Designing Under- and Near-Balanced Coiled-Tubing Drilling by Use of Computer Simulations

This paper describes the use of two computer simulation utilities to design underbalanced or near-balanced coiled tubing drilling: one is a steady-state design module and the other is a transient wellbore simulator developed for coiled-tubing operations. The steady-state design module provides various design parameters for a drilling operation, and the transient wellbore simulator predicts the outcome from a particular design for the entire operation. Simulation results show the desired under or near balance can only be achieved by certain combinations of liquid and gas rates when using foam as the drilling fluid or using gas injection through gas-lift mandrel or parasite string. The effects of cuttings loading, depth of injection point and reservoir inflow on the downhole pressure or underbalance are presented and discussed. A procedure for designing under- or near-balanced drilling is described and demonstrated with an example using the computer utilities. A field example is used to show the accuracy of the simulator by comparing the measured and simulated downhole pressures.