Investigation on the Damage Mechanisms of Unconsolidated Sand, Tight Sand and Shale Induced by Cryogenic Temperature

ABSTRACT: Liquid nitrogen (LN) fracturing is a new approach that utilizes the LN cryogenic temperature to induce complex fracture networks in tight reservoirs. Under the cryogenic treatment of LN, the damage and fracture characteristics of different lithologies such as shale, tight sand, and unconsolidated sand are different, and there are currently few scholars conducting relevant research. This article innovatively designed a visualization experimental method for the continuous propagation of fractures induced by cryogenic LN. Based on digital image correlation (DIC) methods, it explored the differences in the damage mechanisms of unconsolidated sand, tight sand and shale induced by LN cryogenic temperature. The results showed that there were no macro fractures in tight sand and unconsolidated sand, only discrete local deformation occurred, while macro induced fractures appeared in shale. Based on damage mechanics, the numerical simulation method and model of damage induced by LN cryogenic temperature for different lithologies were established by COMSOL to simulate the thermal stress-induced damage characteristics of different lithologies. The results show that the cementation mode of rock skeleton is the main reason for the difference of damage mechanism of different lithologies induced by LN cryogenic temperature. The conclusion is that cryogenic LN can effectively induce macro fractures in shale, while inducing micro damage in unconsolidated sand and tight sand. The research results provide a theoretical basis for the design and feasibility of LN fracturing. 1. INTRODUCTION In recent years, with the continuous progress of horizontal wells and segmented fracturing technology, there has been a wave of exploration and development of shale oil and gas resources, especially the shale oil and gas revolution in the United States, which has greatly increased the production of oil and gas resources in the United States, making it a major exporter of oil. The rise of shale oil and gas is profoundly affecting the global petroleum energy landscape. Shale oil and gas reservoirs have extremely low permeability and generally have no natural production capacity. It is well known that stimulated reservoir volume (SRV) has become the key to shale oil and gas extraction in fracturing and transformation. Therefore, how to form more artificial fractures and increase SRV in shale oil and gas reservoir fracturing and transformation is a key and difficult problem (Zoback and Kohli, 2019).