Quantification of mineralogical changes in flooded carbonate under reservoir conditions

Injection of seawater-like brines is one of the most successful EOR methods on the Norwegian Continental Shelf. Aqueous chemistry affects the mechanical strength of chalk. The injected seawater might trigger several mechanisms simultaneously and the importance of each mechanism is not fully understood. The aim of this study is to obtain an improved understanding of EOR mechanisms at pore scale by studying new mineral phases when flooding chalk with MgCl2 at reservoir conditions (130°C, 1 PV/day, 11.3 MPa effective stress). Two chalk cores were investigated, one artificial and one outcrop chalk. FE-SEM, STEM, and EDS-analyses show newly formed magnesite growing on calcite surfaces after 27 days. The Mg/Ca interphase is sharp, no diffusion of elements is observed on Ångström scale, and flooding experiments change the crystallography of phases. Whole-rock geochemistry of the Liége outcrop chalk flooded with MgCl2 for 3 years reveals a MgO content of c. 42 wt.%, but still c. 4 wt.% CaO. STEM mapping shows that CaO impurities are present in MgO dominated phases. These experiments confirm that magnesite grows as a new mineral phase even after short term flooding and that calcium is still present as impurities in the magnesite after long term flooding.