Combining DWS Depressurization Method and Heat Injection to Improve the Hydrate Recovery

摘要

Natural gas hydrate (NGH) is a new kind of clean resource with a large reserve and wide distribution, which is considered as an important alternative energy. While extracting natural gas hydrate using the method of depressurization, the temperature will decrease for the absorption of heat in the process of hydrate decomposition, which may cause the formation of ice or the re-formation of hydrate and further cause the reduction of natural gas recovery; meantime, the decomposition of natural gas hydrate will cause the decrease of formation pressure, as a result, water coning may occur, which will also cause the reduction of natural gas recovery. The DWS (downhole water sink) method has shown that this technology could increase oil production and control water coning when exploiting oil. This paper adopted this technology and made some improvements to fit in the exploitation of natural gas hydrate. The objective of this paper is to determine the improvement degree of natural gas recovery and whether it can effectively control water coning using the method of combining DWS depressurization method and heat injection method. In this paper, the mathematical model of natural gas hydrate exploitation is built first, then numerical simulation is used to compare three natural gas hydrate production methods - common depressurization method, DWS depressurization exploitation method and the method of combining DWS depressurization method and heat injection. The simulation results confirm that the combined method has better recovery by showing a big increase of recovery factor comparing to the other methods. The results also give a large reduction of water cut. The simulation experiments clearly show that the main advantage of using the method of combining DWS depressurization method and heat injection method is that it is easy to control the recovery process and water coning. For this method, production process could be optimized for maximum recovery and minimum water production by seeking a best combination of gas and water production rates.