Standard Drilling Procedures and Processes
Standard Drilling Procedures and Processes
Air rotary drilling: Background Information
The air drilling method uses air as the drilling and circulating fluid. This method requires a compressor with a high volume and pressure to supply sufficient air for effective drilling. The compressed air is pumped through the drill rods to the drilling bit. Once the compressed air exits the bit it collects the cuttings and begins expanding in an upward direction or towards the lower pressure in the borehole. As the air expands its energy output increases, therefore allowing the air to carry cuttings and solids. Once the air exits the borehole the energy dissipates and the cuttings are left on the surface like sand or rock chips. Air is located anywhere, where drilling is required. It is also much more simplistic in character as supposed to a liquid drilling fluid. The characteristics of air can be altered by adding chemical components to the compressed air. By adding water, foam, and surficant we change the air density and volume allowing for improved cleaning of the borehole. Air rotary drilling is best suited for medium to hard drilling in consolidated formations. This method is commonly used on blast holes and some larger water wells.
Typical Mine Layout
Each borehole is engineered prior to drilling. Tiphography, lithology, stratigraphy, depths and deviation constraints affect the choice of equipment, borehole sizes, casing points and inhole tools necessary to confidently complete the project.
Borehole diameters and casing programs vary with stratigraphy, anticipated in-hole conditions, and locality. Typically pilot boreholes will be drilled using percussion rigs to penetrate the weathered zone and barren overburden. Percussion drilling is cheaper and quicker than diamond drilling. The disadvantages of percussion drilling are that the costs to the contractor rapidly escalate when water-bearing fissures are intersected. Yields of over 40000 L/hr are not uncommon. Percussion boreholes tend to deviate more than diamond drilled boreholes; reducing penetration rates to control deviation soon becomes unattractive as daily revenue decreases.
Percussion drilling begins in 380 or 30m to establish the standpipe into solid rock. Drilling continues in 250mm diameter to +/-400m and a 203mm ID welded casing is inserted to isolate the borehole from near-surface, unstable formations. The borehole diameter is then reduced to 200mm and drilling continues to +/- 800m where the diameter is further reduced to 165mm until the target depth. No casing is inserted at 800m because the larger the upper borehole section reduces the pressure loss associated with a small annulus and enables more power to be generated by the hammer for a given volume of air. On completion of the borehole, a casing string is grouted into the borehole using a non-return valve above the bottom casing joint and a wiper plug.
Methane gas is occasionally associated with carbonaceous shales in the upper overburden. Diverters with blue lines are placed on top of the standpipe to isolate the rig and crew from any potential blow out. Foaming additives are used to aid borehole cleaning when required.
Percussion drilling begins in 380 or 30m to establish the standpipe into solid rock. Drilling continues in 250mm diameter to +/-400m and a 203mm ID welded casing is inserted to isolate the borehole from near surface, unstable formations. The borehole diameter is then reduced to 200mm and drilling continues to +/- 800m where the diameter is further reduced to 165mm until the target depth. No casing is inserted at 800m because the larger the upper borehole section reduces the pressure loss associated with a small annulus and enables more power to be generated by the hammer for a given volume of air. On completion of the borehole a casing string is grouted into the borehole using a non-return valve above the bottom casing joint and a wiper plug.
Methane gas is occasionally associated with carbonaceous shale’s in the upper overburden. Diverters with blue lines are placed on top of the standpipe to isolate the rig and crew from any potential blow out. Foaming additives are used to aid borehole cleaning when required.
Typical Borehole Layout
Boreholes have to provide sufficient water for drilling as circulation losses are common and must be protected from contamination by drilling fluids.
Excavations for rig foundations, fluid pits and drainage; access roads, fencing and electrical installations for the site are undertaken by sub-contactors to the drilling company’s specifications.
Surface plant configurations and ratings vary between contractors, the majority of rigs are mechanically or hydrostatically driven, an integral hoist provides lifting capacity, rotation is transmitted through a quill bushing and feed rates are controlled by two hydraulic cylinders with a stroke of +/- 1m. A much smaller population of top drive or rotary table type rigs have been used for deep hole exploration. A hydrostatic drive is preferable as this greatly reduces the shock loading placed on the drill string. Auxiliary draw works may be used, with a more powerful hoist, to aid pulling and lowering of the rod string.
Typical complements are two 5 man crews each consisting of a driller, top hand and three-floor men working on a 12-hour shift five days per week. The crew alternate shifts under the control of an onsite charge hand who is on 24-hour call. Working hours are controlled by government legislation. Each borehole is considered to be an individual mine and is subject to the Mines and Works Act and the control of the Government Mining Engineer. The Act prohibits any mining on certain public holidays and Sundays if a 7-day week operation is required exemptions may be obtained. However, this requires four crews two of which work a twelve-hour shift for seven days and are then rested for seven days.
These vary between project and client and are also dependant upon the anticipated geology and target intersection parameters. For example, a borehole may be restricted to a cone centered on the borehole collar which has a base at the final true vertical depth with a diameter not exceeding one-tenth of the true vertical depth. Borehole deviation can be controlled to an extent by bit selection, drilling practices and the use of stabilizers.
Borehole deviation increases torque and the cost of drilling fluids required for lubrication; reduces equipment life and necessitates additional drilling to intersect a given target.
The doglegs caused by borehole deviation must be carefully monitored and kept within 3* per 30m of borehole wherever possible. This is particularly important in the upper portion of a borehole as the cyclic stress reversals caused by severe doglegs can rapidly cause rod failure and premature casing wear as the string weight below the dogleg increases.
Water-based, low solids drilling fluids were developed from the oil-field mud system and these are used on the majority of rigs. A filming amine type lubricant is used to coat the rods, casing, and sidewall to reduce torque and enable better energy transfer to the bit. The amine also “west” and lubricates thereby increasing bit life and provides corrosion protection for the rod string. Long-chain polyacrylamide viscosifiers enhance the carrying capacity of the fluid and dampen vibration. The shearing characteristics of the viscosifier enable efficient heat transfer and cleaning of the bit. Other additives are used to control pH, bacterial growth and to counter specific problems such as lost circulation, caving, and swelling clays.
Fluid costs may on average represent 5% of a contractor’s drilling cost. This can increase rapidly if high torque, poor formations or lost circulation zones are encountered. The prevention of environmental contamination and the disposal of spent fluids have increased project costs and make efficient management of the system an integral part of the drilling operations.
Geophysical logs are run prior to deflection drilling obtain to maximum information from the borehole. The type of logs vary but generally include dip meter, gamma neutron, VSP, and resistivity. On completion, cement plugs are inserted below the dolomite to prevent any possible water ingress should subsequent mining operations intersect the borehole. Casings are recovered where possible and inspected and graded for future use. Site clearance is affected to drill the drill site in the closest possible condition when establishment commenced.
The cost of drilling deep exploration boreholes varies greatly depending upon the ability to percuss deep pilot-boreholes; the anticipated lithology and related borehole sizes, the final depth and deflection requirements
Specific Points to the Standard Drilling Procedures