Experimental investigation on the propagation of hydraulic fractures in massive hydrate-bearing sediments

Abstract

Hydraulic fracturing is regarded as one of the potential technical means to realize efficient development of natural gas hydrate. Massive-distributed hydrates are widespread around the world, although the solid fluidization method has been proposed to develop this kind of reservoir, the process of this method is complex, and there are still a lot of key problems need to be solved. Meanwhile, when the hydrate decomposed during production, how to maintain formation stability is still an unsolved key problem. Therefore, we consider using hydraulic fracturing technology to exploit this kind of reservoir. On the one hand, the fractures formed by high viscosity fracturing fluid can improve the reservoir permeability. On the other hand, the proppant added during fracturing can play a good supporting role to the formation. In this work, a series of large-size (300 × 300 × 300 mm) hydraulic fracturing physical simulation experiments were conducted under true triaxial stress states to investigate hydraulic fracture propagation law in massive-distributed hydrate-bearing reservoir. The effects of massive hydrate size, approaching angle and fracturing fluid displacement on the fracture propagation were studied. The experimental results indicated that massive hydrate exerts an important influence on hydraulic fracture propagation. Two main massive hydrate-hydraulic fracture interaction modes were observed in our experiments: (a) When the massive hydrate size is small, the hydraulic fractures mainly propagated along the interface between the sediment matrix and the massive hydrate. (b) The hydraulic fractures penetrated the massive hydrate, which usually occurs when hydraulic fracture encountered large-size massive hydrate, and the penetrated position moves from both sides to the central position of the massive hydrate. The approaching angle has a significant effect on the massive hydrate-hydraulic fracture interaction law. Under the condition of high approaching angle, hydraulic fractures easily penetrated the massive hydrate. The influence of fracturing fluid displacement on massive hydrate-hydraulic fracture interaction is closely related to massive hydrate size. When hydraulic fractures encountered the small-size massive hydrate, increasing the displacement has a limited effect on massive hydrate-hydraulic fracture interaction, and the hydraulic fractures still mainly propagated along the interface. When large-size massive hydrates are distributed in the sample, increasing the displacement has a significant effect on the massive hydrate-hydraulic fracture interaction, and hydraulic fractures tend to branch, the complex fracture geometry would be formed. In this study, a new large-size massive hydrate sample preparation method was proposed, and the effects of various factors on the hydraulic fracture propagation law were investigated experimentally, which can provide guidance for the fracturing design of massive hydrate-bearing reservoir.