Natural gas hydrate dissolution kinetics in sandy sediments: Implications for massive water production in field tests

Abstract

The tremendous natural gas hydrate (NGH) reserve in globe makes it a promising future energy. The field test production indicates that massive water production may occur in the depressurization of sandy NGH reservoirs. The dissolution of NGH may play a significant role in this process. Therefore, quantifying the NGH dissolution kinetics and clarifying the relations between NGH dissolution and water production is crucial. In this work, a modified excess water method was developed to prepare quasi-homogenous water-saturated methane hydrate (MH) bearing sediments (HBS). The MH dissolution kinetics were investigated by flowing methane-free water through the HBS. The MH dissolution process could be divided into the rapid stage and the decayed stage. Preferential flow channels may form by the inhomogeneous MH dissolution. Water flux has a significant impact on the MH dissolution kinetics via affecting the mass transfer and the hydrate-water exposure in flow channels. When increasing the water fluxes from 2.0 mL·cm−2·min−1 to 6.1 mL·cm−2·min−1, the MH dissolution rate initially ramped up from 15.4 cm·d−1 to 36.0 cm·d−1 (mass transfer dominates), and then decreased to 28.7 cm·d−1 (flow channels dominate). MH saturation has an influence on dissolution kinetics. The MH dissolution rate increased from 27.1 cm·d−1 to 44.2 cm·d−1 when the MH saturations increased from 11.8 % to 60.1 %. Experimental results indicate that the preferential flow channels by inhomogeneous NGH dissolution could contribute to the massive water production in the depressurization of NGH reservoir. Large pressure gradient in NGH production may accelerate the NGH dissolution and the following massive water production. Homogeneous high-saturation HBS could more facilitate the hydrate-water exposure for dissolution, which could retard the onset of massive water production. This work may ignite some new thoughts on the water control and production optimization strategies in sandy NGH exploitation.