Numerical Study of the Effect of Thermal Stimulation on Gas Production from Marine Hydrate Reservoirs

Abstract

The temperature of hydrate reservoirs will decrease in a gas production process because gas hydrate dissociation is endothermic, leading to a decrease in the gas production rate as sensible heat is exhausted. Heat supply should be an effective way to alleviate the effect of temperature decrease on the production rate, by thermal stimulation, such as hot water injection into a hydrate reservoir. However, the feasibility of thermal stimulation combined with depressurization is controversial. Hence, to investigate the effect of this combined strategy, gas production performance is studied by numerical simulation based on a 2D Cartesian hydrate reservoir model with a two parallel horizontal well configuration. One of the two horizontal wells, at the top of the hydrate-bearing layers, is for gas production, and the other at the bottom of the hydrate-bearing layers is for hot water injection. The influence of the injection rate, temperature of hot water, and permeability of hydrate-bearing layers on gas production is investigated. When only the hot water injection is applied, the released gas, which originates from the dissociation of hydrates around the injection well, tends to be trapped and forms hydrates again. When the production strategy of hot water injection combined with depressurization is engaged, the hydrate dissociation shows a depressurization-dominant feature, and the production is at a relatively high rate, but it seems the hot water injection is still a negative factor in gas production. Because the hot water injection will replenish reservoir pressure and increase the water production rate, the feasibility of depressurization will be compromised. Sensitivity analyses reveal that the increased permeability of hydratebearing layers can enhance gas production almost proportionally. It is also found that increasing the temperature of the injected hot water only has a very limited effect on gas production.

Embedded Discrete Fracture Model for Investigating the Effect of Fractures on Gas Hydrate Production

“…Currently, a Tough+Hydrate simulator is widely employed to predict methane production in hydrate exploitation 15,16 as well as natural evolution of hydrate systems, 17,18 since the simulator has the advantages of high accuracy 19,20 and broad application range (applicable to marine and permafrost hydrates). 21,22 Several studies on fractured media in Tough ("Transport of Unsaturated Groundwater and Heat") simulators are summarized in Table 1. The current family of Tough simulators can handle fractured media by dual-continua models, such as the dual-permeability model and multiple interacting continua (MINC) model.…”

“…Compared with laboratory experiments and field measurements, numerical simulation offers several advantages, including low cost, fast results, repeatability, and the ability to be applied on a large scale. Currently, a Tough+Hydrate simulator is widely employed to predict methane production in hydrate exploitation , as well as natural evolution of hydrate systems, , since the simulator has the advantages of high accuracy , and broad application range (applicable to marine and permafrost hydrates). , …”