Abstract:
An apparatus comprising a probe having an elongated probe casing includes a magnetometer and a permanent magnet having a north-south magnetic axis disposed in an orientation relative to a longitudinal axis of a drill string or pipe casing when the probe is inserted into the drill string or the pipe casing. The permanent magnet is disposed in a non-vertical plane that does not contain the longitudinal axis to induce a magnetic field in the plane from the north of the magnetic axis to the south of the magnetic axis along a portion of a circumference of the drill string or the pipe casing. The magnetometer is to detect magnetization of the drill string or the pipe casing independently of movement of the permanent magnet. Additional apparatus and methods of analysis using the apparatus are disclosed.

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 12/551,061, filed Aug. 31, 2009; which application is a continuation under 35 U.S.C. 111(a) of International Application No. PCT/FR2008/050462, filed Mar. 18, 2008, and published as WO 2008/139070 A1, on Nov. 20, 2008, which claimed priority under 35 U.S.C. 119 to French Patent Application Serial No. 0753921, filed Mar. 20, 2007, which application and publication are incorporated herein by reference in their entirety and made a part hereof. 
    
    
     BACKGROUND 
     Whilst a drill string is being inserted into the ground, or even once this string of drilling rods or a working pipe casing has been installed, various measurements on the drill string or pipe casing may be carried out. For example, measurements can be made to determine whether a rod is stuck by virtue of a cave-in at depth, perhaps several thousand meters from the point of origin of the drilling. 
     It may also be useful to detect the position of the joints of the drill string or of the pipe casing. In fact, a string of drilling rods or a working pipe casing may comprise a collection of rods or tubes—having, for example, lengths of the order of around ten meters, which are screwed onto one another, and the counting of the joints constitutes a fixing of position. 
     It may also be useful to fix the locations of perforations or of zones of weakening, via corrosion of the pipe casings. It may also be useful to know the stress condition at a point in a drill string whilst it is being raised from the point of origin, for example, in order to create a neutral point at a given depth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Characteristics of various embodiments, as well as others, will be set forth in detail in the following description of particular embodiments, which has been drawn up by way of a non-limiting example in connection with the attached Figures, in which: 
         FIG. 1  represents, in schematic manner, a rod or a tube in which a probe according to one embodiment of the present invention is arranged; 
         FIG. 2  is a view along sectional plane A-A in  FIG. 1 ; 
         FIG. 3  is a view along sectional plane B-B in  FIG. 1 ; 
         FIG. 4  represents, in schematic manner, an embodiment variant of a probe according to the present invention; and 
         FIG. 5  represents readings taken with a probe according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In order to carry out various measurements, use can be made of probes that analyse magnetic effects induced in the rods. These probes comprise means for measuring the magnetic field, possibly connected to means for creating a magnetic field. 
     The means for measuring the magnetic field are generally means for measuring the magnetic flux, which operate when the probe is in a state of displacement, the amplitude of the signals received depending on the speed of displacement. 
     The means for creating a magnetic field in a rod or tube (which is generally made of a ferromagnetic material or another material that is capable of acquiring a remanent magnetisation under the influence of the field) are generally means for generating an alternating field or a pulsed field. In some cases, it has been proposed to use coils or rotary magnets as field-creating means. These means are used either in order to obtain a periodic remanent magnetisation in the rod or tube or, more generally, in order to create local zones of magnetisation by applying pulses to a coil periodically while it is being displaced in the drill string or in the pipe casing. 
     Some of these mechanisms are relatively complex and costly, on account of the fact that when it is desired to excite a coil at a great depth in the interior of a drill string it is sometimes necessary to energise this coil by means of a relatively substantial current through conductors of great length, and that, moreover, the bottom of a well may be at an elevated temperature, capable of attaining values higher than 175° C., which considerably limits the energy that can be dissipated in the coil. 
     Another difficulty is that the analysis probe sent into a drill string is generally associated with other elements, including explosives intended to aid the unscrewing of a joint of rods at a chosen place, or to perforate a pipe casing for the purpose of subsequently bringing a well into production. The detonators associated with these explosives are sometimes affected by interference resulting from the application of intense current pulses in their immediate vicinity. Efficient shields increase the cost of the device and complicate its realisation. 
     Embodiments of the invention include devices and processes for analysis of the state of hollow drilling rods (referred to hereinafter simply as ‘rods’) and of working tubes or pipe casings that are used in the field of oil prospecting and oil mining. 
     In some embodiments, a simple probe for analysis of a collection of drilling rods or of working tubes or pipe casings is provided. 
     In some embodiments, mechanisms are provided to:
         detect the location of a sticking-point of rods,   detect the positions of joints of rods or tubes,   detect the positions of perforations and/or of zones of weakening, for example through corrosion, of a tube,   detect a neutral point as far as the stress applied to a drill string is concerned.       

     Thus, in some embodiments, a probe for analysis of a collection of rods or tubes is provided. The probe comprises an elongated casing which bears, at a first end, at least one first magnetometer and, at a position sufficiently remote from the magnetometer, a permanent magnet, the north-south axis of which is perpendicular to the axis of the rods. 
     According to one embodiment of the invention, the probe comprises at least one second magnetometer arranged on the other side of the permanent magnet in relation to the first magnetometer. 
     According to one embodiment of the invention, the magnet is constituted by a collection of magnets. 
     According to one embodiment of the invention, the magnetometer is a magnetometer with magnetoresistors. 
     According to one embodiment of the invention, four magnetometers are distributed on the periphery of the casing. 
     According to one embodiment of the invention, the magnetometer or magnetometers is/are selected and disposed in order to be sensitive to a field in a transverse plane and insensitive to the axial components of the field. 
     One embodiment of the invention comprises a process of analysis of a sticking-zone using the aforementioned probe, in accordance with which the probe is lowered and it is hoisted after having applied a stress, maintained or released, to the string of rods which is made of a magnetostrictive material. 
     One embodiment of the invention comprises a process of analysis of a sticking-zone using the aforementioned probe, comprising the step of reading variations of magnetisation intensity in order to detect a relative variation of thickness or volume of material, resulting, for example, from joints of rods or of pipe casings, from pipe-casing perforations, from centring tools or other accessories, from anomalies and from pipe-casing deterioration, perhaps through the effect of corrosion. 
     One embodiment of the invention comprises a process of analysis of a sticking-zone using the aforementioned probe for the determination of a neutral traction zone, consisting in magnetising a chosen zone of a string of rods and in raising the string of rods while the variations of magnetisation in said zone are detected. 
     Additional details of various embodiments will now be provided. 
     As  FIG. 1  illustrates, a probe casing  1  is dimensioned so as to be capable of being displaced in the interior of a string of rods or tubes  3  by being tied to a cable  5  for traction and for transmission of electrical signals. 
     The probe casing  1  usually comprises various elements other than the analysis elements which will be described below, for example specific drive means, means for spacing the walls of the rods or tubes, means for triggering an explosion, means for processing and for transmission of signals, etc. These elements may not be shown to avoid obscuring various details concerning various embodiments. 
     The magnetic analysis probe casing  1  bears a magnet  6 , the north-south axis of which is orthogonal to the axis of the rods  3 . This casing also bears one or more devices  7  for measuring a magnetic field, for example magnetometers—elements that are capable of measuring the field created by a remanent magnetisation, independently of any movement of the probe, for example Hall-effect sensors or magnetoresistance sensors. These must be distinguished from the usual means for measuring flux, comprising a coil, which can only detect variations of magnetisation and which therefore only function when they are in a state of displacement in relation to a non-constant field. It is useful to provide a configuration such that the direct influence of the magnet on the magnetometers is negligible. By way of example, the axial distance between the magnet and each set of magnetometers may be of the order of 30 cm to 2 meters, preferably from 50 cm to 1 meter, and more preferably on the order of 50 cm. 
       FIG. 2  is a sectional view according to plane A-A in  FIG. 1 , and  FIG. 3  is a sectional view according to plane B-B in  FIG. 1 . 
     As  FIG. 3  shows, the north-south axis of the magnet is in a plane that is perpendicular to the axis of the rods—that is to say that, as shown, this magnet will tend to create two magnetised zones in the form of half-rings in the rod, and the magnetisation vectors M in the rod will be essentially situated in a plane that is perpendicular to the axis of the rod. 
     Each of the magnetometers will be able to measure from 1 to 3 components of the field. Use can be made of one or two of the components of the magnetometers sensitive to a field, said components being situated in a plane that is perpendicular to the axis of the probe. Since the magnetometers are sensitive to a field in a transverse plane and insensitive to the axial components of the field, the influence of the parasitic or stray magnetisations due to external sources (the earth&#39;s field, for example), essentially oriented axially, can be rendered negligible. 
     If use is made, for example, of four magnetometers  7   a ,  7   b    7   c  and  7   d  with the relative orientation between magnet and magnetometers represented in  FIG. 2 , the tangential component of the detected field will be maximal on the two magnetometers  7   a  and  7   c  only and minimal on the two other perpendicular magnetometers. 
     If a rotation of the probe by 90° occurs, the situation will be reversed, with the maximum of the signal on magnetometers  7   b  and  7   d . For intermediate positions, it can be estimated that the sum of the signals will also give a signal of the same order of magnitude. Identical reasoning may be applied in respect of the radial components (at a rotation close to 90°). These components may be used separately or in combination. For this reason, and if there are at least four magnetometers arranged at 90° on the periphery, the relative angular position between the probe and the tube is of little importance. 
     The probe according to the present invention may be used in various ways, according to the measurements desired. 
     In order to detect the location of jamming of a rod, one begins, for example, by lowering the probe in order to magnetise the walls of all the rods continuously, and, before bringing the probe back up, a stress (torsion, traction, compression, or combination of these stresses) will be applied to the drill string. This stress will be capable of being maintained or released before the probe is brought back up. If the rods are made of a magnetostrictive material, the parts having been subjected to the stress will have their magnetisation diminish appreciably, whereas the parts situated below the block will not be affected. In this way, upon being brought back up the zone in which the magnetisation will have varied (will have passed from a positive or negative value to an approximately zero value) will correspond to the zone situated above the sticking-point. It will be noted that this system, which operates continuously, is particularly sensitive, enabling progressive jamming to be detected. 
     The probe may be used to count joints. During descent, just as when being hoisted, the magnetometers  7  can detect a variation in magnetisation when passing each of the joints of rods or tubes  9 , which are often present every 10 meters, more or less, in strings of drilling tubes or in conventional pipe casings. In fact, the remanent magnetisation is different in the region of the joints, since it is a function of the volume of material and of the thickness/diameter ratio. 
     In order to facilitate counting the joints on coming back up, it will be possible, for example, to utilise a probe variant such as that represented in  FIG. 4 , comprising a central magnet  6  and two sets of magnetometers  7 . 1  and  7 . 2  arranged substantially symmetrically in relation to the central magnet  6 . 
     In this way, various embodiments provide a simple mechanism for counting the joints, enabling the positioning of the probe in the drill string or pipe casing to be determined with more precision than by depending solely on the state of winding of the cable for supporting the probe. It is also possible to detect variations in thickness of the rods, which are associated, for example, with deformations, damage, corrosion or perforations. 
     Another application of various embodiments of the invention involves assisting in the unscrewing of a string of drilling rods. In fact, after a jamming of rods occurs, for example, the customary manoeuvre is to screw the rods right home, then to raise the drill string in such a way as to arrive at traction forces and weight forces that are substantially balanced in the region of the joint that it is desired to unscrew and that will then be unscrewed preferentially in relation to the other joints under stress. The probe enables the determination of good traction on the rods to be unscrewed. In fact, once the joint of rods immediately above the sticking-zone has been determined, the probe is displaced around this sticking-point, in order to magnetise the rod, then one of the sets of magnetometers is arranged just above the joint that it is desired to unscrew. Afterwards, a progressive pull is exerted on the rods from the surface, and at the moment when the magnetisation measured in the region of the magnetometers attains a value determined by a prior calibration, it is then known that the forces have been suitably balanced. It will be noted that if too strong a pull has been exerted, a magnetisation of the rods in the zone being considered can be undertaken anew, and a new measurement of decline in the value of magnetisation, associated with the magnetostrictive phenomenon, can be undertaken. 
       FIG. 5  represents examples of magnetisation curves M as a function of the depth d. 
     Curve  20  represents the magnetisation observed in the absence of any polarisation, for example the magnetisation observed on the descent by the probe  7 . 2 , placed lowest, of the embodiment shown in  FIG. 4 . Quite a weak background noise is observed, corresponding to the remanent magnetisation acquired in the earth&#39;s magnetic field. 
     Curve  22  represents the magnetisation resulting from the passage of the magnet  6 , for example the magnetisation observed on the descent by probe  7 . 1  which follows the magnet  6 . This is also what probe  7 . 1  or probe  7 . 2  would indicate on being hoisted. It will be noted that in the region of the joint of rods  9  a variation in magnetisation is observed. It will also be noted that with the magnets that are standard at the present time the signal contrasts very clearly with the background noise associated with the earth&#39;s magnetic field, in practice in a ratio that may be as high as 50. 
     Curve  24  represents the signal observed upon hoisting the probe when there is jamming at a point  26  and when a stress has been applied to the rods from the surface, having the result that, as a consequence of the magnetostriction, the magnetisation is substantially erased where the stress has been applied, enabling a point  26  to be positioned, in the region of which the jamming of a rod has taken place. It is following this that the operations of disassembly noted previously will be able to be carried out. 
     Various embodiments of the invention provide advantages which will be apparent to a person skilled in the art. For example, because the field created by the magnet  6  is situated in a plane transverse to the axis of the rods, the induced magnetisation can be more concentrated than if the magnet were parallel to the axis of the rods, in which case the field lines would be distributed over a larger zone. This helps to achieve a better-focused and more intense signal. 
     It will also be noted that some embodiments provide a simple mechanism for locating joints and therefore for making measurements of depth in a drilling well or operating well. This is made possible because a signal is used that is independent of the speed of displacement—not an alternating signal or pulsed signal. 
     Some embodiments enable measurements of joint location and of blocking-point determination in the course of one and the same pass, even in the presence of rotations due to a twisting of cable. 
     The magnets may comprise samarium-cobalt magnets or neodymium-iron-boron magnets that are capable of creating a magnetic induction of the order of one tesla. It is also be possible to use several magnets, if desired. 
     In addition, it will be noted that, in view of the intensity of the fields provided by modern magnets, use may be made of one and the same probe for the purpose of entering rods having quite different diameters, for example drilling rods with a diameter of 8.75 cm to 12.5 cm (3.5 inches to 5 inches), working pipe casings which are accessed through a tube for bringing up hydrocarbon, the pipe casing having, for example, a diameter of 17.5 cm (7 inches), whereas the tube for bringing up hydrocarbon only has a diameter of 5 cm to 6.1 cm (2 inches to 2⅜ inches). A sensitive system is useful in these situations. 
     In fact, if use is made of a magnet in a probe configured to function within a tube of 6 cm, it has been established that the ratio of the magnetisation created by this magnet to the parasitic magnetisations is greater than 50. If one passes from a tube of 6 cm to a tube of 17.5 cm, the field may be divided by about 25, but even so it remains very large in comparison with the earth&#39;s field, preserving a useful sensitivity for the system. 
     Various advantages result from the association of a fixed permanent magnet of relatively high power with a detector of the magnetometer type. The use, with a fixed magnet, of sensors for measuring the variation of flux, and not of magnetisation sensors, would not provide the same results, since it would then be possible to carry out the measurements only during a displacement of the probe. Likewise, the specific orientation of the magnetisation magnet—perpendicular to the axis of the rod—provides a clear increase in sensitivity. Tests have shown that the gain obtained is greater than 20 in comparison with the arrangement of the magnet along the longitudinal axis of the rods or of the pipe casing.