Mechanisms and technological parameter optimization of near-wellbore laser thermal fracturing for deep coal seams

China boasts abundant deep coalbed methane (CBM) resources, which play a significant role in further CBM production. However, deep coal seams exhibit low porosities and ultra-low permeabilities due to intricate geological conditions. In the drilling process, drilling fluids enter the reservoirs, prone to cause near-wellbore contamination. Although conventional hydraulic fracturing technology tends to create fractures in the direction of the maximum horizontal principal stress, it is challenging for this technology to achieve blockage removing throughout the whole borehole. Laser thermal fracturing technology can break rocks in a short time. Furthermore, it allows for the laser irradiation angle to change freely by regulating mechanical equipment, thus forming radial fractures and reducing near-wellbore contamination. Using the ABAQUS finite element software, this study, establishing a model of laser thermal fracturing of coal seams, explored the fracturing mechanisms and the influence of laser technological parameters. Through analyses of the variation patterns of fracture length and number, this study determined the optimal laser parameters targeting near-wellbore contamination areas. Key findings include: (1) Laser thermal fracturing can cause thermal stress on the surfaces of coal seams through temperature differences, ultimately fracturing coal seams. (2) There was a positive correlation between the fracture number and the laser power and irradiation time. Specifically, the number increased from 10 to 37 as laser power expanded from 400 to 1 000 W. At a laser power of 600 W, the number increases from 24 to 36 as the irradiation time prolonged from 1 to 15 s. In contrast, the fracture number negatively correlated with the laser frequency. With an increase in the laser irradiation distance, the fracture number increased initially and then decreased, peaking at an irradiation distance of 10 cm. (3) The fracture length positively correlated the laser power, irradiation time, and laser frequency but negatively correlated the laser irradiation distance. Among these factors, the laser irradiation time produced the most significant influence on the fracture length, which soared from 1.52 to 57.6 mm as the irradiation time increased from 1 to 5 s. For instance, for samples collected from Hancheng, Shaanxi Province through deep coring, the optimal laser power and irradiation time of laser thermal fracturing for the near-wellbore contamination area of deep coal seams extending within 2 m were 20 kW and 2 280 s, respectively. Compared to hydraulic fracturing, laser thermal fracturing can form more complex but shorter fractures. In practical application, the approach combining hydraulic fracturing with laser thermal fracturing is recommended for blockage removing and permeability enhancement.