STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drill String: Composition and Design 715
Table 4-70
Density Increase with Weighting Materials
Specific
Practical Weight
Material Gravity Increase, Ib/gal
CaCO, 2.7
FeCO, 3.85
BaCO, 4.43
FeP, 5.24
3.5
6.5
8.0
10.0
pressure gradients lower than 0.433 psi/ft (freshwater gradient). Weighted oil
muds cannot be used for completion operations since they form mud plugs while
perforating. Most of the oil-base systems contain asphalt that can plug the
formation and reverse its wetting characteristic (from water-wet to oil-wet).
Moreover, oil-based fluids are expensive. Their use can be justified. However,
in oil-producing formations where water base fluids would cause serious permeability
damage due to clay problems, or high-condensate gas wells oil-base
fluids are feasible. In these formations the produced fluids will clean up the
oil filtrate.
Foam Systems. The preparation, composition, and maintenance of foam
completion and workover fluids is similar to that of foam drilling fluids. The
advantage of foam is the combination of low density and high lifting capacity
at moderate flow rates. The use of foam as a completion fluid can be justified by:
1. Low hydrostatic pressure (0.3 to 0.6 psi).
2. Circulation with returns where other fluids have no returns to the surface.
3. Easy identification of formation fluids.
4. No inorganic solids; other solids are discarded with the foam at the surface.
5. In wells with sand problems, faster operation and more complete sand
removal.
6. The ability to clean out low pressure wells without killing them.
The limitations of foam are (1) operational complexity, (2) high cost, and (3) the
pressure effect on foam consistency, Le., below about 3,000 ft, foam com-presses
to a near liquid form.
DRILL STRING: COMPOSITION AND DESIGN
The drill string is defined here as a drill pipe with tool joints and drill collars.
The drill stem consists of the drill string and other components of the drilling
assembly that includes the kelly, subs, stabilizers, reamers as well as shock
absorbers, and junk baskets or drilling jars used in certain drilling conditions.
The drill stem (1) transmits power by rotary motion from the surface to a rock
bit, (2) conveys drilling fluid to the rock bit, (3) produces the weight on bit
for efficient rock destruction by the bit, and (4) provides control of borehole
direction.
The drill pipe itself can be used for formation evaluation (Drill Stem Testing-
DST), well stimulation (fracturing, acidizing), and fishing operations.