METHOD OF COOLING A MULTIPHASE WELL EFFLUENT STREAM

BACKGROUND OF THE INVENTION

The invention relates to a method of cooling a multiphase well effluent stream.

Such a method is known from OTC paper 17399 "Subsea Gas Compression - Challenges and Solutions" presented by R.Fantoft at the Offshore Technology Conference held in Houston, USA on 2-5 May 2005 and from International patent applications WO30/033870, WO03/035335 and WO 2005/026497. The method known from WO2005/026497 comprises: transferring the multiphase well . effluent mixture via a multiphase well effluent flowline to a gas liquid separator in which the multiphase well effluent mixture is separated into substantially gaseous and liquid fractions;

- transferring the substantially liquid fraction into a liquid flowline in which a liquid pump is arranged;

- transferring the substantially gaseous fraction into a gas flowline in which a gas compressor is arranged;

- protecting the gas compressor against surge by recirculating a recycled gas stream via a gas recycling conduit through the gas compressor in response to detection of the onset of surge at low inlet flowrate to the compressor.

It is desirable to cool the gas prior to compression for reasons of maximizing capacity for a given installed compression power.

It is an object of the present invention to provide an improved method of cooling a multiphase well effluent mixture .

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method of cooling a multiphase well effluent stream, the method comprising:

- separating the multiphase well effluent stream into gas enriched and liquid enriched fractions in a gas liquid separator;

- cooling the liquid enriched fraction in a heat exchanger;

- reinjecting the cooled liquid enriched fraction into the well effluent stream at a location upstream of the gas liquid separator.

The gas liquid separator and heat exchanger may be immersed in (sea)water and the heat exchanger may be cooled by the surrounding (sea)water.

The driving force for the liquid circulation may be provided by the static head between the liquid level in the separator and the injection point. Particular advantages of the method according to the invention are that any gas carry-under to the liquid stream or liquid carry-over to the gas stream are immaterial, hence no level control is needed. The system may therefore consist entirely of static equipment (i.e. requires no pump, no power, no instrumention and no controls) and is therefore extremely robust, solids tolerant and of low cost.

Optionally, the multiphase well effluent stream is transported from one or more gas and/or crude oil

production wells to the gas liquid separator via a multiphase well effluent transportation conduit and the cooled liquid enriched fraction may be reinjected into the multiphase well effluent transportation conduit by means of a jet pump, where the multiphase effluent will be the motive fluid. This will cause a minor drop in pressure of the multiphase effluent.

The gas liquid separator may be a hybrid cyclonic and gravity separator comprising a substantially vertically orientated tubular separation vessel with a liquid outlet near the bottom of the vessel and a gas outlet near the top of the vessel and a substantially tangential multiphase fluid inlet which , is connected to the multiphase we'll effluent ^■ transportation conduit. V ^"

These and other features, embodiments and advantages of the method according to the invention are described in the accompanying claims, abstract and the following detailed description of preferred embodiments in which reference is made to the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.l depicts a schematic view of assembly for use in the method according to the invention; and

FIG.2 depicts a schematic view of a preferred embodiment of the assembly of FIG.l.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG.l depicts a subsea natural gas and/or crude oil production well 1 from which the produced multiphase well effluent stream G+L is transported to a gas liquid

separator 2 via a multiphase well effluent transportation conduit 3, which may be located close to the sea bed 4.

The gas liquid separator 2 comprises a gravity type separation vessel in which a liquid fraction L accumulates at the bottom of the vessel and is discharged into a liquid recycling conduit 5 in which a heat exchanger 6 is arranged in which the recycled liquid is cooled and which recycling conduit discharges recycled cold liquid L _co id into the multiphase well effluent transportation conduit 3, which recycled cold liquid L _co id cools the entire multiphase well effluent stream, including the gaseous fraction, thereby generating a cooled multiphase well effluent stream G+L _CO oi _ed that is discharged via an upper outlet 7 of the gas liquid separator 2.

FIG.2 depicts a preferred embodiment of a gas liquid separator for use in the method according to the invention, wherein the separator comprises a substantially vertically oriented separating vessel 22 into which a multiphase well effluent mixture G+L is fed via a tangential inlet conduit 20 from a multiphase well effluent transportation conduit 23, which is connected to a subsea gas and/or crude oil production well 21. The tangential inlet conduit 20 ensures bulk gas/liquid separation.

In the separator vessel 22 a liquid fraction L accumulates at the bottom of the vessel and is discharged into a liquid recycling conduit 25 in which a heat exchanger 26 is arranged in which the recycled liquid is cooled and which recycling conduit discharges recycled cold liquid L _co i _d into the multiphase well effluent transportation conduit 23, which recycled cold liquid L _co id cools the entire multiphase

well effluent stream, including the gaseous fraction, thereby generating a cooled multiphase well effluent stream (G+L) _co ol _ed that is discharged via an upper outlet 27 of the gas liquid separator 22.

The cold recycled liquid L _co i _d is injected into the conduit 23 through a jet pump 28, which induces the multiphase well effluent stream G+L to suck the recycled cold liquid L _co i _d into the conduit 23, without requiring a recycling pump and such that the recycled cold liquid L _co i _d is intimately mixed with the multiphase well effluent stream G+L and effectively cools said stream.

An advantage of recycling cold liquid into the conduit 23 over arranging a seawater cooled heat exchanger in the conduit 23 itself is that the heat exchanger 6,26 in the liquid recycling conduit is a liquid-liquid heat exchanger, which may be about ten times smaller than a gas-liquid heat exchanger that would be required to cool the potentially predominantly gaseous well effluent stream G+L flowing through the well effluent transportation conduit 3,23. An additional advantage is that the multiphase well effluent may contain solids that could risk significant erosion over time on the heat exchanger if it was arranged in conduit 23. This risk is substantially reduced as the velocity in the cooler 26 is fairly low and it can be arranged such that most of the solids directly leave the separator 20 through conduit 27 rather than be recycled into conduit 25. It may be desired to cool the multiphase well effluent stream if the stream is separated and/or compressed at a location downstream of the heat exchanger 2,22. The flow capacity for given compression suction and discharge pressures will be higher if the temperature of the

compressed gas is lower. Therefore the method according to the invention is suitable for cooling a multiphase well effluent stream in an efficient manner at a subsea location, with a compact liquid-liquid heat exchanger 6,26 and without requiring additional subsea pumping and/or flow regulating means .