Evolution of the Petrophysical and Mineralogical Properties of Two Reservoir Rocks Under Thermodynamic Conditions Relevant for CO2Geological Storage at 3 km Depth

Abstract

Injection of carbon dioxide (CO2 ) underground, for long-term geological storage purposes, is considered as an economically viable option to reduce greenhouse gas emissions in the atmosphere. The chemical interactions between supercritical CO2 and the potential reservoir rock need to be thoroughly investigated under thermodynamic conditions relevant for geological storage. In the present study, 40 samples of Lavoux limestone and Adamswiller sandstone, both collected from reservoir rocks in the Paris basin, were experimentally exposed to CO2 in laboratory autoclaves specially built to simulate CO2 -storage-reservoir conditions. The two types of rock were exposed to wet supercritical CO2 and CO2 -saturated water for one month, at 28 MPa and 90°C, corresponding to conditions for a burial depth approximating 3 km. The changes in mineralogy and microtexture of the samples were measured using X-ray diffraction analyses, Raman spectroscopy, scanning-electron microscopy, and energy-dispersionspectroscopy microanalysis. The petrophysical properties were monitored by measuring the weight, density, mechanical properties, permeability, global porosity, and local porosity gradients through the samples. Both rocks maintained their mechanical and mineralogical properties after CO2 exposure despite an increase of porosity and permeability. Microscopic zones of calcite dissolution observed in the limestone are more likely to be responsible for such increase. In the sandstone, an alteration of the petrofabric is assumed to have occurred due to clay minerals reacting with CO2 . All samples of Lavoux limestone and Adamswiller sandstone showed a measurable alteration when immersed either in wet supercritical CO2 or in CO2 -saturated water. These batch experiments were performed using distilled water and thus simulate more severe conditions than using formation water (brine).