A new frontier technique for nano-analysis on flooded chalk - TERS (Tip Enhanced Raman Spectroscopy)

Understanding the chalk-fluid interactions at the sub-micron scale is one of the most challenging goals in Enhanced Oil Recovery. The grain size of newly grown minerals far below 1 micron asks for a high performing imaging: we present a new methodology using the TERS (Tip Enhanced Raman Spectroscopy), a new frontier technique that combines Raman Spectroscopy with Atomic Force Microscopy, allowing impressively high-resolution chemical analyses down to an outstanding spatial resolution (∼ 20 nm). TERS permits the recognition of minerals thanks to the vibrational mode peaks that are diagnostic of composition and structure. Carbonate-group minerals are easily identified by Raman spectroscopy. First analyses allow us to state that magnesite and calcite could be identified in, respectively, ultra-long-term flooding experiments of chalk at reservoir conditions and in unflooded samples; no dolomite or high Mg-calcite have been found. Few microns squared areas have been imaged by AFM using ultra polished thin sections with a 50 nanometers step. Transmission electron microscopy 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 methodology such as TERS and TEM to fully understand EOR effects at sub-micron scale.