When should engineers take skin friction into account when writing mud specification for an HDD project?

In short, skin friction should always be considered when writing mud specifications for a horizontal directional drilling (HDD) project.

Skin friction is defined as the amount of resistance developed between the surface of a solid and fluid in relative motion. In other words, it is the amount of friction generated when a solid and fluid move past each other. As the fluid flows over the object, it applies frictional forces which impede its movement.

Skin friction is a function of the viscosity of the fluid as well as the object’s surface roughness. The higher the viscosity of the fluid, the greater the amount of friction generated.

Drilling fluid (or mud) in HDD serves many functions, including transporting the cuttings from the wellbore to the surface, maintaining stability in the wellbore, and cooling the drill bit during drilling operations. If the viscosity of the drilling fluid is too high (i.e., the mud is too thick), it can affect drilling operations in several ways.

Firstly, increased skin friction from highly viscous fluids can give rise to high torques and drag issues. This issue is particularly crucial in water-based muds (WBM), which possess a higher coefficient of friction than oil-based muds (OBM). The resistance generated between the fluid and the cutting head can lower the efficiency of the drilling process as more torque must be applied to produce rotary movements of the drill head and drill string.

Additionally, as the fluid is circulated in the borehole, it is in constant contact with the drill string. The higher the skin friction between the fluid and the string, the harder the pumping system has to work to circulate the fluid. Increased skin friction can also make it harder to restart fluid circulation upon ceasing of drilling operations.

Various types of lubricants and drag reducers can be recommended in mud specifications to help overcome skin friction issues.