Reservoir rock often contains mesoscale (i.e., larger than grain scale but smaller than seismic wavelength scale) heterogeneities. These include patchy distribution of different fluid phases in the pore space (e.g., White et al., 1975; Dutta & Odé, 1979a, b), sedimentary layers with different poroelastic properties (Norris, 1993; Gurevich et al., 1997), and open and partially open fractures and faults (Brajanovski et al., 2005; Nakagawa & Schoenberg, 2007). During seismic wave propagation, these heterogeneities can cause local fluid pressure gradients with scales comparable to the pressure diffusion length (or, the wavelength of Biot's slow compressional waves) for given seismic wave frequencies and fluid and rock properties, resulting in large seismic velocity dispersion and attenuation. Therefore, when studying the effect of scCO2 injection on seismic wave propagation in the laboratory, employing waves with appropriate frequencies can be critically important. Additionally, compliant and open fractures are expected to play an important role in controlling migration of injected fluids in reservoir rock. When both fractures and matrix porosity connected to the fractures are present, wave‐induced dynamic poroelastic interactions between these two different types of rock porosity (high‐permeability, high‐compliance fractures and low‐permeability, low‐compliance matrix porosity) can result in complex velocity and attenuation changes in seismic waves as scCO2 invades the fractured rock. Further, these interactions are affected by the orientation of the fractures with respect to the wave propagation, resulting in different signatures of scCO2 invasion on seismic waves. Understanding these relationships can lead to better predictions of scCO2 behavior during geological carbon sequestration and enhanced oil recovery (EOR) based upon seismic monitoring.
In this laboratory study, we examined changes in dynamic elastic moduli and attenuations related to compressional and shear (torsion) waves during scCO2 injection into sandstone core samples. In the following, we will first describe an experimental setup (a modified resonant bar test system), which allows us to conduct laboratory seismic measurements at frequencies of 1–2 kHz, close to the frequencies used for monitoring of scCO2 injection in the field via crosshole tomography (e.g., Ajo‐Franklin et al, 2013). Subsequently, we will present observed changes in the seismic properties of several fractured rock samples during scCO2 injection tests, with concurrently determined distribution and saturation of the scCO2 in the samples via X‐ray CT imaging. Finally, we will discuss correlations between the changes in the seismic properties and the orientation of the fracture with respect to the scCO2 migration (which is coincidental to the wave propagation direction) as well as scCO2 saturation and distribution within the porous, fractured rock.
5.2. EXPERIMENTAL SETUP
5.2.1. Split‐Hopkinson Resonant Bar (SHRB)
Our seismic measurements employed a variant of conventional resonant bar tests, which allows us to determine the dynamic shear and Young's moduli of a core sample in the sonic frequency range near 1 kHz (Nakagawa, 2011). The basic idea behind this apparatus is that the resonance frequencies of a sample are reduced when sample size and mass are artificially increased by an attached foreign object (e.g., Tittmann, 1977). Once the resonances of the resulting “extended” sample are measured, the seismic properties of the original sample are determined via calibration, modeling, and inversion. In our experiments, a cylindrical rock core (~ 38 mm diameter) is jacketed and placed between a pair of stainless steel rods (Fig. 5.1). The resulting long composite bar is suspended by springs in a tubular aluminum cage, which is inserted into a tubular confining cell (pressure vessel). Longitudinal and torsional vibrations are induced in the bar using piezoelectric sources at one end and measured using accelerometers at the other end. Because the geometry of this apparatus is the same as the conventional Split‐Hopkinson Pressure Bar test apparatus (e.g., Kolsky, 1949), it is called the Split‐Hopkinson Resonant Bar (SHRB).
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 10–6 to 10–7 range 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.
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