Optimization of underbalanced hydraulics for an aerated mud system

Abstract

Underbalanced drilling (UBD) has gained popularity during recent years, as it provides a procedure to prevent formation damage, minimise lost circulation risks, and improve the rate of penetration. However, one of the most crucial steps in UBD design is to optimise the drilling hydraulics for the highest performance during the drilling operation. This task is extremely difficult because of the complex nature of the multiphase flow in the UBD system. To accomplish this task, the bottomhole pressure must be calculated. However, the bottomhole pressure, the fluid influx flow rates and the fluid properties along the wellbore are interdependent parameters and can only be derived through a combination of iterative and finite differential methods. It is therefore necessary to use a computer program to carry out the work involved. To achieve the goals of this process, a commercial software package called WELLFLO 8.1.4 was used to model the underbalanced hydraulics. Field data from the Masila Field (Yemen) reservoirs were used as the input parameters for the UBD simulator. Software validation showed good agreement between the measured standpipe pressure and the simulated standpipe pressure with less than 6% average absolute error. The analysis showed that the liquid flow rate is responsible for carrying capacity of the fluid mixture, while the gas phase is responsible for accelerating the liquid phase. Sensitivity analysis proved that the liquid phase density of drilling fluid influences the bottomhole pressure significantly while other drilling parameters such as the rate of penetration, the gas injection density and the choke pressure cause a minimal impact on the bottomhole pressure which plays a significant role in the success of UBD operations. Furthermore, it has been observed that bottomhole pressure, the velocity of the liquid phase and the nozzle size have a strong influence on bit pressure drop.