海洋气水同产井井筒中天然气水合物非平衡生成风险预测

受海水低温环境的影响,气水同产井井筒中天然气水合物(以下简称水合物)非平衡生成问题突出,增加了井筒堵塞等安全事故发生的风险。为此,基于前人建立的甲烷水合物相平衡模型、水合物生成与分解动力学模型,建立了含水天然气采出过程中井筒温度和压力分布模型、海洋气水同产井水合物非平衡生成与分解理论模型;然后基于有限差分法进行数值模拟计算,形成了一套适用于海洋气水同产井生产过程中水合物非平衡生成风险预测方法(以下简称水合物生成风险预测方法);在此基础上,采用某陆上产气井LN-X的数据对该预测方法的可靠性进行了验证,进而研究了不同参数影响下海洋气水同产井井筒中水合物的非平衡生成与分解规律。研究结果表明:①随着日产气量增大或含水率升高,井筒中生成水合物的区域范围减小,同时水合物物质的量也减小,井筒越不易被堵塞,对海洋气水同产井的安全生产越有利;②井口油压越大,井筒中生成水合物的区域范围越大,同时水合物物质的量也越大,越容易堵塞井筒,对海洋气水同产井的安全生产越不利;③不同海面温度下水合物生成区域的范围一致,并且水合物分解区域的范围也一致,水合物物质的量仅在井口位置附近有差异,海面温度越高,水合物物质的量越低。结论认为,所形成的水合物生成风险预测方法可靠,可以为海上气藏的安全生产管理提供理论指导。

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.