Experiment on coal breaking with cryogenic nitrogen jet

Severe leakage of drilling fluid and reservoir damage due to water and solid invasion during well drilling are the key factors that restrict economic exploitation of coal bed methane (CBM) in China. To solve these issues, this paper presents a method of CBM well drilling with the assistance of cryogenic nitrogen jet, in which no aqueous phase and solid phase are contained. To determine the feasibility of coal breaking by cryogenic nitrogen jet and the effects of different parameters, such as nozzle diameter, dimensionless standoff distance, nozzle pressure difference and exposure time, we conducted corresponding rock breaking experiments on artificial and natural coals with the cryogenic nitrogen jet and the water jet. Rock breaking features and the energy consumption by the two jets were compared. The results indicate that the cryogenic nitrogen jet has better rock breaking performances and consumes less energy. Under the same conditions, the average rock erosion volume and the energy consumption with the cryogenic nitrogen jet were 18.34 times larger and 90.8% lower than those with water jet, respectively. Visible netted fractures could be created on the surface of coal samples blasted with the cryogenic nitrogen jet, and thus rock breaking could be facilitated effectively. The outstanding rock breaking performances of cryogenic nitrogen jet are attributed to three mechanisms: high-velocity jet impact, thermal shock and gasification cracking effect. In addition, under our experimental conditions, the erosion volume and depth were proportional to the pressure difference and inversely proportional to the dimensionless standoff distance. When the nozzle diameter changed from 1.5 mm to 3 mm, the increases of erosion volume were 3% at the pressure difference of 5 MPa and 41.7% at the pressure difference of 10 MPa respectively. This indicates that the effect of nozzle diameter is more significant at relatively higher pressure drops. With the increase of exposure time, both of the rock erosion depth and volume rose, while with a smoother trend. The results of this study are expected to promote the application of cryogenic nitrogen jet in CBM well drilling.