13/07/2023

Dr. Chaojie Cheng

16:00 Uhr

Enhancing the utilization of reservoirs for energy production and rare-earth element (REE) extraction holds significant promise for fostering a sustainable and environmentally friendly economy. Prominent avenues for achieving this goal include, such as geothermal energy utilization, aquifer thermal energy storage (ATES), and lithium extraction from geothermal fluids. However, these applications introduce challenges with respect to maintaining reservoir performance (e.g., permeability). Geological formations hosting these reservoirs experience disruptions to their thermal and/or chemical equilibrium due to variations in environmental conditions, consequently impacting clay minerals present within the formations. Therefore, temperature changes and variations in pore fluid salinity may negatively affect the permeability of clay-bearing sandstones with implications for natural fluid flow and geotechnical applications alike. It is imperative to develop a comprehensive understanding of the underlying mechanisms driving these impairments in permeability. Such knowledge will play an important role in minimizing potential risks and ensuring the sustained effectiveness of these reservoirs.

In this study, these factors are investigated for a sandstone dominated by illite as the clay phase. Systematic long-term flow-through experiments were conducted and complemented with comprehensive microstructural investigations and the application of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to explain mechanistically the observed permeability changes. Initially, sample permeability was not affected by low pore fluid salinity indicating strong attraction of the illite particles to the pore walls as supported by electron microprobe analysis (EMPA). Increasing temperature up to 145 °C resulted in an irreversible permeability decrease by 1.5 orders of magnitude regardless of the pore fluid composition (i.e., deionized water and 2 M NaCl solution). Subsequently, diluting the high salinity pore fluid to below 0.5 M yielded an additional permeability decline by 1.5 orders of magnitude, both, at 145 °C and after cooling to room temperature. By applying scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) thermo-mechanical pore throat closure and illite particle migration were identified as independently operating mechanisms responsible for observed permeability changes during heating and dilution, respectively. These observations indicate that permeability of illite-bearing sandstones will be impaired by heating and exposure to low-salinity pore fluids. In addition, chemically induced permeability variations proved to be path dependent with respect to the applied succession of fluid salinity changes.