Apparatus and method for downhole resistivity measurements

1. A downhole tool used for measuring high-resistivity formations in the presence of drilling fluids, the tool comprising:

a body having a longitudinal axis and bore therethrough;

an array of longitudinal electrode segments electrically separated by electrical insulators, wherein an entire cross section of said body comprises at least one electrode segment;

at least one longitudinal electrode segment configured to emit a first electrical current into said formation and measure said first emitted current;

at least one longitudinal electrode segment configured to emit a second electrical current for directing said first emitted current into said formation, and

at least one longitudinal electrode segment configured to receive said first emitted current returning from said formation,

wherein said electrical insulators facilitate measurement of said high-resistivity formations only after substantially all of said first emitted current being conducted between longitudinal electrode segments first passes substantially through said formation or drilling fluids or both.

2. The tool of claim 1 , further comprising one or more azimuthal electrodes disposed on said body configured for making measurements as the tool rotates.

3. The tool of claim 1 , said electrical insulators comprising:

a pin thread on a first body portion configured to correspond with a box thread on a second body portion; and

an insulating material applied on at least one of said pin and box threads.

4. The tool of claim 3 , wherein said insulating material is selected from a group consisting of ceramic, fiberglass, and epoxy.

5. The tool of claim 1 , wherein said electrical insulators have a longitudinal width of at least about 0.1 inches up to about 1 inch.

6. The tool of claim 1 , wherein said electrical insulators are configurable to be shorted to allow adjacent longitudinal electrode segments to behave as a single electrode segment.

7. The tool of claim 6 , further comprising a switch circuit configurable to open and short said electrical insulators for different modes of measurement.

8. The tool of claim 1 , wherein said electrical insulators comprise:

a transmitting circuit generating a first current; and

a receiving circuit generating a second current,

wherein an electrical gap is generated by adjusting said second current so that a net current between said first and second currents is substantially zero.

9. The tool of claim 1 , further comprising one or more electrodes for monitoring longitudinal mud current.

10. The tool of claim 1 , further comprising a sensor for monitoring longitudinal mud current mounted off-center near an outer diameter of said body, wherein said longitudinal mud current produces a magnetic flux near said sensor.

11. The tool of claim 10 , said sensor comprising a coil oriented in a circumferential direction of said body within a groove on an outer diameter of said body, and a shield with substantially circumferential slots therein to allow magnetic flux to penetrate said shield.

12. A method of measuring high-resistivity formations in the presence of drilling fluids, the method comprising:

providing a tool having an array of longitudinal electrode segments electrically separated by electrical insulators, wherein an entire cross section of said tool comprises at least one electrode segment;

emitting and measuring a first electrical current from at least one of said longitudinal electrode segments;

emitting a second electrical current from one of said longitudinal electrode segments for directing said first electrical current into said formation; and

receiving said first emitted electrical current at one or more of said longitudinal electrode segments returning from said formation,

wherein said electrical insulators facilitate measurement of said high-resistivity formations only after substantially all of said first electrical current being conducted between longitudinal electrode segments first passes through said formation or drilling fluids or both.

13. The method of claim 12 , further comprising making measurements with one or more azimuthal electrodes on said tool while rotating.

14. The method of claim 12 , further comprising shorting electrical insulators between adjacent longitudinal electrode segments.

15. The method of claim 12 , further comprising making simultaneous measurements at different frequencies across multiple electrical insulators.