Development of peridynamics-based hydraulic fracturing model for fracture growth in heterogeneous reservoirs
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Oil and gas reservoirs are heterogeneous at different length scales. At the micro-scale mechanical property differences exist due to mineral grains of different composition and the distribution of organic material. At the centimeter or core scale, micro-cracks, sedimentary bedding planes, natural fractures, planes of weakness and faults exist. At the meter or log scale, larger scale bedding planes, fractures and faults are evident in most sedimentary rocks. All these heterogeneities contribute to the complexity in fracture geometry. However, very little research has been conducted on evaluating the effect of these heterogeneities on fracture propagation, primarily due to the absence of a numerical framework capable of incorporating such heterogeneities in fracture growth models. In this dissertation we developed a novel method for simulating hydraulic fractures in heterogeneous reservoirs based on peridynamics and then utilized it to elucidate the complicated fracture propagation mechanisms in naturally fractured, heterogeneous reservoirs. Peridynamics is a recently developed continuum mechanics theory specially developed to account for discontinuities such as fractures. Its integral formulation minimizes the impact of spatial derivatives in the stress balance equation making it particularly suitable for handling discontinuities in the domain. No fluid flow formulation existed in the peridynamics framework since this theory had not been applied to fluid driven fracturing processes. In this dissertation, a new peridynamics fluid flow formulation for flow in a porous medium and inside a fracture was derived as a first step in the development of a peridynamics-based hydraulic fracturing model. In the subsequent section, a new peridynamics-based hydraulic fracturing model was developed by modifying the existing peridynamics formulation of solid mechanics and coupling it with the newly derived peridynamic fluid flow formulation. Finally, new shear failure criteria were introduced into the model for simulating interactions between hydraulic fractures (HF) and natural fractures (NF). This model can simulate non-planar, multiple fracture growth in arbitrarily heterogeneous reservoirs by solving fracture propagation, deformation, fracturing fluid pressure, and pore pressure simultaneously. The validity of the model was shown through comparing model results with analytical solutions (1-D consolidation problem, the KGD model, the PKN model, and the Sneddon solution) and experiments. The 2-D and 3-D interactions behavior between a HF and a NF were investigated by using the newly developed peridynamics-based hydraulic fracturing model. The 2-D parametric study for the interaction between a HF and a NF revealed that, in addition to the well-known parameters (the principal stress difference, the approach angle, the fracture toughness of the rock, the fracture toughness of the natural fracture, and the shear failure criteria of the natural fracture), poroelastic effects also have a large influence on the interaction between a HF and a NF if leak-off is high. The 3-D interaction study elucidated that the height of the NF, the position of the NF, and the opening resistance of the NF have a huge impact on the three-dimensional interaction behavior between a HF and a NF. The effects of different types of vertical heterogeneity on fracture propagation were systematically investigated by using domains of different length scales. This research clearly showed the mechanisms and the controlling factors of characteristic fracture propagation behaviors (“turning”, “kinking”, and “branching”) near the layer interface. In layered systems, the mechanical property contrast between layers, the dip angle and the stress contrast all play an important role in controlling the fracture trajectory. Each of these effects was investigated in detail. The effect of micro-scale heterogeneity (due to varying mineral composition) on fracture geometry was studied next. It was shown that even at the micro-scale, fracture geometry can be quite complex and is determined by the geometry and distribution of mineral grains and their mechanical properties.