2:45 PM - 3:00 PM
[SCG57-05] Dynamics of rock fracture permeability explored through MEQs
Keywords:rock fracture, permeability, surface topography, microearthquake
In the present study, we explore a linkage between fracture permeability change and MEQs. For this purpose, we first prepared heterogeneous aperture distributions for rock fractures with various combination of fracture length (m), l, and shear displacement (m), d, according to the method of Ishibashi et al . Through the analyses of these aperture distributions, scale dependencies of fluid flows through joints, i.e. fractures without shear displacement, and faults, i.e. fractures with shear displacement of d (m), are predicted as followings. Both joint and fault aperture distributions are characterized by a scale-dependent geometric mean and a scale-independent geometric standard deviation of aperture. Changes in the geometric means of joint and fault apertures (mm), em, joint and em, fault, with fracture length (m), l, are approximated by em, joint = 1.3×10-1l0.10 and em, fault = 1.3×10(d /l) 0.59l0.71, whereas the geometric standard deviations of both joint and fault apertures are approximately 3. Fluid flows through both joints and faults are characterized by formations of preferential flow paths (i.e., channeling flows) with scale-independent flow areas of approximately 10%, whereas the joint and fault permeabilities (m2), kjoint and kfault, are scale dependent and are approximated as kjoint = 9.8×10-13l0.16 and kfault = 2.3×10-6(d /l) 1.18l1.08. By coupling these scaling laws with the concept of moment magnitude [Hanks and Kanamori, 1979], quantitative change in mean aperture (em, fault/em, joint) and fracture permeability (kfault/kjoint) are successfully linked with moment magnitude of MEQs (Mw) during hydraulic stimulation for a reservoir as em, fault/em, joint = 1.0×100.35Mw and kfault/kjoint = 116.4×100.46Mw. Validity of the equation will be discussed through comparisons with some data of real field development/experiments (e.g., EGS system in Basel and Soultz-sous-Fôret).
In summary, such linkages may enable rough inverse-mapping of evolving fracture permeabilities using in-situ MEQ data. This mapping will facilitate new insights into transport phenomenon within the Earth’s crust and it relevant to engineering and scientific applications such as the development of geothermal or hydrocarbon reservoirs and clarification of earthquake mechanisms.