Orthotropic shale brittleness index prediction method

Abstract

The invention discloses an orthotropic shale brittleness index prediction method. The method comprises the following steps of measuring longitudinal wave velocities in six directions and transverse wave velocities in three directions; calculating the anisotropic rigidity coefficient of the shale; determining a stiffness matrix CORT of orthotropic shale under the stratum coordinates; determining the anisotropic elastic modulus and Poisson's ratio of the shale; determining a flexibility matrix SORT of orthotropic shale under the stratum coordinates; calculating the shale elasticity modulus and Poisson's ratio under the borehole observation coordinates; and calculating the brittleness index of the orthotropic shale. According to the method, the anisotropy characteristics of the shale are considered, the brittleness index of the orthotropic shale formation is evaluated from the angle of the actual drilling hole, a defect that the traditional shale brittleness index neglects the anisotropy influence is overcome, and the anisotropy characteristics of the shale brittleness index and the characteristics of the shale brittleness index influenced by the hole track can be reflected more accurately; scientific basis and decision support can be provided for drilling and fracturing of the shale gas horizontal well.

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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