Mechanical Interaction of Multiple Fractures-Exploring Impacts of the Selection of the Spacing/Number of Perforation Clusters on Horizontal Shale-Gas Wells

For multistage hydraulic fracturing of horizontal wells with cased-hole completion, multiple perforation clusters are used typically to create multiple fractures in any single stage. How to place these perforation clusters is a critical issue because the number of perforation clusters to be used and the space between them significantly impact how effectively the fractures can be created in the formation. To optimize the spacing of perforation clusters, stress distributions and fracture mechanics need to be well understood. In this study, the displacement-discontinuity method is used to construct a boundary-element model, which is able to analyze the stress distributions around multiple transverse fractures and the geometries of those fractures. With the boundary-element model, multiple cases are investigated for a different number of fractures and fracture spacings. Changes of both minimum and maximum stresses and shear stress around these fractures are illustrated first. It is found that for the cases with more than two parallel fractures, there is a strong stress concentration around the center fractures. The calculated displacements indicate that the created fractures are no longer elliptic-like, and the widths of the center fractures are reduced significantly compared with those of a single fracture. For the case of two parallel fractures, the stress concentration between two fractures also results in asymmetrical fracture shape, but the fracture widths are not reduced significantly. This study indicates that the number and spacing of the fractures need to be selected carefully to create effective fractures with appropriate fracture geometries. The boundary-element model provides a useful tool to relate rock geomechanic properties to stress distribution and fracture geometries for multiple fractures in hydraulic fracturing of horizontal wells, which can be used as a guide to space the perforation clusters.