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Multiphysics Investigation of Geochemical Alterations in Marcellus Shale Using Reactive Core-Floods

2021    Gundogar, Asli S.; Ross, Cynthia M.; Jew, Adam D.; Bargar, John R.; Kovscek, Anthony R.

The interaction of reactive fracture fluid with host shale and formation water plays an important role on fractured reservoir productivity. This study explores the prominent impacts of shale-fluid reactions on flow properties using representative core-flood experiments under confining stress. Alteration of shale is monitored using time-lapse X-ray computed tomography (CT), microCT (mu CT) of samples pre- and post-reaction, and scanning electron microscopy (SEM). The imaging approach is multiscale from nm's to cm's. The samples are clay-rich and partially fractured Marcellus outcrop and carbonate-rich MSEEL (Marcellus Shale Energy and Environmental Laboratory) downhole endmembers. Both samples have distinct microcracks for probing reactive transport in fractures communicating with matrices. A reduction in krypton-accessible CT porosity and liquid permeability was observed for both samples after fracture fluid exposure. Based on SEM-EDS surface analysis, an iron-bearing precipitate formed on and near fracture openings and in the shale matrix of the Marcellus outcrop indicating partial dissolution of pyrite and/or ferruginous dolomite followed by precipitation of iron (hydro)oxide. The compiled images reveal fracture filling with migrated and/or precipitated fine particles. Significant barite scale growth was detected on the reacted MSEEL surfaces together with halite and other (hydro)oxide precipitates resulting from geochemical reactions between the basin-specific injectants and shale minerals. The MSEEL sample experienced substantial calcite dissolution and a corresponding decrease in its bulk density and microcrack openings. Experimental results presented here indicate the significance of fracture fluid composition optimization based on intrinsic shale and resident brine chemistries.