Abstract
In recent years, the exploitation and utilization of offshore oil and gas resources have attracted more attention. In offshore gas reservoir production, wellbore temperature and pressure change continuously when water-bearing natural gas flows upward. The wellbore temperature is also affected by the low-temperature sea water. The combination of temperatures and pressures controlled by the upward flow, and cooling from the surrounding seawater frequently leads to the conditions of temperature and pressure for hydrate formation. This can lead to pipeline blockage and other safety accidents. In this study, we utilize mathematical models of hydrate phase equilibrium, wellbore temperature, wellbore pressure to study hydrate formation and decomposition in offshore gas reservoir production. Numerical solution algorithms are developed and numerical solutions are validated. The sensitivity influence of different parameters on the regions and regularities of hydrate formation and decomposition in wellbores are obtained through numerical simulations. It is found that increased daily gas production, water content, or geothermal gradient in offshore gas reservoir production pipelines results in less hydrate formation in the wellbores. Accordingly, the risk of wellbore blockage decreases and production safety is maintained. Decreased tubing head pressure or seawater depth results in similar effects. The result of this study establishes a set of prediction methods for hydrate formation and decomposition that can be used in the development of guidelines for safe construction design.
Highlights
With the rapid development of the global economy, the discrepancy between oil and gas supply and demand has become more prominent in most countries
Compared with wellbore pressure Pm in the same condition, if Peq is smaller than Pm, the calculated position is in the formation region of hydrate; if Peq is equal to Pm, the calculated position is the critical formation or critical decomposition position of hydrate; if Peq is larger than Pm, the calculated position is in the non-formation or decomposition region of hydrate
In the production of an offshore gas reservoir, based on the phase equilibrium model of natural gas hydrate combined with the wellbore temperature and pressure, it can be judged whether the hydrate forms or decomposes in the pipeline
Summary
With the rapid development of the global economy, the discrepancy between oil and gas supply and demand has become more prominent in most countries. Henry et al [11] established a theoretical model of phase equilibrium for gas hydrate in porous media based on the Van der Waals and Plateeuw model, considering the single-pore-size capillary effect. In 1987, Kim et al [33] used a pressure-temperature-volume (PVT) cell to study hydrate dissociation under various stirring conditions They correlated the decomposition rate of natural gas hydrate to temperature, pressure, and particle surface area. Few studies been carried the regions and regularities of hydrate and decomposition under have the conditions thatout theonwellbore temperature and pressure varyformation with the and under the conditions that the wellbore temperature and pressure vary with the well welldecomposition depth in the production of water-bearing natural gas.
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