Friction factors: What are they for torque, drag, vibration, bottom hole assembly and transient surge/swab analyses?

The term “friction factor,” conventionally defined for fluid flow engineering calculations, is also used for solid mechanics calculations in the drilling industry. At first glance, it may seem to be a misnomer, but there are underlying facts and reasons for the adoption of the term “mechanical friction factor” that is unique to the industry. This term is used as a proxy for the coefficient of friction (COF) or the Coulomb friction, a dimensionless scalar value, in the estimation of torque, drag, and side forces for the directional tendency prediction, static displacement of string for vibration analysis, and moving pipe elastic force for transient swab/surge pressure estimation. Because of larger uncertainties in the modeling of a long drillstring, including cuttings bed, tortuosity, mud properties, fluid viscous effects, wellbore temperature, pressure, and non-uniform geometrical interferences between the drillstring and wellbore, the COF is termed as a friction factor by including the uncertainties involved without defying the mechanics involved. Another important parameter used during the planning stage in combination with the friction factors is the tortuosity factor to provide artificial undulation to the wellbore. The tortuosity factor causes an additional problem, which requires a different friction factor for the analysis, even though the COF between the drillstring and the wellbore has not changed. Because there is no industry standard for quantifying these parameters, a great deal of confusion exists between these parameters and results in the overestimation or underestimation of the frictional drag. This paper describes the details, clear definition, calibration, pitfalls, and context in which they can be used. It presents the simple guidelines and several example calculations. This paper also provides methods for evaluating these parameters with real-time data.