Linking microearthquakes to fracture permeability change: The role of surface roughness

Abstract

Despite its importance, the relation between microearthquakes (MEQs) and changes in hydraulic properties during hydraulic stimulation of a fractured reservoir has rarely been explored, and it is still not well understood. To investigate this relation, we first formulate a plausible scale dependence, where fracture length and shear displacement are variables, for channeling flow through heterogeneous aperture distributions for joints and faults. By combining this formulation with the concept of the seismic moment, we derive quantitative relations between the moment magnitude (Mw) of MEQs and the fracture permeability change in the directions orthogonal to (kfault,⊥/kjoint) and parallel to (kfault,∥/kjoint) the shear displacement, in the form kfault,⊥true/knormaljnormalonormalinormalnnormalt=116.4×100.46Mw and kfault,true/true/true/knormaljnormaloint=13.1×100.46Mw. Despite the simplicity of the derivation, these relations have the potential to explain the results of field experiments on hydraulic stimulation, such as the enhanced geothermal systems at Soultz‐sous‐Fôret and Basel.

Transmissivity Changes and Microseismicity Induced by Small‐Scale Hydraulic Fracturing Tests in Crystalline Rock

“…A systematic correlation between transmissivity enhancement and event numbers, frequency-magnitude distribution, or maximum magnitudes is not visible, unlike what was suggested in the theoretical considerations made by Ishibashi et al (2016) and Gischig et al (2014). However, considering borehole SBH4 only, the largest transmissivity change occurs for the HTPF, where many of the largest seismic events have also occurred.…”

“…Both HF and HS are associated with microseismicity. In the case of HS, seismicity is thought to be closely related to permeability enhancement mechanisms, as pressure‐induced fracture slip is the predominant source of both phenomena (Evans et al, ; Häring et al, ; Ishibashi et al, ). For HF, the source of seismicity is not as evident and is being discussed in literature.…”

Friction‐Stability‐Permeability Evolution of a Fracture in Granite

“…Reductions in the RMS and the vertical interval result from breakage and crushing of the fracture surfaces during shear (Wang & Scholz, ). Considering that the fracture asperities play a significant role in maintaining the void space within fractures (Ishibashi, Watanabe, et al, ; Jaeger et al, ), the destruction of these asperities will likely reduce the fracture permeability. Some of the wear products will likely further reduce permeability by flow occlusion (Faoro et al, ; Giwelli et al, ), though the production of wear products is not quantified.…”

“…This roughness is manifest from asperities that may self‐prop the fracture during shearing and result in connected pathways of new or enhanced porosity. Similar mechanisms have been explored via numerical modeling of fracture flow (Ishibashi, Watanabe, et al, ; Matsuki et al, ). Counter observations note decreases in fracture permeability/transmissibility with shearing when the initial geometries of the fracture surfaces are relatively smooth (Faoro et al, ; Giwelli et al, ).…”

Frictional stability‐permeability relationships for fractures in shales

“…The permeability of faults is known to change during shear deformation due to the rearrangement and destruction of bridging asperities [ Elsworth and Goodman , ]. When shear deformation occurs on a fracture, permeability may increase due to significant dilation [ Barton et al ., ; Ishibashi et al ., ] or decrease as a result of progressive formation of gouge [ Faoro et al ., ]. During fracture shearing, the frictional strength of the fracture is affected by the state of true area of solid‐solid contact between the displacing surfaces [ Dieterich , ].…”