Investigation on coal damage and fracture extension law of liquid nitrogen injection pre-cooling and fracturing under true triaxial stress

Abstract

To more accurately describe the coal damage and fracture evolution law during liquid nitrogen (LN2) fracturing under true triaxial stress, a thermal–hydraulic-mechanical-damage (THMD) coupling model for LN2 fracturing coal was developed, considering the coal heterogeneity and thermophysical parameters of nitrogen. The accuracy and applicability of model were verified by comparing with LN2 injection pre-cooling and fracturing experimental data. The effects of different pre-cooling times and horizontal stress ratios on coal damage evolution, permeability, temperature distribution, and fracture characteristics were analyzed. The results show that the permeability and damage of the coal increase exponentially, while the temperature decreases exponentially during the fracturing process. As the pre-cooling time increases, the damage range of the coal expands, and the fracture propagation becomes more pronounced. The initiation pressure and rupture pressure decrease and tend to stabilize with longer pre-cooling times. As the horizontal stress ratio increases, fractures preferentially extend along the direction of maximum horizontal principal stress, leading to a significant decrease in both initiation and rupture pressures. At a horizontal stress ratio of 3, the initiation pressure drops by 48.07%, and the rupture pressure decreases by 41.36%. The results provide a theoretical basis for optimizing LN2 fracturing techniques and improving coal seam modification.

1. Introduction

Coalbed methane (CBM) is natural gas contained in coal seams and is a clean and efficient energy resource [1,2]. The efficient development of gas resources is critical for maintaining global energy security [3]. Most deep coal seams typically exhibit poor permeability, strong gas adsorption capacity, high resistance to desorption and seepage, making conventional borehole unsatisfactory for extraction and production of CBM [4,5]. The application of conventional hydraulic fracturing has significantly increased CBM production [6,7]. However, water intrusion into coal seams can cause mineral swelling, thereby further reducing CBM flow capacity [8,9]. To mitigate the drawbacks of conventional hydraulic fracturing, several water-free fracturing technologies have been developed, including gas fracturing, LCO2 (Liquid carbon dioxide) fracturing, foam fracturing, and cryogenic liquefied gas fracturing [10].

Liquid nitrogen (LN2) has gained significant attention as an innovative water-free fracturing fluid because of its capacity to enhance fracture complexity and lower rupture pressure [11]. McDaniel et al. [12] were the first to successfully apply LN2 in shale fracturing, thereby demonstrating its feasibility. Grundmann et al. [13] conducted a field tests of LN2 fracturing of the Devonian shale in Kentucky, USA. Sequentially, extensive experimental studies have demonstrated the potential benefits of LN2 fracturing [14]. At present, most researches have focused on experimentally revealing the effects of LN2 fracturing on coal properties. During the LN2 fracturing process, microfractures in coal extend and propagate under the combined effects of thermal stress, pore pressure, and the expansion force of LN2 vaporization [15]. The coal mechanical properties decrease accordingly, impacting parameters such as mechanical strength, porosity, permeability, and thermal conductivity [16].