Experimental Study on Friction Coefficient of Gas–Oil Two-Phase Flow in a Large Annulus

Summary The friction coefficient is an important factor that affects the accurate calculation of wellbore annular pressure distribution and is of great significance for the safety of drilling operations. To date, investigations of the friction coefficient mainly focused on low-viscosity liquids (such as water and kerosene). Thus, the obtained friction coefficients have poor applicability in the calculation of gas–oil-based mud two-phase flow. This study reports gas–oil two-phase flow experiments for different viscosities (16–39 mPa·s) in the annulus of a large wellbore, performed using an experimental wellbore (Φ100×Φ60×12 000 mm). The gas–liquid mixture Reynolds number ranges from 500 to 10,000. The results reveal a consistent trend for the variation of the friction coefficient under different flow patterns. For the same mixture Reynolds number, a larger liquid viscosity corresponds to a smaller variation of the friction coefficient among different flow patterns. The larger the superficial liquid velocity, the greater the friction coefficient. Based on the dimensionless analysis of the experimental data, a model for the calculation of the friction coefficient of gas–oil two-phase flow in a large annulus is established, and its prediction error relative to the experimental data is found to be less than 30%. This study can provide a basis for accurate calculations of gas–oil-based mud two-phase flow in drilling wellbores.