Transient hydraulics and multiphase kick tolerance study to improve design of narrow margin well

Abstract

Hydraulic and well control studies are the essential parts of well construction planning, especially for drilling of complex and challenging wells with narrow drilling margins. However, the complete applications of dynamic hydraulic analysis and multiphase kick tolerance studies in well design are scanty, which result in ineffective mud pressure management and extra cost spent on unnecessary casing strings, due to excessive emphasis on previous practices (steady-state model) with liberal sprinkling of safety factors. This research project was set out clearly to improve the well design for narrow margin field, in terms of hydraulics and well control. A deductive quantitative method constitutes major part of the research methodology, in which simulation of real case studies and interpretation were conducted. The dynamic hydraulics simulated equivalent circulating density (ECD) was compared with steadystate results in terms of accuracy and extensiveness in providing a good well design. In addition, the single bubble kick tolerance results which are commonly used by the industry in spreadsheet format were compared with the multiphase model results. Sensitivity studies were performed to understand the effect of each of the operational or well design parameters towards primary and secondary well control. As compared to steady-state hydraulics, transient model covers important parameters like pressure and temperature dependent fluid properties, thermophysical properties, detailed geometry description and operational effects, thus it is more representative to the operational ECD. Meanwhile, multiphase kick model is proven to be more effective for the evaluation of kick tolerance as it is able to provide the information of pressure development during a well control operation, from initial influx and shut-in until influx is circulated out of the well at the surface. This includes all phase transitions including dissolving of a gas kick in oil based mud and breakout of free gas when the gas contaminated mud reaches the bubble point at shallower depth in the well. The flow model is much more accurate and reliable than the over-conservative traditional single bubble theory.