A better understanding of seismic dispersion and attenuation of acoustic waves in rocks is important for quantitative interpretation of seismic data, as well as for relating seismic data, sonic-log data, and ultrasonic laboratory data. In the present work, a new laboratory setup is described, allowing for combined measurements of quasi-static deformations of rocks under triaxial stress, ultrasonic velocities, and dynamic elastic stiffness (Young's modulus and Poisson's ratio) at seismic frequencies.The setup has been used mainly for the study of shales. For such rocks, it is crucial that the saturation of the samples is preserved, which requires fast sample mounting. Design of our setup together with a technique that was developed for rapid mounting of strain gages onto the sample and subsequent sealing of the sample allows for sample preservation, which is of particular importance for shales.The performance of the new experimental setup and sample-mounting procedure is demonstrated with test materials (Aluminum and PEEK) as well as two different shale types (Mancos shale and Pierre shale). Furthermore, experimental results are presented that demonstrate the capability of measuring the impact of saturation, stress and stress-path on seismic dispersion. For the tests with Mancos and Pierre shale, large dispersion (up to 50% in Young's modulus normal to bedding) was observed. Increased water saturation of Mancos shale results in strong softening of the rock at seismic frequencies, while hardening is observed at ultrasonic frequencies due to an increase in dispersion, counteracting the rock softening. Poisson's ratio of Mancos shale strongly increases with level of saturation but appears to be nearly frequency independent. We have found that the different types of shale exhibit different stress sensitivities during hydrostatic loading, and also that the stress sensitivity is different at seismic and ultrasonic frequencies.

Previous studies found a significant increase of acoustic velocities between seismic and ultrasonic frequencies (seismic dispersion) for shales, which would have to be taken into account when comparing seismic or sonic field data with ultrasonic measurements in the laboratory. We have executed a series of experiments performed with a partially saturated Mancos shale and a Pierre shale I in which the influence of water saturation on acoustic velocities and seismic dispersion was investigated. The experiments were carried out in a triaxial setup allowing for combined measurements of quasistatic rock deformation, ultrasonic velocities, and dynamic elastic stiffness at seismic frequencies under deviatoric stresses. Prior to testing, the rock samples were preconditioned in desiccators at different relative humidities. For both shale types, we present and analyze the experimental results that demonstrate strong saturation and frequency dependence of dynamic Young’s moduli, Poisson’s ratios, and Thomsen’s anisotropy parameters, as well as P- and S-wave velocities at seismic and ultrasonic frequencies. The observed effects can be attributed to water adsorption and capillary pressure that are functions of several factors including water saturation. Water adsorption results in a reduction of surface energy and grain-contact stiffness. The capillary pressure affects the effective stress and possibly also the effective pore-fluid modulus, which may be approximated by Brie’s empirical model. Reasonable fits to the low-frequency seismic data are obtained by accounting for these two effects and applying the anisotropic Gassmann model. The strong increase in dispersion with increasing water saturation is attributed to local flow involving adsorbed (bound) water, but a quantitative description is yet to be provided.

Knowledge about the stress sensitivity of elastic properties and velocities of shales is important for the interpretation of seismic time-lapse data taken as part of reservoir and caprock surveillance of both unconventional and conventional oil and gas fields (e.g. during 4-D monitoring of CO 2 storage). Rock physics models are often developed based on laboratory measurements at ultrasonic frequencies. However, as shown previously, shales exhibit large seismic dispersion, and it is possible that stress sensitivities of velocities are also frequency dependent. In this work, we report on a series of seismic and ultrasonic laboratory tests in which the stress sensitivity of elastic properties of Mancos shale and Pierre shale I were investigated. The shales were tested at different water saturations. Dynamic rock engineering parameters and elastic wave velocities were examined on core plugs exposed to isotropic loading. Experiments were carried out in an apparatus allowing for static-compaction and dynamic measurements at seismic and ultrasonic frequencies within single test. For both shale types, we present and discuss experimental results that demonstrate dispersion and stress sensitivity of the rock stiffness, as well as P-and S-wave velocities, and stiffness anisotropy. Our experimental results show that the stress-sensitivity of shales is different at seismic and ultrasonic frequencies, which can be linked with simultaneously occurring changes in the dispersion with applied stress. Measured stress sensitivity of elastic properties for relatively dry samples was higher at seismic frequencies however, the increasing saturation of shales decreases the difference between seismic and ultrasonic stress-sensitivities, and for moist samples stress-sensitivity is higher at ultrasonic frequencies. Simultaneously, the increased saturation highly increases the dispersion in shales. We have also found that the stress-sensitivity is highly anisotropic in both shales and that in some of the cases higher stress-sensitivity of elastic properties can be seen in the direction parallel to the bedding plane.

The pioneering work of Mike Batzle and his colleagues has provided a fundamental understanding of mechanisms behind dispersion and attenuation of elastic waves in fluid-saturated rocks. It also has made way for a realization that these phenomena need to be accounted for in a better way when interpreting seismic and sonic data from the field. Laboratory experiments have formed the basis for new insight in the past and will continue to do so. Here, examples of experimental observations that give direct or indirect evidence for dispersion in sand, sandstone, and shale are presented. Ultrasonic data from compaction tests show that Biot flow is the most likely dispersion mechanism in pure unconsolidated sand. Strong shale dispersion has been identified through low-frequency and low-strain quasistatic measurements and through a novel technique based on static loading and unloading measurements. In shale and sandstone containing clay, there is evidence for water weakening. A comparative study shows an example where the stress dependences of P-and S-wave velocities at seismic frequencies exceed those measured by traditional ultrasonic methods.

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Chemical reactions with reservoir fluids and geology related in-situ stress changes may cause damages to cement sealing material in plugged and abandoned oil, gas and CO2 wells. To avoid leakages, a legitimate monitoring technique is needed that could allow for early warning in case such damages occur. In this paper, we test the utility of oil and gas well cement with a conductive filler in sensing stress changes. To this end, we have measured the resistance response of Portland G—oil and gas well cement with carbon nanofibers (CNF) to axial load during uniaxial compressive strength test. Simultaneously, the microseismicity data were collected. The resistance of the nanocomposite was measured using two-point method in the direction of loading. The resistance changes were correlated with acoustic emission events. A total of four different material response regions were distinguished and the resistivity and acoustic emission changes in these regions were described. Our results suggest that the two complementary methods, i.e., acoustic emission and resistance measurements, can be used for sensing stress state in materials including well cement/CNF composites. The results suggest that the well cement/CNF composites can be a good candidate material to be used as a transducer sensing changes in stress state in, e.g., well plugs up to material failure.

Cells probe their environments by extending protrusions: this process is mediated by the polymerization of actin gels at the edge of cells. Although their molecular components have been widely studied, their mesoscopic properties remain to be characterized. In this paper, we show that cell adhesion modulates actin gel dynamics. By changing the grafting density of fibronectin on a surface, we changed the adhesion strength of a cell on this surface. We found that the length of filopodia, the speeds of their growth and the speeds of retrograde flows were non-monotonic functions of the grafting density of fibronectin. The minima of the length and speeds of filopodia and the maximum of the speeds of retrograde flows are found at the same fibronectin density; this implies that there are strong correlations between these parameters. We used a simple model to predict that retrograde flows show non-monotonic behaviors because integrin-fibronectin binding mediates actomyosin and friction forces applied to actin gels. This model also predicts that connectivity of actin gels is responsible for the strong correlations between retrograde flows and filopodial growth. Altogether, our study investigates how actomyosin forces and friction with the substrate influence actin gel dynamics in living cells.

Seismic monitoring of reservoir and overburden performance during subsurface CO2 storage plays a key role in ensuring efficiency and safety. Proper interpretation of monitoring data requires knowledge about the rock physical phenomena occurring in the subsurface formations. This work focuses on rock-stiffness and elastic-velocity changes of a shale overburden formation caused by both reservoir-inflation induced stress changes, and leakage of CO2 into the overburden. In laboratory experiments, Pierre shale I core plugs were loaded along the stress path representative for the in-situ stress changes experienced by caprock during reservoir inflation. Tests were carried out in a triaxial compaction cell combining three measurement techniques and permitting for determination of: (i) ultrasonic velocities; (ii) quasi-static rock deformations, (iii) dynamic elastic stiffnesses at seismic frequencies within single test; which allowed to quantify effects of seismic dispersion. In addition fluid-substitution effects connected with possible CO2 leakage into the caprock formation were modelled by the modified anisotropic Gassmann model. Results of this work indicate that: (i) stress sensitivity of Pierre shale I is frequency dependent; (ii) reservoir inflation leads to the increase of the overburden Young's modulus and Poisson's ratio; (iii) in-situ stress changes mostly affects the P-wave velocities; (iv) small leakage of the CO2 into the overburden may lead to the velocity changes which are comparable with one associated with geomechanical influence; (v) non-elastic effects increase stress sensitivity of an acoustic waves; (iv) both geomechanical and fluid substitution effects would create significant time shifts which should be detectable by time-lapse seismic.