Investigating the Geochemical Alterations in an Aquifer Due to Long-Term Sequestration of CO2 Using Time-Lapse Seismic Information

The effects of chemical interaction between injected CO₂, brine, and formation rocks are often ignored in sequestration studies because carbonate rocks, which are reactive, make up only a small proportion of the potential reservoirs. The paper provides a case-study example of simulating and quantifying geochemical alterations of the reservoir rock at the Cranfield field injection site and then using rock physics models to translate the altered petrophysical properties into seismic responses. The study quantifies the long-term geochemical effects of CO₂ injection on the seismic response and conversely, presents an approach to invert the reservoir regions contacted by the CO₂saturated brine based on the observed seismic response.

Petrophysical properties and mineralogy interpreted from well logs were used within a geostatistical modeling scheme in order to characterize a reservoir. CMG-GEM, a commercial compositional simulator, was used to simulate CO₂ injection into the reservoir over a two-year period, and the geochemical effects were observed for the subsequent 148 years. A comparative simulation ignoring reactions was also performed using the same data. Subsequently, rock physics models and forward seismic models were used to model the porosity and saturation changes and their corresponding seismic signatures at different time steps for both the reaction and no-reaction cases. Because of uncertainties in the reservoir model characterized using geostatistical methods, more models were generated and refined through a model selection processes. The model selection procedure utilizes the observed well data and reference seismic map to select a subset of models. The flow simulation of CO₂injection and forward seismic modeling were repeated for the newly generated reservoir models, and the seismic responses were compared for the reaction and non-reaction cases.

The study showed that the effects of geochemical reactions on petrophysical properties and resultant spatial distribution of fluid saturation were visible in the seismic response. The major differences in seismic responses were detected in regions of the reservoir where significant amount of minerals were dissolved and precipitated. Major differences in the seismic response were also observed along the top of the reservoir due to the reactions caused by the buoyant CO₂ plume. The presence of carbonate facies, even in small proportion, plays an important role in geochemical reactions and their effect is manifested at the seismic scale. The unique model selection methodology presented in this paper is efficient at detecting the important features in the seismic and injection response that is induced by the geochemical alterations occurring in the reservoir. The results of this time-lapse study can provide new interpretation of events observed in time-lapse seismic data that might lead to a better assessment of leakage pathways and other risks.