Effect of geological layers on hydrate dissociation in natural gas hydrate reservoirs

Abstract

Naturally occurring reservoirs of methane gas hydrate have been found in arctic and marine environments around the world. They are an attractive source of energy as natural gas can be produced from these hydrate deposits by depressurization or thermal stimulation. Reservoir simulations are used to find the optimum production strategies for methane gas hydrate reservoirs. Most of these simulation are carried out for homogeneous hydrate reservoirs in absence of substantial well data. These gas hydrate reservoirs are inherently heterogeneous because of the geological settings of the hydrate bearing sediments. Majority of the heterogeneity comes from the depositional layering at different geological time scales. Examples are Mount Elbert, block 818 in Gulf of Mexico, Walker Ridge 313 Site. The effects of geological layering on gas production and hydrate dissociation fronts in the reservoir are still unknown. In the present work, a 3-dimensional, multiphase, multi-component, thermal simulator is used to study layered gas hydrate reservoir, underlain by an aquifer, with cross-flow between the layers. Gas production from the reservoir and gas hydrate dissociation fronts in the reservoir are found for different aquifer permeability, different layering and well completions. In layered confined reservoirs, recovery is found to be dependent on (a) the total volume of the hydrate present in the reservoir which governs the enthalpy requirement for dissociation, (b) depressurization potential of the reservoir which depends on the effective permeability and (c) the enthalpy available for dissociation which depends on the reservoir enthalpy and heat transfer from over and under-burden. The layering suggests the positions and progress of the dissociation fronts and affects the gas recovery depending on the aquifer permeability. This effect is seen because of the pressure variation in different layers and enthalpy available for hydrate dissociation. In unconfined reservoirs the effect of layering on dissociation front positions is found to be insignificant as the pressure variation in the layers is insignificant because of the unconfined aquifer. A weakly un-confined aquifer also renders the depressurization ineffective.