Pressure Control by Mud Density

A serious problem—the enormous waste of natural gas in drilling by the cable-tool method in Oklahoma—led to the general acceptance of "mud laden fluid" as a means of controlling pressure. In May 1913, Pollard and Heggem11 11 of the U.S. Bureau of Mines, demonstrated the practicality of

Figure 2-3. This mud mixer used shortly after the turn of the century served the same purpose as today's mixers. (From Gray,89 World OH.)

adding mud to holes drilled with cable tools to "seal each gas-bearing stratum as it is encountered by drilling with the hole full of mud-laden fluid."

In a more definitive survey, Lewis and McMurray13 stated that mud-laden fluid is "a mixture of water with any clayey material which will remain suspended in water for a considerable time and is free from sand, lime cuttings or similar materials." The mixture's measured specific gravity was 1.05 to 1.15. Mud consistency, they said, should be such as to seal with little penetration of the sand. The distance of mud penetration into the sand was reported to depend on the mud's consistency, the pressure applied, and the sand's porosity. The barrier formed within the sand was maintained principally by the mud pressure in the hole. Advantages in using mud-laden fluids were: ( I) reduction in number of casing strings, (2) protection of upper sands while drilling was continued, (3) prevention of migration between casings, (4) allowing recovery of casing, and (5) protection of the casing from corrosion. This survey related the properties of the mud to its performance and emphasized the economic importance of the control of the drilling fluid.

Most drillers continued to ignore the practical benefits to be derived from mud as a means of pressure control. In 1922, B.K. Stroud,14 supervisor tit the Mineral Division of the Louisiana Department of Conservation, wrote. "They go by the consistency of the mud fluid rather than by its actual weight." He pointed out the hazard of gas-cutting of thick mud and strongly recommended that "drillers should frequently weigh samples of mud." He stated that success or failure in drilling a well in the Monroe (La.) gas field depended upon the control of the gas pressure by heavy mud.

Stroud cited laboratory tests on cement, galena and iron oxide (Fe203) as materials for increasing the density of mud. Iron oxide could be used to raise mud density to 16 lb/gal (specific gravity of 1.86) without an excessive increase in consistency. Iron oxide (hematite) was used successfully in several fields. In the fall of 1922, barite was used to make a heavy mud. This product was pigment-grade barite from Missouri.15

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