Samuel Chapman

Bubbles attenuate elastic waves at seismic frequencies: First experimental evidence

The migration of gases from deep to shallow reservoirs can cause damageable events. For instance, some gases can pollute the biosphere or trigger explosions and eruptions. Seismic tomography may be employed to map the accumulation of subsurface bubble-bearing fluids to help mitigating such hazards. Nevertheless, how gas bubbles modify seismic waves is still unclear. We show that saturated rocks strongly attenuate seismic waves when gas bubbles occupy part of the pore space. Laboratory measurements of elastic wave attenuation at frequencies <100 Hz are modeled with a dynamic gas dissolution theory demonstrating that the observed frequency-dependent attenuation is caused by wave-induced-gas-exsolution-dissolution (WIGED). This result is incorporated into a numerical model simulating the propagation of seismic waves in a subsurface domain containing CO2-gas bubbles. This simulation shows that WIGED can significantly modify the wavefield and illustrates how accounting for this physical mechanism can potentially improve the monitoring and surveying of gas bubble-bearing fluids in the subsurface.

Seismic attenuation in partially saturated Berea sandstone submitted to a range of confining pressures

Using the forced oscillation method, we measure the extensional-mode attenuation and Youngs modulus of a Berea sandstone sample at seismic frequencies (0.5–50 Hz) for varying levels of water saturation (~0–100%) and confining pressures (2–25 MPa). Attenuation is negligible for dry conditions and saturation levels <80%. For saturation levels between ~91% and ~100%, attenuation is significant and frequency dependent in the form of distinct bell-shaped curves having their maxima between 1 and 20 Hz. Increasing saturation causes an increase of the overall attenuation magnitude and a shift of its peak to lower frequencies. On the other hand, increasing the confining pressure causes a reduction in the attenuation magnitude and a shift of its peak to higher frequencies. For saturation levels above ~98%, the fluid pressure increases with increasing confining pressure. When the fluid pressure is high enough to ensure full water saturation of the sample, attenuation becomes negligible. A second series of comparable experiments reproduces these results satisfactorily. Based on a qualitative analysis of the data, the frequency-dependent attenuation meets the theoretical predictions of mesoscopic wave-induced fluid flow (WIFF) in response to a heterogeneous water distribution in the pore space, so-called patchy saturation. These results show that mesoscopic WIFF can be an important source of seismic attenuation at reservoir conditions.

Laboratory measurements of seismic attenuation and Young's modulus dispersion in a partially and fully water-saturated porous sample made of sintered borosilicate glass: Porous sample made of sintered borosilicate glass

We measured the extensional-mode attenuation and Young’s modulus in a porous sample made of sintered borosilicate glass at microseismic to seismic frequencies (0.05–50 Hz) using the forced oscillation method. Partial saturation was achieved by water imbibition, varying the water saturation from an initial dry state up to 99%, and by gas exsolution from an initially fully water-saturated state down to 99%. During forced oscillations of the sample effective stresses up to 10 MPa were applied. We observe frequency-dependent attenuation, with a peak at 1–5 Hz, for 99% water saturation achieved both by imbibition and by gas exsolution. The magnitude of this attenuation peak is consistently reduced with increasing fluid pressure and is largely insensitive to changes in effective stress. Similar observations have recently been attributed to wave-induced gas exsolution–dissolution. At full water saturation, the left-hand side of an attenuation curve, with a peak beyond the highest measured frequency, is observed at 3 MPa effective stress, while at 10 MPa effective stress the measured attenuation is negligible. This observation is consistent with wave-induced fluid flow associated with mesoscopic compressibility contrasts in the sample’s frame. These variations in compressibility could be due to fractures and/or compaction bands that formed between separate sets of forced-oscillation experiments in response to the applied stresses. The agreement of the measured frequency-dependent attenuation and Young’s modulus with the Kramers–Kronig relations and additional data analyses indicate the good quality of the measurements. Our observations point to the complex interplay between structural and fluid heterogeneities on the measured seismic attenuation and they illustrate how these heterogeneities can facilitate the dominance of one attenuation mechanism over another.

Seismic Attenuation in Fluid-saturated Rock: Experimental Evidence of a Gas Dissolution-exsolution Mechanism

Laboratory measurements of seismic attenuation were performed on a synthetic rock sample using the forced oscillation method. Studying first the effect of partial water saturation, significant and frequency-dependent attenuation was only observed at near full water saturation and at low pore fluid pressures (≤0.6 MPa). Increasing the confining pressure and fluid pressure equally, thus keeping the effective stress unchanged, caused a significant decrease in attenuation. For fluid pressures >2.5 MPa attenuation was negligible and frequency independent. Additional measurements at different effective stresses showed that the mechanism responsible for the observed frequency-dependent attenuation is not sensitive to the effective stress. The physical mechanism that can potentially explain these results is wave-induced-gas-exsolution-dissolution (WIGED) occurring in response to fluid pressure variations, rather than wave-induced fluid flow (WIFF).