Laboratory observations of altered porous fluid flow behavior in Berea sandstone induced by low-frequency dynamic stress stimulation

It has been observed repeatedly that low-frequency (1–500 Hz) seismic stress waves can enhance oil production from depleted reservoirs and contaminant extraction from groundwater aquifers. The physics coupling stress waves to fluid flow behavior in porous media is not understood, although numerous physical mechanisms have been proposed to explain the observations. To quantify the effects of low-frequency, dynamic-stress stimulation on multiphase fluid flow and in situ particle behavior in porous media, laboratory experiments were conducted with a core flow stimulation apparatus that allows for precise control and measurement of applied stress and strain, static confinement, and fluid flow parameters. Results are reported for experiments on stimulated single-phase and two-phase fluid flow behavior in 2.54-cm-diameter Berea sandstone cores. For all experiments, stimulation was applied to the cores in the form of sinusoidal, axial, mechanical stress coupled to the solid porous matrix at frequencies of 25 to 75 Hz. Applied stress RMS amplitudes ranged from 300 to 1200 kPa and, at these levels, produced coupled, pore-pressure fluctuations of much less than 1.2 to 4.8 kPa, respectively. During single-phase brine flow, stimulation increased the absolute permeability of the rock by 10–20%. This was caused by mobilizing in situ clay particles that were partially plugging the pore throats. During two-phase, steady-state, constant-rate flow of oil-brine and decane-brine mixtures, stimulation caused significant changes in the bulk fluid pressure drop across the core. The pressure changes showed a strong dependence on the viscosity of the nonwetting fluid phase (oil or decane) relative to the wetting phase (brine). This may indicate that relative changes in the mobility of wetting versus nonwetting fluid phases were induced by the dynamic stress. Under the specific experimental conditions used, pore-scale particle perturbation and altered wettability are possible physical mechanisms that can explain the results.