Active vocabulary. There are two main types of onshore drilling: percussion, or 'cable tool' drilling and rotary drilling.

Drilling into the Earth in the hopes of uncovering valuable resources is nothing new. In fact, the digging of water and irrigation wells dates back to the beginning of recorded history. The first recorded instance of the practice of 'drilling' holes in the ground came about around 600 B.C., when the Chinese developed a technique of repeatedly pounding bamboo shoots capped with metal bits into the ground. This crude technology was the first appearance of what is now known as 'percussion drilling'; a method of drilling that is still in use today. Much advancement has been made since these first bamboo drilling implements, with the realization of the value and increased demand for subsurface hydrocarbons.

There are two main types of onshore drilling: percussion, or 'cable tool' drilling and rotary drilling.

The basic concept for cable tool drilling consists of repeatedly dropping a heavy metal bit into the ground, eventually breaking through rock and punching a hole through to the desired depth. The bit, usually a blunt, chisel shaped instrument, can vary with the type of rock that is being drilled. Water is used in the well hole to combine with all of the drill cuttings, and is periodically bailed out of the well when this 'mud' interferes with the effectiveness of the drill bit. Despite the historical significance of cable tool drilling, modern drilling activity has shifted mainly towards rotary drilling methods.

Rotary drilling uses a sharp, rotating drill bit to dig down through the Earth's crust. Much like a common hand held drill, the spinning of the drill bit allows for penetration of even the hardest rock. The idea of using a rotary drill bit is not new. In fact, archeological records show that as early as 3000 B.C., the Egyptians may have been using a similar technique. Leonardo Da Vinci, as early as 1500, developed a design for a rotary drilling mechanism that bears much resemblance to technology used today.

During the middle and late 20th century, rotary drilling became the preferred penetration method for oil and gas wells. Probably the greatest advantage of rotary drilling over cable tooling is that the wellbore is kept full of liquid during drilling. A weighted fluid (drilling mud) is circulated through the well bore to serve two important purposes. By its hydrostatic pressure, it prevents the entry of the formation fluids into the well, thereby preventing “blowouts” and gushers. In addition, the drilling mud carries the crushed rock to the surface, so that drilling is continuous until the bit wears out.

Rotary drilling techniques have enabled wells to be drilled to depths of more than 9,000 metres. Formations having fluid pressures greater than 1,400 kilograms per square centimetre (20,000 pounds per square inch) and temperatures greater than 250° C (480° F) have been successfully penetrated.

Many kinds of rotary drilling rigs are available, particularly offshore where the marine environment plays an important role in rig design. Two broad categories of rig are those that work on land and those that work offshore.

Land rigs look much alike, although details vary. A major difference is their size, and the size determines how deep the rig can drill. Well depths range from a few hundred or thousand metres to tens of thousands of metres. The depth of the formation that contains, or is believed to contain, oil and gas controls well depth. Classified by size, land rigs are light duty, medium duty, heavy duty, and very heavy duty. Table 1 arranges them according to this scheme and shows the depths to which they can drill.

A rig can drill holes shallower than its maximum rated depth. For example, a medium-duty rig could drill a 750-metre hole, although a light-duty rig could also drill it. On the other hand, a rig cannot drill too much beyond its maximum depth, because it cannot handle the heavier weight of the drilling equipment required for deeper holes.

Another feature land rigs share is portability. A rig can drill a hole at one site, be disassembled if required, moved to another site, and be reassembled to drill another hole. Indeed, land rigs are so mobile that one definition terms them “portable hole factories”. The definition sounds odd, but it is accurate.