Experimental evaluation of gas production from hydrate-bearing sediments via combined hydraulic fracturing and depressurization method

Abstract

Marine hydrate-bearing sediments (HBS) in the Nankai Trough and the South China Sea, characterized by high fines content and high hydrate saturation, are typically associated with very low porosity and permeability, which greatly undermines the hydrate exploitation efficiency. Inspired by the exploitation techniques of coals and shale gases, hydraulic fracturing could potentially be an effective way to improve the overall permeability of HBS and accordingly its gas production efficiency. This paper introduces a novel experimental study on the enhancement of gas production from HBS via combined hydraulic fracturing and depressurization method. The main properties examined are the viscosity of fracturing fluid and the perforated length of production well. Substantial improvement in gas production by hydraulic fracturing was observed, in terms of both the peak and long-term production rates. The most remarkable increase in peak production rate can be up to 90.4% and only half the time was required to achieve a total gas production of 70%. The optimal fluid viscosity of 500 mPa·s was identified in the present experiments. Fracturing fluids with lower viscosities would lead to only small fractures and limited increase in the overall permeability, while that with higher viscosities somewhat inhibit gas flow along fractures, both against the achievement of high gas production efficiency. In particular, sediment subsidence and sand production would be exacerbated at the presence of hydraulic fractures. Furthermore, a greater well perforated length was conductive to fracturing fluid discharge and thus facilitating gas production efficiency, in terms of not only shortening the fluid flow path but also alleviating the sand production. This study on hydraulic fracturing for HBS offers novel insights into enhancing the gas production efficiency and revealing potential engineering risks in practical applications.