What is Drilling?
Drilling in its broadest sense is any process that involves making a circular hole in a solid material or hard surface using a boring tool. With thousands of applications across industries, drilling is undoubtedly one of the greatest inventions of human civilization.
What is a Drilling Machine?
A drilling machine is any of a variety of devices used for drilling. All types of drilling machines feature a drill bit that removes parts of the material it drills into to make a hole. This drill bit most typically rotates around its axis to achieve this.
There are several common types of drilling machines used in workshops, factories, and construction sites, based on the work they are used for:
- Upright sensitive drill press: This is a type of drilling machine that is used for light-duty jobs and is operated by hand.
- Upright drill press: A heavy-duty cousin of the upright sensitive drill press, it can be operated either manually or using a power source as it is used for larger holes in harder or larger materials.
- Radial arm drill press: Possibly the most versatile drilling machine in workshops, the radial arm drill press moves over the material that needs to be drilled rather than the operator having to move the material to position it for drilling.
- Special-purpose drilling machines: These, as the name suggests, are used for special tasks, such as drilling two dozen holes at once (the gang drill and the multispindle drill press, for example) or drilling very small holes (the micro-drill press).
Types of Drilling
From micro-holes to oil wells and mining tunnels, drills are used for making extensive range of holes. Based on size, we distinguish between three main types of drilling:
Microdrilling: This type of drilling refers to the drilling of holes with a diameter of less than 0.5 mm. For such tiny diameters, drillers use high spindle-speed drills that can revolve at more than 10,000 RPM.
Deep drilling: The sort of drilling that is used in the oil and gas industry, deep drilling refers to making holes that are a lot deeper than they are wide. The ratio between their diameter and their depth is often more than 300:1.
Trepanning: This is a type of drilling where the ratio between diameter and depth is the opposite of deep drilling. Strictly speaking, it is not even proper drilling as it involves a cutting machine removing a wide-diameter disc from a hard surface such as a metal sheet, for example.
Types of Drilling Operations
Outside construction, workshops and the home, drilling is most popularly used in mining, science—for taking rock or soil samples—water supply, and in oil and gas exploration and production. In these industries, the drilling machines used are a lot larger and fall into one of two categories: percussive and rotary. Based on this distinction, there are two main types of industrial drilling:
Percussion drilling is used in mining operations to break down the rock. Here, the industry distinguishes between down-the-hole drilling and top hammer drilling.
Down-the-hole drilling involves a hammer driven by compressed air along a drill pipe. DTH drilling is used for breaking down harder rocks. In these drills, the hammer is attached to the bottom of the drill string and hits the rock with a high frequency.
Top hammer drilling is another subtype of percussion drilling, which involves s rig with a piston that delivers the percussive force to a hammer as in DTH but here, the drill also revolves around its axis in addition to delivering hammer strikes to the rock that needs to be drilled through.
Rotary drilling is the most popular sort in oil drilling and in gas drilling, and it is also used in mining. There is no hammer action in rotary drilling, but a lot of rotating action, hence the name. The drill bit in a rotary drill spins on its axis to remove bits of rock and create a hole. In modern rotary drills, both the drill bit and the drill shaft—collectively the drill string—spin around their respective axes.
One major advantage of rotary drilling over percussive drilling is that the drill bit can be cooled by adding fluid to the drill shaft. This allows for longer continuous drilling because the drill bit does not need to stop rotating to cool off.
Drilling operations in mining
The mining industry distinguishes between three types of drilling.
Exploration drilling: This involves drilling for rock sample collection. The purpose of collecting samples is to examine the quality of the mineral resources in the rock. It is also called core drilling, after the shape of samples that are collected. These are cylinders of rock that are removed from the formation with a special-purpose drill that operates like a hole saw.
Technical drilling: Technical drilling encompasses the drilling operations that accompany the development of an underground mine, including drilling for drainage, slope stabilization, and foundation testing before production begins.
Blast hole drilling: This is the process of drilling a hole into the rock and packing it with explosives in order to loosen the rock and make the ore suitable for excavation. Blast hole drilling, sometimes also called production drilling, is used in both surface mining (open-pit) and underground mining, both for exploration and production purposes.
Drilling machinery in mining
Depending on the purpose of the drilling operation, there are several types of drilling rigs used in mining, generally grouped in two categories:
- Face rigs are used to drill mining tunnels and in metal ore mines. The size of the drill used depends on the size of the mine and its depth. Some of these rigs are specifically designed to drill blast holes and others are made to operate in very narrow tunnels. Yet others are designed to drill deep holes and excavate tons of ore.
- Bolting rigs, or mining bolters, are machines that can both drill holes and then install safety bolts in them in the ceiling and walls of underground mining tunnels. Some of these are designed as more versatile rigs that can be used both for drilling holes for safety bolting and for longhole drilling—drilling for ore.
Choosing the appropriate drilling rig in mining depends on a number of factors, including the type of rock that will be drilled into, the size of the mineral prospect, and the sort of hole the miner wants to drill—exploration, blast hole, production hole, or a mining tunnel.
Drilling operations in oil
Oil and natural gas drilling is done for one of three purposes:
Exploration: When a deposit has been identified as a prospect that may contain commercial amounts of hydrocarbons, an exploration well needs to be drilled to confirm or reject the presence of these hydrocarbons.
Appraisal: Once the presence of oil and gas has been established, a well is drilled to see whether they are indeed in quantities that would justify commercial exploitation of the field.
Production: Once a prospect has been confirmed as an oil- and gas-bearing field, production wells are drilled in the locations with the greatest concentration of hydrocarbons.
To drill an oil or gas well, exploration and production companies use drilling rigs. There are some differences between these based on whether they are used for onshore or offshore drilling, but both onshore and offshore rigs share their main components: a drill bit, a drill string to which the bit is attached, and a rotary table that rotates the drill string and the bit.
Besides the drill string and the bit, drilling equipment also typically includes blowout preventers, drill pipes, mud tanks and pumps, a centrifuge, and a set of devices that separate the gas from the oil and the sand and water from the hydrocarbons.
Drilling rig components
Some components of drilling rigs vary depending on location—offshore or onshore—and on the type of drilling to be performed. Regardless of the type, however, all rigs share several basic components.
- Derrick: The steel tower that supports the drill string, the bit, and associated equipment.
- Drawworks: A set of systems attached to the rig to lift and install the drill string and the drill bit; the swivel, which supports the weight of the string and bit and pressure-seals the opening of the well; and the rotary table and the kelly, which rotate the drill. Related: The Natural Gas Nation Every Exporter Is Targeting
- Kelly drive: A shaft with a square or hexagonal shape that is connected to the rotating table via a bowl-on-a-plate-like component called bushing. The kelly is hollow to accommodate the water or drilling mud pumped into the well to cool the drill bit.
- Rotary table: The component that rotates the drill string, typically made up of a chain and a drive sprocket-wheel.
- Swivel: A handle that is used to lower the drill string and support its weight while rotating. Also, a conduit for the drilling mud.
- Drill pipes: These pipes are connected to the kelly and lowered into the well. They are the connection between the surface equipment on the rig and the subsurface one, that is, the drill string and the bit. After the well is drilled to the target depth, these are taken out.
- Bottomhole assembly: This is the group term for the drill bit and the metal structure right above it whose purpose is to provide enough force for the bit to break the rock.
- Blowout preventer: The blowout preventer is a system of valves that is installed at the opening of the well to ensure against the spontaneous release of oil and gas, and also to keep the drill pipes and the casing of the well from getting blown out of the well when pressure mounts.
- Drill bit: The drilling bit is usually custom made in accordance with the geological characteristics of the prospect. Bits can be loosely grouped into three types: roller bits, which crush the rock; diamond bits, or PDCs, which grind rather than crush the rock and are more durable and faster in soft rock; and directional bits, which have a steerable motor on top to drill in different directions, mostly offshore, where a single wellbore can host several wells but also in shale rock.
Besides these essential components, a drilling site also features a power source—commonly a diesel generator—as well as a mud pit for the drilling mud, a pump to get the mud into the well and then out of it, as well as a centrifuge to separate solids from liquids during the drilling process.
Types of drilling rigs
Drilling rigs, in general, are classified based on the power source they use, the pipe used for the drill string, and by method of rotation.
In oil and gas, drilling rigs are mechanical with regards to the power supply. They are overwhelmingly powered by a diesel engine.
As for the pipe, oil and gas rigs use coiled tubing—a long flexible metal pipe that connects the derrick to the bottomhole assembly.
This is the case across oil rigs. However, there is a marked difference between onshore and offshore drilling when it comes to types of rigs. For onshore sites, most rigs look alike. Offshore, it all depends on the depth to be drilled.
Offshore, there are eight types of drilling rigs that are used in different depth ranges.
Fixed platforms: Fixed platforms get their name from the steel or concrete legs that attach them to the seafloor. These rigs are used in shallow waters of up to 1,700 feet, or 530 meters.
Jackup rigs: These platforms have legs like fixed rigs, but these legs can be raised to move the platform from one place to another and then lowered to attach it again to the seafloor. A subtype of fixed platforms, jackup rigs are mostly used in shallow waters as well.
Compliant towers: These are similar to fixed platforms but with a narrower tower (the offshore equivalent of the derrick) so the platform can move sideways with the wind and the waves. This greater flexibility makes them suitable for greater depths than fixed platforms of up to 3,000 feet or about 900 meters.
Semi-submersible platforms: Semi-subs are floating structures standing on pontoon-like legs that are submerged in the water. They are the platform of choice for drilling locations with harsh weather conditions, and they can be used in both shallow and deep waters with depths of up to 10,000 feet, or 3,000 meters.
Based on the type of anchoring used, semi-subs can be: tension-leg and sea star platforms, which are both connected to the seafloor with flexible steel legs; and spar platforms, which float on the water on top of a hollow cylindrical hull that keeps the platform in place.
FPSOs: Floating production, storage, and offloading systems are the most versatile type of offshore drilling platforms. They can operate in shallow or medium-depth waters, like semi-submersibles, or go into ultradeep waters like a drillship. However, because they combine production with storage and offloading, they are most often used in medium-depth waters of up to 6,000 feet, or 1,800 meters, where this combination makes the most economic sense.
Drillships: Drillships are used for drilling in deep waters where semi-subs cannot operate. To stay in place, the drillship uses various kinds of anchoring. Anchoring types include multiple anchors and dynamic positioning: a system of propellers and thrusters that keep a vessel in place automatically through a computer-controlled program.
The drilling process
Before going into the stages of the well-drilling process, here is a quick overview of the types of wells that oil and gas exploration and production companies drill. They distinguish between:
Wildcat well: The very first well drilled on a new prospect—one that has been clearly defined in terms of geology as an oil and gas prospect, but which lies outside of any known oil and gas fields.
Appraisal well: A well drilled to determine the extent and size of a discovery made through seismic imaging; in other words, to assess the characteristics of a proven hydrocarbon deposit.
Production well: A well drilled in the sweet spots of a field with proven commercial reserves of oil and/or gas to extract the hydrocarbons for commercial purposes.
The drilling process can be loosely divided into three main stages: preparation, spudding, and completion.
Onshore, the land plot on which the well will be drilled needs to be leveled so the crew can assemble the derrick, and roads need to be built to bring in all the drilling equipment.
Mud pits and storage pits then need to be dug for any solids separated from the liquid in the drilling process. In some jurisdictions, mud pits are banned and used mud is instead collected in metal reservoirs that need to be put together during the preparation stage.
Finally, the crew will build what in the industry is called a cellar: the place where the well will be drilled and where all the drilling machinery will be assembled.
Offshore, none of this applies. There, everything happens on and from the platform, where the drilling equipment is delivered by air or by boat, and installed on the platform itself from where the drill string is lowered into the water and into the seabed.
This is when the actual drilling begins. It starts with the drilling of a conductor hole: a shallow hole of between 100 and 200 feet, or 30-40 meters that is cemented in place or lined with so-called conductor casing (steel piping). The purpose of the conductor hole is to prevent the top of the wellbore from collapsing in on itself.
Once the conductor hole is in place and cased, the drilling proper begins. The derrick is assembled over the wellbore and the drilling equipment is assembled.
Drilling proper takes place in stages. The first stage of drilling proper is drilling a hole of some 300-500 feet, or 100-250 meters, whose aim is to first seal off the well from the surface aquifers in the area and stabilize the top of the wellbore. Related: Global LNG Markets Are Circling The Drain
This is done by installing well casing—steel pipes—in the wellbore and then pouring cement into them via the drill string. Once the cement reaches the bottom of the hole, it is pushed back up along the sides of the casing, fixing the wellbore in place and insulating it from the aquifers. This casing-and-cementing process is repeated for every stage of the drilling process.
This first proper drilling stage is the most important one because it is the stage where the blowout preventer is installed on the casing pipe to prevent a blowout from the well. Once this is done and the BOP is activated, drilling continues down to the desired depth. In shale oil drilling and in shale gas drilling, there is an additional stage of lateral—also called directional or horizontal—drilling once the maximum desired depth is achieved.
During the drilling process, water—or more commonly, drilling mud—is injected into the wellbore to cool the drill bit and also to flush out the rock cuttings that the drill bit produces and that, if they accumulate around it, would force it to stop drilling. These cuttings are collected in pits or reservoirs on the surface.
Once the last section of the well has been drilled, cased, and cemented, the well is completed and ready to start producing. The typical conventional well remains open-hole, meaning the end of the wellbore remains open into the reservoir of hydrocarbons and they flow straights up.
Unconventional wells, on the other hand, need to be perforated to allow the hydrocarbons to begin flowing in. This is called cased-hole completion and involves one additional stage in the drilling process, during which a perforation gun is inserted into the wellbore to perforate the casing, so the hydraulic fracturing liquid can be pumped through the holes and fracture the surrounding shale rock to release the hydrocarbons.
Drilling an oil well takes several months and continues around the clock. It is a complex, labor-intensive process, often performed in challenging environments such as deep waters or even the Arctic. Yet the whole oil and gas industry depends on this process, as does a lot of the world’s energy supply.
By Tom Kool of Oilprice.com
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