A Prediction Method of Orthotropic Shale Brittleness Index

Abstract

The invention discloses a method for predicting the brittleness index of orthotropic shale, which includes measuring the longitudinal wave velocity in six directions and the shear wave velocity in three directions; calculating the anisotropic stiffness coefficient of shale; Stiffness matrix C of heterogeneous shale ORT ; Determine the anisotropic elastic modulus and Poisson's ratio of the shale; determine the flexibility matrix S of the orthotropic shale in the formation coordinates ORT ; Calculation of shale elastic modulus and Poisson's ratio under the wellbore observation coordinates; calculation of brittleness index of orthotropic shale. The invention considers the anisotropy characteristics of shale, evaluates the brittleness index of orthotropic shale formation from the perspective of actual drilling, overcomes the defect that the traditional shale brittleness index ignores the influence of anisotropy, and can more accurately reflect the shale brittleness Exponential anisotropy and its influence on well trajectory can provide scientific basis and decision-making support for shale gas horizontal well drilling and fracturing.

Description

technical field [0001] The invention relates to a method for predicting the brittleness index of orthotropic shale, belonging to the technical field of drilling and completion engineering. Background technique [0002] The brittleness of shale is one of the important properties of rock, and the shale brittleness index is an important basis for wellbore stability analysis, hydraulic fracturing segmentation and optimization. In the process of hydraulic fracturing, the brittleness index is a key parameter to evaluate the shale's compressibility. In addition to controlling the difficulty of fracture extension in the rock, it is also a factor that affects the development of fractures. [0003] For the evaluation of shale brittleness index, the commonly used methods include: shale brittleness evaluation method based on mineral components and shale brittleness evaluation method based on elastic parameters. For deep shale, due to the significant anisotropy of shale and the developm...

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Problems solved by technology

For deep shale, due to the significant anisotropy of shale and the development of a large number of vertical and orthogonal micro-cracks in the shale formation, the shale exhibits orthotropic characteristics, perpendicular to the bedding plane and parallel to the bedding plane. The brittleness index in the direction of the bedding plane and the direction perpendicular to the development of microcracks is usually inconsistent, and most of the current research focuses on the transversely isotropic shale, ignoring the brittle anisotropy of orthotropic shale, which has It may lead to distortion or even serious errors in the evaluation results of shale brittleness index

[0004] However, in the existing methods, the shale brittleness evaluation method based on mineral components cannot evaluate the brittle anisotropy of shale formations, while the method based on elastic parameters to evaluate the brittleness index can only evaluate the brittleness characteristics perpendicular to the bedding plane. Neither can be applied to orthotropic shale formations

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