Impact of Fractures on CO2Storage Monitoring: Keys for an Integrated Approach

Abstract

The monitoring of CO2 storage in fractured reservoirs (depleted hydrocarbon fields or brine aquifers) requires the study of the impact of fracturation and fluid substitution on seismic data. Seismic data can provide information about the additional compliance due to the fractures and the fluids through the analysis of seismic azimuthal anisotropy with an appropriate rock physics model. We introduce a rock physics model built in collaboration with geologists, providing a realistic description of fractured media. This model concerns fractured geological media in the presence of fluids characterized by some degree of matrix porosity, the presence of pore fluids, connected and/or non-connected fractures, the presence of several fracture sets, and an inherent seismic anisotropy. The direct application of this rock model shows that the P -wave anisotropy value measured through seismic data can be explained by several sets of different parameters such as the fracture density, the pore fluid compliance or the porosity. The presence of inherent layer-induced anisotropy can also modify the P -wave anisotropy and thus the interpretation of this value in terms of fluid substitution in a fractured porous medium. As far as fluid substitution monitoring is concerned, if seismic data are acquired before and after this substitution, a change in the P -wave anisotropy value can be linked to the modification of the compliance of the fluid content in the same medium exhibiting the same fracture network and the same porosity. This relative value can only be correctly interpreted in terms of fluid substitution provided we have some constraints on a few of the parameters involved in the P -wave anisotropy value such as the porosity, and a rough idea of the level of normalized fracture compliance. Then, a multidisciplinary approach is mandatory to constrain these parameters. For instance, borehole and outcrop geological information can give the upper limit of the fracture density expected at depth in the same formation. Furthermore, rock mechanics helps in understanding the fracturation state at depth to identify the predominant fractures in regard to the interpretation of seismic anisotropy in terms of fluid substitution inside the fracture network.