Geochemical alteration of wellbore cement by CO2 or CO2 + H2S reaction during long‐term carbon storage

Abstract

AbstractCement samples were reacted with CO2‐saturated synthetic groundwater, with or without added H2S (1 wt.%), at 50°C and 10 MPa for up to 13 months (CO2 only) or for up to 3.5 months (CO2 + H2S) under static conditions. After the reaction, X‐ray computed tomography (XCT) images revealed that calcium carbonate (CaCO3) precipitation occurred extensively within the fractures in the cement matrix, while micro‐fractures with aperture size <∼50 μm at the cement‐basalt interface were completely sealed by CaCO3 precipitation. Exposure of a fractured cement sample to CO2‐saturated groundwater (50°C and 10 MPa) over a period of 13 months demonstrated progressive healing of cement fractures by CaCO3 precipitation. After reaction with CO2 + H2S‐saturated groundwater, CaCO3 precipitation also occurred within the cement fracture as well as along the cement‐basalt caprock interfaces. X‐ray diffraction analysis showed that major cement carbonation products of the CO2 + H2S‐saturated groundwater were calcite, aragonite, and vaterite, all consistent with cement carbonation by CO2‐saturated groundwater. While pyrite is thermodynamically favored to form, due to the low H2S concentration it was not identified by XRD in this study. The cement alteration rate into neat Portland cement samples by CO2‐saturated groundwater was similar at ∼0.02 mm/d based on XCT images, regardless of the cement‐curing pressure and temperature (P‐T) conditions, or the presence of H2S in the groundwater. The experimental results imply that wellbore cement with micro‐fractures within the cement matrix or along the cement‐caprock interface (<∼200 μm aperture under current experimental conditions) is likely to be healed by CaCO3 precipitation during exposure to CO2‐ or CO2 + H2S‐saturated groundwater. Published 2016. This article is a U.S. Government work and is in the public domain in the USA