Land rigs

Land rigs range greatly in size and design.

Small rigs which are vehicle-mounted are sometimes powered by the same engine that powers the wheels to transit the vehicle, connected to the draw works, rotary table, and mud pumps by a series of chains, gears, and belts.
Rigs which are a bit larger may have diesel engines connected mehcanically to the equipment. The diesel engines on larger rigs power electrical generators which in turn power electric motors on the equipment.
A large land rig may use from 1000 to 1500 hp.

Land rigs can have either 'mast-type' derricks, or 'derrick-type' derricks. Rigs with masts can usually be transported on the back of a specially designed truck. When at the wellsite the mast can be unfolded (winched up), telescoped, or bolted together. Unlike proper derricks, masts usually are not capable of racking back drill pipe; water well drilling rigs (that are really the same as oil drilling rigs but smaller as they drill a lot shallower, predominantly use masts.

Although they are reasonably maneouvreable, you can't just site a land rig directly above where you think there will be oil - you need to prepare the site first:

First, there needs to be access to the rig site - if there are no roads then these will have to be prepared, or else the rig can be transported in sections via heavy lift helicopters.
Then, the 'pad' is prepared - the ground must be cleared, level and stable - so this may entail cutting a clearing in a forest, ensuring there's suitable drainage pathways to prevent flooding, and pouring concrete foundations.
A 'cellar' is prepared - this is a pit in the ground beneath the rig in which the BOP will sit - it is in a pit so the substructure isn't unnecessarily high off the ground. Another pit, the reserve pit, is also required - Waste mud will be returned to this pit; it will be plastic lined to prevent contamination of the soil. A final 'flare pit' will be dug - a boom extends from the poorboy degasser on the rig to the flare boom. In the event of hydrocarbons to surface (e.g. when circulating out a kick), the hydrocarbon returns are directed to the flare boom where they are burnt.
The substructure is a box like structure which houses many components of the rig, like the draw works, dog house, rotary table, and drill floor.

$20,000 per day is a ball-park figure for the cost of renting a medium-large land rig with drill crew (the day rate varies with demand, with demand varying with the oil price); the cost of support services (transport, operator personnel, rental equipment, consumables like drill bits) will increase the oil company's actual expenditure per day (this overall expenditure per day is called the 'spread rate').

So we have determined that we need:

  • A drill bit to cut the rock
  • A drill string to manipulate the bit, and provide a way of circulating fluid down hole
  • A drilling fluid, or mud, to evacuate the cuttings, stabilise the wellbore, and prevent formation fluid influxes (kicks)

There are also a great number of other components used, like:
  • Drill collars - heavier and stiffer than drill pipe, these allow lots of weight to be set down on the bit so it can drill more efficiently
  • Measurement while drilling (MWD) tools - these send pulses back to surface through the mud so the drilling team know in which direction and at what angle the hole is being drilled.
  • Logging while drilling (LWD) tools - these measure properties of the rocks as you?re drilling so you get a better understanding of what the formation is like (they can often tell you if you've found oil).
  • Directional drilling assemblies - used to control the direction in which the hole is being drilled.

So you have your drillstring, mud, bit, and all the other things you need. Do you just drill drill drill until you reach your target? Nope - you need to change your mud weight as you drill deeper or else you'll get kicks (which are dangerous) or fracture the formation therefor lose mud.


Formation pressure usually increases with depth - there is more 'overburden' rock on top of it so the compressive force is higher. "What does this have to do with anything?", you might be thinking, "because at depth the hydrostatic pressure of the mud will be increasing too" (think of swimming to the deep end in a pool - as you go under your ears get sore with the pressure, and as you go deeper it gets sorer as the hydrostatic pressure increases - the hydrostatic pressure of a column of fluid is dependant on its density x vertical height).
The problem is that the change in formation pressure with depth is different to the rate at which the mud hydrostatic changes with depth - the formation pressure will usually increase faster, therefore if you don't change the mud weight there'll come a depth where your formation pressure will be higher than your mud hydrostatic - you'll get a kick, or the hole will collapse.

So why not just make the mud weight higher than the largest pore pressure you're going to see in the well? Because of the formation breakdown pressure (or fracture pressure). This is pressure that the formation can take before cracks are induced, and you start losing your drilling fluid. This is not good - you can lose so much mud that:

  • Your mud hydrostatic pressure will be reduced so you might let in a kick (which will reduce your hydrostatic further, so a larger kick, until maybe a blow out occurs)
  • You might not be able to get cuttings out of the hole so can't drill ahead
  • You might lose so much of your expensive mud that you have nothing left to make it heavy (so you're back to the potential kick situation)

So, basically - losses are not good. Your mud weight can be a range between your pore pressure and fracture gradients. But your pore pressure gradient will eventually be greater than your mud weight... but luckily your fracture gradient will increase at a similar rate to your pore pressure gradient. So your mud weight window can move along a bit, if you can isolate the formation you've just drilled with the previous mud weight.

To be able to change your mud weight to drill deeper, you have to run casing (after you've pulled the drill string out of the hole):
Casing consists of joints of steel pipe about 40ft long which screw together into a casing string. This casing string is a couple of inches smaller in diameter than your hole. It is run across the formation you want to isolate - the bottom of the casing will be a few feet above the bottom of your hole. Then you pump cement down the casing until it goes up between the outside of the casing and the wellbore (you either run pipe down to the bottom of the casing or use wiper plugs to ensure the inside of the casing walls don't get covered in cement). The cement sets the casing in place, and also prevents formation fluids from moving into the area between the casing and the borehole.
Once the cement has set, you run in with a smaller sized drill bit through your casing, and could continue to drill until you either reach your target or have to set more casing.

You'll usually only drill a couple of hundred feet below the surface before you have to set your first casing string - called the conductor - because the formations near the surface aren't subject to as much compression (overburden) as the ones below so they are loose ('unconsolidated'); the shallower-set casing strings also protect groundwater reservoirs (that might be used for drinking water or irrigation) from contamination with oil and mud.Instead of drilling you might pile-drive the conductor (basically hammer it into the rock).

Then after the conductor you'll usually have to set 'surface casing' - casing that doesn't go too deep, upon which you can sit your blowout preventer. You'll usually drill your top hole sections (the holes made prior the BOP is installed) with water, and periodically pump small volumes of high-viscosity mud ('hi-vis') to ensure all the cuttings are cleaned out of the hole.

The blowout preventer (nearly always called a BOP - pronounced 'Bee-Oh-Pee') is another essential piece of equipment. It is attached to the top of the surface casing string. It will have at least three sets of rams which are fixed to pistons. Each set of rams can be activated individually - when the rams are operated, the pistons push the ram closed, sealing in the wellbore.

The three types of rams that will be present on a BOP are:
  • Pipe rams - these are used if there is drill pipe in the hole. They have semi-circular cuts on each half so that they can seal around the pipe. Your pipe rams need to be suitable for the size of drill pipe you have in the hole.
  • Blind/shear rams - these have no gaps for drillpipe. Blind rams just close over a hole with no pipe in it. Shear rams sever any pipe that's across them, but don't necessarily seal the wellbore. Blind/shear rams shear the drill pipe and also isolate the well. Pipe shearing is only performing as a last resort, if there is great danger (for example if control of the well has been lost and a blowout is imminent or in progress).
  • Annular BOP - also called a 'bag' preventer - this is a rubber/elastomer packer which is pushed upwards by a piston. It can close around nearly everything you put in the hole - drillpipe or casing of all sizes, and they can usually seal a hole which has no string in it also. Despite their versatility, sometimes you need to support the weight of your drillstring on a ram - your annular preventer is not designed for this, hence the need for pipe rams as well.

When the rams are closed, you still need a way to circulate the fluid in the well (to get a kick out). This is done through using 'choke' and 'kill' lines, which have outlets (that are usually closed) below the rams. To circulate out a kick (e.g. an influx of gas into your wellbore) you normally pump down the drillstring and up the annulus (the space between the drillstring and the wellbore), coming to surface through your choke line.

So... you have a drill bit on the end of your drill string (with drill collars to provide weight to your bit, and measurement/logging while drilling tools to tell you in which direction you're drilling and what the formation is like), and you're pumping mud down the middle of the string and up the annulus to surface. Your formation pressure is increasing, getting closer to your mud weight, so you pull the drill bit to surface, then run and cement casing. Then you make up a bit with a smaller diameter (which will fit inside the casing you?ve just set), and you drill deeper, and repeat the process until you reach your target.
Obviously, you need something to control this equipment and provide the power required. This is the job of the drilling rig and surface equipment.

Rigs and surface equipment

There are several types of rig, with the one you'll use depending mostly on where you are drilling (on land, in shallow water, deep water), but also to a lesser extent on the type of well you're going to drill (like a shallow well or extended reach).
The basic drilling equipment on each rig type is broadly the same. You need a way to lift the pipe, to rotate it, and pump mud. Up, down, turn, pump.

Lifting the string:
The drillstring is suspended in the derrick. This is the big tower. In it there is a draw works, which is a big cable drum which winds in and out the drill line at the driller's command. The drill line goes up to the top of the derrick to a fixed set of pulleys called the crown block. It then runs down the middle of the derrick to another set of pulleys which can travel - this is called the travelling block. To the travelling block is fixed a big motor called a top drive (if it is a modern system - a system common on older, or more basic, rigs uses a rotary table and kelly).The top drive screws into the drill string. So you reel the drilling line in or out with the draw works, it raises or lowers the travelling block (with the top drive) which raises and lowers the drill string to which it is connected.

Rotating the drill string:
The drill string is connected to the top drive. The top drive has a motor, which turns the string.

Mud is stored in 'mud pits' on the rig - these are just big tanks which have a series of interconnecting pipes and valves to allow each pit to store a different type of mud if required. The mud is sucked from the mud pits to the mud pumps - big pumps, usually 3 of them on each rig - which then direct it to the standpipe manifold.
The standpipe manifold is just a series of interconnected pipes with valves that can be manipulated to control where the mud goes - for normal drilling it will go up the standpipe (a pipe that goes to the top of the derrick). The standpipe will be connected to the top drive via a flexible hose (has to be flexible as the top drive moves as you lift and lower the pipe). It goes through the top drive, down the drillstring and out the bit nozzles, up the annulus, and then at the top of the annulus is the flowline.
The flowline is just a large diameter pipe that is almost perpendicular to the annulus - the mud in the annulus gets to this level, runs down the flowline (which is sloping slightly), and into the header box. The header box is just a big trough that allows the returning mud to be distributed evenly between the shakers. The shakers are machines that vibrate - they have mesh screens, where the size of the mesh is chosen to allow mud to pass through, and keep the cuttings above the screens so that they are bounced to the front of the shakers to be dumped. There will be a few shakers (3 to 5 usually), with each one having two or three rows of screens - the top one will be the coarsest mesh (the 'scalper') and take away the big cuttings, with the row beneath it being finer meshed, and the one below even finer still. When the mud (with cuttings removed) drops all the screens it goes back to the pits, though on its way it can go through further conditioning via a sand trap (which is just a pit which the mud flows through allowing any residual sand to drop to the bottom, and the clean mud to pass over the top edge), or machinery like a centrifuge (which removes very fine particles from the mud), or a degasser (which removes gas which may be entrained in the mud).
Now that it's back at the pits, it'll be taken by the mud pumps and circulated down the hole again.