Behavior of Depressurization in Type III Hydrate Reservoirs

Hydrate reservoirs have been categorized as Types I, II, and III: Type I has underlying free gas, Type II has underlying free water, and Type III is sandwiched by impermeable formations (i.e., there is no underlying mobile phase beneath the hydrate layer). The updip portion of the Mount Elbert prospect in Alaska is one example of a Type III hydrate reservoir. 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 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 behavior 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 1D similarity solutions in Type III reservoirs. (A similarity solution of a PDE is a solution that depends on one variable which itself is made up of the individual independent variables that the PDE depended on.) The results of this study indicate that the behavior 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, for the first time it is shown that the solution to this problem is also identical to a traveling-wave solution, which could offer another type of similarity solution often observed in diffusive/reactive problems that exhibit frontal behavior and sharp gradients. (The traveling-wave solution or convective similarity solution is a type of similarity solution in which the similarity variable is x - vt, with v being the constant characteristic speed. This type of solution exists for the problems in which the profiles of the dependent variables, such as pressure or saturation, advance in time in the form of traveling waves without changing shape and velocity.) 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 improved understanding could lead to simplifying the modeling of the nonlinear mechanisms involved in the process of gas production from hydrates.