Numerical Investigation on Gas Hydrate Production by Depressurization in Hydrate-Bearing Reservoir

Abstract

Gas hydrates are anticipated to be a promising energy source with global distribution underneath the oceans and in permafrost regions. Hydrate dissociation referring to the phase transition of the solid gas hydrate into gas and liquid water, can occur due to altered environmental conditions like increase in temperature, decrease in pressure, or the injection of hydrate inhibitors. Numerical modeling work suggested that energy efficiency of the thermal simulation and inhibitor injection to dissociate gas hydrate in geological formations are quite low, and possibly negative. Depressurization is expected to be effective in many situations; however, the hydrate bearing sediments should be well enough confined by low permeability strata for the depressurization to be effective. In current study, numerical analyses were performed using the open-source reservoir simulator HYDRATERESSIM (HRS) to investigate methane gas hydrate dissociation behavior and production by depressurization, where the system pressure is lowered below the hydrate three-phase (gas-MH-water) stability pressure. Two-dimensional (2D) axisymmetric radial coordinates are used for the numerical simulations with local grid refinement. This paper presents the approach and results of numerical investigation conducted for a hypothetical hydrate-bearing reservoir in an offshore deep water site. A comprehensive parametric sensitivity study was undertaken to assess the effects of (1) various reservoir properties including sediment permeability (absolute permeability and relative permeability function), hydrate distribution, pore compressibility, porosity formation heterogeneity and thermal conductivity; and (2) operational condition on hydrate dissociation and production. Gas productivity and advance of hydrate dissociation front with time after the commencement of depressurization are discussed. This numerical study highlights the need and importance to obtain an accurate evaluation of reservoir properties from different data source, including seismic data, logging while drilling (LWD) data, core samples, field and laboratory testing data and drilling information, on gas hydrate production prediction. It can provide useful insights on the planning and design of a field production well test.