*4.4. Mud Density*

Mud density is one of the key operating parameters for maintaining a stable wellbore condition during the drilling operation. Using an inappropriate mud density can lead to the risk of formation fracture or/and kick [23]. Figure 10 illustrates the changes in the TDR and fluid temperature in the annulus with various mud densities. The minimum TDR depth in all three cases is around 1000 m (Figure 10a). The minimum depth is evidenced by the similar intersection points between the fluid temperature curves and the formation temperature line (Figure 10b). As the mud density becomes heavier, the TDR increases, but the increasing interval is insignificant. Mud density is the minimal influential factor among operational parameters investigated previously: circulation rate, time, and injecting temperature. Changing mud density does not impact thermal disturbance; thus, the uncertainty of temperature measurement after drilling seldom increases.

**Figure 10.** The sensitivity analysis of thermal disturbance with various mud densities: (**a**) TDR and (**b**) fluid temperature in the annulus.
