Designing Under and Near Balanced Coiled Tubing Drilling Using Computer Simulations

Abstract

Abstract 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. Introduction It has long been recognized that a key to improving the recovery of reserves is to minimize the reservoir damage created while drilling. Thus, the main objective of drilling close to or under balance is to increase the productivity of the well by reducing formation damage due to the invasion of the formation by the drilling fluid and/or fines. Additional advantages of underbalanced drilling are that higher penetration rates are obtained than with overbalanced drilling and the risk of differential sticking is reduced. Coiled tubing is particularly suitable for under balanced or near balanced drilling because of the improved well control in a coiled tubing system and because coiled tubing allows continuous drilling while maintaining the underbalance. When planning an underbalanced drilling operation, the magnitude of the desired underbalance is a primary consideration. If the downhole pressure is too low, there may be wellbore stability problems. The amount of underbalance also determines the rate of fluid production from the reservoir and the surface facility should be able to handle the production fluid in the return flow. After the desired amount of underbalance is determined, the next step is to determine how to achieve this underbalance For normal or high pressure reservoirs, underbalance can be achieved by using water, brine or diesel as the drilling fluid. For low pressure reservoirs, nitrogen or produced gas is used to aerate the drilling fluid and reduce downhole pressure. Two injection methods are often used. One is pumping foam or nitrified fluid down the coiled tubing and into the annulus, and the other is injecting nitrogen or produced gas through a parasite string or through existing gas lift mandrels to aerate the liquid column in the annulus. P. 153