Modern drilling practice involves the injection into boreholes of a heavy fluid commonly known as 'mud'. Except in a few instances where suitable substitutes are locally available, this material is made from carefully specified bentonites and serves to counteract hole pressures, to lubricate drill bits, to remove cuttings, and to seal borehole walls.
In order to ensure smooth drilling operations and to compensate for the natural and progressive contamination of the mud, it is also standard practice to introduce additives, of which so-called thinners are examples. The particular function of a thinner is to control the theological properties of mud and, in this way, to maintain normal mudflow without excessive power consumption.
Chemically, thinners can vary widely. But one of the most common types, accounting for as much as 40 per cent of the total 1952 thinner consumption in the United States*, is a complex mixture marketed under the common name "Lignite"**. The active components in this type of thinner are known as humic acids. Since these materials occur in naturally weathered, or artificially oxidized, coals - from which they can, if desired, be extracted with aqueous alkali solution - and since several Alberta coals are known to be particularly rich in humic acids, it was thought pertinent to explore the direct use of such coals are thinners. The investigations reported in the following pages accordingly deal with attempts to survey and characterize some potentially suitable coals and to compare them with two well-established, commercially marked lignite thinners.
Jensen, E.J. (1961): Alberta subbituminous coals as drilling fluid thinners; Research Council of Alberta, RCA/AGS Information Series 33, 22 p.