Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FRICTION PRESSURE REDUCING ACrENTS FOR GASES
'I'he basis of this invention is a method of reducing gas friction pressures
in high-
rate gas pumping operations through the addition of graphite or similar solid
particulate
with lubricating qualities to the gas stream. Also included in this invention
is a method by
which the particulate is introduced to the gas stream.
A secondary application of this invention is the reduction of gas friction
pressures within
a fracture system of a gas producing formation.
Background to the Invention
Wells are drilled and completed for the production of oil and natural gas.
Subsequent to the completion, well stimulation is often required to allow the
wells to
produce at commercial rates. These stimulations may be required initially
before
commercial production is achieved, or may occur at a later stage in the life
of the well, as
a means of restoring well productivity.
A common example of an initial stu-nulation would be the fracturing of a
coalbed
methane formation with nitrogen. In these stimulations, high rate nitrogen is
injected
through coiled tubing into the coalbed fozxnation to create and extend a
fracture systezn.
that allows natural gas that is entrained in the coals to flow to the
wellbore. Common
operations such as this use coiled tubing of outer diameters ranging from 2-
3/8 inch (60.3
millimetres) to 3-1/2 inch (88.9 millimetres). Rates presently used to create
and extend
the fracture system are in the range of 26,500 standard cubic feet per minute
(750
standard cubic metres per minute) to as high as 70,500 standard cubic feet per
mi.n.ute
(2000 standard cubic metres per minute) or higher. At gas rates such as these,
through
relatively small tubulars, friction pressures arc high and can place a
significant burden on
both the pumping equipment and coiled tubing. Fatigue of coiled tubing
increases, and
consequently the life of the coiled tubi.ng decreases, as the pressure at
which the coiled
tubing is incxeased. Minimizing puniping pressures therefore would enhance the
life of a
string of coiled tubing. Additionally, the pressure that a pumping unit is
required to
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operate at determines its power requirements and ene.rgy consumption, and
depending on
the design of the ptunping equipment, higher pressures may lead to increased
wear and
maintenance of the pumping un.it.
Other operations also exist which require the high-rate injection of nitrogen
or
othcr gases_ An example of a stimu.lation occurring later in the life of a
well would be the
injection of nitrogen or a similar gas to remove sand or fill from the
wellbore which is
plugging the wellbore and hindering production. These operations rnay be
conductcd with
tubulars of a wide range in size, often from less than 1.25 inches (31.8
millimetres) to
greater than 2 inches (50.8 millimetres) but due to the amount of fill or
debris to be
removed from the wellbore will require high rates of gas and result in
significant friction
pressures.
For the reasons descrxbed above, it is beneficial to minimize friction
pressure
losses and minunize the pumping pressure in these operations.
Various methods are available to reduce friction pressure losses in pumping
operation.s through tubulars. Where liquids are being pumped, the addition of
a small
volume of friction-reduczng liquid such as a soap or surfactant substance can
signifiaantly
reduce the friction pressure. Where gas is the fluid being pumped, small
volumes of these
liquids can also assist in reducing friction pressure losses. However, in many
operations
where gas is being pumped, the formations being contacted with the gas cannot
tolerate
contact with liquid. This may be due to swelling of clays, plugging of
formation pore
throats, or a variety of other reasons. In such cases, particularly those of a
relatively high
gas rate for the tubular being used, another method of reducing friction
pressure losses
must be found.
Description of the invention
This invention has specific reference to the addition of a solid particulate
to the
gas stream for the purpose of reducing fri.ction pressure losses. Where the
operation is a
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high-rate gas fracturing operation of a formation such as a coalbed methane
formation, it
is desirable for, the solid particulate to be of small particle size so as not
to create any
plugging of the fracture created by the gas injection.
For the purpose of describing the invention, the following embodiment is
described, and illustrated in Figure 1. A subsurface formation (101) has been
penetrated
by a wellbore (102) drilled for the production of natural gas. The wellbore
has been cased
with a steel casing (103) and the casin.g bas been anchored into the earth by
way of
cement (104) between the casing and the earth. The casing is of 4-1/2 inch
(114.3millimetre) outer diameter. The casing has been perforated (105) one or
more
times, at depths commonly in the range of 600 feet (200 metres) from surface
to 1800
feet (600 metres) from surface, to communicate the casing with one or more
subsurface
forniations. The subsurface formations are coalbed methane formations
containing low
pressure natural gas. Coiled tubing (106) is introduced to the wellbore for
the purpose of
providing a means of communicating the perforations with a source of high-rate
and high
pressure gas at surface. High-rate cryogenic nitrogen is delivered from one or
more
nitrogen pumping units (108) through a system of treating iron and valving
(110) to a
rotating joint (103) on the coiled tubing reel. The rotating joirtt allows the
gas to be
pumped into the coil while the coiled tubing is stationary, or while it is
moviztg.
The gas introduced to the coiled tubing is isolated to a specific set of
perforations through
a coiled tubing fracturing tool (107), which uses one or more sets of opposing
cups that
seal against the casing under applied gas pressure to contain gas rate and
pressure
between the cups and force the gas into the set of perforations. The coiled
tubing (106) is
2-7/8 inch outer diameter. Nitrogen gas is pumped at rates of 1200 standard
cubic metres
per minute for the purpose of creating a system of fractures (109) for the
enhanced
production of natural gas_
In this embodiment, pumping pressLtres may attain levels of 5000 pounds per
square inch (35 MegaPascals) or higher, depending on the resident formation
pressure
and the depth of the formation. and the density of wellbore perforations.
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The invention encompasses the addition of a solid particulate as a lubricant,
to the
nitrogen gas stream downstream of the nitrogen pumping unit (108), and
typically
upstream of the coiled tubing rotating joint (103). A,lternatively the
particulate could be
added to the coiled tubing which is fixed inside the coiled tubing reel_ In
this embodiment
of the invention, the solid particulate used to reduce gas friction pressure
losses is
graphite. Other small particulate solids with natural or synthetic lubricating
qualities are
not excluded from this invention.
Figure 2 shows one method of introduction of the particulate to the stream. In
this
case, a length of tubular (201) is connected to the treating iron (202) but
isolated b-om the
treating iron by a first valve or set o#'valves (203). The tubular may be a
joint of tubing or
treating iron, in this case a joi.ttt of treating iron of 2 inch (50.8
millimetre) diameter aztd 4
foot (1.25 metres) length but could be of another length or another diameter.
This valve
or valves, when opened, will expose the tubular to nitrogen gas. A second
valve or set of
valves (204) is located at the end of the tubular to isolate the inside of the
tubular from
the atmosphere. With these valves or sets of valves, both cnds of the tubular
can be
isolated when the valves or sets of valves are closed.
To add the particulate to the gas stream under active nitrogen pumping
operations,
the first valve or set of valves (203) are closed and the second valve or set
of valves (204)
are opened and graphite poured into the tubular. The second valve or set of
valves (204)
are closed and the first valve or set of valves (203) are opened to allow the
graphite to
enter the gas stream. This system can be used to introduce the graphite as a
batch
treatment which can be replenished by reloading the tubular by the method
described
above, or by using a control valve or set of valves as the first valve or set
of valves (203)
the graphite can be introduced to the nitrogen gas as a slow and steady source
until the
tubular is evacuated of graphite.
Another method by which the graphite can be entered into the gas stream is
shown in Figure 3. In this case the upper valve or set of valves as shown in
Figiire 2 is
replaced by a flange cap (301) rated for the treating pressures to be seen in
the operation.
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Introduction is as described above by elosing the first valve or set of valves
(302) to
isolate the tubing or chamber (303) from the gas stream, opening the flange
cap to fi11 the
tubing or chamber with graphite, closing the flange cap and opening the first
valve or set
of valves (302) to allow the graphite to enter the gas stream._
In a third embodiment as shown in Figure 4, an injection device (401) is
attached
to the treating iron (402) and used for the introduction of graphite_ The
injection device
may be a device spccially designed and manufactured for the intxoduction of
graphite, or
may simply be a ball injector. A ball injecto.r is a common device in coiled
tubing
operations and is used for the addition of ball devices to the coiled tubing
under pressure
for operating downhole tools, sealing flow ports, or other such uses. The
injection device
may contain a plurality of ball or injection chambers which can be
electrically or
hydraulically rotated or otherwise activated such that a numbeT of discreet
additions can
be achieved before needing to reload or replenish the device.
Through the methods described above of introducing the graphite into the gas
stream, the graphite provides a coating of lubricant on the inside of the
coiled tubing to
reduce the effective rougbness of the coiled tubing as well as to reduce
interfacial friction
between the gas and the coiled tubing. In the operation described in the
embodiment
above, the coiled tubing is already in the wellborc and the particulate is
added to the
coiled tubing and any excess particulate is exhausted to the wellbore and
potentially the
formation with the gas. In some operations it may be seen as beneficial to
optimize the
coating process by pumping a tubing plug or tubing pig aftcr addition of the
graphite to
provide a more uniform and longer lasting coating, effectively enhancing both
the
lubrication qualities and the longevity of the coating. In this case the
particulate would
be added to a gas stream and pumped through the coiled tubirtg with the coiled
tubing
removcd from the wellbore such that the tubing pig or tubing plug can be
recovered
without it cntering the wellboze_ This method may also be used when it is
undesirable for
excess particulate to enter the formation.
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In some situations it may bc seen as desirable for additional particulate to
be
placed in the formation for the purpose of reducing friction for gas flow
within the
formation itself. The productivity of low pressure gas fozmations can be
significantly
affected by the resistance to flow due to gas friction pressures_ Just as the
graphite or
particulate provides a lubricating coating inside the tubulars, this can also
result in a
lubricating coating on the face of formation fractures. This would be more
prevalent in
coalbed formations in which fractures are created with solid faces, or cleats,
which would
be made smoother with a reduced roughness as a result of the graphite.
The foregoing describes one common embodiment of the invention, and some
variations have also been described throughout this description. Several
modified
embodirueats are obvious, including the use of jointed tubulars rather than
coiled tubing,
the application of this treatment to fracturing operations on sandstone or
carbonate
formations rather than coalbed methane, the application of this treatinent in.
any gas
stimulation operations such as cleanouts or blowdowns, the use of alternate
gases rather
than nitrogen, the use of aIternate lubricating particulates other than
graphite, and
alternate means of introducing the lubricating particulates into the gas
stream. The
description also references certain gas flow rates, tubular diameters and
operating depths
strictly for the intent of providing evidence of application of this invention
to a realistic
operation. This description, therefore, should not be considered to be
exclusive of higher
or lower gas rates, larger or smaller tubulars, or shallower or deeper depths,
or operations
other than fracturing. The invention is inteztded to be applicable to any
situation where it
is desirable to reduce gas friction pressure losses in tubulars. This
invention is intended to
describe the practice and method of adding a lubricating particulate to a gas
stream for
the purpose of friction pressure reduction.
