Study On Multi-phase Flow In Wellbore And Well Control In Deep Water Drilling

Well control is one of the most important problems in deepwater drilling. The theory of multi-phase flow in wellbore is an important part of the well control theorty. As the temperature field is complicate and hydrates form easily, the existent multi-phase flow models can not be used satisfactorily in well control calculation in deepwater drilling operation. So the study on multi-phase flow in wellbore in deep water drilling is very important. The temperature distribution with depth of Nanhai is obtained by regression analysis.And the temperature distribution formulas in wellbore during circulation or non-circulation terms are established based on the thermodynamics theory. Calculation results indicate that the insulating layer should be undamaged and large delivery capacity should be used for maintaining the temperature under circulation condition. When the pumps stop, the temperature in wellbore will be closing to the extraneous temperature with time. The gas production function of hydrate dissolution with time is obtained under the conditions of different temperature and pressure by experiment and data analysis. And the model of hydrate dissolution rate is established.Considered the effect of gas/oil and gas/hydrate phase change, cuttings, and formation fluids et al., multi-phase flow models in the wellbore, riser, and choke line are established under the condition of drilling, shut in and well killing process. The models are comprised of continuity equations, momentum equations and energy equations. And the models are solved by numerical method. Considered the influence of the roughness, the friction factors and frictional pressure drop equations in annulus are obtained under the conditions of gas-liquid two-phase flow. The equations are suitable for both bubble flow and slug flow. The methods for calculating frictional pressure of wellbore, riser, and choke line under the conditions of various flow patterns are also presented. The results of the models can simulate the whole process of well control from kick to well killing. And the parameters can be obtained, such as temperature, pressure in wellbore, volume fraction, flow rate, formation production, stand pipe pressure, choke line pressure and frictional pressure drop et al.On basis of the characters of well control in deepwater, the well control software is developed, and the software is comprised of four modules, which are kick simulation module, kill simulation module, hydrate formation forecasting module and hydrate formation region forecasting module.The well control parameters in deepwater are calculated with the software and the variation of the parameters with the continuance time of kick are analyzed. The influence of many factors on the well control parameters are also studied, such as permeability, initial pressure difference between the wellbore bottom and formation, size and length of the choke line, depth of the water, and the formation or decomposition of the hydrate. The results indicate that the longer continuance kick time is and the larger permeability and initial pressure difference are, the larger pit gain is and the quiker bottom pressure drops, and the rapider adjustment of the choke valve is and the difficulter well control is.Friction loss is proportion to the length of the choke line, and increase rapidly when the diameter of the choke line decreases. Large diameter choke line should be used at specified water depth to decrease the friction loss. If the friction loss of the choke line is too large to using the conventional kill method, dynamic kill method and additional flow rate well control method should be used. The flow pattens in wellbore and choke line are influenced by the formation and the decomposition of the hydrate. The volume faction of gas is influenced effectively by the decomposition of the hydrate when there are large amounts of hydrate in BOP or lines. And the pressure distribution in wellbore and the adjustment of the choke valve are also affected subsequently.