Discrete Element Analysis of Hydraulic Fracturing of Methane Hydrate-Bearing Sediments

Hydraulic fracturing is an important reservoir reconstruction method that may potentially help achieve effective stimulation of natural gas hydrate deposits. To verify some theoretical laws that are difficult to confirm experimentally, a discrete element model of methane hydrate-bearing sediments (MHBSs) was established in this study using the two-dimensional particle flow code software PFC2D, and the hydraulic fracturing of MHBS samples under different conditions was numerically simulated using fluid-mechanical coupling. The minimum breakdown pressure of the MHBS increased as the hydrate saturation increased, but the brittleness of the samples with hydrate saturations below 30% was weak, contrasting the breakdown pressure law of fracturing in the conventional breakdown model. The MHBS samples with 40-60% hydrate saturations could generate an ideal number of main fractures. The higher the pumping pressure of the injected fluid, the shorter was the sample breakdown time of fracturing. According to the favorability of the influence of the distribution model of hydrate in the sediment on fracturing, the hydrate cementing grain contact model could be ranked above the hydrate load-bearing granular frame model. The minimum breakdown pressures of the hydrate cementing grain contact model samples were greater than those of the hydrate load-bearing granular frame model samples. During direct natural fracturing of the MHBS samples, the natural fracture must reach a certain length to produce a new tensile hydraulic fracture that extended the natural fracture. An independent hydraulic fracture could pierce through and further expand the natural fracture. When the vertical stress of each MHBS sample was taken as the maximum principal stress, most fractures generated by fracturing expanded parallel to the direction of the vertical stress, and the failure mode was tensile failure. Permeability enhancement of the fractured MHBS samples decreased with increasing hydrate saturation. These results provide valuable reference for investigating the fracturing of methane hydrate reservoirs.

Automated summary

This study used a discrete element model and numerical simulation to investigate hydraulic fracturing of methane hydrate-bearing sediments. Results show that higher hydrate saturation leads to greater breakdown pressure, samples with 40-60% saturation generate an ideal number of fractures, and higher pumping pressure results in shorter fracturing time. The contact model for hydrate cementing grain is more effective in increasing breakdown pressure compared to the load-bearing granular frame model.