Seismo‐Mechanical Response of Anisotropic Rocks Under Hydraulic Fracture Conditions: New Experimental Insights

S Gehne,P M Benson,M Enfield,K J Dobson,A Barber,N Koor

doi:10.1029/2019jb017342

Abstract

AbstractUnconventional hydrocarbon resources found across the world are driving a renewed interest in mudrock hydraulic fracturing methods. However, given the difficulty in safely measuring the various controlling factors in a natural environment, considerable challenges remain in understanding the fracture process. To investigate, we report a new laboratory study that simulates hydraulic fracturing using a conventional triaxial apparatus. We show that fracture orientation is primarily controlled by external stress conditions and the inherent rock anisotropy and fabric are critical in governing fracture initiation, propagation, and geometry. We use anisotropic Nash Point Shale (NPS) from the early Jurassic with high elastic P wave anisotropy (56%) and mechanical tensile anisotropy (60%), and highly anisotropic (cemented) Crab Orchard Sandstone with P wave/tensile anisotropies of 12% and 14%, respectively. Initiation of tensile fracture requires 36 MPa for NPS at 1‐km simulated depth and 32 MPa for Crab Orchard Sandstone, in both cases with cross‐bedding favorable orientated. When unfavorably orientated, this increases to 58 MPa for NPS at 800‐m simulated depth, far higher as fractures must now traverse cross‐bedding. We record a swarm of acoustic emission activity, which exhibits spectral power peaks at 600 and 100 kHz suggesting primary fracture and fluid‐rock resonance, respectively. The onset of the acoustic emission data precedes the dynamic instability of the fracture by 0.02 s, which scales to ~20 s for ~100‐m size fractures. We conclude that a monitoring system could become not only a forecasting tool but also a means to control the fracking process to prevent avoidable seismic events.

Highlights

Hydrofracturing is a common process in many areas of pure and applied geosciences, such as magma and dike intrusions (e.g., Rubin, 1993; Tuffen & Dingwell, 2005) and the development of mineral veins (e.g., Gudmundsson & Brenner, 2001)
We show that fracture orientation is primarily controlled by external stress conditions and the inherent rock anisotropy and fabric are critical in governing fracture initiation, propagation, and geometry
Five key parameters are defined: (1) the maximum fluid injection pressure; (2) the time of radial deformation; (3) the time of acoustic emission (AE) activity, which itself is defined as the exponential increase in AE hit count rate (AE0; Boone et al, 1991; Martin & Chandler, 1994; Zoback et al, 1977); (4) the time of peak AE activity; and (5) the time at which the fluid pressure starts to decrease rapidly (Prd)

Summary

Introduction

Hydrofracturing is a common process in many areas of pure and applied geosciences, such as magma and dike intrusions (e.g., Rubin, 1993; Tuffen & Dingwell, 2005) and the development of mineral veins (e.g., Gudmundsson & Brenner, 2001). The generation of fresh tensile fracture networks by hydraulic fracturing, usually in low porosity mudrocks, has become a key process for the exploitation of unconventional hydrocarbon resources (Montgomery & Smith, 2010). Controversial, this practice has added significant gas resources to the U.S market and has potential in other regions including across Europe (Andrews, 1986) where significant unconventional resources have been identified. This complexity generates heterogeneities leading to stress localizations that are known to be important for fracture initiation and propagation (Renard et al, 2009; Scholz, 1968), with the effect of bedding planes, significantly influencing the propagation of hydraulic fractures across different lithologies (Chitrala et al, 2010; He et al, 2016)

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