Coal boundary detection using an electric-field borehole telemetry apparatus

The method to detect the relative position of a drill bit in a drill string with respect to a coal seam boundary using an electric-field borehole telemetry apparatus, the steps that include:

• a) providing a measure-while-drilling apparatus that includes inclination sensors, directional sensors, logging sensors of choice and an electric-field telemetry borehole telemetry apparatus, associated with the bit,
• b) providing an insulating gap in lower end extent of the drill string,
• c) applying output voltage derived from the measure while drilling apparatus to the gap maintained in the coal seam to produce a voltage difference between electrical leads at the upper end of the string and in the earth at a distance from said upper end of the string,
• d) operating the electric-field telemetry apparatus for monitoring the inclination, direction and logging parameters,
• e) transmitting to the surface the inclination, direction and logging parameters, including detecting at the surface the data transmitted and monitoring the signal strength received at the surface,
• f) computing the usual drilling parameters needed to guide the drill string along the intended
• path,
• g) determining from the produced voltage including voltage strength received at the surface parameters indicative of bit deviation approaching or penetrating the coal boundary, and making corrections to the generally horizontal direction of drilling maintaining the gap and the terminal end of the drill string and bit, along with said insulating gap, in the coal seam, between upper and lower boundaries thereof, the gap maintained immediately behind the bit.

The method to detect the relative position of a drill bit with respect to a coal seam boundary using an electric-field borehole telemetry apparatus, steps that include:

• a) providing a measure-while-drilling apparatus that includes inclination sensors, directional sensors, logging sensors of choice and an electric-field borehole telemetry apparatus,
• b) within the electric-field borehole telemetry apparatus, in addition to monitoring the inclination, direction and logging parameters, monitoring one or more parameters of the electrical output of the telemetry apparatus,
• c) transmitting to the surface the inclination, direction and logging parameters as well as the one or more parameters of said electrical output by means of the telemetry apparatus,
• d) detecting at the surface the data transmitted and monitoring the signal strength received at the surface,
• e) computing the usual drilling parameters needed to guide the drill string along the intended path,
• f) determining from the one or more transmitted parameters of the electrical output from the downhole apparatus and the signal strength received at the surface parameters indicative of bit approaching or penetrating the coal boundary, and
• g) making corrections to the direction of drilling to maintain the bit in the coal seam,
• h) the method including:
• i) providing an insulating gap in lower end extent of the drill string, directly behind the drill bit thereby to maneuver the gap to travel with the bit in the coal seam,
• ii) applying output voltage derived from the measure while drilling apparatus to the gap maintained in the coal seam in response to bit travel to produce a voltage difference between electrical leads at the upper end of the string and in the earth at a distance from said upper end of the string.