Numerical analysis of the hydrofracturing behaviours and mechanisms of heterogeneous reservoir rock using the continuum-based discrete element method considering pre-existing fractures

Abstract

The continuum-based discrete element method was applied to probe 3D hydrofracturing behaviours. The interaction between hydrofracturing cracks and pre-existing fractures was modelled. Glutenite, a typical heterogeneous rock, was numerically reconstructed using physical parameter tests, computed tomography and imaging processing methods. The effect of pre-existing fractures on hydrofracture propagation was characterized, and the effects of material heterogeneity and horizontal in situ stress differences on fracturing were characterised using fractal theory. Investigation of hydraulic fracture (HF) propagation in heterogeneous rock, and of the mechanism by which complex hydrofracturing crack networks form through numerical simulation, revealed four typical influencing factors. These four, namely mineral particles, in situ stress difference, natural fracture (NF) density, and NF aperture inside the glutenite, were analysed to determine their effect on the HF propagation pattern, and the mechanism of complex fracture network formation in glutenite was determined. A 3D fractal dimension was proposed to characterize the spatial distribution of complex fracture networks. With increasing aperture and density of NFs, the hydrofracturing crack network becomes more complex. Mineral particles hinder the propagation of HFs; analogously, the higher the volume percentage of mineral particles is; the more complex is the hydrofracturing crack network that forms. The HFs generate and propagate near the bilateral tips of the NFs. The HFs will propagate along the direction of maximum horizontal in situ stress, and they are prone to turn into a single crack with greater in situ stress difference. The 3D fractal dimensions were computed and quantitatively reflected the above phenomena.