3. Write down the transcription of the following words and practise their pronunciation:

• hydrostatic pressure

• occurrence

• circulation

• pressure surges

• hydraulics

• equalization

• centrifuges

• viscous

• hydratable

• slough

4. Read and translate the text. While reading, choose the suitable heading for the part of the extract:

1. Density

2. Fluid loss

3. Oil content

4. Solids content

5. Viscosity

Drilling fluid, or mud, has an important effect on hole-making potential and performs other functions vital to cost-effective drilling. Drilling fluid cools and lubricates the bit and drill stem and provides a means for bit cuttings to be circulated out of the hole. The column of mud in the hole supports the walls of the wellbore and prevents the entry of formation fluids into the well. The presence of formation oil and gas can be detected in irregularities in mud volume and weight. Drilling fluid can also be used to drive downhole drilling motors.

The mud program is not the same for every well. In fact, in the course of drilling a well, the composition of the mud can be changed in order to deal with variations in formation properties and mechanical factors that affect the drilling rate. The mud properties with the greatest effect on penetration rate are density, solids content, viscosity, water loss, and oil content.

a) Lightweight mud (less than 10 ppg) permits fast drilling because the hydrostatic pressure of the mud column is kept to a mini­mum. Lower hydrostatic pressure allows formation rock chips that are formed by the action of the bit to be removed with less weight on bit and rotary speed than heavier-weight muds require. In effect, the formation rock drills more easily.

When mud density is too high, a high differen­tial pressure is created between the mud column and the formation, and what is known as a chip hold-down effect is created. In such cases, mechanical energy on the bit must be increased to avoid a substantial drop in the drill­ing rate caused by the bit cutters drilling the same material over and over.

The use of low-weight muds when possible is a definite cost saver. Good drilling rates are achieved, cutting drilling time and bit costs. There are fewer occurrences of lost circulation and other interruptions to the work. Stuck pipe and pressure surges caused by high differential pressure can be eliminated, saving valuable trip­ping time and avoiding unnecessary fishing jobs.

In spite of its potential drawbacks, heavy-­weight mud (16-18 ppg) is necessary in cer­tain drilling situations. Without sufficient hydrostatic pressure to overcome formation pressures, the possibility of a kick exists. Main­taining the proper mud weight and carefully controlling other mud characteristics are the best ways to prevent blowouts.

b) Low-solids fluid (water, where feasible) will enable faster penetration rates than lightweight mud, provided that bit weight, rotary speed, and hydraulics are con­stant. Low-solids mud is usually used in drilling situations in which mud weights are no greater than 10 ppg and circulation rates are high enough to lift cuttings out of the hole.

Small particles of weighting material in the mud can have a detrimental effect on rate of penetration. These particles plug the fractures where a chip has been sheared from the hole bottom, delaying fluid pressure equalization over the chip and holding it in place. In this way, fine solids in the mud contribute to the chip hold-down effect, inhibiting the removal of formation cuttings and decreasing the penetration rate.

Solids content can be kept low by a number of different means. Circulating through the reserve pit allows fine solids to settle out of the mud. Devices like desilters and centrifuges maintain low solids content mechanically. Cen­trifuges are normally used on weighted muds, although they may be used with mud cleaners to maintain minimum mud weights. Chemicals that cause the fine particles to aggregate and settle out as larger particles are also used.

Whenever downhole conditions permit, so-called clear water drilling is common, often to depths of several thousand feet.

c) Increases in viscosity, the mud's resistance to flow, cause circulation pressure losses, which lessen bit hydraulic horsepower (BHHP) and make bottomhole cleaning more difficult. Highly viscous or thick muds are more likely to hold formation chips on bottom than are low-viscosity muds, reducing the penetra­tion rate. Gel strength, the ability of a mud to keep cuttings from settling when circulation is stopped, should also be observed carefully so that a clean hole is maintained.

Sometimes, a quantity of high-viscosity mud is circulated through the system for hole-cleaning purposes. This operation, known as a high-viscosity sweep, is a good way to clear the hole of unwanted junk before setting casing or running a diamond bit.

d) The pressure of the mud column may force some of the fluid component of the mud into cavities in the more permeable forma­tions through which a hole may be drilled. This water loss, or filtration loss, is beneficial in two ways. The initial loss of fluid from the mud, called spurt loss, can make for­mations easier to drill. Over time, this process of filtration allows for the buildup of wall cake (sometimes called filter cake), which is a plasterlike residue of mud solids that forms on the borehole wall. This wall cake seals the wellbore and prevents the loss of whole fluid into the formation.

Excessive fluid loss can cause problems. Buildup of wall cake may cause the drill stem to stick in the wellbore. Certain shales may cave in or slough, creating further problems of stuck pipe and lost circulation. Oil recovery may be inhibited by filtrate in producing formations.

e) The addition of oil to water-base muds can affect penetration rate in certain for­mations. Where high temperatures, sloughing shales, or pipe sticking may be expected, the oil component keeps hole friction to a minimum. In effect, a reduction in friction increases bit weight. Oil in the mud lubricates the bottomhole assembly and keeps the bit from balling up in certain hydratable clays and shales. Oil-base muds may be used to protect producing forma­tions or when special drilling problems such as high temperature, sloughing shales, or stuck pipe are encountered.