The seismoelectric method is a modification of conventional seismic measurements which involves the conversion of an incident poroelastic wave to an electromagnetic signal that can be measured at the surface or down a borehole. This technique has the potential to probe the physical properties of the rocks at depth. The problem is that we currently know very little about the parameters which control seismoelectric conversion and their dependence on frequency and permeability, which limits the development of the seismoelectric method. The seismoelectric coupling coefficient indicates the strength of seismoelectric conversion. In our study, we focus on the effects of the reservoir permeability, porosity and frequency on the seismoelectric coupling coefficient through both experimental and numerical modellings. An experimental apparatus was designed to record the seismoelectric signals induced in water-saturated samples in the frequency range from 1 to 500 kHz. The apparatus was used to measure seismoelectric coupling coefficient as a function of porosity and permeability. The results were interpreted using a microcapillary model for the porous medium to describe the seismoelectric coupling. The relationship between seismoelectric coupling coefficients and the permeability and porosity of samples were also examined theoretically. The combined experimental measurements and theoretical analysis of the seismoelectric conversion has allowed us to ascertain the effect of increasing porosity and permeability on the seismoelectric coefficient. We found a general agreement between the theoretical curves and the test data, indicating that seismoelectric conversion is enhanced by increases in porosity over a range of different frequencies. However, seismoelectric conversion has a complex relationship with rock permeability, which changes with frequency. For the low-permeability rock samples (0–100 × 10⁻¹⁵ m²), seismoelectric coupling strengthens with the increase of permeability logarithmically in the low-frequency range (0–10 kHz); in the high-frequency range (10–500 kHz), the seismoelectric coupling is at first enhanced, with small increases of permeability leading to small increases in size in electric coupling. However, continued increases of permeability then lead to a slight decrease in size and image conversion again. For the high-permeability rock samples (300 × 10⁻¹⁵–2200 × 10⁻¹⁵ m²), the seismoelectric conversion shows the same variation trend with low-permeability samples in low-frequency range; but it monotonically decreases with permeability in the high-frequency range. The experimental and theoretical results also indicate that seismoelectric conversion seems to be more sensitive to the changes of low-permeability samples. This observation suggests that seismic conversion may have advantages in characterizing low permeability reservoirs such as tight gas and tight oil and shale gas reservoirs.

The presence of clay minerals can alter the elastic behaviour of reservoir rocks significantly as the type of clay minerals, their volume and distribution, and their orientation control the shale's intrinsic anisotropic behaviours. Clay minerals are the most abundant materials in shale, and it has been proven extremely difficult to measure the elastic properties of natural shale by means of a single variable (in this case, the type of clay minerals), due to the influences of multiple factors, including water, TOC content and complex mineral compositions. We used quartz, clay (kaolinite, illite and smectite), carbonate and kerogen extract as the primary materials to construct synthetic shale with different clay minerals. Ultrasonic experiments were conducted to investigate the anisotropy of velocity and mechanical properties in dry synthetic and natural shale as a function of confining pressure. Velocities in synthetic shale are sensitive to the type of clay minerals, possibly due to the different structures of the clay minerals. The velocities increase with confining pressure and show higher rate of velocity increase at low pressures, and P-wave velocity is usually more sensitive than S-wave velocity to confining pressure according to our results. Similarly, the dynamic Young's modulus and Poisson's ratio increase with applied pressure, and the results also reveal that E₁₁ is always larger than E₃₃ and ν₃₁ is smaller than ν₁₂. Velocity and mechanical anisotropy decrease with increasing stress, and are sensitive to stress and the type of clay minerals. However, the changes of mechanical anisotropy with applied stress are larger compared with the velocity anisotropy, indicating that mechanical properties are more sensitive to the change of rock properties.

Clay minerals are the most abundant materials in shale. Their presence significantly influences the elastic behavior of reservoir rocks as a function of mineral type, volume, and distribution, and their orientation controls the shale’s intrinsic anisotropic behaviors. Thus, knowing the elastic properties of shale with different types of clay minerals is imperative for fully understanding the seismic properties of the reservoir. However, it is extremely difficult to measure the elastic properties of natural shale by means of a single variable (in this case, the type of clay), due to the influences of multiple factors, including water, total organic carbon content, complex mineral composition, and so on. Thus, we use quartz, clay (kaolinite, illite, and smectite), carbonate, and kerogen extract as the primary materials to construct synthetic shale with different types of clay. Ultrasonic experiments were conducted to study the anisotropy of velocity and mechanical properties (Young’s modulus and Poisson’s ratio) in dry synthetic shale samples as a function of applied axial stress. The results show that the velocity of samples increases with applied pressure and the rate of velocity increase is higher at low pressures. Similarly, the dynamic Young’s modulus and Poisson’s ratio increase with applied pressure; $E_{11}$ is always larger than $E_{33}$, but ${\nu }_{31}$ may be larger or smaller than ${\nu }_{12}$. Furthermore, velocity anisotropy and mechanical anisotropy decrease with the increase of stress and are sensitive to stress and lithology. The closure of large aspect-ratio pores (and/or microcracks) seems to be a dominant mechanism controlling the change of anisotropy. Finally, the changes in mechanical anisotropy under applied stress are larger compared with the changes in velocity anisotropy, indicating that mechanical properties are more sensitive to the changes in rock property.

The laser ultrasonic technique (LUT) has many advantages in dealing with the anisotropy of organic-rich shale over the transducer ultrasonic contacting measurement. We have developed a systematic procedure to estimate anisotropic parameter δ and elastic parameter C13 in organic-rich shale from LUT measurement. A novel filtering method called the structural-oriented space-varying median filter (SOSVMF) is proposed for removing the erratic noise from the recorded data. We apply the proposed algorithm onto one synthetic example and real recorded data in artificial bakelite and organic-rich shale by using LUT to show the successful performance. Compared with the original recorded data, the denoised waveform by SOSVMF will also help to obtain more accurate parameter estimations of the anisotropic parameter δ and elastic parameter C13. More importantly, the presented systematic procedure may extend the application of LUT to the study of attenuation anisotropy in organic-rich shale and scale physical modeling. In addition, the surface-modification method may be an effective method to enhance the laser-ultrasonic conversion efficiency.

Clay minerals are a major component of hydrocarbon reservoir rocks, and they are known to play important roles in the physical and elastic properties of rocks. However, it is difficult to directly measure these properties of single-crystal clays due to their small particle size. Therefore, we have constructed three sets of artificial clay samples with different compaction stresses to investigate the effect of the compaction stress and clay mineralogy on their elastic properties and anisotropy. All of the dry samples are measured by the pulse-transmission method. The results indicate that the compaction stress and clay mineralogy have a significant influence on the physical and elastic properties of the clay samples. The microstructures of clay samples indicate that the clay platelets are aligned almost perpendicularly to the direction of compaction stress, and the ultrasonic velocity analysis validates the assumption of transverse isotropy of our clay samples. The velocities increase with the compaction stress, especially at low stress, which corresponds to the rapid porosity reduction at low stress levels. Velocity anisotropy parameters increase with increasing of compaction stress due to the increase of texture sharpness for clay minerals during the compaction process. The elastic moduli of the clay samples display a significant stress sensitivity and a strong directional dependence, with the Young’s moduli increasing and the Poisson’s ratios decreasing with the compaction stress. A simple theoretical template is used to quantify the orientation distribution functions (ODFs) of clay platelets, and the generalized Legendre coefficients of ODF increase with the increase of compaction stress, especially at low stress. Further, the compressional-wave (P-wave) and shear-wave anisotropy increase with the ODF coefficients W200 and W400, especially P-wave anisotropy.

Based on the assumptions of Hudson theory, we built nine physical models, eight of which contain penny-shaped inclusions used to simulate fractures. The fracture density of the eight fractured model ranges from 2% to 12%. In the case that the fracture size was smaller than the wavelength, P- and S-wave transducers with three different ultrasound frequencies were used to estimate the P- and S-wave velocities of the matrix and inclusions from the travel time picks and distance measurements. The experimentally obtained velocities were substituted into the Hudson’s model to calculate the theoretical velocities and anisotropy of fractured models, which were then compared with the experimental values of velocities and anisotropy. Generally, with the variation of fracture density, the experimental results agreed well with the theoretical estimations within the frequency range from 0.10 MHz to 0.50 MHz. The experimental values of P- and S-wave velocities and anisotropy showed conspicuous dispersion. With the decrease of frequency, P- and S-wave velocities decreased, while the anisotropy increased. Besides, the interaction of fractures in the case of high fracture density can affect the propagation of slow S-wave. When the ratio of S-wave length to fracture diameter (λ_s/D) was lower than 5, a train of interference wave with small amplitude appeared before the first arrival of slow S-wave, which will possibly have influence on the identification of slow S-wave.