Rock Physics—application to Geological Storage of Co2

Abstract

Sequestration of anthropogenic CO2 into underground brine-saturated reservoirs is an immediate option for Australia to reduce CO2 emissions into the atmosphere. Many sites for CO2 storage have been defined within many Australian sedimentary basins. It is anticipated that seismic technology will form the foundation for monitoring CO2 storage within the subsurface, although it is recognised that several other technologies will also be used in support of seismic or in situations where seismic recording is not suitable. The success of seismic monitoring will be determined by the magnitude of the change in the elastic properties of the reservoir during the lifecycle of CO2 storage. In the short-term, there will be a strong contrast in density and compressibility between free CO2 and brine. The contrast between these fluids is greater at shallower depth and higher temperature where CO2 resembles a vapour. The significant change in the elastic moduli of the reservoir will enable time-lapse seismic methods to readily monitor structural or hydrodynamic trapping of CO2 below an impermeable seal. Because the acoustic contrast between brine saturated with CO2 and brine containing no dissolved CO2 is very slight, however, dissolved CO2 is unlikely to be detected by any seismic technology, including high-resolution borehole seismic. The detection of increases in porosity, associated with dissolution of susceptible minerals within the reservoir may provide a means for qualitative monitoring of CO2 dissolution. Conversion of aqueous CO2 into carbonate minerals should cause a detectable rise in the elastic moduli of the rock frame, especially the shear moduli. The magnitude of this rise increases with depth and demonstrates the potential contribution that can be made from repeated shear-wave and multi-component seismic measurements. Forward modelling suggests that the optimal reservoir depth for seismic monitoring of CO2 storage within an unconsolidated reservoir is between 1,000 and 2,500 m. Higher reservoir temperature is also preferred so that free CO2 will resemble a vapour.

Monitoring CO2 response on surface seismic data; a rock physics and seismic modeling feasibility study at the CO2 sequestration site, Ketzin, Germany

“…If this analogy were to be used, effective elastic moduli of a dry rock could be formulated through porosity, average number of contacts per grain, grain dimension, and stiffness. Although most models are an idealistic simplification of real rock, in special cases, they show good agreement with measured data (McKenna et al, 2003).…”

Measurement and monitoring technologies for verification of carbon dioxide (CO2) storage in underground reservoirs

“…Time-lapse seismic does offer potential for useful quantification, but its efficacy is highly site-dependent. In general, storage in shallow, poorlyconsolidated reservoirs is more amenable to seismic quantification than in deeper more lithified rocks (McKenna et al, 2003).…”

A study of CO2 flooding on wave velocities in the Naharkatiya oil reservoir of Upper Assam Basin

“…vc and vs in a homogeneous and isotropic elastic material are defined as: The Gassmann [43,44] equation relates the bulk modulus of the saturated rock to the properties of the rock and the pore fluid,…”