Abstract
The lithology of shale oil reservoir of Lucaogou Formation in Junggar Basin, China shows great variation in a vertical direction and develops bedding planes (BPs). In such formation, rock properties and fabrics have a significant impact on stimulation effects. To clarify the fracture propagation mechanism in a vertically heterogeneous reservoir, an experimental study on fracture propagation in layered rock samples with complex lithology has been conducted. The effect of layers on the height of hydraulic fractures (HFs) was analysed based on triaxial hydraulic fracturing simulation system combined with mineral and mechanical characteristics analysis. The research shows that when the HF is initiated in siltstone layer, it tends to penetrate BPs with the dimensionless fracture height of more than 0.74. When HF is initiated in mudstone layer, the vertical growth of HFs tends to be terminated at the BPs, and thefracture height is constrained. The greater the thickness of the interlayer is, the more likely the HFs tend to be cut off at the interface and propagate along the interface, resulting in the limited fracture height. Under high horizontal stress difference, HFs are relatively straight. Due to the high permeability of BPs and the low viscosity of fracturing fluid, the fluid leakoff into BPs is observed, which is not conducive to the vertical propagation of HFs. Increasing the viscosity of fracturing fluid facilitates HFs to penetrate the high-permeability BPs and improves the vertical stimulated volume of shale oil reservoir.
Highlights
Introduction e shale oil reservoir ofLucaogou Formation in Junggar Basin was deposited in the salinized lake basin sedimentary environment after the closure of residual sea
International scholars have conducted a large number of experimental studies on the mechanism of hydraulic fractures (HFs) propagation [5–9]. e HF propagation behavior in layered rock formations was first studied by Daneshy [10], who found that the HF will penetrate the bonding interface, but the unbonded interface will stop the HF, which is independent of the difference of mechanical properties between the two layers
Fracture propagation behavior in thin-interbedded rock samples was analysed. e findings are summarized as follows: (1) In the high-strength sample, HFs tend to propagate to the upper part of the wellbore. e greater the strength is, the smaller the fracture height would be
Summary
Introduction e shale oil reservoir ofLucaogou Formation in Junggar Basin was deposited in the salinized lake basin sedimentary environment after the closure of residual sea. E influence of BPs on HFs propagation behavior and stimulated volume is not clearly understood. Erefore, it is of great significance to study the interaction mechanism between geological interface and HFs and the theory of fracture propagation in a vertical heterogeneous reservoir. E HF propagation behavior in layered rock formations was first studied by Daneshy [10], who found that the HF will penetrate the bonding interface, but the unbonded interface will stop the HF, which is independent of the difference of mechanical properties between the two layers. It was found that when HFs propagate from rigid media to soft media, the fractures will penetrate the interlayer interface. E fracture mechanics analysis at the interface can be used to judge whether the fracture can penetrate the layer.
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