Electrochemical response of unconsolidated sediment during liquid pore water phase change and its inspiration for gas hydrate saturation calculation

Abstract

Electrical exploration is one of the most effective methods to estimate the location and abundance of hydrates. To better understand the relationship between electrical resistivity and gas hydrate saturation, the conduction mechanism of hydrate-bearing sediment needs to be investigated. Ice-bearing sediments with physical properties similar to hydrate-bearing sediment were synthesized using NaCl solution with different initial salinity. Electrical resistivity was detected using electrochemical impedance spectroscopy. Meanwhile, the distribution of relaxation time (DRT) was utilized to separate the electrochemical processes with various time constants and to analysis the conductive mechanism in depth. It is found that ion migration is the main conductive path of ice-bearing sediments. Salt ion transport is the main conducting path when ice saturation is low. Conversely, hydrogen or salt ion transport through ice is the main conducting path when ice saturation is high. The dual effects of ice on ion transport can be reflected in reducing and enhancing pore water resistivity. Based on the DRT results, increased ice saturation in larger pores hiders salt ion migration and promotes hydrogen ion movement, while the ice-sediment interface in smaller pores may form a highly conductive path. Moreover, it is also found that the DRT can be used to calculate water saturation. A cubic polynomial relationship is suggested between the DRT curve area and ice saturation. The application of electrochemical method to data analysis is very beneficial and can provide valuable information for obtaining more reliable hydrate saturation from electrical resistivity data. The results of this study are helpful to improve gas hydrate saturation interpretation models and provide significant guidance for gas hydrate production strategy design.