A Closer Look at Hydrothermal Alteration and Fluid-Rock Interaction Using Scanning Electron Microscopy

Authors: Bridget LYNNE
Keywords: scanning electron microscopy, hydrothermal alteration, tracking reservoir conditions
Conference: World Geothermal Congress Session: Geology
Year: 2015 Language: English
Abstract: ABSTRACT Traditional techniques routinely used for hydrothermal alteration studies are X-ray diffraction (XRD) and petrographic microscopy. The addition of scanning electron microscopy (SEM) to these standard mineralogical analyses greatly enhances our knowledge and understanding of fluid-rock interactions in our geothermal systems. SEM observations on core from geothermal fields in New Zealand and USA show the advantages of adopting SEM to better understand subsurface processes. SEM imagery reveals detailed nano to micron scale information on crystal-crystal relationships. For example significant changes in subsurface environmental conditions such as cooling after boiling, or an increase in acidity at depth over time, can be tracked using SEM. The advantage of SEM is that it examines intact rock samples while XRD requires samples to be crushed and samples for petrographic thin sections are ground down to produce smooth surfaces. These crushing and grinding processes remove much of the mineralogical detail. SEM imagery also reveals crystal-clay relationships which are important, as in some settings clays attach to and alter crystal surfaces which influence porosity, permeability and rock strength. In other cases the clays do not alter crystal surfaces. Furthermore, SEM findings in this study include detail on clay-clay relationships, such as clay inter-bedding, differences in clay maturity, crystallinity and/or quantity. SEM observations allow fundamental questions to be addressed such as: (1) What influence do crystal and clay morphologies have on permeability? (2) How do crystal and clay inter-relationships affect permeability and/or rock strength? (3) How are the rocks reacting to ongoing changes in subsurface environmental conditions? (4) Are certain lithologies better suited for production or injection? (5) Which stratigraphic units are more likely to compress resulting in subsidence at the surface? SEM is a useful technique to examine fine-scale, fluid-rock interactions on core from production or injection wells.
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