Lost Circulation of Drilling Fluid

Partial or complete loss of drilling-fluid circulation sometimes occurs when hydraulic-rotary coring of rock is performed. This toss results from encountering fractures or high-intergranular permeability in the rock, large solution openings in limestone-associated rocks, and cavernous or scoria-cinder zones in basalts (Teasdale, 1980). Blind coring is sometimes attempted (no drilling fluid circulated to the surface); however, this method could result in stuck or lost drill pipe.

If the coring is being done using a prepared drilling mud, it is often too expensive to continue losing drilling fluid in a lost-circulation zone. Recommendations for regaining circulation of drilling fluid are as follows:

  1. If the loss of drilling fluid zone results from high fracture or intergranular permeability of the rock, it may be possible to regain drilling-fluid circulation by spotting the zone with 75-s to 100-s viscosity, high-yield bentonite. If this alone does not stop the fluid loss, some lost-circulation materials listed in table 3 should be added to the drilling fluid used for spotting the zone of lost drilling-fluid circulation.
  2. The methods suggested in recommendation 1. will often work whenever scoria or cinder zones are encountered in basalts (particularly if one or more of the recommended lost-circulation materials are used in the drilling fluid used for spotting the zone).
  3. circulation zone is of a cavernous nature, it must either be cemented completely off or casing installed through it. If cementing of the cavernous

Table 3.—Lost-circulation materials

Comparatitive products

Table 3.—Lost-circulation materials

Comparatitive products

Baroid

Magcobar

Description

Wall-Nut

Nut-Plug

Crushed granular nuthulls in fine, medium, coarse, and flour grades.

Aspen Fiber

Magco-Fiber

Specially processed blend of aspen fibers of controlled length.

Micatex

Magco-Mica

Crushed muscovite mica flakes in fine and coarse grades.

Hyseal

A blend of various-size particles of ground paper products.

Fibertex

Processed cane fibers.

Jelflake

Cel-O-Seal

Shredded cellophane film

zone is attempted, it should be done by introducing the thickest cement slurry (with a quick-set additive) that will flow down the tremie pipe. The slurry should be added through the tremie pipe in small batches to accomplish a buildup of the cement, instead of pushing it back into the cavernous zone. For a detailed review of cementing practices in core holes, see "Diamond Drill Handbook" (Cumming, 1969). Halliburton and other well-cementing service companies provide many cementing services. If the cementing job is extensive, it would be economical to have one of these service companies (with the proper equipment and expertise to perform all kinds of large cementing jobs) do the cementing.

4. Casing through cavernous, lost-circulation zones might be more economical than cementing off the problem areas and can be done in one of the following ways: (1) if the drilling-in method was used for casing through overburden, the driller may attempt to ream the hole out with the diamond-casing bit and extend the original casing through the lost-circulation zone; or (2) if extending the original casing through the cavernous zone is not possible, then the next smallest size casing would have to be installed and telescoped through the existing casing and set through the lost-circulation zone. This will require that a smaller core barrel be used when coring operations are resumed. Core-drilling

Figure 23.—Illustration of Diamond Core Drill Manufacturers Association W-Series flush-joint casing classification and dimensions (reprinted by permission from Diamond Drill Contracting Co.).

REFERENCE SYMBOLS

CASING DIAMETERS

mm

inches

RX or RW

36.5

1-7/16

EX or EW

46.0

1-13/16

AX or AW

57.1

2-1/4

BX or BW

73.0

2-7/8

NX or NW

88.9

3-1/2

HX or HW

114.3

4-1/2

PX or PW

139.7

5-1/2

SX or SW

168.2

6-5/8

UX or UW

193.6

7-5/8

ZX or ZW

219.0

8-5/8

The DCDMA system of standardizing casing sizes allows a smaller size diameter casing to be used for telescoping a drill hole

The DCDMA system of standardizing casing sizes allows a smaller size diameter casing to be used for telescoping a drill hole

Figure 23.—Illustration of Diamond Core Drill Manufacturers Association W-Series flush-joint casing classification and dimensions (reprinted by permission from Diamond Drill Contracting Co.).

written contract specifications should be written to allow for standardized nesting of flush-joint casing and drill pipe to allow telescoping of casing when core drilling; different designated casing (HW, NW, and so forth) fit one inside the other, with necessary clearances for circulation of fluid and passing through the corresponding designated core barrel (fig. 23).

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