Geophysical Monitoring for Geologic Carbon Storage. Группа авторов

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Figure 5.1 Split‐Hopkinson Resonant Bar. A small (typically 2.5 cm–10 cm long, ~3.8 cm dia), jacketed rock core is placed between a pair of metal rods, and one‐dimensional vibrations of the entire rod assembly are examined for the sample's seismic properties. Hydrostatic confining stress is applied by compressed nitrogen gas inside a tubular pressure vessel, and pore fluids are injected and extracted through ports attached to the metal rods. The tubing for the pore fluids is coiled around the rods, effectively reducing the mechanical coupling between the resonant bar and the pressure vessel.

      5.2.2. Experimental Procedures

A photograph of the experimental setup is shown in Figure 5.2. Each experiment was conducted by first measuring the stress‐dependent seismic properties of a sandstone core sample under mild oven‐dried conditions (60°C overnight). The dry sample was jacketed with a heat‐shrink PVC tubing, and thin lead foil disks were placed at the interfaces between the sample and the stainless steel. The resonant bar test assembly (Fig. 5.1) was then introduced into the tubular pressure vessel, and confining stress was applied up to 9.6 MPa using compressed nitrogen gas for a few cycles at room temperature (~20°C). During these cycles, the pore pressure was maintained at 1 atm (0.1 MPa), and the resonance frequencies and attenuations of the fundamental longitudinal and torsion vibration modes were measured.

      After the dry measurements, the sample was evacuated and injected with low‐pressure CO₂ gas for several cycles to purge the air in the pore space. After the final evacuation step, de‐aired water was slowly injected from one end of the sample. (Note that excessive clay swelling by low‐salinity water can result in disintegration of the rock and loss of permeability. In this experiment, tap water was used after confirming that it caused no significant swelling of clays in the sample.) Once the sample was water saturated, the confining stress and pore pressure were increased to 10.4 MPa and 3.5 MPa, respectively, to ensure dissolution of the remaining CO₂ gas into the pore fluid over ~12 hours. During this period, the temperature of the sample and the pressure cell was also increased up to 60.5°C.

      Prior to the subsequent scCO₂ injection experiment, the confining stress and the pore pressure were increased again, up to 22.1 MPa and 13.6 MPa (8.5 MPa differential stress), while maintaining the sample temperature at 60.5°C. Under these conditions, CO₂ is supercritical, with viscosity 0.040 cP, bulk modulus 0.040 GPa, and density 535 kg/m³ (NIST Webbook, http://webbook.nist.gov/chemistry/fluid/). In comparison, the water has viscosity 0.47 cP, bulk modulus 2.46 GPa, and density 989 kg/m³. The scCO₂ was injected into the core sample at a constant rate, and the pore pressure was controlled using a back‐pressure regulator at the outlet. The injection rate varied from 0.017 to 0.043 mL/min for various test cases, and the sample cross section of 11.4 cm² (slightly larger for a fractured sample). The pore volume replaced by the injected scCO₂ was determined from the weight of the displaced water. The injection was continued until scCO₂ broke through the core and no more water was produced from the system. The long‐term drying effect of the scCO₂ was not examined in this experiment.

Throughout the experiment, the resonance frequencies and attenuations of the system were measured periodically to monitor changes in the seismic properties of the samples. Between the seismic measurements, during the scCO₂ injection tests, X‐ray CT scans were conducted using a modified GE Lightspeed 16 slice medical X‐ray CT scanner to determine the saturation and distribution of scCO₂ in the pore space. Cores were scanned at 120 kV and 160 mA, and the voxel size in the obtained images was set to 193 × 193 × 625 microns. Scans were performed under dry and water‐saturated conditions, in addition to the scans performed periodically during the CO₂ injection tests, to allow computation of CO₂ saturation.