Heat transfer study in the exploitation of argillaceous-silt natural gas hydrate reservoirs

The paper investigates the heat transfer in the exploitation of offshore argillaceous-silt natural gas hydrate reservoirs considering sand control. The temperature prediction model is established in this investigation. The coupled pressure-temperature model of the flowing fluid inside the wellbore is also presented. The sand control for argillaceous-silt natural gas hydrate reservoirs mainly brings two effects into the temperature and pressure profiles simulation, including the moderate sand control and the Joule-Thompson effect. The moderate sand control is reflected by the volume and particle size of produced sand into the wellbore in the mathematical derivation. The Joule-Thompson is considered in the model due to the small diameter of sand control screens. The experiment is designed to measure the pressure drop caused by various mesh diameters of sand control screens. Subsequently, the regression relationship between the pressure drop and the mesh diameter is concluded to calculate the temperature change at the sand control screen. The simulation of heat transfer helps to analyze secondary hydrate formation risk in the wellbore. The proposed model identifies the high-risk location of the secondary hydrate formation. The sand control effect and well geometry effect are discussed quantitatively. The strict sand control in the argillaceous-silt natural gas hydrate (NGH) sediment exploitation reduces the risk of secondary hydrate formation significantly; however, it also has a negative effect on natural gas production. To balance the sand control and the prevention of secondary hydrate formation risk, it is suggested to install the heating electrode in the high-risk location to ensure flow safety with acceptable sand control precision. The heat provided by heat electrodes is calculated by the model presented in this paper. The model can be used to determine the proper arrangement of heat electrodes. The paper also describes the secondary hydrate formation by adjusting the well geometry. The formation depth of the NGH reservoir and the sea interval should be considered in the risk analysis of the secondary hydrate formation. Additionally, the effects of wall thickness and wall thermal properties are evaluated and discussed. It is recommended to optimize the riser thickness with a small thermal conductivity value to minimize the secondary hydrate formation risk, which is more efficient compared to adjusting the cement sheath.