Simulation of microwave stimulation for the production of gas from methane hydrate sediment

Abstract

Natural gas hydrates dissociate via an endothermic process. One of the key requirements for any production technique is to supply the heat necessary for this dissociation. In this study, first, a microwave stimulation model for the production of gas from methane hydrate sediment is developed, which includes mass transport, energy conversion and conservation, and intrinsic kinetic reactions as the governing equations. In addition, the theoretical mixing rule of Lichtenecker and Rother is introduced for calculating the average dielectric data of the sediment containing methane hydrates, which affects the penetration of microwaves into the sediment. Next, simulations are performed for investigating gas production, as well as effects of initial water saturation, initial hydrate saturation, and sediment thermal properties induced by microwave stimulation. Moreover, the energy efficiency ratio is employed in the simulation. The simulation results show that microwave stimulation provides timely energy conversion sufficient for promoting the dissociation of hydrates, with rapid, continuous gas production. Temperature gradients caused by the decrease of the microwave penetration depth appear in the reservoir, leading to a rapid dissociation rate in the upper part of the sediment. The energy efficiency ratio for all simulations ranges between 3.752 and 6.452. Hydrate saturation and the specific heat capacity of porous media are two factors that significantly affect energy efficiency. High hydrate saturation contributes to a rapid gas generation rate and a long rapid gas generation lasting time, leading to a high energy efficiency. A low specific heat capacity of porous media can decrease the loss of heat to the sediment, increase the gas generation rate, and improve energy efficiency. Furthermore, high initial water saturation can cause a rapid decrease in the microwave penetration depth and lead to large temperature gradients in the sediment. The thermal conductivity of porous media mainly affects gas generation in the latter period of hydrate dissociation. Macroscopic heat conduction in the sediment decreases the temperature gradients in the reservoir and promotes homogeneous hydrate dissociation.