Abstract:
An electromagnetic pickup apparatus ( 46 ) and method for use of the same is disclosed. The apparatus ( 46 ) comprises a probe ( 60 ) that is insertable into a bore hole ( 64 ) or a trench ( 70 ) and a conductive backfill composition ( 62 ) that substantially surrounds the probe ( 60 ) having intimate contact with the probe ( 60 ) and the earth, thereby establishing a highly electrically conductive region ( 66 ) between the probe ( 60 ) and the earth.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to downhole telemetry and, in particular to, an electromagnetic signal pickup apparatus that substantially reduces the attenuation of an electromagnetic signal in the region near an electromagnetic pickup probe. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with transmitting downhole data to the surface during measurements while drilling (MWD), as an example. It should be noted that the principles of the present invention are applicable not only during drilling, but throughout the life of a wellbore including, but not limited to, during logging, testing, completing and producing the well. 
     Heretofore, in this field, a variety of communication and transmission techniques have been attempted to provide real time data from the vicinity of the bit to the surface during drilling. The utilization of MWD with real time data transmission provides substantial benefits during a drilling operation. For example, continuous monitoring of downhole conditions allows for an immediate response to potential well control problems and improves mud programs. 
     Measurement of parameters such as bit weight, torque, wear and bearing condition in real time provides for a more efficient drilling operations. In fact, faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of a need to interrupt drilling for abnormal pressure detection is achievable using MWD techniques. 
     At present, there are four major categories of telemetry systems that have been used in an attempt to provide real time data from the vicinity of the drill bit to the surface, namely mud pressure pulses, insulated conductors, acoustics and electromagnetic waves. 
     In a mud pressure pulse system, the resistance of mud flow through a drill string is modulated by means of a valve and control mechanism mounted in a special drill collar near the bit. This type of system typically transmits at 1 bit per second as the pressure pulse travels up the mud column at or near the velocity of sound in the mud. It has been found, however, that the rate of transmission of measurements is relatively slow due to pulse spreading, modulation rate limitations, and other disruptive limitations such as the requirement of mud flow. 
     Insulated conductors, or hard wire connection from the bit to the surface, is an alternative method for establishing downhole communications. This type of system is capable of a high data rate and two way communication is possible. It has been found, however, that this type of system requires a special drill pipe and special tool joint connectors which substantially increases the cost of a drilling operation. Also, these systems are prone to failure as a result of the abrasive conditions of the mud system and the wear caused by the rotation of the drill string. 
     Acoustic systems have provided a third alternative. Typically, an acoustic signal is generated near the bit and is transmitted through the drill pipe, mud column or the earth. It has been found, however, that the very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compounds the signal attenuation problem for drill pipe transmission. 
     The fourth technique used to telemeter downhole data to the surface uses the transmission of electromagnetic waves through the earth. A current carrying downhole data are input to a toroid or collar positioned adjacent to the drill bit or input directly to the drill string. When a toroid is utilized, a primary winding, carrying the data for transmission, is wrapped around the toroid and a secondary is formed by the drill pipe. A receiver, typically a conductive rod or probe, is inserted into the ground at the surface where the electromagnetic signal is picked up and the data carried therein is processed. It has been found, however, that cracking in the earth around the probe and discontinuities in the contact between the probe and the earth create a high resistivity interface between the probe and the earth that significantly attenuates the strength of the electromagnetic waves which, in turn, greatly reduces the allowable distance between the transmitter and the receiver. Additionally, poor contact between the probe and the earth serves as a source of unwanted noise. 
     Therefore, a need has arisen for a system that is capable of telemetering real time data from the vicinity of the drill bit in a deep or noisy well using electromagnetic waves to carry the information to the surface. A need has also arisen for an electromagnetic pickup system that can receive electromagnetic wave from a transmitter at a great distance without attenuating the signal at the interface between the probe and the earth. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a electromagnetic signal pickup apparatus that utilizes a probe for receiving electromagnetic signals carrying information and a method for use of the same. The apparatus and method of the present invention provide for real time communication between downhole equipment and the surface using electromagnetic waves to carry information. The apparatus and method of the present invention overcome the attenuation of the electromagnetic signals caused by the high resistivity interface typically associated with cracking in the earth around the probe and discontinuities in the contact between the probe and the earth, thereby greatly increasing the allowable distance between the transmitter and the electromagnetic signal pickup apparatus. 
     The electromagnetic pickup apparatus of the present invention comprises a probe that is positioned in a bore hole or trench in the earth. The bore hole has a length and diameter sufficient for the insertion of the probe such that the probe is in a spaced apart relationship with the sides of the bore hole creating an annulus therebetween. Alternatively, the trench has a length and depth sufficient for receiving the probe. 
     A conductive backfill composition is placed around the probe to substantially surround the probe and to be in intimate contact with the probe and the earth. The conductive backfill composition thereby establishes a highly electrically conductive region around the probe that fills any crack in the earth and prevents any discontinuities in the electrical contact between the probe and the earth. This highly electrically conductive region of conductive backfill composition substantially reduces the attenuation of a received electromagnetic signal. 
     The conductive backfill composition may comprise a carbonaceous material such as graphite or a calcine fluid petroleum coke. The conductive backfill composition may be sized to less than 16 mesh, i.e., pass through a Tyler standard number 16 sieve. 
     The method of the present invention reduces the attenuation of a received electromagnetic signal. The method comprises drilling a bore hole or digging a trench in the earth and inserting the probe into the bore hole or trench. The probe has a spaced apart relationship with the sides of the bore hole forming an annulus that is filled with a conductive backfill composition. Alternatively, the probe is placed directly in the trench or on a layer of conductive backfill composition already in the trench. The conductive backfill composition is then poured on top of the probe. 
     In either case, the conductive backfill composition is oriented to create intimate contact with the probe and the earth. The conductive backfill composition creates a highly conductive region between the probe and the earth, thereby substantially increasing the allowable distance between the transmitter and the electromagnetic pickup device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic illustration of a drilling rig including a pair of electromagnetic signal pickup devices of the present invention; 
     FIG. 2 top view of a drilling rig including a pair of electromagnetic signal pickup devices of the present invention; 
     FIG. 3 is a cross-sectional view of a prior art electromagnetic pickup device; 
     FIG. 4 is a cross-sectional view of an electromagnetic signal pickup device of the present invention; and 
     FIG. 5 is a cross-sectional view of an electromagnetic signal pickup device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     Referring to FIG. 1, a plurality of electromagnetic signal pickup devices in use during an onshore drilling operation are schematically illustrated and generally designated  10 . A drilling rig  12  is positioned over an oil and gas formation  14  located deep below the earth&#39;s surface  16 . Drilling rig  12  has a derrick  26  and a hoisting apparatus  28  for raising and lowering drill string  30 , including drill bit  32  and electromagnetic transmitter  34 . 
     In a typical drilling operation, drill bit  32  is rotated by drill string  30 , such that drill bit  32  penetrates through the various earth strata, forming wellbore  36 . Measurement of parameters such as bit weight, torque, wear and bearing conditions may be obtained by sensors  38  located in the vicinity of drill bit  32 . Additionally, parameters such as pressure and temperature as well as a variety of other environmental and formation information may be obtained by sensors  38 . The signal generated by sensors  38  may typically be analog, which must be converted to digital data before electromagnetic transmission in the present system. The signal generated by sensors  38  is passed into an electronics package  40  including an analog to digital converter which converts the analog signal to a digital code utilizing “1” and “0” for information transmission. 
     Electronics package  40  may also include electronic devices such as an on/off control, a modulator, a microprocessor, memory and amplifiers. Electronics package  40  is powered by a battery pack including a plurality of batteries, such as nickel cadmium or lithium batteries, which are configured to provide proper operating voltage and current. 
     Once the electronics package  40  establishes the frequency, power and phase output of the information, electronics package  40  feeds the information to transmitter  34 . Transmitter  34  may be a direct connect to drill string  30  or may electrically approximate a large transformer. The information is then carried uphole in the form of electromagnetic wave fronts  44  which travel through the earth. These electromagnetic wave fronts  44  are picked up by one or more electromagnetic pickup devices  46 . Pickup devices  46  are disposed radially about drilling rig  12  and extend into the earth through surface  16  or are disposed beneath surface  16 . Pickup devices  46  are electrically coupled to an electronics package  48 . Electronics package  48  is electrically coupled to drill string  30 . Thus, as electromagnetic wave fronts  44  reach pickup devices  46 , a current is induced in pickup devices  46  that carries the information originally obtained by sensors  40 . 
     The current is fed to an electronics package  48  that may include a variety of electronic devices such as a preamplifier, a demodulator, a plurality of filters, a timer, memory and a microprocessor. Electronics package  48  cleans up and amplifies the signal to reconstruct the original waveform, compensating for losses and distortion occurring during the transmission of electromagnetic wave fronts  44  through the earth. The information originally obtained by sensors  38  may then be further processed making any necessary calculations and error corrections such that the information may be displayed in a usable format. 
     Even though FIG. 1 depicts two pickup devices  46 , it should be understood by one skilled in the art that the number of pickup devices  46  may be increased or decreased without departing from the principles of the present invention. 
     FIG. 2 shows a top view of drilling rig  12  and electromagnetic pickup devices  46 . Drilling rig  12  includes derrick  26  that has drill string  30  positioned therein. Pickup devices  46  are electrically connected via wires  50  to electronics package  48  which is electrically connected to drill string  30  via wire  52  thereby forming a complete circuit. Pickup devices  46  and drill string  30  form electrodes for sensing the voltage therebetween. Pickup devices  46  may be spaced equidistant from drill string  30  in a radial direction or may be located at varying distances from drill string  30 . 
     Even though FIGS. 1 and 2 have described electromagnetic pickup devices  46  with reference to a drilling operation, it should be noted by one skilled in the art that the principles of the present invention are also applicable throughout the lifetime of wellbore  34  including, but not limited to, during logging, testing, completing and producing the well. 
     In FIG. 3, a prior art electromagnetic pickup probe  54  is inserted into the earth through surface  16 . As a result of the insertion process of probe  54 , the periodic moisture content variation of the earth or other environmental factors, discontinuities  56  develop in the contact between probe  54  and the earth which create a high resistivity interface between probe  54  and the earth. In addition, the insertion of probe  54  and the variations in moisture content of the earth causes cracks  58  to develop and propagate which further increases the resistivity of the earth in the area adjacent to probe  54 . The high resistivity in the region directly surrounding probe  54  causes a significant attenuation in the received strength of electromagnetic waves. In fact, the signal strength of electromagnetic waves may be reduced as much as 40% at the earth-probe interface. The significant attenuation of the strength of electromagnetic waves not only requires additional electronics to recreate the original signal, but also reduces the allowable distance between the electromagnetic transmitter and the probe  54 . 
     In FIG. 4, a cross sectional view of electromagnetic pickup device  46  of the present invention is depicted. Electromagnetic pickup device  46  includes a probe  60  and a conductive backfill composition  62  disposed within the earth. Probe  60  may be a rod or tubing constructed from any electrically conductive material such as steel, copper or a copper clad. Probe  60  may alternatively be a magnetic pickup, a toroid or even a bare wire. 
     Conductive backfill composition  62  greatly reduces the resistivity of the region around probe  60  by filling the voids and cracks in the earth that would otherwise create discontinuities in the electrical contact between probe  60  and the earth. By creating a highly conductive region around probe  60 , conductive backfill composition  62  greatly reduces the attenuation of the received electromagnetic wave fronts  44 , thereby reducing the number of electronic devices required to process the information carried by electromagnetic wave fronts  44  and increasing the allowable distance between transmitter  34  and pickup devices  46 . 
     Conductive backfill composition  62  may be formed from a carbonaceous material. For example, conductive backfill composition  62  may be formed of graphite material, a calcine fluid petroleum coke or the like. Calcine fluid petroleum coke is especially advantageous for use as conductive backfill composition  62  due to its characteristic hard, round, uncrushable shape along with its fluid flow characteristic which allows calcine fluid petroleum coke to flow into voids and cracks in the earth while maintaining intimate contact with probe  60 . In addition, calcine fluid petroleum coke is available in a highly uniform structure capable of flowing through a Tyler standard  16  sieve. 
     In operation, bore hole  64  is drilled in the earth to have a length and diameter sufficient to receive probe  60 . For example, the length of bore hole  64  may be between about 1 and 5 feet and is preferably about 3 feet. The diameter of bore hole  64  may be between about 3 and 6 inches and is preferably about 4 inches. Once bore hole  64  is drilled, probe  60  may be inserted therein such that probe  60  has a spaced apart relationship with the sides of bore hole  64  forming annulus  66 . Conductive backfill composition  62  may then be poured into bore hole  64  filling annulus  66  such that annulus  66  becomes a highly conductive region. Once conductive backfill composition  62  is in place, pickup device  46  may receive information carried on electromagentic wave fronts  44  which were radiated into the earth by transmitter  34  from a great distance. Additionally, it should be noted by one skilled in the art that pickup device  46  of the present invention may alternatively be used as a downlink to transmit electromagnetic waves carrying information from the surface downhole to, for example, operate downhole equipment or prompt sensors  40  to obtain information which will be transmitted uphole by transmitter  34  to pickup devices  46 . 
     After a drilling or other operation using electromagnetic pickup device  64  has been completed, conductive backfill composition  62  may be removed from bore hole  64  using a conventional vacuum system. After conductive backfill composition  62  is removed, probe  60  may also be removed and bore hole  64  may be filled with the earth originally removed to create bore hole  64  leaving it in the original state. 
     In FIG. 5, a cross sectional view of electromagnetic pickup device  68  of the present invention is depicted. Electromagnetic pickup device  68  includes a probe  60  and conductive backfill composition  62  disposed within a trench  70  in the earth. Conductive backfill composition  62  greatly reduces the resistivity of the region around probe  60  by filling the voids and cracks in the earth that would otherwise create discontinuities in the electrical contact between probe  60  and the earth. By creating a highly electrically conductive region  72  around probe  60 , conductive backfill composition  62  greatly reduces the attenuation of the received electromagnetic wave fronts  44 , thereby reducing the number of electronic devices required to process the information carried by electromagnetic wave fronts  44  and increasing the allowable distance between transmitter  34  and pickup device  68 . 
     In operation, probe  60  is placed directly in trench  70  or on a layer of conductive backfill composition  62  that has already been poured into trench  70 . Trench  70  has a length and depth sufficient to receive probe  60 , for example, the length of trench  70  may be between about 1 and 5 feet and is preferably about 3 feet while the depth of trench  70  may be between about 3 and 12 inches and is preferably about 6 inches. Once probe  60  is placed in trench  70 , conductive backfill composition  62  may be poured on top of probe  60  in trench  70  such that trench  70  becomes a highly electrically conductive region  72 . Once conductive backfill composition  62  is in place, pickup device  68  may receive information carried on electromagnetic wave fronts  44  which were radiated into the earth by transmitter  34  at a great distance or may transmit electromagnetic waves into the earth. 
     While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.