Drilling fluid (or “mud”) must accomplish or assist with many things at the bottom of a borehole as relates to drilling the well.
Some of those include the following:
- The drilling fluid must remove the cuttings from the bottom of the hole. Cuttings that are not removed are ground finer by the next cutter. Regrinding results in new smaller cuttings and prevents their removal as well as absorbs mechanical energy from the bit that should be applied to breaking fresh new rock.
- The drilling fluid must clean the drill bit itself. Formation cuttings tend to adhere to or wedge between and around the cutting structures. “Sticky” formations such as soft or “gumbo” clays and other shales can also stick to other areas of the bit, stabilizers, and the BHA.
- The drilling fluid must transport the cuttings to the surface with as little regrinding and degradation (mechanical and/or chemical) as possible.
- Oftentimes the fluid must also power downhole equipment such as drilling fluid motors, MWD, and logging while drilling (LWD) tools. This requirement is especially common offshore, where nearly all wells are now drilled with one or more downhole powered-by-the-drilling fluid components in the BHA.
- Related to the above functions for MWD and LWD tools, the fluid must be suitable for transmitting compressional waves used by data transmission systems. Usually this implies a low solids content and very low gaseous content (entrained air, formation gas, etc.).
Optimum use of bit hydraulic energy enables each of these to be accomplished either adequately or if drilling so deep or with insufficient pump power and pressure to do this inadequately, to make the very best use of all available power and energy, even under nonoptimized conditions.