Аннотация:The paper describes an application of critically stressed fractures concept to study activation and reactivation of natural shear fractures existing in rock masses surrounding a hydraulic fracture during its propagation. A simplified numerical model of hydraulic fracture propagation is used to obtain stress fields near a hydraulic fracture. These stress fields are used to determine spatial orientations of natural fractures becoming critically stressed in different points of observation near the hydraulic fracture. The main tendencies of these spatial orientations are analyzed: it is shown that the greatest variation of natural fractures becomes activated at the moment of hydraulic fracture being in the closest vicinity of considered point of observation. The preferable spatial orientation of critically stressed fractures is shown to be in a close connection to the current position of hydraulic fracture's tip. The main parameters governing the natural fractures' activation are discussed providing an opportunity to use the proposed algorithm to optimize hydraulic fracturing in naturally fractured reservoirs.