This paper describes a computer weilbore simulator developed for coiled tubing operations of fill cleanout and unloading of oil and gas wells. The simulator models the transient, multi phase fluid flow and mass transport process that occur in these operations. Unique features of the simulator include a sand bed that may form during fill cleanout in deviated and horizontal wells, particle transport with multiphase compressible fluids, and the transient unloading process of oil and gas wells. The requirements for a computer wellbore simulator for coiled tubing operations are discussed and it is demonstrated that the developed simulator is suitable for modeling these operations.
The simulator structure and the incorporation of sub-modules for gas/liquid two-phase flow, reservoir and choke models, and coiled tubing movement are addressed. Simulation examples are presented to show the sand bed formed in cleanout in a deviated well and the transient unloading results of oil and gas wells.
The wellbore simulator developed in this work can assist a field engineer with the design of coiled tubing operations. By using the simulator to predict the pressure, flow rates, sand concentration and bed depth, the engineer will be able to select the coiled tubing, fluid and schedule of an optimum design for particular well and reservoir conditions.
Coiled tubing is used in many oil field service operations, such as fill cleanouts, well unloading, acid and cement squeezes, logging, and drilling. The more conventional use of coiled tubing in fill cleanout and well unloading accounts for the majority of all coiled tubing services. Fill cleanout involves circulation of fluid through coiled tubing down to the sand fill in the wellbore. The fluid mixes with the sand particles and carries the particles up to the surface through the annulus formed between the coiled tubing and the well string. A schematic plot of fill cleanout with coiled tubing is shown in Fig. 1. The general configuration of well unloading with coiled tubing conveyed nitrogen is similar to that in Fig. 1 except there is no sand fill and the circulation fluid is nitrogen gas. The nitrogen gas is circulated through the coiled tubing down to a certain depth in the wellbore liquid. The nitrogen gas rises in the liquid in the annulus and aerates the liquid column.