Experimental Study on Fluid Production from Methane Hydrate Sediments under the Marine Triaxial Condition

Abstract

Methane hydrate (MH) is potential new energy attracting attention from the scientific and industrial communities. In situ depressurization is considered a high energy efficient method. However, the study of the evolution of fluid production and overburden pressure under an in situ environment was significant to exploit CH4 from methane-hydrate-bearing sediments (MHBS) safely and efficiently. Herein, we investigated gas and water production profiles and evolution of triaxial pressure in MH dissociation. MH samples were synthesized under an aqueous-rich NaCl solution (XNaCl = 3.0 wt %) and triaxial pressure conditions that mimic the marine environment. The results show that the presence of NaCl increased the MH formation time and had no effect on the final hydrate saturation (SH) and methane conversion (XCH4) in the triaxial MH experimental system. Besides, the existence of NaCl posed a slightly positive impact on methane recovery (RG) and had a negative effect on water recovery (RW) in MH dissociation. Interestingly, bottom-hole pressure (BHP) presents a distinct four-stage in situ depressurization pattern, including a sharp decrease stage, a fluctuating stage, a stepwise stage, and a stable stage at last. Most of the gas production was recovered during the fluctuating stage and stepwise stage of MH dissociation. In addition, a decreasing BHP posed a minor impact on gas production and increased water production, reaching a greater deviation value of RW (ΔRW = 7.32% at BHP = 1.0 MPa) between deionized water and NaCl system. The confining pressure presents a similar trend to the BHP under the MH formation and dissociation, while the axial pressure shows a stepwise decreasing trend during the MH dissociation under triaxial condition.