Dispersion measurement using the primary and higher order resonance modes of the DARS system
TANG Genyang, DONG Chunhui, WANG Shangxu, ZENG Xin, REN Baode
1 College of Geophysics , China University of Petroleum-Beijing, Beijing 102249, China 2 Ministry of Education Complex Oil and Gas Reservoir Exploration and Development Engineering Research Center, Beijing 102249, China
The Differential Resonance Acoustic Spectroscopy System (DARS) can be used to measure the compressibility or elastic modulus of rock samples at ~600 Hz. The measurement relies on the first-order resonance mode measurement of the DARS system, while ignoring the use of higher-order resonance modes to obtain information. This article uses the second and third order resonance mode measurements of the DARS system to estimate the bulk modulus of rock samples. Using three reference samples instead of two reference samples, we have obtained calibration parameters with higher accuracy. Through the analysis of the calibration parameters, the measurement process is improved, and a measurement location with higher sensitivity is selected to replace the traditional multi-point measurement. On this basis, a new linear least squares inversion algorithm is proposed to obtain calibration parameters and sample elastic parameters. Using this high-precision inversion method, the bulk modulus of seven artificial sandstone samples with different porosities was obtained at three resonance frequencies. The bulk modulus of these samples showed dispersion from the low-order resonance frequency (~600 Hz) to the third-order resonance frequency (~1800 Hz). For samples with relatively low to medium permeability, the dispersion is large, while for samples with high permeability, the dispersion is weak. This dispersion difference is controlled by permeability, which is due to the different fluidity of the fluid in the sample pores, which causes different samples to fall into different “domains”. For samples with relatively low or medium permeability, the DARS resonance frequency may be within the characteristic frequency range of the dispersion, so the dispersion is strong. For samples with higher permeability, the DARS resonance frequency is in the low frequency range, and the bulk modulus hardly changes with frequency. According to this conclusion, there may be almost no difference in the elastic properties of high permeability reservoir rocks measured at seismic and acoustic logging frequencies. Low-permeability reservoir rocks may have considerable differences in elastic properties in these frequency bands, which may cause a mismatch between seismic and logging reservoir parameters. Therefore, the characteristic frequency and intensity of dispersion may be one of the indicators of permeability. This article adopts the DARS measurement method of multi-order resonance frequencies, which can provide direct dispersion data in the frequency range of seismic-sonic logging, and has great potential for studying the dispersion mechanism of rocks. Dispersion data can also be used to explain the difference in elastic properties between seismic and acoustic frequency bands and provide an important reference for the matching of seismic and logging data.
Key words：
DARS system; artificial sandstone; permeability; bulk modulus; dispersion
TANG Genyang, DONG Chunhui, WANG Shangxu, ZENG Xin, REN Baode. Dispersion measurement using the primary and higher order resonance modes of the DARS system. Petroleum Science Bulletin, 2020, 04: 483-495.
