Friction-Permeability Relationships for Reservoir Caprocks

Abstract:

There is wide concern that pressurized CO₂ fluid has a potential to induce seismicity in the impermeable caprocks that overlie CO₂ injection formations. However, the possible impact of induced seismicity on sustainable CO₂ containment from geological CO₂ sequestration remains unclear because the earthquakes play a significant but mysterious role in influencing the integrity of the caprocks by hypothesized interrelated friction-permeability interaction processes: (1) the earthquakes may occur seismically (i.e., frictionally unstable), enhancing the permeability of faults instantly and leading to potential breaching and loss of inventory; or (2) the earthquakes may occur aseismically (i.e., frictionally stable), closing the aperture of faults and reducing permeability through creep. In this study, we explore these processes through experiments in which we measure the frictional parameters and hydraulic properties using Green River shale sample as an analogue caprock candidate. We observe that fracture permeability declines during shearing while the increased sliding velocity reduces the rate of decline. The physics of these observed behaviors are explored via parametric study and surface measurement of fractures, showing that both permeability and frictional strength are correlated to the fracture asperity evolution that is controlled by the sliding velocity and fracture material. Through the velocity step, the velocity strengthening behavior is observed for Green River shale, suggesting that for Green River shale, only aseismic slip would occur at a low sliding velocity during which the permeability would decrease.

Introduction

Geological storage of carbon dioxide (CO₂) is considered a viable method to significantly reduce CO₂ emissions from energy production and to reduce global impacts on climate (Falkowski et al., 2000). Injection of CO₂ into deep saline aquifers or depleted oil and gas reservoirs has the potential to sequester significant mass of CO₂ in a sustainable manner (Holt et al., 1995; Bachu and Adams, 2003; Orr, 2009; Szulczewski et al., 2012). One key to the success of long-term CO₂ storage is the integrity of the resulting seal of impermeable caprocks that contain the charge to deep saline aquifers and prevents leakage into the atmosphere or potable aquifers (Shukla et al., 2010). However, the presence of preexisting faults and fractures distributed throughout the upper crust may influence the longevity of this storage (Anderson and Zoback, 1982; Curtis, 2002). Fluid injection activities (e.g., hydraulic fracturing, deep disposal of wastewater, enhanced geothermal stimulation, etc.) can reactivate pre-existing faults and induce seismicity (Healy et al., 1968; Raleigh et al., 1976; Kanamori and Hauksson, 1992; McGarr et al., 2002; Shapiro et al., 2006; Majer et al., 2007; Suckale, 2009; Ellsworth, 2013; Walsh and Zoback, 2015; Guglielmi et al., 2015; Im et al., 2016). Likewise, large-scale injection of CO₂ that generates overpressures and decreases effective normal stresses may reactivate preexisting faults in caprocks (Fig. 1). As a result, CO₂-injection induced seismicity may raise the potential that the rupture of caprocks could jeopardize the seal integrity and ultimately result in loss of charge of CO₂ (Chiaramonte et al., 2008; Rutqvist, 2012; Zoback and Gorelick, 2012). Hence, it is of particular interest to understand the evolution of permeability of caprocks as a result of seismic and aseismic deformation in caprocks.