Geophysical Monitoring for Geologic Carbon Storage

The longitudinal piezoceramic source is a single disk, and the torsion source is a group of four pie‐shaped laterally polarized shear piezoelectric slabs. Both are made of Type 5600 Navy V piezoceramics (Channel Industries). These sources are driven selectively to introduce a desired mode of vibration in the sample. At the opposite end of the other steel rod, miniature accelerometers (PCB Piezotronics, 352A24) are attached to measure the resulting vibrations. The longitudinal motion is measured by an axial accelerometer, and the torsion motion by a pair of accelerometers oriented in the tangential directions, in diametrically opposing locations. The torsion vibrations are measured by subtracting the output from one of the torsion sensors from the output of the other, resulting in cancellation of electrical noise and unwanted flexural motions contaminating the measurements. During the experiments presented in this paper, the source amplitude was adjusted so that the strains induced in the samples at resonance were in the 10 –6to 10 –7range to reduce possible nonlinear effects.

To ensure good mechanical coupling, the surfaces of the steel bars and the sample are polished flat, then a thin (30 μm) lead foil disk is placed on the interfaces. These disks are cut with a cross‐shaped pattern at the center to allow distribution of the pore fluid along the interface. With these preparations, typically 3–4 MPa of effective confining stress is sufficient to reduce the additional compliance introduced by the interface to a negligible level. The jacket is made of heat‐shrink PVC, with a thickness ranging from 150 μm to 500 μm. With appropriately machined smooth sample surfaces and with application of sufficient effective confining stress (>~1 MPa), this results in good seal at the jacket‐sample interface.

Currently, our experiments with the SHRB apply confining stress, using high‐pressure nitrogen gas up to ~35 MPa, and introduce and extract fluids into and from the sample through ports attached to the metal rods. The temperature of the system is controlled using both a fluid‐circulating heating/cooling jacket attached to the exterior wall of the pressure vessel (Temco/Corelab, X‐ray transparent, carbon‐fiber‐wrapped, tubular aluminum cell) and film heaters lining the interior wall of the aluminum suspension cage. (Our past experiments have been conducted at temperatures ranging from −15°C up to 65°C.) During the experiments, changes in the distribution of different fluid phases within the sample can be examined using X‐ray CT (Nakagawa et al., 2013), similar to the experiment previously conducted by Cadoret et al. (1995) during conventional resonant bar tests.

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.

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 CO 2gas 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 CO 2gas 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 scCO 2injection 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, CO 2is supercritical, with viscosity 0.040 cP, bulk modulus 0.040 GPa, and density 535 kg/m 3(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/m 3. The scCO 2was 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 cm 2(slightly larger for a fractured sample). The pore volume replaced by the injected scCO 2was determined from the weight of the displaced water. The injection was continued until scCO 2broke through the core and no more water was produced from the system. The long‐term drying effect of the scCO 2was 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 scCO 2injection 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 scCO 2in 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 CO 2injection tests, to allow computation of CO 2saturation.

Figure 5.2 Photograph of an on‐going scCO 2injection experiment. The experiment is conducted within a temperature and pressure controlled, X‐ray transparent tubular pressure cell on a CT examination table. Both seismic (acoustic) properties and fluid phase distribution changes in the sample are measured periodically while scCO 2is injected slowly into the sample. The amount of scCO 2in the sample is determined from the weight of the effluent water, and the pH of the effluent water is monitored during the experiment.

Saturation of the pore space by scCO 2in the samples can be determined from CT images by comparing the CT values (X‐ray absorption values) of water‐saturated and scCO 2‐injected samples. The image resolution (the voxel size) is not sufficient for exactly resolving the pore space geometry. However, from the CT values of 100% water or scCO 2‐saturated samples and a sample saturated by both water and scCO 2( , , and CT Mix, respectively), the average scCO 2saturation value for each voxel is computed by (e.g., Seol & Kneafsey, 2011)