Drilling Fluids

The drilling fluid used for air-rotary coring is aii; but it occasionally has additives, such as water, foam, polymers, and bentonite gels injected to: (1) cut down airborne dust and prevent sediment balling; (2) aid in removal of cuttings at lower air velocity, as well as removal of water in saturated zones, and (3) seal off lost circulation zones. For a more detailed description of these additives and how they are used, see "Water Well Technology" (Campbell and Lehi; 1973, p. 121-125) and commercial catalogs from most drilling-additive supply companies. The velocity and volume of air required to lift cuttings out of the hole are variables that are related to the specific gravity of the particles and to the volume of the annulus formed between the drill pipe and borehole wall. Generally, an average uphole air velocity of 3,000 ft/min is required for drilling materials to remove drilled cuttings having an average specific gravity of 2.6 from the borehole. Also required is some amount of additional air to overcome the effects of whatever volume exists in the annulus plus any unknown air loss to void spaces. Detailed explanation, tables, and formulas for calculating air velocity and volume requirements for drilling various diameter holes with arious diameter drill pipe are available in "Water Well Handbook," (Anderson, 1971, p. 85-90). Air compressors used for air-rotary coring vary (from greater than 300 lb/in.2 down to 40 lb/in.2) in their pressure output; however the industry tends to use higher-volume, lower-pressure compressors.

Methods for removal of the core barrel, removal of core from the inner barrel, and handling of the core are the same, with one exception to that described in the hydraulic-rotary coring section. This exception is in the handling of core that is cut in a suspected or known contaminated area. Contaminated core must be handled extremely carefully to insure personnel safety and to avoid contaminating the cut core. For an outstanding description of the proper way to handle core from this type of environment, see Burgus and Maestas (1975).

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