Rotary Drilling - An Overview
November 1, 2011
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. Archeological records show that as early as 3000 B.C., the Egyptians may have been using a similar technique. Leonardo Di Vinci, as early as 1500, developed a design for a rotary drilling mechanism that bears much resemblance to technology used today. Despite these precursors, rotary drilling did not rise in use or popularity until the early 1900s.
Although rotary drilling techniques had been patented as early as 1833, most of these early attempts at rotary drilling consisted of little more than a mule, attached to a drilling device, walking in a circle. It was the success of the efforts of Anthony Lucas and Patillo Higgins in drilling their 1901 Spindletop well in Texas that catapulted rotary drilling to the forefront of petroleum drilling technology.
While the concept for rotary drilling – using a sharp, spinning drill bit to delve into rock – is quite simple, the actual mechanics of modern rigs are quite complicated. In addition, technology advances so rapidly that new innovations are being introduced constantly.
The basic rotary drilling system consists of four groups of components – the prime movers, hoisting equipment, rotating equipment and circulating equipment – that all combine to make rotary drilling possible.
Prime MoversThe prime movers in a rotary drilling rig are those pieces of equipment that provide the power to the entire rig. Steam engines provided the power to the early drill rigs. Gas and diesel engines became the norm after World War II. Recently, while diesel engines still compose the majority of power sources on rotary rigs, other types of engines also are in use; more so in the oil and gas industry than in the water well sector. Natural gas or gasoline engines commonly are used, as are natural gas- or gasoline-powered reciprocating turbines, which generate electricity on-site. The resulting electricity is used to power the rig itself. The energy from these prime movers is used to power the rotary equipment, the hoisting equipment and the circulating equipment, and, on large rigs, may be used as well to provide incidental lighting, water and compression requirements not directly associated with drilling.
Hoisting EquipmentThe hoisting equipment on a rotary rig consists of the tools used to raise and lower whatever other equipment may go into or come out of the well. The most visible part of the hoisting equipment is the derrick, which serves as a support for the cables (drilling lines) and pulleys (drawworks) that serve to lower or raise the equipment in the well.
For instance, in rotary drilling, the wells are made with long strings of drill pipe extending from the surface down to the drill bit. If a drill bit needs to be changed, either due to wear and tear or a change in the subsurface rock, the whole string of pipe must be raised to the surface.
In deep wells, the combined weight of the drill pipe, drill bit and drill collars may be in excess of thousands of pounds. The hoisting equipment is used to raise all of this equipment to the surface so that the drill bit may be replaced, at which point the entire chain of drill pipe is lowered back into the well. The height of a rig’s derrick often can be a clue as to the depth of the well being drilled. Drill pipe traditionally comes in 20-foot sections, which are joined together as the well is goes deeper and deeper. This means that even if a well is 1,200 feet deep, the drill string must still be taken out in 20-foot sections. However, if the derrick is tall enough, multiple joints of drill pipe may be removed at once, speeding up the process a great deal.
Rotating EquipmentThe rotating equipment on a rotary drilling rig consists of the components that actually serve to rotate the drill bit, which, in turn, sends the hole deeper and deeper into the ground. The rotating equipment consists of a number of different parts, all of which contribute to transferring power from the prime mover to the drill bit itself. The prime mover supplies power to the rotary, which is the device that turns the drill pipe, which, in turn, is attached to the drill bit. A component called the swivel, which is attached to the hoisting equipment, carries the entire weight of the drill string, but allows it to rotate freely.
The drill pipe is manufactured to meet specifications laid out by the American Petroleum Institute and others, which allows for a certain degree of homogeneity for drill pipe across the industry.
Below the drill pipe are drill collars, which are heavier, thicker and stronger than normal drill pipe. The drill collars help to add weight to the drill string, right above the bit, to ensure there is enough downward pressure to allow the bit to drill through hard rock. The number and nature of the drill collars on any particular rotary rig can be altered depending on the down-hole conditions experienced while drilling.
Circulating SystemThe final component of rotary drilling consists of the circulating system. There are a number of main objectives of this system, including cooling and lubricating the drill bit, removing debris and cuttings, and coating the walls of the well with a mud type-cake. The circulating system consists of drilling fluid, which is circulated down through the well hole throughout the drilling process.
The components of the circulating system include drilling fluid pumps, compressors, related plumbing fixtures, and specialty injectors for the addition of additives to the fluid flow stream.
Basic Rotary OperationRotary drilling, as opposed to percussion drilling, cuts by rotating a bit at the bottom of the hole. In addition to rotation, downward pressure must be exerted and continued as the bit cuts it way through the formation.
Part of the art of rotary drilling is to match the bit type and pull-down pressure with the formation, and the use of drilling fluids to maintain circulation to keep the hole clear of cuttings and the bit lubricated and cool. A rotating table turns the drill string via a kelly bar passing through the table and attached to the top joint of the drill string.
Methods of imparting rotation and/or pull-down pressure to the drill bit:
- rotary table, rotary table with pull-down
- hydraulically driven top-head, top-head unit with pull-down chains
(more recent top-head-drive rigs are completely hydraulically driven,
eliminating the need for chains or cables to provide pull-down forces);
- downhole motors (also called downhole turbines), which impart the rotational force directly at the drill bit. The earliest downhole bit-driving device was the Turbodrill, patented in 1873.
When beginning a new hole, and oftentimes during drilling operations, pull-down pressure from the drill rig is applied. This pull-down force is achieved by a screw, cable or chain arrangement, or by hydraulic motors. Hydraulically powered pull-down actions usually are found on more recently manufactured drill rigs, with screw, cable and chain pull-down arrangements more commonly found on older rotary rigs.
The driller controls the pull-down pressure and, thus, the speed of penetration. It must be noted that part of the art of rotary drilling is the matching of pull-down pressure to the formation. Excessive pull-down pressure can damage drill bits, drill pipe and the trueness of the borehole. Thus, applying more pull-down pressure is not always the best drilling practice.
This article is provided through the courtesy of the International School of Well Drilling (see below).
Educating the Drilling IndustryThe International School of Well Drilling (ISWD) was established in May 2002 to serve the needs of the water well, environmental and geotechnical sectors of the drilling industry. Since opening, ISWD has trained students both nationally and internationally. These students have included those seeking to enter the drilling industry, and those currently employed at a junior level in the industry, as well as industry regulators and licensed well drillers seeking to further their knowledge.
Since 2005, the ISWD’s focus has shifted to offering training to those already in the water well drilling industry. Continuing education as a requirement for license renewal is required in 27 states. The remaining states are likely to join the majority over time. ISWD is proud to offer continuing education credits for the well drilling industry online.
ISWD receives calls from operators around the country needing a wide variety of training and consulting services. The school has provided customized training ranging from programs for experienced employees to basic training programs for those new to the industry. Specific consulting projects also are undertaken.
To contact ISWD about training or consulting services, phone 863-648-1565, or send an email to email@example.com.