Seismic stress stimulation mobilizes colloids trapped in a porous rock

Abstract

Low-frequency (1–500 Hz) stress (seismic) waves can alter flow and mass transport behavior in saturated subsurface porous formations. Numerous physical mechanisms have been proposed to explain this phenomenon. Coupling of dynamic stress to in-situ particle (colloid) mobilization is one such mechanism that can produce changes in pore matrix permeability. This phenomenon has broad-ranging impacts on colloid dynamics and mass transport problems over a scale range of microns to kilometers. Porous-flow experiments in sandstone cores demonstrated that in-situ colloidal particles can be released from pores by applying dynamic mechanical stress stimulation at frequencies below 100 Hz. Due to the lower attenuation at these frequencies, relative to ultrasonic, significant dynamic stress in the Earth can penetrate to distances of a kilometer or more. Thus, seismic waves could affect particle mobility in distant oil formations, aquifers and fault systems. Laboratory results are shown for release of in-situ particles from Fontainebleau sandstone induced by applying stress stimulation at 26 Hz. Although enhanced particle release relative to that induced by flowing de-ionized water alone was observed, the permeability of the core was unchanged, indicating that pore throat fouling was insignificant. The behavior of the post-stimulation particle release was distinctly different than the pre-stimulation behavior in that a cyclical pattern with uniform periodicity was observed. This cyclical behavior was observed to be independent of particle size over the range of 50 to 800 nm and is attributed to stimulation causing a long-term change in the distribution of the rate coefficients for release of particles from the pore space. The rate change is likely not due to alteration of particle–wall interactions which are sensitive to particle size. Size-independent release mechanisms that can explain the stimulated rate change are 1) enhanced flushing or squeezing out of particles trapped in dead-end pores, and 2) forced particle detachment and exposing of new detachment sites on the pore walls. The observations presented here are unique in that they indicate sub-pore-scale particle mobility and transport can be influenced by long-wavelength seismic-band stress. Implications on possible field-scale effects and micro-scale physical mechanisms are discussed.