Mechanisms of Hydraulic Fracture Propagation and Multi-well Effect during Multiple Vertical Well Fracturing in Layered Reservoirs with Low Permeability

Abstract

Segmental-layered modeling and fine grid division methods were proposed to construct a three-dimensional largescale layered geologic model and reveal the mechanisms of hydraulic fracture propagation and the well spacing effect in multi-well fracturing in layered reservoirs with low permeability. The hydraulic fracture propagation in layered reservoirs under multi-well fracturing was simulated using a continuum-based discrete element numerical method. The effects of the stratum property difference, in situ horizontal stress difference (HSD), and well spacing on breakdown pressures and fracture propagation were analyzed. The concept of "extension ellipse" was proposed to describe the propagation morphology of fractures, which addresses the difficulty of accurately characterizing the propagation morphology of fractures at the geologic scale. The influence range of the multiwell effect in layered reservoirs was also determined, and the effect mechanisms of stress amplification and attraction and repulsion between fractures on the behavior of hydraulic fracture propagation were revealed. Layered reservoirs and higher in situ HSD restrict the development of hydraulic fractures but greatly reduce the breakdown pressures. The multi-well fracturing mode changes the stress field distribution around the wells and weakens the effects of in situ HSD on fracture propagation. In comparison to homogeneous reservoirs, the range of the multi-well effect in the layered reservoirs is relatively small. At the fracturing site, the well spacing should be appropriately increased in the direction of the maximum horizontal stress and should be reduced in the direction of minimum horizontal stress, helping hydraulic fracture development and improving the fracturing stimulation effect in layered reservoirs. This study addresses the deficiency of considering stratum property differences in the existing large-scale fracturing models, and the results provide a reference for the optimization and design of fracturing simulation in the layered reservoirs with low permeability.