Feasibility evaluation of hydraulic fracturing in hydrate-bearing sediments based on analytic hierarchy process-entropy method (AHP-EM)

Abstract

Natural gas hydrate is an attractive potential as alternative energy source. The initial studies of natural gas hydrate production test have shown that the hydrate productivity via conventional depressurization is still far to reach the commercial level, hence, hydraulic fracturing has been proposed to stimulate the hydrated formation to enhance production. However, the buried depth of hydrate-bearing sediments is generally shallow, and the cementation of sediments is weak. Whether the hydrate-bearing sediments has adequate and commercially viable fracability is a question yet to be answered. In this study, a novel feasibility evaluation model of hydraulic fracturing in hydrate-bearing sediments is developed based on analytic hierarchy process-entropy method. The fracability index is proposed to evaluate the fracability of hydrate-bearing sediments. Representative samples were built based on the physical and mechanical characteristics of Alaska permafrost, and hydraulic fracturing experiments were carried out to evaluate the feasibility of hydraulic fracturing as a stimulation technique. The results showed that the hydrate-bearing sediments with fracability index >0.72 can form fractures under different fracturing fluid viscosity ratio. For hydrate-bearing sediments with fracability index <0.39, when the fracturing fluid viscosity ratio is 1, no fracture network is formed during hydraulic fracturing. However, when the fracturing fluid viscosity ratio is increased to more than 120, fractures may develop. For the hydrate-bearing sediments with fracability index between 0.39 and 0.72, the results are not promising when fracturing fluid viscosity ratio is 1, however, when fracturing fluid viscosity ratio is more than 120, fractures observed to grow. In general, the results indicate that the samples with high fracability index are sweet spots in hydrate sediments in terms of fracability. At low fracability index values, the viscosity of fracturing fluid has a key role in fracability, where increasing the viscosity will result in higher fracability index.