Cryogenic fracturing with liquid nitrogen (LN₂) offers the benefits of reducing the water consumption and adverse environmental impacts induced by water-based fracturing, as well as potentially enhancing the fracture complexity. We performed a series of laboratory experiments to explore the key mechanisms governing the breakdown pressures of shale during cryogenic fracturing. In this study, cylindrical shale samples were pre-conditioned by exposing a borehole to low-temperature LN₂ for a certain time period, and then, the samples were fractured using gaseous N₂ under triaxial stress and a high reservoir temperature. The effects of various key parameters on the breakdown pressure were investigated, including the duration of the low-temperature LN₂ treatment, the confining pressure, the reservoir temperature, and the direction of the shale bedding relative to the borehole axis. The results demonstrate that the injection of low-temperature LN₂ as a pre-fracturing fluid into a borehole can significantly reduce the breakdown pressure of the shale during subsequent nitrogen fracturing. This reduction in breakdown pressure can be further intensified by increasing the duration of the LN₂ pre-conditioning. Without LN₂ pre-conditioning, the breakdown pressure initially increases and then decreases with increasing reservoir temperature. When LN₂ pre-conditioning is applied, the breakdown pressure keeps decreasing with increasing reservoir temperature. As the confining pressure increased, the breakdown pressure increased linearly in the tests with and without LN₂ pre-conditioning. The experimental results demonstrate that LN₂ pre-conditioning before N₂ fracturing is a promising waterless fracturing technique that reduces the breakdown pressure and enhances the fracture complexity.

使用液氮 (LN ₂ ) 进行低温压裂的好处是可以减少水基压裂引起的耗水量和不利的环境影响，并可能增加压裂的复杂性。我们进行了一系列实验室实验，以探索控制低温压裂过程中页岩破裂压力的关键机制。在本研究中，圆柱形页岩样品通过将钻孔暴露于低温 LN ₂一段时间进行预处理，然后在三轴应力和高储层温度下使用气态 N ₂压裂样品。研究了各种关键参数对击穿压力的影响，包括低温 LN ₂的持续时间处理、围压、储层温度以及页岩层理相对于钻孔轴线的方向。结果表明，将低温LN ₂作为压裂前流体注入钻孔可以显着降低后续氮压裂过程中页岩的破裂压力。通过增加 LN ₂预处理的持续时间，可以进一步加强这种击穿压力的降低。在没有 LN ₂预处理的情况下，击穿压力最初会随着储层温度的升高而升高然后降低。当 LN ₂应用预处理后，击穿压力随着储层温度的升高而不断降低。随着围压的增加，在有和没有 LN ₂预处理的测试中，击穿压力线性增加。实验结果表明，在N ₂压裂前进行LN ₂预处理是一种很有前景的无水压裂技术，可以降低破裂压力，提高压裂复杂度。