Study of preparing artificial cores and propagation of hydraulic fractures in plastic sandstones

In this study, a method for preparing the artificial cores of plastic sandstones was introduced; the precise mixing ratios of the materials were presented. The consistency between artificial cores and natural sandstones was tested in terms of mineral components, strength, brittleness, and plasticity. Subsequently, hydraulic fracture experiments were conducted on the artificial cores. These experiments revealed the effects of in-situ horizontal stress difference (HSD) and material plasticity on the hydraulic fracture propagation in plastic sandstones. The results show that, under a constant in-situ stress, the stronger the plasticity, the greater the breakdown pressure of sandstones. The clay minerals increase the compactness of sandstones, leading to the decrease of permeability and breakdown pressure of the sandstone with higher plasticity coefficient. The breakdown pressure in sandstones decreases with in-situ HSD; this influence is not weakened by the plasticity. The plasticity significantly affects the fracture propagation morphology. For a weakly plastic sandstone, a flat and straight main fracture perpendicular to the wellbore is likely to be formed. For a strongly plastic sandstone, a complex fracture surface is more likely to be formed, while stronger plasticity can inhibit the propagation of fractures considerably. Plasticity can also weaken the influence of in-situ HSD on the fracture propagation morphology. Our study provides a way to reveal the propagation mechanisms of hydraulic fractures in plastic sandstones.

Automated summary

This study introduced a method for preparing artificial cores of plastic sandstones and verified their consistency with natural sandstones. The effects of in-situ horizontal stress difference (HSD) and material plasticity on the hydraulic fracture propagation in plastic sandstones were revealed. The results showed that the stronger the plasticity, the greater the breakdown pressure of sandstones. Clay minerals increased the compactness of sandstones, leading to decreased permeability and breakdown pressure in sandstones with a higher plasticity coefficient. The influence of in-situ HSD on the breakdown pressure in sandstones was not weakened by the plasticity. Plasticity significantly affected the fracture propagation morphology, with weakly plastic sandstones likely forming a flat and straight main fracture perpendicular to the wellbore, while strongly plastic sandstones were more likely to form a complex fracture surface and the propagation of fractures was considerably inhibited. Plasticity could also weaken the influence of in-situ HSD on the fracture propagation morphology. This study provides an important way to reveal the propagation mechanisms of hydraulic fractures in plastic sandstones.