Irelative Drilling Rate Per Cent

1 i» 1 • DRILLING MUD

ta

o

> S

OLU

TIO»

e

o

>

0

• •

o

o

T—

<

••

O 10 20 30 40 30 «O TO 60 90 IOO 110 120 130 MO ISO

VISCOSITY - CP. STORMER Fig. 8.15. Laboratory data showing effect of viscosity on drilling rate. After Eckel,30 courtesy API.

O 10 20 30 40 30 «O TO 60 90 IOO 110 120 130 MO ISO

VISCOSITY - CP. STORMER Fig. 8.15. Laboratory data showing effect of viscosity on drilling rate. After Eckel,30 courtesy API.

Filtration Loss

The effect of filtration on the drillability of permeable rocks has been explained by Murray and Cunningham7 in conjunction with their studies on hydrostatic pressure effects. They found that penetration rate was not affected by imposed borehole pressures if such pressures were equalized ahead of the bit. Water, for example, may readily enter a permeable rock ahead of the bit so that no pressure differential exists across the thin element being drilled. Low water loss muds, however, almost instantaneously deposit a tough, low permeability filter cake on the hole bottom, allowing a definite pressure differential to exist. This has two detrimental effects: the dynamic filtration pressure effect already discussed, and the requirement that the bit tooth penetrate the filter cake prior to contacting the rock. Also, the loosened cuttings are trapped in the pasty filter cake mixture and require longer bottom clearance times.

Figure 8.16 shows the observed effect of water loss on the penetration rate in shale. As stated by Cunningham and Goins,81 it is difficult to explain this effect in almost impermeable rocks; however, even the extremely low

-i—^

•--~^outside wati

:rcourse

-

Fig. 8.16. Effect of water loss on drilling rates when drilling a Wilcox shale with a lj-in. bit in a lime base mud. Laboratory data: bit weight — 750 lb; speed — 50 rpm. Each point is average of 24 tests. After Cunningham and Groins,31 courtesy Petroleum Engineer.

WATER LOSS (cc)

Fig. 8.16. Effect of water loss on drilling rates when drilling a Wilcox shale with a lj-in. bit in a lime base mud. Laboratory data: bit weight — 750 lb; speed — 50 rpm. Each point is average of 24 tests. After Cunningham and Groins,31 courtesy Petroleum Engineer.

permeability of shales may allow some pressure equalization effect. Also, the increased starch content used to reduce water loss could conceivably cause the observed behavior. Thus the low fluid loss which is desirable from the formation damage standpoint is undesirable from the standpoint of penetration rate.

Oil Content

It has long been observed in the field that the addition of oil to water base muds almost always improves penetration rate in virtually all types of rocks.32'33 The largest increases appear to occur in the soft rock areas, with smaller increases being noted in hard rock

1

1 •

;

/ /

i

PERCENT OIL

Fig. 8.17. Increase in drilling rate vs. oil concentration. Curve (A) is for a Vicksburg shale; curve (B) is for a Miocene shale (laboratory data). After Cunningham and Goins,31 courtesy Petroleum Engineer.

PERCENT OIL

  1. 8.17. Increase in drilling rate vs. oil concentration. Curve (A) is for a Vicksburg shale; curve (B) is for a Miocene shale (laboratory data). After Cunningham and Goins,31 courtesy Petroleum Engineer.
  2. Generally, these increases have been attributed to better lubrication (increased bit life), better bore-hole conditions (less enlargement, minimum heaving shale troubles, less hole-pipe friction and drag), and less bit balling by hydratable clays and shales. At present, it is considered that prevention of balling is the most important factor, in soft rock areas.

Increased bit life means that less non-productive rig time is expended in making trips; thus the over-all penetration rate is increased. It has also been observed that increases in on bottom drilling rate are also obtained, despite the conflicting facts that oil additions to water base muds generally decrease water loss and increase apparent viscosity. Therefore, other factors exist f RELATIVE DRILLING RATE PER CENT

eor f RELATIVE DRILLING RATE PER CENT

0 10 20 30

Was this article helpful?

0 0

Post a comment