Transient pore pressure response to confining stress excursions in Berea sandstone flooded with an aqueous solution of CO₂

Abstract

We measured the pore pressure response due to carbon dioxide (CO 2 ) gas bubble nucleation and growth in a Berea sandstone core flooded with an initially subsaturated aqueous solution of CO 2 , in response to a rapid drop in confining stress, under conditions representative of a confined aquifer. A portion of the CO 2 in the Earth's crust, derived from volcanic, magmatic, and biogenic sources, dissolves in groundwater. Sudden reductions in confining stress in the Earth's crust occur due to dilational strain generated by the propagation of seismic Rayleigh and P waves, or aseismic slip in the near field of earthquakes. A drop in confining stress produces a proportional drop in pore fluid pressure. When the pore fluid contains dissolved CO 2 , the pore pressure responds to a drop in confining stress like it does in the dissolved gas-free case, until the pore pressure falls below the bubble pressure. Gas bubble nucleation and diffusive growth in the pore space trigger spontaneous, transient buildup of the pore fluid pressure, and reduction of effective stress. We measured the rate of pore fluid pressure buildup in the 100 s immediately following the confining stress drop, as a function of the saturation with respect to CO 2 at the lowest pore pressure realized during the confining stress drop, using five different CO 2 partial pressures. The rate scales with the saturation with respect to dissolved CO 2 , from 10 kPa/min at 1.25 to 166 kPa/min at 1.8. The net pore pressure rise was as large as 0.7 MPa (100 psi) over 5 h.

Experimental evidence for seismically initiated gas bubble nucleation and growth in groundwater as a mechanism for coseismic borehole water level rise and remotely triggered seismicity

“…The outlet valve from the core was then closed and the pore fluid pump remained in communication with the core for 48 h at 2.758 MPaG (400 psig). This solution yielded a bubble pressure of 1.882 MPaG (273 psig) in the Berea sandstone core [ Crews and Cooper , ]. After setting the initial pore fluid pressure and confining stress, the inlet valve to the core was closed and the pore fluid pressure was monitored for stability.…”

“…In the present study, an initially undersaturated aqueous CO 2 solution is subjected to oscillations in pore fluid pressure at frequencies < 1 Hz. The mechanism for bubble growth in this study is a phase transition caused by an instantaneous supersaturation with respect to dissolved CO 2 gas, which is different from rectified diffusion, as the supersaturation mechanism for bubble growth does not rely on an oscillating fluid pressure [e.g., Crews and Cooper , ].…”

“…At pore fluid pressures equal to or below p b , dissolved CO 2 can come out of solution, forming gas bubbles according to H 2 CO 3 (aq) → H 2 O(l) + CO 2 (g). Under confined conditions, p p will increase as newly formed gas bubbles displace and compress the liquid in the pore space [ Crews and Cooper , ].…”

“…Taking the difference between and yields an expression for the gas‐free effective stress response as an explicit function of , Equations and can be compared to the observed responses to determine the influence of CO 2 gas bubble growth in the pore space [ Crews and Cooper , ].…”

Effects of different fluids on microcrack propagation in sandstone under true triaxial loading conditions

“…CO 2 , CH 4 , N 2 , He, water, brine) migration during mechanical testing with various rock types (e.g. sandstone, mudstone, coal) and to examine their flow and strength behaviors . Younane et al .…”