Abstract
The complexity of hydraulic fractures (HF) significantly affects the success of reservoir reconstruction. The existence of a bedding plane (BP) in shale impacts the extension of a fracture. For shale reservoirs, in order to investigate the interaction mechanisms of HF and BPs under the action of coupled stress-flow, we simulate the processes of hydraulic fracturing under different conditions, such as the stress difference, permeability coefficients, BP angles, BP spacing, and BP mechanical properties using the rock failure process analysis code (RFPA2D-Flow). Simulation results showed that HF spread outward around the borehole, while the permeability coefficient is uniformly distributed at the model without a BP or stress difference. The HF of the formation without a BP presented a pinnate distribution pattern, and the main direction of the extension is affected by both the ground stress and the permeability coefficient. When there is no stress difference in the model, the fracture extends along the direction of the larger permeability coefficient. In this study, the in situ stress has a greater influence on the extension direction of the main fracture when using the model with stress differences of 6 MPa. As the BP angle increases, the propagation of fractures gradually deviates from the BP direction. The initiation pressure and total breakdown pressure of the models at low permeability coefficients are higher than those under high permeability coefficients. In addition, the initiation pressure and total breakdown pressure of the models are also different. The larger the BP spacing, the higher the compressive strength of the BP, and a larger reduction ratio (the ratio of the strength parameters of the BP to the strength parameters of the matrix) leads to a smaller impact of the BP on fracture initiation and propagation. The elastic modulus has no effect on the failure mode of the model. When HF make contact with the BP, they tend to extend along the BP. Under the same in situ stress condition, the presence of a BP makes the morphology of HF more complex during the process of propagation, which makes it easier to achieve the purpose of stimulated reservoir volume (SRV) fracturing and increased production.
Highlights
In recent years, hydraulic fracturing has been extensively applied to increase the production rate and to realize long-term and stable yield in ultra-low-permeability shale reservoirs
On the basis of a large number of studies conducted by our research group about the influences of the spatial location of natural fractures (NFs), the angle between NFs and the maximum principal stress, the length of NFs, the mechanical properties of NFs, the angle between bedding plane (BP) and the maximum principal stress, BP spacing, and the mechanical properties of BP on initiation and propagation of the network cracks around the well [37,38,39], we further studied the effect of BP angle, BP spacing, BP compressive strength, and BP elastic modulus on the law of crack propagation considering permeability coefficient differences between the matrix and the BP of shale formation, and quantitatively analyzed initiation pressure and total break pressure under different BP angles
This is because the permeability coefficient of a BP far exceeds that of the matrix; in addition, the strength of the mechanical parameters of the BP is relatively low, so the fracture system of the stratification shale will become more complex after making contact with BPs, which warrants further study
Summary
Hydraulic fracturing has been extensively applied to increase the production rate and to realize long-term and stable yield in ultra-low-permeability shale reservoirs. Compared to the traditional fracturing volume techniques, the formation of fractures during shale formation is more complex owing to the existence of geological discontinuities such as natural fractures (NFs), bedding planes (BPs), and faults [1]. The ability to effectively control the fracture formation as well as to make effective fractures remains a key problem in stimulated reservoir volumes (SRVs). A large number of studies have analyzed and researched fracture pressure under different conditions using different theoretical models [2,3,4,5,6,7]. Chenevert and Mclamore [8,9,10] reported that the compressive strength of layered rock such as shale was a function of the confining pressure and the orientation of the anisotropic
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