Abstract

A new experimental model has been designed to simulate the influence of a natural fracture network on the propagation geometry of hydraulic fractures in naturally fractured formations using a triaxial fracturing system. In this model, a parallel and symmetrical prefracture network was created by placing cement plates in a cubic mold and filling the mold with additional cement to create the final testing block. The surface of the plates will thus be weakly cemented and form prefractures. The dimension and direction of the prefractures can be controlled using the plates. The experiments showed that the horizontal differential stress \Updelta \sigma and the angle \Updelta \theta between the maximum horizontal principal in situ stress and the prefracture are the dominating factors for the initiation and propagation of hydraulic fractures. For \Updelta \theta = 90\circ and \Updelta \sigma \ge 2{\text{ MPa}} or \Updelta \theta = 60\circ and \Updelta \sigma \ge 4{\text{ MPa}}, the direction of the initiation and propagation of the hydraulic fractures are consistent with or deviate from the normal direction of the prefracture. When the hydraulic fractures approach the prefractures, the direction of the hydraulic fracture propagation will be consistent with the normal direction of the prefracture. Otherwise, the hydraulic fracture will deflect and perpendicularly cross the parallel and symmetric prefracture network. For \Updelta \theta = 90\circ and Updelta \sigma < 2{\text{ MPa}},\,\Updelta \theta = 60\circ, and Updelta \sigma < 4{\text{ MPa}}or Updelta \theta = 45\circ and \Updelta \sigma = 4  8{\text{ MPa}}, before the hydraulic fracture and the prefractures intersect, the direction of the hydraulic fracture propagation remains unchanged, and the prefractures open or dilate when the hydraulic fracture propagates to the intersection point, forming a complicated hydraulic fracture network with the propagation region of the overall hydraulic fracture network taking the shape of an ellipse. In this condition, the complexity level of the hydraulic fracture is controlled by the net pressure, the compressive normal stress acting on the prefractures, the shearing strength and the cohesion strength of the planes of weakness. The conclusions of this research are inconsistent with the formulation of the approach angle that has been widely accepted by previous studies. The principle of hydraulic fracture propagation is that it follows the least resistance, the most preferential propagation, and the shortest propagation path. �� 2013 SpringerVerlag Wien.