Abstract
The brittleness prediction of shale formations is of interest to researchers nowadays. Conventional methods of brittleness prediction are usually based on isotropic models while shale is anisotropic. In order to obtain a better prediction of shale brittleness, our study firstly proposed a novel brittleness index equation based on the Voigt–Reuss–Hill average, which combines two classical isotropic methods. The proposed method introduces upper and lower brittleness bounds, which take the uncertainty of brittleness prediction into consideration. In addition, this method can give us acceptable predictions by using limited input values. Secondly, an anisotropic rock physics model was constructed. Two parameters were introduced into our model, which can be used to simulate the lamination of clay minerals and the dip angle of formation. In addition, rock physics templates have been built to analyze the sensitivity of brittleness parameters. Finally, the effects of kerogen, pore structure, clay lamination and shale formation dip have been investigated in terms of anisotropy. The prediction shows that the vertical/horizontal Young’s modulus is always below one while the vertical/horizontal Poisson’s ratio (PR) can be either greater or less than 1. Our study finds different degrees of shale lamination may be the explanation for the random distribution of Vani (the ratio of vertical PR to horizontal PR).
Highlights
Shale reservoir exploration has become an important research area in recent years
This study attempts to use the brittleness index as a “modulus.” By assuming that the coupling geometry of each mineral component is in either series or parallel as shown in Fig. 3, considering that mineral coupling details are missing and the input parameters are limited, the construction of brittleness upper and lower limits allows the prediction of the reasonable range of rock brittleness
Based on our shale rock physics model analysis, our result indicates this observed phenomenon may due to the physical properties of shale
Summary
Shale reservoir exploration has become an important research area in recent years. The industrial scale production of shale oil/gas has led to a revolution in hydrocarbon. The existing brittleness index formulas are subdivided into the mineralogy method or the elastic parameter method; very few methods consider both two categories simultaneously. Both methods have their own advantages and disadvantages. The common way to evaluate shale brittleness is based on using isotropic parameters (i.e., Young’s modulus and Poisson’s ratio), since these can be calculated from measured elastic wave velocities from geophysical data. We intend to construct a rock physics model, which can take the different causes of shale anisotropy into consideration. Following on from this, we construct rock physics templates based on the model in order to analyze the sensitivity of brittleness parameters. We discuss anisotropic brittleness in terms of physical properties (lamination, pore structure, etc.)
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