Laboratory investigation on cryogenically induced fractures in shale with beddings

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 crite-rion (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 propa-gations 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.

Automated summary

Hydraulic fracturing is crucial for shale oil and gas production, and using liquid nitrogen (LN2) as a fracturing agent shows promise. This study combines laboratory experiments and numerical simulations to analyze the cryogenic fracturing mechanisms of LN2 in shale. The experiments use digital image correlation to record the fracturing processes and observe three typical modes of induced fractures in shale with beddings. A numerical model is developed to simulate the thermal-mechanical coupling and analyze the applicability of different fracture criteria. The findings provide references for shale fracturing treatments with LN2.