We have developed a new modeling approach for the complex-valued P-wave modulus of a rock saturated with two-phase fluid accounting for the variation with frequency and water saturation. Our method is based on the dynamic-equivalent-medium approach theory, which predicts P-wave modulus dispersion due to mesoscopic-scale wave-induced fluid flow (WIFF). Although the application of the original theory was limited to small fluctuation media, we have extended it to also be applicable for high-fluctuation media such as partially saturated rock. Our modification and extension consists of two components. The first is introducing a scaling by the rigorous bounds for P-wave velocity dispersion by mesoscopic-scale WIFF. The second is to develop a model representing the effective patch size of stiffer fluid that controls the location of the dispersion curve. We have found that the spatial correlation length of heterogeneity of saturated rock used in the original theory does not appropriately capture the effective heterogeneity scale responsible for mesoscale pressure diffusion. Its variation with saturation can be properly accounted for by the proposed patch-sized variation model. The comparison of the theoretical prediction with the published laboratory velocity and attenuation measurements suggests that our approach predicts the wave properties for high-fluctuation media with reasonable accuracy. The effect of mesoscopic-scale pressure diffusion is significant and the amount of velocity dispersion and attenuation is large in high-fluctuation media; therefore, our extension will improve quantitative characterization of, for example, a [Formula: see text]-sequestrated reservoir either by P-wave velocity or attenuation.
Herein, we have proposed a single-step preparation of topological gels using vinyl-modified βcyclodextrin (V-β-CyD) and isoprene. Copolymerization of V-β-CyD and isoprene in an aqueous solution resulted in gelation due to V-β-CyD acting as a novel type of copolymer chain cross-linker. The vinyl moiety of V-β-CyD becomes a part of the copolymer, while the β-CyD moiety of V-β-CyD simultaneously incorporates the isoprene component of the copolymer. V-β-CyD is capable of two different modes of cross-linking at each end, i.e., chemically bonding and mechanically interlocking. Due to the shape of the cross-linking point, we refer to it as figure-of-six cross-linking. Nuclear magnetic resonance (NMR) analysis showed that the gel contained V-β-CyD and isoprene in an approximately 1:0.3 stoichiometry. The relatively high content of β-CyD was reflected in the character of the gel; the gel swelled in dimethylformamide (DMF) which is a good solvent of β-CyD. A fluorometric analysis using 6-(p-toluidino)-2-naphthalenesulfonic acid (TNS) showed that the appended β-CyD was able to accommodate guest molecules. Introduction of an additional vinyl monomer into the gel was also successful. Addition of 4-vinylphenylboronic acid to the preparation procedure yielded a sugar-responsive gel that swelled in the presence of D-fructose.
A two‐dimensional walkaway vertical seismic profiling survey using distributed acoustic sensing was conducted at an onshore site in Japan. The maximum depth and the deviation of the observation well were more than 4,000 m and 81 degrees, respectively. Among the several methods for installing fibre optic cables, we adopted the inside coiled tubing method, in which coiled tubing containing a fibre optic cable is deployed. The signal‐to‐noise ratio of the raw shot gather was low, possibly due to poor coupling between the fibre optic cable and the subsurface formation resulting from the fibre optic cable deployment method and the existence of considerable tubewave noise. Nevertheless, direct P‐wave arrivals, P–P reflections and P–S converted waves exhibited acceptable signal‐to‐noise ratios after careful optimization of gauge length for distributed acoustic sensing optical processing and the application of carefully parameterized tubewave noise suppression. One of the challenges in current distributed acoustic sensing vertical seismic profile data processing is the separation of P‐ and S‐waves using only one‐component measurements. Hence, we applied moveout correction using two‐dimensional ray tracing. This process effectively highlights only reflected P‐waves, which are used in subsequent subsurface imaging. Comparison with synthetic well seismograms and two‐dimensional surface seismic data confirms that the final imaging result has a sufficiently high quality for subsurface monitoring. We acquired distributed acoustic sensing vertical seismic profile data under both flowing conditions and closed conditions, in which the well was shut off and no fluid flow was allowed. The two imaging results are comparable and suggest the possibility of subsurface imaging and time‐lapse monitoring using data acquired under flowing conditions. The results of this study suggest that, by adopting the inside coiled tubing method without drilling a new observation well, more affordable distributed acoustic sensing vertical seismic profile monitoring can be achieved in fields such as CO2 capture and storage and unconventional shale projects, where monitoring costs have to be minimized.
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