Abstract
Recent examples of hydraulic fracture diagnostic data suggest complex, multi-stranded hydraulic fractures geometry is a common occurrence. This reality is in stark contrast to the industry-standard design models based on the assumption of symmetric, planar, bi-wing geometry. The interaction between pre-existing natural fractures and the advancing hydraulic fracture is a key condition leading to complex fracture patterns. Performing hydraulic fracture design calculations under these less than ideal conditions requires modeling fracture intersections and tracking fluid fronts in the network of reactivated fissures. Whether a hydraulic fracture crosses or is arrested by a pre-existing natural fracture is controlled by shear strength and potential slippage at the fracture intersections, as well as potential debonding of sealed cracks in the near-tip region of a propagating hydraulic fracture. We present a complex hydraulic fracture pattern propagation model based on the Extended Finite Element Method (XFEM) as a design tool that can be used to optimize treatment parameters under complex propagation conditions. Results demonstrate that fracture pattern complexity is strongly controlled by the magnitude of anisotropy of in situ stresses, rock toughness, and natural fracture cement strength as well as the orientation of the natural fractures relative to the hydraulic fracture. Analysis shows that the growing hydraulic fracture may exert enough tensile and shear stresses on cemented natural fractures that they may be debonded, opened and/or sheared in advance of hydraulic fracture tip arrival, while under other conditions, natural fractures will be unaffected by the hydraulic fracture. Detailed aperture distributions at the intersection between fracture segments shows the potential for difficulty in proppant transport under complex fracture propagation conditions.
Introduction
Large volumes of natural gas are stored in low-permeability fractured reservoirs around the world. Because of the low permeability of these formations and the low conductivity of the natural fracture networks, stimulation techniques such as hydraulic fracturing are necessary to make economic production possible. The low conductivity of the natural fracture system could be caused by occluding cements that precipitated during the diagenesis process (Laubach 2003, Gale et al. 2007). The fact that natural fractures might be sealed by cements does not mean that they can be ignored while designing well completion processes, however. Cemented natural fractures can still act as weak paths for fracture growth.
New diagnostic tools developed during the last decade strongly suggest multiple fracture propagation or multi-stranded hydraulic fractures in naturally fractured reservoirs (Fisher et al. 2005). Dynamic fracture mechanics theories (Freund 1990) indicate that crack tip branching will occur only in cases where fracture propagation speed is comparable to the seismic velocity of the material (more precisely, the Rayleigh wave speed). However, field data demonstrate that hydraulic fractures propagate much more slowly than seismic wave speeds (Valkó and Economides, 1995), so multi-branched fracturing should not occur in a homogeneous, isotropic, intact rock mass. On the other hand, the present day in-situ tectonic stress direction can be rotated from the time of the formation of natural fractures (Laubach et al. 2004). So, natural fractures are not necessarily aligned with the present day direction of maximum compressive stress. Thus, natural fractures may not be parallel with the hydraulic fracture and might be intersected by the hydraulic fracture. Intersection with geological discontinuities such as joints, bedding planes, faults and flaws in reservoirs might render fractures non-planar and multi-stranded.
