Risk prediction of non-equilibrium formation of natural gas hydrate in the wellbore of a marine gas/water-producing well

Influenced by the low temperature of seawater, non-equilibrium formation of natural gas hydrate (hereinafter referred to as hydrate) in the wellbore of a gas/water-producing well is prominent, which increases the risks of safety, such as wellbore blockage. In this paper, a wellbore temperature and pressure distribution model and a theoretical model of non-equilibrium hydrate formation and decomposition in marine gas/water-producing wells in the production process of water-bearing natural gas were established on the basis of the previously established methane hydrate phase equilibrium model and hydrate formation and decomposition dynamics model. Then, numerical simulation calculation was carried out on the basis of finite difference method, and thus a set of risk prediction method for the non-equilibrium hydrate formation (hereinafter referred to as hydrate formation risk prediction method) suitable for the production process of marine gas/water-producing well was developed. On this basis, the reliability of the prediction method was verified by using the data of one certain onshore gas production well LN-X, and then the non-equilibrium hydrate formation and decomposition laws in the wellbore of marine gas/water-producing wells under the influences of different parameters were studied. And the following research results were obtained. First, with the increase of daily gas production or water cut, both the hydrate formation area in the wellbore and the amount of hydrate substances decrease. As a result, wellbore blockage is less likely to happen, which is more favorable for the safe production of marine gas/water-producing wells. Second, the higher the wellhead tubing pressure is, the larger the hydrate formation area in the wellbore is and the greater the amount of hydrate substance is. As a result, the wellbore is blocked more easily, which is more unfavorable for the safe production of marine gas/water-producing wells. Third, even under different sea surface temperatures, the range of hydrate formation area is consistent, and so is the range of hydrate decomposition area. The amount of hydrate substance is only different near the wellhead. The higher the sea surface temperature, the lower the amount of hydrate substance. In conclusion, the proposed hydrate formation risk prediction method is reliable, and the research results can provide theoretical guidance for the safe production management of offshore gas reservoirs.