Laboratory observations of permeability enhancement by fluid pressure oscillation of in situ fractured rock

Abstract

We report on laboratory experiments designed to investigate the influence of pore
\npressure oscillations on the effective permeability of fractured rock. Berea sandstone
\nsamples were fractured in situ under triaxial stresses of tens of megapascals, and
\ndeionized water was forced through the incipient fracture under conditions of steady and
\noscillating pore pressure. We find that short‐term pore pressure oscillations induce
\nlong‐term transient increases in effective permeability of the fractured samples. The
\nmagnitude of the effective permeability enhancements scales with the amplitude
\nof pore pressure oscillations, and changes persist well after the stress perturbation.
\nThe maximum value of effective permeability enhancement is 5 × 10-16 m2 with a
\nbackground permeability of 1 × 10−15 m2; that is, the maximum enhanced permeability
\nis 1.5 × 10−15 m2. We evaluate poroelastic effects and show that hydraulic storage
\nrelease does not explain our observations. Effective permeability recovery following
\ndynamic oscillations occurs as the inverse square root of time. The recovery indicates that
\na reversible mechanism, such as clogging/unclogging of fractures, as opposed to an
\nirreversible one, like microfracturing, is responsible for the transient effective permeability
\nincrease. Our work suggests the feasibility of dynamically controlling the effective
\npermeability of fractured systems. The result has consequences for models of earthquake
\ntriggering and permeability enhancement in fault zones due to dynamic shaking from
\nnear and distant earthquakes.