Abstract
Tight-sandstone reservoirs have a complex pore structure with microcracks and intergranular pores, which have a significant impact on the seismic properties. We have performed ultrasonic measurements at different confining pressures for 15 tight-gas sandstone samples of the Xujiahe formation in Western Sichuan Basin, and have available well-log and seismic data of this area. The aim of this work is to estimate the porosity and crack properties for variable pressure conditions. The EIAS (equivalent inclusion-average stress) model is adopted to compute the high- and low-frequency bulk and shear moduli as a function of crack aspect ratio and (soft) and (stiff) porosities. Then, we use the EIAS-Zener anelastic model to obtain the wave properties as a function of frequency, and compare results with those of the constant Q (Kjartansson) one for verification of the robustness of the approach. The corresponding P-wave impedance, density and phase velocity ratio (VP/VS) are computed in order to built 3D rock‐physics templates (RPTs) at the ultrasonic, well-log and seismic frequency bands. The methodology is applied to a survey line crossing two wells, which together with the laboratory experiments, provide calibration suitable data. The estimated stiff porosity and crack porosity and density are consistent with the available data and actual production records, indicating that 3D RPTs provide a useful interpretation tool in seismic exploration and prospect evaluation.
Highlights
Unconventional gas resources offer significant gas production growth potential
Tight-gas sandstones represent an important part of the unconventional production and abundant reserves are yet to be developed (Khlaifat et al, 2011)
The crack porosity, average crack aspect ratio and quality factors and phase velocities were obtained as a function of frequency by fitting experimental data at the unrelaxed and relaxed states
Summary
Unconventional gas resources offer significant gas production growth potential. Tight-gas sandstones represent an important part of the unconventional production and abundant reserves are yet to be developed (Khlaifat et al, 2011). The crack porosity, average crack aspect ratio and quality factors and phase velocities were obtained as a function of frequency by fitting experimental data at the unrelaxed and relaxed states. We used VP/VS, impedance and density to build 3D RPTs, based on the EIAS model and the Zener frequency kernels, to characterize the relation between elastic properties and reservoir attributes, i.e., equant porosity and crack porosity and aspect ratio (Carcione, 2014). Ultrasonic compressional and shear-wave velocities (VP and VS) at full-gas (nitrogen) were obtained by picking the first arrivals of waveforms with 1 MHz dominant frequency These samples have low porosity and permeability, with a maximum of 13.91% and 1.37 mD, respectively. The previous models are used to relate the crack properties (aspect ratio and porosity) and the frequency-dependent elastic moduli, phase velocities and quality factors. The methodology is reliable, and is expected to provide an important basis for further rock-physics studies and field applications of seismic exploration
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