Today, shale reservoirs are completed with multiple hydraulic fractures. For efficiency reasons, hydraulic fracturing is typically "staged", each stage comprising of multiple perforation clusters with the aim of initiating one hydraulic fracture per cluster. Cluster spacing is an important parameter to optimize, but the following is still unanswered: What is the minimum spacing that can be achieved between hydraulic fractures, given that both formation heterogeneity and stress shadow play a governing role? This paper uses a new model for the simultaneous initiation and propagation of hydraulic fractures from a horizontal well. Based on this model and observed field results from a well completed with 92 single-entry fracture treatments, new insights on the lack of robustness associated with multiple cluster plug-and-perf completion are presented. Without a large perforation friction pressure drop, there is a minimum spacing of clusters below which, fractures are "shut down" by the stress shadow effect. The stress shadow can, however, be mitigated with a large perforation pressure drop when there is no stress variation between clusters. Moreover, significant stress variations along clusters can be overcome by engineering the limited entry technique (choking down clusters in low stress zones), provided that the corresponding stress variations are known.

The limited entry technique however fails to enable hydraulic fracture initiation and development from all of the perforation clusters if there is a significant variation of near-wellbore pressure drop associated with the complexity of the fracture geometry in the near-wellbore when re-orienting toward the far-field in-situ stress direction.

Downhole pressure measurements from an experimental well completed with 92 single-entry fractures show large variations in near-wellbore pressure drop, highly correlated with the variations in breakdown pressures even though the stress variation estimated from the mineralogy are very small along the lateral. Had this well been completed with stages of 4 clusters each, a significant portion of the clusters would not have initiated and most of the slurry would have been taken by less than half of the clusters. Using the single-entry fracturing technique, each of the perforation clusters has initiated and propagated fractures as designed.

You can access this article if you purchase or spend a download.