WO2002063135A1 - A method and a sea-based installation for hydrocarbon processing - Google Patents

A method and a sea-based installation for hydrocarbon processing Download PDF

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Publication number
WO2002063135A1
WO2002063135A1 PCT/NO2002/000038 NO0200038W WO02063135A1 WO 2002063135 A1 WO2002063135 A1 WO 2002063135A1 NO 0200038 W NO0200038 W NO 0200038W WO 02063135 A1 WO02063135 A1 WO 02063135A1
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WIPO (PCT)
Prior art keywords
gas
separator
separated
well
plant
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PCT/NO2002/000038
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French (fr)
Inventor
Morten Sanderford
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Navion Asa
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Publication of WO2002063135A1 publication Critical patent/WO2002063135A1/en

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Classifications

    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/01—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells specially adapted for obtaining from underwater installations
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/34—Arrangements for separating materials produced by the well
    • E21B43/40—Separation associated with re-injection of separated materials
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/34—Arrangements for separating materials produced by the well
    • E21B43/36—Underwater separating arrangements

Definitions

  • This invention relates to a method for processing/handling hydrocarbons from subsea wells, on board a surface vessel or other sea-based installation, said hydrocarbon processing/handling comprising i.a. separation of the unprocessed, possibly water-containing hydrocarbons into single fractions, the- heavy fraction thereof, the oil, being the desired end product which is stored, and the gas fractions and possible water constituting the problematic products of the method, which are partly utilized as consumer gas/fuel for devices etc. incorporated in the plant for the practicing of the method, and partly may - as excess gas - be returned and re-injected into the subsea reservoir, in accordance with the introductory part of Claim 1.
  • the invention relates to a plant for carrying out the method.
  • NO patent application No. 2000 2356 discloses a method and devices in processing plants for the sea-based handling of hydrocarbons in the form of oil with associated gas and possibly containing- water in smaller quantities.
  • At least one separating operation is carried out on the seabed, wherein gas is separated from oil, and wherein hydrocarbons after separation of part of the gas are carried by means of a riser up to a surface vessel/production ship, a floating platform or another sea-based installation.
  • the method according to NO patent application No. 2000 2356 is thereby essentially distinguished by the fact that an unprocessed well flow at s subsea level is subjected to a first separating step in a first separator installed on the seabed, controlled/run in such a way that the liquid hydrocarbons which are output therefrom and led to the surface vessel through said riser, will contain an essentially predetermined percentage of residual gas which may be separated from said liquid hydrocarbons on board the surface vessel in a second separating step in an second separator installed on board.
  • the residual gas separated in said second separator is used as consumer gas/fuel for driving subsea and surface plant components incorporated in a plant for the handling/process- ⁇ ing of said hydrocarbons from the subsea well, the term "handling/processing" being meant to cover i.a. the recovery of the hydrocarbons, the first step separation at seabed level and the partial re-injection of gas into reservoirs carried out there, transport to surface level and the second step separation, storing of the heavy fraction and utilization of separated residual gas as fuel for driving various components incorporated in the processing plant. Any excess gas from the second separating step aboard the surface vessel and water separated in a surface position may, according to the above-mentioned NO patent application, be returned to the subsea reservoir and re-injected there.
  • Residual gas separated in the surface position used as consumer gas may, by the method according to said NO patent application, be used for example as fuel for driving electric generators for generating electrical power which is used in its turn to drive electrically driven components, such as the first separator on the seabed, multiphase pumps etc. and the second separator etc.
  • the total plant comprising a subsea part and a surface-based part and the riser connecting them, is distinguished in that the subsea separator (first separator) is arranged to be controlled and run in such a way that it outputs oil with an essentially predetermined percentage of associated residual gas.
  • This oil with associated residual gas is delivered aboard the surface vessel, where it is separated in the second separator in order for storable oil and consumer gas . to be achieved, as described earlier.
  • the separation accuracy of the subsea separator is set in such a way that it is ensured that in a specific period of time an amount of gas is carried aboard the surface vessel together with the oil, essentially corresponding to the power consumption of the total processing plant above and under water during said period of time. Thereby excess gas, which should be returned to the reservoir and re-injected there, is avoided.
  • This known method is quite labour-intensive, and the total plant is expensive and partly of a complex construction.
  • Known technique comprises i.a. NO B 152 730, NO B 166 145, NO C 173 838, NO 180 350 and US 4,960,433. Similar methods and plants are illustrated and described in US patent documents No. 3,221,816, 3,556,218 and 3,608,630.
  • the state of the art is generally based on at least one subsea separator incorporated in a subsea plant of a partly complex structure, the method comprising several labour-intensive, expensive operational steps .
  • the aim has been to achieve substantial simplifications while maintaining and keeping certain advantageous features of the method/plant according to NO patent application No. 2000 2356.
  • This involves, i.a., the above-mentioned partial utilization of associated gas which is dissolved in the oil recovered and is carried with the oil up to the surface craft and aboard it to be subjected to separation.
  • the entire hydrocarbon production is carried up to the surface vessel through a tubing line/hose or other suitable hose/pipeline connection.
  • a swivel connection In this transport system there may be installed a swivel connection.
  • Aboard the surface vessel or other sea-based installation is mounted a two-stage separator in which gas (light gas fraction) is separated from the hydrocarbons carried aboard in the first separation step, to be returned to the well and re-injected into the reservoir, and in which at least a portion of gas separated from the second stage of the separator (heavier gas) may be burnt in a steam boiler for the production of steam to be used subsequently as driving fluid.
  • gas light gas fraction
  • a portion of the steam generated there is utilized as fuel in essentially the same manner as described in the above- mentioned NO patent application No. 2000 2356; in this case to drive a steam turbine connected to an electric generator which serves in its turn to generate electrical power for use in the processing plant.
  • Another portion of the steam produced is used to heat the processing plant, for example to preheat hydrocarbons before they are carried into the two- stage separator.
  • a third heavy gas portion, heavier excess gas, is disposed of by deposition into the reservoir, by return and re-injection as by the lighter gas fractions.
  • the hydrocarbons carried aboard are separated into lighter and heavier fractions, corresponding to light gas fractions and heavy gas and oil (the heavy fraction), respectively.
  • Oil is, as mentioned, the desired end product which is to be stored, whereas the separated gases, as mentioned - apart from the potential driving fluid thereof - (providing properties in the processing) represent problem products.
  • Associated water if any, represents in its entirety a component of no value in the well flow carried aboard.
  • gas separated in the first separating step is preferably compressed and then returned to the subsea well into the annulus thereof, which is formed for example between tubing and outside casing/wellbore wall and similar, to be re-injected into the reservoir.
  • the compression is advantageous as it reduces, to a substantial degree, the gas volume that should be dealt with and disposed of.
  • this separated light gas is returned to the well and re-injected through an umbilical cord, in principle of the kind which is often referred to as an "umbilical", in the form of a collection of coextensive hose lengths known in themselves, encapsulated in a coaxial outer hose, and which is used conventionally for the supply of chemicals to subsea wells.
  • umbilicals will normally comprise hydraulic hoses, electrical cables etc., but for the object of the invention there are, of course, hose lengths with through bores for the transport of gas separated in a surface position to the receiving injection portion at the Gas separated in the second separating step, heavy gas, at least as far as some of the heavy gas is concerned, is retained and utilized as fuel in the steam production mentioned above.
  • Water carried with the hydrocarbons aboard the surface vessel is advantageously separated according to the invention in both the first and second separating steps.
  • This separated water on its part, is contaminated with hydrocarbons and represents at this stage a contamination source/fluid which requires purification before it may be let into the sea.
  • This contaminated associated water separated from the hydrocarbons carried aboard is also purified in a typical two-stage purification process. First in a hydrocyclone and then in a skimmer.
  • the purified water is of high purity and may be let into the sea. Oil separated from the water by means of the hydrocyclone and the skimmer may with advantage be recirculated to the second stage of the two-stage separator.
  • said separator may optimally be adjusted so that gas from the second separating step is separated here in an amount which ensures production of sufficient amounts of steam for the subsequent generation of electrical power and thermal energy.
  • Such an adjustment may be carried out in the easiest manner, according to the invention, by a throttling upstream of the separator, wherein there is connected in a pipeline system/circuit a pressure regulator in the form of a choke valve upstream of an inlet heater in the hydrocarbon supply pipeline of the separator. 7
  • Such umbilicals often have a central passage, for example of a coarser bore than those of the peripheral hose lengths surrounding the central passage and normally extending along the whole length of the umbilical. Gas from the first stage of the two-stage separator may for example be re-injected into the reservoir through the central passage of the umbilical.
  • This gas is supplied to the well through the umbilical and is injected into the reservoir through the annulus of the producing well.
  • the utilization of the umbilical for downward transport of gas to the well, and the annulus of the well as a "transport means" for returned gas which is to be deposited into the reservoir, provides obvious technical advantages which will be discussed later.
  • Another advantage of the re-injection of separated light gas into the reservoir may be seen in the maintenance or approximate maintenance of the desired pressure condition in the reservoir.
  • a further advantage of returning and re-injecting separated gas, especially light gas from the first stage of the separator, into the reservoir is that the surroundings are not strained as by the flaring/burning of separated gas which is not to be utilized.
  • the wells are placed in a producing state in that they are drilled for the developing of a known, commercial petroleum deposit, the underwater development work normally including drilling, completion and installation of wellheads, underwater manifolds and flowlines/gathering lines.
  • the underwater development work normally including drilling, completion and installation of wellheads, underwater manifolds and flowlines/gathering lines.
  • hydraulically operated downhole pumps should be installed.
  • the invention provides, i.a., the following advantages:
  • annulus is defined as the space between a tubing string, normally containing the produced fluid, and the casing;
  • Variations in the flow rate and composition of the produced fluid may be handled in such a way that the processing plant on board the ship is affected minimally in a negative direction.
  • An important feature of the method according to the invention consists in balancing the pressure in the two separating steps, so that the amount of residual gas carried in the crude oil to the second step separation will correspond to the gas amount necessary for the generation of power.
  • By varying the pressure and temperature conditions of the first separation step it is ensured that the required amount of gas is achieved in the second step separation.
  • Low pressure or high temperature in the first separation step will reduce the amount of residual gas in the second separation step.
  • high pressure and low temperature will cause an increase in the amount of residual gas in the crude oil, thus increasing the amount of gas separated in the separation of the last step.
  • the invention may be utilized with advantage for use in connection multifunctional wells working continuously in several modes. Firstly, the well will produce hydrocarbons which are carried up to the surface through a pipe string.
  • An exemplary embodiment of the invention is illustrated in the accompanying single drawn figure showing schematically a plant for processing hydrocarbons recovered from a producing subsea well 10 with associated gas and water carried from the well 10 through a tubing string 12 with an angled upper portion 12' up to the surface to a two-stage separator 14, 16 installed on a surface vessel/platform/installation (not shown), and comprising in the non-limiting embodiment shown two separators connected in series, a first separator 14 and a downstream second separator 16.
  • a pressure regulator is installed, for example in the form of a choke valve 18, and between that and the first step separator 14 an inlet heater 20 for pre-heating hydrocarbons before they flow from the tubing line/supply pipeline 12 into the first step separator 14.
  • a turbine-driven downhole pump 24 may be used instead of a gas lift in order to favour the recovery from the reservoir.
  • the downhole pump 24 is placed in the production zone 22' of the reservoir and is driven by said turbine 26. As it is possible to place the pump in the production zone 22 ' of the reservoir, the efficiency of the well stimulation is maximized.
  • the downhole turbine 26 is driven by means of high-pressure fluid, either in the form of hydraulic fluid in a closed, loop 28 or by a slipstream of the well fluid pumped back for re- injection into the reservoir 22.
  • the high-pressure fluid is pumped down from an electrically driven subsea pump 30 to the downhole turbine 26 and causes the rotor(s) thereof to be rotated and provide the rotary motions necessary for the downhole pump 24.
  • the subsea pump 30, for its part, takes care of the establishment and maintenance of the pressure head of the fluid driving the turbine pump.
  • the electrical drive motor for the subsea pump 30 is identified by 32.
  • excess (light) gas separated in the first separating step 14 is returned to and re-injected into the reservoir 22 through the annulus 34 of the well 10 by means of an umbilical 36 which is central in the present invention and thus forms an important feature thereof.
  • Light gas separated in the first separator 14 is supplied to the upstream end of the umbilical 36 through a gas outlet line 38 coming from the first separator 14, and a piston compressor 40 driven by a motor 42.
  • this gas return primarily to the annulus 34 of the producing subsea well 10 for subsequent re-injection into the reservoir 22, will take place either in the central passage of the umbilical 36 or be divided between two or more of the peripheral passages thereof.
  • the use of an umbilical for the injection/re-injection of gas in the connection in question provides advantages beyond those already mentioned: (i) The umbilical 36 may be attached to the production riser 12, and a swivel connector 44 may be connected at the connection point. This reduces the installation cost;
  • the thermal energy inherent in the well fluid will contribute to ensuring a flow in the gas injection line with respect to hydra ization.
  • the hydratization temperature by the gas injection pressure will be about 25-30 °C, and the flow temperature of the well fluid will be between 50-75°.
  • the first separator 14 works at a pressure of 5-15 bars and a temperature of 65-70 °C
  • the second separator 16 works at a pressure of 1.1-1.2 bars and a temperature of 65- 70 °C.
  • the overboard plant of the FPSO-unit thus comprises the following main components : (i) Inlet heater 20, separators 14 and 16 for two-step separation of three-phase fluid, devices for treating/purifying produced water and utility devices;
  • the well fluid When the well fluid comes up to the overboard plant, the well fluid is first preheated in the inlet heater 20, in which steam is used as heating medium. This pre-heating serves to favour the water/oil separation and to achieve the necessary TVP-criteria for the crude oil.
  • the preheated well fluid After having passed through the inlet heater 20, the preheated well fluid will enter said first three-phase fluid separator 14, in which oil, water and gas will be separated in accordance with conventional technique. After degasification and dehydration in this separator 14, the oil is directed to the second step separator 16 for a final degasification and dehydration. Any residual water left in the crude oil will be carried with the oil into the cargo tanks for a final dehydration of the crude oil. Before the crude oil enters the cargo tanks, it will be pumped through a measuring station and cooled to avoid excessive evaporation within the cargo tanks. Produced water from the second step separator 16 is pumped to hydrocyclones 50 to be treated together with water from the first-time separator 14. After the hydrocyclones the produced water enters a final degasification tank/skimmer 52 before being let out into the sea. Recovered oil will be directed to the second step separator 16.
  • An important feature of the invention is connected to the choice of operating temperature of the first step separator 14.
  • the pressure is set to such a value that the amount of residual gas in the oil will correspond to the amount of fuel gas needed for the steam boilers 46.
  • Separated (light) gas from the first step separator 14 is directed to the compressor line 40 consisting of two electrically driven piston compressors connected in parallel or possibly a single compressor.
  • the key capacity factor will be the volumetric flow rate, provided that the motor is powerful enough.
  • the coolers may be based on air or sea-water as the cooling medium. Condensate from scrubbers will be directed back to the separation line or injected into the gas.
  • the injection pressure by the re-injection of light gas will vary from 250 to 400 bars, so that power of several MW will be required.
  • the gas will be compressed to 250-400 bars to ensure injection into the formation and to reduce the volumetric flow rate.
  • the internal diameter required for a gas injection pipe may be reduced, and the passages of the umbilical 36 may be utilized.
  • the gas is directed to the umbilical manifold for injection.
  • the steam generating device 46 incorporated in the generating plant 48 for electrical power may with advantage consist of two gas-fired boilers, in typical cases generating steam of 18 bars in order to feed a steam turbine 54 and the inlet heater 20 for unprocessed crude oil with associated gas and water.
  • the actual generator of the generating plant 48 driven by the steam turbine 54, is identified by 56.
  • Electrical power from the generator 56 of the generating plant 48 drives the pumps of the surface plant and the motor 42 of the gas compressor 40 and the separating steps/separators 14 and 16 in the surface plant and also the pumps in the subsea plant.
  • the C0 2 -tax will be balanced as a consequence of the higher density and calorific value of the gas, and may even balance the use of a low-efficiency power generating system.
  • the umbilical 36 may, in a known manner, receive and transport various chemicals which it is desirable to inject into the reservoir 22 from storage tanks 58, from which branch pipes lead to the umbilical 36 and are in fluid communication therewith.
  • These branch pipelines may for example carry MeOH, boiler composition, emulsion breaker and wax inhibitor.

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Abstract

A method and a sea-based installation for handling/processing hydrocarbons which are recovered from subsea wells (10) and transported up to the installation comprising a surface vessel etc, have been explained. The well flow is separated in to steps in a manner known in itself, and gas separated in the first separating step (14) is compressed in a compressor (40) connected to a multi-passage umbilical (36) which returns said separated gas to the annulus (34) of the subsea well for re-injection into the reservoir (22). The separator (14) of the first step is equipped with a pressure regulator (18) for the setting and adjusting of the working pressure of the separator (14), so that the pressure is kept at a value ensuring that the amount of residual gas in the oil separated in the separator (16) of the second separating step essentially corresponds to the amount of fuel gas necessary for driving the processing plant of the installation on surface and seabed levels, incorporated underwater components and components at surface level.

Description

A method and a sea-based installation for hydrocarbon processing
This invention relates to a method for processing/handling hydrocarbons from subsea wells, on board a surface vessel or other sea-based installation, said hydrocarbon processing/handling comprising i.a. separation of the unprocessed, possibly water-containing hydrocarbons into single fractions, the- heavy fraction thereof, the oil, being the desired end product which is stored, and the gas fractions and possible water constituting the problematic products of the method, which are partly utilized as consumer gas/fuel for devices etc. incorporated in the plant for the practicing of the method, and partly may - as excess gas - be returned and re-injected into the subsea reservoir, in accordance with the introductory part of Claim 1.
Similarly the invention relates to a plant for carrying out the method.
NO patent application No. 2000 2356 discloses a method and devices in processing plants for the sea-based handling of hydrocarbons in the form of oil with associated gas and possibly containing- water in smaller quantities.
In accordance with the method according to this NO patent application at least one separating operation is carried out on the seabed, wherein gas is separated from oil, and wherein hydrocarbons after separation of part of the gas are carried by means of a riser up to a surface vessel/production ship, a floating platform or another sea-based installation.
The method according to NO patent application No. 2000 2356 is thereby essentially distinguished by the fact that an unprocessed well flow at s subsea level is subjected to a first separating step in a first separator installed on the seabed, controlled/run in such a way that the liquid hydrocarbons which are output therefrom and led to the surface vessel through said riser, will contain an essentially predetermined percentage of residual gas which may be separated from said liquid hydrocarbons on board the surface vessel in a second separating step in an second separator installed on board.
The residual gas separated in said second separator is used as consumer gas/fuel for driving subsea and surface plant components incorporated in a plant for the handling/process- ■ ing of said hydrocarbons from the subsea well, the term "handling/processing" being meant to cover i.a. the recovery of the hydrocarbons, the first step separation at seabed level and the partial re-injection of gas into reservoirs carried out there, transport to surface level and the second step separation, storing of the heavy fraction and utilization of separated residual gas as fuel for driving various components incorporated in the processing plant. Any excess gas from the second separating step aboard the surface vessel and water separated in a surface position may, according to the above-mentioned NO patent application, be returned to the subsea reservoir and re-injected there.
Residual gas separated in the surface position used as consumer gas, may, by the method according to said NO patent application, be used for example as fuel for driving electric generators for generating electrical power which is used in its turn to drive electrically driven components, such as the first separator on the seabed, multiphase pumps etc. and the second separator etc.
According to NO patent application No. 2000 2356, the total plant comprising a subsea part and a surface-based part and the riser connecting them, is distinguished in that the subsea separator (first separator) is arranged to be controlled and run in such a way that it outputs oil with an essentially predetermined percentage of associated residual gas. This oil with associated residual gas is delivered aboard the surface vessel, where it is separated in the second separator in order for storable oil and consumer gas . to be achieved, as described earlier.
For optimum utilization of the method and plant as disclosed in NO 2000 2356 the separation accuracy of the subsea separator is set in such a way that it is ensured that in a specific period of time an amount of gas is carried aboard the surface vessel together with the oil, essentially corresponding to the power consumption of the total processing plant above and under water during said period of time. Thereby excess gas, which should be returned to the reservoir and re-injected there, is avoided. This known method is quite labour-intensive, and the total plant is expensive and partly of a complex construction.
Known technique comprises i.a. NO B 152 730, NO B 166 145, NO C 173 838, NO 180 350 and US 4,960,433. Similar methods and plants are illustrated and described in US patent documents No. 3,221,816, 3,556,218 and 3,608,630. The state of the art is generally based on at least one subsea separator incorporated in a subsea plant of a partly complex structure, the method comprising several labour-intensive, expensive operational steps .
In NO patent application 2000 2356 the processing of the well flow takes place, as mentioned, first at a subsea level and then in a surface position on board the production ship. The use of a subsea separator and the return and re-injection of water, separated therefrom, and part of the gas content of the hydrocarbons into the reservoir by means of an immersed multiphase pump, are central in this method, together with the in situ utilization of part of the produced gas for energy production.
Also in the method and plant according to the present invention it is desirable to keep some of the gas which is dissolved in oil recovered from the subsea well, and brought aboard the surface vessel as associated gas and separated there from the oil, for the generation of electricity and heat through i.a. electrical current generators.
In the present invention, the aim has been to achieve substantial simplifications while maintaining and keeping certain advantageous features of the method/plant according to NO patent application No. 2000 2356. This involves, i.a., the above-mentioned partial utilization of associated gas which is dissolved in the oil recovered and is carried with the oil up to the surface craft and aboard it to be subjected to separation.
According to the present invention the entire hydrocarbon production is carried up to the surface vessel through a tubing line/hose or other suitable hose/pipeline connection. In this transport system there may be installed a swivel connection.
Aboard the surface vessel or other sea-based installation is mounted a two-stage separator in which gas (light gas fraction) is separated from the hydrocarbons carried aboard in the first separation step, to be returned to the well and re-injected into the reservoir, and in which at least a portion of gas separated from the second stage of the separator (heavier gas) may be burnt in a steam boiler for the production of steam to be used subsequently as driving fluid.
A portion of the steam generated there is utilized as fuel in essentially the same manner as described in the above- mentioned NO patent application No. 2000 2356; in this case to drive a steam turbine connected to an electric generator which serves in its turn to generate electrical power for use in the processing plant. Another portion of the steam produced is used to heat the processing plant, for example to preheat hydrocarbons before they are carried into the two- stage separator. A third heavy gas portion, heavier excess gas, is disposed of by deposition into the reservoir, by return and re-injection as by the lighter gas fractions. In the first stage of said separator the hydrocarbons carried aboard are separated into lighter and heavier fractions, corresponding to light gas fractions and heavy gas and oil (the heavy fraction), respectively. Oil is, as mentioned, the desired end product which is to be stored, whereas the separated gases, as mentioned - apart from the potential driving fluid thereof - (providing properties in the processing) represent problem products. Associated water, if any, represents in its entirety a component of no value in the well flow carried aboard.
According to the present invention, gas separated in the first separating step, light gas, is preferably compressed and then returned to the subsea well into the annulus thereof, which is formed for example between tubing and outside casing/wellbore wall and similar, to be re-injected into the reservoir.
The compression is advantageous as it reduces, to a substantial degree, the gas volume that should be dealt with and disposed of.
According to the invention this separated light gas is returned to the well and re-injected through an umbilical cord, in principle of the kind which is often referred to as an "umbilical", in the form of a collection of coextensive hose lengths known in themselves, encapsulated in a coaxial outer hose, and which is used conventionally for the supply of chemicals to subsea wells. Such umbilicals will normally comprise hydraulic hoses, electrical cables etc., but for the object of the invention there are, of course, hose lengths with through bores for the transport of gas separated in a surface position to the receiving injection portion at the Gas separated in the second separating step, heavy gas, at least as far as some of the heavy gas is concerned, is retained and utilized as fuel in the steam production mentioned above.
Water carried with the hydrocarbons aboard the surface vessel is advantageously separated according to the invention in both the first and second separating steps. This separated water, on its part, is contaminated with hydrocarbons and represents at this stage a contamination source/fluid which requires purification before it may be let into the sea.
This contaminated associated water separated from the hydrocarbons carried aboard is also purified in a typical two-stage purification process. First in a hydrocyclone and then in a skimmer.
The purified water is of high purity and may be let into the sea. Oil separated from the water by means of the hydrocyclone and the skimmer may with advantage be recirculated to the second stage of the two-stage separator.
Herein said separator may optimally be adjusted so that gas from the second separating step is separated here in an amount which ensures production of sufficient amounts of steam for the subsequent generation of electrical power and thermal energy. Such an adjustment may be carried out in the easiest manner, according to the invention, by a throttling upstream of the separator, wherein there is connected in a pipeline system/circuit a pressure regulator in the form of a choke valve upstream of an inlet heater in the hydrocarbon supply pipeline of the separator. 7
interfaces with channels etc. in the reservoir surrounding the petroleum-producing subsea well. Such umbilicals often have a central passage, for example of a coarser bore than those of the peripheral hose lengths surrounding the central passage and normally extending along the whole length of the umbilical. Gas from the first stage of the two-stage separator may for example be re-injected into the reservoir through the central passage of the umbilical.
In a sea-based oil-recovering, hydrocarbon-separating and gas-returning/re-injecting plant with transport pipeline connection (round-trip pipeline) between a subsea well and a processing plant on board a surface vessel, platform or other installation, there are at all times at least two separate connections/pipe strings, namely a pipeline/hose in the form of a tubing string transporting a well flow constantly in a direction from the bottom upwards, and said umbilical, which forms a kind of return pipe string for re-injection gas, in which the direction of flow - once light gas is starting to separate from the heavier hydrocarbon fractions in the first separating step of the two-stage separator - is running at all times from the top downwards. This gas is supplied to the well through the umbilical and is injected into the reservoir through the annulus of the producing well. The utilization of the umbilical for downward transport of gas to the well, and the annulus of the well as a "transport means" for returned gas which is to be deposited into the reservoir, provides obvious technical advantages which will be discussed later. Another advantage of the re-injection of separated light gas into the reservoir may be seen in the maintenance or approximate maintenance of the desired pressure condition in the reservoir. A further advantage of returning and re-injecting separated gas, especially light gas from the first stage of the separator, into the reservoir is that the surroundings are not strained as by the flaring/burning of separated gas which is not to be utilized.
In a sea-based plant/installation of the kind that the present invention relates to, the wells are placed in a producing state in that they are drilled for the developing of a known, commercial petroleum deposit, the underwater development work normally including drilling, completion and installation of wellheads, underwater manifolds and flowlines/gathering lines. To enhance the flow conditions and favour the conditions of the reservoir in general, and to avoid gas lift, it has been decided that hydraulically operated downhole pumps should be installed. The hydrocarbons produced are directed through flowlines/gathering lines and a riser for example to a surface vessel or other floating craft/sea-based installation, such as an FPSO-ship or an FPSO-unit (FPSO = Floating Production and Storage of Oil).
It has earlier been pointed out that compression of light gas separated in the first separating step is of advantage. By balancing the residual gas in the oil from the first step separation with the required fuel gas flow rate from the second step separation, compression of residual gas from the second step separation is not necessary.
The invention provides, i.a., the following advantages:
(i) By injecting/re-injecting gas through the annular volume/annulus of a producing well, there is no need for injection wells. The annulus is defined as the space between a tubing string, normally containing the produced fluid, and the casing;
(ii) By injecting/re-injecting gas through an umbilical, there will be no need for flowlines/gathering lines and riser for the gas injection/re-injection;
(iii) Easy requirements to the fuel/driving gas. As the burners of the steam boiler make few and moderate/low demands on the gas quality, it is unproblematic to utilize gas from the second step separation as fuel in the steam production plant. As gas from the separation of the last step serves as consumer gas in the steam plant, the awkward heavy gas and possible build-up of recirculating condensate caused by the heavy gas are eliminated. Compression of the heavy separated gas is not necessary, and condensation of heavy gas and buildup of recirculated fluid may be avoided;
(iv) Variations in the flow rate and composition of the produced fluid may be handled in such a way that the processing plant on board the ship is affected minimally in a negative direction.
An important feature of the method according to the invention consists in balancing the pressure in the two separating steps, so that the amount of residual gas carried in the crude oil to the second step separation will correspond to the gas amount necessary for the generation of power. By varying the pressure and temperature conditions of the first separation step it is ensured that the required amount of gas is achieved in the second step separation. Low pressure or high temperature in the first separation step will reduce the amount of residual gas in the second separation step. On the other hand, high pressure and low temperature will cause an increase in the amount of residual gas in the crude oil, thus increasing the amount of gas separated in the separation of the last step.
In a normal processing line the residual gas is compressed and enters into the compression line. By compression of the heavy residual gas, condensation and build-up of gas may occur in circuits and "drown" the process in C2-C5 components. By burning the C2-C5 components instead of recirculating them in the process, the recirculation problems are avoided.
Normally gas injection wells will have to be drilled for the deposition of gas. If the central largest passage of the umbilical is utilised for the return/re-injection of light gas separated in the first separating step in the separator aboard the surface vessel, other passages in the umbilical may be used to increase the amount of gas injected per time unit. Thereby a gas injection riser string and subsea gathering lines/flowlines will be made redundant.
The invention may be utilized with advantage for use in connection multifunctional wells working continuously in several modes. Firstly, the well will produce hydrocarbons which are carried up to the surface through a pipe string.
Secondly, the well will be stimulated in that a downhole pump is employed.
An exemplary embodiment of the invention is illustrated in the accompanying single drawn figure showing schematically a plant for processing hydrocarbons recovered from a producing subsea well 10 with associated gas and water carried from the well 10 through a tubing string 12 with an angled upper portion 12' up to the surface to a two-stage separator 14, 16 installed on a surface vessel/platform/installation (not shown), and comprising in the non-limiting embodiment shown two separators connected in series, a first separator 14 and a downstream second separator 16.
In the upper portion 12' of the tubing line 12 a pressure regulator is installed, for example in the form of a choke valve 18, and between that and the first step separator 14 an inlet heater 20 for pre-heating hydrocarbons before they flow from the tubing line/supply pipeline 12 into the first step separator 14.
To assist in the production from a low-pressure reservoir 22 a turbine-driven downhole pump 24 may be used instead of a gas lift in order to favour the recovery from the reservoir. The downhole pump 24 is placed in the production zone 22' of the reservoir and is driven by said turbine 26. As it is possible to place the pump in the production zone 22 ' of the reservoir, the efficiency of the well stimulation is maximized.
The downhole turbine 26 is driven by means of high-pressure fluid, either in the form of hydraulic fluid in a closed, loop 28 or by a slipstream of the well fluid pumped back for re- injection into the reservoir 22.
The high-pressure fluid is pumped down from an electrically driven subsea pump 30 to the downhole turbine 26 and causes the rotor(s) thereof to be rotated and provide the rotary motions necessary for the downhole pump 24. The subsea pump 30, for its part, takes care of the establishment and maintenance of the pressure head of the fluid driving the turbine pump.
The electrical drive motor for the subsea pump 30 is identified by 32.
In order, i.a., to save a gas injection riser, in which the gas flow for re-injection purposes has a course from the top downwards, excess (light) gas separated in the first separating step 14 is returned to and re-injected into the reservoir 22 through the annulus 34 of the well 10 by means of an umbilical 36 which is central in the present invention and thus forms an important feature thereof.
Light gas separated in the first separator 14 is supplied to the upstream end of the umbilical 36 through a gas outlet line 38 coming from the first separator 14, and a piston compressor 40 driven by a motor 42.
Depending on the configuration of the umbilical 36 this gas return, primarily to the annulus 34 of the producing subsea well 10 for subsequent re-injection into the reservoir 22, will take place either in the central passage of the umbilical 36 or be divided between two or more of the peripheral passages thereof.
The use of an umbilical for the injection/re-injection of gas in the connection in question, provides advantages beyond those already mentioned: (i) The umbilical 36 may be attached to the production riser 12, and a swivel connector 44 may be connected at the connection point. This reduces the installation cost;
(ii) By attaching the gas injection umbilical to the tubing line, the thermal energy inherent in the well fluid will contribute to ensuring a flow in the gas injection line with respect to hydra ization. In typical cases the hydratization temperature by the gas injection pressure will be about 25-30 °C, and the flow temperature of the well fluid will be between 50-75°.
(iii) When high-pressure gas is used, the temperature of the gas will increase during expansion, so that hydratization is counter-acted.
(iv) By unexpected formation of hydrates, these may be removed, e.g. by melting, by flowing a hot medium in other passages in the umbilical and/or by relieving pressure.
The first separator 14 works at a pressure of 5-15 bars and a temperature of 65-70 °C, whereas the second separator 16 works at a pressure of 1.1-1.2 bars and a temperature of 65- 70 °C.
The overboard plant of the FPSO-unit thus comprises the following main components : (i) Inlet heater 20, separators 14 and 16 for two-step separation of three-phase fluid, devices for treating/purifying produced water and utility devices;
(ii) Two steam-producing boilers, one 46 of them incorporated in a plant 48 for the generation of electrical power;
(iii) A steam turbine for generating power;
(iv) A gas compressor 40;
(v) A flare system;
(vi) Overboard support arrangements and devices for connections, transfers and transitions.
When the well fluid comes up to the overboard plant, the well fluid is first preheated in the inlet heater 20, in which steam is used as heating medium. This pre-heating serves to favour the water/oil separation and to achieve the necessary TVP-criteria for the crude oil.
After having passed through the inlet heater 20, the preheated well fluid will enter said first three-phase fluid separator 14, in which oil, water and gas will be separated in accordance with conventional technique. After degasification and dehydration in this separator 14, the oil is directed to the second step separator 16 for a final degasification and dehydration. Any residual water left in the crude oil will be carried with the oil into the cargo tanks for a final dehydration of the crude oil. Before the crude oil enters the cargo tanks, it will be pumped through a measuring station and cooled to avoid excessive evaporation within the cargo tanks. Produced water from the second step separator 16 is pumped to hydrocyclones 50 to be treated together with water from the first-time separator 14. After the hydrocyclones the produced water enters a final degasification tank/skimmer 52 before being let out into the sea. Recovered oil will be directed to the second step separator 16.
An important feature of the invention is connected to the choice of operating temperature of the first step separator 14. The pressure is set to such a value that the amount of residual gas in the oil will correspond to the amount of fuel gas needed for the steam boilers 46.
Separated (light) gas from the first step separator 14 is directed to the compressor line 40 consisting of two electrically driven piston compressors connected in parallel or possibly a single compressor. By the use of at least one compressor of the piston type, the key capacity factor will be the volumetric flow rate, provided that the motor is powerful enough. The higher the operating pressure of the first step separator 14, the higher the capacity of the piston compressor 40.
Depending on the vessel, the coolers may be based on air or sea-water as the cooling medium. Condensate from scrubbers will be directed back to the separation line or injected into the gas.
Depending on the reservoir and the structure and configuration of the underwater plant, the injection pressure by the re-injection of light gas will vary from 250 to 400 bars, so that power of several MW will be required. For 1 MMSCMD (35 mmscfd) estimated power of about 6 MW is required for an injection pressure of 350 bars. The gas will be compressed to 250-400 bars to ensure injection into the formation and to reduce the volumetric flow rate. By reducing the volumetric flow rate, the internal diameter required for a gas injection pipe may be reduced, and the passages of the umbilical 36 may be utilized.
From the compressor(s) 40 the gas is directed to the umbilical manifold for injection.
The steam generating device 46 incorporated in the generating plant 48 for electrical power, may with advantage consist of two gas-fired boilers, in typical cases generating steam of 18 bars in order to feed a steam turbine 54 and the inlet heater 20 for unprocessed crude oil with associated gas and water. The actual generator of the generating plant 48 driven by the steam turbine 54, is identified by 56.
Electrical power from the generator 56 of the generating plant 48 drives the pumps of the surface plant and the motor 42 of the gas compressor 40 and the separating steps/separators 14 and 16 in the surface plant and also the pumps in the subsea plant.
According to the invention there is no compression of gas from the second separation, which works at near atmospheric pressure, and thereby at least one compression step is saved.
By eliminating the flow of the heavy residual gas, less condensation occurs in the compression line, while at the same time the need for power is reduced. By using heavy fuel gas the C02-tax will be balanced as a consequence of the higher density and calorific value of the gas, and may even balance the use of a low-efficiency power generating system.
Along with its important task as a return/re-injection pipeline for light gas from the first separating step, the umbilical 36 may, in a known manner, receive and transport various chemicals which it is desirable to inject into the reservoir 22 from storage tanks 58, from which branch pipes lead to the umbilical 36 and are in fluid communication therewith. These branch pipelines may for example carry MeOH, boiler composition, emulsion breaker and wax inhibitor.

Claims

C L A I M S
A method for processing/handling hydrocarbons/petroleum recovered from subsea wells (10) and transported up to a surface vessel or corresponding sea-based platform, installation or plant working at surface level, wherein in a surface position at least one separating operation is carried out, in which a portion of separated gas is returned and re-injected into the reservoir, and another portion of separated gas from a second separating operation is converted into another form of energy, for example electrical power or thermal energy to be used for driving components incorporated in a plant for practicing the method, c h ar a c ter i z ed i n that the two-step separation is carried out in a surface position, and that said portion of gas separated in the first separating step and not used, is first carried to a compression device (40) including at least one compressor, and compressed before being returned through one or more passages in an umbilical (36) leading from surface level to seabed level, to the annulus (34) of the subsea well (10) for re-injection into the reservoir (22), and that the pressure of the first separating step (14) is set to such a value that the amount of residual gas in the oil, which is separated in the second separating step (16), will essentially correspond to the amount of fuel gas necessary in order to drive the above-mentioned components .
A method as claimed in claim 1, c h ar ac te r i z ed i n that associated water, if any, is separated from the hydrocarbons carried aboard in the first (14) as well as the second separating step (16), and that the oil-contaminated water separated from the heavy fraction of the hydrocarbons, the oil, is treated in two or more steps, preferably first in a hydrocyclone (50) and then in a skimmer (52).
3. A method as claimed in claim 2, c h ar ac ter i z e d i n that oil separated from the water in the treatment process thereof, is recirculated to the second stage (16) of the separating device.
4. A method as claimed in claim 1, c h ar ac ter i - z e d i n that the setting of the working pressure of the first separating step is carried out by means of a choke valve (18) which works as a pressure regulator and is installed in the tubing string immediately upstream of the first separator (14).
5. A method as claimed in claim 4, c har ac ter i z e d i n that the unprocessed hydrocarbons carried aboard are preheated (in 20) before being transported further into the first separator (14).
6. A method as claimed in any one of the preceding claims, c h ar ac t eri z e d i n that the gas separated from other hydrocarbons in the second separating step is subjected to an energy form conversion process for the generation of electrical power and thermal energy which are used to drive components in the plant arranged, above and under water, for the practicing of the method.
7. A sea-based installation, plant or vessel for processing/handling hydrocarbons/petroleum from a subsea well (10), comprising tubing string (12) for transporting an unprocessed well flow from the well (10) to the well fluid processing equipment (14, 16 etc.) of the plant on surface level, comprising separating devices, in which there is arranged in addition to the tubing string (12) at least one pipe string extending between the surface position and seabed level for excess gas to be returned to the reservoir (22) and re-injected into it, c har acter i z ed in that said installation comprises a multi-passage umbilical (36) which is connected by way of a compression device (40) for the excess gas to an outlet pipeline (38) from the separator (14) of the first separating step, and extends down to the subsea well (10) where its outlet mouth(s) is (are) in fluid communication with the annulus (34) of the well, which serves as a transition chamber by the re-injection into the reservoir (22), and that the separator (14) of the first separating step has a pressure regulator (18) arranged thereto for the setting of the working pressure of this separator (14), so as to adjust the amount of gas separated in the second separator (16) so that it corresponds to the amount of gas needed for said energy/power generation.
A sea-based installation in accordance with claim 7, c h ar ac ter i z e d i n that the pressure regulator for the first separator (14) comprises an adjustable choke valve (18).
A sea-based installation in accordance with claims 7 and 8, c har acter i z ed in that in the inlet pipeline portion (12') of the first separator (14), which is in free fluid communication with the tubing string (12) carrying the well flow from the subsea well (10), there is installed an inlet heater (20) for preheating the well flow before it is directed into the first separator (14), said inlet heater (20) being provided with thermal energy from the steam boiler(s) (46) incorporated in the power-generating device (48).
A sea-based installation in accordance with claim 7 or
8, characteri z ed in that the installation comprising a subsea and a surface plant, includes an energy-generating device (48) which includes, in addition to the steam boiler(s) (46), a connected steam turbine (54) and a generator (56) connected thereto.
PCT/NO2002/000038 2001-02-05 2002-01-30 A method and a sea-based installation for hydrocarbon processing WO2002063135A1 (en)

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