Experimental investigation of shale breakdown pressure under liquid nitrogen pre-conditioning before nitrogen fracturing

Abstract

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.