Tensile and Shear Mechanical Characteristics of Longmaxi Shale Laminae Dependent on the Mineral Composition and Morphology

Lihui Li,Ming Wang,Xiaolin Huang,Beixiu Huang,Xiao Li

doi:10.3390/en13112977

Abstract

Laminae are well developed in shale and generally influence fracture propagation during hydraulic fracturing. Hence, comprehensively understanding the tension and shear behaviors of shale laminae is crucial. There have been limited systematic studies thus far on the tensile and shear strength as well as fracture morphology of shale laminae. In this study, the Lower Silurian Longmaxi Shale (China) was investigated via Brazilian tensile and angle-varied plate shear tests. Five lamina types were tested, i.e., calcite (Cal), pyrite (Py), organic-enriched (Oc), the interface between Cal and Oc (Cal-Oc), and the interface between Py and Oc (Py-Oc) laminae. Results showed that the tensile strength was in the range 0.43–8.22 MPa, mainly in the order of Cal > Py > Cal-Oc > Py-Oc > Oc. The modes of fracture morphology were highly related to the occurrence, continuity, and mineralogy fillings of laminae. Shear strength parameters were within the range 22.50–29.64 MPa for cohesion and 37.29–43.60° for internal friction angle. Fracture surface roughness was strongly related to its cohesion. Calcite laminae considerably influenced the tensile fracturing of shale, suggesting that the geometry and properties of calcite lamina should receive more attention during the design of shale gas exploration.

Highlights

Shale is generally characterized by layered structures and consequent intrinsic anisotropy owing to distinct sequential sedimentation [1]
The microcomputer-controlled electronic universal testing machine MTS-SANS CMT integrated with three sets of shear clamps was employed to conduct the Brazilian tensile and angle-varied plate shear tests
The microcomputer-controlled electronic universal testing machine MTS-SANS CMT integrated with three sets of shear clamps was employed to conduct the Brazilian tensile and Tensile Strength angle-varied plate shear tests

Summary

Introduction

Shale is generally characterized by layered structures and consequent intrinsic anisotropy owing to distinct sequential sedimentation [1]. The frequently developed sandy laminae in shale have a significant influence on the accumulation [2], storage, and permeability [3] of shale gas as well as on the mechanical properties of shales [4,5]. Outcrop observation and core analysis have indicated that black shale deposited in marine environments was abundant with calcite-/pyrite-rich laminae and lenses as well, such as the Longmaxi Shale [6] and the Devonian Horn River Shale [7]. Quantitative moduli mapping of organic and inorganic components in shale at a nanometer scale demonstrated that the Young’s modulus of different components is distinguishable and varies significantly, ranging from 15 MPa for organic matter to nearly 100 MPa for quartz, except for pyrite being the stiffest in the range 250–312 MPa [10,11]. The heterogeneity of mineralogical components is a Energies 2020, 13, 2977; doi:10.3390/en13112977 www.mdpi.com/journal/energies

Objectives

Methods

Results

Discussion

Conclusion