Nuclear magnetic resonance apparatus for measuring properties of a fluid stream flowing within a pipeline has one or more magnet systems for applying magnetic field to the fluid stream and also has means for inducing and observing magnetic resonance within the fluid stream as it passes through a said magnetic field. The apparatus may also include a polarizing magnetic field upstream of the magnetic field in which resonance is observed. The fluid stream may be hydrocarbon from an underground reservoir. In order to guard against accumulation of magnetisable iron debris particles entrained in the fluid flow, the apparatus comprises one or more upstream traps having a magnetic field to attract and hold solid magnetizable material and an exit path for the removal of the solid magnetizable material so that it does not continue towards any polarizing field and the field where resonance is observed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to British Application No. GB1217402.5 filed 28 Sep. 2012, which is incorporated herein by reference in its entirety.

BACKGROUND

There have been a number of proposals to use nuclear magnetic resonance (NMR) variously referred to as magnetic resonance imaging (MRI) for examining a flowing stream of fluid in the pipeline. The fluid may be liquid hydrocarbon or a mixture of hydrocarbon and aqueous phases, such as the flow produced from an underground reservoir. Some gas may be present in the fluid. Use of NMR for such flow measuring/monitoring was disclosed in U.S. Pat. Nos. 3,191,119 and 3,419,715 and has been the subject of other patents since then, including U.S. Pat. Nos. 6,046,587, 7,501,819, 7,852,074 and 7,872,474. In a number of these documents the apparatus is shown as having a pipeline which passes through the magnetic field used (together with radiofrequency signals) to induce and observe magnetic resonance within the pipeline. The apparatus may also include polarizing magnets upstream of the magnets used to bring about magnetic resonance. NMR apparatus such as described in these documents has the attraction that it can be carried out without requiring any parts to be placed in the pipeline.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below. This summary is not intended to limit the scope of the subject matter claimed.

The present inventors have recognized that such NMR apparatus is at risk of becoming a trap for magnetizable particulate material which is present in the wellbore flow. Such particles could accumulate within a magnetic field provided by the apparatus. This may be detrimental to flow in the pipeline and/or detrimental to operation and accuracy of the NMR apparatus. The term “magnetizable material” is used here to denote material which can be drawn to and held by a magnetic field. Such materials may be ferromagnetic or ferrimagnetic and may or may not have a magnetic field of their own. Solid magnetizable material encountered in a flow of liquid may well be particles of, or containing, iron.

In the case of fluid flowing from an underground hydrocarbon reservoir, the magnetizable particulates may predominantly comprise iron debris which is small fragments of iron broken off or scraped like iron filings from the drillstring and wellbore tools and left in the wellbore while drilling and completing the well. The magnetizble material may also include small objects lost in the well and may include magnetite (mixed iron (ii) and iron (iii) oxide, Fe3O4) which is ferrimagnetic and may be present as an impurity in materials such as barite used as weighting agent.

The present inventors envisage trapping magnetizable particulates upstream of magnetic resonance equipment used to examine a flow of fluid. Disclosed herein is NMR apparatus for measuring properties of a fluid stream flowing within a pipeline (and which may originate from an underground reservoir) comprising one or more magnet systems for applying one or more magnetic fields to the fluid stream and means for inducing and observing magnetic resonance within the fluid stream as it passes through a said magnetic field, wherein the apparatus comprises one or more traps having a magnetic field to attract and hold solid magnetizable material and an exit path for the removal of the solid magnetizable material

Also disclosed herein is a method of intercepting solid magnetizable material entrained in a fluid flow in a pipeline upstream of a magnetic resonance spectrometer, comprising: directing the flowing fluid upstream of the spectrometer through at least one trap having a magnetic field to attract and hold solid magnetizable material and an exit path for the removal of the solid magnetizable material.

The magnetic resonance section of the apparatus may be of known type. The apparatus may include a polarising magnetic field to polarise nuclear spins before the flowing liquid enters the magnetic field in which magnetic resonance is induced and observed. If a polarising magnetic field is provided the trapping of magnetizable solids may take place upstream of the polarizing magnetic field. As disclosed in U.S. Pat. No. 4,259,638 a polarising magnetic field may be provided by a magnet in which superconducting properties give a magnetic field of high strength. However, the constructional arrangement to incorporate a superconducting magnet may add to the inconvenience and difficulty of removing any solid magnetizable material which reaches the polarizing magnetic field, thus making it even more advantageous to intercept and remove solid magnetizable material.

A trap for magnetizable solids as disclosed herein utilises a magnetic field to arrest the travel of the magnetizable solids entrained in the fluid stream. Removal of the magnetizable solids from the path of flow may be done by taking the magnetizable solids into a branch from the pipeline and configuring this branch to provide an exit path or it may be done by temporarily disconnecting a portion of pipeline or a branch pipe in which the magnetizable solids have accumulated. Both of these approaches can remove the magnetizable material without allowing it to travel on towards a polarising magnet or the location at which magnetic resonance is induced and observed.

When removing accumulated magnetizable solids, the magnetic field which was used to arrest the magnetizable solids may be removed, for instance by moving permanent magnets or turning off an electromagnet, or the portion of pipeline or branch pipe in which the magnetizable solids have accumulated may be moved out of the magnetic field.

Trapping of magnetizable material may be carried out using a single trap, in which case the flow of liquid in the pipeline may be stopped temporarily when removal of accumulated magnetizable solids is required. Alternatively, trapping of magnetizable material may be carried out using two or more traps so that one trap is available to collect material while already-trapped material is being removed from another. One possibility is to provide two traps in parallel, with valves to direct flow selectively through one or the other. Another possibility is to provide two traps connected in sequence so that flow passes through both and each trap is configured for drawing the trapped material into an exit portion which branches off the path of flow, and allows removal of material while flow continues.

DETAILED DESCRIPTION

Embodiments of the apparatus and method disclosed herein and further features which may be used will now be described with reference to the accompanying drawings. This description is exemplary in nature and is not intended to limit the scope of the subject matter disclosed and claimed.

FIG. 1shows a general arrangement of NMR apparatus for examining properties of fluid flowing along a pipeline10in the direction indicated by arrows. In the portion A, the pipeline10is made of non-magnetic electrically insulating material such as fibre reinforced polymer. The pipeline10extends through a uniform magnetic field between a pair of permanent magnets11. Within this field there is at least one radio-frequency coil12encircling the pipeline and used to emit radiofrequency pulses to induce magnetic resonance and also to receive signals from nuclei undergoing resonance. The magnets11, the coil12and associated electronics for creating radiofrequency pulses, observing radiofrequency emissions and storing data may all be conventional in construction and operation.

Upstream of this portion A of the apparatus there is a polarizing portion B. Here too the pipeline10is made of non-magnetic material although electrically conducting material such as aluminium or stainless steel may be used. Magnets13provide a magnetic field to polarize resonant nuclei in the liquid flowing in the pipeline10, before the liquid reaches the magnetic resonance portion A. These magnets may be made of material with so-called high-temperature superconducting properties (superconductivity above 77 K) such as yttrium barium copper oxide (YBCO), bismuth strontium calcium copper oxide (BSCCO) and the materials mentioned in W02007/045929 and in Coombs et al, Superconductor Sci. & Tech. Volume 21, article 034001 (2008). The portion C is a trapping section for magnetizable material in the flowing fluid. Possible embodiments of the trapping section C will next be described.

FIG. 2shows a simple possibility for trapping magnetizable solids. A branch pipe14is connected to the pipeline10through a gate valve16and is closed at its other end by gate valve17. The branch pipe14is surrounded by a solenoid coil18. In operation with valve16open, valve17closed and the solenoid coil18energised, the magnetic field within the solenoid coil attracts magnetizable solids out of the fluid stream in the pipeline10into the branch pipe14. Periodically the valve16is closed, the coil18is turned off and magnetizable solids which have collected in the branch pipe14are discharged through valve17. During this removal of accumulated magnetizable solids, flow along the pipeline10may be temporarily halted by closing valves19.

FIG. 3shows a possible arrangement of the trapping section C with traps in parallel. The incoming pipeline splits into two parts22which lead to valves24,25. Further on the two flow paths pass through valves26,27and then rejoin. Flow in the rejoined path28continues into the polarizing section B and the magnetic resonance section A. Between the valves24and26and between the valves25and27there are lengths of pipe30,31made of non-magnetic material such as aluminium or stainless steel and attached at flanges32,33. These lengths of pipe30,31pass through magnetic fields transverse to the pipe created by pairs of magnets, one above and one below the pipe. The magnets above the pipes are indicated34inFIG. 3. While flow is passing through a pipe30or31, the magnetic field draws any magnetizable contaminants to the pipe wall and holds them there. Thus, each length of pipe30,31functions as a trap for magnetizable contaminants.

The valves24-27are used to direct flow alternately through the lengths of pipe30and31. So, when valves24and26are open and flow passes through pipe30, the valves25and27may be closed. The length of pipe31can then be temporarily detached by unbolting at the flanges33, removed from the magnetic field, cleaned out and replaced. Subsequently, when valves25and27are open to direct flow through pipe31, the valves24and26can be closed, allowing pipe30to be unbolted at flanges32, removed, cleaned out and replaced.

FIG. 4shows another possible arrangement for the trapping section C. The pipeline40follows a serpentine path and has two traps41,42in sequence at low points of the pipeline. Each of these traps has a gate valve44opening and closing connection between the pipeline40and a length of pipe46connected at flanges48. To provide a magnetic field for trapping magnetizable contaminants, a probe consisting of rod shaped steel pole piece50attached to a permanent magnet52is mounted so that the pole piece50extends through a seal54into the pipe46.

Each probe is movable between two positions. As shown in the trap41at the left ofFIG. 3, the probe can be pushed inwardly towards the pipeline40so that the distal part56of the pole piece50projects through the gate valve44into the pipeline40. In this position the magnetic field from the magnet52attracts any magnetizable material in the flowing stream onto the pole piece50and holds them on it.

When it is desired to remove accumulated contaminants from one of the traps, the probe is withdrawn longitudinally to a position as shown in the trap42at the right ofFIG. 3. The distal part56of the pole piece50has moved down into the pipe46, taking accumulated magnetizable material with it. The valve44can then be closed and the pipe46can be detached at the flanges48allowing the pipe46and the probe50,52to be removed, cleaned of accumulated magnetizable material and then replaced.

The two traps41,42are operated alternately. At all times one or other of the traps has its magnetic pole piece50pushed in, as shown in trap41at the left ofFIG. 3, so that it can attract and retain magnetizable material. Periodically, the probe52,50of one or other of the two traps is moved to the position as shown by the trap42, for the accumulated magnetizable material to be removed from it.

FIGS. 5 and 6show a further possibility for a trapping section C. A pipeline60carrying flow in the direction shown by arrows has two traps at position62and two more at position64.FIG. 6is a cross section at position62showing that the two traps at this position extend downwardly at an angle at each side of the pipeline60. The traps are somewhat similar to the traps inFIG. 4. Each trap communicates with the pipeline60through a gate valve44and has a pipe46attached at flanges48. The traps have probes comprising electromagnets69and pole pieces66,67which extend through seals54. The probes are movable between two positions. A pushed-in position is shown by pole piece67and also at the right ofFIG. 5. In this position the distal part68of the pole piece extends across the pipeline60and its magnetic field attracts and holds any magnetizable solid material. The pole piece can also be drawn back into the pipe46, through gate valve44, as illustrated by pole piece66. When the pole piece is drawn back, as in the case of pole piece66, it carries accumulated magnetizable material with it into the pipe46. After a pole piece has been withdrawn as shown by pole piece66, the gate valve44is closed and the electromagnet69for that pole piece is switched off. Accumulated magnetizable material on the pole piece66then falls off into the pipe46, and can be discharged through a valve71.

The four traps are operated in a sequence such that at any time at least one trap has its pole piece pushed in and extending across the pipeline60. While the pole pieces of the two traps at position62are moved to allow one of the traps to be cleaned out, one trap at position64always has its pole piece pushed in and functioning to collect magnetizable material. Correspondingly one trap at position62has its pole piece in the pushed in position throughout the time that pole pieces of the traps at position64are moved between their two positions.

FIG. 7shows a single trap with some resemblance to the traps inFIG. 4. As inFIG. 3, a low point of pipeline40is connected to a branch pipe46through a gate valve44. A probe consisting of a pole piece73and an electromagnet69is located above the pipeline and extends into the pipeline through a seal75, so that the distal part76of the pole piece73, magnetised by the electromagnet69arrests and holds magnetizable solids entrained in the flow stream along the pipeline40. Periodically, flow is stopped by closing valves78. The pole piece73and the electromagnet69are lowered, so that the distal part76of the pole piece carrying magnetizable solids accumulated on it passes through the open valve44into the branch pipe46. The electromagnet69is then switched off, so that the magnetizable solids fall from the pole piece73into the branch pipe. The pole piece and electromagnet are then raised back to the position shown, with the electromagnet still switched off. The valve44is closed and the magnetizable solids in the branch pipe46are discharged by opening the valve79.

FIG. 8shows a further possibility for a single trap. The pipeline80has a downwardly inclined tubular section82at the lower end of which a branch pipe46is connected to the pipeline80through a gate valve44. A magnetic field to arrest magnetizable solids is provided by a pair of permanent magnets84located at either side of the inclined section82. These magnets84are supported on pivoted arms86. Periodically, to remove accumulated magnetizable solids from the inclined section82, flow is stopped with by closing valves88and the arms86and magnets84are swung to put the magnets84at the position shown by a dotted circle87, with the magnetic field now extending through the branch pipe46. This movement of the magnetic field allows and assists the accumulated magnetizable solids to slide down the inclined section82into the branch pipe46. The gate valve44is then closed, the magnets84are then swung back to their original position and the magnetizable solids are discharged from the branch pipe46by opening valve89.

It will be appreciated that the diagrams shown here are schematic and do not show the equipment used to move the magnets52,69and pole pieces50,66,6773between positions, nor the mechanical handling equipment used to move detached pipes30,31or46. Many modifications are possible and features used in one embodiment illustrated here may be utilised in another embodiment. Specifically, the single traps shown inFIGS. 2, 7 and 8could be used in an arrangement with two traps in parallel or in sequence. Any of the trapping sections shown inFIGS. 2 to 8could be used with the resonance section A but without a polarising section B if so desired. Magnetic fields of any of the traps could be provided by electromagnets instead of permanent magnets, or could be provided by movably mounted permanent magnets in place of electromagnets. All such modifications are intended to be included within the scope of this disclosure.