Hydrate reservoirs have been categorized as Type I, II, and III; Type I with underlying free-gas, Type II with underlying freewater, and Type III that is sandwiched by impermeable formations. The most common type of hydrate reservoirs are probably Type III reservoirs, where there is no underlying mobile phase beneath the hydrate layer. Depressurization in Type III reservoirs is characterized by difficulty in reducing pressure over a large region because of limited available surface area for decomposition and low permeability in the hydrate. This is unlike to be the case in Type I and II reservoirs, where pressure could be reduced across a large surface area between the hydrate and the underlying free phase.
A 3D numerical model incorporating heat and fluid flow, along with kinetics of decomposition and (re)formation of hydrate and ice, is developed in this paper. Next, the solution behaviour of Type III hydrate reservoirs in response to application of the depressurization technique is studied, with the goal of understanding the interactions between fluid and heat flow and their effects on the decomposition region. This is achieved by exploring for similarity solutions in Type III reservoirs.
The results of this study indicate that the behaviour of Type III reservoirs is sometimes close to that of diffusion problems, suggesting that a similarity solution exists. This has also been shown to be the case in the literature. However, under some other conditions, it is shown that the solution to this problem is also identical to a travelling wave solution, which is another type of similarity solution often observed in diffusive-reactive problems that exhibit frontal behaviour and sharp gradients. Conditions leading to development of these two types of similarity solutions are identified.
The contribution of this work is in identifying the different solution regimes in Type III hydrate reservoirs. This could help to understand and simplify the modeling of governing mechanisms involved in the process of gas production from the most common type of hydrates.