Attenuation methods for quantifying gas saturation in organic-rich shale and tight gas formations

Geophysics ◽  
2020 ◽  
pp. 1-44
Author(s):  
Qiaomu Qi ◽  
Li-Yun Fu ◽  
Jixin Deng ◽  
Junxing Cao
Keyword(s):  
Wave Attenuation ◽  
Rock Physics ◽  
Accurate Estimate ◽  
Neutron Density ◽  
Median Frequency ◽  
Tight Gas ◽  
S Wave ◽  
Gas Saturation ◽  
Fluid Saturation ◽  
Positive Linear Correlation

Quantitative interpretation of waveform attenuation for determining petrophysical properties remains one of the most challenging problems associated with rock physics. In this study, we extract $P$- and $S$-wave attenuations from monopole and dipole waveforms by median-frequency shift and multi-frequency inversion methods, respectively. Two effective methods are then proposed to compute gas saturation in organic-rich shale and tight gas formations from the full-waveform sonic attenuations. Crossplots of the $P$-to-$S$ wave attenuation ratio ( Qp-1/ Qs-1) and core gas saturation show a positive linear correlation. The Qp-1/ Qs-1 and neutron-density porosity difference exhibit an identical log trend across different formations. The coincidence of these two different hydrocarbon indicators implies that the Qp-1/ Qs-1 is most sensitive to pore-fluid saturation and less affected by variations in lithology. In the first method, the core-calibrated Qp-1/ Qs-1 yields an accurate estimate of gas saturation. The second method is suited for the absence of core saturation data, which employs the probability distribution of Qp-1/ Qs-1 for the evaluation of gas saturation. Compared to conventional resistivity methods, the proposed attenuation method, as a non-electric approach, provides more accurate gas saturation prediction for low-resistivity reservoir rocks. Finally, we analyze the characteristics of attenuation-saturation relations in low porosity rocks and discuss the possible attenuation mechanisms.

Rock-physics modeling of ultrasonic P- and S-wave attenuation in partially frozen brine and unconsolidated sand systems and comparison with laboratory measurements

Geophysics ◽  
2019 ◽  
Vol 84 (5) ◽  
pp. MR153-MR171 ◽  
Author(s):  
Linsen Zhan ◽  
Jun Matsushima
Keyword(s):  
Ultrasonic Wave ◽  
Wave Attenuation ◽  
Freezing Point ◽  
Rock Physics ◽  
Ultrasonic Waves ◽  
P Wave ◽  
Dominant Factor ◽  
S Wave ◽  
High Attenuation ◽  
Unconsolidated Sand

The nonintuitive observation of the simultaneous high velocity and high attenuation of ultrasonic waves near the freezing point of brine was previously measured in partially frozen systems. However, previous studies could not fully elucidate the attenuation variation of ultrasonic wave propagation in a partially frozen system. We have investigated the potential attenuation mechanisms responsible for previously obtained laboratory results by modeling ultrasonic wave transmission in two different partially frozen systems: partially frozen brine (two phases composed of ice and unfrozen brine) and unconsolidated sand (three phases composed of ice, unfrozen brine, and sand). We adopted two different rock-physics models: an effective medium model for partially frozen brine and a three-phase extension of the Biot model for partially frozen unconsolidated sand. For partially frozen brine, our rock-physics study indicated that squirt flow caused by unfrozen brine inclusions in porous ice could be responsible for high P-wave attenuation around the freezing point. Decreasing P-wave attenuation below the freezing point can be explained by the gradual decrease of squirt flow due to the gradual depletion of unfrozen brine. For partially frozen unconsolidated sand, our rock-physics study implied that squirt flow between ice grains is a dominant factor for P-wave attenuation around the freezing point. With decreasing temperature lower than the freezing point, the friction between ice and sand grains becomes more dominant for P-wave attenuation because the decreasing amount of unfrozen brine reduces squirt flow between ice grains, whereas the generation of ice increases the friction. The increasing friction between ice and sand grains caused by ice formation is possibly responsible for increasing the S-wave attenuation at decreasing temperatures. Then, further generation of ice with further cooling reduces the elastic contrast between ice and sand grains, hindering their relative motion; thus, reducing the P- and S-wave attenuation.

scholarly journals Statistical characterization of gas-patch distributions in partially saturated rocks

Geophysics ◽  
2009 ◽  
Vol 74 (2) ◽  
pp. WA51-WA64 ◽  
Author(s):  
Julianna Toms-Stewart ◽  
Tobias M. Müller ◽  
Boris Gurevich ◽  
Lincoln Paterson
Keyword(s):  
Correlation Function ◽  
Autocorrelation Function ◽  
Correlation Length ◽  
Wave Attenuation ◽  
Length Scales ◽  
Gas Saturation ◽  
Partially Saturated ◽  
Statistical Measures ◽  
Fluid Saturation ◽  
Limestone Sample

Reservoir rocks are often saturated by two or more fluid phases forming complex patterns on all length scales. The objective of this work is to quantify the geometry of fluid phase distribution in partially saturated porous rocks using statistical methods and to model the associated acoustic signatures. Based on X-ray tomographic images at submillimeter resolution obtained during a gas-injection experiment, the spatial distribution of the gas phase in initially water-saturated limestone samples are constructed. Maps of the continuous variation of the percentage of gas saturation are computed and associated binary maps obtained through a global thresholding technique. The autocorrelation function is derived via the two-point probability function computed from the binary gas-distribution maps using Monte Carlo simulations.The autocorrelation function can be approximated well by a single Debye correlation function or a superposition of two such functions. The characteristic length scales and show sensitivity (and hence significance) with respect to the percentage of gas saturation. An almost linear decrease of the Debye correlation length occurs with increasing gas saturation. It is concluded that correlation function and correlation length provide useful statistical information to quantify fluid-saturation patterns and changes in these patterns at the mesoscale. These spatial statistical measures are linked to a model that predicts compressional wave attenuation and dispersion from local, wave-induced fluid flow in randomly heterogeneous poroelastic solids. In particular, for a limestone sample, with flow permeability of 5 darcies and an average gas saturation of [Formula: see text], significant [Formula: see text]-wave attenuation is predicted at ultrasonic frequencies.

Seismic estimation of fluid saturation based on rock physics: A case study of the tight-gas sandstone reservoirs in Ordos Basin

2021 ◽  
pp. 1-47
Author(s):  
Chao Li ◽  
Peng Hu ◽  
Jing Ba ◽  
José M. Carcione ◽  
Tianwen Hu ◽  
...  
Keyword(s):  
Ordos Basin ◽  
Rock Physics ◽  
Gas Production ◽  
P Wave ◽  
Quantitative Prediction ◽  
Rock Fragment ◽  
Tight Gas ◽  
Fluid Saturation ◽  
Sandstone Reservoirs ◽  
Tight Gas Sandstone

Tight-gas sandstone reservoirs of the Ordos Basin of China are characterized by high rock-fragment content, dissimilar pore types and a random distribution of fluids, leading to strong local heterogeneity. We model the seismic properties of these sandstones with the double-double porosity (DDP) theory, which considers water saturation, porosity and the frame characteristics. A generalized seismic wavelet is used to fit the real wavelet and the peak frequency-shift method is combined with the generalized S-transform to estimate attenuation. Then, we establish rock-physics templates (RPTs) based on P-wave attenuation and impedance. We use the log data and related seismic traces to calibrate the RPTs and generate a 3D volume of rock-physics attributes for the quantitative prediction of saturation and porosity. The predicted values are in good agreement with the actual gas production reports, indicating that the method can be effectively applied to heterogeneous tight-gas sandstone reservoirs.

scholarly journals Rock Physical Modeling and Seismic Dispersion Attribute Inversion for the Characterization of a Tight Gas Sandstone Reservoir

2021 ◽  
Vol 9 ◽  
Author(s):  
Han Jin ◽  
Cai Liu ◽  
Zhiqi Guo ◽  
Yiming Zhang ◽  
Cong Niu ◽  
...  
Keyword(s):  
Rock Physics ◽  
Wave Dispersion ◽  
P Wave ◽  
Tight Gas ◽  
Tight Sandstone ◽  
P Wave Dispersion ◽  
Frequency Dependent ◽  
Gas Saturation ◽  
Avo Inversion ◽  
Tight Gas Sandstone

Gas identification using seismic data is challenging for tight gas reservoirs with low porosity and permeability due to the complicated poroelastic behaviors of tight sandstone. In this study, the Chapman theory was used to simulate the dispersion and attenuation caused by the squirt flow of fluids in the complex pore spaces, which are assumed to consist of high aspect-ratio pores (stiff pores) and low aspect-ratio microcracks (soft pores). The rock physics modeling revealed that as the gas saturation varies, P-wave velocity dispersion and attenuation occurs at seismic frequencies, and it tends to move to high frequencies as the gas saturation increases. The velocity dispersion of the tight gas sandstone causes a frequency-dependent contrast in the P-wave impedance between the tight sandstone and the overlying mudstone, which consequently leads to frequency-dependent incidence reflection coefficients across the interface. In the synthetic seismic AVO modeling conducted by integrating the rock physics model and the propagator matrix method, the variations in the amplitudes and phases of the PP reflections can be observed for various gas saturations. The tests of the frequency-dependent AVO inversion of these synthetic data revealed that the magnitude of the inverted P-wave dispersion attribute can be used to indicate gas saturation in tight sandstone reservoirs. The applications of the frequency-dependent AVO inversion to the field pre-stacked seismic data revealed that the obtained P-wave dispersion attribute is positively correlated with the gas production from the pay zone at the well locations. Thus, the methods of the rock physics modeling and the frequency-dependent AVO inversion conducted in this study have good potential for the evaluation of the gas saturation in tight gas sandstone reservoirs.

Digital rock physics applied to squirt flow

Geophysics ◽  
2021 ◽  
pp. 1-40
Author(s):  
Simón Lissa ◽  
Matthias Ruf ◽  
Holger Steeb ◽  
Beatriz Quintal
Keyword(s):  
Numerical Model ◽  
Wave Attenuation ◽  
Stokes Equations ◽  
Solid Phase ◽  
Fluid Pressure ◽  
Rock Physics ◽  
S Wave ◽  
Digital Rock Physics ◽  
Digital Rock ◽  
Dry Conditions

We present a workflow for computing the seismic wave moduli dispersion and attenuation due to squirt flow in a numerical model derived from a micro X-Ray Computed Tomography image of cracked (through thermal treatment) Carrara marble sample. To generate the numerical model, the image is processed, segmented and meshed. The finite-element method is adopted to solve the linearized, quasi-static Navier-Stokes equations describing laminar flow of a compressible viscous fluid inside the cracks coupled with the quasi-static Lamé-Navier equations for the solid phase. We compute the effective P- and S-wave moduli in the three Cartesian directions for a model in dry conditions (saturated with air) and for a smaller model fully saturated with glycerin and having either drained or undrained boundary conditions. For the model saturated with glycerin, the results show significant and frequency-dependent P- and S-wave attenuation and the corresponding dispersion caused by squirt flow. Squirt flow occurs in response to fluid pressure gradients induced in the cracks by the imposed deformations. Our digital rock physics (DRP) workflow can be used to interpret laboratory measurements of attenuation using images of the rock sample.

scholarly journals S-wave attenuation in the Marmara Region, northwestern Turkey

1998 ◽  
Vol 25 (14) ◽  
pp. 2733-2736 ◽  
Author(s):  
Horasan Gündüz ◽  
Kaşlilar-Özcan Ayşe ◽  
Boztepe-Güney Aysun ◽  
Türkelli Niyazi
Keyword(s):  
Wave Attenuation ◽  
Marmara Region ◽  
S Wave ◽  
Northwestern Turkey

P- and S-wave attenuation logs from monopole sonic data

Geophysics ◽  
2000 ◽  
Vol 65 (3) ◽  
pp. 755-765 ◽  
Author(s):  
Xinhua Sun ◽  
Xiaoming Tang ◽  
C. H. (Arthur) Cheng ◽  
L. Neil Frazer
Keyword(s):  
Wave Attenuation ◽  
Fluid Velocity ◽  
P Wave ◽  
Reservoir Rock ◽  
Rock Properties ◽  
S Wave ◽  
Intrinsic Attenuation ◽  
Modified Method ◽  
Receiver Array ◽  
Attenuation Profiles

In this paper, a modification of an existing method for estimating relative P-wave attenuation is proposed. By generating synthetic waveforms without attenuation, the variation of geometrical spreading related to changes in formation properties with depth can be accounted for. With the modified method, reliable P- and S-wave attenuation logs can be extracted from monopole array acoustic waveform log data. Synthetic tests show that the P- and S-wave attenuation values estimated from synthetic waveforms agree well with their respective model values. In‐situ P- and S-wave attenuation profiles provide valuable information about reservoir rock properties. Field data processing results show that this method gives robust estimates of intrinsic attenuation. The attenuation profiles calculated independently from each waveform of an eight‐receiver array are consistent with one another. In fast formations where S-wave velocity exceeds the borehole fluid velocity, both P-wave attenuation ([Formula: see text]) and S-wave attenuation ([Formula: see text]) profiles can be obtained. P- and S-wave attenuation profiles and their comparisons are presented for three reservoirs. Their correlations with formation lithology, permeability, and fractures are also presented.

Sign in / Sign up

Export Citation Format

Share Document