Gas hydrates will dissociate to gas and water at pressures below the equilibrium pressure or temperatures above the equilibrium temperature. Moreover, some hydrate reservoirs have considerable amounts of mobile gas and water coexisting with the hydrates in the formation. This implies, fluid diffusivity in such reservoirs is predominantly multiphase and hence addressing multiphase flow in such reservoirs becomes very vital.

Conventionally, most well testing models address diffusivity in porous media by assuming a single/dominant flowing phase or addressing the fluid phases separately. If a huge discrepancy exists between the fluid saturations, the fluid with the highest saturation could denote the dominant flowing phase, depending on the fraction flow model, and the correction with the multiphase model becomes trivial, as also seen with most conventional gas reservoirs with low water saturation. However, if the saturations of the different phases do not vastly differ from one another, multiphase well testing models give a more accurate prediction of reservoir behavior. The definition of the multiphase dimensionless time becomes very essential in predicting the diffusivity of the fluids and for well test designs.

The multiphase flow models developed here are based on mass balance and volumetric material balance approaches. The total mobility is derived for both the mass balance and volumetric material balance models, further related to fraction flow models. The total mobility model developed by Perrine, used by most reservoir engineering calculations is not addressed in detail for this work, as this does not fulfill mass conservation considered in this paper. Due to the non-linearity of the diffusivity equation, the multiphase diffusivity model addressed here includes pseudo-pressure and pseudo-time integrals. The solutions to the diffusivity equations for multiphase flow are given for both approaches. The differences between both models and their limitations are addressed and highlighted with illustrative examples.

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