Active vocabulary. 1. circulating system система циркуляции 2. drilling fluid (mud) буровой раствор 3. weighting materials

The circulating system consists of drilling fluid, which is circulated down through the well hole throughout the drilling process, performing a number of functions, and circulating equipment.

Typically, liquid drilling fluids are used. The most common liquid drilling fluid, known as ‘mud’, may contain clay, chemicals, weighting materials, water, oil, or gases. ‘Air drilling’ is the practice of using gasses as the drilling fluid, rather than a liquid. Gases used include natural gas, air, or engine exhaust. Air drilling can significantly cut down on drilling time, as well as drilling fluid costs. The drilling fluid, much like the bit, is custom designed and chosen depending on what type of subsurface conditions are expected or experienced.

The drilling fluid chosen must have a number of properties to allow it to accomplish its tasks. It must be light and thin enough, on the one hand, to circulate through the drill bit, and heavy enough to carry drill cuttings away from the bit and back to the surface, on the other hand; as well as control upward pressure that may be experienced in the well to prevent blowouts. In addition, the drilling fluid must be thick enough to coat the wellbore with a cake, which serves to temporarily seal the walls of the well until casing can be installed.

Circulating equipment includes the mud pump, the discharge line, the standpipe, the rotary hose, the swivel, and the jelly, the drill pipe, the drill collars, the bit, the annulus, the return line, the shale shaker, the desilter, the desander, the mud tanks, and the suction line.

The mud pump takes mud from the mud tanks and sends it out a discharge line to a standpipe. The standpipe is a steel pipe mounted vertically on one leg of the mast or derrick. Mud flows out of the standpipe and into the rotary hose, which is connected to the swivel and then goes down the kelly, the drill stem, out the bit. It does a sharp U-turn and heads back up the hole in the annulus carrying the cuttings made by the bit.

Finally, the mud leaves the hole through a steel pipe called the “mud return line” and falls over a vibrating, screenlike device called the “shale shaker”. It acts like a sifter and screens out the cuttings. Except in environmentally sensitive areas on land, the cuttings fall into the reserve pit, the earthen pit excavated when the site was being prepared. In areas where the contractor cannot use a reserve pit because of the environmental reasons, the shaker dumps the cuttings into a special receptacle. Later, the cuttings are properly disposed of. The mud drains back into the mud tanks where the mud pump recycles it downhole. The circulating system is essentially a closed system. The system circulates the mud over and over throughout the drilling of the well. From time to time, however, crew members may add water, clay, or other chemicals to make up for losses or to adjust the mud’s properties as the hole drills into new and different formations.

Several pieces of auxiliary equipment keep the mud in good shape. The shale shaker sifts out the normal-sized cutting. Sometimes, though, the bit creates particles so small that they fall through the shaker with the mud. So, after the mud passes through the shaker, the system sends the mud through desanders, desilters, mud cleaners, and mud centrifuges. These pieces of equipment remove fine particles, or small solids, to keep them from contaminating the drilling mud.

A degasser removes small amounts of gas that enter the drilling mud as it circulates past a formation that contains gas. A degasser is used when the amount of gas is not enough to make the well a producer; instead, it is just enough to contaminate the mud. The driller cannot recirculate this gas-cut mud back into the hole because the gas makes the mud lighter, or less dense. If the mud gets too light, the well can kick – formation fluids under pressure can enter the hole. If not handled properly, a kick can lead to a blowout. To avoid this a device known as the “blowout preventer” is situated on the well casing below the deck of the rig.

The blowout preventer can consist of hydraulically powered devices that can seal off the well quickly and completely, preventing any potential for a well blowout should extreme downhole pressures be encountered. Pressure release systems are also installed to relieve the great pressure that can be experienced in a blowout situation.

Once the well has been drilled, and if the supervising engineers believe that the targeted formation contains commercially viable quantities of oil or natural gas, well completion is performed to allow the well to become productive.