scholarly journals Numerical Investigation of the Effect of Heterogeneous Pore Structures on Elastic Properties of Tight Gas Sandstones

2021 ◽  
Vol 9 ◽  
Author(s):  
Zhiqi Guo ◽  
Xiaoying Qin ◽  
Yiming Zhang ◽  
Cong Niu ◽  
Di Wang ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Elastic Properties ◽  
Mean Value ◽  
Tight Gas ◽  
Ratio Model ◽  
Empirical Formulas ◽  
Tight Sandstone ◽  
Pore Model ◽  
The Third ◽  
Tight Gas Sandstone

Strong heterogeneity of pore microstructures leads to complicated velocity-porosity relationships in tight sandstone that cannot be well explained by conventional empirical formulas. To better understand the effect of complex pore structures on elastic properties of tight gas sandstone, we compared three rock physics models. In the first model, we used a single aspect ratio value to quantify varied pore geometry in the tight sands. In the second model, complex pore space was equivalent to the combination of high-aspect-ratio round pores (stiff pores) and low-aspect-ratio compliant microcracks (soft pores). In the third multiple pore-aspect-ratio model, pore spaces are represented using a set of pores with varied values of aspect ratio following statistical normal distribution. Modeling results showed that complex velocity-porosity relationships could be interpreted by the variations in pore aspect ratio in the first model, by the fraction of soft pores in the second model, and by the mean value and variance in the third statistical model. For a given mean value in the third model, higher variance of the multiple pore-aspect-ratio indicated stronger heterogeneity of pore spaces. Further studies on rock physical inversion showed that, compared with the first single pore-aspect-ratio model, the second dual-pore model gave better prediction in shear wave velocity by regarding the soft pore fraction as a fitting parameter. This finding revealed that the dual-pore model could be a more realistic representation of tight sandstone. The third statistical model showed comparable precision in the prediction of shear wave velocity compared with the dual-pore model; however, uncertainty existed for simultaneously determining mean value and variance of pore aspect ratio. On the basis of the dual-pore model, we evaluated the elastic modulus of dry frames of the tight sandstone using logging data in a borehole. Compared with empirical formulas, such as the Krief methods, the method in this paper provided a more rigorous way to determine elastic properties of dry frames for the tight sandstone. Comparisons of rock physical modeling methods offer a better understanding of the microstructures controlling the elastic behaviors of tight gas sandstone.

scholarly journals Estimation of the Pore Microstructure of Tight-Gas Sandstone Reservoirs with Seismic Data

2021 ◽  
Vol 9 ◽  
Author(s):  
Wei Cheng ◽  
Jing Ba ◽  
José M. Carcione ◽  
Mengqiang Pang ◽  
Chunfang Wu
Keyword(s):  
Seismic Data ◽  
Velocity Ratio ◽  
P Wave ◽  
Seismic Exploration ◽  
Variable Pressure ◽  
Well Log ◽  
Tight Gas ◽  
Tight Sandstone ◽  
Sandstone Reservoirs ◽  
Tight Gas Sandstone

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.

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.

scholarly journals Linking elastic and electrical properties of rocks using cross-property DEM

2021 ◽  
Author(s):  
P A Cilli ◽  
M Chapman
Keyword(s):  
Electrical Properties ◽  
Aspect Ratio ◽  
Elastic Properties ◽  
Rock Physics ◽  
Intermediate Step ◽  
Ratio Analysis ◽  
Pore Shape ◽  
Ratio Model ◽  
Property Relations ◽  
Elastic Data

Summary Combining electrical and elastic measurements can be instrumental in lowering the uncertainty of subsurface characterisation. Many commonly used rock physics relations for joint electrical-elastic properties are at least partly empirical, and often rely on the estimation of porosity as an intermediate step. We combine differential effective medium schemes which relate respectively elastic and electrical properties to porosity and pore shape. The resulting expressions are independent of porosity, depending only on pore aspect ratio. Analysis of published joint electrical-elastic data shows that a single aspect ratio model performs well for clean sandstones, allowing us to model Vp/Vs ratios as a function of resistivity. Clay-bearing sandstones are more complex, but our modelling can still identify the correct trends. We speculate about the potential to extend our approach to produce additional cross-property relations.

Correlations Between the Values of Maternal Glycemia from the Last Trimester of Pregnancy and Fetal Birth Weight

2013 ◽  
Vol 20 (3) ◽  
pp. 259-265
Author(s):  
Monica Vereş ◽  
Aurel Babeş ◽  
Szidonia Lacziko
Keyword(s):  
First Trimester ◽  
Mean Value ◽  
Fetal Macrosomia ◽  
Entire Group ◽  
Third Trimester ◽  
The Third ◽  
Group I ◽  
Fetal Birth Weight ◽  
First Trimester Of Pregnancy ◽  
The Mean

Abstract Background and aims: Gestational diabetes represents a form of diabetes diagnosed during pregnancy that is not clearly overt diabetes. In the last trimester of gestation the growth of fetoplacental unit takes place, thus maternal hyperglycemia will determine an increased transplacental passage, hyperinsulinemia and fetal macrosomia. The aim of our study was that o analyzing the effect of maternal glycemia from the last trimester of pregnancy over fetal weight. Material and method: We run an observational study on a group of 46 pregnant women taken into evidence from the first trimester of pregnancy, separated in two groups according to blood glucose determined in the third trimester (before birth): group I normoglycemic and group II with hyperglycemia (>92mg/dl). Results: The mean value of third trimester glycemia for the entire group was of 87.13±22.03. The mean value of the glycemia determined in the third trimester of pregnancy was higher in the second group (109.17 mg/dl) in comparison to the first group (74.,21 mg/dl). The ROC curve for third trimester glycemia as fetal macrosomia appreciation test has an AUC of 0.517. Conclusions: Glycemia determined in the last trimester of pregnancy cannot be used alone as the predictive factor for fetal macrosomia.

Quantifying tight-gas sandstone permeability via critical path analysis

2016 ◽  
Vol 92 ◽  
pp. 316-322 ◽  
Author(s):  
Behzad Ghanbarian ◽  
Carlos Torres-Verdín ◽  
Todd H. Skaggs
Keyword(s):  
Path Analysis ◽  
Critical Path ◽  
Tight Gas ◽  
Critical Path Analysis ◽  
Sandstone Permeability ◽  
Tight Gas Sandstone

scholarly journals Study on the additional support length requirements of single-span bridges due to skew using a simplified method

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Suiwen Wu ◽  
Junfeng Jia ◽  
Chiyu Jiao ◽  
Junfei Huang ◽  
Jianzhong Li
Keyword(s):  
Aspect Ratio ◽  
Response Spectrum ◽  
Skew Angle ◽  
Empirical Formulas ◽  
Design Response Spectrum ◽  
Skew Bridges ◽  
Simplified Method ◽  
Additional Support ◽  
Linear Manner ◽  
Support Length

AbstractSkew bridges with seat-type abutments are frequently unseated in earthquakes due to large transverse displacements at their acute corners. It is believed these large displacements are due to in-plane rotation of the superstructure. Lack of detailed guidelines for modeling of skew bridges, many current design codes give empirical expressions rather than theoretical solutions for the additional support length required in skew bridges to prevent unseating. In this paper, a parametric study has been carried out to study the influence of skew angle, aspect ratio and fundamental periods of bridges on the additional support length requirements of single-span bridges due to skew using a shake table experiment validated Simplified Method, which is capable of simulating gap closure based on response spectrum analysis. This method is developed based on the premise that the obtuse corner of the superstructure engages the adjacent back wall during lateral loading and rotates about this corner until the loading reverses direction. A design response spectrum specified in AASHTO LRFD Specifications was employed to represent the design-level earthquakes. The results show the additional length required to prevent unseating due to skew increases with the skew angle in an approximately linear manner when the angle is less than a critical value and decreases for angles above this value. This critical skew angle increases with the aspect ratio approximately in a linear manner and shows negligible dependence on the fundamental periods of the bridges, and combination of span length and width. In addition, the critical skew angle varies between 58° and 66°, when the aspect ratio is varied from 3.0 to 5.0. The results also show that the empirical formulas for minimum support length requirements of skew bridges in current codes and specifications can not accurately reflect the influence of skew.

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