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Abstract Non-aqueous drilling fluids (such as synthetic-based mud) are frequently used to drill one or more sections of an oil/gas well to reduce drilling problems such as shale sloughing, wellbore stability, and stuck pipe. However, solubility of formation gas in such fluids makes early gas detection and thereby the well control process very challenging. This is of particular concern in deep offshore wells, in which large amount of gas can be dissolved in non-aqueous drilling fluids under high pressure and temperature conditions. The gas remains in solution until the bubble point is reached. Thereafter, a sudden release of gas at shallow depth can compromise wellbore and riser integrity, particularly when the gas has passed the blow out preventer installed at the seafloor. An advanced planning tool to simulate the transient multi-phase phenomena associated with gas kicks in non-aqueous drilling fluids is therefore highly desirable.
This paper presents a novel and comprehensive hydraulic model with associated calculation routines and software to simulate a gas kick in non-aqueous drilling fluids. A transient drift-flux approach based on conservation of mass and momentum was applied in association with appropriate algebraic closure equations and sophisticated friction and choke models. Advanced numerical schemes, where applied previously, have been modified to handle the mass transfer between the liquid (mud) and gas phases. In addition, PVT models have been included to investigate and predict the effect of gas solubility in various types of drilling fluids.
The calculation routines contained in a new software tool predict crucial parameters during well construction such as pit gain, gas break out location and void fraction, annular pressure profile, kick tolerance, choke opening, flow-out, standpipe and casing pressures. Simulation results generated using the tool are presented here for both water-based and synthetic-based muds to illustrate the impact of gas solubility on kick behavior. The tool can handle several other complexities which occur during a well control incident such as multiple influxes from one or several formations, dynamic well control (suitable for managed pressure drilling), automated choke control, sudden pump startup/shutdown, non-Newtonian drilling fluids, arbitrary wellbore path, lost circulation, etc.
Applying advanced numerical schemes associated with relevant PVT models and several types of boundary conditions makes the tool comprehensive, unique, robust, and efficient for well control analysis for a variety of complex drilling scenarios, particularly deepwater wells. As such, it has the potential to enhance well control operations and well design, thereby enhancing rig safety and reducing non-productive time and cost associated with well control-related events.
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