Acoustic-electrical properties and rock physics models for shale-oil formations:prediction of reservoir properties of interbedded sandstone and shale layers

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.

Brittleness index and seismic rock physics model for anisotropic tight-oil sandstone reservoirs

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.

Prediction of brittleness based on anisotropic rock physics model for kerogen-rich shale

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.

The construction of shale rock physics model and brittleness prediction for high-porosity shale gas-bearing reservoir

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.

Rock physics inversion based on an optimized MCMC method

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.

Simplifying the Integration of Petrophysics and Rock-Physics to Identify Hydrocarbon Bearing Rocks on Seismic

A considerable effort has been made in the literature for quality assurance (QA) and quality checking (QC) of the petrophysical log data for computation of reservoir rock property parameters. Well log data plays an integral role in building a rock physics model for quantitative interpretation (QI) work. A poor-quality rock physics model may lead to significant financial and HSSE implications by drilling wells in undesired locations. Historically, a variety of techniques have been used including histograms and cross plots for reviewing the feasibility of petrophysical logs for QI work. However, no attempt has ever been made to introduce a simplified workflow. This paper serves two-fold. It provides a simplified step by step approach for building a petrophysics/rock physics model. A case study has been presented to compare the synthetic seismogram generated from the simplified workflow with the actual seismic trace at well locations. Secondly, the paper shows how a few key cross plots and rock property parameters provide adequate information to validate petrophysical data, distinguish overburden and reservoir sections, and to help identify fluids and saturation trends within the reservoir sands. In the mentioned case study, the robustness of the simplified rock physics model has helped seismic data to successfully distinguish hydrocarbon bearing reservoir sands from non-reservoir shales.

The constructing of pore structure factor in carbonate rocks and the inversion of reservoir parameters

With a more complex pore structure system compared with clastic rocks, carbonate rocks have not yet been well described by existing conventional rock physical models concerning the pore structure vagary as well as the influence on elastic rock properties. We start with a discussion and an analysis about carbonate rock pore structure utilizing rock slices. Then, given appropriate assumptions, we introduce a new approach to modeling carbonate rocks and construct a pore structure algorithm to identify pore structure mutation with a basis on the Gassmann equation and the Eshelby-Walsh ellipsoid inclusion crack theory. Finally, we compute a single well＇s porosity using this new approach with full wave log data and make a comparison with the predicted result of traditional method and simultaneously invert for reservoir parameters. The study results reveal that the rock pore structure can significantly influence the rocks＇ elastic properties and the predicted porosity error of the new modeling approach is merely 0.74%. Therefore, the approach we introduce can effectively decrease the predicted error of reservoir parameters.

An overview of the thermography-based experimental studies on roadway excavation in stratified rock masses at CUMTB

This paper presents an overview of experimental investigations conducted at China University of Mining and Technology Beijing(CUMTB) on roadway excavation using large-scale geomechanical model tests.The simulated sedimentary rocks are composed by alternating layers of sandstone, mudstone and coal seam inclined at varied angles with respect to the horizontal including 0°, 45°, 60°, and 90°. During the excavation, infrared thermography was employed to detect the thermal response of the surrounding rocks under excavation. The obtained raw thermograms were processed using denoising algorithm, data reduction procedure and Fourier analysis. The infrared temperature(IRT) characterizes the overall rock response; the processed thermal images represent the structural behavior, and the Fourier spectrum describes damage development in the frequency domain. Deeper understanding was achieved by the comparative analyses of excavation in differently inclined rock masses using the image features of IRTs, thermal images and Fourier spectra.

Establishment and application of the anisotropic shale-rock physical model in the observation coordinate system

No shale-rock physical model has been established in the observation coordinate system.To this end,this paper carried out anisotropic wave velocity tests on shale rock and compared the Thomsen,Daley,and Berryman solutions to characterize anisotropic acoustic wave velocity.Finally,the Daley solution was selected.Based on basic rock physical models,such as SCA and DEM methods,and combined with the Daley solution,an anisotropic shale-rock physical model was established in the observation coordinate system and applied in Well B1 in the Luzhou area,Sichuan Basin.Our research conclusions were as follows:1.for the samples from the same core,the P-wave velocities in three directions were in the order VP11>VP45>VP33,shear wave velocity VS11 was the largest,but VS33 and VS45 did not follow the law of Vs33>Vs45 for some samples;2.the Daley solution,which not only considers the accuracy requirements but also has a complete expression of P-,SV-,and SH-waves,is most suitable for characterization of anisotropic wave velocity in this study area;3.the rock physical model constructed in the observation coordinate system has high accuracy,in which the absolute value of the relative error of the P-wave slowness was between 0%and 5.05%(0.55%on average),and that of shear-wave slowness was between 0%and 6.05%(0.59%on average);4.the acoustic waves recorded in Well B1 in the observation coordinate system were very different from those in the constitutive coordinate system.The relative difference of the P-wave was between 6.76%and 30.84%(14.68%on average),and that of the S-wave was between 7.00%and 23.44%(13.99%on average).The acoustic slowness measured in the observation coordinate system,such as in a deviated well or a horizontal well section,must be converted to the constitutive coordinate system before it can be used in subsequent engineering applications;5.the anisotropic shale-rock physical model built in the observation coordinate system proposed in this paper can provide basic data and guidance for subsequent pore pressure prediction,geomechanical modeling,and fracturing stimulation design for deviated and horizontal wells.

Influence of Gas Hydrate on the Acoustic Properties of Sediment: A Comprehensive Review with a Focus on Experimental Measurements

In recent years, natural gas hydrate has attracted increasing attention worldwide as a potential alternative energy source due to its attributes of wide distribution, large reserves, and low carbon. Since the acoustic characteristics of hydratebearing reservoirs clearly differ from those of adjacent formations, an acoustic approach, using seismic and acoustic logging, is one of the most direct, effective and widely used methods among the identification and characterization techniques for hydrate reservoir exploration. This review of research on the influence of hydrate(content and distribution) on the acoustic properties(velocity and attenuation) of sediments in the past two decades includes experimental studies based on different hydrate formation methods and measurements, as well as rock physics models. The main problems in current research are also pointed out and future prospects discussed.

Multi-exponential model to describe pressure-dependent P-and S-wave velocities and its use to estimate the crack aspect ratio

We present new quantitative model describing the pressure dependence of acoustic P-and S-wave velocities.Assuming that a variety of individual mechanisms or defects(such as cracks,pore collapse and grain crushing)can contribute to the pressure-dependent change of the wave velocity,we order a characteristic pressure to all of them and allow a series of exponential terms in the description of the(Pand S-waves)velocity-pressure function.We estimate the parameters of the multi-exponential rock physical model in inversion procedures using laboratory measured P-and S-wave velocity data.As is known,the conventional damped least squares method gives acceptable results only when one or two individual mechanisms are assumed.Increasing the number of exponential terms leads to highly nonlinear ill-posed inverse problem.Due to this reason,we develop the spectral inversion method(SIM)in which the velocity amplitudes(the spectral lines in the characteristic pressure spectrum)are only considered as unknowns.The characteristic pressures(belonging to the velocity amplitudes)are excluded from the set of inversion unknowns,instead,they are defined in a set of fixed positions equidistantly distributed in the actual interval of the independent variable(pressure).Through this novel linear inversion method,we estimate the parameters of the multi-exponential rock physical model using laboratory measured P-and S-wave velocity data.The characteristic pressures are related to the closing pressures of cracks which are described by well-known rock mechanical relationships depending on the aspect ratio of elliptical cracks.This gives the possibility to estimate the aspect ratios in terms of the characteristic pressures.

Estimation of S-wave Velocity for Gas Hydrate Reservoir in the Shenhu Area,North South China Sea

Estimation of S-wave velocity using logging data has mainly been performed for sandstone, mudstone and oil and gas strata, while its application to hydrate reservoirs has been largely overlooked. In this paper we present petxophysical methods to estimate the S-wave velocity of hydrate reservoirs with the P-wave velocity and the density as constraints. The three models used in this paper are an equivalent model （MBGL）, a three-phase model （TPBE）, and a thermo-elasticity model （TEM）. The MBGL model can effectively describe the internal relationship among the components of the rock, and the estimated P-wave velocities are in good agreement with the measured data （2.8% error）. However, in the TPBE model, the solid, liquid and gas phases axe considered to be independent of each other, and the estimation results are relatively low （46.6% error）. The TEM model is based on the sensitivity of the gas hydrate to temperature and pressure, and the accuracy of the estimation results is also high （3.6% error）. Before the estimation, the occurrence patterns of hydrates in the Shenhu area were examined, and occurrence state one （the hydrate is in solid form in the reservoir） was selected for analysis. By using the known P-wave velocity and density as constraints, a reasonable S-wave velocity value （ranging from 400 to 1100 m s 1 and for a hydrate layer of 1100 m s 1） can be obtained through multiple iterations. These methods and results provide new data and technical support for further research on hydrates and other geological features in the Shenhu area.

Shale Gas Characterization of Sembar Formation, Khipro Area, Pakistan

This study pertains to the evaluation of shale gas and rock physics properties of this area with respect to its total organic content of Sember Formation, Khiproarea, Pakistan. We use well logs data for this study. The Khipro area is prominent in the Lower Indus Basin for its hydrocarbon (oil and gas) structural traps. In shale gas evaluation, TOC of Sember Formation is estimated. The analysis has been done with the help of the wire line data of the well Bilal North-01. The presence of shale gas in the study area is analyzed with the help of different techniques. Rock physics and petrophysical analysis have been done in order to get the properties of the area related to the shale gas evaluation.

Seismic rock physical modelling for gas hydrate-bearing sediments

There are ambiguities and uncertainties in the recognition of gas hydrate seismic reflections and in quantitative predictions of physical information of natural gas hydrate reservoirs from seismic data. Rock physical modelling is a bridge that transforms the seismic information of geophysical observations into physical information, but traditional rock physics models lack descriptions of reservoir micro-structures and pore-filling materials. Considering the mineral compositions and pore microstructures of gas hydrates, we built rock physical models for load-bearing and pore-filling gas hydrate-bearing sediments,describe the mineral compositions, pore connectivity and pore shape using effective media theory, calculated the shear properties of pore-filling gas hydrates using Patchy saturation theory and Generalized Gassmann theory, and then revealed the quantitative relation between the elastic parameters and physical parameters for gas hydrate-bearing sediments. The numerical modelling results have shown that the ratios of P-wave and S-wave velocities decrease with hydrate saturation, the P-wave and S-wave velocities of load-bearing gas hydrate-bearing sediments are more sensitive to hydrate saturation, sensitivity is higher with narrower pores, and the ratios of the P-wave and S-wave velocities of pore-filling gas hydrate-bearing sediments are more sensitive to shear properties of hydrates at higher hydrate saturations. Theoretical analysis and practical application results showed that the rock physical models in this paper can be used to calculate the quantitative relation between macro elastic properties and micro physical properties of gas hydrate-bearing sediments, offer shear velocity information lacking in well logging, determine elastic parameters that have more effective indicating abilities, obtain physical parameters such as hydrate saturation and pore aspect ratios, and provide a theoretical basis and practical guidance for gas hydrate quantitative predictions.

Shear wave velocity prediction:A review of recent progress and future opportunities

Shear logs,also known as shear velocity logs,are used for various types of seismic analysis,such as determining the relationship between amplitude variation with offset(AVO)and interpreting multiple types of seismic data.This log is an important tool for analyzing the properties of rocks and interpreting seismic data to identify potential areas of oil and gas reserves.However,these logs are often not collected due to cost constraints or poor borehole conditions possibly leading to poor data quality,though there are various approaches in practice for estimating shear wave velocity.In this study,a detailed review of the recent advances in the various techniques used to measure shear wave(S-wave)velocity is carried out.These techniques include direct and indirect measurement,determination of empirical relationships between S-wave velocity and other parameters,machine learning,and rock physics models.Therefore,this study creates a collection of employed techniques,enhancing the existing knowledge of this significant topic and offering a progressive approach for practical implementation in the field.

Seismic Methods for Exploration and Exploitation of Gas Hydrate

Seismic and rock physics play important roles in gas hydrate exploration and production.To provide a clear cognition of the applications of geophysical methods on gas hydrate,this work presents a review of the seismic techniques,rock physics models,and production methods in gas hydrate exploration and exploitation.We first summarize the commonly used seismic techniques in identifying the gas hydrate formations and analyze the limitations and challenges of these techniques.Then,we outline the rock physics models linking the micro-scale physical properties and macro-scale seismic velocities of gas hydrate sediments,and generalize the common workflow,showing the frequently-used procedures of building models with detailed analysis of the potential uncertainties.Afterwards,we summarize the production techniques of gas hydrate and point out the problems regarding the petrophysical basis and abnormal seismic responses.In the end,considering the geological and engineering problems,we come up with several aspects of using geophysical techniques to solve the problems in gas hydrate exploration and production,hopefully to provide some important clues for future studies of gas hydrate.