Application of Tip-Enhanced Raman Spectroscopy for the nanoscale characterization of flooded chalk

One of the most challenging goals of flooded chalk analyses used in Enhanced Oil Recovery (EOR) is to reach high-resolution mineralogical data, in order to detect the composition of new crystals grown after brine injections, with ≤1 μm grain size. Understanding how flooding affects chemical induced compaction, mechanical strength permeability, and porosity is paramount in EOR related investigations. Magnesite formation is the most pervasive process when MgCl₂ is injected into chalk, and the submicrometric grain size of the new minerals requires a high performing imaging technique and a new methodological approach: in our study, we present the first attempt of applying Tip-Enhanced Raman Spectroscopy (TERS) to rock and mineral samples. It is a new frontier technique that couples Raman Spectroscopy with Atomic Force Microscopy, allowing impressively high-resolution topography and mineralogical maps. Two long term experiments have been analyzed, where chalk cores were flooded for 718 days and 1072 days, at reservoir conditions comparable to hydrocarbon reservoirs in chalk at the Norwegian continental shelf. Few microns squared areas have been imaged by Atomic Force Microscopy using ultra-polished thin sections. First analyses identified a less pervasive secondary growth of magnesite in the 718 days test and an almost pure magnesite composition in the 1072 days test. Transmission Electron Microscopy (TEM) has been employed to confirm the results of TERS and add dark and bright field grain imaging to the investigations. This confirms the need for high-resolution methodologies such as TERS and TEM to fully understand the EOR effects at submicron-scale.