Using Combined X-ray Computed Tomography and Acoustic Resonance to Understand Supercritical CO2 Behavior in Fractured Sandstone

Abstract:

Distribution of supercritical (sc) CO₂ has a large impact on its flow behavior as well as on the properties of seismic waves used for monitoring. Simultaneous imaging of scCO₂ distribution in a rock core using X-ray computed tomography (CT) and measurements of seismic waves in the laboratory can help understand how the distribution evolves as scCO₂ invades through rock, and the resulting seismic signatures. To this end, we performed a series of laboratory scCO₂ core-flood experiments in intact and fractured anisotropic Carbon Tan sandstone samples. In these experiments, we monitored changes in the CO₂ saturation distribution and sonic-frequency acoustic resonances (yielding both seismic velocity and attenuation) over the course of the floods. A short-core resonant bar test system (Split-Hopkinson Resonant Bar Apparatus) custom fit into a long X-ray transparent pressure vessel was used for the seismic measurements, and a modified General Electric medical CT scanner was used to acquire X-ray CT data from which scCO₂ saturation distributions were determined. The focus of the experiments was on the impact of single fractures on the scCO2 distribution and the seismic properties. For this reason, we examined several cases including 1. intact, 2. a closely mated fracture along the core axis, 3. a sheared fracture along the core axis (both vertical and horizontal for examining the buoyancy effect), and 4. a sheared fracture perpendicular to the core axis. For the intact and closely mated fractured cores, Young’s modulus declined with increasing CO₂ saturation, and attenuation increased up to about 15% CO₂ saturation after which attenuation declined. For cores having wide axial fractures, the Young’s modulus was lower than for the intact and closely mated cases, however did not change much with CO₂ pore saturation. Much lower CO₂ pore saturations were achieved in these cases. Attenuation increased more rapidly however than for the intact sample. For the core-perpendicular fracture, the Young’s modulus decreased quickly with increasing CO₂ saturation. Attenuation increased with increasing CO2 saturation until the CO2 front reached the fracture, after which it fell to below that for the brine-saturated case, increasing again as the CO₂ invaded the downstream core region.