Abstract
Abstract Tight sandstones have low porosity and permeability and strong heterogeneities with microcracks, resulting in small wave impedance contrasts with the surrounding rock and weak fluid-induced seismic effects, which make the seismic characterization for fluid detection and identification difficult. For this purpose, we propose a reformulated modified frame squirt-flow (MFS) model to describe wave attenuation and velocity dispersion. The squirt-flow length (R) is an important parameter of the model, and, at present, no direct method has been reported to determine it. We obtain the crack properties and R based on the DZ (David-Zimmerman) model and MFS model, and how these properties affect the wave propagation, considering ultrasonic experimental data of the Sichuan Basin. The new model can be useful in practical applications related to exploration areas.
Highlights
Tight reservoirs are becoming important in seismic exploration [1,2,3], with tight sandstones playing a dominant role in China, since they are widely distributed
The results show that R increases with increasing crack density and permeability and decreases with increasing crack aspect ratio
We have proposed a rock-physics model to estimate the squirtflow length (R) of tight sandstones, based on experiment data
Summary
Tight (oil and gas) reservoirs are becoming important in seismic exploration [1,2,3], with tight sandstones playing a dominant role in China, since they are widely distributed. The P-wave velocity predicted by the M-J model is greater than that of the Biot model at high frequencies and greater than the Gassmann velocity at low frequencies This equation is not applicable to the case of gas-saturated or dry rocks. Dvorkin et al [40] obtained the complex modulus of fluid-saturated rocks based on the BISQ mode by considering a one-dimensional radial flow This theory is consistent with the Gassmann velocity at the low-frequency limit. The P-wave velocity obtained with this model is higher than the theoretical maximum value at high frequencies (when all the cracks are closed, and the P-wave velocity value is determined by the Biot model) [41]. A semiempirical formula of R for tight sandstones is proposed
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