Altering shale permeability by cold shock

Abstract

The matrix permeability of shale is on the order of nanodarcy (nD), and hydrocarbon recovery from it is economical only after hydraulic fracturing. Another fracturing technique, which is often overlooked, is cryogenic fracturing. This study determines the effects of cold shock on shale permeability by focusing on the sub-100-nm pore throats that control access to a significant fraction of the pore space in the matrix. We extracted shale samples, heated them to mimic various formation burial depths, and shocked them in liquid nitrogen. Subsequently, we measured their capillary pressures. This study presents a conceptual model to shed light on the heterogeneous deformations at a sub-100-μm scale. Also, it uses a network modeling approach to determine the permeability enhancement based on the pore-throat size distributions obtained from the capillary pressure measurements. It demonstrates that cold shock enhances permeability by 20%, significantly lower than the typical values reported in the literature where the effects of subsurface temperature have been neglected. The enhancement depends non-monotonically on the temperature difference applied. Moreover, the results indicate that there may be an optimal scenario to maximize the enhancement that can only be determined experimentally. The reason for the incapability of the purely analytical approach is also discussed.