On the Impact of Temperature on Shale Swelling and Expansion: A Diffuse Double-Layer Perspective

Abstract

Using a newly developed thermo-linear swelling test, this work investigated the influence of temperature on shale swelling and shrinkage and Debye-Hückel length alterations within the diffuse double layer. Furthermore, the effect of temperature, dielectric constant of water, and ionic strength of aqueous solutions on the Debye-Hückel length and subsequent shale swelling and shrinkage was explored. It was found that, for dilute solutions, the product of temperature and dielectric constant of water (T×εr) did not considerably change for temperatures ranging from 25°C to 90°C. When shale communicated with 12% and 20% weight by weight (w/w) NaCl and CaCl2 solutions, the percent change in shale shrinkage as temperature incrementally increased from 25°C to 90°C is quite high and cannot be ignored. Results proposed that the dielectric constant of water may have been reduced by both temperature and ionic strength of solution, all of which caused a greater decrease in the Debye-Hückel length and consequent shale shrinkage. As for saturated NaCl and CaCl2 solutions, shale exhibited swelling performance at moderate temperatures (up to 50°C for NaCl and up to 70°C for CaCl2) followed by shrinkage afterward. This could be credited to the development of repulsive forces between similar ions within the diffuse double layer, which may have overwhelmed their electrostatic screening effect on charged clay surfaces. At higher temperatures, the influence of dielectric constant of water on the decreasing Debye-Hückel length may have softened the repulsion action generated by ions.