Laboratory investigation on cryogenically induced fractures in shale with beddings

Abstract

Hydraulic fracturing is essential for effective production of shale oil and gas reservoirs. Liquid nitrogen (LN2) fracturing is promising to enhance oil and gas production. However, the cryogenically fracturing mechanisms of LN2 in shale are still not clear, which are analyzed through combining the laboratory experiments and numerical simulations in this work. Laboratory fracturing shales using the cryogenic LN2 are carried out with the digital image correlation (DIC) recording the fracturing processes. Three typical modes of induced fractures in shale with beddings are observed, including the splitter (mode-S), arrester (mode-A) and divider (mode-D). The mechanisms and detailed fractures of the three modes are analyzed, which strongly depend on the weak plane and thermal conductivity anisotropy. A numerical model is developed to simulate the thermal-mechanical coupling, based on which the applicability of the tensile strength criteria (TSC) and fracture toughness criterion (FTC) are analyzed. It shows that TSC and FTC determine the lower and upper bounds of the induced fracture lengths. TSC is suitable to specify the lengths of mode-A fractures, while FTC is applicable to specify the lengths of mode-S and mode-D fractures. The developed model is used to simulate the branch fracture propagations in a shale reservoir in Triassic Yanchang Formation under the in-situ temperature and stress conditions. The results suggest that mode-D fractures are mostly induced by the cryogenic LN2, which can form a complex fracture network in shale. The findings on fracture lengths and modes can provide references for the shale fracturing treatments with LN2.