Abstract:
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.

Description:
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, Fe 3 O 4 ) 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of NMR apparatus including a trapping section for magnetizable particulates; 
         FIG. 2  shows one form of trap; 
         FIG. 3  shows a trapping section with traps in parallel; 
         FIG. 4  shows a trapping section with traps in series; 
         FIG. 5  is a side view of a length of pipeline with two trapping positions; and 
         FIG. 6  is a cross section on line VI-VI of  FIG. 5 ; 
         FIG. 7  shows another form of trap; and 
         FIG. 8  shows another form of trap. 
     
    
    
     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. 1  shows a general arrangement of NMR apparatus for examining properties of fluid flowing along a pipeline  10  in the direction indicated by arrows. In the portion A, the pipeline  10  is made of non-magnetic electrically insulating material such as fibre reinforced polymer. The pipeline  10  extends through a uniform magnetic field between a pair of permanent magnets  11 . Within this field there is at least one radio-frequency coil  12  encircling the pipeline and used to emit radiofrequency pulses to induce magnetic resonance and also to receive signals from nuclei undergoing resonance. The magnets  11 , the coil  12  and 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 pipeline  10  is made of non-magnetic material although electrically conducting material such as aluminium or stainless steel may be used. Magnets  13  provide a magnetic field to polarize resonant nuclei in the liquid flowing in the pipeline  10 , 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. &amp; 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. 2  shows a simple possibility for trapping magnetizable solids. A branch pipe  14  is connected to the pipeline  10  through a gate valve  16  and is closed at its other end by gate valve  17 . The branch pipe  14  is surrounded by a solenoid coil  18 . In operation with valve  16  open, valve  17  closed and the solenoid coil  18  energised, the magnetic field within the solenoid coil attracts magnetizable solids out of the fluid stream in the pipeline  10  into the branch pipe  14 . Periodically the valve  16  is closed, the coil  18  is turned off and magnetizable solids which have collected in the branch pipe  14  are discharged through valve  17 . During this removal of accumulated magnetizable solids, flow along the pipeline  10  may be temporarily halted by closing valves  19 . 
       FIG. 3  shows a possible arrangement of the trapping section C with traps in parallel. The incoming pipeline splits into two parts  22  which lead to valves  24 ,  25 . Further on the two flow paths pass through valves  26 ,  27  and then rejoin. Flow in the rejoined path  28  continues into the polarizing section B and the magnetic resonance section A. Between the valves  24  and  26  and between the valves  25  and  27  there are lengths of pipe  30 ,  31  made of non-magnetic material such as aluminium or stainless steel and attached at flanges  32 ,  33 . These lengths of pipe  30 ,  31  pass 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 indicated  34  in  FIG. 3 . While flow is passing through a pipe  30  or  31 , the magnetic field draws any magnetizable contaminants to the pipe wall and holds them there. Thus, each length of pipe  30 ,  31  functions as a trap for magnetizable contaminants. 
     The valves  24 - 27  are used to direct flow alternately through the lengths of pipe  30  and  31 . So, when valves  24  and  26  are open and flow passes through pipe  30 , the valves  25  and  27  may be closed. The length of pipe  31  can then be temporarily detached by unbolting at the flanges  33 , removed from the magnetic field, cleaned out and replaced. Subsequently, when valves  25  and  27  are open to direct flow through pipe  31 , the valves  24  and  26  can be closed, allowing pipe  30  to be unbolted at flanges  32 , removed, cleaned out and replaced. 
       FIG. 4  shows another possible arrangement for the trapping section C. The pipeline  40  follows a serpentine path and has two traps  41 ,  42  in sequence at low points of the pipeline. Each of these traps has a gate valve  44  opening and closing connection between the pipeline  40  and a length of pipe  46  connected at flanges  48 . To provide a magnetic field for trapping magnetizable contaminants, a probe consisting of rod shaped steel pole piece  50  attached to a permanent magnet  52  is mounted so that the pole piece  50  extends through a seal  54  into the pipe  46 . 
     Each probe is movable between two positions. As shown in the trap  41  at the left of  FIG. 3 , the probe can be pushed inwardly towards the pipeline  40  so that the distal part  56  of the pole piece  50  projects through the gate valve  44  into the pipeline  40 . In this position the magnetic field from the magnet  52  attracts any magnetizable material in the flowing stream onto the pole piece  50  and 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 trap  42  at the right of  FIG. 3 . The distal part  56  of the pole piece  50  has moved down into the pipe  46 , taking accumulated magnetizable material with it. The valve  44  can then be closed and the pipe  46  can be detached at the flanges  48  allowing the pipe  46  and the probe  50 ,  52  to be removed, cleaned of accumulated magnetizable material and then replaced. 
     The two traps  41 ,  42  are operated alternately. At all times one or other of the traps has its magnetic pole piece  50  pushed in, as shown in trap  41  at the left of  FIG. 3 , so that it can attract and retain magnetizable material. Periodically, the probe  52 ,  50  of one or other of the two traps is moved to the position as shown by the trap  42 , for the accumulated magnetizable material to be removed from it. 
       FIGS. 5 and 6  show a further possibility for a trapping section C. A pipeline  60  carrying flow in the direction shown by arrows has two traps at position  62  and two more at position  64 .  FIG. 6  is a cross section at position  62  showing that the two traps at this position extend downwardly at an angle at each side of the pipeline  60 . The traps are somewhat similar to the traps in  FIG. 4 . Each trap communicates with the pipeline  60  through a gate valve  44  and has a pipe  46  attached at flanges  48 . The traps have probes comprising electromagnets  69  and pole pieces  66 ,  67  which extend through seals  54 . The probes are movable between two positions. A pushed-in position is shown by pole piece  67  and also at the right of  FIG. 5 . In this position the distal part  68  of the pole piece extends across the pipeline  60  and its magnetic field attracts and holds any magnetizable solid material. The pole piece can also be drawn back into the pipe  46 , through gate valve  44 , as illustrated by pole piece  66 . When the pole piece is drawn back, as in the case of pole piece  66 , it carries accumulated magnetizable material with it into the pipe  46 . After a pole piece has been withdrawn as shown by pole piece  66 , the gate valve  44  is closed and the electromagnet  69  for that pole piece is switched off. Accumulated magnetizable material on the pole piece  66  then falls off into the pipe  46 , and can be discharged through a valve  71 . 
     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 pipeline  60 . While the pole pieces of the two traps at position  62  are moved to allow one of the traps to be cleaned out, one trap at position  64  always has its pole piece pushed in and functioning to collect magnetizable material. Correspondingly one trap at position  62  has its pole piece in the pushed in position throughout the time that pole pieces of the traps at position  64  are moved between their two positions. 
       FIG. 7  shows a single trap with some resemblance to the traps in  FIG. 4 . As in  FIG. 3 , a low point of pipeline  40  is connected to a branch pipe  46  through a gate valve  44 . A probe consisting of a pole piece  73  and an electromagnet  69  is located above the pipeline and extends into the pipeline through a seal  75 , so that the distal part  76  of the pole piece  73 , magnetised by the electromagnet  69  arrests and holds magnetizable solids entrained in the flow stream along the pipeline  40 . Periodically, flow is stopped by closing valves  78 . The pole piece  73  and the electromagnet  69  are lowered, so that the distal part  76  of the pole piece carrying magnetizable solids accumulated on it passes through the open valve  44  into the branch pipe  46 . The electromagnet  69  is then switched off, so that the magnetizable solids fall from the pole piece  73  into the branch pipe. The pole piece and electromagnet are then raised back to the position shown, with the electromagnet still switched off. The valve  44  is closed and the magnetizable solids in the branch pipe  46  are discharged by opening the valve  79 . 
       FIG. 8  shows a further possibility for a single trap. The pipeline  80  has a downwardly inclined tubular section  82  at the lower end of which a branch pipe  46  is connected to the pipeline  80  through a gate valve  44 . A magnetic field to arrest magnetizable solids is provided by a pair of permanent magnets  84  located at either side of the inclined section  82 . These magnets  84  are supported on pivoted arms  86 . Periodically, to remove accumulated magnetizable solids from the inclined section  82 , flow is stopped with by closing valves  88  and the arms  86  and magnets  84  are swung to put the magnets  84  at the position shown by a dotted circle  87 , with the magnetic field now extending through the branch pipe  46 . This movement of the magnetic field allows and assists the accumulated magnetizable solids to slide down the inclined section  82  into the branch pipe  46 . The gate valve  44  is then closed, the magnets  84  are then swung back to their original position and the magnetizable solids are discharged from the branch pipe  46  by opening valve  89 . 
     It will be appreciated that the diagrams shown here are schematic and do not show the equipment used to move the magnets  52 ,  69  and pole pieces  50 ,  66 ,  67   73  between positions, nor the mechanical handling equipment used to move detached pipes  30 ,  31  or  46 . Many modifications are possible and features used in one embodiment illustrated here may be utilised in another embodiment. Specifically, the single traps shown in  FIGS. 2, 7 and 8  could be used in an arrangement with two traps in parallel or in sequence. Any of the trapping sections shown in  FIGS. 2 to 8  could 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.