ABSTRACT:

Continental shale gas is a new field of unconventional oil and gas resources. Systematic research on the rock mechanical properties of continental shale after being soaked with fracturing fluid is extremely scarce. For the first time, 7 types of continental shale cores of different strata with 30 m depth variations in the Sichuan Basin were obtained. Two sets of tri-axial compression experiments before and after soaked were compared. Three quantitative indicators were used to evaluate the macroscopic fracture characteristics of samples: the 3D scanner was used to scan the main failure surface, and the root mean square was used to calculate scanned point cloud data; a simpler index for calculating roughness based on area variation was established; the surface crack morphology of the sample was drawn by high-resolution photos, and the crack complexity was quantitatively described based on box-counting method in fractal dimension. The results show that the impact mechanism of fracturing fluid immersion on the shale at different sampling locations is different, which can be summarized as two types: lubrication dominated and softening dominated. It provides certain guiding significance for the variation of mechanical properties and brittleness assessment of continental shale under the influence of fracturing fluid immersion.

1. Introduction

In recent years, typical shale gas resources has developed rapidly, the oil and gas theory and fracturing technology have been upgraded, and the field of exploration and development is constantly innovating and making breakthroughs. With the increase in demand for energy and unconventional resource exploration (Jiang et al., 2017), it is very urgent and important to drive the technological development of various unconventional oil and gas resources including different types of shale gas to improve the energy structure and resource shortage. Continental shale gas resources, as a brand-new field of unconventional energy strategy, are still in the stage of geological evaluation and industrialized mining experiment exploration (Dong et al., 2016)(Zhai et al., 2018).

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