Fractured core experiments to study water-rock-cement interaction under CO2 storage conditions
In wellbores used for CO2 injection in Geological Carbon Storage (GCS), Portland cement is placed between the steel casing and the surrounding rock to prevent gas or fluid leakages. During and after CO2 injection, the resulting CO2-rich acid water may deteriorate the cement and favour undesired leaking. The objective of this study is to understand the processes that take place in the contact between Portland cement and sedimentary rocks under GCS conditions. In this study, artificially fractured cores made of a half cement cylinder and a half sedimentary rock cylinder (limestone, sandstone and marl) were reacted with synthetic brines at 25ºC and 10-3.4 bar CO2 and 60ºC and 130 bar CO2 (atmospheric and supercritical CO2 conditions, respectively) by means of percolation experiments. Variation in the aqueous chemistry (concentrations of Ca, SO4, Mg, K, Na, Si, and Al) was monitored over time. At the end of the experiments, the fractured cores were examined by SEM-EDS, XRD, and XCMT to evaluate the changes in the mineralogical content and structure. Results showed a pH increase in all injected solutions: up to 7-11 in the atmospheric experiments and up to 6 under supercritical CO2 conditions. Ca and Si output concentrations were also higher than the initial ones, whereas a sulphate deficit was observed. Dissolved calcium and silicon and removal of sulphate were higher under supercritical CO2 conditions. Overall, under all conditions, the release of Ca and Si was attributed to dissolution of portlandite and C-S-H from the cement, and the sulphate depletion was caused by gypsum precipitation. The SEM-EDS analyses showed a significant alteration of the cement and rock surfaces along the fracture walls. On the rock side, dissolution of calcite and precipitation of aragonite and gypsum were detected. On the altered cement wall, precipitation of aragonite and gypsum were identified. Under atmospheric conditions, an extensive layer of a hydrated Mg phase was formed. Under supercritical CO2 conditions, an increase in cement porosity and fracture aperture was visible. The observed chemical and mineralogical changes allow us to compare the hydrogeochemical response of artificially fractured cores by changing temperature and PCO2. Under CO2 supercritical conditions, the main chemical processes were dissolution of calcite and gypsum precipitation. Cement degradation occurred through dissolution of portlandite and C-S-H. Under atmospheric conditions, similar alteration processes occurred although at a slower rate, along with the formation of a Mg hydrated layer on the cement and rock fracture walls. These experiments are providing evidence of the reactivity of cement and sedimentary rock fractures to be considered in the management of GCS systems. Moreover, 2D reactive transport simulations of the variations in the experimental aqueous chemistry and core mineralogy will be used to quantify the kinetics of the acid brine-cement-rock interaction and to evaluate the integrity of the cemented annulus in GCS wellbores. Acknowledgements This study was financed by projects CGL2017-82331-R (Spanish Ministry of Economy and Competitiveness), with contribution of FEDER founds, and 2017SGR 1733 (Catalan Government).
| Main Authors: | , , , |
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| Format: | actas de congreso biblioteca |
| Published: |
2019-03-25
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| Online Access: | http://hdl.handle.net/10261/204463 |
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