Permeability Variations in Illite‐Bearing Sandstone: Effects of Temperature and NaCl Fluid Salinity

Abstract

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. 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.

Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)

“…For particles of similar size, this would lead to a difference of drag forces between the two fluid compositions of about 4%, which is unlikely to explain the lower permeability obtained with NaCl compared to DI. In addition, high-salinity fluid would increase the stability of mobile particles due to the increase in the electrostatic force [35]. In this case, with the assumption of fines migration, permeability should present less reduction upon fluid flow with NaCl, which is opposite to our experimental results.…”

“…We noticed that the initial aperture changes of sample SM3 were significantly larger than that of the other two samples. However, the aperture closure rate ∆𝑏 ̇= 𝑛𝛼𝑡 𝑛−1 , of sample SM3 decreased much faster and was smaller than that of SM1 and SM2 after about 10 hours duration (Figure Another possibility is fines migration-induced permeability decline, commonly occurring in porous media [35]. In our samples, fine particles are attached to the fracture surfaces (Figure 10), which may have been transported upon fluid flow, causing permeability to decrease.…”

“…This suggests that a chemical equilibrium within the fracture aperture was attained during no flow periods leading to a high concentration of the pore fluid and less efficient pressure solution, resulting from limiting the diffusion from contacts to the pore fluids. Another possibility is fines migration-induced permeability decline, commonly oc-curring in porous media [35]. In our samples, fine particles are attached to the fracture surfaces (Figure 10), which may have been transported upon fluid flow, causing permeability to decrease.…”

“…In addition, sufficient dissolved components can be detected in the effluents due to pressure solution-enhanced solubility [17,19,24,43]. For the other mechanisms, i.e., free-face dissolution, precipitation [25,61], and fines migration [35], no obvious deformation is required in conjunction with fracture permeability changes at a relatively small scale.…”

“…All of the former processes imply a change in the fracture void space, i.e., the fracture aperture, available for fluid flow and thereby yield either a decrease (pressure solution, stress corrosion, and precipitation) or increase (free-face dissolution) in fracture permeability. In addition, fine particle migration-induced permeability reduction was commonly observed in porous media, where fines/clay particles attached to grain surfaces are susceptible to fluid flow [35]. Fines migration may also occur in (re-)activated rock fractures, where gouge material may lead to clogging of the main flow pathways, resulting in permeability reduction [5].…”