Inclusion of mass transfer terms in the AUSMV transient flow model
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Gas contamination of an oil-based drilling mud while performing drilling operations, having influx of formation gas into the borehole in the form of a gas kick; pose a potential hazard to the personnel, environment and the drilling equipment. This danger grow worse when bottomhole conditions are such that the gas completely dissolves into the oil-based drilling mud and quickly evolves as the gas-cut oil-based drilling mud is circulated up the well. It is therefore crucial to have the ability to understand and model the phenomenon of gas solubility in a flowing well scenario. The first part of this thesis gives an introduction to well control in general, before going deeper into High-Pressure High-Temperature well control. In addition, gas solubility in oil-based mud is presented, trying to bring into light the various factors affecting gas solubility in oil-based mud and how important it is to understand the behavior of gas contaminated mud. An extensive literature study has been performed to give an overview of the various challenges that may be encountered during drilling operations, and the advancements in well control to diminish these challenges. Since deep water wells with narrow operational windows is currently more common than before, one of the most critical areas for development in well control safety is early kick detection. Being able to model a precise and consistent kick detection system seems to be the common denominator to reduce the High-Pressure High-Temperature issues. The second part of the thesis is an attempt to introduce the ability to include mass transfer into the AUSMV scheme. The system is modified to fit a water and steam system, looking at the phase transition between water liquid and water vapor. How the conservation variables are updated needed to be modified as we introduce a new source term to the original AUSMV scheme. First, fixed values for the mass transfer is used to experiment with the AUSMV scheme in a horizontal pipe. The main purpose of this simulation was to see whether the AUSMV scheme could handle the introduction of mass transfer. Simulation showed a significant change to the whole system, as gas is being generated during the simulation. The system is initially stagnant, with no gas present in the well. During the simulation, the temperature increases gradually, eventually leading to boiling of the water liquid and generating gas. The gas is affected by the temperature of the system and will begin to expand, subsequently forcing the system to start flowing towards the outlet of the pipe. The second simulation is an experimental case using Rohsenows’ correlation as a mass transfer equation in a horizontal pipe. Other correlations had to be added in order to solve Rohsenows’ correlation: the evaporation energy of water and the interfacial tension between liquid water and water vapor. This makes the mass transfer more complex, and helps to test the ability of the AUSMV scheme to handle mass transfer even further. To make the simulation more realistic, a boiling point criterion has been introduced to the model. This criterion makes the mass transfer equation dependent on both pressure and temperature. When the pressure in the pipe increases due to gas generation, the boiling point temperature of the liquid also increases. The third simulation is performed in a vertical well with a fixed numerical value for the mass transfer. For this simulation the objective was to see how the AUSMV performed in a vertical case with the inclusion of mass transfer. The simulation was modified to force the liquid to vaporize in the upper sections of the well so that the bottomhole pressure is reduced. When the bottomhole pressure drops it can cause a secondary kick to occur. The fourth simulation is a comparison of first and second order accuracy method and also comparison of different grid adjustments. The objective of this simulation was to see how the end result changes when using different accuracy methods or by refining the grids in the simulation.
Master's thesis in Petroleum engineering