An advanced simulator for investigation and planning of coiled tubing (CT) operations has been developed. In this paper, various models pertinent to the fluid flow within the CT and in the annular space between the CT and the well wall are presented, as well as various mechanical models pertinent to the placement of the CT in the well. Some verification tests have been carried out which match well to calculations by the simulator.
The paper gives a summary of flow models and mechanical models used in the simulator to describe CT operations. Most of the models are taken from published literature. In addition some models, e.g., for tortuosity and friction back-calculation, have been especially derived.
The main calculations of the simulator concern friction force between CT string and well wall, and frictional pressure drop of pumped fluids within and outside the CT. Parameters and effects of practical importance are computed, e.g., hook-load, bottom force, curvature effects in the frictional pressure drop when the CT is on the reel, cable inside the CT, pick-up and slack-off, effects pertinent to drilling with CT, etc. Buckling and lock-up check are implemented separately as one of the main functions.
To verify some aspects of the simulator, tests were performed at RF-Rogaland Research's Ullrigg Drilling & Well test centre in Stavanger. The friction coefficients back calculated by the simulator, calculated result of well end force with buckling effect, and calculated frictional pressure loss in the CT with water match well to the test results.
A one dimensional model is used to calculate the pressure distribution as a function of measured depth (MD). Integral versions of mass and momentum balance equations form the basis of the model.
The formulas used correspond to the solutions for fully developed and incompressible flow in a duct with a fluid with constant physical properties. A layered fluid can be defined to take into account the variation of the physical properties due to pressure and temperature variations along the well. Four rheological models are considered, the ones for Newtonian, Power-law, Bingham-Plastic and Robertson-Stiff fluids. The average wall shear stress is calculated by using analytical expressions for laminar flow and experimental correlations for turbulent flow for the Fanning friction factor. Practically, the rheology data from the field are readings from a Fann viscometer. In the CT simulator, the Fann rheology measurements are transferred to model parameters fitting the four supported rheology mod