Unit 4 Weight on Bit and Rotary Speed

Special Considerations

1. Before reading the text, study the vocabulary:

bottomhole cleaning	очистка забоя
bit hydraulics	гидравлическая характеристика долота
indiscriminately	неразборчиво
crushing-chipping action	дробяще-скалывающее действие
finite time	ограниченное время
to fracture	раздроблять
to cor­respond	соответствовать, соотноситься
to shatter	разбить
drill string	бурильная колонна
drill collar	утяжеленная бурильная труба
derrick	буровая вышка
lifting sub	втулка с выступающими плечиками (навернутая на конец утяжеленной бурильной трубы для соединения с элеватором)
safety clamp	предохранительный хомут
stand	свеча бурильных труб
round trip	рейс, спускоподъемная операция (подъем инструмента, смена долота, опорожнение колонковой трубы и спуск инструмента на забой)
horsepower	мощность
torque	вращающий момент
slanted hole	наклонная скважина
dogleg	резкое искривление ствола скважины
flexible shaft	гибкая, эластичная колонна
joint	муфта
twistoff	поломка (бурильных труб) вследствие скручивания
feasible	возможный, выполнимый
smooth running	мягкая проходка
downhole motor	забойный двигатель

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

psi (pounds per square inch) – фунтов на квадратный дюйм

rpm (revolutions per minute) – оборотов в минуту

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

• indiscriminately

• finite time

• torque

• centrifugal forces

• severely

• unevenly

• hydraulics

4. Read and translate the text.

The mechanical factors of bit weight and rotary speed must be coordinated with bit selection in order to achieve optimal drilling rates. Generally speaking, an increase in either bit weight or rotary speed will increase the rate of penetration, provided that bottomhole cleaning (bit hydraulics) is attended to properly. But weight on bit and rotary rpm should not be increased indiscriminately. Overworking the bit wears the bit's bearings and cutting structures. Such drilling practices can also cause hole devia­tion. When these added costs are considered, the intended benefits of such high-energy drill­ing are lost. And since least-cost drilling is the objective, bit weight and rotary speed are usu­ally applied in inverse proportion to each other.

Most soft, shaly formations respond to varia­tions in rotary speed, and bit weights are light (3,000-7,000 psi bit diameter). The shearing action of soft-formation bits is well-suited for rotary speeds of up to 250 or even 300 rpm. Although increases in weight may increase the rate of penetration, the chances for hole devia­tion also increase, especially as the bit becomes dull.

In hard formations, heavy weights (5,000-10,000 psi bit diameter) are required for the bit's crushing-chipping action to overcome the compressive strength of the rock. Because a certain amount of finite time is required for the bit cutting structure to fracture the rock, the rate of increase in the drilling rate does not cor­respond directly with rotary speed increases. Excessive rotary speed can result in shock loads that can shatter bit teeth and cause drill string connections to fail. Rotary speed may vary between 30 and 80 rpm.

In calculating the appropriate bit weight and rotary speed, the capability of the rig must be considered. Additional weight requirements call for additional drill collars, which are expensive to buy and maintain and require extra handling time on round trips. Drill collars are heavy and hard to handle in the derrick and at the rotary, where lifting subs and safety clamps have to be used. Ten stands (thirty collars) or more are sometimes used, and several hours may be required to break out and make up the collars during a round trip.

Increased weight and rpm increase rotary torque, or resistance to turning. Higher torque means that more horsepower is required to operate the rotary. Here again, the rig's power system may impose limitations. Long-tooth bits require more horsepower than do short-tooth bits for the same formations. Slanted holes and wells with sharp doglegs also require more rotary power because of the greater friction between the drill string and the wall of the well.

The combined factors of high rotary speed, rotary torque, and extreme weight on bit can create great stress on the drill string. Smooth running is important. Since the drill string is a flexible shaft, a certain amount of irregularity is to be expected. But if centrifugal forces are severely unbalanced, hole friction will increase and the bit may wear unevenly. In such instances, joint failure, twistoffs, tooth breakage, and bit bearing failure may result.

The level of energy (weight and rpm) imposed on insert bits is generally lower that that required on steel-tooth bits to avoid insert chippage and breakage. Improvements in insert grades and shapes have allowed for higher rotary speeds in soft formations and higher weights in hard forma­tions than were previously possible.

Diamond bits are best operated under much lower weights than roller cone bits; rotary speeds are relatively high. Weights usually range from 350-750 psi of hole area, but even greater weight may be used, depending on the drilling situation. The value of diamond bits and the nature of their cutting action require the use of the lowest weight at which good penetration can be achieved. Rotary speeds are usually in the 100-rpm range, but when downhole motors are used, speeds from 200 up to 1,000 rpm are feasible. With smooth running, proper hydraulics, and optimum weight, diamond bit drilling rates usually increase in direct proportion to rotary speed.

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

Rotary rpm; the intended benefits of high-energy drill­ing; least-cost drilling; to apply in inverse proportion; to cause drill string connections to fail; additional weight requirements; to require extra handling time on round trips; to break out and make up the collars; to increase rotary torque; wells with sharp doglegs; to be severely unbalanced; to avoid insert chippage and breakage.

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

Достичь оптимальной скорости бурения; увеличить скорость проходки; нагрузка на долото; количество оборотов ротора в минуту; перенапряжение долота; вызывать искривление скважины; намеченная прибыль; цель; хорошо подходить для чего-то; преодолеть; ограниченное время; разбить зубья долота; бурильная колонна.

7. Find the synonyms:

• choice

• limited time

• to break, to crack

• roughly

• worn bit

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

to ball up	the lowest or deepest part of a well.
dogleg	to collect a mass of sticky consolidated material, usually drill cuttings, on drill pipe, drill col­lars, bits, and so forth.
twistoff	the column of drill pipe with attached tool joints that transmits fluid and rotational power from the kelly to the drill collars and bit.
drill string	a complete break in pipe caused by rota­tional force wrenching damaged pipe apart.
bottomhole	a short change of direction in the wellbore, frequently resulting in the formation of a key seat.