With the huge growth in the stimulation of naturally fractured formations, it is clear that the industry needs new hydraulic fracturing simulation tools beyond the limits imposed by pseudo3D fracturing model. Discrete element models (DEM), in which both matrix block behavior and fracture behavior are explicitly modeled, offer one option for the specific modeling of hydraulic fracture creation and growth in naturally fractured formation without, for example, the assumption of bi-planar fracture growth.In this paper, we show the results of the successful realistic simulations of fluid injection into a given NFR using the 3DEC DEM model. The simulations included coupled fluid flow-deformation analysis, failure type and extent calculations, as well as a series of parametric analyses. The parameters investigated included: 1) injection rate and its effect on the overall injection results, and 2) fluid viscosity, which had a significant influence on the ratio of tensile (mode 1) failure versus shear failure.
In horizontal well shale completions, multiple stages, each often with multiple clusters, are used to provide sufficient stimulated area to make an economic well. Each created hydraulic fracture alters the stress field around it. 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. The effects include higher net pressures, smaller fracture widths and changes in the associated complexity of the stimulation. The level of microseismicity is also altered by stress shadow effects. For example, it is commonly seen that the number of microseismic events is significantly reduced from the toe to the heal of the well, where the first frac stage is conducted at the toe of the well.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 fracture spacing, shale rock mechanical properties, and the in-situ stress ratio. In addition, utilizing the inherent ability of discrete element models to evaluate shear and tensile failure along fracture surfaces, shear failure, as a proxy for microseismicity, is evaluated as a function of fracture-induced stress and stress shadowing. The results of the study provide a means to optimize shale completions by understanding the effect of stress ratio, rock mechanical parameters, and hydraulic fracture spacing on the stress shadow effect and the potential for changing fracture complexity.
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.
In this work, the effect of fracture network connectivity on hydraulic fracturing effectiveness was investigated using a discrete element numerical model. The simulation results show that natural fracture density can significantly affect the hydraulic fracturing effectiveness, which was characterized by either the ratio of stimulated natural fracture area to hydraulic fracture area or the leakoff ratio. The sparse DFN cases showed a flat microseismic distribution zone with few events, while the dense DFN cases showed a complex microseismic map which indicated significant interaction between the hydraulic fracture and natural fractures. Further, it was found that the initial natural fracture aperture affected the hydraulic fracturing effectiveness more for the dense natural fracture case than for the sparse less dense case. Overall, this work shows that fracture network connectivity plays a critical role in hydraulic fracturing effectiveness, which, in-turn, affects treating pressures, the created microseismicity and corresponding stimulated volume, and well production.
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