The accurate forecast of wellbore temperature and pressure during intervention operations is critical to the long-term exploitation of deep-water oil and gas resources. A wellbore temperature prediction model was constructed in the seawater and formation sections based on Hasan and Kerber’s gas-liquid two-phase flow heat transfer model and integrated with the features of intervention operations. Simultaneously, the heat exchange produced by the change in fluid flow rate caused by the tool placement was evaluated. The wellbore pressure prediction model is developed in the intervention tool and intervention operation sections, and the friction gradient is modified by introducing the intervention operation.The established temperature and pressure model is coupled and solved iteratively, and the disturbance law of the wellbore temperature and pressure field induced by the intervention operation is investigated. The results show that the pressure in the wellbore gradually decreases when the tool is removed; the temperature at the mudline of the wellbore with high gas production is higher, and the wellhead pressure is low; the tool size is weakly sensitive to the wellbore; and the wellbore temperature and pressure are weakly sensitive to the running speed. The aforesaid findings have some guiding relevance for the design of the intervention operation plan, as well as the prediction and prevention of hydrate in the wellbore during deep-water gas well cleaning and blowout.

In the development of deepwater high temperature and pressure gas wells, frequent intervention operations are required. Wellbores near the seabed have the risk of producing massive hydrates and plugging the wellbore. Based on the non-steady state model, a temperature-pressure coupling model of deepwater gas well intervention was established, and a physical simulation experiment of hydrate formation under deepwater gas well intervention was carried out by using the self-developed hydrate deposition simulation reaction device. Intervention operation of deepwater high temperature and high pressure gas wells. With reference to the test data of a real well in the South China Sea, the hydrate formation rate was analyzed under the influence of different production rates, running speed and tool size, temperature and pressure distribution of deepwater gas wells. The operation rules of deepwater gas well intervention and the formation rules of hydrates are obtained. Simulation prediction results can be used to predict the timing and location of hydrate plugging under intervention operations.

Hydrate control research is of great significance to the efficient development of deepwater gas wells. A deepwater gas well intervention wellbore hydrate suppression experimental device was built to simulate the field intervention operation and monitor the hydrate generation in the kettle in real time. The physical simulation experiments were conducted for the injection concentration, injection volume, and injection rate of the glycol inhibitor for different working conditions of the intervention operation. The results show that the hydrate inhibition effect is good when the injection concentration of glycol solution is 40%, the hydrate plugging in the wellbore is slow when the injection volume is 20 mL, and the hydrate inhibition is effective when the injection rate is 4 mL/min. The injection of glycol inhibitor can effectively delay the time required for wellbore hydrate plugging and ensure the intervention operation. The above research results provide ideas for in-wellbore hydrate control in deepwater gas well intervention operations in the field.

In order to explore the characteristics of the hydrate decomposition behavior at the pore scale, this study carries out a pore-scale experimental study of methane hydrate decomposition based on the high-pressure visual model under the etched glass. A mathematical model is also constructed to analyze the behavioral characteristic of the self-preservation effect and memory effect during the hydrate decomposition period. This study draws the following conclusions: (1) The self-preservation effect and memory effect exist during the methane hydrate decomposition lead by depressurization, which generally inhibits the hydrate decomposition process. (2) The hydrate self-preservation effect is a transition of the surface water film’s phase state to inhibit the methane hydrate decomposition, in which the liquid phase transforms into a metastable “quasiliquid film”. (3) The multiple syntheses induced by the hydrate memory effect are a periodic attenuation process. The times of synthesis are a critical factor affecting the hydrate gas production and decomposition rate. (4) The self-preservation and memory effects during the hydrate decomposition period are associated with each other. The two are correlated at some degree with playing a dominant role alternately at different stages. The self-preservation effect is an abnormal behavior of the hydrate, which refers to the hydrate’s transition from the solid phase to the gas-liquid mixed phase. The memory effect is another abnormal behavior, which refers to the transition from the gas-liquid mixed phase to the solid phase. The sustaining pressure drop is the key reason of the disappearance of the two effects. This research was aimed at providing a theoretical basis for the exploitation and optimization of marine natural gas hydrates.

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