U.S. patent application number 12/990426 was filed with the patent office on 2011-07-14 for mid water gas lift.
Invention is credited to Jean Luc Legras, Jean Francois Saint-Marcoux.
Application Number | 20110168399 12/990426 |
Document ID | / |
Family ID | 39638278 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110168399 |
Kind Code |
A1 |
Saint-Marcoux; Jean Francois ;
et al. |
July 14, 2011 |
MID WATER GAS LIFT
Abstract
Methods and apparatuses for the production of hydrocarbons from
an offshore reservoir are disclosed. The hydrocarbons are produced
using at least one spur (or flowline), and at least one riser
conduit to transport said hydrocarbons from the seafloor to a point
at or near the sea surface, wherein an artificial lift method, such
as the injection of a lifting gas, is used to help transport said
hydrocarbons from the reservoir to said point at or near the
surface, and wherein said artificial lift method is applied to said
hydrocarbons at a point along the at least one riser conduit,
substantially above said seafloor.
Inventors: |
Saint-Marcoux; Jean Francois;
(London, GB) ; Legras; Jean Luc; (La Ciotat,
FR) |
Family ID: |
39638278 |
Appl. No.: |
12/990426 |
Filed: |
May 4, 2009 |
PCT Filed: |
May 4, 2009 |
PCT NO: |
PCT/IB2009/053467 |
371 Date: |
January 19, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61049808 |
May 2, 2008 |
|
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Current U.S.
Class: |
166/345 ;
166/344 |
Current CPC
Class: |
E21B 17/18 20130101;
E21B 43/01 20130101; E21B 17/01 20130101; E21B 43/122 20130101 |
Class at
Publication: |
166/345 ;
166/344 |
International
Class: |
E21B 17/01 20060101
E21B017/01; E21B 43/01 20060101 E21B043/01 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 6, 2009 |
GB |
0810355.8 |
Claims
1. A method of production of hydrocarbons from an offshore
reservoir using at least one spur or flowline to transport
hydrocarbon wellfluid from a wellhead to the bottom of at least one
riser conduit, and using said at least one riser conduit to
transport said hydrocarbon wellfluid from the seafloor to a point
at or near the sea surface, wherein an artificial lift method is
used to help transport said hydrocarbon from the reservoir to said
point at or near the surface, and wherein said artificial lift
method is applied to said hydrocarbon wellfluid at a point along
the at least one riser conduit, substantially above said
seafloor.
2. A method as claimed in claim 1 wherein the hydrocarbon wellfluid
comprises crude oil.
3. A method as claimed in claim I wherein the hydrocarbon well
fluid comprises a mixture of crude oil, water and natural gas.
4. A method as claimed in any claim 1 wherein said at least one
riser conduit forms part of a riser tower having a top, said riser
tower comprising one or more rigid conduits supported in a tower
structure and extending in use from a foundation connecting
structure on the seabed to a point below the sea surface and
wherein there are provided one or more flexible conduits extending
from said tower structure to connect said tower structure to a
surface structure, and wherein there is further provided a buoyancy
device attached to said top of said tower structure, such that in
use said buoyancy device is located above and exerts an uplift
buoyancy force on said riser tower.
5. A method as claimed in claim 1 wherein said artificial lift
method comprises a gas lift method, where gas is injected into the
hydrocarbon wellfluid to be recovered, at a point on said riser
conduit substantially above the seafloor, so as to reduce its
density.
6. A method as claimed in claim 5 wherein said injection is made at
a point where hydrostatic pressure is below a cricondenbar of the
hydrocarbon wellfluid.
7. A method as claimed in claim 6 wherein said injection is made at
a point where pressure in the riser conduit is below a bubble point
pressure of the hydrocarbon wellfluid.
8. A method as claimed in claim 5 wherein said gas for injection is
transported from the surface to the injection point by a dedicated
pipe.
9. A method as claimed in claim 5 wherein said gas for injection is
transported from the surface to the injection point at least partly
in an annulus of a pipe-in-pipe structure, a central pipe
comprising a main conduit for transporting said hydrocarbon
wellfluid.
10. A riser apparatus for the production of hydrocarbons from an
offshore well comprising at least one riser conduit adapted to
transport hydrocarbon wellfluid from a spur on the seafloor to a
point at or near the sea surface, and an artificial lift recovery
mechanism to help transport said hydrocarbon wellfluid from the
well to said point at or near the surface, wherein said artificial
lift recovery mechanism operates on said hydrocarbon wellfluid at a
point along the at least one riser conduit disposed in use to be
substantially above the seafloor.
11. A riser apparatus as claimed in claim 10, said apparatus
comprising a riser tower having a top comprising one or more rigid
conduits supported in a tower structure and adapted in use to
extending from a connecting structure on the seabed to a point
below the sea surface and wherein there are provided one or more
flexible conduits extending from said tower structure to connect
said tower structure to a surface structure, and wherein there is
further provided a buoyancy device attached to the top of said
tower structure, such that in use said buoyancy device is located
above and exerts an uplift force on said riser tower.
12. A riser apparatus as claimed in claim 11, wherein the
artificial lift recovery mechanism comprises for at least one of
said rigid conduits, at least one inlet for the injection of gas,
in use, at a point substantially above said seabed.
13. A riser apparatus as claimed in claim 12 adapted such that in
use said injection is made at a point where the hydrostatic
pressure is below a cricondenbar of the hydrocarbon wellfluid.
14. A riser apparatus as claimed in claim 13 adapted such that in
use said injection is made at a point where pressure in the riser
conduit is below a bubble point pressure of the hydrocarbon
wellfluid.
15. A riser apparatus as claimed in claim 12 comprising a dedicated
pipe for carrying from the surface the gas for injection at the
injection point.
16. A riser apparatus as claimed in claim 12 wherein at least one
of said one or more rigid conduits comprises a pipe-in-pipe
structure for at least a portion of its length, an annulus of said
pipe-in-pipe structure being adapted for carrying from the surface
the gas for injection at the injection point, a central pipe of
said pipe-in-pipe structure comprising a main conduit for
transporting said hydrocarbon wellfluid.
Description
[0001] This invention relates to apparatus and methods of obtaining
liquids from wells, and in particular the obtaining of hydrocarbon
deposits such as crude oil from wells located at ultra deepwater
sites.
[0002] Any liquid-producing reservoir, such as an oil reservoir,
will have a `reservoir pressure`, and consequently some level of
energy or potential that will force fluid to areas of lower energy
or potential. After time the pressure in the reservoir decreases as
the reservoir is being depleted. Depending on the depth of the
reservoir (deeper wells result in a higher pressure requirement)
and density of the fluid (heavier mixture results in higher
pressure requirement), the reservoir may or may not have enough
potential to push the fluid to the surface by itself. Some
hydrocarbon production reservoirs have sufficient pressure to
produce oil and gas naturally in the early phases of production.
However, at some point, most well operators will implement an
artificial lift plan to continue and/or to increase production.
[0003] Artificial lift or assisted recovery involves the use of
artificial means to increase the flow of liquids of a production
well, such as crude oil, to the surface. This is usually achieved
by a mechanical device inside the well, such as a pump and/or
decreasing the weight of the liquid/gas mixture via high pressure
gas. Artificial lift is needed in wells when there is insufficient
pressure in the reservoir to lift the liquid to the surface, but is
also often used in naturally flowing wells to increase the flow
rate above what would flow naturally. The produced fluid can be oil
and/or water, typically with some amount of gas included. The
artificial lift provides additional energy to the system so that
the fluids can be lifted to surface at a reasonable flow rate. In
deepwater (500-1800 m) and ultradeepwater (1800 m or deeper), well
production is gathered in manifolds and circulated through
flowlines (or spurs) before entering a riser.
[0004] One particular artificial lift method commonly used is gas
lift wherein gas is injected into the well or, in deepwater, at the
riser base. The Injected gas "aerates" the fluid to reduce its
density enabling the formation pressure to lift the oil column and
forces the fluid out of the well bore. The additional benefit of
riser base gaslift is that it stabilizes the multiphase flow in the
spur. Gas may be injected continuously or intermittently, depending
on the producing characteristics of the well and the arrangement of
the gas-lift equipment. Gas lift techniques and apparatus are well
known and therefore are not described in detail herein.
[0005] The inventors have recognised that one drawback with gas
lift techniques is that they may be ineffective in ultra deep water
(defined as greater than 6000 ft or 1829 m). In ultra deep water
the bubble point or even the cricondenbar (that is the maximum
pressure that two phases can co-exist) of the wellfluid may be
lower than the hydrostatic pressure at the seafloor. Given that the
pressure distribution in a well/flowline/riser is somewhat around
the hydrostatic pressure, the wellfluid will remain in liquid phase
in the well, the flowline, and even in the lower part of the riser.
The gas and the liquid phase exist concurrently only in the riser
at some distance above the seafloor. The inventors have therefore
recognised that conventional riser base gas lift is not effective
at such water depths. In such situations, one solution may be to
boost the flow at the well base, by some form of pumping means.
However, this would increase cost and complexity significantly.
[0006] It is the aim of the invention to attempt to address the
abovementioned drawback.
[0007] In a first aspect of the invention there is provided a
method of production of hydrocarbons from an offshore reservoir
using at least one spur (or flowline) to transport said
hydrocarbons from a wellhead to the bottom of said at least one
riser conduit and said at least one riser conduit to transport said
hydrocarbons from the seafloor to a point at or near the sea
surface, wherein an artificial lift method is used to help
transport said hydrocarbons from the reservoir to said point at or
near the surface, and wherein said artificial lift method is
applied to said hydrocarbons at a point along the at least one
riser conduit, substantially above said seafloor.
[0008] The hydrocarbon wellfluid may comprise crude oil, or a
mixture of crude oil, water and natural gas.
[0009] Said at least one elongate conduit may form part of a riser
tower structure comprising one or more rigid conduits supported in
a tower structure and extending from a foundation connecting
structure on the seabed to a point below the sea surface and
wherein there is provided one or more flexible conduits extending
from said tower structure to connect said tower structure to a
surface structure, and wherein there is further provided a buoyancy
device attached to top of said tower structure, such that said
buoyancy device is located above and exerts an uplift buoyancy
force on said riser tower.
[0010] Said artificial lift method may comprise a gas lift method
where gas is injected into the hydrocarbons to be recovered so as
to reduce their density. Said injection may be made at a point
where the hydrostatic pressure is below the cricondenbar of the
wellfluid. Preferably said injection is made at a point where the
pressure in the riser is below the bubble point pressure of the
wellfluid. Said gas for injection may be transported from the
surface to the injection point by a dedicated pipe, or in the
annulus of a pipe-in-pipe structure, the central pipe comprising
the main conduit for transporting said hydrocarbons.
[0011] In a further aspect of the invention there is provided
apparatus for carrying out the methods of the invention. Said
apparatus may include riser apparatus, and in particular, a riser
tower structure comprising one or more rigid conduits supported in
a tower structure and extending from a connecting structure on the
seabed (such as a pile) to a point below the sea surface and
wherein there is provided one or more flexible conduits extending
from said tower structure to connect said tower structure to a
surface structure, and wherein there is further provided a buoyancy
device attached to the top of said tower structure, such that said
buoyancy device is located above and exerts an uplift force on said
riser tower, wherein at least one of said rigid conduits has at
least one inlet for the injection of gas at a point substantially
above said seabed.
[0012] Said apparatus may comprise a dedicated pipe for carrying
from the surface the gas for injection at the injection point or
alternatively said at least one elongate conduit may comprise a
pipe-in-pipe structure for at least a portion of its length, the
annulus of said pipe-in-pipe structure for carrying from the
surface the gas for injection at the injection point, the central
pipe comprising the main conduit for transporting said
hydrocarbons.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Embodiments of the invention will now be described, by way
of example only, by reference to the accompanying drawings, in
which:
[0014] FIG. 1 shows a type of riser structure in an offshore oil
production system;
[0015] FIG. 2 shows a cross section of a pipe arrangement suitable
for carrying out gas lift for assisted recovery according to an
embodiment of the invention; and
[0016] FIGS. 3a and 3b show further alternative of pipe
arrangements suitable for carrying out gas lift for assisted
recovery according to other embodiments of the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0017] The riser tower is a particular type of known riser
structure which typically includes either one or a number of
conduits for the transport of fluids. In particular it relates to
apparatus for buoyancy tensioning of offshore deepwater structures.
It finds particular application in tensioning a slender, vertical
or near-vertical, bottom-anchored, submarine structure, such as a
riser or a bundle of risers (said structure including a structural
member which may, or may not, be one of the risers) or an
umbilical.
[0018] Tensioning is the act of ensuring that a marine structure
doesn't experience excursions from its nominal upright position
that would fall outside the acceptable limits, even in extreme
weather conditions, the said limits being possibly defined with
reference to environmental conditions. There should always be
sufficient tension to ensure stability, no matter the weight of the
structure and the weight of the pipelines/risers hanging off the
structure.
[0019] The structure may form part of a so-called hybrid riser,
having an upper and/or lower portions ("jumpers") made of flexible
conduit. U.S. Pat. No. 6,082,391 (Stolt/Doris) proposes a
particular Hybrid Riser Tower consisting of an empty central core,
supporting a bundle of riser pipes, some used for oil production
and some used for water and gas injection. This type of tower has
been developed and deployed, for example, in the Girassol field off
Angola, and in the Greater Plutonio field off Angola. Syntactic
foam blocks surrounds the core and the riser pipes and separates
the hot and cold fluid conduits. Further background has been
published in papers "Hybrid Riser Tower: from Functional
Specification to Cost per Unit Length" by J-F Saint-Marcoux and M
Rochereau, DOT XIII Rio de Janeiro, 18 Oct. 2001 and "Girassol
Field Development--Total Elf Fina--Riser Tower Installation" OTC
2002 number 14211 by Vincent Alliot & Olivier Care. Updated
versions of such risers have been proposed in WO 02/053869 A1, from
which it is known to use a vertical riser bundle where the
production lines are individually insulated and where the syntactic
foam function is buoyancy only.
[0020] FIG. 1 illustrates such riser tower assemblies in use. It
shows a floating offshore structure 128 fed by riser bundles 112,
114, which are supported by subsea buoys 124, 126. Spurs (or
flowlines) 116 extend from the bottom of the riser bundle to the
various well heads 100. In subsea production the oil from the wells
is gathered at manifolds and then circulates from the manifolds in
a spur 116 (or flowline) to a riser bundle 112, 114. Often two
spurs and two risers are arranged in a loop to allow circular
pigging. The floating structure is kept in place by mooring lines
(not shown), attached to anchors (not shown) on the seabed. The
example shown is of a type known generally from the Girassol
development, mentioned above.
[0021] Each riser bundle is supported by the upward force provided
by its associated buoy 124. Flexible jumpers 132 are then used
between the buoys and the floating structure 128. The tension in
the riser bundles is a result of the net effect of the buoyancy
combined with the weight of the structure and risers in the
seawater. The skilled person will appreciate that the bundle may be
a few metres in diameter, but is a very slender structure in view
of its length (height) of for example 1.8 km or more in ultra deep
water applications. The structure must be protected from excessive
bending and the tension in the bundle is of assistance in this
regard.
[0022] Hybrid Riser Towers (HRTs), such as those described above,
have been developed as monobore structures or as structures
comprising a number, in the region of four to twelve, of risers
arranged around a central structural core.
[0023] Conventional riser tower structures will often incorporate
one or several gaslift lines for the channelling of gas from the
surface to the well or the riser base, where the gas is injected
into the oil/wellfluid to be recovered. This injection of gas
considerably lowers the density of the oil being recovered so that
it can be lifted, through the main riser, to the surface under the
influence of the reservoir pressure.
[0024] One feature of such riser tower arrangements described
above, the inventors of the present invention have determined, is
that they are particularly suited for injection of gas for gas lift
at a mid-water point 150. The injection of gas for gas lift part
way up the riser may allow gas lift techniques to be used for low
cricondenbar fluids at large water depths, as explained previously.
As a gas lift injection element adds mass and results in a local
change of the riser's mechanical properties, it may be more
difficult to accommodate a gas lift injection system along the
riser length of highly dynamic risers such as flexible risers or
catenary riser configurations. The inventors have realised that
this would not be such an issue for riser tower configurations.
[0025] The mid-water gas lift gas Injection should be made at such
a level, significantly above the well head and sea floor, where the
pressure in the riser is low enough for the gas to "aerate" the oil
effectively, thus reducing the weight of the oil in the riser above
the gas injection point. This, of course, has the effect of
reducing the total weight of the oil in the riser, and
consequently, in many cases, enabling recovery under reservoir
pressure alone, or else increasing recovery rate.
[0026] FIG. 2 shows a single riser HRT with main riser conduit 200
and associated gas lift line 210 which allows mid-water gas lift
gas injection. Around both lines is insulation 220. Furthermore the
gap between the two lines is filled with an injected gel or high
viscosity material 230. A sleeve maintains the gel and gas lift
line in place. The mid-water gas injection is effected by a
connection being made between the two lines, at a suitable point
somewhere along the length of the riser, some way above the seabed
so to inject the gas into the wellfluid.
[0027] FIGS. 3a and 3b show alternative arrangements to allow
mid-water injection of gas lift gas, in riser tower arrangements
where the gas lift gas is carried from the surface in the annulus
of a pipe-in pipe type arrangement, the recovered oil being carried
to the surface in the central pipe.
[0028] FIG. 3a shows a riser pipe section comprising a central pipe
300, outer pipe 310, annulus 320, multiple injection holes 330 and
passageways 350. This pipe section therefore enables the gas to be
injected from the annulus into the wellfluid at 45.degree., via
said passageways 350 and through said injection holes 330.
[0029] FIG. 3b shows a riser pipe section comprising a central pipe
300, outer pipe 310, annulus 320 and a pipe connection 340
connecting the annulus space 320 to the central pipe 300. This pipe
connection 340 enables the injection of the gas, carried in the
annulus 320, into the wellfluid in the central pipe 300. (It can be
seen that, in both FIGS. 3a and 3b, there is no need for an outer
pipe below the gas injection point).
[0030] In both examples of FIG. 3, these pipe riser sections will
be located somewhere along the length of the riser, some way above
the seafloor, such that the gas injection is again made at a
suitable point where it will be effective, taking into account of
the surrounding hydrostatic pressure.
[0031] There are other advantages to mid-water gas lift methods (or
in performing other artificial lift methods mid-water), which may
make them attractive even in depths where hydrostatic pressures may
not pose such a problem. Such arrangements have the advantages of
simplifying the apparatus, as gas lift lines (or pipe-in-pipe
structures), for example, would only be needed along part of the
riser length. Also energy requirements may be reduced as (for gas
lift) the gas would no longer need to be channelled all the way to
the wellhead, or the riser base.
[0032] The above embodiments are for illustration only and other
embodiments and variations are possible and envisaged without
departing from the spirit and scope of the invention. For example
the above embodiments have all been described in terms of riser
tower structures, which are particularly suitable. However,
mid-water artificial lift techniques may also be applicable to
other riser structures. Also, while the main embodiments have been
directed at gas-lift methods in particular, the invention is
equally applicable to other mid-water artificial lift
techniques.
* * * * *