Hydraulic fracture diagnostics have highlighted the potentially complex natural of hydraulic fracture geometry and propagation. This has been particularly true in the cases of hydraulic fracture growth in naturally fractured reservoirs, where the induced fractures interact with pre-existing natural fractures. A simplified numerical model has been developed to account for mechanical interaction between pressurized fractures, and to examine the simultaneous propagation of multiple (>2) hydraulic fracture segments. Fracture intersection is presumed to communicate the hydraulic fracturing fluid to the natural fracture, which then takes up the continued propagation. Simulations for multi-stage horizontal well treatments and single stage vertical well treatments show that fracture pattern complexity is strongly controlled by the magnitude of the hydraulic fracture net pressure relative to the in situ horizontal differential stress as well as the geometry of the natural fractures. Analysis of the neartip stress field around a hydraulic fracture also indicates that induced stresses may be high enough to debond sealed natural fractures ahead of the arrival of the hydraulic fracture tip. IntroductionComplex hydraulic fracture geometry has become more evident with the widespread application of improved fracture diagnostic technology. 1 Multi-stranded fracture propagation from vertical wells has been confirmed by coring, 2,3 while microseismic data in naturally fractured reservoirs such as the Barnett Shale suggests significant diversion of hydraulic fracture paths due to intersection with natural fractures. 1 Apparent interaction between a propagating hydraulic fracture and pre-existing natural fractures seems to be the key component explaining why some reservoirs exhibit more complex behavior. 4 There are several possibilities for the interaction between hydraulic and natural fractures. The likelihood of intersection between a hydraulic and natural fracture is partly a function of orientation. If the hydraulic and natural fracture directions are parallel, intersection is less likely, but there can still be interaction between close, en echelon overlaps of fractures, and the natural fractures may be reactivated by being within the process zone (region of altered stress) around the crack tip. If the natural fractures are orthogonal to the present-day hydraulic fracture direction, the propagating hydraulic fracture is likely to cross a large number of natural fractures as it propagates through the reservoir. For these cases of direct intersection, the hydraulic fracture could propagate across the natural fracture plane without deviation and without additional leak-off, a possible outcome for strongly cemented natural fracture planes. Even if the hydraulic fracture propagates across the natural fracture, the stress induced by the hydraulic fracture could open the natural fracture enough for it to divert fracturing fluid and increase the leak-off. If the fluid diverted into the natural fracture becomes significant, the natural ...
Hydraulic fracturing is recognized as the main stimulating technique to enhance recovery in tight fissured reservoirs. These fracturing treatments are often mapped by use of hypocenters of induced microseismic events. In some cases, the microseismic mapping shows asymmetry of the induced-fracture geometry with respect to the injection well. In addition, the conventional theories predict fracture propagation along a path normal to the least compressive in-situ stresses, whereas in some cases the microseismic data suggest fracture propagation parallel to the minimum compressive stress. In this paper, we present an extended-finiteelement-method (XFEM) model that can simulate asymmetric fracture-wing development as well as diversion of the fracture path along natural fractures. Simulation results demonstrate the sensitivity of the fracture-pattern geometry to differential stress and natural-fracture orientation with respect to the in-situ maximum compressive stress. We examine the properties of sealed natural fractures that are common in formations such as the Barnett shale and show that they may still serve as weak paths for hydraulic-fracture beginning and/or diversion. The presented model predicts faster fracture propagation in formations where natural fractures are favorably aligned with the tectonic stresses.
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.
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