Abstract:
A downhole magnetic hollow core dipole antenna has a high permeability material magnetic core composed of laminated sections placed around a section of drill pipe and running substantially along the length of the pipe. The magnetic core is then surrounded by electrically conductive windings, which in turn are surrounded by a protective sleeve which, if conductive, is split to prevent power-robbing eddy current generation.

Description:
FIELD OF THE INVENTION 
     This invention is directed to a magnetic dipole antenna for use downhole in gas and oil wells. The invention also includes a well bore system which includes the downhole magnetic dipole antenna. 
     BACKGROUND OF THE INVENTION 
     The technology of drilling gas and oil wells has advanced significantly in recent years. Part of this advancement involves new and improved techniques for drilling non-vertical (i.e. horizontal and other directional) wells. One advantage of horizontal and other directional drilling is that it enables a greater portion of the well bore to be exposed to gas or oil-producing strata, which tend to be disposed more horizontally than vertically. This enables more gas or oil to be produced from the directional well, than from a similar vertical well. 
     When drilling non-vertical well bores, it is common practice to use downhole sensors to measure the orientation of the well bore. The well orientation information gathered during drilling must be transmitted to the surface. Conventional downhole sensors used to measure well orientation include a three-axis accelerometer used to measure roll and inclination of the well bore, and a three-axis magnetometer (which functions as an electronic compass) to measure the well bore azimuth. Information on the well bore has been transmitted to the surface of the earth using a wireline, a measurement while drilling (MWD) mud pulser, or an electric dipole. 
     The conventional transmission methods and devices have certain disadvantages. Wireline systems, which use a coaxial high strength cable to connect the downhole sensors to the surface, require the use of a wireline truck. Wireline trucks are expensive, both to buy and operate. Also, the wireline must be cut and reconnected to enable the insertion of drill pipe at the surface as the well is drilled down. 
     MWD methods require changing the downhole fluid dynamics to propagate pressure pulses to the surface. The pressure pulses are used to encode the downhole information. MWD systems are expensive to buy and operate, and do not work well in some formations which the circulation is lost or poor. 
     The electric dipole transmission method creates a downhole dipole by electrically isolating a portion of the drill pipe and impressing a voltage across it. This method is relatively simple and inexpensive. However, the technique does not work when there is a moderately conducting formation above the dipole, which shorts the dipole signal. Also, this technique cannot be used inside casing, because casing shorts out the signal. 
     Magnetic dipole antenna transmission has been proposed to eliminate the above shortcomings but has yet to be perfected for practical usage. 
     Thus, there is a need or desire for a technique for transmitting downhole data to the surface via a magnetic dipole antenna which is relatively simple and inexpensive, provides strong signal and which can be used in a wide variety of environments. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for transmitting downhole data to the surface using a magnetic dipole. The magnetic dipole has the advantages of the electric dipole technique of being simple and inexpensive to use. Yet the magnetic dipole eliminates the disadvantage of short circuiting in certain environments. 
     The magnetic dipole includes an elongated metallic cylinder composed of laminations with high magnetic permeability, and an excitation coil wound around the cylinder. The cylinder can be fastened around a drill pipe for mechanical strength, wrapped with excitation coils and covered with a thin protective sleeve. The sleeve is preferrably split longitudinally when composed of a conductive material. The dipole is preferably about as long as a section of drill pipe, e.g. ten meters, because this increases its strength. 
     The dipole can be energized by a supply of electricity removed from the dipole location by several dipole lengths. The supply of electricity can be at, above or below the earth&#39;s surface, and can be connected to the dipole by an electric transmission wire. At least one orientation sensor capable of measuring well bore orientation (e.g. inclination and azimuth) is provided in electronic communication with the dipole. 
     Data on the well bore can be transmitted to the surface by energizing the dipole and employing phase shift key (PSK), or other known modulation techniques. The dipole can be energized with a frequency of about 2 to about 10 Hz, and preferably about 3 Hz for instance. The magnetic downhole transmission power and duration can be enhanced over standard battery pack power by employing a downhole hydraulic power generator. 
     Magnetic signals from the dipole can be detected at the top of the bore hole using a magnetic field sensor. More than one sensor can be used for increased accuracy to reduce environmental noise and increase range. 
     With the foregoing in mind, it is a feature and advantage of the invention to provide a magnetic dipole antenna for improved downhole monitoring of the orientation of a well bore during drilling of a gas or oil well. 
     It is also a feature and advantage of the invention to provide an improved method for monitoring the orientation of a well bore during drilling of an oil or gas well. 
     It is also a feature and advantage of the invention to provide a well which includes an improved magnetic dipole antenna in a downhole location. 
     The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are intended to be illustrative rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a transverse cross section of a downhole magnetic dipole antenna according to one embodiment of the present invention. 
     FIG. 2 is a perspective view of one embodiment of the laminated magnetic core of the present invention. 
     FIG. 3 is a perspective view an alternative embodiment of the laminated magnetic core of the present invention and showing other features of the antenna in phantom. 
     FIG. 4 is a perspective view of an alternative embodiment of the laminated magnetic core of the present invention. 
     FIG. 5 is a prespective view of an alternative embodiment of the laminated magnetic core of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a drill pipe  11  having a central bore  12  is fitted with a hollow core magnetic dipole antenna assembly  13  having a laminated magnetic core  15  surrounding the drill pipe  11 . Surrounding the magnetic core  15  are windings  17  for inducing a field into the magnetic core  15 . Surrounding the windings  17  is an outer protective sleeve  19 . It will be appreciated that FIG. 1 is somewhat schematic and that the scale of parts, arrangement of windings, etc., will be constructed and arranged according to known techniques by the ordinarily skilled artisan as desired or necessary to the application. 
     The antenna of the preferred embodiment is most easily constructed to surround virtually an entire length of drill pipe, giving the antenna a high aspect ratio of length to diameter. The laminated magnetic core  15 , as further detailed below, is preferably constructed from high magnetic permeability material such as coated steel laminations to render the core non-electrically conductive. The magnetic core should preferably not conduct to the drill pipe and can be insulated therefrom by known coatings or coverings (not shown). If the outside layer of the magnetic core can conduct to the surrounding layer of the core the inside layer must be electrically insulated from the pipe. The cross sectional area of the magnetic core and the magnetic permeability of the materials may be dictated by practical considerations including performance and cost. 
     The protective sleeve  19  is selected of material suitable to protect the windings  17  in the drilling environment, and may, e.g., be composed of steel, fiberglass or other suitably abrasion resistant material. If the material of the cover  19  is electrically conductive, the cover  19  should be slotted, as at  21 , to prevent a shorted turn from conducting induced eddy currents which would significantly reduce the signal strength of the antennae. 
     As seen in FIGS. 2 through 5 the laminated core has several alternative constructions. For ease of illustration, FIGS. 2 through 5 show short sections of the long magnetic core  15  of the antennae which surrounds a section of drill pipe  11 , as seen in phantom in FIG.  3 . If the laminations are made from oriented material, the magnetic-easy axis should be oriented along the length of the antenna. FIG. 2 shows an embodiment in which the laminations, collectively  23 , are bent into the form of first and second semicircles  25 ,  27  with each semicircle covering one-half of the circumference of the drill pipe. The laminations  23  may need to be held in place during construction by means of a strong adhesive, as the laminations are not likely to be bent to the exact radius needed. Also shown is a clamp  29  which can be used during assembly. The two flanges  31 ,  33  on the clamp would be held together by screws or other means. 
     FIG. 3 shows the case in which the laminations  23  are bent into nearly complete circles, with a gap  37  in the circumference. Also shown is one of a set of spacers  39  applied to the outermost lamination  41 . Such spacers could be used during assembly to spread successive outer laminations enough to allow them to be slid over the underlying laminations and drill pipe  11 . The spacers could also take the form of a single long strip instead of a set of discrete spacers. This arrangement would usually be glued together, but the laminations would generally be bent to a slightly smaller radius than is needed to fit, so that when the spacers are removed the corresponding lamination would spring tightly to the underlying laminations and drill pipe. 
     FIG. 4 shows the case in which the laminations  23  are flat strips of varying widths, and all oriented with the flat sides to each other. The stacked laminations are shown to be in two semicircular sections with a space in between. Each section is preferably assembled separately and then the two bound together over the drill pipe as the final step. 
     FIG. 5 illustrates the arrangement with the laminations arranged radially out from the central drill pipe. Because the circumference of the magnetic core increases with radius, the laminations achieve a higher packing density if the arrangement is broken into at least two shells  43 ,  45  as shown. Each shell would contain more laminations along the circumference than the preceding inner shell. 
     Therefore, it will be appreciated from study of the present invention disclosure that a hollow core magnetic antenna may be utilized without inducing eddy currents to the drill pipe and without reducing fluid flow through the drill pipe. The present antenna provides increased diameter and length of magnetic material added around the drill pipe to provide increased signal strength. Further, by utilizing magnetic material apart from the drill pipe material, the magnetic material may be selected to be of higher magnetic permeability while still utilizing the drill pipe for mechanical rigidity.