Prediction And Study Of Bottom Hole Pressure In Deep Water Drilling And Shut-in Process

With the world's growing demand for energy, the exploration degree of oil and gas in onshore and offshore shallow water is increasing steadily, and deepwater have become an important alternative field in global hydrocarbon resources. Under the influence of marine environment, well control turns to be the main risk in deepwater drilling. In order to improve the safety of deepwater well control and ensure the safety of wellbore while shut-in, the changes of wellbore pressure need to describe more realistic. Therefore, research in bottom hole pressure is of great significance in deepwater drilling while shut-in.This paper presents annular bubble's force analysis during its expansion and detachment. Mathematical models of bubble volume have been built. The influences of pore throat diameter, bottom hole pressure difference, drilling mud density and surface tension on bubbles generated size have been calculated respectively. On the basis of the thermodynamics, heat exchange mechanism of fluid between the formation and the sea has been investigated. According to the first law of thermodynamics, wellbore static temperature field has been established and solved by analytical method. Wellbore temperature distribution at an arbitrary moment has been analyzed. Combining equation of state of real gas with wellbore static temperature field, governing equations for the volume variations of bubble rising in the annulus has been established. The expansion rule of gas rising along the wellbore has been studied under shut-in conditions. In the light of continuity equation and governing equations for the volume variations of bubble, a wellbore afterflow model has been established with the consideration of compressibility of gas and drilling fluid under shut-in conditions. Taking account of the gas expansion, compressibility of gas and drilling fluid and the filtration of drilling fluid, a gas slippage model has been constructed. A calculating model of bottom hole pressure has been built considering wellbore afterflow and gas slippage comprehensively. The influences of overflow volume, formation permeability, gas slippage velocity and cake permeability on bottom hole pressure have been analyzed built on top of theoretical research. The gas-liquid-solid water-hammer wave speed has been derived from the momentum equation and continuity equation with consideration of gas content. The influences of gas content, solid content and the ratio between casing-wall thickness and inside diameter of casing on water-hammer wave speed have been dissected. The annular space fundamental equation of water hammer has been deduced according to Newton second law. The annular space continuity equation of water hammer has been established through the law of mass conservation. The law of water hammer pressure variation with time has been calculated in the case of different valve closing time and different gas content.The simulation results show that the initial generation volume of bubble increases with the increase of the pore throat diameter, hole pressure difference and surface tension and the decrease of drilling mud density. In stratigraphic section, the gas expansion is slow, the gas into the throttle pipe is rapidly expanding, the closer to the wellhead, the greater the volume expansion. With the increase of rest time, the fluid temperature change trend between the throttle pipe and the stratigraphic is opposite. During the shut-in, time of recovery of bottom hole pressure to formation pressure increases with an increase in overflow volume and an decrease in formation permeability and gas slippage velocity. The effect of cake permeability on the bottom hole pressure is greater than that from the bottom hole pressure, which is more beneficial to accurately describe the change of bottom hole pressure. Gas content is an important factor in the impact of water hammer wave speed, a small amount of gas content will greatly reduce the water hammer wave speed, while the gas content exceeds a critical value, the water hammer wave speed to have little changes with the increase of gas content. The maximum water hammer pressure at the blowout preventer is greater than that of the casing shoe. The water hammer pressure increases with the increase of valve closing time and the decrease of gas content.