Shale Rock Physics Model Research Articles

A new rock physics model for fractured oil shale reservoirs in Mahu oil field

The exploration of unconventional reservoirs such as oil shale has become a focus of research in the oil/gas industry, but due to the diversity of lithology and structural complexity of shale reservoirs, the study of their rock physics laws is a challenge. By analyzing the physical, lithologic, and fluid characteristics of oil shale reservoirs in the Mahu area of Xinjiang, China, we adopted a variety of effective-medium theories to carry out rock physics modeling. We analyzed the differences in the calculation results of various theoretical models, and finally constructed a set of rock physics modeling processes suitable for oil shale reservoirs. The analysis shows that the calculation results of Voigt-Reuss-Hill (VRH) and Hashin-Shtrikman (HS) average for mixed mineral matrix are very similar, but there are certain differences between the upper and lower bounds calculated by them. The upper and lower bounds of HS average are closer than VRH average. We used the Kuster-Toksoz effective-medium, differential effective-medium (DEM), and self-consistent approximation models to construct the rock skeleton and analyzed the differences of the models. The results of the three models are almost identical, but their assumptions and limitations differ. We used DEM in the final shale model, considering the large porosity and the order of inclusion filling. According to the fracture distribution characteristics of oil shale reservoirs, we used an inclined fracture model to describe the fractures with different structural characteristics. Finally, the established shale rock physics model was used to calculate the actual logging data, and the results of elastic parameters are consistent with the measured data. Through the inversion of the fracture parameters, the inversion results are consistent with the measured results as a whole, indicating that the model has certain applicability to the simulation of oil shale reservoirs.

Direct Geostatistical Seismic Amplitude Versus Angle Inversion for Shale Rock Properties

The purpose of seismic reservoir characterization is to predict the spatial distribution of the subsurface rock properties from a set of direct and indirect measurements. Commonly, rock property volumes are obtained in a two-steps approach. First, the elastic properties are inverted from seismic reflection data and then used to compute rock properties volumes by applying a calibrated rock physics models. Such an approach is not only time consuming but may also lead to biased results as the uncertainties related to seismic inversion may not be propagated through the entire geomodeling workflow. Here, we propose an iterative geostatistical shale rock physics seismic AVA inversion method to invert seismic reflection data directly for shale rock properties. The workflow consists of three main steps starting from shale properties model generation of brittleness, total organic carbon (TOC), and porosity using stochastic sequential simulation and cosimulation and calculation of a volume of shale. In the following step, elastic property volumes are calculated based on a calibrated shale rock physics model using the self-consistent approximation. The elastic models are then used for the calculation of the synthetic seismic data. In the final step, the misfit between synthetic and real data is calculated and used as part of the stochastic update of the model parameter space. The whole process is repeated until a minimum misfit between the observed and synthetic seismic is achieved. The proposed method is successfully tested on an onshore Lower Paleozoic shale gas reservoir in Northern Poland, and the predicted shale rock properties agree with those observed at a blind-well location.

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.

A rock physics modelling algorithm for simulating the elastic parameters of shale using well logging data

As a high-resolution geophysical method employed by the oil and gas industry, well logging can be used to accurately investigate reservoirs. Challenges associated with shale gas reservoir exploration increase the importance of applying elastic parameters or velocity at the logging scale. An efficient shale rock physics model is the foundation for the successful application of this method. We propose a procedure for modelling shale rock physics in which an appropriate modelling method is applied for different compositions of shale rock. The stiffnesses of the kerogen and fluid (oil, gas or water) mixture are obtained with the Kuster-Toksöz model, which assumes that the fluid is included in the kerogen matrix. A self-consistent approximation method is used to model clay, where the clay pores are filled with formation water. The Backus averaging model is then used to simulate the influence of laminated clay and laminated kerogen. Elastic parameter simulations using well logging data show the importance of treating the volume fractions of laminated clay and kerogen carefully. A comparison of the measured compressional slowness and modelled compressional slowness shows the efficiency of the proposed modelling procedure.

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 (BI) 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 selfconsistent 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 BI. 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.

A shale rock physics model for analysis of brittleness index, mineralogy and porosity in the Barnett Shale

We construct a rock physics workflow to link the elastic properties of shales to complex constituents and specific microstructure attributes. The key feature in our rock physics model is the degrees of preferred orientation of clay and kerogen particles defined by the proportions of such particles in their total content. The self-consistent approximation method and Backus averaging method are used to consider the isotropic distribution and preferred orientation of compositions and pores in shales. Using the core and well log data from the Barnett Shale, we demonstrate the application of the constructed templates for the evaluation of porosity, lithology and brittleness index. Then, we investigate the brittleness index defined in terms of mineralogy and geomechanical properties. The results show that as clay content increases, Poisson's ratio tends to increase and Young's modulus tends to decrease. Moreover, we find that Poisson's ratio is more sensitive to the variation in the texture of shales resulting from the preferred orientation of clay particles. Finally, based on the constructed rock physics model, we calculate AVO responses from the top and bottom of the Barnett Shale, and the results indicate predictable trends for the variations in porosity, lithology and brittleness index in shales.