Drilling Fluid Properties

Mud Characteristics

1. Before reading the text, study the vocabulary:

wellbore	ствол скважины
solids content	содержание твердой фазы
viscosity	вязкость
water loss	водоотдача
chip hold-down effect	эффект оседание частиц
stuck pipe	прихваченная труба
pressure surges	скачки давления
fishing jobs	ловильные работы
drawback	помеха, препятствие, недостаток
kick	выброс, резкое повышение давления
blowout	разрыв (колонны, трубы, резервуара); внезапный выброс, фонтан (из скважины во время бурения)
detrimental effect	отрицательное воздействие
to plug	закупоривать
reserve pit	запасной резервуар
desilter	отстойник
to aggregate	накапливаться
bit hydraulic horsepower (BHHP)	гидравлическая мощность на долоте
bottomhole cleaning	очистка забоя
high-viscosity sweep	высоковязкое очищение
casing	обсадная колонна
spurt loss	мгновенная фильтрация
wall cake	глинистая корка (образующаяся на стенках ствола скважины в результате фильтрации бурового раствора); фильтрационный осадок
residue	осадок
to seal	уплотнять
to cave in	обрушиваться
to slough	осыпаться
to inhibit	препятствовать
bottomhole assembly	компоновка инструмента
hydratable clay	водонасыщенная, гидрофильная глина

2. Pay attention to the abbreviation used in the text:

Ppg (pounds per gallon) –фунт на галлон

What is measured in gallons? Do you know the difference between English and American values of gallon? Find it out.

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:

Density

Fluid loss

Oil content

Solids content

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.

5. Give the adequate Russian variants for the following word combinations:

An important effect on hole-making potential; functions vital to cost-effective drilling; a means for bit cuttings to be circulated out of the hole; to deal with variations in formation properties; to avoid a substantial drop in the drill­ing rate; a definite cost saver; in spite of potential drawbacks; to have a detrimental effect on rate of penetration; to delay fluid pressure equalization over the chip; whenever downhole conditions permit; to allow for the buildup of wall cake; a plasterlike residue of mud solids.

6. Find the English equivalents for the following words and word combinations:

Охлаждать и смазывать долото; предотвратить попадание пластового флюида; забойный двигатель; влиять на скорость проходки; избегать значительного падения скорости бурения; уменьшать время бурения и стоимость эксплуатации долота; прихват; перепады давления; создавать эффект оседания частиц; избегать ненужных ловильных работ; преодолеть пластовое давление; выброс; предотвратить разрыв колонны; иметь отрицательный эффект; закупоривать трещины (в скважине); поддерживать малое содержание твердой фазы (в буровом растворе); уменьшать гидравлическую мощность на долоте; усложнять очистку забоя; задавливать обсадные трубы; ненужные металлические обломки; добыча нефти; образование сальника.

7. Find the synonyms:

- to decrease

- oil production

- cuttings

- considerable decline

- needless

- to prevent

- to allow

8. Match the words with their definitions and give their Russian equivalents:

1. chip hold-down effect	a.the liquid circulated through the wellbore during rotary drilling and workover operations. In addi­tion to its function of bringing cuttings to the surface, it cools and lubricates the bit and drill stem, protects against blowouts by holding back subsurface pressures, and deposits a mud cake on the wall of the borehole to prevent loss of fluids to the formation.
2. kick	b.a centrifugal device for removing very fine particles, or silt, from drilling fluid to keep the amounts of solids in the fluid at the lowest possible point.
3. mud	c.an entry of water, gas, oil or other formation fluid into the wellbore during drilling. It occurs because the pressure exerted by the column of drilling fluid isn’t great enough to overcome the pressure exerted by the fluids in the formation drilled.
4. spurt	d.the holding of formation rock chips in place as a result of high differential pressure in the wellbore. This effect limits the cutting action of the bit by retarding circulation of bit cuttings out of the hole.
5. desilter	e.the initial loss of drilling mud solids by filtration, making formations easier to drill.

9. Look through the text and find words which correspond to the given definitions:

1. The measure of hydraulic power expended through the bit nozzles for cleaning the bit cutters and the hole bottom.

2. The escape of the liquid part of a drilling mud into permeable formations.

3. The sheath of mud solids that forms on the wall of the hole when liquid from mud filters into the formation.

4. A measure of the resistance of a liquid to flow. Resistance is brought about by the internal fric­tion resulting from the combined effects of cohesion and adhesion.

5. Drilling operations in which water is used as the circulating fluid.

10. Complete the following sentences:

1. In the course of drilling a well, the composition of the mud can be changed in order to ...

2. Lower hydrostatic pressure allows formation rock chips ...

3. When a chip hold-down effect is created mechanical energy on the bit must be increased to ...

4. Stuck pipe and pressure surges caused by ...

5. Heavy-­weight mud is necessary in cer­tain drilling situations when without sufficient hydrostatic pressure ...

6. Low-solids fluid (water, where feasible) will enable faster penetration rates than ...

7. Fine solids in the mud contribute to ...

8. A high-viscosity sweep, operation when a quantity of high-viscosity mud is circulated through the system, is a good way to ...

9. The pressure of the mud column may force ...

10. Oil in the mud ...

11. Answer the questions:

1. What functions does drilling fluid perform?
2. Whatmud properties have the greatest effect on penetration rate?
3. Why doeslightweight mud permit fast drilling?
4. What happens when mud density is too high?
5. How can you prove that the use of low-weight muds when possible is a definite cost saver?
6. What are the best ways to prevent blowouts?
7. What drilling situations is low-solids mud used in?
8. What detrimental effect on rate of penetration can small particles of weighting material in the mud have?
9. How can solids content be kept low?
10. What can increases in mud’s viscosity cause?
11. What ways is water loss beneficial in?
12. What problems can excessive fluid loss create?

Work in groups. Choose one of the characteristics that affect the rate of penetration and write down true/false statements. When everyone is ready, exchange your statements and ask your groupmates to express their points of view. Ask them to prove why they agree or disagree with your statement.