Methane hydrate reformation in porous media with methane migration

Abstract

The hydrate reformation that occurs in natural gas hydrate (NGH) exploration reduces mining efficiency and safety. To elucidate the hydrate formation/reformation characteristics during NGH exploration, methane hydrate (MH) was formed/reformed in two different modes to simulate mining of NGH sediment. The effects of residual water, residual MH and methane flow rate on MH reformation in a porous medium were investigated experimentally. Magnetic resonance imaging (MRI) was used to analyze MH saturation and distribution in the porous medium. In reformation, a positive correlation exists between the hydrate saturation (Sh) increment and MH dissociation. Moreover, the percentage of reformation Sh increment increases with the methane injection rate. That demonstrates MH dissociation by depressurization improves the contact area of gas-liquid and enhance the nucleation rate, which contributes to hydrate reformation. In addition, the residual Sw and MH reformation rate maximum (Rmax) are positively correlated in the rapid-reformation period. According to MRI images, crack-like pathways exist in the porous medium after MH dissociates completely in the first experimental mode. However, constantly flowing methane in the MH reformation process can render the water distribution uniform after MH dissociation in the second experimental mode. That means the methane flow affects the capillary force distribution then further influences the pore water distribution in porous medium.