Evolution of Friction and Permeability in a Propped Fracture under Shear

Abstract

We explore the evolution of friction and permeability of a propped fracture under shear. We examine the effects of normal stress, proppant thickness, proppant size, and fracture wall texture on the frictional and transport response of proppant packs confined between planar fracture surfaces. The proppant-absent and proppant-filled fractures show different frictional strength. For fractures with proppants, the frictional response is mainly controlled by the normal stress and proppant thickness. The depth of shearingconcurrent striations on fracture surfaces suggests that the magnitude of proppant embedment is controlled by the applied normal stress. Under high normal stress, the reduced friction implies that shear slip is more likely to occur on propped fractures in deeper reservoirs. The increase in the number of proppant layers, from monolayer to triple layers, significantly increases the friction of the propped fracture due to the interlocking of the particles and jamming. Permeability of the propped fracture is mainly controlled by the magnitude of the normal stress, the proppant thickness, and the proppant grain size. Permeability of the propped fracture decreases during shearing due to proppant particle crushing and related clogging. Proppants are prone to crushing if the shear loading evolves concurrently with the normal loading.

Friction of Longmaxi Shale Gouges and Implications for Seismicity During Hydraulic Fracturing

“…The best quality shale reservoirs are typically located in both TOC‐rich and brittle mineral (quartz)‐rich shales (Loucks & Ruppel, 2007; Xu et al, 2019). High TOC content is a prerequisite for high intrinsic gas content and high brittleness favors the production of long and pervasive fractures during hydraulic fracturing that potentially enhance reservoir permeability (Zhang et al, 2017). However, an increase in the proportion of brittle minerals may have an adverse effect on the stability of preexisting faults and promote seismic response during fault reactivation (Figure 15).…”

Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

“…Considering geothermal reservoirs with high thermal gradients (40 ℃/km or more) as representative, the estimated temperature at 4-5 km depth can reach up to 160-200 ℃ (Beckers et al, 2014;Olasolo et al, 2016) and the frictional properties of epidote gouge could promote unstable sliding at both hydrostatic and elevated pore uid pressures. Conversely, for faults containing epidote/chlorite mixed gouges, even a low chlorite content can reduce fault frictional strength and promote failure (Zhang et al, 2017) and high proportions of epidote will maintain velocity weakening response and promote potentially unstable fault slip. Our results therefore imply that fault instability could be facilitated by the presence of epidote and such metamorphic transformations, allowing gouge composition to exert a subtle but signi cant control on fault stability.…”

Permeability Evolution of an Intact Marble Core during Shearing under High Fluid Pressure

“…While these empirical studies seek to quantify the permeability and frictional sliding characteristics of fracture surfaces with different roughness during the shearing process, the fracture surface stiffnesses and elastic moduli of these materials are much lower than those of a natural hard rock sample. Other ways to create rough fracture surfaces include splitting rocks such as granite, sandstone, marble, and shale [19][20][21] or preroughened surfaces using other methods to obtain rough fracture surfaces [22,23]. Then, through shear flow experiments, it is possible to explore the effects of parameters such as normal stress, shear displacement, and surface roughness on the evolution of fracture permeability during the shear process.…”

“…If the initial roughness of the fracture surface is high, then the fracture permeability may be enhanced during shear slip [26,27]. Conversely, when the initial fracture surface is relatively smooth, the fracture permeability decreases with increased shearing [5,22,28]. In addition, Fang et al [6] pointed out that the permeability evolves in a fluctuating pattern for significantly rough fractures.…”