True Triaxial Hydraulic Fracturing Experiments of Clayey-Silty Hydrate-Bearing Sediments

Hydraulic fracturing has been proposed as a potential stimulation technology to enhance gas production of a marine hydrate reservoir. However, the buried depth of the hydrate reservoir is shallow and the cementation degree of hydrate-bearing sediments is weak. Whether a hydrate reservoir can be developed by hydraulic fracturing is still a controversial hot issue. This research performed a series of true triaxial hydraulic fracturing experiments on clayey-silty hydrate-bearing sediments. Experimental results can reflect the fracture morphology after hydraulic fracturing and hydrate decomposition. The influence of hydrate saturation, fracturing fluid injection rate, fracturing fluid viscosity, and in situ stress on hydraulic fracturing results was analyzed, and the fracability of the hydrate reservoir was further evaluated. Experimental results show that the cementation strength of hydrate-bearing sediments plays a decisive role in fracability. A high fracability index indicates greater feasibility of hydraulic fracturing in the hydrate reservoir. Because there are clay components in hydrate-bearing sediment samples (HBSS), clay is severely argillated by fracturing fluid, resulting in the destruction of uncemented HBSS. Weakly cemented HBSS shows brittleness, and the fracturing curve has obvious fracture pressure. Although fractures are formed in most weakly cemented HBSS after hydraulic fracturing, fractures remain stable without collapse after hydrate decomposition. There are still some samples that fail to form fractures after hydraulic fracturing and hydrate decomposition, indicating that hydrate decomposition affects hydraulic fracturing results. Fractures can be formed and maintained after hydrate decomposition when the fracability index of the hydrate reservoir is greater than 0.48, which is greater than the previous threshold of 0.39.