In horizontal well shale completions, multiple stages, each often with multiple clusters, are used to provide sufficient stimulated area to make an economic well. However, each created hydraulic fracture alters the stress field around it, and when hydraulic fractures are placed close enough together, the well-known stress shadow effect occurs in which subsequent fractures are affected by the stress field from the previous fractures. Further, new completion techniques, like simultaneous fracturing and zipper fracs, have been proposed to take advantage of stress shadows to enhance production and are therefore dependent upon a proper understanding of stress shadow effects.
In recent years, several papers, by the authors and others, have been published attempting to describe the stress field associated with hydraulic fracturing. Often, the authors focus on the change in the stress field as indicative of increasing ‘complexity’ where the evaluations are based upon the common modeling of multiple hydraulic fractures assuming a parallel, planar hydraulic fracture geometry. However, for hydraulic fracturing in unconventional plays, parallel, planar hydraulic fractures are unlikely to occur. If there are few natural fractures – or if these are tight with narrow initial apertures - whether pumped simultaneously (as in multiple clusters per frac stage) or sequentially (one stage pumped after the other), the stress shadow effect will force subsequent fractures to grow away from the first fracture. Alternatively, if there are many, hydraulically open natural fractures, the stress shadow effect may be muted by the many open natural fractures and reduced length and aperture of the main hydraulic fracture.
In this paper, we present the results of a numerical evaluation of the effect of multiple hydraulic fractures on stress shadowing as a function of the natural fractures, hydraulic fracture spacing, rock mechanical properties, and in-situ conditions. The results of the study provide a quantitative means to optimize shale completions by understanding the effect of hydraulic fracture spacing on the stress shadow effect and the potential for changing fracture complexity. In addition, the results show the relationship between stress shadowing and microseismic events along multi-stage horizontal wells, which allows for better interpretation of the microseismic data.