Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

Yusong Wu,Xiao Li,Bo Zheng,Jianming He

doi:10.3390/en9121088

Yusong Wu, Xiao Li + Show 2 more

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https://doi.org/10.3390/en9121088

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Abstract

With the exploitation of shale gas booming all over the world, more and more studies are focused on the core technology, hydraulic fracturing, to improve commercial exploitation. Shale gas resources in China are enormous. In this research, a series of tests were carried out with samples of black organic-rich shale from the Lower Silurian Longmaxi formation, south China. Samples were drilled from different directions and were subjected to uniaxial and triaxial condition with various confining pressures, aiming at studying its rock mechanics properties, so as to provide basis for research and breakthrough of hydraulic fracturing technology. According to the results of the study, the development and distribution of shale’s bedding planes significantly impact its mechanical properties. Shale samples show obvious brittle characteristics under low confining pressure, and its mechanical behavior begins to transform from brittle to plastic characteristics with increasing confining pressure. Shale samples with different inclinations (β) have different sensitivities to the confining pressure. As a result, samples with 45° inclinations (β) are least sensitive. The strength of bedding planes is significantly lower than that of shale matrix, and tensile failure and shear failure generally tend to occur along the bedding planes. When hydraulic fracturing was conducted in shale formation with depth less than 2.25 km, corresponding to original in-situ of 60 MPa, cracks will preferably occur at first along the inclination (β) angle of 45° from the maximum principal stress, and the failure mode is most likely to be shear failure without volumetric strain. And, different modes of failure will occur at different locations in the reservoir, depending on the orientation of bedding inclined from the principle stress, which can probably explain the phenomenon why there are fractures along and cross the bedding planes during hydraulic fracturing treatment. When hydraulic fracturing was conducted in shale formation with depth greater than 2.25 km, hydraulic fractures may not crack along the bedding surfaces to some extent.

Highlights

Shale has received much attention in recent years due to its gas bearing properties, which offer important economic benefits [1,2,3]
Shale gas extraction requires the use of hydraulic fracturing technology [4], a technology that can benefit from understanding the mechanical properties [5,6]
Longmaxi marine shale was used to drill along different angles and conduct tests under various confining pressures

Summary

Introduction

Shale has received much attention in recent years due to its gas bearing properties, which offer important economic benefits [1,2,3]. Shales generally exhibit low porosity and permeability [7,8,9,10] Better grasp of these characteristics helps to improve the fracturing process. Swan et al presented evidence for a great rate effect as seen in triaxial compression tests on comparatively soft, saturated shale from Kimmeridge Bay, Dorset, UK. Ibanez et al investigated the mechanical properties and deformation mechanisms of illite-rich shale in triaxial compression experiments at varying confining pressures, temperatures and strain rates [16]. Kwon et al investigated mechanical and fluid transport properties of brine-saturated illite shale from the Wilcox Formation to examine the effects of strain rate on fracture strength and to determine the conditions over which rates of deformation and internal fluid flow compete [17]. From experiments performed with outcrop and overburden shales, Martin et al

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