Quantitative prediction of reservoir properties(e.g., gas saturation, porosity, and shale content) of tight reservoirs is of great significance for resource evaluation and well placements. However, the complex pore st...Quantitative prediction of reservoir properties(e.g., gas saturation, porosity, and shale content) of tight reservoirs is of great significance for resource evaluation and well placements. However, the complex pore structures, poor pore connectivity, and uneven fluid distribution of tight sandstone reservoirs make the correlation between reservoir parameters and elastic properties more complicated and thus pose a major challenge in seismic reservoir characterization. We have developed a partially connected double porosity model to calculate elastic properties by considering the pore structure and connectivity, and to analyze these factors' influences on the elastic behaviors of tight sandstone reservoirs. The modeling results suggest that the bulk modulus is likely to be affected by the pore connectivity coefficient, while the shear modulus is sensitive to the volumetric fraction of stiff pores. By comparing the model predictions with the acoustic measurements of the dry and saturated quartz sandstone samples, the volumetric fraction of stiff pores and the pore connectivity coefficient can be determined. Based on the calibrated model, we have constructed a 3D rock physics template that accounts for the reservoir properties' impacts on the P-wave impedance, S-wave impedance, and density. The template combined with Bayesian inverse theory is used to quantify gas saturation, porosity, clay content, and their corresponding uncertainties from elastic parameters. The application of well-log and seismic data demonstrates that our 3D rock physics template-based probabilistic inversion approach performs well in predicting the spatial distribution of high-quality tight sandstone reservoirs in southwestern China.展开更多
The carbonate reservoirs in the Ordovician Majiagou Formation of the Ordos Basin have undergone complex geological evolution,resulting in high-quality dolomite reservoirs that exhibit strong heterogeneity.Neglecting t...The carbonate reservoirs in the Ordovician Majiagou Formation of the Ordos Basin have undergone complex geological evolution,resulting in high-quality dolomite reservoirs that exhibit strong heterogeneity.Neglecting the fundamental factor of reservoir genetic mode,conventional rock physics experiments cannot accurately determine the seismic elastic responses of the target rock.Here,a set of carbonate samples from different sedimentary environments were selected elaborately based on geological and logging data.Subsequently,systematic petrological and rock physics measurements were conducted to investigate the variation of rock physics properties from both macro-geological and microstructural perspectives.The measurement results illustrate that the microstructures in carbonate rocks are influenced by tectonic-sedimentary patterns and sea level fluctuation.Various rock types are observed:pore type dolomitic gypsum,argillaceous dolomite,and microcrystal dolomite in restricted-evaporative lagoon environments;dissolved pore type and crack-dissolved pore type dolomite in mound-shoal environments;and dissolved pore type gypsum dolomite in platform flat environments.Furthermore,the mineral components as the load-bearing frame and the pore structure jointly control the elastic properties.Samples with the same lithology exhibit similar load-bearing frames,leading to a strong statistical relationship between VPand VS.Concerning the pore structure,dissolved pores formed by atmospheric freshwater dissolution during the penecontemporaneous period have high stiffness,minimally affecting the elastic properties of reservoirs.Conversely,the lower stiffness of microcracks resulting from tectonic rupture significantly decreases the P-wave impedance and Poisson's ratio of dry samples,while increasing the Poisson's ratio of water-saturated samples.These findings enable the accurate recognition of the seismic elastic characteristics of high-quality dolomite reservoirs in moundshoal environments,thus providing a rock physics experimental basis for improving the precision of seismic reservoir prediction in the study area.展开更多
Rock physics modeling is implemented for shales in the Luojia area of the Zhanhua topographic depression. In the rock physics model, the clay lamination parameter is introduced into the Backus averaging theory for the...Rock physics modeling is implemented for shales in the Luojia area of the Zhanhua topographic depression. In the rock physics model, the clay lamination parameter is introduced into the Backus averaging theory for the description of anisotropy related to the preferred alignment of clay particles, and the Chapman multi-scale fracture theory is used to calculate anisotropy relating to the fracture system. In accordance with geological features of shales in the study area, horizontal fractures are regarded as the dominant factor in the prediction of fracture density and anisotropy parameters for the inversion scheme. Results indicate that the horizontal fracture density obtained has good agreement with horizontal permeability measured from cores, and thus confirms the applicability of the proposed rock physics model and inversion method. Fracture density can thus be regarded as an indicator of reservoir permeability. In addition, the anisotropy parameter of the P-wave is higher than that of the S-wave due to the presence of horizontal fractures. Fracture density has an obvious positive correlation with P-wave anisotropy, and the clay content shows a positive correlation with S-wave anisotropy, which fully shows that fracture density has a negative correlation with clay and quartz contents and a positive relation with carbonate contents.展开更多
Unlike previous theories with velocity and/or elastic modulus averaging, we use a three-phase porous rock physics model developed by Santos for analyzing the seismic response of two immiscible fluids in saturated poro...Unlike previous theories with velocity and/or elastic modulus averaging, we use a three-phase porous rock physics model developed by Santos for analyzing the seismic response of two immiscible fluids in saturated porous media. Considering reservoir reference pressure and coupling drag of two fluids in pores, the effects of frequency, porosity, and gas saturation on the phase velocities of the P-and S-waves are discussed in detail under field conditions. The effects of porosity and gas saturation on Vp/Vs are also provided. The data for our numerical experiments are from a sample of deep volcanic rock from Daqing. The numerical results show that the frequency dispersion effect can be ignored for deep volcanic rocks with low porosity and low permeability. It is concluded that for deep volcanic rocks the effect of gas content in pores on Vp/Vs is negligible but the effect of porosity is significant when there is a certain amount of water contained in the pores. The accurate estimate of lithology and porosity in this case is relatively more important.展开更多
Seismic inversion is one of the most widely used technologies for reservoir prediction. Many good results have been obtained but sometimes it fails to differentiate the lithologies and identify the fluids. However, se...Seismic inversion is one of the most widely used technologies for reservoir prediction. Many good results have been obtained but sometimes it fails to differentiate the lithologies and identify the fluids. However, seismic prestack elastic inversion based on rock physics modeling and analysis introduced in this paper is a significant method that can help seismic inversion and interpretation reach a new quantitative （or semi-quantitative） level from traditional qualitative interpretation. By doing rock physics modeling and forward perturbation analysis, we can quantitatively analyze the essential relationships between rock properties and seismic responses and try to find the sensitive elastic properties to the lithology, porosity, fluid type, and reservoir saturation. Finally, standard rock physics templates （RPT） can be built for specific reservoirs to guide seismic inversion interpretation results for reservoir characterization and fluids identification purpose. The gas sand distribution results of the case study in this paper proves that this method has unparalleled advantages over traditional post-stack methods, by which we can perform reservoir characterization and seismic data interpretation more quantitatively and efficiently.展开更多
We developed an anisotropic effective theoretical model for modeling the elastic behavior of anisotropic carbonate reservoirs by combining the anisotropic self-consistent approximation and differential effective mediu...We developed an anisotropic effective theoretical model for modeling the elastic behavior of anisotropic carbonate reservoirs by combining the anisotropic self-consistent approximation and differential effective medium models.By analyzing the measured data from carbonate samples in the TL area,a carbonate pore-structure model for estimating the elastic parameters of carbonate rocks is proposed,which is a prerequisite in the analysis of carbonate reservoirs.A workflow for determining elastic properties of carbonate reservoirs is established in terms of the anisotropic effective theoretical model and the pore-structure model.We performed numerical experiments and compared the theoretical prediction and measured data.The result of the comparison suggests that the proposed anisotropic effective theoretical model can account for the relation between velocity and porosity in carbonate reservoirs.The model forms the basis for developing new tools for predicting and evaluating the properties of carbonate reservoirs.展开更多
Rock physics inversion is to use seismic elastic properties of underground strata for predicting reservoir petrophysical parameters.The Markov chain Monte Carlo(MCMC)algorithm is commonly used to solve rock physics in...Rock physics inversion is to use seismic elastic properties of underground strata for predicting reservoir petrophysical parameters.The Markov chain Monte Carlo(MCMC)algorithm is commonly used to solve rock physics inverse problems.However,all the parameters to be inverted are iterated simultaneously in the conventional MCMC algorithm.What is obtained is an optimal solution of combining the petrophysical parameters with being inverted.This study introduces the alternating direction(AD)method into the MCMC algorithm(i.e.the optimized MCMC algorithm)to ensure that each petrophysical parameter can get the optimal solution and improve the convergence of the inversion.Firstly,the Gassmann equations and Xu-White model are used to model shaly sandstone,and the theoretical relationship between seismic elastic properties and reservoir petrophysical parameters is established.Then,in the framework of Bayesian theory,the optimized MCMC algorithm is used to generate a Markov chain to obtain the optimal solution of each physical parameter to be inverted and obtain the maximum posterior density of the physical parameter.The proposed method is applied to actual logging and seismic data and the results show that the method can obtain more accurate porosity,saturation,and clay volume.展开更多
Brittleness analysis becomes important when looking for sweet spots in tightoil sandstone reservoirs. Hence, appropriate indices are required as accurate brittleness evaluation criteria. We construct a seismic rock ph...Brittleness analysis becomes important when looking for sweet spots in tightoil sandstone reservoirs. Hence, appropriate indices are required as accurate brittleness evaluation criteria. We construct a seismic rock physics model for tight-oil sandstone reservoirs with vertical fractures. Because of the complexities in lithology and pore structure and the anisotropic characteristics of tight-oil sandstone reservoirs, the proposed model is based on the solid components, pore connectivity, pore type, and fractures to better describe the sandstone reservoir microstructure. Using the model, we analyze the brittleness sensitivity of the elastic parameters in an anisotropic medium and establish a new brittleness index. We show the applicability of the proposed brittleness index for tight-oil sandstone reservoirs by considering the brittleness sensitivity, the rock physics response characteristics, and cross-plots. Compared with conventional brittleness indexes, the new brittleness index has high brittleness sensitivity and it is the highest in oil-bearing brittle zones with relatively high porosity. The results also suggest that the new brittleness index is much more sensitive to elastic properties variations, and thus can presumably better predict the brittleness characteristics of sweet spots in tight-oil sandstone reservoirs.展开更多
Digital rock physics(DRP)is a paramount technology to improve the economic benefits of oil and gas fields,devise more scientific oil and gas field development plans,and create digital oil and gas fields.Currently,a si...Digital rock physics(DRP)is a paramount technology to improve the economic benefits of oil and gas fields,devise more scientific oil and gas field development plans,and create digital oil and gas fields.Currently,a significant gap is present between DRP theory and practical applications.Conventional digital-core construction focuses only on simple cores,and the recognition and segmentation effect of fractures and pores of complex cores is poor.The identification of rock minerals is inaccurate,which leads to the difference between the digital and actual cores.To promote the application of DRP in developing oil and gas fields,based on the high-precision X-ray computed tomography scanning technology,the U-Net deep learning model of the full convolution neural network is used to segment the pores,fractures,and matrix from the complex rock core with natural fractures innovatively.Simultaneously,the distribution of rock minerals is divided,and the distribution of rock conditions is corrected by X-ray diffraction.A pore—fracture network model is established based on the equivalent radius,which lays the foundation for fluid seepage simulation.Finally,the accuracy of the established a digital core is verified by the porosity measured via nuclear magnetic resonance technology,which is of great significance to the development and application of DRP in oil and gas fields.展开更多
Due to the huge differences between the unconventional shale and conventional sand reservoirs in many aspects such as the types and the characteristics of minerals,matrix pores and fluids,the construction of shale roc...Due to the huge differences between the unconventional shale and conventional sand reservoirs in many aspects such as the types and the characteristics of minerals,matrix pores and fluids,the construction of shale rock physics model is significant for the exploration and development of shale reservoirs.To make a better characterization of shale gas-bearing reservoirs,we first propose a new but more suitable rock physics model to characterize the reservoirs.We then use a well A to demonstrate the feasibility and reliability of the proposed rock physics model of shale gas-bearing reservoirs.Moreover,we propose a new brittleness indicator for the high-porosity and organic-rich shale gas-bearing reservoirs.Based on the parameter analysis using the constructed rock physics model,we finally compare the new brittleness indicator with the commonly used Young’s modulus in the content of quartz and organic matter,the matrix porosity,and the types of filled fluids.We also propose a new shale brittleness index by integrating the proposed new brittleness indicator and the Poisson’s ratio.Tests on real data sets demonstrate that the new brittleness indicator and index are more sensitive than the commonly used Young’s modulus and brittleness index for the high-porosity and high-brittleness shale gas-bearing reservoirs.展开更多
The construction of a shale rock physics model and the selection of an appropriate brittleness index （B/） are two significant steps that can influence the accuracy of brittleness prediction. On one hand, the existin...The construction of a shale rock physics model and the selection of an appropriate brittleness index （B/） are two significant steps that can influence the accuracy of brittleness prediction. On one hand, the existing models of kerogen-rich shale are controversial, so a reasonable rock physics model needs to be built. On the other hand, several types of equations already exist for predicting the BI whose feasibility needs to be carefully considered. This study constructed a kerogen-rich rock physics model by performing the self- consistent approximation and the differential effective medium theory to model intercoupled clay and kerogen mixtures. The feasibility of our model was confirmed by comparison with classical models, showing better accuracy. Templates were constructed based on our model to link physical properties and the BL Different equations for the BI had different sensitivities, making them suitable for different types of formations. Equations based on Young＇s Modulus were sensitive to variations in lithology, while those using Lame＇s Coefficients were sensitive to porosity and pore fluids. Physical information must be considered to improve brittleness prediction.展开更多
Kerogen content and kerogen porosity play a significant role in elastic properties of organic-rich shales. We construct a rock physics model for organic-rich shales to quantify the effect of kerogen content and keroge...Kerogen content and kerogen porosity play a significant role in elastic properties of organic-rich shales. We construct a rock physics model for organic-rich shales to quantify the effect of kerogen content and kerogen porosity using the Kuster and Toks6z theory and the selfconsistent approximation method. Rock physics modeling results show that with the increase of kerogen content and kerogen-related porosity, the velocity and density of shales decrease, and the effect of kerogen porosity becomes more obvious only for higher kerogen content. We also find that the Poisson＇s ratio of the shale is not sensitive to kerogen porosity for the case of gas saturation. Finally, for the seismic reflection responses of an organic-rich shale layer, forward modeling results indicate the fifth type AVO re- sponses which correspond to a negative intercept and a positive gradient. The absolute values of intercept and gradient increase with kerogen content and kerogen porosity, and present predictable variations associated with velocities and density.展开更多
Ultrasonic coda waves are widely usea to stuay hign-trequency scattering, however, ultrasonic coda waves are strongly affected by interference from by boundary-reflected waves. To understand the effect of boundary-ref...Ultrasonic coda waves are widely usea to stuay hign-trequency scattering, however, ultrasonic coda waves are strongly affected by interference from by boundary-reflected waves. To understand the effect of boundary-reflected waves, we performed ultrasonic experiments using aluminum and shale samples, and the rotating staggered-mesh finite-difference method to simulate the wavefield. We analyzed the wavefield characteristics at the different receiving points and the interference characteristics of the boundary-reflected waves with the ultrasonic coda wave, and the effect of sample geometry on the ultrasonic coda waves. The increase in the aspect ratio of the samples delays the interference effect of the laterally reflected waves and reduces the effect on the ultrasonic coda waves. The main waves interfering with the ultrasonic coda waves are laterally reflected PP-, PS-, PPP-, and PPS-waves. The scattering and attenuation of the high-frequency energy in actual rocks can weaken the interference of laterally reflected waves with the ultrasonic coda waves.展开更多
Total organic carbon (TOC) prediction with elastic parameter inversions has been widely used in the identification and evaluation of source rocks. However, the elastic parameters used to predict TOC are not only deter...Total organic carbon (TOC) prediction with elastic parameter inversions has been widely used in the identification and evaluation of source rocks. However, the elastic parameters used to predict TOC are not only determined by TOC but also depend on the other physical properties of source rocks. Besides, the TOC prediction with the elastic parameters inversion is an indirect method based on the statistical relationship obtained from well logs and experiment data. Therefore, we propose a rock physics model and define a TOC indicator mainly affected by TOC to predict TOC directly. The proposed rock physics model makes the equivalent elastic moduli of source rocks parameterized by the TOC indicator. Combining the equivalent elastic moduli of source rocks and Gray’s approximation leads to a novel linearized approximation of the P-wave reflection coefficient incorporating the TOC indicator. Model examples illustrate that the novel reflectivity approximation well agrees with the exact Zoeppritz equation until incident angles reach 40°. Convoluting the novel P-wave reflection approximation with seismic wavelets as the forward solver, an AVO inversion method based on the Bayesian theory is proposed to invert the TOC indicator with seismic data. The synthetic examples and field tests validate the feasibility and stability of the proposed AVO inversion approach. Using the inversion results of the TOC indicator, TOC is directly and accurately estimated in the target area.展开更多
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 dis...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 diff raction,and ultrasonic and electrical experiments at diff erent 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 diff erent 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 diff erent 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 eff ectively 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.展开更多
Tight sandstone gas reservoirs have characteristics of low porosity and low permeability,complex pore structure.In the paper,we consider the pore and micro-fracture systems in tight sandstone reservoir and perform roc...Tight sandstone gas reservoirs have characteristics of low porosity and low permeability,complex pore structure.In the paper,we consider the pore and micro-fracture systems in tight sandstone reservoir and perform rock physics modeling of sandstone gas reservoirs based on Chapman.Then we analyze the effects of relaxation time,and difference fluid types on dispersion and attention,and seismic responses.We found that the magnitude of the relaxation time is related to the gas saturation and the viscosity coefficient of the fluid.The magnitude of the relaxation time affects the frequency range of the velocity dispersion and attenuation,and the type of fluid affects the magnitude of the velocity dispersion and attenuation.As the relaxation time decreases,the calculated amplitude deceases.And the amplitude and the waveform of the synthetic seismic record are related to different fluid types and formation thickness.展开更多
Sokor Formation reservoir intervals are intrinsically anisotropic,heterogeneous and with a characteristic of Low Contrast Low Resistivity(LCLR)log responses in parts of the Termit basin.Discriminating sands from shale...Sokor Formation reservoir intervals are intrinsically anisotropic,heterogeneous and with a characteristic of Low Contrast Low Resistivity(LCLR)log responses in parts of the Termit basin.Discriminating sands from shales/mudstones and hydrocarbon sands from brine sands as well as accurately evaluating the distribution of relevant reservoir properties using conventional seismic interpretation are complicated,and undermines reservoir characterization in such reservoirs.To enhance reservoir evaluation and reduce development planning and production risks,rock physics analysis was intergrated into the petrophysical workflow,which fed higher fidelity inputs into a post stack seismic inversion workflow.Rock Physics Diagnostics(RPD)analysis revealed that the reservoir interval of interest has grain size distribution of different lithologies,which is related to the environment of deposition and burial history,and could be best described by the constant cement sand model.The rock physics analyses revealed that facies were most effectively discriminated based on their Vp/Vs ratios and acoustic impedance.Particularly,hydrocarbon saturated sandstones,brine saturated shaly sandstones and shales/mudstones which exhibit similar acoustic impedance characteristics,were clearly discriminated by their Vp/Vs.The inverted seismic attributes as well as Seismic Based-Rock Physics Templates(RPT),clearly delineated the hydrocarbon fields,predicted new prospects beyond the existing well locations,which could be considered for field appraisal or development opportunities in the basin.These results demonstrate the value of the robust application of rock physics diagnostic modeling and seismic inversion in quantitative reservoir characterization and may be quite useful in undrilled locations in the basins and fields with similar geology.展开更多
The stability of the drilled wellbores in carbonate formations is of great importance.This paper is a new approach to determine the elastic properties of the wellbore based on rock physics methods using the Hashin-Sht...The stability of the drilled wellbores in carbonate formations is of great importance.This paper is a new approach to determine the elastic properties of the wellbore based on rock physics methods using the Hashin-Shtrikman bounds to determine the wellbore stability.Initially,the static elastic parameters were determined by two different methods,namely the rock physics method and DSI log,followed by clustering through Multi-Resolutional Graph-Based clustering(MRGC)and calculating geomechanical-units(GMUs)of each method separately.The obtained results from DSI log and rock physic methods were then compared followed by determining the wellbore stability.The results showed that the correlation between shear and compressional wave velocity obtained from the petrophysical method with the measured values of shear and pressure wave velocity in the well was 0.94 and 0.90,respectively,which shows that the petrophysical method can estimate these two logs with high accuracy,and it can be a suitable alternative instead of using the Dipole Shear Sonic Imager(DSI).It was observed that the geomechanical units of elastic parameters calculated by proposed rock physics method are in good agreement with those obtained from DSI log.Rock-physic method can be a good alternative for when expensive DSI logs are missing.展开更多
Tight sandstone reservoirs have strong heterogeneity and complex gas-water relationship,causing diffi culty in quantitatively predicting water saturation.Deep learning,combined with rock physics analysis and geostatis...Tight sandstone reservoirs have strong heterogeneity and complex gas-water relationship,causing diffi culty in quantitatively predicting water saturation.Deep learning,combined with rock physics analysis and geostatistics theory,was used to predict water saturation in tight sandstone,focusing on the P_(sh)^(8) in the GFZ area of the Ordos Basin.Results show that:Starting with actual wells where porosity and saturation results are obtained from log interpretations,the relationship between reservoir parameters(porosity and saturation)and elastic properties(P-wave velocity,S-wave velocity,and density)is established through the development of a rock physics model suitable for the region.Under the constraints of geostatistical laws,such as background trends of elastic and reservoir parameters and the vertical variations in logging curves,reservoir conditions(including porosity,saturation,and thickness)are simulated to generate numerous pseudowells and corresponding seismic gathers modeled using the Zoeppritz equation.A convolution neural network is used to train the target curve and predict the target body.The predicted water saturation of the P_(sh)^(8) shows strong agreement with the results from two blind wells,providing a reliable basis for understanding the water saturation(Sw)of tight sandstone.展开更多
Estimating gas enrichments is a key objective in exploring sweet spots within tight sandstone gas reservoirs.However,the low sensitivity of elastic parameters to gas saturations in such formations makes it a significa...Estimating gas enrichments is a key objective in exploring sweet spots within tight sandstone gas reservoirs.However,the low sensitivity of elastic parameters to gas saturations in such formations makes it a significant challenge to reliably estimate gas enrichments using seismic methods.Through rock physical modeling and reservoir parameter analyses conducted in this study,a more suitable indicator for estimating gas enrichment,termed the gas content indicator,has been proposed.This indicator is formulated based on effective fluid bulk modulus and shear modulus and demonstrates a clear positive correlation with gas content in tight sandstones.Moreover,a new seismic amplitude variation versus offset(AVO)equation is derived to directly extract reservoir properties,such as the gas content indicator and porosity,from prestack seismic data.The accuracy of this proposed AVO equation is validated through comparison with the exact solutions provided by the Zoeppritz equation.To ensure reliable estimations of reservoir properties from partial angle-stacked seismic data,the proposed AVO equation is reformulated within the elastic impedance inversion framework.The estimated gas content indicator and porosity exhibit favorable agreement with logging data,suggesting that the obtained results are suitable for reliable predictions of tight sandstones with high gas enrichments.Furthermore,the proposed methods have the potential to stimulate the advancement of other suitable inversion techniques for directly estimating reservoir properties from seismic data across various petroleum resources.展开更多
基金supported by the National Natural Science Foundation of China (42104121)the Scientific Research and Technology Development Project of the CNPC (2021DJ0606)。
文摘Quantitative prediction of reservoir properties(e.g., gas saturation, porosity, and shale content) of tight reservoirs is of great significance for resource evaluation and well placements. However, the complex pore structures, poor pore connectivity, and uneven fluid distribution of tight sandstone reservoirs make the correlation between reservoir parameters and elastic properties more complicated and thus pose a major challenge in seismic reservoir characterization. We have developed a partially connected double porosity model to calculate elastic properties by considering the pore structure and connectivity, and to analyze these factors' influences on the elastic behaviors of tight sandstone reservoirs. The modeling results suggest that the bulk modulus is likely to be affected by the pore connectivity coefficient, while the shear modulus is sensitive to the volumetric fraction of stiff pores. By comparing the model predictions with the acoustic measurements of the dry and saturated quartz sandstone samples, the volumetric fraction of stiff pores and the pore connectivity coefficient can be determined. Based on the calibrated model, we have constructed a 3D rock physics template that accounts for the reservoir properties' impacts on the P-wave impedance, S-wave impedance, and density. The template combined with Bayesian inverse theory is used to quantify gas saturation, porosity, clay content, and their corresponding uncertainties from elastic parameters. The application of well-log and seismic data demonstrates that our 3D rock physics template-based probabilistic inversion approach performs well in predicting the spatial distribution of high-quality tight sandstone reservoirs in southwestern China.
基金supported by the National Natural Science Foundation of China(41774136,42474149)。
文摘The carbonate reservoirs in the Ordovician Majiagou Formation of the Ordos Basin have undergone complex geological evolution,resulting in high-quality dolomite reservoirs that exhibit strong heterogeneity.Neglecting the fundamental factor of reservoir genetic mode,conventional rock physics experiments cannot accurately determine the seismic elastic responses of the target rock.Here,a set of carbonate samples from different sedimentary environments were selected elaborately based on geological and logging data.Subsequently,systematic petrological and rock physics measurements were conducted to investigate the variation of rock physics properties from both macro-geological and microstructural perspectives.The measurement results illustrate that the microstructures in carbonate rocks are influenced by tectonic-sedimentary patterns and sea level fluctuation.Various rock types are observed:pore type dolomitic gypsum,argillaceous dolomite,and microcrystal dolomite in restricted-evaporative lagoon environments;dissolved pore type and crack-dissolved pore type dolomite in mound-shoal environments;and dissolved pore type gypsum dolomite in platform flat environments.Furthermore,the mineral components as the load-bearing frame and the pore structure jointly control the elastic properties.Samples with the same lithology exhibit similar load-bearing frames,leading to a strong statistical relationship between VPand VS.Concerning the pore structure,dissolved pores formed by atmospheric freshwater dissolution during the penecontemporaneous period have high stiffness,minimally affecting the elastic properties of reservoirs.Conversely,the lower stiffness of microcracks resulting from tectonic rupture significantly decreases the P-wave impedance and Poisson's ratio of dry samples,while increasing the Poisson's ratio of water-saturated samples.These findings enable the accurate recognition of the seismic elastic characteristics of high-quality dolomite reservoirs in moundshoal environments,thus providing a rock physics experimental basis for improving the precision of seismic reservoir prediction in the study area.
基金sponsored by the National Natural Science Foundation of China under Grants 41404090,U1262208,and U1663207the Foundation of the Sino PEC Key Laboratory of Shale Oil/Gas Exploration and Production Technology under Grants No.G5800-15-ZS-WX039the project under Grants No.G5800-15-ZS-WX004
文摘Rock physics modeling is implemented for shales in the Luojia area of the Zhanhua topographic depression. In the rock physics model, the clay lamination parameter is introduced into the Backus averaging theory for the description of anisotropy related to the preferred alignment of clay particles, and the Chapman multi-scale fracture theory is used to calculate anisotropy relating to the fracture system. In accordance with geological features of shales in the study area, horizontal fractures are regarded as the dominant factor in the prediction of fracture density and anisotropy parameters for the inversion scheme. Results indicate that the horizontal fracture density obtained has good agreement with horizontal permeability measured from cores, and thus confirms the applicability of the proposed rock physics model and inversion method. Fracture density can thus be regarded as an indicator of reservoir permeability. In addition, the anisotropy parameter of the P-wave is higher than that of the S-wave due to the presence of horizontal fractures. Fracture density has an obvious positive correlation with P-wave anisotropy, and the clay content shows a positive correlation with S-wave anisotropy, which fully shows that fracture density has a negative correlation with clay and quartz contents and a positive relation with carbonate contents.
文摘Unlike previous theories with velocity and/or elastic modulus averaging, we use a three-phase porous rock physics model developed by Santos for analyzing the seismic response of two immiscible fluids in saturated porous media. Considering reservoir reference pressure and coupling drag of two fluids in pores, the effects of frequency, porosity, and gas saturation on the phase velocities of the P-and S-waves are discussed in detail under field conditions. The effects of porosity and gas saturation on Vp/Vs are also provided. The data for our numerical experiments are from a sample of deep volcanic rock from Daqing. The numerical results show that the frequency dispersion effect can be ignored for deep volcanic rocks with low porosity and low permeability. It is concluded that for deep volcanic rocks the effect of gas content in pores on Vp/Vs is negligible but the effect of porosity is significant when there is a certain amount of water contained in the pores. The accurate estimate of lithology and porosity in this case is relatively more important.
文摘Seismic inversion is one of the most widely used technologies for reservoir prediction. Many good results have been obtained but sometimes it fails to differentiate the lithologies and identify the fluids. However, seismic prestack elastic inversion based on rock physics modeling and analysis introduced in this paper is a significant method that can help seismic inversion and interpretation reach a new quantitative （or semi-quantitative） level from traditional qualitative interpretation. By doing rock physics modeling and forward perturbation analysis, we can quantitatively analyze the essential relationships between rock properties and seismic responses and try to find the sensitive elastic properties to the lithology, porosity, fluid type, and reservoir saturation. Finally, standard rock physics templates （RPT） can be built for specific reservoirs to guide seismic inversion interpretation results for reservoir characterization and fluids identification purpose. The gas sand distribution results of the case study in this paper proves that this method has unparalleled advantages over traditional post-stack methods, by which we can perform reservoir characterization and seismic data interpretation more quantitatively and efficiently.
基金supported by the National Natural Science Foundation of China(No.41274136)
文摘We developed an anisotropic effective theoretical model for modeling the elastic behavior of anisotropic carbonate reservoirs by combining the anisotropic self-consistent approximation and differential effective medium models.By analyzing the measured data from carbonate samples in the TL area,a carbonate pore-structure model for estimating the elastic parameters of carbonate rocks is proposed,which is a prerequisite in the analysis of carbonate reservoirs.A workflow for determining elastic properties of carbonate reservoirs is established in terms of the anisotropic effective theoretical model and the pore-structure model.We performed numerical experiments and compared the theoretical prediction and measured data.The result of the comparison suggests that the proposed anisotropic effective theoretical model can account for the relation between velocity and porosity in carbonate reservoirs.The model forms the basis for developing new tools for predicting and evaluating the properties of carbonate reservoirs.
基金supported by the National Natural Science Foundation of China(No.42174146)CNPC major forwardlooking basic science and technology projects(No.2021DJ0204).
文摘Rock physics inversion is to use seismic elastic properties of underground strata for predicting reservoir petrophysical parameters.The Markov chain Monte Carlo(MCMC)algorithm is commonly used to solve rock physics inverse problems.However,all the parameters to be inverted are iterated simultaneously in the conventional MCMC algorithm.What is obtained is an optimal solution of combining the petrophysical parameters with being inverted.This study introduces the alternating direction(AD)method into the MCMC algorithm(i.e.the optimized MCMC algorithm)to ensure that each petrophysical parameter can get the optimal solution and improve the convergence of the inversion.Firstly,the Gassmann equations and Xu-White model are used to model shaly sandstone,and the theoretical relationship between seismic elastic properties and reservoir petrophysical parameters is established.Then,in the framework of Bayesian theory,the optimized MCMC algorithm is used to generate a Markov chain to obtain the optimal solution of each physical parameter to be inverted and obtain the maximum posterior density of the physical parameter.The proposed method is applied to actual logging and seismic data and the results show that the method can obtain more accurate porosity,saturation,and clay volume.
基金supported by the National 973 project(Nos.2014CB239006 and 2011CB202402)the National Natural Science Foundation of China(Nos.41104069 and 41274124)+1 种基金Sinopec project(No.KJWX2014-05)the Fundamental Research Funds for the Central Universities(No.R1401005A)
文摘Brittleness analysis becomes important when looking for sweet spots in tightoil sandstone reservoirs. Hence, appropriate indices are required as accurate brittleness evaluation criteria. We construct a seismic rock physics model for tight-oil sandstone reservoirs with vertical fractures. Because of the complexities in lithology and pore structure and the anisotropic characteristics of tight-oil sandstone reservoirs, the proposed model is based on the solid components, pore connectivity, pore type, and fractures to better describe the sandstone reservoir microstructure. Using the model, we analyze the brittleness sensitivity of the elastic parameters in an anisotropic medium and establish a new brittleness index. We show the applicability of the proposed brittleness index for tight-oil sandstone reservoirs by considering the brittleness sensitivity, the rock physics response characteristics, and cross-plots. Compared with conventional brittleness indexes, the new brittleness index has high brittleness sensitivity and it is the highest in oil-bearing brittle zones with relatively high porosity. The results also suggest that the new brittleness index is much more sensitive to elastic properties variations, and thus can presumably better predict the brittleness characteristics of sweet spots in tight-oil sandstone reservoirs.
基金Science and Technology Cooperation Project of the CNPC-SWPU Innovation Alliance(No.2020CX010501)National Science and Technology Major ProjectNational Natural Science Foundation of China Petrochemical Joint Fund Project(U1762107)
文摘Digital rock physics(DRP)is a paramount technology to improve the economic benefits of oil and gas fields,devise more scientific oil and gas field development plans,and create digital oil and gas fields.Currently,a significant gap is present between DRP theory and practical applications.Conventional digital-core construction focuses only on simple cores,and the recognition and segmentation effect of fractures and pores of complex cores is poor.The identification of rock minerals is inaccurate,which leads to the difference between the digital and actual cores.To promote the application of DRP in developing oil and gas fields,based on the high-precision X-ray computed tomography scanning technology,the U-Net deep learning model of the full convolution neural network is used to segment the pores,fractures,and matrix from the complex rock core with natural fractures innovatively.Simultaneously,the distribution of rock minerals is divided,and the distribution of rock conditions is corrected by X-ray diffraction.A pore—fracture network model is established based on the equivalent radius,which lays the foundation for fluid seepage simulation.Finally,the accuracy of the established a digital core is verified by the porosity measured via nuclear magnetic resonance technology,which is of great significance to the development and application of DRP in oil and gas fields.
文摘Due to the huge differences between the unconventional shale and conventional sand reservoirs in many aspects such as the types and the characteristics of minerals,matrix pores and fluids,the construction of shale rock physics model is significant for the exploration and development of shale reservoirs.To make a better characterization of shale gas-bearing reservoirs,we first propose a new but more suitable rock physics model to characterize the reservoirs.We then use a well A to demonstrate the feasibility and reliability of the proposed rock physics model of shale gas-bearing reservoirs.Moreover,we propose a new brittleness indicator for the high-porosity and organic-rich shale gas-bearing reservoirs.Based on the parameter analysis using the constructed rock physics model,we finally compare the new brittleness indicator with the commonly used Young’s modulus in the content of quartz and organic matter,the matrix porosity,and the types of filled fluids.We also propose a new shale brittleness index by integrating the proposed new brittleness indicator and the Poisson’s ratio.Tests on real data sets demonstrate that the new brittleness indicator and index are more sensitive than the commonly used Young’s modulus and brittleness index for the high-porosity and high-brittleness shale gas-bearing reservoirs.
基金supported by the NSFC and Sinopec Joint Key Project(No.U1663207)National Science and Technology Major Project(No.2017ZX05049-002)National 973 Program(No.2014CB239104)
文摘The construction of a shale rock physics model and the selection of an appropriate brittleness index （B/） are two significant steps that can influence the accuracy of brittleness prediction. On one hand, the existing models of kerogen-rich shale are controversial, so a reasonable rock physics model needs to be built. On the other hand, several types of equations already exist for predicting the BI whose feasibility needs to be carefully considered. This study constructed a kerogen-rich rock physics model by performing the self- consistent approximation and the differential effective medium theory to model intercoupled clay and kerogen mixtures. The feasibility of our model was confirmed by comparison with classical models, showing better accuracy. Templates were constructed based on our model to link physical properties and the BL Different equations for the BI had different sensitivities, making them suitable for different types of formations. Equations based on Young＇s Modulus were sensitive to variations in lithology, while those using Lame＇s Coefficients were sensitive to porosity and pore fluids. Physical information must be considered to improve brittleness prediction.
基金supported by the National Natural Science Foundation of China under Grants U1262208the National Natural Science Foundation of China under Grants 41404090
文摘Kerogen content and kerogen porosity play a significant role in elastic properties of organic-rich shales. We construct a rock physics model for organic-rich shales to quantify the effect of kerogen content and kerogen porosity using the Kuster and Toks6z theory and the selfconsistent approximation method. Rock physics modeling results show that with the increase of kerogen content and kerogen-related porosity, the velocity and density of shales decrease, and the effect of kerogen porosity becomes more obvious only for higher kerogen content. We also find that the Poisson＇s ratio of the shale is not sensitive to kerogen porosity for the case of gas saturation. Finally, for the seismic reflection responses of an organic-rich shale layer, forward modeling results indicate the fifth type AVO re- sponses which correspond to a negative intercept and a positive gradient. The absolute values of intercept and gradient increase with kerogen content and kerogen porosity, and present predictable variations associated with velocities and density.
基金supported by the Strategic Leading Science and Technology Programme(Class B)of the Chinese Academy of Sciences(No.XDB10010400)
文摘Ultrasonic coda waves are widely usea to stuay hign-trequency scattering, however, ultrasonic coda waves are strongly affected by interference from by boundary-reflected waves. To understand the effect of boundary-reflected waves, we performed ultrasonic experiments using aluminum and shale samples, and the rotating staggered-mesh finite-difference method to simulate the wavefield. We analyzed the wavefield characteristics at the different receiving points and the interference characteristics of the boundary-reflected waves with the ultrasonic coda wave, and the effect of sample geometry on the ultrasonic coda waves. The increase in the aspect ratio of the samples delays the interference effect of the laterally reflected waves and reduces the effect on the ultrasonic coda waves. The main waves interfering with the ultrasonic coda waves are laterally reflected PP-, PS-, PPP-, and PPS-waves. The scattering and attenuation of the high-frequency energy in actual rocks can weaken the interference of laterally reflected waves with the ultrasonic coda waves.
基金The authors acknowledge the sponsorship of National Natural Science Foundation of China(42174139,41974119,42030103)Laoshan Laboratory Science and Technology Innovation Program(LSKj202203406)Science Foundation from Innovation and Technology Support Program for Young Scientists in Colleges of Shandong Province and Ministry of Science and Technology of China(2019RA2136).
文摘Total organic carbon (TOC) prediction with elastic parameter inversions has been widely used in the identification and evaluation of source rocks. However, the elastic parameters used to predict TOC are not only determined by TOC but also depend on the other physical properties of source rocks. Besides, the TOC prediction with the elastic parameters inversion is an indirect method based on the statistical relationship obtained from well logs and experiment data. Therefore, we propose a rock physics model and define a TOC indicator mainly affected by TOC to predict TOC directly. The proposed rock physics model makes the equivalent elastic moduli of source rocks parameterized by the TOC indicator. Combining the equivalent elastic moduli of source rocks and Gray’s approximation leads to a novel linearized approximation of the P-wave reflection coefficient incorporating the TOC indicator. Model examples illustrate that the novel reflectivity approximation well agrees with the exact Zoeppritz equation until incident angles reach 40°. Convoluting the novel P-wave reflection approximation with seismic wavelets as the forward solver, an AVO inversion method based on the Bayesian theory is proposed to invert the TOC indicator with seismic data. The synthetic examples and field tests validate the feasibility and stability of the proposed AVO inversion approach. Using the inversion results of the TOC indicator, TOC is directly and accurately estimated in the target area.
基金supported by the National Natural Science Foundation of China (Nos.41974123,42174161)the Jiangsu Innovation and Entrepreneurship Plan and the Jiangsu Province Science Fund for Distinguished Young Scholars (grant no.BK20200021).
文摘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 diff raction,and ultrasonic and electrical experiments at diff erent 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 diff erent 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 diff erent 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 eff ectively 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.
基金sponsored by the CNOOC research project (No.YXKY-2019-ZY-04)。
文摘Tight sandstone gas reservoirs have characteristics of low porosity and low permeability,complex pore structure.In the paper,we consider the pore and micro-fracture systems in tight sandstone reservoir and perform rock physics modeling of sandstone gas reservoirs based on Chapman.Then we analyze the effects of relaxation time,and difference fluid types on dispersion and attention,and seismic responses.We found that the magnitude of the relaxation time is related to the gas saturation and the viscosity coefficient of the fluid.The magnitude of the relaxation time affects the frequency range of the velocity dispersion and attenuation,and the type of fluid affects the magnitude of the velocity dispersion and attenuation.As the relaxation time decreases,the calculated amplitude deceases.And the amplitude and the waveform of the synthetic seismic record are related to different fluid types and formation thickness.
文摘Sokor Formation reservoir intervals are intrinsically anisotropic,heterogeneous and with a characteristic of Low Contrast Low Resistivity(LCLR)log responses in parts of the Termit basin.Discriminating sands from shales/mudstones and hydrocarbon sands from brine sands as well as accurately evaluating the distribution of relevant reservoir properties using conventional seismic interpretation are complicated,and undermines reservoir characterization in such reservoirs.To enhance reservoir evaluation and reduce development planning and production risks,rock physics analysis was intergrated into the petrophysical workflow,which fed higher fidelity inputs into a post stack seismic inversion workflow.Rock Physics Diagnostics(RPD)analysis revealed that the reservoir interval of interest has grain size distribution of different lithologies,which is related to the environment of deposition and burial history,and could be best described by the constant cement sand model.The rock physics analyses revealed that facies were most effectively discriminated based on their Vp/Vs ratios and acoustic impedance.Particularly,hydrocarbon saturated sandstones,brine saturated shaly sandstones and shales/mudstones which exhibit similar acoustic impedance characteristics,were clearly discriminated by their Vp/Vs.The inverted seismic attributes as well as Seismic Based-Rock Physics Templates(RPT),clearly delineated the hydrocarbon fields,predicted new prospects beyond the existing well locations,which could be considered for field appraisal or development opportunities in the basin.These results demonstrate the value of the robust application of rock physics diagnostic modeling and seismic inversion in quantitative reservoir characterization and may be quite useful in undrilled locations in the basins and fields with similar geology.
文摘The stability of the drilled wellbores in carbonate formations is of great importance.This paper is a new approach to determine the elastic properties of the wellbore based on rock physics methods using the Hashin-Shtrikman bounds to determine the wellbore stability.Initially,the static elastic parameters were determined by two different methods,namely the rock physics method and DSI log,followed by clustering through Multi-Resolutional Graph-Based clustering(MRGC)and calculating geomechanical-units(GMUs)of each method separately.The obtained results from DSI log and rock physic methods were then compared followed by determining the wellbore stability.The results showed that the correlation between shear and compressional wave velocity obtained from the petrophysical method with the measured values of shear and pressure wave velocity in the well was 0.94 and 0.90,respectively,which shows that the petrophysical method can estimate these two logs with high accuracy,and it can be a suitable alternative instead of using the Dipole Shear Sonic Imager(DSI).It was observed that the geomechanical units of elastic parameters calculated by proposed rock physics method are in good agreement with those obtained from DSI log.Rock-physic method can be a good alternative for when expensive DSI logs are missing.
基金Supported by:CNPC Major Project "Research on Key Technologies for Enhanced Oil Recovery in Tight Sandstone Gas Reservoirs"(No. 2023ZZ25)Gansu Provincial Science and Technology Major Project"Research and Application of Key Technologies for Geophysical Prediction of Natural Gas Reservoirs in Longdong Area"(No. 23ZDGA004)PetroChina Changqing Oilfield Company'Qingshimao gas field water-bearing gas reservoir 3D seismic fine interpretation and well position support'(No.2023QCPJ33)。
文摘Tight sandstone reservoirs have strong heterogeneity and complex gas-water relationship,causing diffi culty in quantitatively predicting water saturation.Deep learning,combined with rock physics analysis and geostatistics theory,was used to predict water saturation in tight sandstone,focusing on the P_(sh)^(8) in the GFZ area of the Ordos Basin.Results show that:Starting with actual wells where porosity and saturation results are obtained from log interpretations,the relationship between reservoir parameters(porosity and saturation)and elastic properties(P-wave velocity,S-wave velocity,and density)is established through the development of a rock physics model suitable for the region.Under the constraints of geostatistical laws,such as background trends of elastic and reservoir parameters and the vertical variations in logging curves,reservoir conditions(including porosity,saturation,and thickness)are simulated to generate numerous pseudowells and corresponding seismic gathers modeled using the Zoeppritz equation.A convolution neural network is used to train the target curve and predict the target body.The predicted water saturation of the P_(sh)^(8) shows strong agreement with the results from two blind wells,providing a reliable basis for understanding the water saturation(Sw)of tight sandstone.
基金supported by the National Natural Science Foundation of China(Grant Nos.42074153 and 42274160)。
文摘Estimating gas enrichments is a key objective in exploring sweet spots within tight sandstone gas reservoirs.However,the low sensitivity of elastic parameters to gas saturations in such formations makes it a significant challenge to reliably estimate gas enrichments using seismic methods.Through rock physical modeling and reservoir parameter analyses conducted in this study,a more suitable indicator for estimating gas enrichment,termed the gas content indicator,has been proposed.This indicator is formulated based on effective fluid bulk modulus and shear modulus and demonstrates a clear positive correlation with gas content in tight sandstones.Moreover,a new seismic amplitude variation versus offset(AVO)equation is derived to directly extract reservoir properties,such as the gas content indicator and porosity,from prestack seismic data.The accuracy of this proposed AVO equation is validated through comparison with the exact solutions provided by the Zoeppritz equation.To ensure reliable estimations of reservoir properties from partial angle-stacked seismic data,the proposed AVO equation is reformulated within the elastic impedance inversion framework.The estimated gas content indicator and porosity exhibit favorable agreement with logging data,suggesting that the obtained results are suitable for reliable predictions of tight sandstones with high gas enrichments.Furthermore,the proposed methods have the potential to stimulate the advancement of other suitable inversion techniques for directly estimating reservoir properties from seismic data across various petroleum resources.
