Dilation of rock describes the increase in joint aperture during shear; which opens up additional space in the joint and can change its transmissivity, which in turn changes the overall permeability of the rock mass. Source mechanisms computed from recorded microseismic events during hydraulic fracturing show a significant amount of shear failure during treatment. Therefore, the ability to properly account for shear-induced dilation is of considerable importance in assessing the transmissivity of fracture networks and hydrocarbon deliverability of stimulated reservoirs. In this study, 22 dilation responses from direct shear tests under constant normal loads were analyzed for their effect on fracture transmissivity. The cubic law amplifies the dilation effect on transmissivity, however, onset of this amplification is delayed compared to the onset of dilation, and the magnitude of the delayed response is related to surface roughness.
During hydraulic fracturing operations, where fluids and proppants are injected at depth to increase the permeability of a reservoir, focal mechanisms computed from recorded microseismic events show a significant number of shear slippage during treatment (Warpinski & Du, 2010). The shear slippage can be the result of destabilization of hydraulically connected pre-existing joints or stress changes caused by the injected fluid pressure. In the case of hydraulically connected joints, the introduction of fluid increases the pore pressure and counteracts the compressive stress that is keeping the joints in place. This results in the slippage of favorably oriented joints with respect to the principal stresses, often referred to as hydroshearing (Rinaldi et al., 2015). In the case of stress changes, high pressure created by the injection fluid ahead of the fluid front alters the local stress and causes shear slippage in fluid free joints (Figure 1).
Joint surfaces are often rough meaning the surfaces have waviness and unevenness. Therefore, when sheared, the roughness of the surfaces generates a lever motion that prop the contacting fracture surfaces away from each other. This process is called dilation (Figure 2). This propping motion creates additional conduits in which fluids can flow. According to the cubic law, doubling the joint aperture increases the transmissivity by a factor of eight, and considering that shear displacement is permanent in-situ and therefore the additional aperture stays open, shear slippage during hydraulic fracturing can further enhance the permeability of unconventional reservoirs and in turn its hydrocarbon deliverability.
