3D traveltime inversion of induced micro-seismic events to construct a detailed velocity model

Abstract

Micro-seismic events are produced within a hydrocarbon reservoir as a result of production activities such as hydrocarbon extraction, fluid injection and/or rock fracturing caused by alterations of local stress or pore pressure. The micro-seismic events are characterized by high frequency which can be used to give a hi-resolution image of the reservoir. However, they are generally too small in magnitude to be detected on the surface due to seismic wave attenuation through the over-burden. The energy generated as a result of the induced micro-earthquakes propagates as a compressional wave at the P-wave speed followed by a shear wave at the slower S-wave speed. Such waves can be conveniently recorded by tri-axial geophones located close enough to injection wells. The tri-axial sensors are suitably located within monitoring wells to prevent seismic wave attenuation. The most common mechanism of induced micro-seismicity is tensile failure in hydraulic fracturing. However, the majority of recorded induced micro-earthquakes are thought to be caused by shear slippages (failure) along planes of weakness within the reservoir as a result of alterations in local stress or pore-pressure. Thus, the presence of micro-seismicity at a given location is taken to indicate a pressure relationship, but not necessarily a high-permeability connection, between that location and the injection well (Phillips et al., 1998). Furthermore, the above described model based on the origin of the sources predicts little shear-slip seismicity along fractures that open in tensile mode, potentially the most conductive flow paths (Phillips et al., 1998). In addition, it does not reveal the complexity of the reservoir in the inactive zones within the reservoir. And above all, it assumes that the velocity model is given and true. In this work, I present a method of using micro-seismic events resulting from the production activities within a hydrocarbon reservoir to construct a comprehensive velocity model.