Origin of Strong Nonlinear Dependence of Fracture (Joint) Spacing on Bed Thickness in Layered Rocks: Mechanical Analysis and Modeling. Issue 3 (12th March 2021)

• Record Type: Journal Article
• Authors: Chemenda, A. I.; Lamarche, J.; Matonti, C.; Bazalgette, L.; Richard, P.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 3(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-03-12
• Subjects: dilation bands -- elastoplastic modeling -- fracture spacing -- fracturing -- joints -- sedimentary basins; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2020JB020656
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Fracturing (jointing) affects large volumes of rocks and strongly impacts their mechanical stability and permeability. In layered sedimentary basins, the fractures typically form subparallel sets with different fracture spacing S . This parameter has been extensively studied both in the field and in mechanical models in relation to the thickness T of the fractured layers. A linear relation was suggested by most studies. A few geological investigations involving wide ranges of T values have shown that the T ( S ) relation is generally strongly nonlinear and that in thick beds ( T >∼1.5 m), S practically does not depend on T . The origin of this relation remained hitherto unexplained. Here we obtained such a relation in 2‐D finite‐difference models of 3‐layer sedimentary sequences where layers interact through the interfaces of strength R . The middle layer has elastoplastic properties with coupled tensile and shear failure mechanisms. The number of fractures forming in this layer increases under horizontal extension to a saturation value S depending on R . At T < ∼0.5 m, the increase of S with T is near‐linear. At ∼1 m < T < 2 m, it is strongly nonlinear; and S changes little at larger T where S is more than certain value S cr . The increase of ratio D = T / S with T is, on the contrary, close to linear. This behavior is explained by the fact that for D >∼2, the stress state in the upper and lower horizons of the middle layer between fractures spaced 2 S cr apart is practically independent of T . Key Points: Fractures are generated in a competent elastoplastic layer sandwiched between two incompetent elastic layers in FD numerical models As in nature, fracture spacing S increases nonlinearly with the thickness T of the fractured layer, and at T >∼1 m, S changes very little It is because the stress is practically independent of T in the upper and lower horizons of the thick layer where fractures are initiated