Lubrication Process at the Wall in Foam Flow Application to Pressure Drop Estimation While Drilling UBD Wells
Authors: Peysson Y. , Herzhaft B. 
Published: March 2010

Abstract
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 bottom hole 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.

Introduction
Underbalanced drilling is recognized as one of the most effective solutions to prevent formation damage, differential sticking or fluid losses. When drilling underbalanced, the pressure of the drilling fluid is maintained at a value below the formation pressure, and for this purpose, low density fluids are used. Foam, defined as a mixture of a gas phase and a foaming solution, is one of the most versatile aerated fluids. It allows very low densities from 0.2 s.g., and is one of the most efficient fluids for lifting the cuttings and cleaning the wellbore. However, it is very important when drilling underbalanced to keep the bottom hole pressure in an adequate range in real time. Indeed, foam, as other underbalanced fluids, is not designed to create a mud cake at the wall. Therefore, a short overbalanced period can lead to the deep penetration of a filtrate into the reservoir, severely reducing the expected productivity. One of the key features of a successful underbalanced operation is control of the bottom hole pressure. Wellbore hydraulic simulation is essential as well, to design the operation and for monitoring the drilling phase or evaluating changes in the drilling program(1). While pressure estimation for aerated fluids is often realized through multiphase modelling (with estimation of the different phase's velocities), it is possible to consider flowing foam as a fluid with a given rheology that varies with the pressure, temperature and quantity of gas. Predicting the bottomhole pressure while drilling with foam implies knowledge of foam rheological properties that depend on the foam density; the foam density being, itself, a function of the frictional and hydrostatic pressure.