Modeling of Interaction of Hydraulic Fractures in Complex Fracture Networks
- Ruiting Wu (Schlumberger) | Olga Kresse (Schlumberger) | Xiaowei Weng | Charles-Edouard Cohen (Schlumberger) | Hongren Gu (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Hydraulic Fracturing Technology Conference, 6-8 February, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 3 Production and Well Operations, 4.2 Pipelines, Flowlines and Risers, 2.5.2 Fracturing Materials (Fluids, Proppant), 2.5.1 Fracture design and containment, 4.1.2 Separation and Treating, 2.2.2 Perforating, 4.1.5 Processing Equipment, 1.2.2 Geomechanics, 5.8.2 Shale Gas, 5.5 Reservoir Simulation, 5.8.6 Naturally Fractured Reservoir, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.1.7 Seismic Processing and Interpretation
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A recently developed unconventional fracture model (UFM) is able to simulate complex fracture network propagation in a formation with pre-existing natural fractures. Multiple fracture branches can propagate simultaneously and intersect/cross each other. Each open fracture exerts additional stresses on the surrounding rock and adjacent fractures, which is often referred to as "stress shadow?? effect. The stress shadow can cause significant restriction of fracture width, leading to greater risk of proppant screenout. It can also alter the fracture propagation path and drastically affect fracture network patterns. It is hence critical to properly model the fracture interaction in a complex fracture model.
A method for computing the stress shadow in a complex hydraulic fracture network is presented. The method is based on an enhanced 2D Displacement Discontinuity Method with correction for finite fracture height. The computed stress field is compared to 3D numerical simulation in a few simple examples and shows the method provides a good approximation for the 3D fracture problem. This stress shadow calculation is incorporated in the UFM. The results for simple cases of two fractures shows the fractures can either attract or repel each other depending on their initial relative positions and compares favorably with an independent 2D non-planar hydraulic fracture model.
Additional examples of both planar and complex fractures propagating from multiple perforation clusters are presented, showing that fracture interaction controls the fracture dimension and propagation pattern. In a formation with small stress anisotropy, fracture interaction can lead to dramatic divergence of the fractures as they tend to repel each other. However, even when stress anisotropy is large and fracture turning due to fracture interaction is limited, stress shadowing still has a strong effect on fracture width, which affects the injection rate distribution into multiple perforation clusters, and hence overall fracture network geometry and proppant placement.
Multi-stage stimulation has become the norm for unconventional reservoir development. However, one of the primary obstacles to optimizing completions in shale reservoirs has been the lack of hydraulic fracture models that can properly simulate complex fracture propagation often observed in these formations. A complex fracture network model, referred to as Unconventional Fracture Model (UFM), had recently been developed (Weng et al., 2011, Kresse et al, 2011). The model simulates the fracture propagation, rock deformation, and fluid flow in the complex fracture network created during a treatment. The model solves the fully coupled problem of fluid flow in the fracture network and the elastic deformation of the fractures, which has similar assumptions and governing equations as conventional pseudo-3D fracture models. Transport equations are solved for each component of the fluids and proppants pumped. A key difference between UFM and the conventional planar fracture model is being able to simulate the interaction of hydraulic fractures with pre-existing natural fractures, i.e. determine whether a hydraulic fracture propagates through or is arrested by a natural fracture when they intersect and subsequently propagates along the natural fracture. The branching of the hydraulic fracture at the intersection with the natural fracture gives rise to the development of a complex fracture network. A crossing model that is extended from the Renshaw and Pollard (1995) interface crossing criterion, applicable to any intersection angle, has been developed (Gu and Weng 2010) and validated against the experimental data (Gu et al., 2011), and is integrated in the UFM.
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