Wellbore cement fracture evolution at the cement–basalt caprock interface during geologic carbon sequestration Hun Bok Jung a , Senthil Kabilan a , James P. Carson a , Andrew P. Kuprat a , Wooyong Um a , Paul Martin a , Michael Dahl a , Tyler Kafentzis a , Tamas Varga a , Sean Stephens a , Bruce Arey a , Kenneth C. Carroll b , Alain Bonneville a , Carlos A. Fernandez a,⇑ a Pacific Northwest National Laboratory, P.O. Box 999, 902 Battelle Boulevard, Richland, WA 99354, United States b New Mexico State University, Las Cruces, NM 88003, United States article info Article history: Available online 9 May 2014 Editorial handling by M. Kersten abstract Composite Portland cement–basalt caprock cores with fractures, as well as neat Portland cement col- umns, were prepared to understand the geochemical and geomechanical effects on the integrity of wellb- ores with defects during geologic carbon sequestration. The samples were reacted with CO 2 –saturated groundwater at 50 °C and 10 MPa for 3 months under static conditions, while one cement–basalt core was subjected to mechanical stress at 2.7 MPa before the CO 2 reaction. Micro-XRD and SEM–EDS data col- lected along the cement–basalt interface after 3-month reaction with CO 2 –saturated groundwater indi- cate that carbonation of cement matrix was extensive with the precipitation of calcite, aragonite, and vaterite, whereas the alteration of basalt caprock was minor. X-ray microtomography (XMT) provided three-dimensional (3-D) visualization of the opening and interconnection of cement fractures due to mechanical stress. Computational fluid dynamics (CFD) modeling further revealed that this stress led to the increase in fluid flow and hence permeability. After the CO 2 -reaction, XMT images displayed that calcium carbonate precipitation occurred extensively within the fractures in the cement matrix, but only partially along the fracture located at the cement–basalt interface. The 3-D visualization and CFD mod- eling also showed that the precipitation of calcium carbonate within the cement fractures after the CO 2 -reaction resulted in the disconnection of cement fractures and permeability decrease. The perme- ability calculated based on CFD modeling was in agreement with the experimentally determined perme- ability. This study demonstrates that XMT imaging coupled with CFD modeling represent a powerful tool to visualize and quantify fracture evolution and permeability change in geologic materials and to predict their behavior during geologic carbon sequestration or hydraulic fracturing for shale gas production and enhanced geothermal systems. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Carbon dioxide (CO 2 ) capture and storage technology in deep geologic formations including oil and gas reservoirs, deep saline aquifers, basalt formations, coal seams, and salt caverns has been proposed to decrease atmospheric CO 2 concentrations and mitigate global warming. During geologic carbon storage, potential CO 2 leakage can occur through wellbores, and damage underground sources of drinking water and consequently human health, as well as the ecosystem (Bruant et al., 2002; Little and Jackson, 2010; Wilkin and Digiulio, 2010). Portland cement is commonly consid- ered as a sealing material for carbon storage sites. During typical well construction, cement slurry is placed in the annulus between steel casing and formation rocks to prevent vertical fluid migration and to provide mechanical support (Nelson and Guillot, 2006). Hydrated products formed by mixing Portland cement with water are a semi-amorphous gel-like calcium–silicate–hydrate (C–S–H), and a crystalline phase of portlandite [Ca(OH) 2 (s)] (Neville, 2004; Nelson and Guillot, 2006). Potential leakage pathways of stored CO 2 may occur at the interface between casing and cement, cement plug and casing, and cement and host rock, or through the cement pore spaces and fractures (Gasda et al., 2004; Jung et al., 2013; Jung and Um, 2013). Wellbore cement may contain fractures and defects because of changes in pressure and temperature within the wellbore during field operation, cement shrinkage during dehydration, mechanical shock from pipe tripping, poor cement slurry placement, and residues of drilling mud and drill cuttings (Zhang and Bachu, 2011). After well completion, changes in down- hole conditions can also induce sufficient stresses to damage the http://dx.doi.org/10.1016/j.apgeochem.2014.04.010 0883-2927/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 (509) 371 7020; fax: +1 (509) 375 2186. E-mail address: carlos.fernandez@pnnl.gov (C.A. Fernandez). Applied Geochemistry 47 (2014) 1–16 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem