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Abstract
The Moxa Arch Anticline is a regional-scale northwest-trending uplift in western Wyoming and it has been chosen for CO2 capture and storage. The Nugget Sandstone is a deep saline aquifer that has been a candidate for CO2 storage. In this paper we compare the amount of mineral and solution trapping in comparison with dynamic hysteresis trapping based on composi-tional simulation. To the best of our knowledge this is the first paper to computationally assess the chemical trapping in the Nugget formation and to compare these three trapping mechanisms against each other.

Reaction-path and kinetic modeling of CO2–brine–mineral reactions in the Nugget formation was investigated to probe the factors that affect capacity for CO2 chemical trapping. The solution and precipitation trapping of CO2 are functions of tem-perature, pressure, CO2 fugacity and brine composition. The geochemical simulation of this system was explored in order to assess how mineralogy might change and the relative importance of mineral and solution trapping phenomena through time. After 30 years, 0.06 g of CO2 per kg of reacted rock is sequestered as mineral phases and solution trapping amounts to 0.11 g/kg rock. In comparison, a recent computational study of the Rose Run sandstone, Ohio indicates a much higher (30 times higher) mineral trapping capacity, mainly because of the presence of glauconite as an iron source for siderite formation. The total hysteresis trapping in our study is 0.14 g/kg rock based on compositional simulation for the same period of time.

These results reveal that mineral trapping in the Nugget formation is not significant but that total chemical trapping might be as high as 80% of hysteresis trapping. Therefore, the contribution and importance of chemical trapping in CO2 sequestration should be taken into account for assessment of CO2 sequestration.

Introduction
CO2 sequestration is becoming one of the hot topics cross all disciplines. Most of countries in the world are spending large amount of money to investigate CO2 sequestration feasibility and shortcomings e.g. cap rock leakage. Underground forma-tion became the target for CO2-sequestraion such as abundant oil and gas fields, coal bed methane and saline aquifer. The saline aquifers have the maximum capacity. For instance in North America, saline aquifers capacity is 8±4 Billion metric tons, around 94% of total capacity while the mature hydrocarbon reservoirs has 4% of total capacity (DOE and NETL, Car-bon and Sequestration Atlas for the USA and Canada, 2008). All saline aquifers which have salinity above 10000 ppm are acceptable for CO2-sequestration based on U.S. Environmental Protection Agency. There have been a lot of researches on CO2-Sequestration in last decades all over the world; The Netherlands (Lohuis, 1993), Alberta basin, Canada (Bachu et al., 1994; Gunter et al., 2004; Cantucci et al., 2009), North Sea (Korbol and Kaddour, 1995) and USA (Zerai et al., 2006; Han et al., 2009).

Once CO2 is injected it might be trapped in different ways. Significant portion of CO2 might be trapped beneath caprock if caprock integrity is not compromised. CO2 can be dissolve in water which also donated as solubility trapping and also it might hydraulically trapped (hysteric trapping) and/or it might react with a rock and produce carbonate minerals (mineral trapping). Mineral trapping is the most stable trapping mechanism and there are a few studies examine the reaction of CO2 with host rock. These studies are mostly done by geologist while engineers have generally probe the other trapping mechan-isms. In fact, there is not robust interdisciplinary study that combines these two main disciplines and compares them. Howev-er, Nghiem (2009) simulated the trapping mechanisms with GEM software (CMG package).