Method for measuring shale brittleness index based on rock stress-strain curves and ultrasonic longitudinal wave speed

Abstract

The invention provides a method for measuring a shale brittleness index based on triaxial compression complete stress-strain curves and ultrasonic longitudinal wave speed. The method comprises the following steps: obtaining a triaxial compression complete stress-strain curve and the ultrasonic longitudinal wave speed of each time point in an experimental process by adopting dynamic and static combined triaxial compression experiment; determining a time point when a shale microcrack starts to be damaged by utilizing a change curve of the longitudinal wave speed; according to the change of the longitudinal wave speed and the shape of the complete stress-strain curve, dividing the complete stress-strain curve into four stages, namely, a stage of microcrack closure, a stage from compaction to starting of microcrack damage, a stage from microcrack extension to damage instability and a stage after rock damage instability; by utilizing the complete stress-strain curve, calculating the unit volume energy absorbed by a shale test piece in the corresponding stage; calculating the shale brittleness index by utilizing the ratio of the unit volume energy absorbed in the elastic stage to the absorbed total unit volume energy. The method is a comprehensive calculation method considering mechanical properties of shale in each stage and improves the accuracy and reasonability of rock brittleness evaluation.

Description

technical field [0001] The invention relates to the technical field of rock mechanics measurement, in particular to a method for measuring rock brittleness index. Background technique [0002] my country's shale gas reserves are abundant and have great development potential, but the mechanical properties of shale rocks are very different from conventional sandstones. The brittleness of shale is the reflection of the mechanical properties of shale. At present, there are many characterization methods for shale brittleness. , part of the brittleness calculation formula is based on the statistics of field results, and part of the calculation formula only studies the brittleness under uniaxial conditions, which is very different from the field brittleness. Therefore, it is of great significance to establish scientific shale gas reservoir brittleness experimental methods and calculation formulas for shale drilling and fracturing evaluation. [0003] Brittleness is a comprehensive ...

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Problems solved by technology

[0007] (3) Ignoring the influence of reservoir conditions on brittleness

At present, most strength-based brittleness index calculation formulas do not consider the actual pressure conditions of the reservoir. The actual shale brittleness under the reservoir depth condition is quite different from the actual calculated brittleness.

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