ORIGINAL PAPER A Framework for Fracture Network Formation in Overpressurised Impermeable Shale: Deformability Versus Diagenesis Sotiris Alevizos 1 • Thomas Poulet 1,2 • Mustafa Sari 1 • Martin Lesueur 1,2 • Klaus Regenauer-Lieb 1 • Manolis Veveakis 1,2 Received: 5 November 2015 / Accepted: 25 April 2016 Ó Springer-Verlag Wien 2016 Abstract Understanding the formation, geometry and fluid connectivity of nominally impermeable unconven- tional shale gas and oil reservoirs is crucial for safe unlocking of these vast energy resources. We present a recent discovery of volumetric instabilities of ductile materials that may explain why impermeable formations become permeable. Here, we present the fundamental mechanisms, the critical parameters and the applicability of the novel theory to unconventional reservoirs. We show that for a reservoir under compaction, there exist certain ambient and permeability conditions at which diagenetic (fluid-release) reactions may provoke channelling locali- sation instabilities. These channels are periodically inter- spersed in the matrix and represent areas where the excess fluid from the reaction is segregated at high velocity. We find that channelling instabilities are favoured from pore collapse features for extremely low-permeability forma- tions and fluid-release diagenetic reactions, therefore pro- viding a natural, periodic network of efficient fluid pathways in an otherwise impermeable matrix (i.e. frac- tures). Such an outcome is of extreme importance the for exploration and extraction phases of unconventional reservoirs. Keywords Shale gas  Diagenetic reactions  Reservoir compaction  Channelling localisation instability List of symbols b Arrhenius number for micromechanical processes ‘ H McKenzie’s compaction length (m) k Hydro-mechanical coefficient g Chemo-mechanical coefficient a Thermal conductivity (W m 1 K 1 ) Dh Enthalpy of the reaction (J mol 1 )  ij Strain tensor _  ref Reference strain rate (s 1 ) _  vp d Deviatoric visco-plastic strain rate (s 1 ) _  vp v Volumetric visco-plastic strain rate (s 1 ) k s Thermal expansion coefficient of solid (K 1 ) l f Fluid viscosity (Pa s) q,[q s , q f ] Density [solid, fluid] (kg m 3 ) r ij ,[r 0 ij ] Stress tensor [effective stress] (Pa) / Porosity C e ijkl Elasticity tensor (Pa) C Specific heat capacity (m 2 K 1 s 2 ) E Activation energy (J mol 1 ) K c Ratio of forward over reverse pre- exponential constants M,[M i ] Molar mass [of species i] (kg mol 1 ) Q,[Q F , Q R ] Activation enthalpy [forward, reverse] (J mol 1 ) R Universal gas constant (J K 1 mol 1 ) T Temperature (K) k,[k þ , k  ] Pre-exponential factor [forward, reverse] (s 1 ) k p Permeability (m 2 ) p,[p c ] Volumetric mean stress [value at yield] (Pa) & Manolis Veveakis e.veveakis@unsw.edu.au 1 School of Petroleum Engineering, UNSW Australia, Sydney, Australia 2 CSIRO Mineral Resources, North Ryde, NSW, Australia 123 Rock Mech Rock Eng DOI 10.1007/s00603-016-0996-y