| With the increasing energy demand and the increasing difficulty of exploration on land and in shallow water areas, deep-water oil & gas exploration and exploitation will surely become an important part in our petroleum industry. Affected by deep water and complex ocean environment, the major risk of deep-water drilling is the well control risk. Once a blowout happens in a deep-water well, the damages to personnel on the platform and ocean environment are irreparable. To ensure the drilling safety, it's necessary to study the flow rules of multiphase flow in deep-water wellbores.Based on the heat and thermodynamics theory, the wellbore circulation temperature field prediction model was established. Then, taking a deep-water well as an example, the change of wellbore temperature field with time and well depth was calculated. The solubility calculation model was established which could calculate the gas solubility in both water-based drilling fluids and oil-based drilling fluids. Based on the solubility calculation model, the gas solubility under different pressure and temperature was calculated. Based on the principle of mass conservation and momentum conservation, the continuity equations and the momentum equation, taking the gas solubility into account was established. Combing the basic control equations with the auxiliary equations, initial condition and boundary condition, the multiphase flow model for deep-water wellbores was finally established. The model was solved by finite-difference method and the forefront of multiphase flow was being tracked during the whole solution process. The change rules of gas solubility, air phase volume fraction, mixed phase velocity, pit gain, backflow volume and bottom-hole pressure in both oil-based drilling fluid and water-based drilling fluid during the gas kick were simulated and comparatively analyzed. The change rules of bottom-hole pressure, pit gain and backflow volume under different flow rates, kick rates and drilling fluid oil-water ratios for oil-based drilling fluid were simulated. Based on the simulation results, the effect of kick detection by monitoring pit gain and backflow volume was evaluated and the sensitivity analysis for bottom-hole pressure was also carried out. The change multipoint pressures in the wellbore with time before the forefront of multiphase flow reach the riser was simulated. Then, based on the simulation results, the effect of kick detection by measuring multipoint pressures in the wellbore in real time was evaluated. According to the theoretical model, the software for deep-water gas kick simulation was written.The simulation results showed that the gas kick in oil-based drilling fluid was concealed and dramatic. Before the gas evolved from the oil-based drilling fluid, the bottom-hole pressure, pit gain, backflow volume did not change significantly. When the gas began to evolve from the oil-based drilling fluid, the fore mentioned parameters changed quickly. To water-based drilling fluid, the fore mentioned parameters would change significantly from the beginning of the gas entering the wellbore. Small flow rates and low oil-water ratios would do benefit to detect the kick. Once a kick was detected, large flow rates were recommended to discharge the gas quickly and improve the control on the bottom-hole pressure. The traditional kick detection methods of monitoring the pit gain and backflow volume had hysteresis, so the kick alarm should be decreased. Detecting a kick by measuring multipoint pressures in the wellbore in real time was an effective method of early kick detection, but this method relied on accurate annular pressure prediction and real-time and accurate pressure measurements.The multiphase flow model established in this paper could be used to predict flow rules of multiphase flow in deep-water wellbores when a kick happened with both oil-based which provided theoretical support for wellbore pressure control and the choices of early kick detection methods and its kick alarms. |
PDF Full Text Request