Abstract

Laminated shale reserves various natural weak planes involving some bedding planes, natural fractures and mineral dykes. The obvious phenomenon of the interactions between hydraulic fractures and bedding weak planes were observed involving arrest, crossing, and diversion in previous works. Moreover, the bedding angles, cement mineral compositions of natural weak planes, and net pressure of hydraulic fracture were considered as the primary factors to determine the type of interaction. However, the rock failure modes and characterization of weak planes from micromechanics has been yet addressed. In this paper, a Particle Flow Code (PFC) model on natural weak planes of laminated shale was proposed by considering micromechanical properties including the cohesive strength, stiffness, elastic modulus, and friction angle. The PFC model was validated by the three-point bending tests on a typical laminated shale. The influence of weak plane angles, elastic modulus, and strength of weak planes on tensile fracture propagation were obtained. Their interaction types and dominant failure mode from tensile to shear were analyzed. It concludes that fractures divert in the inclined angles and low strength of natural weak planes, crossing occurs in the opposite case.

Introduction

Shale oil and gas formation normally reserves various natural weak planes involving the weak bedding planes, natural fractures and mineral dykes. It is evident that the natural weak planes affect hydraulic fracture geometry. The obvious phenomenon of the interactions between hydraulic fractures and bedding weak planes were observed involving arrest, crossing, and diversion in previous works. Moreover, the bedding angles, cement mineral compositions of natural weak planes, and net pressure of hydraulic fracture were considered as the primary factors to determine the type of interaction. However, the rock failure modes and characterization of weak planes from micromechanics has been yet addressed.

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