Search

Geometric heterogeneities in tight reservoir rocks saturated with a fluid mixture may exhibit different scale distribution characteristics. Conventional models of rock physics based on poroelasticity, which usually consider single‐scale pore structure and fluid patches, are inadequate for describing elastic wave responses. A major challenge is to establish the relationship between the wave response at different spatial scales and frequencies. To address this problem, three sets of observational data over a wide frequency range were obtained from a tight oil reservoir in the Ordos Basin, China. Ultrasonic measurements were made on eight sandstone samples at partial oil‐water saturation at 0.55 MHz. Data from six borehole measurements and seismic profiles were acquired and analyzed at about 10 kHz and 30 Hz, respectively. Analysis of the cast thin sections shows that dissolution pores and microcracks generally develop, with fractal dimensions of the pores ranging from 2.45 to 2.67 for the samples with porosities between 5.1% and 10.2%. Compressional wave velocity and attenuation were estimated from the observed data. The results show that the velocity dispersion from seismic to ultrasonic frequencies is 10.02%, mostly occurring between sonic and ultrasonic frequencies. The attenuation is stronger at higher oil saturation. The relationships between velocity, attenuation, and wavelength were established and can be used for further forward modeling and seismic interpretation studies. A partial saturation model has been derived based on effective differential medium theory and a double double‐porosity model, assuming that the medium contains fractal cracks and fluid patches. The effects of scale and saturation on wave responses are prevalent. Modeling results consistent with observed data show that the radii of cracks and fluid patches range from 0.1 μm to 2.8 mm, affecting ultrasonic, acoustic, and seismic attenuation. The multiscale data and proposed model quantify the relationship between fracture and fluid distributions and attenuation and could be useful for upscaling to the reservoir scale. The study helps improve the understanding of seismic wave propagation in partially saturated rocks, which has potential applications in seismic exploration, hydrocarbon production in reservoirs, and CO2 sequestration in aquifers. Plain Language Summary: The physical properties of the rock and fluid can be inferred from the measured elastic wave responses and energy dissipation characteristics. However, the effects of heterogeneities of different sizes and at different frequencies can hinder studies to quantify wave responses in a partially saturated porous medium, which are usually based on laboratory measurements. A major problem is the difference between observed frequencies and scales: megahertz in the laboratory, 10 of kilohertz in the borehole scale, and hertz in the seismic exploration scale. In this work, the frequency‐ and saturation‐dependent compressional velocity and attenuation are investigated using three geophysical data sets from the same tight reservoirs. A strong velocity dispersion over the measured frequency range is observed. The stronger attenuation at partial saturation may be caused by the multiscale heterogeneities of the pore structure and fluid patch distribution. A fractal poroelasticity model is developed by gradually inserting inclusions of different sizes with compliant pores and liquid patches into a homogeneous host skeleton. The wave responses are significantly affected by scale distribution and saturation. The proposed model, verified by the measured data, can be useful in interpreting the anelasticity of tight heterogeneous reservoirs in a broadband range. Key Points: We investigate how fluid saturation affects wave attenuation and dispersion over a wide frequency range in tight sandstone layersA partial saturation model describes the fractal properties of the layersThe size of the cracks and fluid patches are determined by applying the fractal model to the multiscale data [ABSTRACT FROM AUTHOR]

Tight gas sandstone reservoirs play an important role in the field of exploration and development of unconventional oil/gas resources. However, these reservoirs typically exhibit low-porosity and poor-permeability, with pore structures of strong heterogeneity comprising connected and isolated pores. The traditional rock physics models (such as the Gassmann model) are established under the assumption of fully connected pores, which cannot reasonably describe the complexity in tight reservoirs. In this regard, this work proposes a rock physics modeling approach aimed at tight sandstone reservoirs based on a reformulated Xu–White model. Specifically, the differential effective medium model and Xu–White model are combined to analyze the influences of isolated pores on the dry rock skeleton. To characterize the general effects due to the different pore types on rock elastic moduli, the volume content of connected pores is defined as a pore-connectivity parameter. The simulated annealing algorithm is applied to invert the parameter, which is treated as a weighting coefficient for correcting the wet rock elastic moduli, thereby improving the precision of rock physics modeling. The proposed approach is tested and verified by using the log data of the work area of Sichuan Basin, West China. The result shows that, compared with the conventional method, the P- and S-wave velocities predicted by the proposed method are consistent with the log data. In addition to the inversion results of the conventional petrophysical parameters, the inverted pore-connectivity parameter can be a good auxiliary attribute that assists locating potential targets for tight gas sandstone reservoirs. [ABSTRACT FROM AUTHOR]

Seismic wave propagation is affected by wave‐induced local fluid flow between stiff pores and multiscale fractures. To investigate this phenomenon, forced vibration (1–100 Hz) and ultrasonic (106 Hz) measurements are performed on two tight sandstones with complex pore geometry in dry and water‐saturated scenarios. Porosity, permeability, and ultrasonic velocities were also measured at different differential pressures. The results indicate that the nonlinear behavior of these properties is strongly influenced by the presence of cracks, and the correlations between the permeability/ultrasonic velocities and porosity are different. A wave propagation model is then developed in which penny‐shaped cracks are inclusions introduced stepwise into a porous medium to describe the wave anelasticity in a wide frequency band at different pressures. As a result, the model provides good agreement with the measured P‐wave velocity dispersion, and the pore aspect ratio spectrum and crack radii are determined. We then compare the estimated crack radii and pore size distributions from nuclear magnetic resonance spectroscopy. Published data of a tight sandstone and a low porosity sandstone in the frequency range (2–200, 106) and (1–3,000) Hz at different differential pressures are also analyzed to validate the model. The aspect ratios, volume fractions, and radii of pores/cracks are used to describe the measured permeability. The present work can provide new insights into the geophysical properties of reservoir rocks with complex pore geometry. Plain Language Summary: Multiscale cracks exert an important influence on the dispersion and attenuation of broadband acoustic waves. To better understand the underlying mechanism, we measured frequency‐dependent elastic moduli and pressure‐dependent porosity, permeability, and ultrasonic wave velocities of two tight sandstones. These data show that velocity dispersion is observable in water‐saturated rock and that permeability and ultrasonic velocities are differentially correlated with porosity. A proposed wave propagation model, in which penny‐shaped cracks are inclusions introduced stepwise into a porous medium, successfully describes the experimental data and other published data on tight sandstone and low‐porosity sandstone, and aspect ratios, volume fractions, and radii of pores/cracks are determined. Key Points: We propose a model to investigate the effect of multiscale cracks on seismic waves in tight sandstonesThe pore aspect ratio and crack‐radius spectra are estimated at different differential pressuresThe permeability is obtained according to the pore geometry [ABSTRACT FROM AUTHOR]

In recent years, the Yanchang shale-oil formations of the Ordos Basin are rich in reserves with complex lithology and structure characteristics, low porosity and low permeability, and weak anomalies for oil and water discriminations, have been the key targets of unconventional oil/gas resource exploration and development in the relevant areas. The joint acoustic-electrical (AE) properties can be used to interpret reservoir lithology, mineralogy, pore structure, and fluid saturation. To conduct tests of thin section analysis, X-ray diffraction, and ultrasonic and electrical experiments at different pressures and saturation degrees, cores from the shale-oil formations in the Q area of the basin are collected. The variations in AE properties with respect to clay content, porosity, pressure (microfracture), and saturation are analyzed. The experimental results indicate that the rock physics behaviors of sandstones with different clay contents vary significantly. The AE properties of clean sandstones are basically dependent on the microfractures (pressure), while for muddy sandstones, the clay content is an important factor affecting the responses. The target reservoir consists of interbedded sandstone and shale layers. The AE equivalent medium equations and the Gurevich theory are applied to establish the joint models for the different lithologies and simulate the variations in AE properties with respect to fluid type, pore structure, and mineral components. The three-dimensional joint templates of clean and muddy sandstones, as well as shale, are developed based on the elastic and electrical attributes and then calibrated using the experimental and well-log data. The reservoir properties are estimated with the templates and validated by the log data. The results indicate that the joint templates based on lithology characteristics can effectively characterize the properties of interbedded sandstone and shale layers. Furthermore, the combined application of AE data provides more beneficial information for the assessment of rock properties, leading to precise estimates that conform with the actual formation conditions. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs basically have low porosity and permeability, a complex pore structure and a heterogeneous distribution of immiscible fluids. With the development of theoretical models, it is common to characterize rock properties, i.e., pore structure, microfractures, fluid type and saturation, etc., based on acoustic and electrical properties. We have taken four tight sandstone samples and performed X-ray diffraction and cast thin section analyses. We measure porosity and permeability as well as ultrasonic properties and electrical conductivity at different confining pressures and fluid saturations. These measurements show that the P-wave velocity, P-wave attenuation and conductivity strongly depend on the type and saturation of the fluid and the microstructure of the rock. We propose a combined acoustic-electrical model based on the concept of equivalent medium and on the double porosity, patchy saturation and squirt flow models. We then create rock physics templates calibrated with wellbore log data to estimate fluid saturation and equant and soft porosities, which are well corroborated by gas production reports. This work demonstrates the link between combined acoustic-electrical responses and rock properties and provides an effective approach for applications in reservoirs. [ABSTRACT FROM AUTHOR]

The prediction of the pore geometrical properties is important in the exploration and development of tight-sandstone hydrocarbon reservoirs. To investigate this topic, we have measured the porosity, permeability, P- and S-wave velocities, electrical conductivity, and axial and radial strains as a function of differential (confining minus pore) pressure of tight-sandstone samples, collected from the Zhongjiang gas field of Sichuan, in West China. The results show that the closure of cracks with pressure highly affects these properties. Then, we propose a multiphase reformulated differential effective-medium (R-DEM) model that employs the unified pore geometry (the same pores or cracks with different aspect ratios and volume fractions) for both elastic and electrical modeling. The model gives the pressure-dependence of the P- and S-wave velocities and electrical conductivity, and the experimental porosity and static moduli are used as constraints to estimate the pore geometry. The model describes the elastic properties of sandstones saturated with nitrogen gas, and the electrical conductivity when the pore fluid is brine. The prediction of the wet-rock S-wave velocities is less accurate, due to the presence of shear stiffening and weakening effects. Furthermore, we compare the results with those of the joint elastic-electrical inversion by using the dynamic instead of the static stiffness modulus. The results show that the latter provides a better agreement between theory and experiment. Subsequently, we show that the pore geometry estimated from the elastic or the electrical measurements separately (unjoint inversion) present discrepancies, indicating that a joint inversion is required. The published experimental data are also used to illustrate the model, and the results are satisfactory. • We have measured the porosity, velocities, electrical conductivity, and axial and radial strains of tight sandstones. • The data are interpreted by a multiphase reformulated differential effective-medium modelwith a unified pore geometry. • The results show that the proposed model successfully explains those properties. [ABSTRACT FROM AUTHOR]

Reservoir parameters, such as porosity, saturation, and clay volume, are directly related to the hydrocarbon-bearing properties of potential reservoirs, of which estimation is one of the ultimate goals of data processing for seismic exploration. However, the complex pore structure of tight sandstone presents difficulty in evaluating reservoir properties. This study proposes a novel reservoir parameter inversion method intended for tight sandstone reservoirs. The method builds the forward operator by combining the rock physics model and the seismic reflectivity equation, enabling the direct inversion of reservoir parameters from observed seismic data. In particular, the (sand- and clay-related) pore aspect ratios of rock frame are treated as internal variables, which are iteratively updated during the inversion process. The varying aspect ratios address the complex pore structure, which facilitates the improved accuracy in rock physics modeling of tight sandstone reservoirs. Besides, the Bayesian inversion constrained by the prior Gaussian mixture model considers the complex prior distributions of reservoir parameters in different lithofacies, which stabilizes the inversion process and achieves the optimal solution. The method is tested by the synthetic data which exhibits less uncertainty. The application to the field data from tight sandstone gas reservoirs in southwestern China demonstrates the method has the good capability of indicating the gas-bearing areas. • Seismic rock physics inversion method is proposed for tight sandstone reservoirs. • Reservoir property parameters can be directly inverted from seismic data. • Varying pore aspect ratio describes the complex pore structures in the inversion. • The estimated reservoir parameters can be good indicators for gas-bearing areas. [ABSTRACT FROM AUTHOR]