WO1988007212A1 - Well logging system employing focused current in measuring resistivity while drilling - Google Patents

Well logging system employing focused current in measuring resistivity while drilling Download PDF

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Publication number
WO1988007212A1
WO1988007212A1 PCT/US1988/000034 US8800034W WO8807212A1 WO 1988007212 A1 WO1988007212 A1 WO 1988007212A1 US 8800034 W US8800034 W US 8800034W WO 8807212 A1 WO8807212 A1 WO 8807212A1
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WO
WIPO (PCT)
Prior art keywords
current
insulating sleeve
drill string
drive
node
Prior art date
Application number
PCT/US1988/000034
Other languages
French (fr)
Inventor
James Edward Meisner
Roy Earl Squire
Ronald Anthony Hall
Andrew Goodwin Brooks
Original Assignee
Exploration Logging, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Exploration Logging, Inc. filed Critical Exploration Logging, Inc.
Publication of WO1988007212A1 publication Critical patent/WO1988007212A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/18—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging
    • G01V3/20—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with propagation of electric current
    • G01V3/24—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with propagation of electric current using ac

Definitions

  • this invention relates to systems for electrical logging during well-drilling operations; more particularly, it relates to such a system employing focused current in measuring formation resistivity.
  • __ suitable with relatively low conductivity mud involve induc ⁇ tion of electromagnetic waves, and generally require complex electronic circuitry.
  • MWD measurement-while-drilling
  • the adverse environ ⁇ mental conditions involve high temperature and shock.
  • mud is circulated under high pressure to flow down through the drill string to the drill bit and then to flow back up in the annular space between the drill string and the wall of the borehole, carrying cuttings to the surface.
  • Various elements of an MWD system must be contained in an instrument housing, which is a sealed pressure vessel or barrel, so as to be protected from exposure to the high pressure drilling mud.
  • any element of the MWD system that is exposed to the upwardly flowing mud, and the whipping action of the drill string against the inner wall of the borehole must be extremely abrasion resistant.
  • a wireline system In contrast to an MWD system, far fewer problems need to be addressed in a well logging system, commonly called a wireline system, that is used while drilling opera ⁇ tions are suspended. Because the mud is stationary while drilling operations are suspended, various elements of a wireline system are not subjected to the adverse conditions discusse above. One minor exception is that downhole temperature is somewhat higher while the mud is stationary than while the mud is circulating and to some extent providing cooling.
  • the environmental conditions of use of a wireline system in addition to being generally more benign, enable substantially more control over distribution of electrical power.
  • a generator In a wireline system, a generator is located at the surface, and the electric power it generates is easily supplied to downhole electronics.
  • the tool body has an array of longitudinally-spaced outer electrodes, each electrode having a cylindrical shape, there being three such electrodes in the LL3 version and seven such electrodes in the LL7 version.
  • the electrode that is in the middle of the array is called either the exploring electrode or the measuring electrode.
  • all the electrodes operate at the same potential relative to a system ground potential, so that current flowing through the measuring electrode flows radially with respect to the longitudinal axis of the tool body for a substantial dis- tance through a surrounding region of the formation before curving to flow toward the system ground potential.
  • the current flowing through the measuring electrode is focused or generally confined to a disk-shaped region of the surrounding formation.
  • the practice has been to employ an on-surface source of electrical power.
  • a common practice has been to employ multiple connectors to supply power, with the measuring electrode being connected to its on-surface source of power by a dedicated conductor allocated solely to the measuring electrode.
  • the power source for the measuring electrode has further been a common practice for the power source for the measuring electrode to produce a con ⁇ stant-magnitude current. With the magnitude of the current flowing through the measuring electrode being fixed, a measurement of the potential difference between the poten ⁇ tial at the measuring electrode and the system ground pro ⁇ vides the information needed for calculating a resistance value.
  • an "apparent resistivity" can be determined directly from the resistance value.
  • Apparent resistivity is the resistivity of a homo ⁇ geneous medium having the same resistance value and geometr involved in the measurement.
  • the true formation resistivit is a function of apparent resistivity and mud resistivity, which may easily be independently determined.
  • resistivity measurement system that have been developed in the past, including wireline systems and MWD systems,- generally have a limited dynamic range that falls short by an order of magnitude relative to a desirable dynamic range of 10,000 to 1.
  • a first electrode is connected to a constant current source; second and third electrodes are connected across a voltmeter; and a fourth electrode is connected to form part of a circuit path through which some of the current flowing through the first electrode (and through the forma- tion) returns to the constant current source.
  • the fifth electrode is intended to form part of a second return circuit path to the constant current source.
  • Grosso teaches that the sum of the currents flowing in the two return paths will equal the magnitude of the current produced by the constant current source.
  • Grosso further teaches employing an ammeter to measure the current flowing in one of the two return paths. Based on simultaneous measurements of voltage drop between the second and third electrodes, and of current flowing in a return path, a resistance value can be calculated.
  • each of the MWD systems Grosso has a limit ⁇ ed dynamic range.
  • Each of these systems is plagued by additional problems relating to borehole effects such as highly conductive muds, large borehole diameters and high formation-to-mud resistivity contrasts. In effect, these systems in some circumstances measure little more than the resistivity of the mud rather than the resistivity of the formation. Further, these systems suffer from poor bed boundary resolution and depth of investigation.
  • the short normal has four electrode rings in an array, the middle two being connected to a voltmeter, and the outer two being connected such that the drilling mud, in parallel with the formation, define a circuit load for a constant- magnitude current source.
  • One of the advantages of the construction method taught in the More '393 patent is that a damaged insulating sleeve can relatively quickly replaced in the field. The need for such replacement arises even though the insulating sleeve is extremely rugged; however, the extremely adverse downhole conditions to which it is subjected in use are such that the insulating sleeve has a much more limited life than the drill string segment.
  • the downhole conditions during drilling involve high temperature, high pressure, shock, and abras ⁇ ion. Further, the upflowing mud impregnates the insulating sleeve, with the result that the insulation resistance of the sleeve decreases.
  • This invention provides an improved system for meas ⁇ uring formation resistivity while a drilling operation is occurring.
  • the system can be defined in terms of structures on and within a drill string segment which preferably is a drill collar section directly connected to the drill bit.
  • the drill string segment contains an electronics package.
  • An insulating sleeve is provided to cover a portion of the drill string segment.
  • Three electrode bands are arranged on the insulating sleeve.
  • the three electrode bands are referred to herein as an upper guard band, a middle or measurement band, and a lower guard band.
  • the bands are arranged on the insulating sleeve in a configuration or array to enable focusing of current flowing through the measurement band.
  • the electronics package includes drive circuit means for applying a drive voltage across a drive node and a return node.
  • the system includes means providing a supply current flow path extending from the drive node. A portion of the supply current flow path is embedded in the insulat ⁇ ing sleeve to extend longitudinally along a portion of the drill string segment.
  • this and other longi ⁇ tudinally extending and embedded conductors exit the insulator sleeve close to the upper end of the sleeve.
  • Each conductor in the sleeve-embedded cable assembly is connected to a respective conductor that extends within a wireway defined in the wall of the drill string segment, the conductors within this wireway being referred to herein collectively as a wireway-contained cable assembly.
  • the junction of these series-connected cable assemblies is located just below a wear ring.
  • the system of this invention not only is adapted to provide those advantages but also provides significant improvements attributable to the circuit means in the electronics pack ⁇ age, and electrical components within the insulating sleeve, and the cooperation between them.
  • the system of this invention includes current distribution means embedded in the insulat ⁇ ing sleeve for distributing the supply current among bran ⁇ ches that extend to the three electrode bands to provide for communication with upper, middle, and lower regions of the formation surrounding the drill string segment to complete a load circuit defined between the drive node and the return node.
  • the loading of the drive circuit means is a function of the formation resistivity.
  • the extent to which mud resistivity influences the loading, especially with higher formation-to-mud resistivity contrasts, is advantageously substantially less than prior art MWD system employing electrodes or insulating sleeves. This advantage is attributable to the focused current operation that is achievable in an MWD system in accord with this invention.
  • the system includes both voltage measuring means and current measuring means.
  • the system produces both voltage representing and current-representing signals to provide data for determining formation resis ⁇ tivity.
  • the dynamic range of variable resistivity that is readily measured involves a ratio of 10,000 to 1.
  • the current measuring means As to the current measuring means, a portion of it is within the electronics package, another portion is em ⁇ bedded in the insulating sleeve and defines a current-sens ⁇ ing means, and another portion forms part of an overall serial-interconnection cable assembly extending from the current-sensing means embedded in the insulating sleeve to the portion of the current measuring means within the elec ⁇ tronics package.
  • FIG. 1 shows an overall system for simultaneously drilling and logging a well, in which a logging collar includes a formation-resistivity measuring system according to this invention
  • FIG. 2 is an elevation view showing exterior features of the logging collar of the drill string, such exterior features including a replaceable insulating sleeve on which there is an array of electrode bands;
  • FIG. 3 is an elevation view showing exterior features of an elongated assembly that the logging collar contains, which elongated assembly includes an instrument housing for an electronics package;
  • FIG. 4 schematically shows conductors and electrical connectors, and electrical components that are on and em ⁇ bedded in the insulating sleeve, and that are intercon ⁇ nected in the presently preferred embodiment of this inven ⁇ tion to define various current flow paths including a supply current path and branches, and sensing signal paths;
  • FIG. 5 shows mechanical features of components in ⁇ cluding bands, conductors, and a transformer that are pref ⁇ erably fabricated as a subassembly and then integrated into the replaceable insulating sleeve during its assembly;
  • FIG. 5A is an enlarged view of the transformer shown in FIG. 5;
  • FIG. 6 is a view taken on line 6-6 of FIG. 7, and shows a longitudinal cross-section showing a portion of the insulating sleeve and the array of electrode bands;
  • FIG. 7 is a view taken on line 7-7 of FIG. 2;
  • FIG. 8 is an enlarged cross-section taken of area 8 of FIG. 3;
  • FIG. 9 is a block and schematic diagram of circuitry contained in the electronics package for performing electronic functions of the resistivity measurement system of this invention.
  • FIG. 10 is a schematic showing the preferred arrange ⁇ ment of the power amplifier of FIG. 9.
  • a well 1 is being drilled into the earth under con ⁇ trol of surface equipment including a rotary drilling rig 3.
  • rig 3 in ⁇ cludes a derrick 5, derrick floor 1 , draw works 9, hook 11, swivel 13, kelly joint 15, rotary table 17, and drill string 19 that includes drill pipe 21 secured to the lower end of kelly joint 15 and to the upper end of a section of drill collars including an upper drill collar 23 and a lower drill collar 25.
  • a drill bit 26 is carried by the lower end of drill collar 25.
  • Drilling fluid (or mud, as it is commonly called) is circulated from a mud pit 27 through a mud pump 29, past a desurger 31, through a mud supply line 33, and into swivel 13.
  • the drilling mud flows down through the kelly joint and drill string, and through jets (not shown) in the lower face of the drill bit.
  • the drilling mud flows back up through the annular space between the outer surface of the drill string and the inner surface of the borehole to be circulated to the surface where it is returned to the mud pit through a mud return line 35.
  • a shaker screen (not shown) separates formation cuttings from the drilling mud before it returns to the mud pit.
  • the overall system of FIG. 1 uses mud pulse telemetr techniques to communicate data from downhole to the surface while the drilling operation takes place.
  • a transducer 37 in mud supply line 33. This transducer generates electrical signals in response to drilling mud pressure variations, and these electrical signals are transmitted by a surface conductor 39 to a surface electronic processing system 41.
  • U.S. Patent No. 4,216,536 to More
  • mud pulse telemetry techniques provide for communicating data to the surface about numerous down ⁇ hole conditions sensed by well logging transducers or meas- urement systems that ordinarily are located on and within the drill collar nearest the drill bit.
  • the drill collar 25 is preferably nearest the drill bit, as shown in FIG. 1, and is hereinafter referred to as the logging collar 25.
  • the mud pulses that define the data propagated to the surface are produced by equipment (shown in FIG. 3) within logging collar 25.
  • the collar-contained equipment shown in FIG. 3 comprises a pressure pulse generator 43 operat ⁇ ing under control of electronics contained within an instru ment housing 45 to allow drilling mud to vent through an orifice 46 extending through the logging collar wall.
  • a measurement system embodying this invention in ⁇ cludes electronics contained in instrument housing 45, and contains elements contained in and on logging collar 25. Some of these elements on logging collar 25 are indicated in FIG. 1, and include an insulating sleeve 47 that sur- rounds a longitudinally extending portion of drill string segment 49 of logging collar 25. Three electrode bands are arranged on the insulating sleeve to define an electrod array. These bands are referred to herein as upper guard band 51, middle or measurement band 53, and lower guard band 55.
  • the insulating sleeve is configured to define an upper insulating ring 57 and a lower insulating ring 59.
  • the ring 57 separates upper guard band 51 from measurement band 53
  • ring 59 separates lower guard band 55 from measurement band 53.
  • the array configuration of these electrode bands enables focusing of current flowing through measurement band 53 upon application to the bands of appropriate elec ⁇ trical potentials relative to the potential of drill string segment 49.
  • current flowing through measurement band 53 follows current flow paths that, within a generally disk-shaped region of the surrounding formation, radiate generally perpendicularly with respect to the axis of the drill string. To so focus the measurement current requires that all three electrode bands be maintained at approximately the same instantaneous potential.
  • the flow of current through the upper band follows current flow paths that curve upwardly to return through a upper portion of drill string segment 49, and the flow of current through the lower band follows current flow paths that curve downwardly to return through a lower portion of the drill string segment.
  • Electrical resistance of a body is a function of the resistivity of the material making up the body and the shape of the body. Resistivity may be expressed as the ratio of voltage gradient (e.g., voltage difference per meter), to current density (e.g., amperes per square meter), and is generally expressed in units of ohm-meters.
  • the cross sectional area through which the radiating current flows increases with distance away from the axis.
  • less and less contribution to the total resistance is made by a given portion of a body the further such portion is from the axis.
  • most of the total resistance, defined by the formation between the measuring electrode band and the return potential is contributed by a generally disk shaped region.
  • depth of invest ⁇ igation refers to the diameter of such a disk-shaped regio involved in contributing 50% of such total resistance.
  • the depth of investigation provided by an embodiment of this invention depends on numerous factors. For an 8- inch diameter logging collar, the depth of investigation is approximately 24 inches. To achieve this performance, the overall length of the insulating sleeve is approximatel 4 meters; the exposed length of each of the guard bands is approximately 30 cm.; the exposed length of the measurement band is approximately 7 cm., and the exposed length of each of the insulating rings is approximately 3.5 cm.
  • logging collar 25 includes a wear- ring 61 against which the upper annular face of insulating sleeve 47 abuts.
  • a wireway or cable tunnel 63 In the portion of the wall of logging collar extending upwardly from the wear ring, there is a wireway or cable tunnel 63.
  • the More '393 patent discloses a suitable construction for the wear ring and the wireway, and discloses advantages of such construc- tion.
  • the wireway is defined by an elongated tubular condui 63C (FIG. 8) , approximately 8 feet long, that is spot welde into a groove formed into the outer face of the wall of the logging collar.
  • An epoxy covering surrounds the conduit and fills in the groove.
  • wireway 63 there is a multi-conductor wireway cable 64 (FIG. 4) that extends from a lower interconnection end just below wear ring 61, through wireway 63, to an upper connection end at a serial port 65.
  • the cable 64 forms part of an overall wiring means for interconnecting, at one end, drive circuit and measuring means within instru ⁇ ment housing 45, and, at the other end, the electrode bands and associated electrical structure on and embedded in insulating sleeve 47.
  • wireway cable 64 includes a wireway supply conductor 67 and three wireway sensing-signal conductors 68, 69, and 70.
  • the wireway supply conductor forms part of a current supply path emanating from a drive node 72 within an electronics package 73 (FIG. 8) contained in instrument housing 45.
  • the wireway sensing-signal conductors 68, 69, and 70 form part of a current-sensing means for producing a current-representing signal used in providing data for determining formation resistivity.
  • the electric power for the circuitry within instru- ent housing 45 and for pressure pulse generator 43 is generated by a turbine-driven generator sub-assembly 75 which is positioned at the upper end of the collar-contained equipment shown in FIG. 3.
  • the equipment of FIG. 3 is an assembly including turbine-driven-generator sub-assembly 75 at the upper end of the assembly, pressure pulse generator 43, a sensor block 76 that fits between pressure pulse generator 43 and instrument housing 45, and conventional supporting means (not shown) to hold the overall interior assembly in place within the hollow interior of logging collar 25.
  • the in ⁇ strument housing has centralizer rings, such as centralizer rings 71 and 73 for orienting the instrument housing within logging collar 25, spaced from its interior cylindrical surface to provide a passageway for downflowing drilling mud.
  • sensor block 76 has opposite side surfaces 79 that have a convex shape to abut portions of the interior cylindrical surface of drill string segment 49.
  • the remaining side surfaces of sensor block 76, which extend between side surfaces 79, are generally flat and parallel to each other, and are accordingly spaced from the interior cylindrical surface of the drill string segment.
  • a torque connection sub-assembly 83 provides cabling for . electrically interconnecting upper bulkhead connector 81 to a lower bulkhead connector 85.
  • the cabling that is electrically interconnected by the upper and lower bulkhead connectors is electrically connected to cabling within instrument housing 45 and contains numerous conductors in addition to conductors that provide interconnections of components of the resistance measuring system of this inven ⁇ tion.
  • the numerous conductors in this cabling include conductors connected to a mud resistivity and temperature sensor 86 for providing signals useful in determining true formation resistivity on the basis of data about apparent resistivity and mud resistivity.
  • FIG. 9 there will now be described a block diagram of that portion of the circuitry included in the presently preferred embodiment of this invention that is contained within the electronics package.
  • An un- regulated d.c. potential Vu is applied to a d.c. voltage regulator 90 which produces a regulated d.c. potential V R .
  • the unregulated potential Vy is derived in conventional manner from the output of turbine-driven generator 75 (FIG. 3).
  • the unregulated d.c. potential is subject to variation in the range between about 20 volts and 24 volts with respect to ground potential defined at return node 74.
  • the magnitude of VT depends upon the amount of current drawn by various circuits in the electronic package in addition to the circuitry forming part of the resistivity measurement system.
  • the output of power amplifier 94 is applied to drive node 72.
  • the voltage that power amplifier 94 applies across drive node 72 and return node 74 is referred to herein as V Q .
  • V Q The voltage that power amplifier 94 applies across drive node 72 and return node 74 is referred to herein as V Q .
  • V Q has a generally square wave shape, with a frequency or pulse repetition rate of ap ⁇ proximately 1000 Hz (1 KHz) .
  • a conductor 97 constitutes part of the supply current flow path defined by end-to-end connected conductors within the interconnecting wiring means that extends through the torque connection, etc., to and through the wireway, and through the upper portion of insulating sleeve 47, as shown in FIG. 4.
  • the output frequency of power amplifier 94 is con ⁇ trolled in accord with conventional digital circuit techni ⁇ ques.
  • clocking pulses are generated, preferably by a crystal controlled oscillator not shown, and distributed generally throughout the electronics package for controlling the timing of a variety of circuits.
  • a crystal controlled oscillator not shown
  • the desired output frequency of approximately 1 KHz is readily obtained.
  • FIG. 9 there are shown two input switching signals, each operating at approximately 1 KHz and having opposite phases, ⁇ A and ⁇ B, that are produced by such conventional digital techniques. These two switching signals are applied to power amplifier 94.
  • the circuitry thus far described with reference to FIG. 9 forms the preferred embodiment of a circuit means for applying a drive voltage across drive node 72 and return node 74.
  • circuitry of FIG. 9 further includes circuitry forming a voltage measuring means for producing a voltage- representing signal V F .
  • This circuitry includes a trans- former 99 having a center-tapped secondary that applies oppositely-phased input signals to a conventional integrate circuit synchronous detector 101.
  • the detector 101 also receives the ⁇ A and ⁇ B signals to provide for synchronously demodulating the input signals to produce an input signal for a conventional low pass filter 103.
  • the output of low pass filter 103 is applied to a buffer amplifier 105 that produces the voltage-representing signal Vp.
  • the circuitry of FIG. 9 further includes circuitry forming part of a current measuring means for producing a current-representing signal Ip.
  • This circuitry includes a conventional integrated circuit synchronous detector 106 responsive to oppositely-phased current-sensing input sig ⁇ nals that are referenced to the return node.
  • the three conductors that are connected to the signal input of ⁇ ynch- ronous detector 106 are conductors 107, 108, and 109.
  • the detector 106 also receives the ⁇ A and ⁇ B signals to provide for synchronously demodulating the current-sensing input signals to produce an input signal for a conventional low pass filter 110.
  • the output of low pass filter 110 is applied to a buffer amplifier 111 that produces the current- representing signal Ip.
  • the complementary input signals ⁇ A and ⁇ B are applied to resistors 112 and 113 respectively.
  • Resistor 112 is connected in series with the base emitter junction of n-p-n switching transistor 114.
  • the emitter electrode of transistor 114 is connected to the return node 74 so as to operate at ground potential.
  • the collector electrode of transistor 114 is connected through a collector load resistor 115 to the DC regulated voltage V R .
  • a zener diode 117 is connected across the collector electrode of transis ⁇ tor 114 and the return node.
  • a n-p-n power transistor 119 has its base electrode connected to the junction of the collector electrode of transistor 114 and the cathode of zener 119.
  • the emitter electrode of transistor 119 is connected to one end of a current limiting resistor 121, the other end of which is connected to return node 74.
  • the collector electrode of transistor 119 is con ⁇ nected to one end of a primary winding 122 of a transformer 123.
  • the opposite end of primary winding 122 is connected to one end of a primary winding 125 on a transformer 127.
  • the opposite end of primary winding 125 is connected to the regulated voltage R.
  • the above-described transistors, zener diode, and primary windings form one-half of a bal ⁇ anced circuit arrangement, this half of the arrangement being responsive to the ⁇ A signal; the other half of the balanced arrangement is responsive to the complementary signal ⁇ B and similarly includes two transistors, viz, n- p-n switching transistor 129 and n-p-n power transistor 131, a zener diode 133, a current limiting resistor 135, and primary windings 138 and 139.
  • Transformer 123 has a single secondary winding 141, and transformer 127 has a single secondary winding 143.
  • the secondary windings are connected in series, with one end of the series connection being connected to return node 74 (i.e., at ground potential), and with the opposite end connected to drive node 72.
  • transsistors 114 and 129 alternately switch on and off in response to the signals ⁇ A and ⁇ B.
  • the zener diodes and the current-limiting resistors are selected to limit the current flowing through the collector electrode of the "on" power transistor to a maximum of one amp.
  • each zener diode has a zener breakdown voltage of 2.4 volts
  • each current-limiting resistor has a resistance of 1.8 ohms.
  • the base-emitter junction voltage drop of the "on" transistor is approximately 0.6 volts at the maximum output current level, so that a maximu voltage of 1.8 volts is developed across the 1.8 ohm resis ⁇ tor to limit the collector current to 1 amp.
  • the drive voltage Vrj produced at drive node 72 has a generally square-wave shape, a frequency or pulse repeti ⁇ tion rate of approximately 1 KHz, and a magnitude that depends upon the resistance load presented to it. That resistance load varies depending upon the resistivity of the formation surrounding the array of band electrodes. The resistance load presented with an apparent resistivity of 1 ohm-meter is approximately 0.44 ohms.
  • This resistance load constitutes the parallel combination of the resistance loads presented by the upper guard band and upper region of the formation (approximately 1 ohm) , by the measurement guard band and the disk-shaped region of the formation (ap ⁇ proximately 5 ohms) , and by the lower guard band and the lower region of the formation (approximately 1 ohm) .
  • FIG. 5 shows the three electrode bands and interconnected electrical com- ponents that form a sub-assembly that is connected together on a mandrel and then incorporated into the insulating sleeve while it is being made.
  • the sub-assembly of FIG. 5 includes, among other components, upper guard band 51, measurement band 53, and lower guard band 55.
  • Each band is made by spot welding together two thin-wall steel tubes, each made from thin, flat steel sections that are rolled into cylindrical shape and joined along a longitudinally extending seam.
  • the inner of the two tubes projects outwardly at opposite ends, and defines tangs 145 for fixing the orientation of the bands in place upon assembly into the insulating sleeve.
  • the tangs are regularly arranged, except that a tang is removed so as not to interfere with routing of conductors that also form part of the sub-assembly of FIG. 5.
  • Conductor 151 is one of the string of conductors that are interconnected in series as schematically shown in FIG. 4, to define a supply current path connected to drive node 72 within the elec ⁇ tronics package.
  • conductors 157, 158, and 159 is also, as schematically shown in FIG. 4, connected in a separate circuit path involved in providing the oppositely- phased current-sensing input signals that are referenced to the return node.
  • FIG. 5 a portion of the cylindrical stainless steel wall of each band is broken away to show interior features including a copper-plated pad or conductor-connec ⁇ tion areas.
  • the interior facing surface of upper guard band 51 has such a copper-plated pad 161, preferably located near its lower end 162 (on the right in the view of FIG. 5) .
  • the interior facing surface of measurement band 53 has such a copper-plated pad 163, preferably located midway between ends 165 and 167 of its exposed length.
  • the inter- ior facing surface of lower guard band 55 has such a copper plated pad 169, preferably located near its upper end 171.
  • the supply conductor 151 has one of its ends sold ⁇ ered to pad 161.
  • supply conductor 151 is a #14 gauge copper wire having an insulating jacket or cover- ing to minimize leakage of current in use through a mud- impregnated insulating sleeve from the supply conductor to the drill string segment.
  • Each of two other conductors has an end soldered to copper pad 161 of upper guard band 51.
  • One of these two conductors viz, conductor 153, has its opposite end sold ⁇ ered to copper pad 169 on the inside of lower guard band 55.
  • conductor 153 is made of 1/4 inch wide flat braid copper wire, electrically equivalent to 14 AWG, and has its own insulating jacket or covering.
  • the other of the two conductors that has an end soldered to copper pad 161 of upper guard band 51 is conductor 155, which has its opposite end soldered to copper pad 163 on the inside of measurement band 53.
  • conductor 155 is also made of 3/8 inch wide flat braid copper wire, electrically equivalent to 12 AWG, and has its own insulating jacket or covering.
  • Conductor 155 defines a two-turn primary winding for transformer 150, which is shown separately on in FIG. 5A.
  • Transformer 150 has a toroidal magnetic core, having high initial permeability and low hysteresis loss.
  • Magnetic cores having such desireable characteristics are suitably made of Super- alloy, and are available from Magnetics, Inc.
  • its outer diameter is 0.784 inches
  • the diameter of its opening is 0.477 inches
  • its thickness is 0.170 inches.
  • the transformer secondary around this core is bifilar wound to provide a total of 1000 turns, 500 on each side of a center tap.
  • the outer diameter is 0.95 inches
  • the thickness is 0.26 inches.
  • each separator ring has an exposed length of 3.75 cm.
  • transformer 150 is placed over a partially-wrapped insulating sleeve at the longitudinal position at which separator ring 57 is to be defined, and potted in place. Further wrappings are then made so that transformer 150 is embedded within ring 57.
  • Conductor 158 is electrically connected to the center tap of transformer 150, and conductors 157 and 159 are connected to the opposite ends of the secondary winding.
  • transformer 150 and the six above-described conductors are embedded in the insulating sleeve.
  • tangs 145 of each band are embedded in insulating sleeve 47.
  • Each of four of the sleeve-embedded conductors, viz, supply conductor 151 and current-sensing conductors 157, 158, and 159, ex ⁇ tends upwardly away from upper guard band 51 (to the left in the view of FIG. 6) .
  • each of the four upwardly extending conductors has a length of approx ⁇ imately 8 feet, and its upper end exits from insulating sleeve 47 through an opening near its top end adjacent wear ring 61.
  • the free end of each of these four upwardly extending conductors is soldered to the end of a correspond- ing wireway conductor to define soldered connections SC1- SC4 (FIG. 4).
  • the More '393 patent discloses a suitable arrangemen for effecting electrical connection between conductors in a wireway or cable tunnel extending above a wear ring, and conductors embedded within a replaceable insulating sleeve.
  • the conductors within wireway 63 exit the wireway inwardly just above wear ring 61 and pass between the inside of the wear ring and the outside of the drill string segment.
  • drill string segment has a shoulder that is just above, and is protected by, wear ring 61.
  • the outside diameter of drill string segment 49 is slightly less at its lower end that at the position where wear ring 61 em ⁇ braces it, to define a slight taper to facilitate instal ⁇ lation of replaceable insulating sleeve 47.
  • FIG. 7 shows such splines and channels in engage ⁇ ment, for example at 180.
  • braided conductor within the insulating sleeve.
  • the braided conductor can be flattened out, and can be embedded in the insulating sleeve to extend along a longitudinal portion spaced from both the interior surface that contacts the drill string segment and from the exterior surface that is exposed to the upflow- ing drilling mud.
  • the dual-measurement feature is also advantageous, particularly in combination with the current-limiting fea ⁇ ture of the output power amplifier.
  • the voltage-represent ⁇ ing and current-representing signals provided to conven ⁇ tional signal processing circuits used in the mud-pulse telemetry system, define data for determining apparent resistivity over a wide dynamic range.
  • a branching node can be defined to provide a symmetric arrangement involving two current-sensing trans- formers.
  • an upper transformer can be embedded in ring 57 and a lower transformer can be em ⁇ bedded in ring 59.
  • An elongated braided conductor having its opposite ends soldered to pads 161 and 163, can be turned around the core of the upper transformer to serve as its primary winding.
  • Another elongated braided conduc ⁇ tor having its opposite ends soldered to pads 163 and 169, can be turned around the core of the lower transformer to serve as its primary winding.
  • the branching node can be defined at a soldered junction, embedded in.the insulat ⁇ ing sleeve rather than on a pad of a band, with this junc ⁇ tion connecting the end of supply conductor 97 to the braid- ed-wire branch paths to pads 161 and 169.
  • the secondaries of such upper and lower transformers can each be wound to define centertaps, with six secondary conductors being provided to supply to demodulation circuitry two current- sensing signals representing current flow in respective paths to the measurement band, and enable making multiple readings such as the average of the two signals and the difference between them.
  • the secondaries can be wound without centertaps, and can be connected in series with each other to sum the signals they provide.

Abstract

A formation resistivity system is disclosed for use while a drilling operation is occurring. The system contains three electrode bands (51, 53, 55) arranged on an insulating sleeve (47) of a drill string member (25). An electronics package (73) contained in the drill string member includes a drive voltage circuit (90, 94) to provide through a current distribution circuit (64) a focused current to the electrode bands. Formation resistivity data is obtained by a voltage measuring circuit (104) and a current measuring circuit (106) contained in the electronics package. An additional portion (150) of the current measuring circuit is arranged in the insulating sleeve.

Description

10 WELL LOGGING SYSTEM EMPLOYING FOCUSED CURRENT
IN MEASURING RESISTIVITY WHILE DRILLING
BACKGROUND OF THE INVENTION
1. Field of The Invention
15 In general, this invention relates to systems for electrical logging during well-drilling operations; more particularly, it relates to such a system employing focused current in measuring formation resistivity.
2. The Prior Art
20 Data concerning how the resistivity of a formation varies with well depth provides useful clues in exploring for oil and gas bearing beds. Over the years, many formation-resistivity measuring systems have been developed. The known systems may be classified in numerous ways. One
25 such way categorizes systems on the basis of whether the measurement is made while drilling. Another such way cate¬ gorizes systems on the basis of suitability for use either with relatively high conductivity drilling fluid ("mud") or with relatively low conductivity mud. Systems that are
__ suitable with relatively low conductivity mud involve induc¬ tion of electromagnetic waves, and generally require complex electronic circuitry.
Measuring while drilling has significant, long-recog¬ nized advantages, including the advantage of providing
35 data immediately, and further including the advantage of providing data before drilling mud has had sufficient time to invade the formation and affect the resistivity of the formation.
Such advantages, however, can be gained only if the measurement-while-drilling (MWD) system is capable of with¬ standing the extremely adverse environmental conditions prevailing down hole while drilling. The adverse environ¬ mental conditions involve high temperature and shock. Further, during the drilling operation, mud is circulated under high pressure to flow down through the drill string to the drill bit and then to flow back up in the annular space between the drill string and the wall of the borehole, carrying cuttings to the surface. Various elements of an MWD system must be contained in an instrument housing, which is a sealed pressure vessel or barrel, so as to be protected from exposure to the high pressure drilling mud. Further, any element of the MWD system that is exposed to the upwardly flowing mud, and the whipping action of the drill string against the inner wall of the borehole, must be extremely abrasion resistant.
Another problem arises in an MWD system because the electrical power for the MWD system is-generated within a downhole drilling segment by a turbine-driven generator, the turbine being driven by the downflowing mud. Because the electrical power is generated inside the drill string segment while high pressure drilling mud is circulating down inside the drill string segment and up about its out¬ side, complexities arise in distributing electrical power and electrical signals to various components of the MWD system.
In contrast to an MWD system, far fewer problems need to be addressed in a well logging system, commonly called a wireline system, that is used while drilling opera¬ tions are suspended. Because the mud is stationary while drilling operations are suspended, various elements of a wireline system are not subjected to the adverse conditions discusse above. One minor exception is that downhole temperature is somewhat higher while the mud is stationary than while the mud is circulating and to some extent providing cooling. The environmental conditions of use of a wireline system, in addition to being generally more benign, enable substantially more control over distribution of electrical power. In a wireline system, a generator is located at the surface, and the electric power it generates is easily supplied to downhole electronics.
The literature concerning wireline systems includes two papers authored by Hubert Guyod. One of these, titled "The Shielded Electrode Method," appears in the December 1951 issue of World Oil, at pages 111-116. The other, titled "Factors Affecting the Responses of Laterolog-Type Logging Systems (LL3 and LL7)," appears in the February 1964 issue of the Journal of Petroleum Technology, at pages 211-219. These papers explain the principles of operation of a wireline system that has been produced in two versions, one called a Guard Electrode βonde or LL3, and the other called a Laterolog or LL7. In each version, a cylindrical tool body is suspended in a borehole. The tool body has an array of longitudinally-spaced outer electrodes, each electrode having a cylindrical shape, there being three such electrodes in the LL3 version and seven such electrodes in the LL7 version. In each version, the electrode that is in the middle of the array is called either the exploring electrode or the measuring electrode. Ideally, all the electrodes operate at the same potential relative to a system ground potential, so that current flowing through the measuring electrode flows radially with respect to the longitudinal axis of the tool body for a substantial dis- tance through a surrounding region of the formation before curving to flow toward the system ground potential. In other words, the current flowing through the measuring electrode is focused or generally confined to a disk-shaped region of the surrounding formation. In wireline tools operating in accord with the prin¬ ciples of operation of a focused-current producing array, the practice has been to employ an on-surface source of electrical power. A common practice has been to employ multiple connectors to supply power, with the measuring electrode being connected to its on-surface source of power by a dedicated conductor allocated solely to the measuring electrode. It has further been a common practice for the power source for the measuring electrode to produce a con¬ stant-magnitude current. With the magnitude of the current flowing through the measuring electrode being fixed, a measurement of the potential difference between the poten¬ tial at the measuring electrode and the system ground pro¬ vides the information needed for calculating a resistance value. Using a factor based on the geometry of the current flow paths for a focused current measurement, an "apparent resistivity" can be determined directly from the resistance value. Apparent resistivity is the resistivity of a homo¬ geneous medium having the same resistance value and geometr involved in the measurement. The true formation resistivit is a function of apparent resistivity and mud resistivity, which may easily be independently determined.
The focused-current wireline systems made in accord with the practices discussed above have been subject to several problems in addition to the basic problems (such as mud invasion into the formation) that affect wireline systems generally. Such additional problems include a so- called "Delaware effect" problem, and a problem relating to limited dynamic range in measuring resistivity.
The "Delaware effect" problem arises in circumstance in which the borehole extends below an extremely high resis tivity formation, and into a much lower resistivity system. A wireline system that does not provide for an independent current return path below the multi-electrode array produce an inaccurately-high resistivity measurement while the extremely high-resistivity formation is in the path taken . by current flowing upwardly away from the measuring elec¬ trode. In other words, the resolution of such a wireline system in delineating narrow beds can be adversely affected. It is of course desirable to provide for delineating narrow beds, such as beds less than one foot thick. It also is desirable to provide wide dynamic range because formation-to-formation resistivity can vary in a wide range, such as 0.1 ohm-meters to 1000 ohm-meters; that is, a dy¬ namic range of 10,000 to 1. Resistivity measurement system that have been developed in the past, including wireline systems and MWD systems,- generally have a limited dynamic range that falls short by an order of magnitude relative to a desirable dynamic range of 10,000 to 1.
Whereas the general concept of a focused-current producing array has been developed for practical use in wireline systems, other techniques have been employed in MWD systems in which environmental conditions and various obstacles preclude using the practices that have been devel¬ oped for wireline systems. U.S. Patent No. 4,570,123 to Grosso explains the principles of operation of two MWD formation resistivity systems. Each system employs an annular ring of insulation having annular electrodes on it at longitudinally spaced- apart locations. In one system, there are four such elec- trodes; in the other, five such electrodes. In either system, a first electrode is connected to a constant current source; second and third electrodes are connected across a voltmeter; and a fourth electrode is connected to form part of a circuit path through which some of the current flowing through the first electrode (and through the forma- tion) returns to the constant current source. As explained in the Grosso patent, it is possible in a four-electrode system for a leakage current path to exist and to conduct an undesirable amount of leakage current that can cause an inaccurate measurement. In the five-electrode system Grosso teaches, the fifth electrode is intended to form part of a second return circuit path to the constant current source. Grosso teaches that the sum of the currents flowing in the two return paths will equal the magnitude of the current produced by the constant current source. Grosso further teaches employing an ammeter to measure the current flowing in one of the two return paths. Based on simultaneous measurements of voltage drop between the second and third electrodes, and of current flowing in a return path, a resistance value can be calculated.
Like the focused-current wireline systems discussed above, each of the MWD systems Grosso describes has a limit¬ ed dynamic range. Each of these systems is plagued by additional problems relating to borehole effects such as highly conductive muds, large borehole diameters and high formation-to-mud resistivity contrasts. In effect, these systems in some circumstances measure little more than the resistivity of the mud rather than the resistivity of the formation. Further, these systems suffer from poor bed boundary resolution and depth of investigation.
Such shortcomings are widely recognized. Neverthe¬ less, because of the basic advantages of measuring while drilling, extensive use has been made of such MWD systems. Further, many developments have been made with respect to making the electrically insulated sleeves used in these prior art MWD systems. U.S. Patent No. 4,483,393 to More et al. (More '393 patent) discloses among other things a particularly advantageous method of making an insulating sleeve. Insulating sleeves made as described in the More *393 patent have been used in an MWD formation-resistivity measurement system sometimes called a "short normal" system. The short normal has four electrode rings in an array, the middle two being connected to a voltmeter, and the outer two being connected such that the drilling mud, in parallel with the formation, define a circuit load for a constant- magnitude current source. One of the advantages of the construction method taught in the More '393 patent is that a damaged insulating sleeve can relatively quickly replaced in the field. The need for such replacement arises even though the insulating sleeve is extremely rugged; however, the extremely adverse downhole conditions to which it is subjected in use are such that the insulating sleeve has a much more limited life than the drill string segment. As set forth above, the downhole conditions during drilling involve high temperature, high pressure, shock, and abras¬ ion. Further, the upflowing mud impregnates the insulating sleeve, with the result that the insulation resistance of the sleeve decreases.
SUMMARY OF THE INVENTION
This invention provides an improved system for meas¬ uring formation resistivity while a drilling operation is occurring.
The system can be defined in terms of structures on and within a drill string segment which preferably is a drill collar section directly connected to the drill bit. The drill string segment contains an electronics package. An insulating sleeve is provided to cover a portion of the drill string segment. Three electrode bands are arranged on the insulating sleeve. The three electrode bands are referred to herein as an upper guard band, a middle or measurement band, and a lower guard band. The bands are arranged on the insulating sleeve in a configuration or array to enable focusing of current flowing through the measurement band. The electronics package includes drive circuit means for applying a drive voltage across a drive node and a return node. The system includes means providing a supply current flow path extending from the drive node. A portion of the supply current flow path is embedded in the insulat¬ ing sleeve to extend longitudinally along a portion of the drill string segment.
In the preferred embodiment, this and other longi¬ tudinally extending and embedded conductors, collectively referred to as a sleeve-embedded cable assembly, exit the insulator sleeve close to the upper end of the sleeve. Each conductor in the sleeve-embedded cable assembly is connected to a respective conductor that extends within a wireway defined in the wall of the drill string segment, the conductors within this wireway being referred to herein collectively as a wireway-contained cable assembly. The junction of these series-connected cable assemblies is located just below a wear ring. The advantages of such a wear ring in conjunction with a replaceable insulating sleeve assembly are disclosed in the More '393 patent.
The system of this invention not only is adapted to provide those advantages but also provides significant improvements attributable to the circuit means in the electronics pack¬ age, and electrical components within the insulating sleeve, and the cooperation between them.
More particularly, the system of this invention includes current distribution means embedded in the insulat¬ ing sleeve for distributing the supply current among bran¬ ches that extend to the three electrode bands to provide for communication with upper, middle, and lower regions of the formation surrounding the drill string segment to complete a load circuit defined between the drive node and the return node. In operation, the loading of the drive circuit means is a function of the formation resistivity. The extent to which mud resistivity influences the loading, especially with higher formation-to-mud resistivity contrasts, is advantageously substantially less than prior art MWD system employing electrodes or insulating sleeves. This advantage is attributable to the focused current operation that is achievable in an MWD system in accord with this invention. A further significant and advantageous feature of the system of this invention resides in a dual measurement approach. In particular, the system includes both voltage measuring means and current measuring means. The system produces both voltage representing and current-representing signals to provide data for determining formation resis¬ tivity. In the preferred embodiment, the dynamic range of variable resistivity that is readily measured involves a ratio of 10,000 to 1. As to the current measuring means, a portion of it is within the electronics package, another portion is em¬ bedded in the insulating sleeve and defines a current-sens¬ ing means, and another portion forms part of an overall serial-interconnection cable assembly extending from the current-sensing means embedded in the insulating sleeve to the portion of the current measuring means within the elec¬ tronics package.
The foregoing and other features and advantages of the invention are described in detail below and recited in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an overall system for simultaneously drilling and logging a well, in which a logging collar includes a formation-resistivity measuring system according to this invention;
FIG. 2 is an elevation view showing exterior features of the logging collar of the drill string, such exterior features including a replaceable insulating sleeve on which there is an array of electrode bands; FIG. 3 is an elevation view showing exterior features of an elongated assembly that the logging collar contains, which elongated assembly includes an instrument housing for an electronics package;
FIG. 4 schematically shows conductors and electrical connectors, and electrical components that are on and em¬ bedded in the insulating sleeve, and that are intercon¬ nected in the presently preferred embodiment of this inven¬ tion to define various current flow paths including a supply current path and branches, and sensing signal paths; FIG. 5 shows mechanical features of components in¬ cluding bands, conductors, and a transformer that are pref¬ erably fabricated as a subassembly and then integrated into the replaceable insulating sleeve during its assembly; FIG. 5A is an enlarged view of the transformer shown in FIG. 5;
FIG. 6 is a view taken on line 6-6 of FIG. 7, and shows a longitudinal cross-section showing a portion of the insulating sleeve and the array of electrode bands; FIG. 7 is a view taken on line 7-7 of FIG. 2; FIG. 8 is an enlarged cross-section taken of area 8 of FIG. 3;
FIG. 9 is a block and schematic diagram of circuitry contained in the electronics package for performing electronic functions of the resistivity measurement system of this invention; and FIG. 10 is a schematic showing the preferred arrange¬ ment of the power amplifier of FIG. 9.
DETAILED DESCRIPTION
By way of introduction, there will now be described, with reference to FIG. l an overall simultaneous drilling and logging system that incorporates a focused-current resistivity measurement system according to this invention. A well 1 is being drilled into the earth under con¬ trol of surface equipment including a rotary drilling rig 3. In accord with a conventional arrangement, rig 3 in¬ cludes a derrick 5, derrick floor 1 , draw works 9, hook 11, swivel 13, kelly joint 15, rotary table 17, and drill string 19 that includes drill pipe 21 secured to the lower end of kelly joint 15 and to the upper end of a section of drill collars including an upper drill collar 23 and a lower drill collar 25. A drill bit 26 is carried by the lower end of drill collar 25.
Drilling fluid (or mud, as it is commonly called) is circulated from a mud pit 27 through a mud pump 29, past a desurger 31, through a mud supply line 33, and into swivel 13. - The drilling mud flows down through the kelly joint and drill string, and through jets (not shown) in the lower face of the drill bit. The drilling mud flows back up through the annular space between the outer surface of the drill string and the inner surface of the borehole to be circulated to the surface where it is returned to the mud pit through a mud return line 35. A shaker screen (not shown) separates formation cuttings from the drilling mud before it returns to the mud pit.
The overall system of FIG. 1 uses mud pulse telemetr techniques to communicate data from downhole to the surface while the drilling operation takes place. To receive data at the surface, there is a transducer 37 in mud supply line 33. This transducer generates electrical signals in response to drilling mud pressure variations, and these electrical signals are transmitted by a surface conductor 39 to a surface electronic processing system 41. As explained in U.S. Patent No. 4,216,536 to More
(More '536 patent), mud pulse telemetry techniques provide for communicating data to the surface about numerous down¬ hole conditions sensed by well logging transducers or meas- urement systems that ordinarily are located on and within the drill collar nearest the drill bit. The drill collar 25 is preferably nearest the drill bit, as shown in FIG. 1, and is hereinafter referred to as the logging collar 25. The mud pulses that define the data propagated to the surface are produced by equipment (shown in FIG. 3) within logging collar 25. The collar-contained equipment shown in FIG. 3 comprises a pressure pulse generator 43 operat¬ ing under control of electronics contained within an instru ment housing 45 to allow drilling mud to vent through an orifice 46 extending through the logging collar wall.
Each time pressure pulse generator 43 causes such venting, a negative pressure pulse is transmitted to be received by surface transducer 37. An alternative conventional arrange ment generates and transmits positive pressure pulses. A measurement system embodying this invention in¬ cludes electronics contained in instrument housing 45, and contains elements contained in and on logging collar 25. Some of these elements on logging collar 25 are indicated in FIG. 1, and include an insulating sleeve 47 that sur- rounds a longitudinally extending portion of drill string segment 49 of logging collar 25. Three electrode bands are arranged on the insulating sleeve to define an electrod array. These bands are referred to herein as upper guard band 51, middle or measurement band 53, and lower guard band 55. The insulating sleeve is configured to define an upper insulating ring 57 and a lower insulating ring 59. The ring 57 separates upper guard band 51 from measurement band 53, and ring 59 separates lower guard band 55 from measurement band 53. The array configuration of these electrode bands enables focusing of current flowing through measurement band 53 upon application to the bands of appropriate elec¬ trical potentials relative to the potential of drill string segment 49. As schematically and generally indicated in FIG. 1, current flowing through measurement band 53 follows current flow paths that, within a generally disk-shaped region of the surrounding formation, radiate generally perpendicularly with respect to the axis of the drill string. To so focus the measurement current requires that all three electrode bands be maintained at approximately the same instantaneous potential. With the electrode bands being maintained at the same or approximately the same instantaneous potential, the flow of current through the upper band follows current flow paths that curve upwardly to return through a upper portion of drill string segment 49, and the flow of current through the lower band follows current flow paths that curve downwardly to return through a lower portion of the drill string segment. Electrical resistance of a body is a function of the resistivity of the material making up the body and the shape of the body. Resistivity may be expressed as the ratio of voltage gradient (e.g., voltage difference per meter), to current density (e.g., amperes per square meter), and is generally expressed in units of ohm-meters.
For a disk shaped body of uniform resistivity having current following radially, the cross sectional area through which the radiating current flows increases with distance away from the axis. Thus, less and less contribution to the total resistance is made by a given portion of a body the further such portion is from the axis. Accordingly, regardless of the overall lengths and shapes of the paths taken by the generally focused current, most of the total resistance, defined by the formation between the measuring electrode band and the return potential, is contributed by a generally disk shaped region. The term "depth of invest¬ igation" refers to the diameter of such a disk-shaped regio involved in contributing 50% of such total resistance.
The depth of investigation provided by an embodiment of this invention depends on numerous factors. For an 8- inch diameter logging collar, the depth of investigation is approximately 24 inches. To achieve this performance, the overall length of the insulating sleeve is approximatel 4 meters; the exposed length of each of the guard bands is approximately 30 cm.; the exposed length of the measurement band is approximately 7 cm., and the exposed length of each of the insulating rings is approximately 3.5 cm.
As indicated in FIG. 2, logging collar 25 includes a wear- ring 61 against which the upper annular face of insulating sleeve 47 abuts. In the portion of the wall of logging collar extending upwardly from the wear ring, there is a wireway or cable tunnel 63. The More '393 patent discloses a suitable construction for the wear ring and the wireway, and discloses advantages of such construc- tion. In the presently preferred embodiment of this inven¬ tion, the wireway is defined by an elongated tubular condui 63C (FIG. 8) , approximately 8 feet long, that is spot welde into a groove formed into the outer face of the wall of the logging collar. An epoxy covering surrounds the conduit and fills in the groove.
In wireway 63, there is a multi-conductor wireway cable 64 (FIG. 4) that extends from a lower interconnection end just below wear ring 61, through wireway 63, to an upper connection end at a serial port 65. The cable 64 forms part of an overall wiring means for interconnecting, at one end, drive circuit and measuring means within instru¬ ment housing 45, and, at the other end, the electrode bands and associated electrical structure on and embedded in insulating sleeve 47. With reference to the schematic of FIG. 4, wireway cable 64 includes a wireway supply conductor 67 and three wireway sensing-signal conductors 68, 69, and 70. The wireway supply conductor forms part of a current supply path emanating from a drive node 72 within an electronics package 73 (FIG. 8) contained in instrument housing 45. Current flowing from drive node- 72, through the current supply path to be distributed via branch circuit paths so as to flow through the formation surrounding the electrode bands, returns to a return node 74 within electronics pack- age 73. The wireway sensing-signal conductors 68, 69, and 70 form part of a current-sensing means for producing a current-representing signal used in providing data for determining formation resistivity.
The electric power for the circuitry within instru- ent housing 45 and for pressure pulse generator 43 is generated by a turbine-driven generator sub-assembly 75 which is positioned at the upper end of the collar-contained equipment shown in FIG. 3.
The equipment of FIG. 3 is an assembly including turbine-driven-generator sub-assembly 75 at the upper end of the assembly, pressure pulse generator 43, a sensor block 76 that fits between pressure pulse generator 43 and instrument housing 45, and conventional supporting means (not shown) to hold the overall interior assembly in place within the hollow interior of logging collar 25. The in¬ strument housing has centralizer rings, such as centralizer rings 71 and 73 for orienting the instrument housing within logging collar 25, spaced from its interior cylindrical surface to provide a passageway for downflowing drilling mud.
As shown in detail in FIG. 8, sensor block 76 has opposite side surfaces 79 that have a convex shape to abut portions of the interior cylindrical surface of drill string segment 49. The remaining side surfaces of sensor block 76, which extend between side surfaces 79, are generally flat and parallel to each other, and are accordingly spaced from the interior cylindrical surface of the drill string segment. Thus, there is a passageway provided for down- flowing drilling mud. Within sensor block 76, means are provided for elec¬ trically interconnecting wireway cable 64, which terminates at serial port 65, to an upper bulkhead connector 81. A torque connection sub-assembly 83 provides cabling for . electrically interconnecting upper bulkhead connector 81 to a lower bulkhead connector 85. The cabling that is electrically interconnected by the upper and lower bulkhead connectors is electrically connected to cabling within instrument housing 45 and contains numerous conductors in addition to conductors that provide interconnections of components of the resistance measuring system of this inven¬ tion. The numerous conductors in this cabling include conductors connected to a mud resistivity and temperature sensor 86 for providing signals useful in determining true formation resistivity on the basis of data about apparent resistivity and mud resistivity.
Some of the electrical components of the resistivity measurement system of this invention are contained within instrument housing 45 and are mounted on circuit boards supported in tubular electronics package 73, an end portion of which is shown in FIG. 8.
With reference to FIG. 9, there will now be described a block diagram of that portion of the circuitry included in the presently preferred embodiment of this invention that is contained within the electronics package. An un- regulated d.c. potential Vu is applied to a d.c. voltage regulator 90 which produces a regulated d.c. potential VR. The unregulated potential Vy is derived in conventional manner from the output of turbine-driven generator 75 (FIG. 3). In operation, the unregulated d.c. potential is subject to variation in the range between about 20 volts and 24 volts with respect to ground potential defined at return node 74. The magnitude of VT depends upon the amount of current drawn by various circuits in the electronic package in addition to the circuitry forming part of the resistivity measurement system.
As is conventional, there is available in the elec¬ tronics package other, lower-voltage and lower output power, d.c. regulated power supplies for use by integrated cir¬ cuits. These lower-voltage, separate, d.c. regulated power supplies are used to supply operating current to all the circuitry involved in FIG. 9 except for a current-limited power amplifier 94, which is described in detail below with reference to FIG. 10.
The output of power amplifier 94 is applied to drive node 72. The voltage that power amplifier 94 applies across drive node 72 and return node 74 is referred to herein as VQ. Preferably, it has a generally square wave shape, with a frequency or pulse repetition rate of ap¬ proximately 1000 Hz (1 KHz) . A conductor 97 constitutes part of the supply current flow path defined by end-to-end connected conductors within the interconnecting wiring means that extends through the torque connection, etc., to and through the wireway, and through the upper portion of insulating sleeve 47, as shown in FIG. 4. The output frequency of power amplifier 94 is con¬ trolled in accord with conventional digital circuit techni¬ ques. As is customary within electronics packages in logg¬ ing collars, clocking pulses are generated, preferably by a crystal controlled oscillator not shown, and distributed generally throughout the electronics package for controlling the timing of a variety of circuits. Using conventional digital circuits for frequency division, the desired output frequency of approximately 1 KHz is readily obtained. In FIG. 9 there are shown two input switching signals, each operating at approximately 1 KHz and having opposite phases, øA and øB, that are produced by such conventional digital techniques. These two switching signals are applied to power amplifier 94. The circuitry thus far described with reference to FIG. 9 forms the preferred embodiment of a circuit means for applying a drive voltage across drive node 72 and return node 74.
' he circuitry of FIG. 9 further includes circuitry forming a voltage measuring means for producing a voltage- representing signal VF. This circuitry includes a trans- former 99 having a center-tapped secondary that applies oppositely-phased input signals to a conventional integrate circuit synchronous detector 101. The detector 101 also receives the øA and øB signals to provide for synchronously demodulating the input signals to produce an input signal for a conventional low pass filter 103. The output of low pass filter 103 is applied to a buffer amplifier 105 that produces the voltage-representing signal Vp.
The circuitry of FIG. 9 further includes circuitry forming part of a current measuring means for producing a current-representing signal Ip. This circuitry includes a conventional integrated circuit synchronous detector 106 responsive to oppositely-phased current-sensing input sig¬ nals that are referenced to the return node. The three conductors that are connected to the signal input of εynch- ronous detector 106 are conductors 107, 108, and 109.
The detector 106 also receives the øA and øB signals to provide for synchronously demodulating the current-sensing input signals to produce an input signal for a conventional low pass filter 110. The output of low pass filter 110 is applied to a buffer amplifier 111 that produces the current- representing signal Ip.
With reference to FIG. 10 there will now be described the presently preferred embodiment of current-limited power amplifier 94. The complementary input signals øA and øB are applied to resistors 112 and 113 respectively. Resistor 112 is connected in series with the base emitter junction of n-p-n switching transistor 114. The emitter electrode of transistor 114 is connected to the return node 74 so as to operate at ground potential. The collector electrode of transistor 114 is connected through a collector load resistor 115 to the DC regulated voltage VR. A zener diode 117 is connected across the collector electrode of transis¬ tor 114 and the return node.
A n-p-n power transistor 119 has its base electrode connected to the junction of the collector electrode of transistor 114 and the cathode of zener 119. The emitter electrode of transistor 119 is connected to one end of a current limiting resistor 121, the other end of which is connected to return node 74. The collector electrode of transistor 119 is con¬ nected to one end of a primary winding 122 of a transformer 123. The opposite end of primary winding 122 is connected to one end of a primary winding 125 on a transformer 127. The opposite end of primary winding 125 is connected to the regulated voltage R. The above-described transistors, zener diode, and primary windings form one-half of a bal¬ anced circuit arrangement, this half of the arrangement being responsive to the øA signal; the other half of the balanced arrangement is responsive to the complementary signal øB and similarly includes two transistors, viz, n- p-n switching transistor 129 and n-p-n power transistor 131, a zener diode 133, a current limiting resistor 135, and primary windings 138 and 139.
Transformer 123 has a single secondary winding 141, and transformer 127 has a single secondary winding 143. The secondary windings are connected in series, with one end of the series connection being connected to return node 74 (i.e., at ground potential), and with the opposite end connected to drive node 72. in operation, transsistors 114 and 129 alternately switch on and off in response to the signals øA and øB. The zener diodes and the current-limiting resistors are selected to limit the current flowing through the collector electrode of the "on" power transistor to a maximum of one amp. Suitably, each zener diode has a zener breakdown voltage of 2.4 volts, and each current-limiting resistor has a resistance of 1.8 ohms. The base-emitter junction voltage drop of the "on" transistor is approximately 0.6 volts at the maximum output current level, so that a maximu voltage of 1.8 volts is developed across the 1.8 ohm resis¬ tor to limit the collector current to 1 amp.
The drive voltage Vrj produced at drive node 72 has a generally square-wave shape, a frequency or pulse repeti¬ tion rate of approximately 1 KHz, and a magnitude that depends upon the resistance load presented to it. That resistance load varies depending upon the resistivity of the formation surrounding the array of band electrodes. The resistance load presented with an apparent resistivity of 1 ohm-meter is approximately 0.44 ohms. This resistance load constitutes the parallel combination of the resistance loads presented by the upper guard band and upper region of the formation (approximately 1 ohm) , by the measurement guard band and the disk-shaped region of the formation (ap¬ proximately 5 ohms) , and by the lower guard band and the lower region of the formation (approximately 1 ohm) .
In circumstances in which high resistivity forma¬ tions, including formations having as high a resistivity as 1000 ohm-meters, are being measured, far less than the maximum or current-limited amount of current is drawn from the power amplifier, and its output voltage, Vp, alternates from positive to negative values set primarily by the mag¬ nitude of the regulated voltage rather than by the load resistance. The magnitude of this alternating polarity, generally square-wave shaped, drive voltage decreases with decreasing resistivity of the formation, with the magnitude * involved for relatively low-resistivity formations being set primarily by the parallel load resistance.
Reference is now made to FIG. 5, which shows the three electrode bands and interconnected electrical com- ponents that form a sub-assembly that is connected together on a mandrel and then incorporated into the insulating sleeve while it is being made.
The sub-assembly of FIG. 5 includes, among other components, upper guard band 51, measurement band 53, and lower guard band 55. Each band is made by spot welding together two thin-wall steel tubes, each made from thin, flat steel sections that are rolled into cylindrical shape and joined along a longitudinally extending seam. The inner of the two tubes projects outwardly at opposite ends, and defines tangs 145 for fixing the orientation of the bands in place upon assembly into the insulating sleeve. The tangs are regularly arranged, except that a tang is removed so as not to interfere with routing of conductors that also form part of the sub-assembly of FIG. 5. In addition to the three bands, the sub-assembly of FIG. 5 includes a current-sensing transformer 150, and six conduc¬ tors 151, 153, 155, 157, 158, and 159. Conductor 151 is one of the string of conductors that are interconnected in series as schematically shown in FIG. 4, to define a supply current path connected to drive node 72 within the elec¬ tronics package. Each of conductors 157, 158, and 159 is also, as schematically shown in FIG. 4, connected in a separate circuit path involved in providing the oppositely- phased current-sensing input signals that are referenced to the return node.
In FIG. 5, a portion of the cylindrical stainless steel wall of each band is broken away to show interior features including a copper-plated pad or conductor-connec¬ tion areas. The interior facing surface of upper guard band 51 has such a copper-plated pad 161, preferably located near its lower end 162 (on the right in the view of FIG. 5) . The interior facing surface of measurement band 53 has such a copper-plated pad 163, preferably located midway between ends 165 and 167 of its exposed length. The inter- ior facing surface of lower guard band 55 has such a copper plated pad 169, preferably located near its upper end 171. The supply conductor 151 has one of its ends sold¬ ered to pad 161. Preferably, supply conductor 151 is a #14 gauge copper wire having an insulating jacket or cover- ing to minimize leakage of current in use through a mud- impregnated insulating sleeve from the supply conductor to the drill string segment.
Each of two other conductors has an end soldered to copper pad 161 of upper guard band 51. One of these two conductors, viz, conductor 153, has its opposite end sold¬ ered to copper pad 169 on the inside of lower guard band 55. Preferably, conductor 153 is made of 1/4 inch wide flat braid copper wire, electrically equivalent to 14 AWG, and has its own insulating jacket or covering. The other of the two conductors that has an end soldered to copper pad 161 of upper guard band 51 is conductor 155, which has its opposite end soldered to copper pad 163 on the inside of measurement band 53. Preferably, conductor 155 is also made of 3/8 inch wide flat braid copper wire, electrically equivalent to 12 AWG, and has its own insulating jacket or covering.
Conductor 155 defines a two-turn primary winding for transformer 150, which is shown separately on in FIG. 5A. Transformer 150 has a toroidal magnetic core, having high initial permeability and low hysteresis loss.
Having such characteristics has the advantage of minimizing the impedance of the primary winding. Magnetic cores having such desireable characteristics are suitably made of Super- alloy, and are available from Magnetics, Inc. As to the dimensions of the core used in the presently preferred embodiment, its outer diameter is 0.784 inches, the diameter of its opening is 0.477 inches, and its thickness is 0.170 inches. The transformer secondary around this core is bifilar wound to provide a total of 1000 turns, 500 on each side of a center tap. As to the dimensions of the transformer with this core and winding, the outer diameter is 0.95 inches, and the thickness is 0.26 inches. As set forth above in describing features of insulating sleeve 47, each separator ring has an exposed length of 3.75 cm. (approximately one and one-half inches) in the presently preferred embodiment. During the making of insulating sleeve 47 on a mandrel by steps of the kind disclosed in the More '393 patent, transformer 150 is placed over a partially-wrapped insulating sleeve at the longitudinal position at which separator ring 57 is to be defined, and potted in place. Further wrappings are then made so that transformer 150 is embedded within ring 57.
Conductor 158 is electrically connected to the center tap of transformer 150, and conductors 157 and 159 are connected to the opposite ends of the secondary winding.
With the sub-assembly of FIG. 5 being in place after assembly into the insulating sleeve, transformer 150 and the six above-described conductors are embedded in the insulating sleeve. As shown in FIG. 6, tangs 145 of each band are embedded in insulating sleeve 47. Each of four of the sleeve-embedded conductors, viz, supply conductor 151 and current-sensing conductors 157, 158, and 159, ex¬ tends upwardly away from upper guard band 51 (to the left in the view of FIG. 6) . In the presently preferred embodiment, each of the four upwardly extending conductors has a length of approx¬ imately 8 feet, and its upper end exits from insulating sleeve 47 through an opening near its top end adjacent wear ring 61. The free end of each of these four upwardly extending conductors is soldered to the end of a correspond- ing wireway conductor to define soldered connections SC1- SC4 (FIG. 4).
The More '393 patent discloses a suitable arrangemen for effecting electrical connection between conductors in a wireway or cable tunnel extending above a wear ring, and conductors embedded within a replaceable insulating sleeve. In the presently preferred embodiment of this invention, , the conductors within wireway 63 exit the wireway inwardly just above wear ring 61 and pass between the inside of the wear ring and the outside of the drill string segment.
Each of the conductors that are soldered together to form respective junctions, below the wear ring, of four circuit paths extending to the electronics package, has its own insulating jacket or covering, and a separate insulating soldering sleeve covers each solder junction or joint. Further in accord with the teachings of the More '393 patent, drill string segment has a shoulder that is just above, and is protected by, wear ring 61. The outside diameter of drill string segment 49 is slightly less at its lower end that at the position where wear ring 61 em¬ braces it, to define a slight taper to facilitate instal¬ lation of replaceable insulating sleeve 47. Further, there are four longitudinally extending channels along the taper¬ ing portion of drill string segment 49, and mating splines defined on the inside of insulating sleeve 47. The cross- section of FIG. 7 shows such splines and channels in engage¬ ment, for example at 180.
One advantage of using a braided conductor within the insulating sleeve is that the braided conductor can be flattened out, and can be embedded in the insulating sleeve to extend along a longitudinal portion spaced from both the interior surface that contacts the drill string segment and from the exterior surface that is exposed to the upflow- ing drilling mud. Using such a braided copper conductor to define a branch circuit path, from the branching node defined at pad 161 on band 53 to pad 169 on band 55, main¬ tains the two bands at approximately the same instantaneous potential even at high current flows involved with low- resistivity formations such as about 0.1 ohm-meters, because the series resistance of the braided conductor is only about 0.0015 ohms. Likewise, using such a braided conductor as the primary winding of current-sensing transformer 150, introduces an acceptably low resistance into the branch circuit path from the branching node defined at pad 161 to pad 163 on band 53, such that measurement band 53 also is maintained at an approximately the same instantaneous poten¬ tial as the two guard bands. The inductance presented by the primary of the transformer is acceptably low also.
The dual-measurement feature is also advantageous, particularly in combination with the current-limiting fea¬ ture of the output power amplifier. The voltage-represent¬ ing and current-representing signals, provided to conven¬ tional signal processing circuits used in the mud-pulse telemetry system, define data for determining apparent resistivity over a wide dynamic range.
The above-described specific embodiment of this invention is presently preferred, and is subject to numerous modifications within the scope of this invention, some of which modifications have been mentioned above. Further alternatives include modifications to the current-distribu¬ tion means of the presently preferred embodiment. For example, instead of defining the branching node at pad 161 on upper band 51, a branching node can be defined to provide a symmetric arrangement involving two current-sensing trans- formers.
To provide such symmetry, an upper transformer can be embedded in ring 57 and a lower transformer can be em¬ bedded in ring 59. An elongated braided conductor, having its opposite ends soldered to pads 161 and 163, can be turned around the core of the upper transformer to serve as its primary winding. Another elongated braided conduc¬ tor, having its opposite ends soldered to pads 163 and 169, can be turned around the core of the lower transformer to serve as its primary winding. The branching node can be defined at a soldered junction, embedded in.the insulat¬ ing sleeve rather than on a pad of a band, with this junc¬ tion connecting the end of supply conductor 97 to the braid- ed-wire branch paths to pads 161 and 169. The secondaries of such upper and lower transformers can each be wound to define centertaps, with six secondary conductors being provided to supply to demodulation circuitry two current- sensing signals representing current flow in respective paths to the measurement band, and enable making multiple readings such as the average of the two signals and the difference between them. Alternatively, the secondaries can be wound without centertaps, and can be connected in series with each other to sum the signals they provide.

Claims

WHAT IS CLAIMED IS t
1. A formation resistivity measurement system for use in a measuring while drilling operation, the system comprising: a drill string segment containing an electronics package, and having an insulating sleeve, and having upper, middle, and lower electrode bands arranged on the insulation sleeve in a configuration to enable focusing of current flowing through the middle electrode band; the electronics package including drive circuit means for applying a drive voltage across a drive node and a return node; means providing a supply current flow path extending from the drive node, a portion of the supply current flow path being defined by a conductor embedded in the insulating sleeve to extend longitudinally along a portion of the drill string segment; current-distribution means embedded in the insulating sleeve for distributing the supply current among branches that extend to the upper, middle, and lower electrode bands to provide for communicating with upper, middle, and lower regions of the formation surrounding the drill string seg¬ ment to complete a load circuit defined between the drive node and the return node; voltage measuring means and current measuring means for producing voltage-representing and current-representing signals to provide data for determining formation resis¬ tivity; and a portion of the current measuring means being within the electronics package, another portion being embedded in the insulating sleeve and defining a current-sensing means, and another portion being an interconnections cable extend¬ ing between the current-sensing means embedded in the in¬ sulating sleeve and the portion of the current measuring means within the electronics package.
2. A system according to claim 1, wherein the drive circuit means includes means for reducing the drive voltage as a function of the supply current.
3. A system according to claim 2, wherein the drive circuit means includes means for producing synchronizing signals having opposite phases, and a balanced configura¬ tion of transformer drive stages responsive to the synchronizing signals.
4. A system according to claim 3, wherein the vol¬ tage measuring means includes synchronous detector means controlled by the synchronizing signals to demodulate the drive voltage.
5. A system according to claim 1, wherein the drive circuits means includes cyclically operating switching circuit means for causing the drive voltage to have a gen¬ erally square-wave shape.
6. A system according to claim 5, wherein the drive circuit means includes an oscillator for controlling the cyclically operating switching means.
7. A system according to claim 1, wherein the cur¬ rent-distribution means includes an elongated braided con¬ ductor having one end connected to the upper electrode band and the opposite end connected to the lower electrode band.
8. A system according to claim 1, wherein the cur¬ rent-sensing means includes a step-up transformer having a braided conductor for defining the primary of the trans¬ former.
9. A system according to claim 1, wherein a portion of the drill string segment above the insulating sleeve and a portion of the drill string segment below the insulat¬ ing sleeve provide independent paths through which current can return to the return node.
10. A system according to claim 9, wherein the upper annular end of the insulating sleeve abuts a wear ring having means for communicating with a wireway extending longitudinally within a portion of the drill string segment above the wear ring, and wherein the conductor for carrying the supply current extends through the wireway.
UY:gh
PCT/US1988/000034 1987-03-16 1988-02-03 Well logging system employing focused current in measuring resistivity while drilling WO1988007212A1 (en)

Applications Claiming Priority (2)

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US2593787A 1987-03-16 1987-03-16
US025,937 1987-03-16

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GB2338306A (en) * 1998-06-05 1999-12-15 Schlumberger Holdings Method and apparatus for controlling the effect of contact impedance in a logging-while-drilling environment
EP2943818A4 (en) * 2014-03-21 2017-03-22 Halliburton Energy Services, Inc. Electromagnetic formation evaluation tool apparatus and method
US10190412B2 (en) 2016-05-11 2019-01-29 Halliburton Energy Services, Inc. Determining subterranean-formation resistivity using an electromagnetic telemetry system

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Publication number Priority date Publication date Assignee Title
GB2338306A (en) * 1998-06-05 1999-12-15 Schlumberger Holdings Method and apparatus for controlling the effect of contact impedance in a logging-while-drilling environment
GB2338306B (en) * 1998-06-05 2000-12-27 Schlumberger Holdings Method and apparatus for controlling the effect of contact impedance on a galvanic tool in a logging-while-drilling application
US6373254B1 (en) 1998-06-05 2002-04-16 Schlumberger Technology Corporation Method and apparatus for controlling the effect of contact impedance on a galvanic tool in a logging-while-drilling application
EP2943818A4 (en) * 2014-03-21 2017-03-22 Halliburton Energy Services, Inc. Electromagnetic formation evaluation tool apparatus and method
US10190412B2 (en) 2016-05-11 2019-01-29 Halliburton Energy Services, Inc. Determining subterranean-formation resistivity using an electromagnetic telemetry system

Also Published As

Publication number Publication date
GB2209607A (en) 1989-05-17
EP0310633A4 (en) 1989-06-14
EP0310633A1 (en) 1989-04-12

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