6. Answer the questions:

1. What formations were insert bits designed for?

2. Why was it not desirable to use them in soft-formations?

3. What was the main problem?

4. What drilling characteristics can the usage of these bits improve?

5. What do the shape and placement of the inserts depend on?

6. What bit features are required for drilling soft shales, lime­stones, clay?

7. What bits are required to maintain constant penetration through the abrasive streaks?

8. What are bits used in hard limestones, dolomite, chert, and hard shales characterized by?

9. What factors help to prolong life of the bit?

Diamond bits

1. Before reading the text, study the vocabulary

diamond bit	Алмазное буровое долото
watercourse	Промывочный канал, поток
cross-pad watercourse	jhgjhfgjfgfjfjfk
groove	Выемка, желобок
anhydrite	Ангидрид
steep	Крутой
feeder slots	Дозатор/канавка
radial flow	Радиальное течение
velocity	Скорость
junk	Металлические обломки
salvageable	Извлекаемый
matrix	Матрица алмазной коронки
dislodging	Смещение

2. Pay attention to the pronunciation of the following words:

• anhydrite

• pressure

• configuration

• desirable

• quartzitic sandstone

• junk

• salvageable

• circulation

3. Read and translate the text

When a roller cone bit's progress fails below a preestablished rate, e.g. 3-5 feet/hour, a diamond bit may be the best alternative. Again, the choice is one of economics, since a diamond bit may cost up to four or five times as much as an insert bit. In spite of their expense, diamond bits, under certain conditions, can be the most cost-effective choice of bit.

The diamond bit is basically conical in shape, a radically different design from that of a roller cone bit. The size and arrangement of diamonds and the pattern of the watercourses, which are grooves in the bit face that govern the flow of drilling fluid across the bit, determine the bit's suitability for drilling a particular formation. Bits for soft formations such as marl, salt, anhydrite, chalk, and soft limestones have large-sized diamonds that are set in a cone that is relatively steep. The watercourses for soft-formation diamond bits may be in either of two commonly used configurations. The cross-pad watercourse has alternating high-pressure feeder slots and low-pressure collector slots to direct the flow of the drilling fluid. The radial flow configuration, in which drilling fluid is routed from the center of the bit to its outer diameter, permits high mud velocity, which is desirable in soft-formation drilling.

In medium-strength formations such as sandy shale, limestone, and dolomite, medium-sized diamonds are set in a radial pattern on a cone that is shallower and more rounded than the soft-formation bit cone. For hard sand­stones, siltstone, quartzitic sandstone, and volcanic rock, a rounded, shallow cone prevents the damage to the center of the bit that can occur on a deeper cone when extreme weight is applied. The diamonds on hard-formation bits are small and are set in dense patterns to guard against shearing, which may occur in such highly compressive and abrasive rocks.

When diamond bits are used, a hole free of junk must be maintained to avoid breaking the diamonds, which are salvageable. In fact, up to 50 percent of the bit's cost can be recovered, provided the bit is not destroyed. Circulation must be watched carefully so that the bit will remain cool and not ball up. An overheated bit can cause cuttings to burn into the matrix, dislodging the diamonds.