Types of Drilling Rigs
A drilling rig is a structure housing equipment used to drill for water, oil, natural gas from underground reservoirs or to obtain mineralcore samples. The term can refer to a land-based rig, a marine-based structure commonly called an 'offshore rig' or a structure that drills oil wells called an 'oil rig'. The term correctly refers to the equipment that drills oil wells or extracts mineral samples, including the rig derrick (which looks like a metal frame tower).
Sometimes a drilling rig is also used to complete (prepare for production) an oil well. However, the rig itself is not involved with the extraction of the oil, its primary function is to make a hole in the ground so that the oil can be produced.
Laypeople may refer to the structure which sits on top offshore wells as a 'rig', but this is not correct. The correct name for the structure in a marine environment is platform. A structure upon which wells produce is a production platform. A floating vessel upon which a drilling rig sits is a floating rig or semi-submersible rig because the whole purpose of the structure is for drilling.
Drilling rigs can be small and portable such as those used in mineral exploration drilling, or huge, capable of drilling through thousands of metres of the Earth's crust; large "mud pumps" are used to circulate drilling mud (slurry) through the drill bit and the casing, for cooling and removing the "cuttings" whilst a well is drilled; hoists in the rig can lift thousands of tons of pipe; other equipment can force acid or sand into reservoirs to facilitate extraction of the oil or mineral sample; and permanent living accommodation and catering for crews which may be greater than a hundred people in number. Marine rigs may operate many hundreds of miles or kilometres offshore with infrequent crew rotation.
Until the advent of internal combustion engines in the late 19th century, the primary method for drilling rock involved muscle power be it human or animal. Rods were turned by hand, using clamps attached to the rod. The rope and drop method invented in China utilized a steel rod or piston raised and dropped vertically via a rope. Mechanised versions of this persisted until about 1970, utilising a cam to rapidly raise and drop what, by then, was a steel cable.
In the 1970s, outside of the oil and gas industry, roller bits utilising mud circulation were replaced by the first efficient pneumatic reciprocating piston RC drills, and became essentially obsolete for the majority of shallow drilling, and are now only used in certain situations where rocks preclude other methods. RC drilling proved much faster and efficient, and continues to improve with better metallurgy deriving harder, more durable bits, and compressors delivering higher air pressures at higher volumes, enabling deeper and faster penetration. Diamond drilling has remained essentially unchanged since its inception.
Mobile drilling rigs
In early oil exploration, drilling rigs were semi-permanent in nature often being built on site and left in place after the completion of the well. In more recent times drilling rigs are expensive custom built machines that are capable of being moved from well to well. Some light duty drilling rigs are similar in nature to a mobile crane though these are more usually used to drill water wells. Larger land rigs must be broken apart into multiple sections and loads in order to move to a new location, a process which can often take weeks.
Small mobile drilling rigs are also used to drill or bore piles. Rigs can range from 100 ton continuous flight auger (CFA) rigs to small air powered rigs used to drill holes in quarries, etc. These rigs use the same technology and equipment as the oil drilling rigs, just on a smaller scale.
The drilling mechanisms outlined below differ mechanically in terms of the machinery used, but also in terms of the method by which drill cuttings are removed from the cutting face of the drill and returned to surface.
Drilling rig classification
There are many types and designs of drilling rigs, depending on their purpose and improvements; many drilling rigs are capable of switching or combining different drilling technologies.
by power used
electric - rig is connected to a power grid usually produced by its own generators
mechanic - rig produces power with its own (diesel) engines
hydraulic - most movements are done with hydraulic power
pneumatic - pressured air is used to generate small scale movements
by pipe used
cable - a cable is used to slam the bit on the rock (used for small geotechnical wells)
conventional - uses drill pipes
coil tubing - uses a giant coil of tube and a downhole drilling motor
single - can drill only single drill pipes, has no vertical pipe racks (most small drilling rigs)
double - can store double pipe stands in the pipe rack
triple - can store stands composed of three pipes in the pipe rack (most large drilling rigs)
quad - can store stands composed of four pipes in the pipe rack
by method of rotation
no rotation (most service rigs)
rotary table - rotation is achieved by turning a square pipe (the kelly) at drill floor level.
top-drive - rotation and circulation is done at the top of the drillstring, on a motor that moves along the derrick.
by position of derrick
conventional - derrick is vertical
slant - derrick is at an angle (this is used to achieve deviation without an expensive downhole motor)
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 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, soilengineering 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 mineralexploration, 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.
Although RC drilling is air-powered, water is also used, mainly to reduce dust but also when collaring a new hole. A mud called super foam is mixed with water and pumped into the rod string, down the hole. This helps to bring up the sample to the surface by making the sand stick together. When the drill reaches hard rock, a collar is put down the hole around the rods which is normally PVC piping. Occasionally the collar may be made from metal casing. Collaring a hole is needed to stop the walls from caving in and bogging the rod string at the top of the hole. Collars may be up to 60 metres deep, depending on the ground, although if drilling through hard rock a collar may not be necessary.
Reverse circulation rig setups usually consist of a support vehicle, an auxiliary vehicle, as well as the rig itself. The support vehicle, normally a truck, holds diesel and water tanks for resupplying the rig. It also holds other supplies needed for maintenance on the rig. The auxiliary is a vehicle, carrying an auxiliary engine and a booster engine. These engines are connected to the rig by high pressure air hoses. Although RC rigs have their own booster and compressor to generate air pressure, extra power is needed which usually isn't supplied by the rig due to lack of space for these large engines. Instead, the engines are mounted on the auxiliary vehicle. Compressors on an RC rig have an output of around 1000cfm/500psi.
Limits of the technology
Drill technology has advanced steadily since the 19th century. However, there are several basic limiting factors which will determine the depth to which a bore hole can be sunk.
All holes must maintain outer diameter; the diameter of the hole must remain wider than the diameter of the rods or the rods cannot turn in the hole and progress cannot continue. Friction caused by rotation will tend to reduce the outside diameter of the drill bit. This applies to all drilling methods, except that in diamond core drilling and oil well drilling the use of thinner rods and casing may permit the hole to continue. Casing is simply a hollow sheath which protects the hole against collapse during drilling, and is often made of metal or PVC. Often diamond holes will start off at a large diameter and when outside diameter is lost, thinner rods put down inside casing to continue, until finally the hole becomes too thin. Alternatively, the hole can be reamed.
For percussion techniques, the main limitation is air pressure. Air must be delivered to the piston at sufficient pressure to activate the reciprocating action, and in turn drive the head into the rock with sufficient strength to fracture and pulverise it. With depth, volume is added to the in-rod string, requiring larger compressors to achieve operational pressures. Secondly, groundwater is ubiquitous, and increases in pressure with depth in the ground. The air inside the rod string must be pressurised enough to overcome this water pressure at the bit face. Then, the air must be able to carry the rock fragments to surface. This is why depths in excess of 500 m for reverse circulation drilling are rarely achieved, because the cost is prohibitive and approaches the threshold at which diamond core drilling is more economic.
Diamond drilling can routinely achieve depths in excess of 1,200 m. In cases where money is no issue, extreme depths have been achieved because there is no requirement to overcome water pressure. However, circulation must be maintained to return the drill cuttings to surface, and more importantly to maintain cooling and lubrication of the cutting surface.
Without sufficient lubrication and cooling, the matrix of the drill bit will soften. While diamond is the hardest substance known to man at 10 on the Mohs hardness scale, it must remain firmly in the matrix to achieve cutting. Weight on bit, the force exerted on the cutting face of the bit by the drill rods in the hole above the bit, must also be monitored.
One final phenomenon limiting drilling only became apparent during deep drilling of an attempted Mohole.
Causes of deviation
All drill holes deviate from their plan. This is because of the torque of the turning bit working against the cutting face because of the flexibility of the steel rods and especially the screw joints, because of reaction to foliation and structure within the rock, and because of refraction as the bit moves from different rock units of varying rheology. Additionally, inclined holes will deviate upwards usually because the drill rods will lie against the bottom of the bore, causing the drill bit to be slightly inclined from true. It is because of deviation that drill holes must be surveyed if deviation will impact on the usefulness of the information returned. Often oil companies will use a process of controlled deviation called directional drilling.