Increased likelihood of induced seismicity in highly overpressured shale formations

Ryan Schultz,David W Eaton

doi:10.1093/gji/ggy167

Abstract

SUMMARYFluid-injection processes such as disposal of saltwater or hydraulic fracturing can induce earthquakes by increasing pore pressure and/or shear stress on faults. Natural processes, including transformation of organic material (kerogen) into hydrocarbon and cracking to produce gas, can similarly cause fluid overpressure. Here, we document two examples from the Western Canada Sedimentary Basin where earthquakes induced by hydraulic fracturing are strongly clustered within areas characterized by pore-pressure gradient in excess of 15 kPa m−1. Despite extensive hydraulic-fracturing activity associated with resource development, induced earthquakes are virtually absent in the Montney and Duvernay Formations elsewhere. Statistical analysis suggests a negligible probability that this spatial correlation developed by chance. This implies that, in addition to known factors such as anthropogenic pore-pressure increase and proximity to critically stressed faults, high in situ overpressure of shale formations may also represent a controlling factor for inducing earthquakes by hydraulic fracturing. On a geological timescale, natural pore-pressure generation may lead to fault-slip episodes that regulate the magnitude of formation overpressure.

Highlights

Large-volume disposal of produced brine (Keranen et al 2014; Schultz et al 2014; Weingarten et al 2015) or hydraulic fracturing (Schultz et al 2015a, 2017; Atkinson et al 2016; Bao & Eaton 2016; Lei et al 2017) can induce earthquakes by increasing pore pressure or stress on faults (Ellsworth 2013; Segall & Lu 2015)
Necessary conditions for the occurrence of injection-induced seismicity include a source of elevated pore pressure, a proximal, critically stressed fault and a pathway for fluid pressure to propagate from the injection site to the fault (Ellsworth 2013; Eaton 2018)
Two major resource fairways in the Western Canada Sedimentary Basin are considered in this study, both prone to induced seismicity from hydraulic fracturing (BCOGC 2014; Bao & Eaton 2016; Mahani et al 2017; Schultz et al 2017)

Summary

INTRODUCTION

Large-volume disposal of produced brine (Keranen et al 2014; Schultz et al 2014; Weingarten et al 2015) or hydraulic fracturing (Schultz et al 2015a, 2017; Atkinson et al 2016; Bao & Eaton 2016; Lei et al 2017) can induce earthquakes by increasing pore pressure or stress on faults (Ellsworth 2013; Segall & Lu 2015). Necessary conditions for the occurrence of injection-induced seismicity include a source of elevated pore pressure, a proximal, critically stressed fault and a pathway for fluid pressure to propagate from the injection site to the fault (Ellsworth 2013; Eaton 2018). The most prolific parts of a fairway often coincide with areas of highest overpressure (Cander 2012)

DATA AND METHODOLOGY

Findings

DISCUSSION

CONCLUSIONS