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.

The development of many shale plays has been met with significant challenges, including low production rates and rapid production declines, and many ideas and concepts have been tried to address these. Multi-well completions, like ‘Simulfracs’ and ‘Zipper fracs’ for example, have been attempted as a means to increase production, but these have met with only limited success. The authors have shown in previous work that multi-well completions increase the in-situ stress field around the created hydraulic fractures, which has the effect of stabilizing natural fractures and weakness planes (making them more resistant to shear). Further, the authors have also shown in previous work that the increase in shear from the tip of propagating hydraulic fractures, in a Zipper frac multi-well completion for example, is a complicated factor of well spacing, frac spacing, hydraulic fracture length (for a given well spacing), natural fracture mechanical properties, and in-situ stress. In this paper, we present the results of a discrete element model numerical study of multi-well completions simulated in a fully hydro-mechanical coupled fashion. Building upon the previous work of the authors and others, the influence of changes in the in-situ pressure are considered in order to more completely understand the mechanical interactions between propagating hydraulic fractures and natural fractures during multi-well completions. The study includes a parametric study of well configuration, in-situ stress conditions, in-situ pressure, and mechanical properties on the ability to enhance natural fracture shear from multi-well completions and increase hydrocarbon production. The quantitative results of the study provide a direct means to consider when multi-well completions may help increase hydrocarbon production. Further, the results of the study also provide a means to optimize the application and design of multi-well completions as a function of the in-situ stresses, in-situ pressure, the mechanical properties of the natural fractures and weakness planes, and well configuration, which, ultimately, should lead to improved well economics.

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There have been extensive industry efforts to understand the geophysical implicationsand limitationsof microseismic analyses; however, a critical issue that is often overlooked is the geomechanics of the rock failure that is represented by microseismicity. Recall that microseismicity is the acoustic representation of rock failure, whether tensile failure or shear failure, which is driven by the coupled hydro-thermo-mechanical effects of injecting cool fluids at high rates into naturally fractured formations. Often overlooked in the analysis of microseismic data is the stress and deformational effects at the tip of a propagating fracture that cause a significant percentage of the total microseismic record. Previous publications, for example, have noted that at the horizontal leading edge of a propagating fracture, the dominant shear is in a horizontal plane. Conversely, at the upper and lower vertical leading edge of the propagating fracture, vertical shear has been reported to dominate. These would be expected to not only cause a different microseismic response, but also, likely, a different stimulation response.In this paper, we present a detailed numerical evaluation of the stresses generated at the tip of a propagating hydraulic fracture under varying field conditions. The simulations were performed with a finite difference continuum code. The results of the simulations show that field conditions and position along the perimeter of the propagating hydraulic fracture can significantly impact local stresses, which will have then have an impact on the generated microseismicity. The results of the work will allow for a better interpretation of field microseismicity for completion optimization.