Numerical simulation of gas production behavior of class Ⅰ, class Ⅱ and class Ⅲ hydrate reservoirs for different well locations

The exploitation of natural gas hydrate, an unconventional natural gas, has drawn attention on a global scale due to increasing imbalance between energy supply and demand. Horizontal wells are considered to have a better production performance, while vertical wells have been mostly employed in previous field tests and laboratory research. Therefore, based on the representative properties of three kinds of hydrate reservoirs, a hydrate decomposition model is built to study the gas generation behaviors of various hydrate reservoirs with different horizontal well-spacing strategies in this work. The results show that for Class I hydrate reservoirs, placing the production well near the free gas layer can improve gas generation by 47%, and GWR (gas-water ratio) to 346.0 and maintain higher temperature in hydrate reservoir than that with placing the well in the hydrate reservoir. For Class Ⅱ hydrate reservoirs, placing the producing well away from the free water layer can cut down water production by 92%, while gas production could be also elevated, causing GWR increasing to 175.8. In Class Ⅲ reservoirs, the reduction of 30% in water production, and increment in GWR from 81.5 to 118.0 can be achieved by placing the well on the top of the hydrate layer, rather than that on the bottom. In addition, the presence of free gas layer, free water layer can promote the pressure-drop interface spread, provide sensible heat, and then accelerate hydrate decomposition further.