Well Control Aspects of Riserless Drilling
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This master thesis has investigated feasibility of effective well control in riserless drilling. An attempt was undertaken to design a realistic model of riserless well using advanced multiphase simulation tool and evaluate well control aspects as loss of primary well control, kick detection and well killing operations. One of the most significant findings in this study is that the well control procedures, which were specially developed for riserless and dual gradient application, do not work as expected. Modified Driller’s method and detection of shut-in drill pipe pressure after well is shut with HCV did not show desired results during simulations and must be revised. With unknown formation pressure it is impossible to circulate kick out of the well and pump the kill mud into the wellbore safely. And both Driller’s and Wait and Weight methods are not applicable. Therefore, well control operation in riserless drilling cannot be executed safely and in controlled manner using current technologies and procedures. Kick detection in riserless system is similar to conventional drilling. Almost all conventional kick detection methods are applicable in riserless drilling and allow detecting the kick effectively. The speed of the subsea pump also proved to be an effective method, which can be used in addition to conventional ones. However, it is not possible to perform a flow check and fluid fill-ups on trips to verify that kick is taken. It was also shown that the new equipment with complex control logic–hydraulic control valve is a very critical element of riserless drilling system. It is installed at the bottom of the drill string, which creates restriction for its maintenance. Effectiveness of the well control relies solely on HCV in the situation when the well must be shut-in. Therefore, the fact that HCV is the single point of failure is considerable disadvantage of the riserless system. The results of this study indicate that new well control procedures for riserless drilling require a high level of understanding of wellbore fluids influence on conditions in the well. Therefore, further development and utilization of model that was established in OLGA is important. It is also recommended that further research be undertaken in the following areas: Back-up emergency procedures for failure of the hydraulic control valve, subsea pump or both of them. Alternative well control procedures for the situation when the bit is not on the bottom. Research and development of the subsea pump so that new system would have high degree of operational flexibility to act not only as a pump but also as an analog of subsea choke during drilling, tripping and well control operations. Prior to development of the technical solutions and procedures it is highly recommended to enhance accuracy of the model and simulation software. OLGA multiphase phase flow simulator is intensively validated against field data for subsea multiphase flow lines, production riser systems and gas condensate pipelines. For the drilling systems, the extensive validation campaign against the field data is required to increase the confidence in the simulations results. This could be achieved through the cooperation between leading oil companies via JIPs. In addition operator training simulators could potentially help with such a complex and dynamic phenomena. The verified (for long vertical risers/pipes, with viscous mud) multiphase flow simulator could be integrated to the drilling simulator. This online system could potentially predict the behavior of the flow during the gas kick in the system during the drilling operation and will provide more time to react (as analogy to the OLGA/Hysys or Leda/K-Spice online integrated systems for the process/transport systems).