Underground drilling for gas drainage is a steering problem in a space that gives no second chance. The hole has to stay inside a coal seam that may be a few metres thick, follow a trajectory that avoids old workings, and hit a target that is defined in three dimensions. An inclinometer MWD system for mining measures inclination, azimuth and tool face while the rod is still in the hole, so the operator can correct the path before the next rod is added rather than after the hole is finished. On a drainage job the difference is measured in metres of seam, and the crew sees it on the console rather than in a report a week later.
What an Inclinometer MWD Measures Underground
The probe measures inclination from -90 to +90 degrees, azimuth and tool face from 0 to 360 degrees, and reports total magnetic field and gravitational acceleration alongside. Those last two are diagnostic: a sudden change in total field usually means magnetic interference from equipment or geology, and a change in gravity suggests the probe is not sitting where the operator thinks it is. Real-time display of the trajectory lets the driller adjust the bend direction and the drilling parameters on the current rod. The probe has to pass inside the rod string, so its 35 mm body is matched to the drill rod in use rather than to the hole size.
Probe Accuracy and Communication Distance
Accuracy is plus or minus 0.2 degrees in inclination and plus or minus 1.0 degrees in azimuth and tool face. Communication distance exceeds 1,500 meters, which is what allows the console to stay at the rig floor while the probe works at the bottom of a long drainage hole. The probe is 35 mm in diameter and 950 mm long, weighs 5.2 kg, and runs on 15 to 18 volts DC with a two-wire current output between 7 and 130 mA. That current loop is also the reason the cable and the connector are checked before every shift; a wet joint shows up as a drifting reading before it shows up as a failed survey.
Intrinsically Safe Equipment for Coal Mines
The system is built for the underground environment: a flameproof and intrinsically safe computer, an intrinsically safe probe, keyboard and data converter, and certification documents that include the coal safety certificate and the explosion-proof certificate. The probe is externally powered with dual independent signal channels, and it is held by a plug-in triple-wing clamp so it can be assembled and broken down at the face without special tools. Water pressure resistance is at least 12 MPa, which covers the flushing pressures used in drainage drilling. In a gassy roadway the certification matters as much as the accuracy: the computer, probe, keyboard and converter are all supplied as intrinsically safe items with the documents the mine inspector will ask for.
Steering a Gas Drainage Hole
The working method is short-interval survey and correction. Drill a rod, take a survey, compare the inclination and azimuth with the plan, and adjust the bend before the next rod. In a seam that dips, the correction is mostly in inclination; in a hole that is crossing a disturbed zone, azimuth drift is the bigger risk. Because the data is live, the operator sees the deviation while it is small enough to correct with a bend change rather than a sidetrack. In a thin seam the inclination correction is small and frequent, and the crew that surveys every rod holds the hole inside the seam while the crew that surveys occasionally spends the shift correcting.
Survey every rod through the target interval, not every second or third.
Watch total magnetic field for interference near pumps and steel structures.
Record the survey station depth with the rod count so the two tallies agree.
Inclinometer MWD Checks Before the Shift
The equipment is only as good as the shift preparation. Charge or power the probe, check the cable and the connector for water ingress, confirm the clamp is seating the probe squarely, and run a surface check of the console against a known inclination. Operating temperature spans -40 to 75 degrees C, so a probe that has been stored in a cold roadway needs time to stabilise before the first survey. A spare probe and cable on site matter more than a faster drill rate.
An inclinometer MWD system turns a drainage hole from a guessed path into a measured one. Survey at the rod, correct at the bend, and the hole stays in the seam where the gas is.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
