The rock mechanical properties of lacustrine shales: Argillaceous shales versus silty laminae shales

Rock mechanical properties are of crucial importance to shale reservoirs as they directly affect the production efficiency and hydraulic fracturing design. However, limited data exists for rock mechanical properties in lacustrine shales. We thus conducted a series of petrophysical and rock mechanical tests on lacustrine shales to study the difference between argillaceous shales and silty laminae shales. We observed distinctly different rock mechanical properties for these two types shales. Moreover, these two lithologies also showed significantly different Young's moduli and Ultimate strength. Mixed illite/smectite and organic matter content strongly affected the rock mechanical properties of the lacustrine shales and 3D x-ray imaging indicated that the argillaceous shales contained more open bedding fractures, transverse sub-fractures, and a complicated fracture network. In contrast, the silty laminae shales only contained a few open bedding fractures and sub-fractures propagating throughout the samples. It is concluded that the texture and micro-structure in the study shale reservoir determines their fracture complexity. We also conclude that lacustrine shales with high clay content can be extensively fractured and potentially achieve economical production, especially in case of argillaceous shales. The results of this study can therefore help optimize hydraulic fracturing and improve enhanced gas recovery in lacustrine shale reservoirs.