Laboratory seismic monitoring of supercritical CO₂ flooding in sandstone cores using the Split Hopkinson Resonant Bar technique with concurrent x‐ray Computed Tomography imaging

Abstract

A B S T R A C TAccurate estimation of CO 2 saturation in a saline aquifer is essential for the monitoring of supercritical CO 2 injected for geological sequestration. Because of strong contrasts in density and elastic properties between brine and CO 2 at reservoir conditions, seismic methods are among the most commonly employed techniques for this purpose. However the relationship between seismic (P-wave) velocity and CO 2 saturation is not unique because the velocity depends on both wave frequency and the CO 2 distribution in rock. In the laboratory, we conducted measurements of seismic properties of sandstones during supercritical CO 2 injection. Seismic responses of small sandstone cores were measured at frequencies near 1 kHz, using a modified resonant bar technique (Split Hopkinson Resonant Bar method). Concurrently, saturation and distribution of supercritical CO 2 in the rock cores were determined via x-ray CT scans. Changes in the determined velocities generally agreed with the Gassmann model. However, both the velocity and attenuation of the extension wave (Young's modulus or 'bar' wave) for the same CO 2 saturation exhibited differences between the CO 2 injection test and the subsequent brine re-injection test, which was consistent with the differences in the CO 2 distribution within the cores. Also, a comparison to ultrasonic velocity measurements on a bedded reservoir rock sample revealed that both compressional and shear velocities (and moduli) were strongly dispersive when the rock was saturated with brine. Further, large decreases in the velocities of saturated samples indicated strong sensitivity of the rock's frame stiffness to pore fluid.

Effect of brine-CO2 fracture flow on velocity and electrical resistivity of naturally fractured tight sandstones

“…By considering fluid flow through matrix, a number of research conducted geophysical measurements to investigate properties of brine-and CO 2 -saturated sandstones (such as Lei and Xue, 2009;Nakatsuka et al, 2010;Alemu et al, 2013;Chen et al, 2013;Nakagawa et al, 2013;Omolo, 2015;Tran, 2015;Falcon-Suarez et al, 2016a). Laboratory measurements showed that resistivity of porous sandstones is profoundly influenced (increased to about five times) when CO 2 displaced brine in the matrix (Nakatsuka et al, 2010;Alemu et al, 2013;Omolo, 2015;Tran, 2015).…”

“…Moreover, while theoretical models could estimate CO 2 saturation for clean, homogeneous rocks, they failed to provide accurate results for heterogeneous shaly reservoir rocks (Nakatsuka et al, 2010;Alemu et al, 2013). It is demonstrated that variations in porosity, matrix heterogeneity and the layering relative to flow direction can considerably influence the CO 2 distribution pattern during drainage (Lei and Xue, 2009;Alemu et al, 2013;Nakagawa et al, 2013;Falcon-Suarez et al, 2016a). Acoustic velocity indicates a less sensitivity to fluid distribution pattern compared to resistivity (Alemu et al, 2013).…”

An experimental study of acoustic responses on the injection of supercritical CO2 into sandstones from the Otway Basin

“…The long-term effect of brine/scCO 2 on the mechanical properties of sandstones is studied by Zemke et al (2010). Nakagawa et al (2013) implement a resonance bar technique for measurement of acoustic response of sandstones core during CO 2 flooding at seismic frequencies. Adam and Otheim (2013) experimentally measure bulk and shear moduli dispersion of basalts saturated with water and liquid CO 2 at seismic frequencies.…”

Saturation-path dependency of P-wave velocity and attenuation in sandstone saturated with CO2 and brine revealed by simultaneous measurements of waveforms and X-ray computed tomography images

“…The conventional drainage and imbibition experiments thus suggest the nonuniqueness in the relationship between seismic velocity (or attenuation) and the ratio of wetting fluid to nonwetting fluid. Nakagawa et al (2013) also show faster velocities and smaller attenuation of extensional waves during CO 2 drainage than during water imbibition. However, the large wavelength compared with the fluid distribution scale in their experiment could not reveal the effect of the small-scale inhomogeneity of fluid saturation on velocity/attenuation.…”