Abstract
Understanding hydraulic fracturing mechanisms in heterogeneous laminated rocks is important for designing and optimizing well production, as well as for predicting shale gas production. In this study, a micromechanics-based numerical approach was used to understand the physical processes and underlying mechanisms of fracking for different strata orientations, in-situ stresses, rock strengths, and injection parameters. The numerical experiments revealed a very strong influence of the pre-existing weakness planes on fracking. Geological models for rock without weakness planes and laminated rock behave very differently. Most simulated fractures in the rock without weakness planes were caused by tensile failure of the rock matrix. In an intact rock model, although a radial damage zone was generated around the injection hole, most of the small cracks were isolated, resulting in poor connectivity of the fracture network. For rock models with pre-existing weakness planes, tension and shear failure of these structural planes formed an oval-shaped network. The network was symmetrically developed around the injection well because the strength of the pre-existing weakness planes is generally lower than the rock matrix. The research shows that the angular relations between the orientation of the structural planes and the maximum horizontal stress, as well as the in-situ stress ratios, have significant effects on the morphology and extent of the networks. The strength of the pre-existing weakness planes, their spacing, and the injection rate can dramatically influence the effectiveness of hydraulic fracturing treatments.
Highlights
Hydraulic fracturing is an important technology for shale gas developement [1]
This technology depends on multiple parameters, and fracture formation is dominated by several factors, e.g., the in-situ stress, the rock mass properties, and the injection rates [1,2]
Liu et al [45] studied the influence of natural fractures on the propagation geometry of hydraulic fractures through a tri-axial fracturing system and found that the horizontal differential stress and the angle between the maximum horizontal principal in situ stress and the pre-fracture are the dominating factors for the initiation and propagation of hydraulic fractures. He et al [46] studied the hydraulic fracturing through the displacement discontinuities method and found that the pre-existing fractures were favorable to the formation of the hydraulic fracture network
Summary
Hydraulic fracturing is an important technology for shale gas developement [1]. This technology depends on multiple parameters, and fracture formation is dominated by several factors, e.g., the in-situ stress, the rock mass properties, and the injection rates [1,2]. Ehlers et al [31] studied hydraulic fracturing processes based on the theory of porous media and a phase-field fracture model Despite this arsenal of insights, techniques, and specialized knowledge, there is still a lack of understanding of the dynamic fracture propagation and the failure mechanisms in shale formations. To study coupled flow-damage problems, a coupled flow-stress-damage (FSD) model was put forward by Tang et al [38,39,40,41,42] to simulate the fracture process under boundary stress and hydraulic conditions This model was improved to incorporate heterogeneity in micro-mechanical properties [33]. Hydrofracturing simulations were conducted to assess the influences of the in-situ stress ratio, the mechanical properties of the discontinuities, the discontinuity spacing, and the injection rate on the evolution of hydraulic fractures
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