Barrel yield for bentonite drilling fluids is an oilfield term for measuring the viscosifying capabilities of a bentonite clay and is defined as the number of 42 gallon barrels of 30 to 35 centipoise (or cP, a measure of viscosity) drilling fluid that 1 ton (2,000 pounds or 40 50-pound bags) of bentonite clay will produce. For example, 1 ton of 200 bbl yield bentonite will produce approximately 8,500 U.S. gallons of 30 to 35 viscosity slurry. So what does this mean to the everyday small to medium driller who utilizes between 500 and 2,000 gallons of slurry a day? Probably not much until a rep approaches them and points to the fact that they are using 200 bbl drilling fluid product versus a 220 bbl high yield bentonite drilling fluid product. Horizontal directional drilling (HDD) bentonite drilling fluid products, for example, which vary from 150 bbl to as high as 220 bbl, are specifically designed to meet the demands of horizontal directional drilling.
The biggest difference between high yield bentonite drilling fluid products and drilling fluid products specifically designed for HDD applications is in gel strength and fluid loss/filtration control. HDD specialty drilling fluid products contain additives to enhance gel strength (to suspend drill cuttings to carry them out of the hole) and filter cake qualities/filtration control (which enables a drilling fluid to stabilize or keep the hole open), whereas extra high yield bentonite products do not have enhanced gel strength and filter cake/fluid loss additives, and require higher viscosities to achieve the same properties.