Imaging pre-existing natural fractures using microseismic data

Abstract

In hydraulic fracturing treatments, locating not only hydraulic fractures but also any pre-existing natural fractures and faults in a subsurface reservoir is very important. Hydraulic fractures can be tracked by locating microseismic events, but to identify the locations of natural fractures, an additional technique is required. In this paper, we present a method to image pre-existing fractures and faults near a borehole with virtual reverse vertical seismic profiling data or virtual single-well profiling data (limited to seismic reflection data) created from microseismic monitoring using seismic interferometry. The virtual source data contain reflections from natural fractures and faults, and these features can be imaged by applying migration to the virtual source data. However, the imaging zone of fractures in the proposed method is strongly dependent on the geographic extent of the microseismic events and the location and direction of the fracture. To verify our method, we produced virtual reverse vertical seismic profiling and single-well profiling data from synthetic microseismic data and compared them with data from real sources in the same relative position as the virtual sources. The results show that the reflection travel times from the fractures in the virtual source data agree well with travel times in the real-source data. By applying pre-stack depth migration to the virtual source data, images of the natural fractures were obtained with accurate locations. However, the migrated section of the single-well profiling data with both real and virtual sources contained spurious fracture images on the opposite side of the borehole. In the case of virtual single-well profiling data, we could produce correct migration images of fractures by adopting directional redatuming for which the occurrence region of microseismic events is divided into several subdivisions, and fractures located only on the opposite side of the borehole are imaged for each subdivision.