Chaoyi Wang

Frictional stability-permeability relationships for fractures in shales: Friction-Permeability Relationships

There is wide concern that fluid injection in the subsurface, such as for the stimulation of shale reservoirs or for geological CO2 sequestration (GCS), has the potential to induce seismicity that may change reservoir permeability due to fault slip. However, the impact of induced seismicity on fracture permeability evolution remains unclear due to the spectrum of modes of fault reactivation (e.g., stable versus unstable). As seismicity is controlled by the frictional response of fractures, we explore friction-stability-permeability relationships through the concurrent measurement of frictional and hydraulic properties of artificial fractures in Green River shale (GRS) and Opalinus shale (OPS). We observe that carbonate-rich GRS shows higher frictional strength but weak neutral frictional stability. The GRS fracture permeability declines during shearing while an increased sliding velocity reduces the rate of permeability decline. By comparison, the phyllosilicate-rich OPS has lower friction and strong stability while the fracture permeability is reduced due to the swelling behavior that dominates over the shearing induced permeability reduction. Hence, we conclude that the friction-stability-permeability relationship of a fracture is largely controlled by mineral composition and that shale mineral compositions with strong frictional stability may be particularly subject to permanent permeability reduction during fluid infiltration.

Influence of weakening minerals on ensemble strength and slip stability of faults

We explore the impact of phyllosilicate (weak but velocity-strengthening) in a majority tectosilicate (strong but velocity-weakening) matrix in bulk shear strength and slip stability of faults. Numerical simple-shear experiments using a Distinct Element Model (DEM) are conducted on both uniform mixtures of quartz and talc analogs and on textured mixtures consisting of a talc layer embedded in a quartz matrix. The mechanical response of particles is represented by a linear-elastic contact model with a slip weakening constitutive relation representing the essence of rate-state friction. The weight percentage of the talc in the uniform mixtures and the relative thickness of the talc layer in the textured mixtures are varied to investigate the transitional behavior of shear strength and slip stability. Specifically, for uniform mixtures, ~50% reduction on bulk shear strength is observed with 25% talc present, and a dominant influence of talc occurs at 50%; for textured mixtures, a noticeable weakening effect is shown at a relative layer thickness of 1-particle, ~50% shear strength reduction is observed with 3-particles, and a dominant influence occurs at 5-particles. In terms of slip stability, a transition from velocity-weakening to velocity-strengthening is observed with 10% to 25% talc present in the uniform mixtures or with 3-particles to 5-particles in the textured mixtures. In addition, further analysis suggest that quartz has a high tendency towards dilation, potentially promoting permeability; while talc dilates with increased slip rate, but compacts rapidly when slip rate is reduced, potentially destroying permeability. The simulation results match well with previous laboratory observations.

Evolution of Friction and Permeability in a Propped Fracture under Shear

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 shearing-concurrent 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.

Mineralogical Controls on Frictional Strength, Stability, and Shear Permeability Evolution of Fractures

Massive fluid injection into the subsurface can inducemicroearthquakes by reactivating preexisting faults or fractures as seismic or aseismic slip. Such seismic or aseismic shear deformations may result in different modes of permeability evolution. Previous experimental studies have explored frictional stability-permeability relationships of carbonate-rich and phyllosilicate-rich samples under shear, suggesting that friction-permeability relationship may be primarily controlled by fracture minerals. We examine this relationship and identify the role of mineralogy (i.e., tectosilicate, carbonate, and phyllosilicate content) using direct-shear experiments on smooth saw-cut fractures of natural rocks and sintered fractures with distinct mineralogical compositions. These results indicate that the friction-permeability relationship is controlled by mineralogy. Frictional strength and permeability change upon reactivation decrease with phyllosilicate content but increase with tectosilicate content. In contrast, the reverse trend is observed for frictional stability (a-b). However, the permeability change decreases with carbonate content while both frictional strength and stability increase. The permeability change always decreases with an increase in frictional stability. This relationship implies a new mechanical-hydrochemical coupling loop via a linkage of frictional properties, mineralogy, and permeability.

Induced Seismicity and Permeability Evolution in Gas Shales, CO 2 Storage and Deep Geothermal Energy

Contemporary methods of energy conversions that reduce carbon intensity include sequestering CO2, fuel switching to lower-carbon sources, such as from gas shales, and recovering deep geothermal energy via EGS. In all of these endeavors, either maintaining the low permeability and integrity of caprocks or in controlling the growth of permeability in initially very-low-permeability shales and geothermal reservoirs represent key desires. At short-timescales of relevance, permeability is driven principally by deformations – in turn resulting from changes in total stresses, fluid pressure or thermal and chemical effects. These deformations may be intrinsically stable or unstable, result in aseismic or seismic deformation, with resulting changes in permeability conditioned by the deformational mode. We report observations, experiments and models to represent the respective roles of mineralogy, texture, scale and overpressures on the evolution of friction, stability and permeability in fractured rocks – and their interrelationships. The physics of these observed behaviors are explored via parametric studies and surface measurement of fractures, showing that both permeability and frictional strength are correlated to the fracture asperity evolution that is controlled in-turn by the sliding velocity and fracture material.