The effects of chemical interaction between injected CO2, brine, and formation rocks are often ignored in sequestration studies because chemical reactions are assumed to be localized to carbonate rocks that make up only a small proportion of the potential reservoirs. It is conjectured in this work that long-term exposure of certain types of clays and cement material to CO2-brine mixtures can induce chemical reactions and subsequent alteration of rock properties that can be subsequently detected in time-lapse seismic surveys. This is demonstrated using a case-study structured after the Cranfield field injection site. Geochemical alterations of the reservoir rock are quantified by performing reactive transport simulations and subsequently rock physics models are used 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 infer the reservoir regions contacted by the CO2-saturated brine based on the observed seismic response and a unique model selection algorithm.

Time-lapse or passive seismic monitoring is an effective method for mapping the progress of the CO2 plume through the subsurface. But, because of the lack of resolution of the seismic information, it is necessary to use the seismic information together with prior geologic knowledge about the surface in order to identify if there is any migration of CO2 into regions that might be deemed sensitive e.g. overlying aquifers or faults. Because of uncertainties in the prior geologic description of the reservoir, the feasibility of implementing a model selection process is explored in this work. The model selection procedure utilizes the observed well data and reference seismic map to select a subset of models that represent the posterior geologic uncertainty. The flow simulation of CO2 injection and forward seismic modeling were repeated on the posterior models, and the seismic responses were compared for the reaction and non-reaction cases.

The study showed that the effects of geochemical reactions and the resultant impact on petrophysical properties and fluid saturation distribution were visible in the seismic response. Major differences in seismic responses were detected in regions of the reservoir where significant amount of minerals were dissolved and precipitated. These regions were at 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 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.

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