Investigating the impact of carbon dioxide (CO2) injection and leakage on seismic wave velocity behavior: An analytical and Early detection approach using the Gassmann equation

Researchers commonly employ seismic analyses and fluid-substitution techniques to track subsurface carbon
capture storage (CCS) and map the likely distribution of CO₂ within the storage reservoir. The fluid-substitution
rock-physics model leverages well-log data and injection-simulation outputs to infer gas distribution, based on a
seismic velocity model. This work introduces new insights for making more precise predictions about how CO₂
injection — and subsequent CO₂–brine–rock interactions — alter the geophysical and petrophysical properties of
reservoir rocks, which in turn supports earlier detection of potential leaks. Including a dedicated model for CO₂
leakage improves the fidelity of geophysical predictions by simulating how leak-induced changes affect seismic
wave speeds, thereby allowing the identification of distinctive velocity anomalies associated with leak zones. By
carefully choosing relevant physical parameters — such as fluid saturation, pressure, and gas composition — and
tracking their influence on wave propagation, the model offers a more realistic representation of subsurface
conditions and strengthens early warning of possible leakage pathways. We developed a complete workflow for
rock-physics modelling that combines geological and geophysical interpretations. This includes deriving elastic
parameters from measured data, generating synthetic seismic data, and applying fluid-substitution analysis
informed by CO₂ injection simulation results. We then apply the Gassmann equation to construct a new postinjection
velocity model, incorporating other elastic parameter changes to assess the detection threshold of
seismic velocity alterations. Because the Gassmann theory reliably models P-wave velocity in sandstone reservoirs
saturated with mixtures of supercritical CO₂ and brine, it enables forecasting how seismic responses vary
with different levels of gas saturation. Because acoustic impedance is sensitive to gas saturation changes,
observed deviations from expected impedance values may reveal abnormal fluid distributions — potentially
indicating zones of CO₂ leakage.