ELECTROMAGNETIC BOREHOLE LOGGING APPARATUS AND METHOD

Abstract: An apparatus and method for determining the dielectric constant and/or resistivity of an earth formation which is subject to invasion of fluid, for example drilling fluid from a borehole. A transmitter (17) transmits electromagnetic energy at about 20 Mhz. from a first location in the borehole. Three receivers (22, 23, 24) are spaced at locations succes­ sively further away from the transmitter. Secondary field ratio means (140, 154, 154A, 168, 170) derives the ratio of the magnitudes of the secondary electromagnetic field at the two nearest of the three receivers, and a phase difference means (130) derives the phase difference between the received ener­ gies at the two furthest of the three receivers. The dielectric constant and the resistivity are determined from the secon­ dary field amplitude ratio and the phase difference. The use of the secondary field ratio improves the depth of investigation.

Description: ELECTROMAGNETIC BOREHOLE LOGGING APPARATUS AND METHOD This invention relates to electromagnetic borehole logging, and in particular to an apparatus and method for determining the dielectric constant and/or resistivity of an earth formation in the vicinity of a borehole, the earth formation being subject to invasion of fluid such as a drilling fluid from the borehole. The problem of electromagnetic investigation of earth formations in which there is a zone invaded by a drilling fluid from the borehole and a non-invaded zone, has been discussed in US-A-4185238. That patent disc].oses a dielectric and resistivity well logging system utilising a transmitter coil with four receiver coils spaced longitudinally from the transmitter coil and having spacings of 69 cms., 132 cms., i90 cms., and 254 cms. from the transmitter coil. Our co-pending Patent Application No. of even date herewith, claiming priority from US Serial No. 480641, discloses a logging system using one transmitter and three receiver coils and which provides substantially the same information as the aforementioned US Patent but with fewer elements. The system of our co-pending Application is referred to as the short ratio, long phase system. The present invention permits improvements over the short ratio, long phase system by increasing the depth of investigation, as will be described below in connection with Figure 1. According to the present invention there is provided apparatus for determining the dielectric constant and/or resistivity of an earth formation in the vicinity of a borehole, the earth formation being subject to invasion of fluid from the borehole, said apparatus characterised by: means for transmitting electromagnetic energy into the earth formation from a first location in the borehole at a frequency which permits the electromagnetic energy to propagate through the surrounding earth formation, means for receiving electromagnetic energy at three locations in the borehole spaced successively longitudinally apart from the transmitting means and for providing signals representative of the total electromagnetic fields at those three locations, and means for determining the dielectric constant and/or resistivity of said earth formation in accordance with said signals and with portions of said signals representative of the secondary electromagnetic fields at two or more of the three receiving locations. In another aspect the invention provides a well logging method for determining the dielectric constant and/or resistivity of an earth formation in the vicinity of a borehole, the borehole being subject to invasion of fluid from the borehole, said method characterised by: transmitting electromagnetic energy into the earth formation from a first location in the borehole at a frequency which permits the electromagnetic energy to propagate through the surrounding earth formation, receiving electromagnetic energy at second, third and fourth locations in the borehole spaced successively longitudinally further away from said first location, providing signals representative of the total electromagnetic fields at the-three receiving locations in accordance with the received energy, and determining the dielectric constant and/or resistivity of said earth formation in accordance with said signals and with portions of said signals representative of the secondary electromagnetic fields at two or more of the second, third and fourth locations. An embodiment of the invention will now be described, by way of Example, with reference to the accompanying drawings, in which: Figure 1 is a plot of the response of a well logging tool constructed in accordance with the present invention along with the response of the well logging tool of a prior art system and the response of a short ratio, long phase well logging system. Figure 2 is a graphical plot of a secondary field ratio versus phase difference for a well logging system constructed in accordance -with an embodiment of the present invention. Figure 3 is a simplified block diagram of a short ratio, long phase well logging system constructed in accordance with the present invention. The short ratio, long phase system may be improved upon to increase the depth of investigation, as shown in Figure 1, by making a secondary field ratio (SFR) measurement in place of the total field amplitude ratio. The total field signals at each receiver coil consist of contributions from both a primary and a secondary electromagnetic field. The primary field is that part of the received signal which results directly from the electromagnetic field generated by the transmitter coil The transmitted electromagnetic field also excites eddy currents in the surrounding formation. These eddy currents consist of both conductive and displacement currents. The displacement current density is proportional to the dielectric constant of the surrounding material, while the conductive current density is proportional to the conductivity of the surrounding material. Both the conductive and displacement currents are themselves a source of an electromagnetic field1 which is called the secondary field. The present invention determines the primary field with the sonde located in air and then uses this signal to cancel out the primary field part of the total field so that only the secondary field is being measured. The results of this technique are shown in Figure 1 and it can be seen that this method is considerably less affected by invasion than the aforementioned short ratio, long phase system. A plot utilising the short-spaced secondary field ratio, long-spaced phase method is shown in Figure 2. This plot relates the 20 megahertz secondary field ratio and the 20 megahertz phase difference measurements to the formation resistivity and dielectric constant values. Referring now to Figure 3, a well logging sonde 11 whose main body member is preferably constructed of fiberglass or some other non-conductive material of sufficient strength characteristics, is shown suspended by a well logging cable 12 in an uncased well borehole 13 filled with borehole fluid 14 and is surrounded by earth formations 15 whose dielectric constant and resistivity properties are to be measured. Within the lower portion of the well logging sonde 11 is housed a transmitter electronic section 16 and an associated transmitting coil 17. Transmitting coil 17 is a wire wound on a non-conducting machinable ceramic material. Transmitting coil 17 is energised at a preferred frequency of 20 megahertz as hereinafter described, to transmit electromagnetic energy into earth formations 15. Receiver coils 22, 23 and 24 which are helically wound on machinable ceramic forms comprise tuned resonant circuits which are sensitive to a frequency of 20 megahertz. Receiver coils 22, 23 and 24 are located at 61, 102 and 142 cms., respectively, from transmitter coil 17 in sonde 11. Transmitter coil 17 and receiver coils 22, 23 and 24 are electrostatically shielded as indicated by the dotted line boxes around the coils. The coil spacings just recited are intended as being illustrative only and it will be appreciated by those skilled in the art that other operating frequencies in the range of interest for practicing the invention and other coil spacings than these may be used without departing from the inventive concept. Transmitter section 16 includes a transmitter 30 which may be in an off state, a lower power operational state, or a high power operational state as determined by an operator. Transmitter 30 is used to energise transmitter coil 17. Transmitter section 16 includes also transmitter mode select means 32 which provides a control signal to transmitter 30, to control the state of transmitter 30, in response from a signal from a photo detector and amplifier 34. Transmitter 30 is energised by DC voltage from battery 36. An operator at the surface determines the transmitter mode and provides, as hereinafter explained, a modulated signal down well logging cable 12 to a demodulator 40 located in the upper section of sonde 11 which demodulates the signal. and provides it to a light emitting diode 44. Light emitting diode 44 provides a corresponding light output through a fiber optic cable 46 which passes through the coils 17, 22, 23 and 24 and is then converted back into an electrical signal by photo detector and amplifier 34 located in transmitter section 16. The purpose of using fiber optic cabling is to allow the state of transmitter 30 to be controlled without creating electromagnetic interference in receiver coils 22, 23 and 24. The signals from coils 22, 23 and 24 are provided to amplifiers 48, 49 and 50, respectively, of receiver electronics 54. The outputs of receivers 48, 49 and 50 are provided to RF mixers 66, 67 and 68, respectively, where the 20 megahertz signals are heterodyned to a lower frequency preferably 1 kilohertz by action of a local oscillator 73. An automatic frequency control circuit 75 maintains the intermediate frequency locked to a frequency of 1 kilohertz reference signal provided by a 1 kilohertz reference source 78 The outputs of mixers 66, 67 and 68 are provided to voltage controlled oscillators 83, 84 and 85, respectively, which converts the mixers signals to frequency modulated signals. Voltage controlled oscillators 83, 84 and 85 provide frequency modulated signals having carrier frequencies of 20, 45 and 86 kilohertz, respectively. These carrier frequencies were chosen to provide adequate separation of the modulated carriers to allow for low pass, high pass and band pass filtering at the surface and further to fal] within the maximum transmission capability of the logging cable. The frequency modulated signals provided by voltage controlled oscillators 83, 84 and 85 are provided to summing amplifier means 90 where they are summed and provided to a cable driver 92. Cable driver 92 provides the sum signal from summing means 90 to cable 12 which conducts it uphole to the surface electronics 100 on the surface. As noted earlier, transmitter 30 may be in any one of three states. The transmitter operatipnal state is selected at the-surface to allow high power operation of transmitter 16 only while actual logging and calibrations are in process. This permits the standard battery back to operate within its power capabilities while providing a factor of 10 increase in transmitter input power during logging. Transmitter 30 state is selected by sending a dual tone signal from tone generator 105 of surface electronics 100 to cable 12 where it is conducted downhole to demodulator 40 in sonde 11. As explained previously in the discussion of transmitter section 16, the signals are then convey ed to photo detector 34 via fiber optic cable and thence to transmitter mode select 32 which controls the mode of transmitter 30. Surface electronics 100 also includes a low pass filter 110, a band pass filter 111, and a high pass filter 112 which filters the signal from cable 12 to provide reproductions of the signals from voltage control oscillators 83, 84 and 85, respectively to phase locked loop demodulators 120, 121 and 122, respectively. The output of phase locked loop demodulators 120, 121 and 122 are reproductions of the signals provided by receiver coils 22, 23 and 24, respectively, each signal having a frequency of I kilohertz The signals from phase locked loop demodulators 120, 121 and 122 are provided to phase means 130 and 140 which provide a signal representative of the phase differences between the signals received by coils 23 and 24 and by coils 22 and 23, respectively Phase locked loop demodulators 120, 121 and 122 provide their output signals also to rectifier means 154, 154A, and 154B, respectively. The outputs of phase means 130 and 140 and rectifier means 154 and 154A are provided to an analog input/output means 168. Analog input/output means 168 provides them as digital signals to microprocessor means 170. Analog input/output means 178 receives pulses from sheave wheel 150 and provides corresponding digital signals to microprocessor means 170 where depth shifting of data may be done if so desired. Microprocessor means 170 provides digital signals to analog input/output means 168, representative of the dielectric constant and of the resistivity of the earth formations, which in turn provides analog signals. The signals from rectifier means 154, 154A and 154B; from phase means 130 and 140; and from analog input/output means 168 are provided to amplifiers 155A, 155B, 155C, 155D, 155F, 155G and 155H; which amplifiers the signals and provides them to recorder means 149. Microprocessor means 170 performs the calculation of SFR and then uses this value together with the 9L 8 phase difference to calculate the dielectric constant value. The SFR is calculated in the following manner: EMI9.1 where SFR is the magnitude of the secondary field ratio and H and H5 are the field strength values at the medium and short-spaed demodulator outputs. These signals are proportional to the original signals at the medium-spaced and short-spaced receiver coils, respectively. The field strength amplitudes are normally measured relative to their values in air. Thus equation (1) becomes: EMI9.2 where AM and AS are the values of the electromagnetic field strength in air at the medium-spaced and short-spaced receiver coils, respectively. This can be reduced to the form: EMI9.3 Solving for the magnitude of (3) gives: EMI9.4 EMI9.5 and P = e-o5. The 10 phase angle results from the phase shift in air between the medium and short spaced receiver coils. The microprocessor 170 is programmed to solve equation (4). The system is calibrated by placing sonde 11 in air and adjusting the SM - OS phase value to 10 and the SFR to zero. In equation (4), this is accomplished by setting P=l degree and F=l. The factor F is adjusted by varying the d.c. output of the rectifier means 154 in the short channel. Microprocessor means 170 combines the SFR data calculated from the electromagnetic fields at the medium and short-spaced coils with the phase difference data measured between the long and medium-spaced coils. This is done by programming microprocessor means 170 to interpret the crossplot shown in Figure 2. This crossplot relates the measured values of SFR and phase difference to the formation electrical parameters of resistivity and dielectric constant. The crossplot represents the solution of the Helmholtz wave equation in a cylindrical geometry for the coil spacings, frequency and borehole parameters chosen. The phase measurement is depth-shifted by microprocessor means 170 to align the two measure points vertica3ly. The transmitting coil 17 is preferably energised at a frequency in the range of from 10 to 60 megahertz, preferab]y 20 megahertz as described above. The present invention hereinbefore described is a resistivity and dielectric constant measurement method and apparatus using secondary field ratio measurement as well as a phase shift measurement. It should be noted that a3.though the phase differences have been described above as being derived from the total fields received by receiver coils 22, 23 and 24, the phase differences may be as easily derived from the secondary fields of the total fields received by receiver coils 22,23 and 24. There is no substantial difference in the result of phase difference measurement by the alternative routes.

Claims: CLAIMS 1. Apparatus for determining the dielectric constant and/or resistivity of an earth formation (15) in the vicinity of a borehole (13), the earth formation being subject to invasion of fluid (14) from the borehole, said apparatus characterised by: means (16,17) for transmitting electromagnetic energy into the earth formation from a first location in the borehole at a frequency which permits the electromagnetic energy to propagate through the surrounding earth formation, means (22,23,24) for receiving electromagnetic energy at three locations in the borehole spaced successively longitudinally apart from the transmitting means (16,17) and for providing signals representative of the total electromagnetic fields at those three locations, and means (54,100) for determining the dielectric constant and/or resistivity of said earth formation in accordance with said signals including portions of said signals representative of the secondary electromagnetic fields at two or more of the three receiving locations. 2. Apparatus according to claim 1 characterised in that the determining means (54,100) includes: secondary field ratio means (140,154,154A,168, 170) for deriving the ratio of the magnitudes of the secondary electromagnetic fields at the two (22,23) of said three receiving locations nearest the transmitting means (16,17), and first phase difference means (130) for deriving the phase difference between said signals provided by the respective said receiving means (23,24) at the two of said three locations furthest away from the transmitting means (16,17). 3. Apparatus according to claim 1 characterised in that the determining means (54,100) includes: secondary field ratio means (140,154,154A,168, 1.70) for deriving the ratio of the magnitudes of the secondary electromagnetic fields at the two (22,23) of said three receiving locations nearest the transmitting means (16,17), and first phase difference means for deriving the phase difference between the portions of said signals representative of the secondary electromagnetic fields at the two of said three locations furthest away from the transmitting means (16,17). 4. Apparatus according to claim 2 or claim 3 characterised in that said secondary field ratio means (140,154,154A,168,170) includes second phase difference means (340) for deriving the phase difference between said signals provided by the respective said receiving means (22,23) at the two of said three receiving locations nearest to the transmitting means (16,17). 5. Apparatus according to claim 2 or claim 3 characterised in that said secondary field ratio means (140,154,154A,168,170) includes second phase difference means for deriving the phase difference between the portions of said signals representative of the secondary electromagnetic fields at the two of said three locations nearest the transmitting means (16,17). 6. Apparatus according to claim 4 or claim 5 characterised in that said determining means (54,100) includes means (168,170) for determining the resistivity and the dielectric constant of the earth formation in accordance with the derived secondary field ratio and the phase differences. 7. Apparatus according to any one of claims I to 6 characterised in that the frequency of the electromagnetic energy transmitted by said transmitting means (16,17) lies within the range of from 10 megahertz to 60 megahertz. 8. A well logging method for determining the dielectric constant and/or resistivity of an earth formation (15) in the vicinity of a borehole (13), the borehole being subject to invasion of fluid (14) from the borehole, said method characterised by transmitting electromagnetic energy into the earth formation from a first location in the borehole at a frequency which permits the electromagnetic energy to propagate - through the surrounding earth formation, receiving electromagnetic energy at second, third and fourth locations in the borehole spaced successively longitudinally further away from said first location, providing signals representative of the total electromagnetic fields at the three receiving locations in accordance with the received energy, and determining the dielectric constant and/or resistivity of said earth formation in accordance with said signals including portions of said signals representative of the secondary electromagnetic fields at two or more of the second, third and fourth locations. 9. A method according to claim 8 characterised in that the determining step inc]udes: deriving the ratio of the magnitudes of the secondary electromagnetic fields at the second and third locations, and deriving a first phase difference between the signals provided from the third and fourth locations. 10. A method according to claim 8 characterised in that the determining step includes: deriving the ratio of the magnitudes of the secondary electromagnetic fields at the second and third locations, and deriving a first phase difference between the portions of said signals representative of the secondary electromagnetic fields at the third and fourth locations. 11. A method according to claim 9 or claim 10 characterised in that the step of deriving the magnitude ratio includes deriving a second phase difference between the signals provided from the second and third locations. 12. A method according to claim 9 or claim 10 characterised in that the step of deriving the magnitude ratio includes deriving a second phase difference between the portions of said signals representative of the secondary electromagnetic fields at the second and third locations. 13. A method according to claim 11 or claim 12 characterised in that the determining step includes providing signals representative of the resistivity and dielectric constant of the earth formation in accordance with the derived secondary field ratio and the phase differences. 14. A method according to any one of claims 8 to 13 characterised in that the transmitting frequency lies in the range of from 10 to 60 megahertz.

Inventors: COX, PERCY TERRELL, JOHNSON, DONALD LLOYD, JR., WARREN, WAYNE FORD
Applicants: TEXACO DEVELOPMENT CORPORATION
Application year: 1984
Main classification: G01V3
IPC codes: G01V3