The role of the interbed thickness on the step-over fracture under overburden pressure

Multilayer jointing in tight oil and gas reservoirs is a useful indicator for the prediction of fluid mobility. Since pore pressure balances the principal minor horizontal stress at depth, joint nucleations, considered as sub-vertical, are mainly driven by the overburden pressure. We demonstrate, with a mixed criterion combining necessary conditions of stress and energy, how heterogeneity such as an interbed promotes initiation of a fracture by a step-over mechanism, referring to critical characteristics such as the interbed thickness. A specific initiation criterion emerges, giving rise to a size effect due to the interbed thickness, and is compared to a typical homogeneous rock failure criterion. Jointing over an interbed is enhanced by large interbed thicknesses, even for moderate effective confining stresses. Size effect is reversed according to the relative importance of singular effects (effective minor stress normal to the fracture plane) and structural effect (parallel loading effective conditions).