Interference Fracturing: Non-Uniform Distributions of Perforation Clusters that Promote Simultaneous Growth of Multiple Hydraulic Fractures

One of the important hurdles in horizontal well stimulation is the generation of hydraulic fractures (HFs) from all perforation clusters within a given stage despite the challenges posed by stress shadowing and reservoir variability. In this paper we use a newly-developed, fully coupled, parallel-planar 3D HF model to investigate the potential to minimize the negative impact of stress shadowing and thereby to promote more uniform fracture growth across an array of HFs by adjusting the location of the perforation clusters. In this model the HFs are assumed to evolve in an array of parallel planes with full 3D stress coupling while the constant fluid influx into the wellbore is dynamically partitioned to each fracture so that the wellbore pressure is the same throughout the array. The model confirms the phenomenon of inner fracture suppression due to stress shadowing when the perforation clusters are uniformly distributed. Indeed, the localization of the fracture growth to the outer fractures is so dominant that the total fractured area generated by uniform arrays is largely independent of the number of perforation clusters. However, numerical experiments indicate that certain non-uniform cluster spacings promote a profound improvement in the even development of fracture growth. Identifying this effect relies on this new model’s ability to capture the full hydro-dynamical coupling between the simultaneously evolving HFs in their transition from radial to PKN-like geometries. Introduction Horizontal wells often have a significant proportion of non-producing perforation clusters (Miller and Waters 2011). One issue is the non-uniformity of reservoir properties, including the in-situ stress, along the well (e.g. Baihly et al. 2010, Cipolla et al 2011). Another issue is the well-known phenomenon known as “stress shadowing” that refers to suppression of some hydraulic fractures by the compressive stresses exerted on them by neighboring hydraulic fractures (e.g. Gemanovich et al. 1997, Fisher et al. 2004, Abass et al. 2009, Meyer and Bazan 2011). Using a fracture array model it has been recently shown that the competition between the stress shadow effect and the drive of the system to minimize the energy dissipated via viscous fluid flow leads to a stable spacing for contained hydraulic fractures (i.e. growing in length L but with a fixed height H) that is ~1.2-2.5 times the height (Bunger et al. In Press). However, the fracture spacing used in practice is significantly smaller than that of these energy-minimizing configurations, which ensure stable fracture growth without localization. Indeed, most horizontal wells are fractured in stages that involve an array of perforation clusters distributed within a total zone length Z that is usually similar to, or somewhat shorter than, the contained fracture height. Hence, while the work of Bunger et al. (In Press) is consistent with past observations of multiple hydraulic fracture growth from stages that are long relative to the fracture height (Fisher et al. 2004), it does not preclude the possibility that localization to 1 or 2 fractures might be inevitable due to stress shadowing when the zone length is similar to or less than the fracture height. The issue of stress shadowing is, in a sense, more problematic than the issue of reservoir heterogeneity. Efforts to choose perforation cluster locations in an integrated way with reservoir characterization show some promising improvements (Cipolla et al. 2011). However, if localization due to stress shadowing is inevitable and uncontrollable by engineering design, then the uniformity of stimulation that can be achieved is fundamentally limited even for a mechanically homogeneous reservoir. It is therefore vital to address both potential sources of stimulation non-uniformity in a complimentary manner. This study uses a newly-developed parallel-planar 3D hydraulic fracturing simulator that includes full coupling between fluid flow, fracture growth, and elastic deformation of the rock via the implicit level set algorithm developed by Peirce and Detournay (2008). The model expands on this prior work by including the non-local, fully 3D stress interaction among multiple hydraulic fractures. Furthermore, the total fluid injected into the wellbore is assumed to be constant while the fluid influx to each hydraulic fracture is partitioned dynamically in such a way that the wellbore pressure is constrained to be equal across all fractures in the array. This numerical approach is therefore distinct from so-called pseudo-3D models that rely on a local elasticity expression and therefore cannot appropriately track the growth of, and interaction among, the hydraulic fractures through the transition from radial to blade-like geometries, where, in the limit of zero height growth, the latter is approximated by the classical PKN solution (Perkins and Kern 1961, Nordgren 1972). The model is also distinct from models of multiple fracture growth that assume a uniform pressure within the fractures, a temporally constant pressure at the inlet, and/or uniform distribution of fluid among the fractures. Because it is able to capture the fully-coupled mechanics including the radial to PKN-like transition with an energyminimizing partitioning of fluid, the new Planar3D simulator is appropriate for investigating fracture designs that promote simultaneous growth from multiple perforation clusters in a homogeneous reservoir. The model demonstrates that choosing specific, non-uniformly spaced perforation cluster locations can actually reduce the negative impact of stress shadowing in