Developed by Sasol Coal, the technology creates an underground ‘picture’ of coal reserves that is 90% complete, as opposed to the conventional accuracy rate of 30%, reports Sasol Coal directional drilling manager Conrad Kahts.
The Germiston, Gauteng, company is the constructor of a specialised directional drillrig used for the new exploration technology.
The biggest problem in South Africa when planning a coal-mine is to determine where dolerite intrusions and faults are.
Dolerite is a fine-grained mafic (containing ferromagnesian minerals) igneous rock with a chemical and mineralogical structure similar to basalt.
Even extensive conventional vertical drilling from the surface cannot reveal these intrusions or faults.
“In-seam horizontal drilling from established underground operations can reveal these intrusions and faults, but a substantial investment in the form of a shaft is needed for this type of exploration.
“One can then still be faced with a coal seam that is not feasible to mine, such as the one adjacent to Majuba power station in Mpumalanga,” reports Smith Capital Equipment group technical director Roger Briggs.
“With many of Sasol Coal’s mines located in the Secunda region, where the geology is a serious factor, a new technology was needed to assist in mine planning,” reports Kahts.
Research proved that directional drilling is a technology which can be used to determine the scope of a coal seam accurately, as well as all intrusions and faults, without first sinking a shaft.
“Directional drilling from the surface resembles a conventional system in that it requires a drillrig and a drill bit.
“However, unlike conventional vertical drilling, the drill bit can be steered in virtually any desired direction underground by an in-hole or down-hole motor and with the help of a computer-based steering control system,” explains Kahts.
Directional drilling in its first stage resembles conventional core drilling in that drilling starts from the surface and proceeds at a relatively steep angle.
Once the drill bit has reached a predetermined depth, it can be programmed to be diverted or deflected from its vertical position into one of a virtually endless choice of curved bore-hole directions.
Drill-bit rotation is not actuated by rod rotation in the conventional way.
Instead, a steerable down-hole motor provides the necessary rotational force to keep the bit progressing through its host-rock strata.
Steering is enabled through a slight bend behind the down-hole motor and the drilling bit.
This bend’s position is monitored continuously, and can be adjusted at the surface by an operator turning the drill rods.
When the rods are turned, the down-hole bend turns as well, and the new orientation of the slight bend steers the drill bit in a new direction.
The slight bend has a predetermined angle that can be adjusted as required.
It is adjusted by the operator before drilling starts, and can be changed when the motor is brought back to the surface.
The position of the dolerite intrusion is detected when the drill rate is reduced dramatically because of the greater resistance created by the hardness of dolerite.
The thickness of the dolerite is determined by drilling through it with a diamond-impregnated bit.
The strike direction of a dolerite intrusion is determined by drilling deflecting boreholes on both sides of the first borehole.
Three points of contact are needed to establish the direction of the dolerite intrusion or fault.
“We are encouraged by our results so far, with directional drilling enabling the company to achieve dolerite and related fault-detection accuracy rates of between 85% and 95%.
“This is a substantial increase from the previous rates of 30% to 35% achieved with other methods,” says Kahts.
Manufacturing of the rig by Smith Capital Equipment started in 1996, with the specialised drillrig commissioned in April last year. A second rig was commissioned last month.
“Some of the features of the drillrig are that it is powered by a diesel generator set, and all hydraulic and water systems are powered by electric motors, and a knuckle-boom crane is used to set up the rig and move carousels between their storage on the crane trailer and the operating position on the rig.
Briggs enthuses that these features reduce costs and maintenance requirements.
“When changing the carousels, with each carrying 280 m of drill rods, the crane is also used, making the operation safer and easier,” he adds.