Response of the Paleocene sand reservoir and seal of the Nini Field to CO₂ - results of batch experiments. Project Greensand Phase 1 WP2

Storage of CO₂ in the subsurface is one mean of effectively remove CO₂ from the atmosphere and reduce climatic changes. The Nini West Field, Danish Central Graben, represents a potential CO₂ storage site. A series of batch experiments have been conducted to investigate the hydrogeochemical reactions between CO₂, and the Paleocene sandstone reservoir of the Nini West Field, Danish Central Graben, and the overlying seal mudstone. Cleaned and uncleaned samples from the oil leg were investigated. To distinguish between the reactions caused by disequilibrium with the brine and those originating from CO₂ exposure, experiments with CO₂ saturated brine as well as brine without CO₂ were performed. Water samples were extracted continuously during the experiments to monitor the changes in the chemical composition of the brine after 4, 10, 14, 26, 46, 61 and 83 days of experimental run. Comprehensive post experimental back-calculations to experimental conditions were made to estimate the true chemical concentrations at experimental conditions.
Mineralogical changes during the experiments were identified by petrographic analysis of the rock material prior to and after termination of the experiments. Subsequently, geochemical models were made to help identify the chemical reactions between CO₂ and reservoir rock material and between CO₂ and the seal rock mudstone. The models were calibrated against the observed changes in the brine composition measured.
For both reservoir and seal, the most noticeable changes in the water chemistry are a decrease in the pH and an increase in the alkalinity and the aqueous concentrations of Fe2+, Mn and Si.
For the reservoir, the results of the geochemical modelling show that the presence of CO₂ leads to:
• A significant increase in the initial dissolution of calcite followed by reprecipitation of calcite. The model does not predict calcite reprecipitation in the absence of CO₂. Calcite dissolution is not confirmed by the petrographic analysis, but is never the less crucial to fit the modelled concentrations of the components of the carbonate system (pH, alkalinity and Ca) to the measured values.
• An increase in the dissolution of Mn-bearing siderite followed by precipitation of a pure Fe-siderite. The model does not predict reprecipitation of siderite in the absence of CO₂. Alternatively, the relatively lower release of Fe to solution compared to Mn could potentially result from dissolution of a siderite with a different chemical composition or reprecipitation of ironhydroxides.
• Dissolution of an Fe-containing mineral. In the model berthierine has been applied.
• Mass balance considerations of the CO₂ and reservoir system indicate that only small fractions of the reservoir rock react during the experiment. Thus, the composition of the sandstone after the experiments calculated by mass balance considerations of the results of the geochemical model equals that determined by point counting prior to the experiment.

The presence of hydrocarbons does not seem to affect the nature of the dissolution/precipitation processes between the Paleocene sandstone reservoir of the Nini West Field and CO₂. The presence of hydrocarbons may affect the reaction rates; however, it is uncertain if this is due to heterogeneities or that the hydrocarbons block some of the reactive sites on the minerals.
The results of the geochemical model indicate that for the seal, the presence of CO₂ may lead to:
• Increased dissolution of calcite and siderite.
• Dissolution of an Fe-bearing mineral. The identification of this mineral is uncertain. In the model illite was used.
• It is unclear whether heulandite dissolves or precipitates in the presence of CO₂ the petrographic analysis contradicts the model.
• Based on mass balances it is estimated only an insignificant fraction of the seal mud-stone has reacted during the experiment. Thus, the composition of the seral mud-stone after the experiments calculated by mass balance considerations of the results of the geochemical model equals that determined by point counting prior to the experiment.

The results of this study suggest that only minor constituents of the reservoir and seal rock material is reactive towards CO₂. In both the reservoir and seal experiments reactions associated with the initial brine chemical composition and the rock matrix were observed suggesting that the starting brine composition estimated from the produced water should be altered in future experiments. We suggest using the chemical composition measured at the end of the experiments i.e., at day 83 without CO₂ as a starting point in new experiments in order to be as close to equilibrium conditions in the reservoir and seal.