Hydraulic and thermal controls on gas production from methane hydrate reservoirs

Abstract

This study explores the controls of fluid flow and heat transport on gas production from methane hydrate reservoirs under depressurization. We find that effective water permeability of hydrate-bearing sediments plays a primary role in determining the reservoir response and gas production. Effective water permeability determines the velocity of water flow and the speed of pressure propagation. These propagation speeds give rise to two distinct behaviors of hydrate dissociation, which in turn lead to two distinct regimes of gas production. In reservoirs with high effective water permeability (> ${10}^{-16}$ m² or >10⁻¹ mD), the low-pressure wave propagates rapidly, creating a wide, laterally expansive zone of hydrate dissociation. This large dissociation zone leads to a broad region where free methane gas is released, supporting a high-rate gas production at the wellbore. The hydrate-dissociation zone continuously expands with time until it reaches the lateral edge of the reservoir, enhancing both the gas production rate and the cumulative gas-to-water production ratio. Increased heat conduction further accelerates the gas production rate. Conversely, in reservoirs with low effective water permeability (< ${10}^{-18}$ m² or <10⁻³ mD), pressure propagation is more restricted, which limits the extent of hydrate dissociation to a narrow interface. This confined dissociation zone results in a significantly lower gas production rate, with minimal increase over time and a declining gas-to-water production ratio. These conclusions are based on a multiphase flow, multicomponent reactive transport numerical model applied to hydrate reservoirs with initial effective permeabilities spanning five orders of magnitude. Our findings highlight the importance of accurately characterizing the effective permeability of hydrate-bearing sediments, particularly in the context of methane hydrate presence and sediment compaction, to assess the viability of gas hydrates as an energy resource.