الرئيسية
Drill types
There are a variety of drill mechanisms which can be used to sink a borehole into the ground. Each has its advantages and disadvantages, in terms of the depth to which it can drill, the type of sample returned, the costs involved and penetration rates achieved. There are two basic types of drills—ones which produce rock chips or ones which produce Core sample.
Auger drilling
Auger drilling is achieved by means of a helical screw which is driven into the ground with rotation; the earth is lifted up the borehole by the blade of the screw. Auger drilling is used for well drilling, fence construction, soil engineering and geochemistry reconnaissance work in exploration for mineral deposits. In some cases, mine shafts are dug with auger drills. Small augers can be mounted on the back of a utility truck, with large augers used for sinking piles for bridge foundations.
Auger drilling is restricted to generally soft, unconsolidated material or weak, weathered rock. It is cheap and fast.
Air core drilling
Air core drilling and related methods use hardened steel or tungsten blades to bore a hole into rock. The drill bit has three blades arranged around the bit head, which cut the rock. The rods are hollow and contain an inner tube which sits inside the hollow outer rod barrel. The drill cuttings are removed by injection of compressed air into the hole via the hollow inner rod. The cuttings are then blown back to surface via the outer space inside the barrel where they are collected if needed, or discarded. Drilling continues with the addition of rods to the top of the drill string. Air core drilling can occasionally produce small chunks of cored rock.
This method of drilling is used to drill the weathered regolith, as the drill rig and steel or tungsten blades cannot penetrate fresh rock. where possible, air core drilling is preferred over RAB drilling as it provides a more representative sample. Air core drilling can achieve depths approaching 200 metres in good conditions as the cuttings are removed inside the rods and are less likely to clog. However, this method is more costly and slower than RAB.
Cable tool drilling
Cable tool rigs are a traditional way of drilling small bore water wells in rural areas of the USA. Commonly called "spudders" these rigs raise and drop the bit to finely crush the rock surface. Then the bit is raised and a bail is lowered; if the borehole is dry, water is added. The bail receives the water and rock fragments, then closes, allowing them to be raised and removed. The bit is again lowered until contact is made with the rock surface, upon which it is again raised and dropped until additional bailing is possible. Cable tool rigs are simpler (thus cheaper) than similarly sized rotary rigs, but much slower and not capable of deep depths, since the tools have to be raised and lowered so often.
Diamond core drilling
Damond core drilling utilises an annular diamond-impregnated drill bit attached to the end of hollow drill rods to cut a cylindrical core of solid rock. The diamonds used are fine to microfine industrial grade diamonds. They are set within a matrix of varying hardness, from brass to high-grade steel. Matrix hardness, diamond size and dosing can be varied according to the rock which must be cut. Holes within the bit allow water to be delivered to the cutting face. This provides three essential functions; lubrication, cooling, and removal of drill cuttings from the hole.
Diamond drilling is much slower than reverse circulation (RC) drilling due to the hardness of the ground being drilled. Drilling of 1200 to 1800 metres is common and at these depths, ground is mainly hard rock. Diamond rigs need to drill slowly to lengthen the life of drill bits and rods, which are very expensive.
Core samples are retrieved via the use of a lifter tube, a hollow tube lowered inside the rod string by a winch cable until it stops inside the core barrel. As the core is drilled, the core lifter slides over the core as it is cut. An overshot attached to the end of the winch cable is lowered inside the rod string and locks on to the backend, located on the top end of the lifter tube. The winch is retracted, pulling the lifter tube to the surface. The core does not drop out the inside of the lifter tube when lifted because a lifter ring located at the bottom of the tube allows the core to move inside the tube but not fall out.
Diamond rigs can also be part of a multi-combination rig. Multi-combination rigs are a dual setup rig capable of operating in either a reverse circulation (RC) and diamond drilling role (and not at the same time). This is a common scenario where exploration drilling is being performed in a very isolated location. The rig is first setup to drill as an RC rig and once the desired metres are drilled, the rig is setup for diamond drilling. This way the deeper metres of the hole can be drilled without moving the rig and waiting for a diamond rig to setup on the pad
Oil well drilling
Oil well drilling utilises three-cone roller, fixed-cutter diamond, or diamond-impregnated drill bits to wear away at the cutting face. This is preferred because there is no need to return samples to surface for assay as the objective is to strike a formation containing oil or natural gas. Sizable machinery is used, enabling depths of several kilometres to be penetrated. Rock chips are carried to surface in bentonite and barite impregnated muds and logged; the process is known as mud logging. Another form of well logging is electronic and is frequently employed to evaluate the existence of possible oil and gas deposits in the well hole.
The drilling and production of oil and gas pose a safety risk and a hazard to the environment from the ignition of the entrained gas causing dangerous fires and also from the risk of oil leakage polluting water, land and groundwater. For these reasons, redundant safety systems and highly trained personnel are required by law in all countries with significant production.
Percussion rotary air blast drilling (RAB)
RAB drilling is used most frequently in the mineral exploration industry. The drill uses a pneumatic reciprocating piston to energetically drive a heavy drill bit into the rock. The drill bit is hollow, solid steel and has ~20 mm thick tungsten rods protruding from the steel matrix as 'buttons'. The tungsten buttons are the cutting face of the bit.
The cuttings are blown up the outside of the rods and collected at surface. Air or a combination of air and foam are used to lift the cuttings.
RAB drilling is used primarily for mineral exploration, water bore drilling and blast-hole drilling in mines, as well as for other applications such as engineering, etc. RAB produces lower quality samples because the cuttings are blown up the outside of the rods and can be contaminated from contact with other rocks. RAB drilling rarely achieves more than 150 metres depth as encountering water rapidly clogs the outside of the hole with debris, precluding removal of drill cuttings from the hole.
Reverse circulation (RC) drilling
RC drilling is similar to air core drilling, in that the drill cuttings are returned to surface inside the rods. The drilling mechanism is a pneumatic reciprocating piston known as a hammer driving a tungsten-steel drill bit. RC drilling utilises much larger rigs and machinery and depths of up to 500 metres are routinely achieved. RC drilling ideally produces dry rock chips, as large air compressors are used to dry the rock out ahead of the advancing drill bit. RC drilling is slower and costlier but achieves better penetration than RAB or air core drilling; it is cheaper than diamond coring and is thus preferred for most mineral exploration work.
Reverse circulation is achieved by blowing air down the rods, the differential pressure creating air lift of the water and cuttings up the inner tube which is inside each rod. It reaches the bell at the top of the hole, then moves through a sample hose which is attached to the top of the cyclone. The drill cuttings travel around the inside of the cyclone until they fall through an opening at the bottom and are collected in a sample bag.
The most commonly used RC drill bits are 5-8 inches in diameter and have round metal 'buttons' that protrude from the bit, which are required to drill through rock and shale. As the buttons wear down, drilling becomes slower and the rod string can potentially become bogged in the hole. This is a problem as trying to recover the rods may take hours and in extreme cases weeks. The rods and drill bits themselves are very expensive, often resulting in great cost to drilling companies when equipment is lost down the hole.
There are a variety of drill mechanisms which can be used to sink a borehole into the ground. Each has its advantages and disadvantages, in terms of the depth to which it can drill, the type of sample returned, the costs involved and penetration rates achieved. There are two basic types of drills—ones which produce rock chips or ones which produce Core sample.
Auger drilling
Auger drilling is achieved by means of a helical screw which is driven into the ground with rotation; the earth is lifted up the borehole by the blade of the screw. Auger drilling is used for well drilling, fence construction, soil engineering and geochemistry reconnaissance work in exploration for mineral deposits. In some cases, mine shafts are dug with auger drills. Small augers can be mounted on the back of a utility truck, with large augers used for sinking piles for bridge foundations.
Auger drilling is restricted to generally soft, unconsolidated material or weak, weathered rock. It is cheap and fast.
Air core drilling
Air core drilling and related methods use hardened steel or tungsten blades to bore a hole into rock. The drill bit has three blades arranged around the bit head, which cut the rock. The rods are hollow and contain an inner tube which sits inside the hollow outer rod barrel. The drill cuttings are removed by injection of compressed air into the hole via the hollow inner rod. The cuttings are then blown back to surface via the outer space inside the barrel where they are collected if needed, or discarded. Drilling continues with the addition of rods to the top of the drill string. Air core drilling can occasionally produce small chunks of cored rock.
This method of drilling is used to drill the weathered regolith, as the drill rig and steel or tungsten blades cannot penetrate fresh rock. where possible, air core drilling is preferred over RAB drilling as it provides a more representative sample. Air core drilling can achieve depths approaching 200 metres in good conditions as the cuttings are removed inside the rods and are less likely to clog. However, this method is more costly and slower than RAB.
Cable tool drilling
Cable tool rigs are a traditional way of drilling small bore water wells in rural areas of the USA. Commonly called "spudders" these rigs raise and drop the bit to finely crush the rock surface. Then the bit is raised and a bail is lowered; if the borehole is dry, water is added. The bail receives the water and rock fragments, then closes, allowing them to be raised and removed. The bit is again lowered until contact is made with the rock surface, upon which it is again raised and dropped until additional bailing is possible. Cable tool rigs are simpler (thus cheaper) than similarly sized rotary rigs, but much slower and not capable of deep depths, since the tools have to be raised and lowered so often.
Diamond core drilling
Damond core drilling utilises an annular diamond-impregnated drill bit attached to the end of hollow drill rods to cut a cylindrical core of solid rock. The diamonds used are fine to microfine industrial grade diamonds. They are set within a matrix of varying hardness, from brass to high-grade steel. Matrix hardness, diamond size and dosing can be varied according to the rock which must be cut. Holes within the bit allow water to be delivered to the cutting face. This provides three essential functions; lubrication, cooling, and removal of drill cuttings from the hole.
Diamond drilling is much slower than reverse circulation (RC) drilling due to the hardness of the ground being drilled. Drilling of 1200 to 1800 metres is common and at these depths, ground is mainly hard rock. Diamond rigs need to drill slowly to lengthen the life of drill bits and rods, which are very expensive.
Core samples are retrieved via the use of a lifter tube, a hollow tube lowered inside the rod string by a winch cable until it stops inside the core barrel. As the core is drilled, the core lifter slides over the core as it is cut. An overshot attached to the end of the winch cable is lowered inside the rod string and locks on to the backend, located on the top end of the lifter tube. The winch is retracted, pulling the lifter tube to the surface. The core does not drop out the inside of the lifter tube when lifted because a lifter ring located at the bottom of the tube allows the core to move inside the tube but not fall out.
Diamond rigs can also be part of a multi-combination rig. Multi-combination rigs are a dual setup rig capable of operating in either a reverse circulation (RC) and diamond drilling role (and not at the same time). This is a common scenario where exploration drilling is being performed in a very isolated location. The rig is first setup to drill as an RC rig and once the desired metres are drilled, the rig is setup for diamond drilling. This way the deeper metres of the hole can be drilled without moving the rig and waiting for a diamond rig to setup on the pad
Oil well drilling
Oil well drilling utilises three-cone roller, fixed-cutter diamond, or diamond-impregnated drill bits to wear away at the cutting face. This is preferred because there is no need to return samples to surface for assay as the objective is to strike a formation containing oil or natural gas. Sizable machinery is used, enabling depths of several kilometres to be penetrated. Rock chips are carried to surface in bentonite and barite impregnated muds and logged; the process is known as mud logging. Another form of well logging is electronic and is frequently employed to evaluate the existence of possible oil and gas deposits in the well hole.
The drilling and production of oil and gas pose a safety risk and a hazard to the environment from the ignition of the entrained gas causing dangerous fires and also from the risk of oil leakage polluting water, land and groundwater. For these reasons, redundant safety systems and highly trained personnel are required by law in all countries with significant production.
Percussion rotary air blast drilling (RAB)
RAB drilling is used most frequently in the mineral exploration industry. The drill uses a pneumatic reciprocating piston to energetically drive a heavy drill bit into the rock. The drill bit is hollow, solid steel and has ~20 mm thick tungsten rods protruding from the steel matrix as 'buttons'. The tungsten buttons are the cutting face of the bit.
The cuttings are blown up the outside of the rods and collected at surface. Air or a combination of air and foam are used to lift the cuttings.
RAB drilling is used primarily for mineral exploration, water bore drilling and blast-hole drilling in mines, as well as for other applications such as engineering, etc. RAB produces lower quality samples because the cuttings are blown up the outside of the rods and can be contaminated from contact with other rocks. RAB drilling rarely achieves more than 150 metres depth as encountering water rapidly clogs the outside of the hole with debris, precluding removal of drill cuttings from the hole.
Reverse circulation (RC) drilling
RC drilling is similar to air core drilling, in that the drill cuttings are returned to surface inside the rods. The drilling mechanism is a pneumatic reciprocating piston known as a hammer driving a tungsten-steel drill bit. RC drilling utilises much larger rigs and machinery and depths of up to 500 metres are routinely achieved. RC drilling ideally produces dry rock chips, as large air compressors are used to dry the rock out ahead of the advancing drill bit. RC drilling is slower and costlier but achieves better penetration than RAB or air core drilling; it is cheaper than diamond coring and is thus preferred for most mineral exploration work.
Reverse circulation is achieved by blowing air down the rods, the differential pressure creating air lift of the water and cuttings up the inner tube which is inside each rod. It reaches the bell at the top of the hole, then moves through a sample hose which is attached to the top of the cyclone. The drill cuttings travel around the inside of the cyclone until they fall through an opening at the bottom and are collected in a sample bag.
The most commonly used RC drill bits are 5-8 inches in diameter and have round metal 'buttons' that protrude from the bit, which are required to drill through rock and shale. As the buttons wear down, drilling becomes slower and the rod string can potentially become bogged in the hole. This is a problem as trying to recover the rods may take hours and in extreme cases weeks. The rods and drill bits themselves are very expensive, often resulting in great cost to drilling companies when equipment is lost down the hole.
