Kaolinite Mobilisation in Sandstone: Pore Plugging vs. Suspended Particles

Abstract

The effect of temperature and salinity on sandstone permeability is critical to the feasibility of heat storage in geothermal aquifers. Permeability reduction has been observed in Berea sandstone when the salinity of the pore water is reduced as well as when the sample is heated. Several authors suggest that this effect is due to kaolinite clay mobilisation from the quartz grain surface; the mobilised particles subsequently plug the pore throats and reduce the permeability irreversibly. The expected hysteresis is observed when the salinity is reduced and increased; however, in contradiction with the throat plugging theory, the effect of heating is found to be reversible with cooling. In laboratory experiments we heated Berea sandstone from 20oC to 80oC and observed a reversible permeability reduction. The permeability of the heated samples increased at higher flow rates. We propose that in this case the mobilised kaolinite particles either remain suspended and thereby increase the fluid viscosity, or form porous aggregates that can be destabilized by hydrodynamic forces. To address how the pore scale distribution of kaolinite relates to the permeability of the entire sample, we relate permeability to the effective specific surface, Sp. The effective specific surface represents the average surface area that resists the flow through the sample of a volume of fluid. We propose that flow paths with a small Sp contribute more than proportionately to the total volume flux. Kaolinite mobilisation in pores with a small Sp diverts fluid flow through pores with a higher Sp, and thereby reduces permeability of the entire sample. In this paper, we use the DLVO theory to compare how temperature and salinity affect the surface interaction forces between quartz and kaolinite, as well as the interaction forces among kaolinite particles to evaluate whether heating can be expected to a) mobilise particles and b) result in kaolinite forming a suspension rather than plugging the pore throats.