46th U.S. Rock Mechanics/Geomechanics Symposium,
2012. American Rock Mechanics Association
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Fractures in wellbore cement and at the cement-host rock interface are potential leakage pathways for long-term carbon sequestration sites. Portland cement exposed to carbon-dioxide-rich brine undergoes a series of diffusion-limited reactions that form distinctive reaction fronts adjacent to the cement surface. This paper outlines a joint experimental and numerical modeling effort investigating the formation of these reaction fronts and their impact on fracture transmissivity. Prepared by LLNL under Contract DE-AC52-07NA27344.
Fractures in wellbore cement and at the cement-host rock interface have been identified as potential leakage pathways for carbon dioxide in long-term sequestration sites [1-5]. In the presence of acidic carbon-dioxide-rich brines, the alkaline Portland cement degrades - causing distinctive layers of reacted material to form adjacent to the cement surface. To explain the formation of the reaction layers, Kutchko and co-workers [4,5] proposed a three stage mechanism for the degradation of Portland cement in the presence of carbon-dioxide rich brines: 1) Portland cement is largely comprised of calcium-silicate hydrate (CSH) and Portlandite (calcium hydroxide or CH) crystals. When the brine comes into contact with the cement, calcium leaches from the calcium hydroxide crystals. 2) Next the dissolved calcium reacts with the carbonic acid in the brine, precipitating calcium carbonate. 3) The calcium carbonate initially protects the CSH from the brine. However, once the acidic brine depletes the calcium carbonate, the CSH dissolves – leaving a high porosity amorphous silicate region with low material strength. As a consequence of these diffusion-limited reactions, the cement-brine boundary becomes divided into distinct regions separated by thin fronts where the reactions take place. Predicting the effect of these regions on fracture transmissivity is non-trivial, as the changes induced in the cement composition affect material strength and, consequently, the geometry of the fracture surface.
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