The circulating system pumps drilling mud in and back out of the well hole. Drilling mud is stored in several steel mud tanks on the ground beside the rig.
Several, rectangular, steel tanks, arranged end-on-end that hold drilling mud on a drilling rig. The tanks are open on the top and connected by pipes. The drilling mud from the shale shakers flows from the shaker tank to the reserve tank to the suction tank where it is pumped back down the well by the mud hogs.————-Mud tank
The drilling mud is kept mixed in the tanks by rotating paddles on a shaft called a mud agitator or by a high-pressure jet in a mud gun. Large pumps driven by the prime movers, called mud hogs, use pistons in cylinders to pump the drilling mud from the mud tank. Mud pumps are either duplex or triplex. A duplex pump uses two double-acting pistons in cylinders that drive the mud on both the forward and backward strokes. A triplex pump uses three single-acting pistons in cylinders that drive the mud only during the forward stroke.
The mudflows from the pumps through a long rubber tube, the mud hose, and into the swivel. The drilling mud then flows down through the hollow, rotating drill string and jets out through the holes in the drilling bit on the bottom of the well. The holes on tri-cone drill bits, called nozzles or jets, are located between each pair of cones (fig. 1). On a diamond bit, the drilling mud flows through the bit into grooves, called watercourses, on the face of the bit, and across the diamonds (fig. 1). The drilling mud picks the rock chips (cuttings) off the bottom of the well and flows up the well in the space (annulus) between the rotating drill string and well walls.
At the top of the well, the mud flows through the BOPs, along with the mud return line and on to a series of vibrating screens made of woven screen cloth in a steel frame called the shale shaker. The shale shaker is located on the mud tanks and is designed to separate the coarser well cuttings from the drilling mud. It can be either single or double deck.
The double-deck shaker has a coarser screen located above a finer screen. The screens are tilted 10˚ from horizontal to cause the cuttings to vibrate down the screen and into the reserve pit. The mud then flows through other solids control devices such as cone-shaped desanders and desilters (fiure 2) that centrifuge the mud to remove finer particles. The mud then flows back into the mud tanks to be recirculated down the well.
The mud tanks are 6 ft (1.8 m) high, up to 8 ft (2.4 m) wide, and are usually 26 ft (7.9 m) long. They have two, three, four, or more compartments. A common mud tank configuration has the shaker tank receiving the drilling mud from the well after the cuttings have been removed. The drilling mud flows from the shaker tank to the reserve tank and then to the suction tank. Drilling mud from the suction tank goes to the mud hogs.
On the shaker tank is a mud gas separator that removes any subsurface gas that was dissolved in the returning drilling mud. Adjacent to the mud tanks but away from the rig is a large earthen pit called the reserve pit (figure 3). It holds discarded mud for reuse and the cuttings from the shale shakers.
Drilling mud is a mixture of special clay with either water (water-based drilling mud), oil (oil-based drilling mud), a mixture of oil and water (emulsion mud), or a synthetic organic matter and water mixture (synthetic-based drilling mud). The water in water-based drilling mud is usually fresh but can be saline.
Oil-based drilling mud is made from diesel, mineral, or synthetic oil and brine. It has excellent bit-lubricating properties and does not affect the formations being drilled. It is, however, expensive, hard to dispose of after drilling, and can be flammable. The emulsion mud with 8 to 12% oil in water has advantages of both. The synthetic-based drilling mud has the oil-based drilling mud advantages and is relatively easy to dispose of. Water-based drilling mud made with freshwater is commonly used on land and synthetic-based drilling mud is commonly used offshore.
Water-based drilling mud is usually made of freshwater and bentonite. Bentonite is a type of clay that forms a colloid and will stay suspended in the water a very long time after agitation has stopped. Drilling mud viscosity and density can be increased by adding more bentonite (mud up) or decreased by adding water (water back). Chemicals mixed with the mud for various effects are called additives. A mass of additives put into drilling mud to remedy a situation is called a pill. Heavier drilling mud used to exert more pressure in the well is made by mixing in high-density substances called weighting material such as barite (BaSO4) or galena (PbS).
Other mud additives include the following:
- alkalinity or pH control agents
- filtrate reducers
- foaming agents
- shale control agents
- surface active agents
- lost-circulation material
The clay and additives are brought onto the drill site in dry sacks and are stored in the mud house. They can be added to the mud in the mud tanks through a hopper, a funnel-shaped device. Drilling mud is described by weight. Freshwater, for reference, weighs 8.3 pounds per gallon. Typical water-based bentonite drilling mud weighs 9 to 10 pounds per gallon. A very heavy drilling mud designed to exert a greater pressure on the bottom of the well can weight 15 to 20 pounds per gallon.
Circulating drilling mud in a well serves several purposes. The mud removes well cuttings from the bottom of the well to allow drilling to continue. Without removing the cuttings, drilling would have to stop every few feet to remove the cuttings that clog up the bottom of the well as had to be done on a cable tool drilling rig. As the mudflows across the bit, it cleans the cuttings from the teeth. The drilling mud also cools and lubricates the bit. In very soft sediments, the jetting action of the drilling mud squirting out of the bit helps cut the well.