Abstract:

During CO2 sequestration, petrophysical and geomechanical alteration of host formations can occur due to mineral dissolution and precipitation. At the core scale, laboratory tests have shown that sandstone or siltstone can be weakened through either naturally or artificially CO2-related alteration, but few quantitative studies have been performed to fully investigate the mechanism. Based on the hypothesis that CO2 alters the integrity of rock structure by changing cements rather than grains, we attempt to explain the degradation of cementation due to long-term contact with CO2 and how it leads to changes in rock mechanical properties. Pore-scale modeling by means of the Discrete Element Method (DEM) is used to perform the numerical simulations. The model is verified against the analytical Cavity Expansion Model (CEM) and validated against an experimental indentation test on sandstone without CO2-alteration. The experimental result for the same sandstone but with CO2-alteration is reproduced by fitting one microscopic parameter of cementation under the same simulation setup. The result demonstrates that the CO2-related degradation of rock properties can be numerically attributed to the weakening of cementation microscopic properties. Our study indicates that it is possible to describe the CO2-related degradation through particle-scale mechanisms.

Introduction

Geologic sequestration of CO2 is proposed as an economically viable approach to mitigate the CO2 emissions from fossil fuel consumption. Experiments at both field and laboratary scales have shown that injected CO2 is able to change the species composition through mineral dissolution and precipitation (Hovorka et al., 2006, 2013; Carroll et al., 2011; Lu et al., 2012). The occurrence of geochemical reactions initiated by the dissolution of CO2 can have either positive or negative impact. On the one hand, the trapping of CO2 could benefit from some of the reactions transforming the dissolved CO2 into new carbonate minerals (Bachu et al., 2007). On the other hand, other reactions may jeopardize the structure integrity of reservoir and caprock formations by altering the petrophysical and geomechanical properties (Rutqvist and Tsang, 2002; Espinoza et al., 2011; Rinehart et al., 2014; Renard et al., 2008). Positive feedback mechanisms leading to rock degradation at CO2 storage sites may facilitate the leakage of CO2 from host formations.

At the core scale, laboratory tests have shown that sandstones and siltstones can be weakened through either naturally or artificially CO2-related alteration (Sun et al., 2016), which may be the result of dissolution of interparticle cement (Major et al., 2014). Core-flood experiments are a platform for the artificially CO2-related alteration but with the limitation of extended experiment durations and complexity (Carroll et al., 2011, 2013; Rinehart et al., 2014). Autoclave experiments are easy to perform whereas the penetration depth of CO2 is limited to millimeters (Rimmelé et al., 2010). Thus, micromechanical tests which can probe the reacted skin provide a diagnostic tool for rocks reacted in autoclave experiments (Sun et al., 2016). Indentation tests have been widely adopted to study the mechanical behavior of different materials (Oliver and Pharr, 2004; Kumar et al., 2012). The assessment of mechanical degradation of the rocks subject to CO2-related alteration can be easily and quickly achieved by indentation tests.

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