Sites with mixed soil conditions and rock can be some of the most challenging to drill.
Bits suitable for such applications may include slant-face rock bits, rotary rock bits and percussive bits.
Percussion may be generated tophole or downhole, with the latter more effective in harder conditions.
Steering may be initiated by the shape of the bit and/or by a side force on the bit created by interference of the downhole drilling assembly within the borehole.
Rotation of the bit may be via single-member drill string, mud motor or by the inner member of a dual-member drill string.
Rotary steerable HDD systems
In mixed soil conditions and rock, rotary steerable HDD drilling systems are generally more productive than drilling systems that cannot continuously rotate the drill bit.
Two popular drilling systems that provide continuous bit rotation are: a single-member drill string supplying high volumes of drilling fluid to a downhole mud motor and; a dual-member drill string where the inner drill string rotates the bit and the outer drill string controls steering.
Dual-pipe rotary steerable HDD systems: A brief history
The major advantage of dual-pipe rotary steerable HDD systems is their ability to continuously rotate the bit while steering.
The first drilling system entered the commercial market nearly two decades ago, in 1995.
Originally, as pipe was added to the drill string, the inner pipe segments had to be screwed together before joining the outer segments.
An early revolutionary improvement allowed segments of the inner pipe (or rod) to automatically couple together as tool-joints of the concentric outer pipe are made up.
This was accomplished via a hexagonal (hex) collar attached to one end of hexagonal-ended inner pipe segments.
The first HDD drilling system to commercially implement this advancement was the Ditch Witch JT2720 All Terrain (AT) unit in 2000, using a solid inner drill string.
A hex-coupled hollow inner drill string became available two years later on the larger capacity JT4020AT.
This allowed additional fluid to be pumped downhole to flush cuttings from the bigger pilot bore it drills.
An even larger unit, the JT100AT, reached the market in 2010 (the JT2720AT was replaced by the JT3020AT in 2007).
In a conventional downhole AT drilling assembly for a dual-pipe drilling system, the inner drill string rotates the bit – typically a tri-cone tungsten carbide insert (TCI) bit in medium-hard rock.
Straight drilling is accomplished by slowly rotating the outer drill string coupled to the downhole housing.
Outer rotation is stopped at the proper clock position when a steering correction is needed.
An interference fit of the downhole housing in the borehole creates a side force on the bit to deviate the bore laterally in the desired direction.
This dual-pipe steering principle is patented.
Mechanical power rotates the bit.
Thus the flow of drilling fluid is only the amount needed to flush cuttings from the borehole – much less than required to power a mud motor.
Recent developments in dual-pipe rotary steerable HDD systems
As noted above, larger and more advanced drill rigs now have dual-pipe drilling systems.
Automated features for controlling pipe-loading, tool joint makeup and various aspects of drilling operations have reduced operator fatigue and improved productivity.
Downhole tooling and drilling methods have evolved to improve reliability, range of application and drilling rate.
A new HDD rock drilling system, evolving from a contractor’s concept, reached the market in 2011.
A downhole air hammer is substituted for a rotary bit on the AT dual pipe drilling assembly to more productively drill harder rock.
Air hammers have been utilised on single-pipe drill strings for several years, but productivity diminishes in the steering mode because drill string rotation generally must revert to an oscillatory motion about the desired clock direction, to create a carving action on the rock.
Whereas on a dual-pipe drill string the hammer and its bit are continuously rotated whether steering or drilling straight hole.
Conventional air hammers require lubrication via injection of special oil into the air stream.
This adds cost and may be an environmental concern in some situations.
The new drilling system utilises a different air hammer concept – water lubricates its non-metallic wear bands instead of oil.
A hydro-cyclone separator limits water passing through this “÷aqua’ air hammer to that needed for lubrication, thereby maintaining high productivity by preventing loss of piston blow energy.
Fluid ejected behind the hammer aids in moving rock cuttings up the borehole.
The propeller-like portion creates a cyclonic, vortex action that separates the majority of the water and ejects it through ports.
Jason Hockran, a rock drilling specialist contractor says “Air hammers are increasingly becoming a more popular alternative in the HDD rock drilling market.
They are very effective for drilling 150 mm å± pilot holes in hard rock more than 15,000 psi [100 MPa], and in certain very hard rock exceeding 25,000 psi [170 MPa], an air hammer will be more effective and efficient than a mud motor.
A limitation with air hammers is that steering is not very effective in unconsolidated formations, and it is sometimes very tricky to keep the drilled hole clean.”
Hockran emphasised that choosing a rock drilling method is project specific – no one tool or method is suited to all types of rock drilling.
Air hammers are not effective in soil and generally are not used for long bores.
Case study: Polycrystalline diamond bit, Tarrawanna Creek, Australia
S & R Pace worked with their client Endeavour Energy to convince the Environment Protection Authority (EPA) to attempt this bore after a previous contractor failed in a disaster two years prior that contaminated the creek, surrounding area and adjoining houses.
The major stipulation was that there be no inadvertent returns and no damage to council or resident’s land.
Crossing of Tarrawanna Creek, Queensland, involved drilling through 140 m of sandstone [above 20,000 psi (100 MPa)] and 100 m of clay, a total distance of 240 m.
The bore had to be 6 m below the water, which was 9 m below the bank where the Ditch Witch JT4020AT entered the ground at a negative 40 pitch.
A PCD bit drilled through both clay and sandstone, the latter at a rate of 20-25 minutes per rod.
Mud pump flow ranged between 53-75 lpm throughout the pilot bore, which was completed in two and a half days.
Backreaming to 550 mm was completed in three steps, culminating in pullback of a bundle of six 140 mm and two 63 mm conduits.
The job was completed successfully with no problems and within the given time frame of 17 days.
Ditch Witch Australia New South Wales Branch Manager Steven Mercer said “PCD bits are used a lot in Australia.
They are very good for sandstone/abrasive rock.
They drill exceptionally well and also last well.
We have seen some PCD bits drill in excess of 1,000 hours before being changed.
The Tri-cone housings do not cope anywhere near as well in the [abrasive] sandstone conditions.
Most of Sydney, Brisbane and Perth are made up of sandstone basins.
PCD bits are no good for drilling in harder rock like granite or basalts.”
Recent improvements in manufacturing and technology may have lowered the cost of a PCD bit while improving the resistance to chipping or breaking of the cutting insert.
However, the expense of a PCD bit still warrants careful evaluation when being considered for any bore – especially bores encountering chunk rock, cobble, and unconfined formations.
Conclusions
Recent advances in directional downhole tooling for dual-pipe drilling systems extend their productivity into harder rock applications.
Rotary downhole tooling can be changed out if productivity diminishes.
Larger drill units have hollow inner drill strings that pass sufficient volumes of air to operate downhole hammers and clear cuttings from the borehole.
Ability to rotate the bit while in the steering mode greatly improves air hammer productivity.
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