Salt Water Muds

Early field experience, as well as laboratory studies, established that benionite was the most practical material for improving the viscosity and wall-building properties of fresh-water muds. As dissolved salt content increased, however, bentonite became progressively less effective. In saturated salt water, bentonite did not swell, and contributed little toward reduction of filtration. To thicken salty muds, bentonite was mixed into fresh water and the resulting thick slurry was added to the salt-water mud. After a short time, however, the salty mud would become thin and additional treatment would be necessary.

Roy Cross and M.F. Cross" found that a type of fuller's earth, mined in southwestern Georgia and northwestern Florida, and consisting principally of the clay mineral attapulgite (palygorskite), could be used to thicken salty water, regardless of salt content. Although this clay greatly improved the cuttings-carrying capability of salty muds, the wall-building properties remained poor. Thick filter cakes formed on porous formations, often causing struck pipe, and caving of shale was common. Drilling progress in the Permian Basin of West Texas was hampered by salt beds. Dome salt in the Gulf Coast created problems.

A laboratory study on the control of filtration of salt water muds led to field tests of tragacanth gum and gelatinized starch in 1939.48 Field experience confirmed laboratory test results which showed great reduction in

1!. Untreated Mud. h. Mud Containing 0.5 Per Cent

Tragacanth Gum.

Figure 2-8. Influence of tragacanth gum on thickness of filter cake: 20 percent Shatter Lake clay in salt solution filtered on sand. (From Gray, etaIS* Copyright 1942 by AIME.)

1!. Untreated Mud. h. Mud Containing 0.5 Per Cent

Tragacanth Gum.

Figure 2-8. Influence of tragacanth gum on thickness of filter cake: 20 percent Shatter Lake clay in salt solution filtered on sand. (From Gray, etaIS* Copyright 1942 by AIME.)

cake thickness (Figure 2-8). The major problems associated with thick filter cakes (stuck drill pipe and inability to run casings to bottom) were generally eliminated. In the United States, the ready availability and lower cost of starch excluded the imported gums from further consideration and salt-starch mud was recognized as an economical solution to problems of salt drilling. Applications of starch as a means for controlling filtration of brackish and fresh water muds spread quickly.40

Muds for "Heaving Shale"

In drilling on piercement-type salt domes along the Gulf Coast in the early 1920s, the problem of heaving shale frequently led to abandonment of the hole before the objective was reached.

The term heaving shale was applied to any shale that sloughed into the hole in excessive quantities and thereby interfered with drilling progress. That the term described an effect, rather than a specific substance, was generally recognized.29 Sensitivity to water was a characteristic of such shales. Some shale samples swelled on contact with water; others broke up into fragments with little indication of swelling.

The difficulties involved in obtaining reliable samples of the trouble-making shale caused much of the investigative work to be done with bentonite instead of shale. The important effect of pressure on shale behavior-was ignored in these studies (See Chapter 8). Consequently, special muds were designed to prevent swelling of bentonite, either by salts dissolved in water, or by substitution of oil for water.

A patent issued in 193150 proposed the use of dissolved salts to reduce the osmotic pressure between fluid in the bore hole and that in the heaving shale. In the following years, several unsuccessful attempts were made to drill with calcium chloride—zinc chloride and sodium chloride—sodium nitrate muds. '1 W.V. Vietti52 of the Texas Company initiated a study to find an aqueous solution that would prevent the disintegration of shale. Based on such tests, sodium silicate was tried at Bryan Mound, Texas, in 1935." Further experimentation led to the issuance of seven patents on compositions of sodium silicate muds. These patents typically claimed, . . the method of preventing the heaving of shale encountered in the well which comprises circulating through the well a drilling mud comprising clay, water, sodium silicates of varying silica to sodium oxide ratios and in varying percentages and containing varying percentages of water-soluble salts."52 Sodium silicate muds were used with some success in a number of problem wells until about 1945. W.F. Rogers devotes a chapter to sodium silicate muds in his second edition (1953)53 but silicate muds are barely mentioned in the Third Edition (1963) * because by that time other types of muds were being used successfully in drilling heaving shale.

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