages.
These journal bearings 12 are supported by support
disks 13 which are perforated (not shown) to permit the
produced gas to flow from an inlet section 15 of the
compressor via the first and subsequent stages of the
compressor 10 towards the outlet section 16.
The housing 5 contains a series of openings 1'7 at the
inlet section 15 of the compressor 10 to permit inflow of
gas from the wellbore into the housing 5.
In view of the significant amount of journal bearings
12 and the elevated temperature and pressure of the
produced gas it would be impractical to lubricate the
journal bearings 12 by a liquid lubricant, in particular
because the large amount of seals required to provide a
fluid barrier at each side of the journal bearings 12
would generate significant friction which would even
further increase the lubricant temperature and which
would also require a significant extra power requirement
for the motor.
It is therefore beneficial to lubricate the journal
bearings 12 with the produced gas. The gas is supplied to
the journal bearings via a small compressor unit 18 which
is mounted in a recess within the gear box section 8 and
a branched high pressure gas supply line 20 which is
shown in dotted lines.
The gas compressor unit 18 may be driven by the
electric motor 6. Alternatively the unit 18 may be driven
by a separate electric motor which supplies the gas to
the journal bearings 12 at a constant pressure even
during start-up or run-down of the mufti-stage rotary
compressor.
The journal bearings 12 may be gas bearings of the
pivoted pad or herringbone groove type. Bearings of this
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type are known per se, and are described, for example, in
the Mechanical Engineers Handbook, Edition 1986,
page 488-567, published by John Wiley & Sons, and are
therefore not described in detail herein.
In the configuration shown in Fig. 1 the thrust
t bearing 21 of the compressor shaft 9 is mounted at the
bottom of the gear box section 8 and is therefore a
conventional oil lubricated trust bearing. However, if
desired, the thrust bearing could also be mounted within
the gas inlet 15 of the compressor 10, in which case the
thrust bearing could be a gas bearing as well.
The bearings 22 of the electric motor 6 are
conventional oil lubricated bearings.
It is preferred to use in the apparatus according to
the invention gas bearings for the journal bearings 12 of
the pump shaft 9 since, as described above, this obviates
the need for a large amount of shaft seals, and moreover
since gas bearings are able to run at much higher
temperatures than oil lubricated bearings.
This is a significant advantage for a downhole gas
compressor since it would be difficult and expensive to
cool the apparatus. However, since it is still required
to use liquid lubricants in the gear box 8 and motor 6 it
is preferred to equip the electric motor 6 with power
control means which limit the power exerted to the pump
shaft 9 such that the discharge temperature of the
compressed gas at the outlet 16 is maintained below
250 C.
The temperature increase of the compressed gas is
beneficial for the reduction of liquid loading in the
;,production tubing 1. However, the production tubing may
be several kilometres long such that a significant
cooling of the gas does occur.
In order to reduce the cooling-off of the produced
gas within the production tubing 1 it is preferred to
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insulate the tubing 1 by creating a low gas pressure in
the annular space 24 that extends between the production
tubing 1 and the well casing 2 from a packer 25 towards
the wellhead (not shown).
Preferably the annular space 24 is first filled with
an inert gas, such as nitrogen, and then evacuated. In '
this way the annular space 24 acts as an efficient
thermal insulator which reduces the cooling of the
produced gas and the condensation of aqueous liquids that
may be present in the gas to a significant extent.
It will be understood that instead of or in addition
to the presence of a vacuum insulation in the annular
space 24 the production tubing 1 may also be insulated by
other insulation means, such as a conventional foam
insulation sleeve.
It is preferred that the electric motor 6 is a
brushless motor having a rotor part 5B comprising
rotatable permanent magnets that produce a first magnetic
field and having the stator part 6A comprising an
armature winding (not shown) which is connected to a
source of electrical current to produce a second magnetic
field, which first and second magnetic fields are capable
of interacting to create an electromagnetic field that
rotates in use the rotor part 6B relative to the stator
25-- part of the motor &.
An electric motor 6 of the above described type is
able to deliver optimum torque at rotor speeds well over
5000 revolutions per minute, which will make the presence
of the gear box 8 obsolete. _
The absence of a gear box 8 and the use of gas
bearings as journal bearings 12 is attractive since it
creates a compact motor and compressor assembly with a
minimum of oil filled compartments which would require
regular replacement of oil and maintenance and inspection y,
of wear prone components such as seals and gaskets.
