Rotary rigs drill the vast majority of wells today, including all medium and deep wells. The rotary rig consists of four major systems. These include the engines, and the hoisting, rotating, and mud systems.
The engines supply the power to the rig. Most local rigs use a single engine to power the drawworks and rotary table. Power is usually transmitted through a modified heavy truck type powershift transmission (automatic). These engines are diesel fueled and are rated between 425-550 hp. The power is used primarily to turn the drill string and raise and lower equipment in the well.
Engines also supply the electricity used on and around the rig. Electrical power is supplied, usually, through two generator sets. The rig can run with one of these units but it would run at close to maximum output at night. The second provides for back-up, and allows for other options. These engines are generally rated at 300-350 hp. They are housed on a skid and are enclosed; this unit is referred to as a "light plant".
Rigs also employ 1 or 2 engines to power the mud pump. Total output varies from 300-500hp. When two engines are used they must be tuned to match and are hooked together through a gear box. One of these engines runs clockwise while the other runs counter-clockwise. Power for most these application is through a clutch with an over centre release mechanism - they can be locked in the released or engaged position.
The hoisting system is used to raise and lower and to suspend equipment in the well. The drilling line (wire rope) is usually braided steel cable about 1 1/8 inches in diameter. It is wound around a reel in the draw works. The engines are connected to the draw works and let the drilling line in or out. The derrick or mast is the steel tower. If the tower comes on a tractor-trailer and is jacked up, it is a mast. If the tower is erected on the site, it is a derrick. The drilling line goes over a pulley, called the crown block, at the top of the derrick, and then down to another pulley called the traveling block. Below the traveling block is a hook to which equipment can be attached. As the drilling line is reeled in or out of the draw works, the traveling block rises and falls in the derrick. This raises and lowers the equipment in the well.
The size of a rig is usually denoted by the number of joints that can be left together when they "trip". On a single trip, each joint must be disconnected, on a double two can be left together, and on a triple ... three. However, there are also variations within these groups as well. As the size increases, so does the amount of load it can handle. Some rigs, usually triples, are fitted with a topdrive unit which lowers the trip capacity by one joint. Most rigs in this area are "doubles".
The rotating system is used to cut the hole. Suspended on the hook directly below the traveling block is the swivel. The swivel is required to allow the drill strings to rotate while connected to the traveling block and as a means to connect the mud hose to the drill string. Without the swivel, mud could not be pumped downhole. Below the swivel is a four- or six-sided pipe called the kelly. The kelly has sides so that it can be gripped and turned. Turning the kelly turns all the pipe in the hole and drills the hole. The rotary table is a circular table in the derrick floor. It is connected to the engines and is used to rotate the kelly. The kelly fits into a device called the kelly bushing, which attaches to the rotating table. The rotary table, kelly bushing, and kelly rotate as a unit in a clockwise direction. "Turning to the right" is a common term for drilling.
Below the kelly is the drill pipe. Steel drill pipe comes in 30-foot sections that are threaded on both ends. Each section of drill pipe is called a joint. The kelly must always be located on top of the drill pipe. After drilling 30 feet, the kelly must be raised and another joint of pipe added below the kelly. This is called making a connection.
Below the drill pipe are larger-diameter pipes called drill collars. Drill collars weigh more than drill pipe and are designed to lower the center of gravity of the drill pipe. This helps control drilling (e.g. making a straight hole) and prevents the pipe from kinking and breaking. Two to twenty drill collars are often used.
The bit screws into the bottom of the drill collars. The most common bit is the tricone bit, which has three rotating cones.
The three interlocking bits rotate.
Bits wear out after 8 to 200 hours of rotation, with an average bit wearing out after about 24 hours. A worn bit can be detected by the noise on the derrick floor that the rotating drill pipe makes and by a decrease in rate of drill penetration. "Making a trip" is necessary for changing the bit. All the pipe is pulled out of the hole (tripping out) and stacked (usually 2 joints high) in the derrick. The bit is then changed and the pipe put back into the hole (tripping in). This takes rig time and costs money. The deeper the well, the longer the trip takes.
The drill string is the rotating pipe and its attachments. This includes the swivel, kelly, drill pipe, drill collars, and bit.
The mud system circulates drilling mud in the hole. Drilling mud is stored in steel mud pits beside the rig. Pumps, called mud hogs, force the drilling mud through a hollow rubber tube. The drilling mud then flows down through the hollow rotating drill string and jets out through holes in the drilling bit on the bottom of the well. The drilling mud picks rock chips (cuttings) from the bottom of the well. It flows up the well in the space between the rotating drill string and well walls (annulus). At the top of the well, the mud flows though the blowout preventers and on to a series of screens called the shale shaker. The shale shaker is designed to separate the cuttings from the drilling mud. Other devices are also used to clean the drilling mud before it flows back into the mud pits.
There's more to drilling than simply rotating the
Circulating drilling mud serves several purposes. The mud removes cuttings from the bottom of the well. As the mud flows across the bit, it cleans cuttings from the teeth. The drilling mud cools the bit from heat generated by the friction of drilling. In very soft sediments, such as in a coastal plain, the jetting action of the drilling mud Crudeying out of the bit on the bottom of the well helps cut the well. The drilling mud also controls pressures in the well and prevents blowouts. At the bottom of the well, there are two fluid pressures. Pressure on fluids in the rock tries to cause the fluids to flow into the well. Pressure exerted by the weight of the drilling mud tries to force the drilling mud into the surrounding rocks. If the pressure on the fluid in the subsurface rock is greater than the pressure of the drilling mud, the water, gas, or oil will flow out of the rock into the well. This often causes the sides of the well to cave or stuff in, trapping the equipment. In extreme cases, it causes a blowout. In order to control subsurface fluid pressure, the weight of the drilling mud is adjusted to exert a greater pressure on the bottom of the well. This is called overbalance, and the drilling mud is then forced into the surrounding rocks. The rocks act as a filter, and the solid mud particles cake to the sides of the well as the fluids enter the rock. This filter or mud cake is very hard. Once the filter cake has formed, the sides of the well are stabilized and subsurface fluids cannot enter the well.
Drilling mud is usually a clay and water mixture. A common drilling mud is made of bentonite clay and is called gel. A heavier drilling mud can be made by adding barite (BaSO,). Various chemicals are also used in different situations. The drilling mud liquid is usually water (freshwater based or salt-water-based) but is sometimes oil-based. Drilling muds are described by their weight. Water weighs 8.3 pounds per gallon. Average bentonite drilling mud weighs from 9 to 10 pounds per gallon. Heavy drilling mud weighs from 15 to 20 pounds per gallon. The heavier the drilling mud, the greater the pressure it exerts on the bottom of the well.
Blowout preventers (BOPS) are designed to close off the well. They are attached to the top of the well below the derrick floor. Two types are often used on the same well. One type is designed to close around the drill pipe to shut off the annulus. The other type closes the well by shearing off the drill pipe with rams.
Air or pneumatic drilling is used in many shallow wells or in cases in which the drilling mud would damage a subsurface reservoir rock. Air is forced down the drill string similar to circulating drilling mud. The air mixes with water, which is always present on the bottom of the well, and forms foam. The foam picks cuttings off the bottom of the well and returns through the annulus to the surface. The foam and cuttings blow out the side of the rig in a blooey line. Air drilling is faster and less expensive than mud drilling but has the disadvantages that it cannot control subsurface pressures and sometimes air mixes with subsurface gas, causing an explosive mixture.
Because of dipping beds of hard and soft rocks, drillers used to have a hard time keeping a well going straight down. If the bit hit a subsurface hard rock layer with a dip greater than 45 degrees, the bit would tend to be deflected down dip. If the hard rock layer dipped less than 45°, the bit would tend to be deflected up dip. The number of degrees a well deviates from vertical is called the deviation and the well is called a crooked hole. Drilling deviations caused by dip deflection can be prevented when drilling in areas where the geologic structure is known. By reducing the bit pressure and allowing the bit to do the work, deviation can be eliminated. In instances where this cannot be done air drilling can be employed. (see above)
Today, drilling contracts often have a clause stipulating that the well deviate no more than a few degrees from vertical. Modern rotary rigs can be controlled so that the well is drilled at a predetermined angle (directional or deviation drilling) and ends up in a predetermined location.
The first device used for directional drilling was a whipstock, a wedge designed to bend the drill string. Steel drill pipe bends 3 to 5'. More modern equipment includes swivel or knuckle joints that gives the drill string flexibility and down-hole motors that drive the drill bit from the bottom of the drill string. Directional surveys or drift logs are often run on wells to determine how much they deviate and where they end up (bottom out). This is usually done by lowering a compass into the well.
In the past, some wells were "accidentally" drilled to drain oil out from under adjacent leases. There are also many legitimate reasons for drilling a crooked hole. If a well is on fire and cannot be approached, a relief well can be drilled at a safe distance from the wild well. The relief well does not have to intersect the wild well in the subsurface, just come close. Heavy drilling mud is then pumped from the relief well through the subsurface rocks and into the wild well to control it. If something breaks off or falls down the well and cannot be removed, the well can be sidetracked around the obstacle. It is more economical to drill a crooked hole to test several potential petroleum reservoirs than to drill several wells to each reservoir. Deviation drilling is also used to overcome a poor drilling location.
More recently, directional drilling and slant drilling have become common in our area. Slant drilling allows more than one (usually 4, 6, or 8) wells to be drilled off the same lease site (pad). Aside from the slant direction, these wells are usually drilled and completed much like vertical wells. Directional drilling (where the direction of the well changes from time to time) is usually associated with some enhanced recovery technique (EOR), such as horizontal wells being drilled for SAGD (steam assisted gravity drainage).