Injection‐Induced Shear Slip and Permeability Enhancement in Granite Fractures

Abstract

Shear stimulation, so‐called hydroshearing, is believed to reactivate critical or near‐critical preexisting fractures by pressurized fluid injection, causing them to slip and dilate for economic production from geothermal and unconventional petroleum reservoirs. However, little or no experiments have been conducted to directly probe the coupled hydromechanical responses and permeability evolution during fracture shearing. In this paper, we present the results of novel injection‐induced shear tests on cylindrical granite samples each containing a single tensile or saw cut fracture. The shear test on rough fractures demonstrates that retainable permeability enhancement can be achieved through dilatant shear slip. The characteristic stick‐slip behavior of the fracture during shearing is also observed. Fracture aperture controlled by effective normal stress dominates the fluid flow in stick state, while in slip state, the major contributor to production increase is the irreversible normal dilation caused by fracture shear slip. The experimental data show that depending on the fracture roughness, the fracture slip process includes two quasi‐static slip intervals with a slip velocity of ~10−9 to ~10−6 m/s and a dynamic slip interval with a slip velocity of ~10−7 to ~10−5 m/s. The fracture slip correlates well with the associated stress relaxation: a faster fracture slip induces a quicker stress relaxation, and vice versa. The influence of surface roughness on the hydromechanical responses during fracture shearing and the shear‐induced asperity degradation are also studied. The various experimental observations on rough and smooth fractures clearly indicate that the key component to shear stimulation is fracture self‐propping by asperities.

Permeability Evolution and Frictional Stability of Fabricated Fractures With Specified Roughness

“…Comparing with previous fracture shearing and fluid flow experiments on smooth natural fractures, the net permeability evolution of smooth surfaces (sample E in Figure 9) shows a similar monotonically declining trend with displacement Im et al, 2018;Wu et al, 2017). The permeability decrease is due to the production of wear products, although dilation is temporarily triggered by the instantaneous increase in shear velocity.…”

Changes of Slip Rate and Slip‐Plane Orientation by Fault Geometrical Complexities During Fluid Injection

“…Furthermore, the direct comparisons of numerical results with measurements of carefully designed experiments (Lei et al, ; Lockner & Byerlee, ) are required. The fluid‐driven shear tests on rough fractures in a lab scale by Ye and Ghassemi () gave results with many features similar to those presented here. For examples, the aseismic slip accompanied by fault dilation, the correlation of stress drop, and fast slip and the stick‐slip behavior of the fault due to the self‐propping of asperities that are similar to the jogs at a finer scale were found in the experiments.…”

“…The segment interaction under shear loading will generate high stress, leading to damage zones, where the parallel segments join. The permeability of these damage zones or cracks, which form dilational segment as discussed by Aki et al (1977), Sibson (1986), and Yamashita (1997), is much larger than that of the original permeability of the connected segments. If no offsets exist between parallel fractures, the high-permeability zone acts as a coplanar fracture array, and otherwise, it is described as a zone of dilational jogs in many studies Rutledge et al, 2004;Sibson, 1986).…”

“…The third sort of GCs consists of branches and nonplanar splay fractures that deviated from the fault plane (Cooke, 1997;Segall & Pollard, 1983;Petit et al, 1999;Zhang & Jeffrey, 2016). The branching fractures also exist with a wide range of sizes and permeability (Yamashita, 1997). These GCs can slip and dilate, and be pressurized by fluid (Phillips, 1972;Petit et al, 1999;Sibson, 2000).…”

“…The linear weakening form of the coefficient of friction with slip, as presented by Ida (1972), can provide a first-order approximation for the slip response (Uenishi & Rice, 2003). Slip weakening is implemented as a displacement-dependent Coulomb friction law that has been found to be useful and numerically efficient (Marone et al, 1992;Yamashita, 1997). Based on the previous studies on injectioninduced dynamic slip nucleation (Azad et al, 2017;Ciardo & Lecampion, 2019;Garagash & Germanovich, 2012;Viesca & Rice, 2012;Zhang, Damjanac, & Maxwell, 2019), the linear slip weakening friction law allows initially stable slip to develop into unstable slip and produces a variety of responses for evolution of the slip and slip zone on a permeable or impermeable fault.…”