Comparative study of the electrical characteristics of hydrate reservoirs before and after gas hydrate trial production in the Muli permafrost area of the Qilian Mountains, NW China

Natural gas hydrate (NGH) is considered one of the most promising alternative energy sources in the 21st century. The exploitation and decomposition of NGHs could cause geological disasters and eco-environmental problems related to climate warming, seawater blue tides, submarine landslides, drilling platform collapses, slope instability, etc. Thus, an investigation of the eco-environmental effects of NGH exploitation and decomposition has scientific and practical significance. In this paper, the electrical characteristics of NGH reservoirs before and after NGH trial production in the Muli permafrost area of the Qilian Mountains are comparatively examined for the first time. The electrical structure models before and after NGH trial production are obtained through two-dimensional and three-dimensional inversion of the NE audio-magnetotelluric (AMT) sounding profile around production test well SK2. Notably, the electrical structure of the NGH reservoir changes greatly. The research indicates that (1) after NGH trial production, the formation resistivity increases remarkably within 200 m and burial depths in the range of 120– 340 m around the production test well; (2) the main reason for the increasing resistivity after trial production is the Joule-Thomson effect caused by depressurization production and the endothermic effect of NGH decomposition, which lead to the formation of ice-bearing strata; afterwards, we explain the formation process of ice-bearing strata after NGH trial production; and (3) we speculate that the formation stability is not adversely affected after NGH trial production. These findings and research results can provide guidance for subsequent geoenvironmental safety risk assessments of NGH development in permafrost areas. • Two AMT profiles are comparatively studied before and after NGH trial production. • The resistivity of NHG reservoir increases significantly after trial production. • Joule-Thomson effect and depressurization endothermic action is the main reason for ice-bearing strata formation. • The formation stability is not adversely affected after NGH trial production.