TEM, SEM and optical microscopy analyses of Berea sandstone cores flooded with sodium silicate polymers for IOR purposes

Abstract

Improved Oil Recovery (IOR) is a “hot topic” in the Oil and Gas industry. Sodium silicate (SS) is a polymer which can be injected into a reservoir as a fluid and due to polymerization it precipitates into a semi rigid gel under given circumstances. However, the knowledge about this polymer on a micron- and nano-scale is restricted. Berea sandstone cores with permeabilities of 500 mD and 200 mD were flooded with SS and studied by various methods- optical light microscopy (OLM), geochemistry (GC), x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and transmission electron microscopy (TEM). At low magnifications (OLM) a white substance was first observed in the pore space in both flooded and unflooded cores– the “before” and “after” flooding had very similar features. Utilizing XRD analysis, the bulk-investigation showed little impact of SS – however, it stated the presence of quartz, kaolinite, orthoclase, albite, and rutile. GC analysis revealed a significant increase in iron (from less than 1% to almost 16%) and in the entire range of trace elements after adding HCl to the SS solution. Furthermore, GC data indicated an increase of SS in the flooded contra the unflooded core material. SEM-EDS provided 3D insight into the appearance of dried SS solution before and after mixing with HCl, showing acicular tube-like crystallization as well as NaCl precipitation after adding HCl. EDS-measurements of the flooded core revealed a dramatic decrease in Na-content compared to the solution before injection – in many cases the content was nearly absent. High resolution scan of a FIB/SEM-sample in a TEM showed the variation of Na-content within a single agglomerated gel-particle grown on a quartz-grain, on a nano-scale. EDS point-scans of the same sample area presented spectacular results with approximately identical Si and O content for quartz and the core of the SS-particle, with no Na present, whilst the rim of the SS-particle contained Na. This phenomenon of extraordinary fractionated gelling with a center and a rim was indicated by TEM-EDS. For further studies, there is a need for systematic testing with a series of experiments conducted on the same sample, as well as a thorough analysis of the effluent. However, the application of routinely used methods for mineralogical changes related to EOR research was highly successful in imaging a fractionation of polymer composition. This will have enormous significance for wettability and surface charge interactions.