AU783668B2 - A trap for particulates - Google Patents

Description

0?-DEC-2001 11:30 FROM A.J.PPRKTO0128399P052 TO 0061262837999 P. 05/25 Regulation 3.2

AUSTRALIA

PATENTS ACT, 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIG]INAL

Name of Applicant: Actual Inventor: Address for service in Australia: Invention Title: VAUSE OEL PRODUCTION SERVICES LTD PETER ALAN VAliSE A J PARK, Level 11, 60 Marcus Clarke Street, Canberra ACT 2601 a TRAP FOR PARTICULATE S The following statement Is a full description of tis invention, including the best method of performing it known to me/us 0?-DEC-2001 11:30 FROM A.J.PPRK T 016879 .62 TO 006126283?999 P.06/25 -2.

FIELD OF THE INVENTION This invention provides a trap for particulates and a method of trapping particulates. In the preferred application of the invention the particulates are small particles known as proppant or frac sand used in some oil field operations.

BACKGROUND OF THE INVENTION In some oil fields, depending on the geology of the field, some or all wells may undergo a frac stimulation to improve the well's production performance.

"Fraccing" involves pumping small sieved, granular silica sand or ceramic carbide spheres (the proppant or frac sand) suspended in a thixotropic pump fluid into the producing rock formation under extreme pressure. The pressure breaks down the rock's structure permitting frac sand to enter tiny fissures opened by the fluid. When the extreme fluid pressure is relaxed, fluid flows back into the well bore and leaves proppant behind in the newly fractured zone, thereby *****increasing the total flow area of the producing formation and increasing well production.

A problem is that not all of the proppant stays in the cracks of the fractured rock. In oil wells most of the excess proppant is carried sedately back to the surface with the returning pump fluid and the "flush flow" oil flowing behind it. In high pressure gas wells the gas, being much lighter and less viscous than pump fluid or oil, breaks through the liquid. As the gas expands on its way to the surface, it increases in velocity. Welhead pressures of up to 5,000 psi are common in n ew gas wells in New Zealand.

Wellhead pressure is contained by a "choke". Typically, the choke is a hardened steel or ceramic coated variable orifice which can hold back wellhead pressure and enable the controlled release of it.

When gas flows through a choke a large pressure drop can occur. A drop in the press=r of the gas means an increase in the velocity of the gas flow. A large pressure drop means a large increase in velocity; quite often to supersonic speed. Even though the proppant particles are 07-DEC-2001 11:31 FROM A.J.PARK T 016879 .72 TO 0061262e3?999 P.07/25 -3small, typically having a diameter of about 0.3mm, when they are entrained in gas moving at high velocity and perhaps being accelerated to supersonic speeds, they became abrasive missiles. Even hardened steel I10mm. thick can be penetrated in seconds.

"Washed out" chokes, wellhead valves and flow lines are very dangerous and also very common.

In the past, wells were "clean up" flowed to a flare pit for whatever time it took for the well to flow cleanly. Frac equipment has been sacrificed knowingly, as required to keep personnel safe.

Such practices are costly and polluting and it is now considered that such practices should be avoided. It was with this in mind that the present invention was devised.

SUMMARY OF THE PNENTION The proppant can be filtered from the gas flow. However, if it is filtered from the gas flow after the choke then that part of the flow line between the wellhead and the choke and especially is between the choke and the filter can still be damaged by the proppant when moving at high velocities. Apart from that, the filter element used to catch the high velocity proppant is itself soon damaged. An object of the present invention is to allow the removal of the proppant before :5 :the choke. This protects the choke. Furthermore, it is an object of the preferred embodiment of the invention to filter out the proppant while the gas is still at a relatively high pressure and a relatively low velocity before the choke. It is at the choke that the drop in pressure occurs causing the gas to flow at high velocities but at that stage the proppant has been removed.

In one aspect, the present invention broadly consists in a trap for particulates, the trap being connected or connectable in a fluid flow line between a wellhead and a choke and having: a casing defining a chamber and having an inlet to the chamber which is directly or indirectly connected or connectable to the wellhead and an outlet from the chamber which is directly or indirectly connected or connectable to the choke; a filter element at the outlet whereby particulates entrained in fluid flowing through the chamber from the inlet to the outlet are retained in the chamber; and access means whereby the casing may be opened to allow accumulated particulates to be cleared from the chamber.

07-DEC-2001 11:31 FROM A.J.PPRK TO 0061262837999 P.08/25 -4- Preferably the wellhead is a wellhead for an oil and/or gas well.

Preferably the particulates comprise frac sand or proppant.

Preferably the trap has an elongated casing defining an elongated chamber and having an inlet end with the inlet to the chamber and an outlet end with the outlet from the chamber.

Preferably the trap has an inlet boss attached at the inlet end of the casing and the inlet to the chamber passes through the inlet boss.

I0 Preferably the inlet boss is screw-threadedly attached to the casing.

Preferably the inlet boss is connected or connectable to an inlet flange which is attached or attachable to an inlet pipe.

Preferably the inlet flange is bolted to the inlet boss.

Preferably a gasket is disposed between the inlet boss and the attached inlet flange.

Preferably the trap has pressure indicating means to indicate the differential pressure across the trap.

Preferably the trap has a pressure gauge to indicate the pressure at the inlet end of the chamber.

Preferably the trap has a pressure gauge to indicate the pressure at the outlet end of the chamber.

Preferably the trap has an outlet boss attached at the outlet end of the casing and the outlet to the chamber passes through the outlet boss.

Preferably the outlet boss is screw-threadedly attached to the casing.

Preferably the outlet boss is connected or connectable to an outlet flange which is attached or 07-DEC-2001 11:31 FROM A.J.PF1RK T 016879 .92 TO 0061262837999 P.09/25 attachable to an outlet pipe. This feature preferably provides the access means, the outlet flange when not attached to the outlet boss being movable away from the casing to open the outlet end of the chamber for access for removal of particulates accumulated in the chamber.

Preferably the outlet flange is bolted to the outlet boss.

Preferably a gasket is disposed between the outlet boss and the attached outlet flange.

Preferably the filter element is attached to the outlet flange so that removal of the outlet flange from the casing also removes the filter element from the casing. The filter element is preferably detachable from the outlet flange to allow its replacement. The filter element may be screw- :threadedly attached to the outlet flange.

Preferably the trap has supporting means to support the weight of the outlet flange and the attached filter element when these are separated from the outlet boss.

Preferably the supporting means comprises a gantry rail attached to the casing so that a distal end thereof extends abovc and beyond the outlet end of the casing, and a traveller on the rail which is connected to the outlet flange.

Preferably the filter element comprises a perforated tube attached at one end to the outlet flange and closed at its distal end and covered between its ends with a tubular screen. Such a filter element may project inwardly into the chamber of the trap from the outlet end thereof. However, the filter element preferably does not project as far as the inlet end of the trap. In other words, it is preferred that the filter element is clear of the inlet to the chamber.

Preferably the filter element is capable of screening out particles having a diameter of at least about 0.3mm. Such particles preferably comprise frac sand or proppant consisting, for example, of ceramic carbide spherical particles.

Preferably the casing and the bosses of the trap are made of high tensile steel. Preferably the flanges and the bolts used to attach them to the bosses are also made of high tensile steel.

07-DEC-2001 11:32 FROM A.J.PARK T 016879 .02 TO 006126283?999 P.10/25 -6- Preferably the trap has no welded joints.

In another aspect, the present invention broadly consists in an oilfield or gasfield installation having: a wellhead; a choke; a fluid flow line between the wellhead and the choke; and a trap for particulates as defined above located in the fluid flow line between the wellhead and the choke, Preferably the installation has a second trap whereby one trap may be operational while the other is closed off to allow it to be emptied of accumulated particulates. The two traps may be connected in parallel between the wellhead and the choke. Alternatively, each trap may be connected to a separate choke.

In yet another aspect the present invention broadly consists in a method of trapping particulates entrained in a fluid flowing through a fluid flow line between a wellhead and a choke, the method having thle steps of: including in the flow line a trap for particulates as defined above; and inducing fluid having particulates entrained therein to flow from the wellhead to the choke and thereby through the trap where the filter element retains the particulates in the chamber.

Preferably the fluid at the wellhead is at high pressure and the fluid is induced to flow in the flow line by the choke being opened to at least some extent.

Preferably, where the pressure of the fluid at the wellhead is such as to enable fluid to flow through the at least partly opened choke at supersonic speeds, the choke is operated so that the fluid flows from the wellhead to the choke at subsonic speeds.

BRIEF DESCRIPTION OF THE DRAWINGS The above has broadly defined the present invention, a preferred embodiment of which will now 07-DEC-2001 11:32 FROM A.J.PPIRK T 016879 .12 TO 0061262e3?999 P.11/25 -7-m be described with reference to the accompanying drawings, in which: Figure 1 is a side view showing the general arrangement of a trap for particulates connected between a welhead and a choke, the position of the filter element in the trap being shown by broken lines; Figure 2 shows a side view, partly broken away, of the filter element; Figure 3 shows a side view, the lower half in cross-section, of the inlet end of the trap for particulates; and Figure 4 shows a side view, the lower half in cross-section, of the outlet end of the trap for particulates.

DETAILED DESCRIPTION OF THE INVENTION Referring to Figure 1, a trap 10 for particulates is located in a fluid flow line between a wellhead 12 and a choke contained within a choke box. The choke box is not shown in the drawings but would usually be attached to the flange 14 at the distal end of the outlet pipe 16 at 20 the outlet end of the trap.

In a preferred embodiment of the invention, the trap 10 comprises an elongated tubular casing 18 defining a chamber 20 and having an inlet end 22 and an outlet end 24. At the inlet end there is an inlet 26 (see Figure 3) which, in use, is directly or indirectly connected to the wellhead 12 or, more particularly, to the so-called Christmas tree structure at the wellhead. At the outlet end there is an outlet 28 (see Figure 4) which, in use, is directly or indirectly connected to the choke, for example, by way of the outlet pipe 16.

In Figure 1, the trap 10 is supported in a horizontal position above the ground 3 0 by two legs 3 2, each having a cradle 34 at its upper end to receive the casing 18 of the trap. The trap is thereby supported at a height to suit the height of the Christmas tree structure at the wellhead 12.

07-DEC-2001 11:33 FROM A.J.PARK T 016879 .22 TO 0061262e3?999 P.12/25 -8- The trap 10 has an inlet boss 36 attached at the inlet end 22 of the casing and the inlet 26 through the chamber 20 passes through the inlet boss. Similarly, the trap has an outlet boss 38 attached at the outlet end 24 of the casing and the outlet 28 to the chamber passes through the outlet boss.

Both the inlet and outlet bosses are preferably screw-threadedly attached to the casing, for example, with standard casing buttress threads that are rated for the selected casing grade.

In the embodiment of trap shown in the drawings, the inlet boss 36 is, in use, connected to an inlet flange 40 which, in use, is attached to an inlet pipe 42 from the wellhead 12. In the illustrated embodiment, the inlet flange is bolted to the inlet boss and a gasket 44 (see Figure 3) is disposed between them to effect a seal. Similarly, at the outlet end of the trap, the outlet boss 38 is, in use, connected to an outlet flange 46 which is, in use, attached to the outlet pipe 16. In the illustrated embodiment, the outlet flange 46 is bolted to the outlet boss 38 with a gasket 48 (see Figure 4) 0:between them to effect a seal.

The caing 18 ispreferably mae from ahigh tensile steel tube, for example, C95 grade steel. Its length and diameter can be chosen to suit considerations such as the expected operating conditions and cost. For example, the length may be in the order of 6-7 metres and the diameter in the order of 120-800mm. The larger sizes of casing can obviously trap more particulate matter 0 o: such as proppant before having to be emptied. However, an advantage of a casing having a 20 length not exceeding 6 metres is that it can be transported in a standard size of container. The inlet and outlet bosses 36 and 38 are preferably machined from blocks of high tensile steel, the outer end of each bos being machined and studded to suit the mating inlet or outlet flange choice.

The bolts used to attach the inlet and outlet flanges 40 and 46 to the inlet and outlet bosses respectively are also preferably made from high tensile steel. Suitable gaskets are A.P.I.

(American Petroleum Institute) ring gaskets made of soft steel.

The use of high tensile steel over weldable steel is preferred for several reasons. The casing made of high tensile steel means that the casing can be thinner and lighter than if weldable steel were used. The inlet and outlet bosses and flanges may also be less bulky and lighter when made of high tensile steel. Furthermore, use of high tensile steel means it is acceptable to have the end bosses 36 and 38 screw-threadedly attached to the casing 18, despite the high pressures that the trap may be exposed to in use. The use of screw-threaded fittings in such a piece of apparatus is 07-DEC-2001 11:33 FROM A.J.PARK T 016879 .32 TO 0061262e37999 P.13/25 -9novel in high pressure situations. These features mean that welded joints can be avoided.

Welded joints can introduce weaknesses into high pressure equipment and involve more work in that heat treating of welds is usually necessary to avoid unduly brittle joints.

From the inlet 26, the fluid flow line opens into the chamber 20. The chamber has a larger cross-sectional area than the inlet and functions as an expansion chamber. Therefore, as oil and/or gas flows from the inlet and into the chamber when the choke is opened, the velocity of the flow reduces. This in turn means a reduction in the velocity of the proppant entrained in the fluid. Because of the density of the proppant some of this may settle out in the chamber before it reaches the outlet end of the chamber.

The trap 10 includes a tubular filter element 50 of a lesser diameter than that of the casing. The filter element shown in Figure 2 comprises a perforated steel tube 52 which has screw threads 54 at one end and a mild steel plug 56 at the other end. The perforated length of the tube 52 is covered by a 0.25mm slot well screen 58 to prevent the 0.3mm frac sand particles from passing through the filter element. The screen may be made of stainless steel. For a trap having a length in the order of 6-7 metres, the filter element may have a length in the order of 2.4-2.5meters and diameter of about 70-80mm but other dimensions may be used.

20 The fact that the filter element projects into the chamber from the outlet end means firstly that the filter element itself does not fill with proppant and secondly that a large filtering surface area is presented to the incoming fluid with its entrained proppant. The filter element does not extend as far as the inlet end of the trap and is therefore clear of the inlet to the chamber. This allows the expansion provided by the chamber to have some effect in reducing the velocity of the proppant before it reaches the filter element. Also, most of the volume of the chamber is available for collection of proppant though the chamber would usually be emptied before it filled to an extent such as to seriously inhibit fluid flow through it.

As best shown in Figure 4, the filter element 50 is screw threadedly attached in the outlet 28 where the outlet passes through the outlet flange 46. This arrangement allows the filter element to be replaced. Furthermore, the direct connection of the filter element to the outlet flange means that when the bolts securing the outlet flange 46 to the outlet boss 38 are removed to allow the 07-DEC-2001 11:34 FROM FA.J.PARK T 016879 .42 TO 0061262837999 P.14/25 outlet flange to be moved away from the outlet boss, the filter element is removed together with the outlet flange. This leaves that part of the outlet 28 passing through the outlet boss open whereby particulate material which has accumulated within the chamber of the trap can be removed. If necessary, the outlet boss can itself be removed from the outlet end of the casing to facilitate this clearing of the chamber. Of course, a valve in the flow line on the wellhead side of the trap, for example, at or towards the wellhead, would be closed before the outlet end of the trap was opened.

In opening the outlet end of the trap and removing the outlet flange 46 and attached filter element 50, their significant weight must be supported. While this can be done manually, the embodiment shown in Figure 1 is provided with a gantry-like crane structure 60 to assist. This structure comprises a rail 62 extending above and beyond the outlet end of the trap. The proximal end of the rail is supported by uprights 64 attached to collars 66 about the casing 18. A traveller crane 68 is movable along the rail. This traveller crane is attached to an eyebolt 70 which is in :15 tur attached to the outlet flange 46. When the bolts attaching the outlet flange to the outlet boss are removed, the weight of the outlet flange and attached filter element is largely borne by the traveller crane which can then be moved out along the rail to facilitate removal of the outlet flange and filter element from the rest of the trap. The flange 14 on the outlet pipe 16 can be 4 disconnected from the choke box to permit this.

A pressure gauge 72 is located at the inlet end of the trap to measure the pressure at the inlet end of the chamber 20. Similarly, a pressure gauge 74 is located at the outlet end of the trap to measure the pressure at the outlet end of the chamber. These pressure gauges can be used to indicate the pressure differential across the trap and thereby indicate when the chamber should be cleared of accumulated particulate materials.

In an oilfield or gasfield application where high pressures are involved, the trap is located upstream of the choke. This ensures that the trap is dealing with fluid (with proppant entrained) at relatively high pressures but at realtively low velocities. Downstream of the choke the fluid pressures are much lower and the velocities much higher but the fluid is no longer carrying proppant with it and so there is much less wear on those downstream parts of the fluid flow line where the fluid travels at high velocities.

07-DEC-2001 11:38 FROM A.J.PlRK T 016879 .52 TO 006126283?999 P.15/25 11 The trap may be constructed on site or else may be constructed off site and subsequently installed.

Because it is necessary for fluid flow through the trap to be stopped when the trap is opened for cleaning, it is desirable that there be at least two traps associated with any wellhead so that one can continue in use while the other is being cleaned. In one embodiment, two traps may be connected in parallel, these having a common choke. In another embodiment each trap may be connected to a separate choke.

The above has described a preferred embodiment of the invention and has indicated a number of possible modifications. However, other modifications can be made without departing from the scope of the invention as has been broadly defined.

:For example, the trap may have other shapes provided that the trap is sufficiently strong in its construction to withstand the high pressures it may be subject to, for example, pressures in excess of 5,000 psi. Furthermore, the capacity of the trap should be sufficient to enable its use for a :15 reasonable period of time before removal of the accumulated particulate matter in the trap becomes necessary. Indeed, it may be desirable that the trap has a capacity sufficient to receive all the flow back proppant volume likely from a particular well. In one well, that volume was in the order of 22 cubic feet.

20 Where the trap has inlet and outlet boss ends, these can be attached to the casing in other ways, for example, by being bolted to flanges attached to the casing or even welded to the casing, though this latter option is not preferred as it makes it difficult to replace the bosses should that become necessary. Similarly, the inlet and outlet flanges 40 and 46 may be attached to the inlet and outlet bosses 36 and 38 by means other than bolts. For example, these attachments could be a means of threaded unions ("hammer" unions) or Unibolts (trade mark). These would allow faster removal and then replacement of the outer flange.

The filter element may use a section of standard down hole Houston brand sand screen commonly run on the bottom of well casings to keep loose formation fines and sand from blocking the flow tubing.

Whereas other filters catch the debris in the filter element, the preferred trap of the present 07-DEC-2001 11:39 FROM A.J.PARK TO 0061262837999 P.16/25 -12invention prevents the debris from entering the filter element. The casing can handle severe pressure and is large enough to catch and contain sufficient debris to prevent cleaning becoming an onerous round the clock chore. The casing traps the proppant and the filter screen ensures that only clean gas/oil passes the trap to the choke.

The trap may be designed for use in other applications and for trapping other types of particulates.

The nature of the filter element would be changed to suit the types and sizes ofparticulates to be filtered.

**ee oo e0

Claims (15)

1. 07-DEC-2001 11:39 FROM A.J.PARK TO 0061262837999 P.17/25 13 CLAIMS 1. A trap for particulates, the trap being connected or connectable in a fluid flow line between a wellhead and a choke and having: a casing defining a chamber and having an inlet to the chamber which is directly or indirectly connected or connectable to the wellhead and an outlet from the chamber which is directly or indirectly connected or connectable to the choke; a filter element at the outlet whereby particulates entrained in fluid flowing through the chamber from the inlet to the outlet are retained in the chamber; and access means whereby the casing may be opened to allow accumulated particulates to be cleared from the chamber. 2. A trap according to claim I wherein the wellhead is a wellhead for an oil and/or gas well. :15 3. A trap according to claim I or 2 wherein the particulates comprise firac sand or proppant. 4. A trap according to any one of the preceding claims wherein the trap has an elongated casing defining an elongated chamber and having an inlet end with the inlet to the chamber and an outlet end with the outlet from the chamber. A trap according to any one of the preceding claims wherein the trap has an inlet boss attached at the inlet end of the casing and the inlet to the chamber passes through the inlet boss. 6. A trap according to claim 5 wherein the inlet boss is screw-threadedly attached to the casing. 7. A trap according to claim 5 or 6 wherein the inlet boss is connected or connectable to an inlet flange which is attached or attachable to an inlet pipe.
2. 8. A trap according to claim 7 wherein the inlet flange is bolted to the inlet boss. 07-DEC-2001 11:39 FROM R.J.PRRK TO 0061262837999 P.18/25 -14-
3. 9. A trap according to claim 7 or 8 wherein a gasket is disposed between the inlet boss and the attached inlet flange. A trap according to any one of the preceding claims wherein the trap has pressure indicating means to indicate the differential pressure across the trap.
4. 11. A trap according to ay one of the preceding claims wherein the trap has a pressure gauge to indicate the pressure at the inlet end of the chamber.
5. 12. A trap according to any one of the preceding claims wherein the trap has a pressure gauge to indicate the pressure at the outlet end of the chamber.
6. 13. A trap according to any one of the preceding claims wherein the trap has an outlet boss attached at the outlet end of the casing and the outlet to the chamber passes through the outlet boss. *14. A trap according to claim 13 wherein the outlet boss is screw-threadedly attached to the casing.
7. 15. A trap according to claim 13 or 14 wherein the outlet boss is connected or connectable to an outlet flange which is attached or attachable to an outlet pipe.
8. 16. -A trap according to claim 15 wherein the outlet flange provides the access means, the outlet flange when not attached to the outlet boss being movable away from the casing to open the outlet end of the chamber for access for removal of particulates accumulated in the chamber.
9. 17. A trap according to claim 15 or 16 wherein the outlet flange is bolted to the outlet boss.
10. 18. A trap according to claim 17 wherein a gasket is disposed between the outlet boss and the attached outlet flange. 07-DEC-2001 11:40 FROM A.J.PARK TO 0061262837999 P.19/25 15
11. 19. A trap according to any one of claims 15-18 wherein the filter element is attached to the outlet flange so that removal of the outlet flange from the casing also removes the filter -element from the casing.
12. 20. A trap according to claim 19 wherein the filter element is detachable from the outlet flange to allow its replacement.
13. 21. A trap according to claim 20 wherein the filter element is screw-threadedly attached to the outlet flange.
14. 22. A trap according to any one of claims 19-21 wherein the trap has supporting means to support the weight of the outlet flange and the attached filter element when these are separated from the outlet boss.
15. 090. :15 23. A trap according to claim 22 wherein the supporting means comprises a gantry rail attached to the casing so that a distal end thereof extends above and beyond the outlet end of the ****casing, and a traveller on the rail which is connected to the outlet flange. A trap according to any one of claims 19.23 wherein the filter element comprises a perforated tube attached at one end to the outlet flange and closed at its distal end and covered between its ends with a tubular screen. A trap according to claim 24 wherein the filter element projects inwardly into the chamber of the trap from the outlet end thereof. 26. A trap according to any one of the preceding claims wherein the filter element is clear of the inlet to the chamber. 27. A trap according to any one of the preceding claim wherein the filter element is capable of screening out particles having a diameter of at least about 0.3mm. 0?-DEC-2001 11:40 FROM ALJ.PARK T 016879 .02 TO 006126283?999 P.20/25 16 28. A trap according to any one of claims 5-9 and any one of claims 13-23 wherein the casing and the bosses of the trap are made of high tensile steel. 29. A trap according to any one of claims 7-9 and any one of claims 15-23 wherein the flanges and any bolts used to attach them to the bosses are made of high tensile steel. A trap according to any one of the preceding claims wherein the trap has no welded joints. 31. A trap substantially as herein described with reference to the accompanying drawings. 32. An oifield or gasfield installation having: a wellhead; a choke; :15 a fluid flow line between the wellhead and the choke; and a trap for particulates as claimed in any one of the preceding claims located in the fluid flow line between the wellhiead and the choke. 33. An installation according to claim 32 wherein the installation has a second trap whereby one trap may be operational while the other is closed off to allow it to be emptied of accumulated particulates. 34. An installation according to claim 33 wherein the two traps are connected in parallel between the wellhead and the choke. An installation according to claim 33 wherein each trap is connected to a separate choke. 36. An installation substantially as herein described with reference to the accompanying drawings. 37. A method of trapping particulates entrained in a fluid flowing through a fluid flow line 07-DEC-2001 11:40 FROM A.J.PARK TO 0061262837999 P.21/25 -17- between a wellhead and a choke, the method having the steps of: including in the flow line a trap for particulates as claimed in any one of claims 1-31; and inducing fluid having particulates entrained therein to flow from the wellhead to the choke and thereby through the trap where the filter element retains the particulates in the chamber. 38 A method according to claim 35 wherein the fluid at the wellhead is at high pressure and the fluid is induced to flow in the flow line by the choke being opened to at least some extent. 39. A method according to claim 36 wherein, where the pressure of the fluid at the wellhead is such as to enable fluid to flow through the at least partly opened choke at supersonic speeds, the choke is operated so that the fluid flows from the wellhead to the choke at subsonic speeds. 40. A method according to claim 35 and substantially as herein described with reference to 15 any embodiment disclosed. S Dated this 7th day of December 2001 By their Patent Attorneys AJ PARK 20 Onbe fo theApplicant