• Energy Research

Abstract A geochemical numerical modeling study was conducted to constrain processes occurring in field and laboratory experiments, simulating CO 2 leakage from geological storage on shallow potable aquifers. A leak was previously physically simulated in a shallow potable aquifer at Vrogum plantation, Western Denmark by injection of 1600 kg of gas phase CO 2 over 72 days. Here, a 1-dimensional reactive transport model was constructed based on field and laboratory results and subsequently used to explore the contributions of various geochemical processes to explain observed results from the carbonate free system. Finite gibbsite derived Al 3+ driven cation exchange is able to explain the majority of water chemistry change observed at Vrogum including: a pulse like effect showing a fast peak and return toward background levels for alkalinity and dissolved ion concentrations; and increasing and persistent acidification via buffering exhaustion. Model processes were supported further by simulation of a batch experiment conducted on the Vrogum glacial sand, employing the same processes and sediment parameters. The fitted reactive transport model was subsequently used to extend predictions and explore various scenarios. Extended predictions suggest the pulse of elevated ions travels with advective flow succeeded by a zone of increasing acidification. Model runs at higher P CO2 (implying greater depths) suggest amplification of effects, i.e., greater peaks and more rapid and severe acidification. Calcite limits acidification, however, induces additional Ca driven ion exchange giving rise to more significant chemistry change. Although a site specific model, results have significant implications for risks posed to water resources from CCS leakage and implementation of MMV programs.

Abstract A shallow CO2 injection experiment was conducted in an unconfined, unconsolidated siliclastic aquifer in western Denmark. The aims were to test injection and sampling systems, confirm the conceptual hydrogeological site model and determine the aquifers potential geochemical response to a larger scale, sustained CO2 injection in order to finalize design of the main release experiment. Food grade CO2 (45 kg in 48 h) was injected at 10 m depth into glacial sand and water chemistry subsequently monitored. Results indicate the injection system effectively delivered CO2 gas into the glacial sand layer where it dissolved and moved with advective flow. Dissolved CO2 was not detected in the aeolian sand (0–5 m depth) indicating prevention of migration due to permeability heterogeneities. Dissolved CO2 in the glacial sand (5–10 m depth) created a plume of depressed pH (5.6–4.7), elevated EC (166–304 μS/cm) and concurrent increases in dissolved ion concentrations. EC was the most effective indicator for presence of dissolved CO2. Ionic concentration changes were generally slight with increases in Al, Ba, K, Na, Mg, Si, Sr and Zn forming the major changes. Water quality was not significantly affected and risks from small scale, short duration CO2 contamination appear minimal for this geological setting.

Abstract The consequences of CO2 leakage from geological sequestration into shallow aquifers must be fully understood before such geo-engineering technology can be implemented. A series of CO2 exposure batch reactor experiments were conducted utilizing 8 sediments of varying composition obtained from across Denmark including; siliceous, carbonate and clay materials. Sediments were exposed to CO2 and hydro-geochemical effects were observed in order to improve general understanding of trace metal mobility, quantify carbonate influence, assess risks attributable to fresh water resources from a potential leak and aid monitoring measurement and verification (MMV) program design. Results demonstrate control of water chemistry by sediment mineralogy and most significantly carbonate content, for which a potential semi-logarithmic relationship with pH and alkalinity was observed. In addition, control of water chemistry by calcite equilibrium was inferred for sediments containing >2% total inorganic carbon (TIC), whereby pH minima and alkalinity maxima of approximately 6 and 20 mequiv./l respectively were observed. Carbonate dominated (i.e. >2% TIC) and mixed (i.e. clay containing) sediments showed the most severe changes in water chemistry with large increases in all major and trace elements coupled to minimal reductions in pH due to high buffering capacity. Silicate dominated sediments exhibited small changes in dissolved major ion concentrations and the greatest reductions in pH, therefore displaying the greatest propensity for mobilization of high toxicity pH sensitive trace species.