Drill stem logging with electromagnetic waves and electrostatically-shielded and inductively-coupled transmitter and receiver elements

• US 4,785,247 A
• Filed: 04/06/1987
• Issued: 11/15/1988
• Est. Priority Date: 06/27/1983
• Status: Expired due to Fees

• Current Assignee: Baroid Technology Incorporated
• Original Assignee: NL Industries Incorporated (Valhi Holding Company)
• Inventors: Grayson, William R., Thompson, Larry W., Meador, Richard A.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US07/036,170
• Time in Patent Office: 589 Days
• Field of Search: 324/323, 324/338-343, 324/356, 324/359
• US Class Current: 324/338
• CPC Class Codes: E21B 47/017 Protecting measuring instruments
• CPC Class Codes: G01V 3/28 using induction coils

Abstract

An apparatus and method for measuring formation parameters by transmitting and receiving electromagnetic signals by antennas disposed in recesses in a tubular housing member and including apparatus for reducing the coupling of electrical noise into the system resulting from conducting elements located adjacent the recesses and housing. More particularly, a method and apparatus employing electrostatic, but not magnetic, shielding of the low impedance antenna systems and inductive signal coupling to enhance the signal at the receiver is disclosed. This system is capable of measuring formation parameters in the presence of conducting bodies, e.g., the drill string, permitting incorporation of the apparatus in a measuring while drilling (MWD) logging system. In a particular embodiment, high frequency electromagnetic energy is used to measure formation resistivity. Small low impedance coils are arranged in the wall or around the outer circumferential surface of a steel drill collar and spaced from the surface a minimum distance to allow efficient propagation and reception. The low impedance receiver antennas are substantially electrostatically, but not magnetically, shielded and include inductive current coupling devices to further improve the signal. In a preferred embodiment, the coils are circularly configured about the collar and longitudinally spaced thereon to serve as transmitting and receiving elements which together with associated circuit components employ high frequency electromagnetic wave propagation techniques to measure formation parameters. The received signals are phase shifted and amplitude attenuated as they pass through the formation, and formation parameters are determined by detecting variations in travel time and attenuation between longitudinally spaced receivers.

Claims

1. A method for use in a borehole for reducing interference and for enhancing an electromagnetic signal received by receiving means on a tubular housing member, comprising:

transmitting an electromagnetic signal into a surrounding formation from a transmitting means arranged in a recess on the exterior of but insulated from a tubular, electrically conductive housing means threaded at each end for threaded engagement in a drill string;

receiving said electromagnetic signal from said surrounding formation at a receiving means, said receiving means spaced longitudinally from said transmitting means and arranged in a recess on the exterior of but insulated from said housing means;

reducing the coupling to said receiving means of the electric field component of the received signal, including noise, by electrostatically, but not magnetically, shielding substantially all of said receiving means;

picking up the received signals from said receiving means by inductive current coupling means positioned within the shielded portion of said receiving means and cooperating with said receiving means; and

communicating a signal indicative of the signal received by said receiving means to a data handling means located within said housing means.

2. The method of claim 1 comprising receiving said signal in said receiving means with a low impedance antenna and picking up said signal from said antenna onto a wire coil forming a first toroidal winding about a first ring means of high magnetic permeability and through the center of which said antenna passes.

3. The method of claim 1 comprising receiving electromagnetic signals from a surrounding formation at a plurality of receiving means each substantially electrostatically, but not magnetically, shielded and spaced longitudinally from said transmitting means, picking up said received signals by a plurality of inductive current coupling means cooperating with each receiving means and transmitting said received signals to a data handling means.

25. The method of claim 1 wherein said housing means is both electrically conductive and magnetic.

26. The method of claim 25 wherein said housing means is a steel drill collar.

4. An apparatus suitable for measuring while drilling formation parameters about a borehole traversing earth formations and containing drilling fluids, comprising:

a tubular, electrically conductive drill collar threaded at each end for threaded engagement in a string of drill pipe above the drill bit and including a longitudinal passage suitable for conveying drilling fluids therethrough;

means for transmitting electromagnetic energy into the space surrounding said drill collar, said transmitting means insulated from but disposed on the exterior of said drill collar;

first means for receiving electromagnetic energy from the space surrounding said drill collar, said first receiving means longitudinally spaced a first distance from said transmitting means and insulated from but disposed on the exterior of said drill collar;

a first receiver electrostatic shield substantially surrounding said first receiving means to reduce the electric field component of the received electromagnetic energy, including electrical noise, said first shield comprising conductive elements substantially surrounding said receiving means, insulated from said drill collar, having at least one gap to prevent eddy currents therein and maintained at the ground potential of said receiving means; and

first inductive coupling means disposed within said electrostatic shield and inductively coupled to said receiving means for coupling the signal from said receiving means to an electrical conductor for communicating to a data handling means.

5. The apparatus of claim 4 wherein said receiving means is a receiving antenna of the low impedance type.

6. The apparatus of claim 4 which also includes:

data handling means disposed within said drill collar.

7. The apparatus of claim 4 wherein said transmitting and receiving means each comprise an annular antenna, said antennas, respectively disposed within first and second longitudinally spaced, annular recesses about the exterior surface of said drill collar.

8. The apparatus of claim 7 wherein said first and second longitudinally spaced recesses extend to a depth sufficient to protect components located therein from damage but not sufficient to appreciably impair the mechanical integrity of said drill collar.

9. The apparatus of claim 8 wherein said conductive elements of said first receiver electrostatic shield comprise

a pair of semicircular conductive shield tubes surrounding the receiving antenna, said tubes having a first pair of ends spaced from one another at the electrical center of the antenna;

and

a conductive case mounted within the second recess and insulated from said housing and having a pair of opposed openings in the side walls thereof through which the receiving antenna extends, the second pair of ends of the semicircular conductive shield tubes being joined to said conductive case around the opposed side wall openings therein to form an electrostatic shield substantially enclosing the receiving antenna to reduce the coupling of electrical noise from conductive elements into the antenna.

10. The apparatus of claim 9 which also includes:

a toroidal ferromagnetic core transformer for matching the impedance between the receiving antenna and the receiver.

11. The apparatus of claim 9 wherein said receiving antenna comprises a single winding coil receiving antenna.

12. The apparatus of claim 9 wherein said inductive signal coupling means comprises a ring means having a toroidal winding of conductive wire about said ring means and said receiving antenna extends through the opening in said ring means.

13. The apparatus of claim 12 wherein the material of said ring means is selected from the group consisting of ferrite and powdered iron.

14. The apparatus of claim 4 further comprising:

a transmitter electrostatic shield substantially surrounding said transmitting means to reduce electrical noise from ground loop currents, said transmitter shield comprising conductive elements substantially surrounding said transmitting means, insulated from said drill collar, having at least one gap to prevent eddy currents therein and maintained at the ground potential of said transmitting means.

15. The apparatus of claim 14 further comprising

a second means for receiving electromagnetic energy from the space surrounding said drill collar, said second receiving means longitudinally spaced a second distance from said transmitting means and insulated from but disposed on the exterior of said drill collar;

a second receiver electrostatic shield substantially surrounding said second receiving means to reduce electrical noise from ground loop currents, said second shield comprising conductive elements substantially surrounding said receiving means, insulated from said drill collar, having at least one gap to prevent eddy currents therein and maintained at the ground potential of said receiving means; and

a second inductive coupling means disposed within said electrostatic shield for coupling the signal from said receiving means to an electrical conductor for communicating to a data handling means.

16. The apparatus of claim 15 wherein said conductive elements of said second receiver electrostatic shield comprise:

a pair of semicircular conductive shield tubes surrounding a second receiving antenna, said tubes having a first pair of ends spaced from one another at the electrical center of the antenna; and

a conductive case mounted within the second recess and insulated from said housing and having a pair of opposed openings in the side walls thereof through which said second receiving antenna extends, the second pair of ends of the semicircular conductive shield tubes being joined to said conductive case around the opposed side wall openings therein to form an electrostatic shield substantially enclosing said second receiving antenna to reduce the coupling of electrical noise from conductive elements into said second antenna.

17. The apparatus of claim 16 wherein all of said conductive elements are copper.

18. The apparatus of claim 14 wherein said conductive elements of said transmitter electrostatic shield comprise:

a pair of semicircular conductive shield tubes surrounding the transmitting antenna, said tubes having a first pair of ends spaced from one another at the electrical center of the antenna; and

a conductive case mounted within the first recess and insulated from said housing and having a pair of opposed openings in the side walls thereof through which the circular loop transmitting antenna extends, the second pair of ends of the semicircular conductive shield tubes being joined to said conductive case around the opposed side wall openings therein to form an electrostatic shield substantially enclosing the transmitting antenna to reduce the coupling of electrical noise into the transmitted signal.

19. The apparatus of claim 4 wherein said receiving antenna has an impedance less than about 100 ohms.

27. The apparatus of claim 4 wherein said drill collar is both electrically conductive and magnetic.

28. The apparatus of claim 27 wherein said drill collar is steel.

20. An apparatus suitable for measuring a formation parameter about a borehole traversing an earth formation and containing drilling fluid, comprising:

a tubular, electrically conductive housing means threaded at each end for threaded engagement in a drill string above the drill bit and including a longitudinal passage suitable for conveying drilling fluids therethrough;

means for transmitting electromagnetic energy into the formation surrounding said housing means, said transmitting means insulated from but disposed about the exterior of said housing means;

first receiving means for receiving electromagnetic energy from said formation, said first receiving means longitudinally spaced a first distance from said transmitting means and insulated from but disposed about the exterior of said housing means;

first shield means substantially surrounding said first receiving means for electrically, but not magnetically, shielding said first receiving means by reducing the coupling of the electric field component of the received signal, including noise, to said first receiving means, said first shield means comprising;

a pair of first conductive shield tubes substantially surrounding said first receiving means and within which said first receiving means is disposed, said first tubes each having a first end, said first ends spaced from one another with a first gap to prevent eddy currents therein, each having a second end, said second ends joined to a first conductive case at a pair of opposed openings in the side walls of said case through which said first receiving means extends, said first shield tubes and said first conductive case insulated from said housing means,

means mounted within said first conductive case for inductively coupling the signal from said first receiving means to an electrical conductor leading to signal handling means, and

means for electrically connecting said first shield tubes and said first conductive case to ground potential while insulating them from said conductive housing to reduce electrical noise from ground loop currents therein.

21. The apparatus of claim 20 wherein said housing means is a tubular, steel pipe section.

22. The apparatus of claim 21 wherein said conductive shield tubes are semi-circular and said gap between said pair of first ends is substantially at the electrical center of said first receiving means.

23. The apparatus of claim 22 further comprising a second electrostatically shielded receiving means longitudinally spaced a second distance from said transmitting means and insulated from but disposed about the exterior of said housing means and wherein said second receiving means and said second shield means comprise components substantially identical to those of said first receiving means and said first shield means.

24. The apparatus of claim 23 wherein said first and second distances are different.

29. The apparatus of claim 20 wherein said housing means is both electrically conductive and magnetic.

30. The apparatus of claim 29 wherein said housing means is a steel drill collar.