When drilling underbalanced, the pressure of the drilling fluid is maintained at a value below the formation pressure. In order to lower the well pressure, specific low density fluids are needed, i.e., gas, aerated mud or foam. Foam is particularly beneficial for drilling mainly due to its low density coupled with good carrying capability, but its use remains hazardous due to the incomplete knowledge of its bottomhole properties, and especially of its flowing properties. Pressure drop estimation is crucial for underbalanced drilling (UBD) operations in order to be able to keep the bottomhole pressure in the adequate range in real time. In this study, we analyze the pressure drop variation with the flow rate in a circular pipe for different foam qualities and formulations. Experimental investigations are realized in a pressure and temperature circular conduct flow. We show that lubrication at the wall plays a crucial role. Indeed, the intrinsic viscosity of the foam can be very high, leading to the development of a water layer at the wall responsible for the lubrication of the flow. A two-phase description of the system allows for the analytical estimation of the pressure drop. The size of the lubricated layer is then deduced and its range of existence is discussed. Main parameters of its formation are also discussed. We show that this lubrication effect can be significant in pressure drop estimation for underbalanced operations.