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The underbalanced drilling technique, which is also known as managed-pressure drilling, is playing an important role in oil and gas sector, as it reduces common conventional drilling problems such as minimal drilling rates and formation damage, differential sticking and lost circulation. Flow regime monitoring is one of the key topics in annular multiphase flow research, particularly for underbalanced drilling technique. Prediction of the prevailing flow regime in an annulus is of particular importance in the design and installation of underbalanced drilling facilities. Especially, for establishing a suitable pressure-drop model based on the characteristics of the active flow regime. The methods of flow regime prediction (or visualisation) in an annulus that are currently in use are very limited, this is evidently due to poor accuracy or they are simply not applicable to underbalanced drilling operation in practice. Therefore, this paper presents a monitoring method, in which Electrical Resistance Tomography (ERT) is used to rapidly image the prevailing flow regime in an annulus with a metallic inner pipe. Experiments were carried out using an air–water flow loop with a test section 50 mm diameter flow pipe. The two-phase air–water flow regimes are visualised in the upward vertical annulus with a radius ratio ( r / R ) 0.4. This paper highlights the visualisation results of only three flow regimes, namely bubble flow, transitional bubble-slug flow and slug flow. The flow regimes are visualised through axial images stacked from 50 mm diameter-pixels of 2D tomograms reconstructed with the Conjugate Gradient Method (SCG). Gas volume fraction profiles within the annular flow channel are also illustrated. The profiles are extracted using the Modified Sensitivity coefficient Back-Projection (MSBP) method with a sensitivity matrix generated from a realstic phantom in the finite element method software. The results are compared with visual observations (e.g. photographs) of the active flow regime at the time of ERT measurements. [ABSTRACT FROM AUTHOR]

• The in-situ heat generation plugging removal agents for gas hydrate is presented. • A new model for in-situ heat generation plugging removal agents is proposed. • Effect of in-situ heat generation system on plugging removal process is identified. • The theoretical model is in good agreement with the experimental results. • The heating temperature, alcohol and electrolyte of in-situ heat generation system are evaluated. Under the condition of low temperature and high pressure, natural gas will form solid gas hydrates in oil and gas wellbore and pipeline, resulting in blockage, pressure increase, production reduction, and even shutdown. Methods such as heat preservation, heating, and injection of inhibitors are often used in engineering to avoid the formation of hydrates. The latter, injection of inhibitors, is the most widely used means of hydrate prevention and control at present. A special inhibitor, namely an in-situ heat generation plugging removal agent, has the advantages of both heating and chemical plugging removal and is a kind of hydrate inhibitor with great potential. In this study, a new mathematical model is developed based on hydrate phase equilibrium thermodynamics and decomposition kinetics, which is used to solve the heat and mass transfer problems in the plugging removal process of the gas hydrate by using in-situ heat generation plugging removal agents. Numerical simulation is used to solve the mathematical model, and the plugging removal process and parameter variation of pure in-situ heat generation system (high/medium/low temperature), electrolyte in-situ heat generation system (containing Na+/Mg2+), and alcohol in-situ heat generation system (containing MeOH/DEG) are studied respectively. Based on the experimental results, the quantitative relationship between the plugging removal time, the dosage, and the concentration of the plugging removal agent is calculated and analyzed. Our proposed model is verified through experimental study and the calculated values of the mathematical model are in good agreement with the experimental values under different working conditions. It is concluded that the rapid removal of hydrates and the prevention of secondary formation of hydrates can be achieved by configuring the high-temperature in-situ heat generation plugging removal agents, and the efficiency of plugging removal can be enhanced by adding appropriate concentration of electrolyte and alcohol without changing the heating property of the original plugging removal agent. This study can provide theoretical guidance for the safety of oil and gas flow and the efficient development of hydrate. [ABSTRACT FROM AUTHOR]