Numerical Analysis for Promoting Uniform Development of Simultaneous Multiple-Fracture Propagation in Horizontal Wells
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Copyright 2017 Society of Petroleum Engineers. Multistage hydraulic fracturing, together with horizontal drilling, plays an important role in the economic development of unconventional reservoirs. However, according to field analysis of stimulation effectiveness, only a small percentage of perforation clusters contribute to most of the well production. One reason for this low effectiveness is that multiple fractures do not take the same amount of fluid and proppant because of the interaction between hydraulic fractures (i.e., stress-shadow effects). Unfortunately, how best to minimize the negative effects of stress shadowing is still not fully understood in the petroleum industry. In this paper, we analyze this problem to promote more-uniform fracture growth by use of a complex hydraulic-fracture-development model. We use our fracture-propagation model, which couples rock deformation and fluid flow in the fracture and horizontal wellbore. The model assumes homogeneous linear-elastic rock medium and does not consider proppant transport in multiple fractures. Partitioning of flow rate between multiple fractures is calculated by use of the principles of flow through an electric-circuit network. Fracture development is dominated by flow-rate distribution, which is controlled by flow resistance within the fractures. For simultaneous multiple-fracture propagation, nonuniform flow-rate distribution is induced because stress-shadow effects exert additional flow resistance on the interior fractures. To balance the extra resistance introduced by the effects, we investigated two adjustment approaches to promote uniform fracture development. The first approach was to adjust perforation number or diameter to increase flow resistance entering the exterior fractures. The second was to mitigate stress-shadow effects through management of nonuniform fracture spacing. We found that decreasing perforation diameter or the number of exterior fractures can divert fluid into the interior fractures and promote even fracture growth. Stress-shadow effects can be mitigated by moving the two interior fractures away from each other and toward the exterior fractures. The key mechanism is to maintain even flow resistance within each fracture and facilitate fractures to obtain the same amount of injection fluid. Furthermore, flow resistance is affected by nearwellbore fracture tortuosity, natural fractures, and stress heterogeneity in the reservoirs. These factors might also be effectively controlled by adding extra flow resistance to pass perforations, provided that this extra resistance is larger than the extra resistance introduced by the factors. This work analyzes the problem of uneven development of multiple fractures along the horizontal wellbore and provides new insights into how to control simultaneous multiple-fracture propagation. It offers potential approaches for engineers to apply in the field to optimize hydraulic-fracturing-treatment design, thereby maximizing well production cost-effectively.
