We developed a system to explore the effects of pressure and fluid viscosity on the dispersion and attenuation of fully saturated tight sandstones, especially at seismic frequencies. Calibration of the new system revealed that the system can operate reliably at frequencies of [2–200, 106] Hz. Tight sandstone with a “crack–pore” microstructure was tested under nitrogen gas (dry), brine, and glycerin saturation. A frequency‐dependent effect was not found for the dry case. However, apparent dispersion and attenuation for the undrained/unrelaxed transition was clearly observed for sample under brine or glycerin saturation, the magnitude of which was largely suppressed by increasing effective pressure. The measurement results illustrated that increasing the fluid viscosity or the effective pressure will shift the dispersion curve to the lower frequency range. A simple squirt‐flow model with dual‐porosity scheme was used to compare with the measurement results. Although the estimated values deviated slightly from the data, the trend fitted the saturated data relatively well, especially at low effective pressures. Therefore, considering the crack–pore microstructure of the tight sandstone, dispersion and attenuation are induced predominantly by the squirt‐flow stiffening effect from cracks to pores.

AC) stimuli. Xiao et al. [30] fabricated a graphene/poly(vinyldene fluoride) (PVDF) bilayer actuator by coating of PVDF solution onto a porous graphene paper. The hybrid film exhibited electrodriven vibration having rapid response rate, large displacement, and durable stability. In those electrothermal actuators, Joule heating was generated when electric current passed through the graphene film. Then the thermal expansion resulted in the large amount of deflection of the graphene/ polymer bilayer film. Polymer actuator is advantageous because of its high sensitivity, low energy input, and larger expansion rate. In this work, reduced graphene oxide (RGO)-based actuator was fabricated by spin-coating a reduced graphene oxide solution onto a polymer substrate obtaining a bilayer structure actuator. Poly dimethylsiloxane (PDMS) or polyethylene (PE) polymer substrate was used to support the electric-heated RGO layer and enlarge the thermal expansion deformation. [31] The bilayer film showed responsive bending motion under DC or AC voltage driving. Several actuate modes were proposed according to the bilayer structure design and driving current control. Experimental Section Preparation of RGOGraphite powder (2 g) was added into 46 mL of concentrated H 2 SO 4 , followed by adding 1 g NaNO 3 into above mixture under stirring and cooling in an ice bath condition. The mixture was continuous stirred while 6 g KMnO 4 was added slowly to keep the temperature of mixture below 5 °C. Then the mixture was kept at 35 °C for 30 min, followed by adding 90 mL deionized water while stirring, and the temperature would rise up to 95 °C. The mixture was kept stirring for a further 30 min, and 100 mL deionized water and 10 mL 30% H 2 O 2 were added in sequence. The oxidized material was then washed with 1:10 (v:v) HCl solution one time and deionized water three times to remove metal ions, followed by centrifugation. The collected product was dried in a vacuum drying oven at 45 °C.To obtain RGO, the as-synthesized GO powder (10 mg) was suspended in 20 mL of distilled water by ultrasonication until a yellowish-brown colloid was obtained. Subsequently, few drops of ammonia solution (35%) were added to increase the pH up to 8 allowing stability of the sheets. 15 µL hydrazine hydrate (0.1 m) solution was added to the above solution and refluxed at 98 °C for 100 min in a water bath, affording the formation Electrothermal Actuator Electrodriven bilayer actuator is designed and fabricated by spin-coating a reduced graphene oxide solution onto a polymer substrate. The bonded interface properties, electrical and thermal conductivities are characterized through scanning electron microscopy and X-ray diffraction. The bilayer actuator exhibits fast and large bending response when a direct current voltage is applied to the graphene layer. Whereas it exhibits oscillation when an alternating current voltage is applied to the bilayer actuator. The effects of the layer structure and the electro-operation methods on the bending motions are studied. Two new ...

We built a broad-frequency-band measurement system for rock elastic parameters based on the stress-strain method following Batzle et al., Geophysics 71, N1-N9 (2006). The system gives strain amplitude anomalies at some measurement frequencies. These anomalies put limitations on the range of the measurement frequency and jeopardize the credibility of the measurement results over a broad frequency band. To overcome these limitations, we investigated the cause of these anomalous strains by numerical model simulations with a finite element method based on the experimental apparatus. Through the systematic analysis of the modeling results, we conclude that the resonances caused by non-axial perturbations lead to such anomalous measurement results. Based on the analysis, we give a solution to reduce the effect of the resonances and shift the first resonance frequency beyond the frequency band of 1-2000 Hz. The enhanced measurement system can provide robust and reliable measurements on the elastic parameters of rocks between 1 and 2000 Hz, which is crucial for a quantitative study of the frequency-dependent phenomenon related to fluid effects. This in turn will provide a powerful tool for the experimental characterization of elastic properties of oil/gas reservoir rocks, thus laying a solid foundation for low-frequency rock physics analysis and quantitative seismic interpretation.

SUMMARY In fully fluid-saturated rocks, two common phenomena are documented both experimentally and theoretically for frequency-dependent elastic moduli and attenuation, that is, the drained/undrained transition and the relaxed/unrelaxed transition. When investigating these transitions with the forced oscillation method in the laboratory, it is crucial to consider the boundary differences between the laboratory and the underground. A 1-D poroelastic numerical model was previously established to describe these differences and their effects; however, the boundary conditions used in the model are actually different from the real experiment case, thus leading to inaccurate predication of the measurement results in a laboratory. In this paper, we established a 3-D poroelastic numerical model with a new set of boundary conditions that better represent the experiment conditions. Furthermore, the 3-D poroelastic modelling results were compared with laboratory measurements under the same boundary conditions, showing a much better fit than the 1-D model. Therefore, the 3-D model provides a more accurate and reliable approach to understand the regimes and transitions of elastic modulus dispersion and attenuation, and thus has great importance in interpreting the measurements of frequency-dependent properties of rocks in the laboratory.