CO2 messes with rock physics

Seismic techniques are the main monitoring tools for CO2 storage projects, especially in saline aquifers with good porosity. The majority of existing commercial and pilot CO2 injections have resulted in clear time-lapse seismic anomalies that can be used for leakage detection as well as refinement of the reservoir models to conform with the monitoring observations. Both tasks are legal requirements imposed on site operators. This paper revisits the rock-physics effects that may play an important role in the quantitative interpretation of seismic data. First, we briefly describe a standard approach to the rock-physics modeling of CO2 injections: Gassmann-type fluid substitution accounts for the presence of compressible CO2 in the pore space, and dissolution/precipitation of the minerals changes the pore volume. For many geologic conditions and injection scenarios, this approach is inadequate. For example, dissolution of the carbonate cement may weaken the rock frame, wave-induced fluid flow between CO2 patches can vary the magnitude of the seismic response significantly for the same saturation, the fluid itself might undergo change, and the seal might act as a sink for CO2. Hence, we critically review the effects of some recent advances in understanding CO2 behavior in the subsurface and associated rock-physics effects. Such a review should help researchers and practitioners navigate through the abundance of published work and design a rock-physics modeling workflow for their particular projects.

Modeling the effects of fracture infill on frequency-dependent anisotropy and AVO response of a fractured porous layer

In a fractured porous hydrocarbon reservoir, wave velocities and reflections depend on frequency and incident angle. A proper description of the frequency dependence of amplitude variations with offset (AVO) signatures should allow effects of fracture infills and attenuation and dispersion of fractured media. The novelty of this study lies in the introduction of an improved approach for the investigation of incident-angle and frequency variations-associated reflection responses. The improved AVO modeling method, using a frequency-domain propagator matrix method, is feasible to accurately consider velocity dispersion predicted from frequency-dependent elasticities from a rock physics modeling. And hence, the method is suitable for use in the case of an anisotropic medium with aligned fractures. Additionally, the proposed modeling approach allows the combined contributions of layer thickness, interbedded structure, impedance contrast and interferences to frequency-dependent reflection coefficients and, hence, yielding seismograms of a layered model with a dispersive and attenuative reservoir. Our numerical results show bulk modulus of fracture fluid significantly affects anisotropic attenuation, hence causing frequency-dependent reflection abnormalities. These implications indicate the study of amplitude versus angle and frequency (AVAF) variations provides insights for better interpretation of reflection anomalies and hydrocarbon identification in a layered reservoir with vertical transverse isotropy (VTI) dispersive media.