Fracture-permeability behavior of shale

Abstract

Geomechanics 23 CO 2 sequestration 24 2 5 a b s t r a c t 26The fracture-permeability behavior of Utica shale, an important play for shale gas and oil, was investi-27 gated using a triaxial coreflood device and X-ray tomography in combination with finite-discrete element 28 modeling (FDEM). Fractures were generated in both compression and in a direct-shear configuration that 29 allowed permeability to be measured across the faces of cylindrical core. Shale with bedding planes 30 perpendicular to direct-shear loading developed complex fracture networks and peak permeability of 31 30 mD that fell to 5 mD under hydrostatic conditions. Shale with bedding planes parallel to shear loading 32 developed simple fractures with peak permeability as high as 900 mD. In addition to the large anisotropy 33 in fracture permeability, the amount of deformation required to initiate fractures was greater for perpen-34 dicular layering (about 1% versus 0.4%), and in both cases activation of existing fractures are more likely 35 sources of permeability in shale gas plays or damaged caprock in CO 2 sequestration because of the 36 significant deformation required to form new fracture networks. FDEM numerical simulations were able 37 to replicate the main features of the fracturing processes while showing the importance of fluid penetra-38 tion into fractures as well as layering in determining fracture patterns. 39 Ó 2015 Published by Elsevier Ltd. 40 41 42 43 Introduction 44 Fracture permeability in shale 1 is crucial to understanding pro-45 duction of hydrocarbon during hydraulic fracturing operations and 46 the trapping of buoyant fluids in reservoirs, including CO 2 sequestra-47 tion projects. However, several studies suggest that the mechanisms 48 that generate permeability and govern fluid flow through fractured 49 shale are poorly understood (e.g., Dewhurst et al., 1999; Nygård et 50 al., 2006; Dusseault and McLennan, 2011; Vincent, 2012; Gomaa et 51 al., 2014). This has consequences including risks that 52 injection-triggered seismicity may allow stored CO 2 to escape 53 through damaged caprock (Zoback and Gorelick, 2012). There are 54 several lines of evidence that suggest that creating long-lasting per-55 meability in shale is difficult. For example, in hydraulic fracturing 56 the use of proppants is apparently required to maintain the perme-57 ability of the generated fracture system. Shale and other mudstone 58 are well known for their tendency toward plastic deformation or 59 creep while under stress that may close or seal fractures. Studies 60 by Kohli and Zoback (2013) show a clear connection between clay 61 and organic content of shale and the tendency toward creep. 62 Extensive studies of shale fracture behavior in European nuclear 63 waste storage programs have observed self-sealing of fractured shale 64 in tunnels as well as in experimental studies (e.g., Bastiaens et al., 65 2007; Davy et al., 2007; Bock et al., 2010). Finally, faults within 66 clay-rich rocks are known to act both as seals and fluid conduits in 67 pe...

Influence of Normal and Shear Stress on the Hydraulic Transmissivity of Thin Cracks in a Tight Quartz Sandstone, a Granite, and a Shale

“…In contrast to the above results, and the other examples cited above (Faoro et al, 2009;Tanikawa et al, 2010, etc. ), the conventional wisdom is that slip on fault and crack surfaces results in an increase in transmissivity, at least when the crack is sufficiently rough and potentially also at low effective normal stresses so that frictional wear debris is less likely to accumulate (e.g., Carey et al, 2015;Co & Horne, 2014;Gale et al, 1990;Guo et al, 2013;Ishibashi et al, 2012;Lee & Cho, 2002;Yeo et al, 1998). Barton et al (1995), Barton et al (1997), and Townend and Zoback (2000) found from televiewer images of borehole walls that higher-temperature fluids preferentially seeped from cracks oriented such that the ratio (τ/σ n ) was high with respect to the regional principal stress directions.…”

On sorption and swelling of CO2 in clays

“…Fractures are important pathways for fluid migration, both within a reservoir and across a seal unit (Carey et al 2015). Even if such pathways are present, either naturally or induced they can be impermeable due to self-sealing.…”

Shale gas and non-aqueous fracturing fluids: Opportunities and challenges for supercritical CO2

“…The ultimate project goal is to make necessary measurements and develop models that can be used to compare different working fluids. Experiments across the pore, core and reservoir scales enable study of (1) fracture propagation in shale [28,40,41], (2) multi-phase fluid flow in fractures and the bulk rock matrix [42][43][44], and (3) how these mechanisms contribute to shale gas production [45]. This includes microfluidic experiments conducted under reservoir pressures and temperatures with geomaterials in order to characterize sweep efficiency and flow blocking.…”