In unconventional shale reservoirs, the quality of completion is driven by the brittle failure, which means that the rock exhibits low plastic deformation. Several authors have proposed to evaluate the failure mode of the rock through a brittleness index that has been associated with the elastic properties and the mineralogical content. However, brittleness index could not represent the failure mode of the rock, because the elastic properties and the mineralogical composition are not related to the failure mechanism. Another concept called fracability index was introduced to define the degree of easiness to which a formation can be fractured; the most fracable intervals are usually spot that require less energy to fracture. Brittlennes and fracability indexes have been incorporated by the industry for the optimization of the completion, although in some cases the efficiency of the hydraulic fracture was not achieved because the role of the stresses on the completion could be omitted. In tectonically active basins, horizontal stresses can be highly influenced by the tectonic component, which in turn is affected by the elastic properties of the formation. In this paper, a case, validated with laboratory data, of selection of completion quality - the best hydraulic stimulation intervals–for Cretaceous formations for a specific area of Magdalena Middle Valley basin in Colombia is presented, based on variables such as the brittleness index, fracability index and stresses. In this scenario, it turns out that the brittlenes and fracability index alone is not enough to identify the best intervals to fracture. Additionally, in the intervals where the brittleness index suggests the completion quality, it was observed that it corresponds to the depths where the minimum horizontal stress increases. Finally, the completion quality could be defined by understanding the relationship of the elastic properties on the brittleness index, fracability index and stresses

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