Analysis on heat and mass transfer law and coal damage during liquid nitrogen injection into coal seam borehole

Abstract

Currently the law of heat and mass transfer and the analysis of coal damage in the process of injecting low-temperature liquid nitrogen (LN2) into the borehole of coal seam are the key problems to be solved urgently for the application of LN2 in the field of coal seam permeability enhancement. By using flowmeter, endoscope, thermocouple, mass balance and ultrasonic detector, the initial accumulation process and liquid level changes of LN2 during its injection into a coal borehole were visualized and analyzed. On this basis, the variation laws of the actual liquid level, net liquid level, and injection efficiency of LN2 in the borehole with time at different injection rates were investigated, and the coal body was ultrasonically detected for damage at three different times. It was found that the initial accumulation of LN2 will form an obvious Leidenfrost effect at the interface of the coal at the bottom of the borehole. With the continuous reduction of the coal temperature at the bottom of the borehole, the radiation heat transfer coefficient decreases, and then the thickness of the vapor film decreases. At different injection rates, the maximum increment of the net level in a 1 m borehole is about 35 cm, which means that when LN2 is injected into a long borehole, it should be injected segmentally, and the increase of injection time of LN2 will lead to the waste of LN2. The ratio of actual liquid level to net liquid level decreases first and then tends to be stable. The ratio of two liquid levels is similar in stable state. The change rule of LN2 injection efficiency with time at different rates is reduction-increase-stability-reduction. The injection efficiency of LN2 is higher at low injection rate. When the injection rate is increased, the nitrogen gas with phase change per unit volume of LN2 will have shorter heat exchange time in the borehole and lower discharge temperature outside the borehole, thus increasing the loss of LN2 and lowering the injection efficiency. Ultrasonic detection results show that as LN2 penetrates the fractures, the cold shock effect can freeze and shrink the fractures in both sides. As a result, the original fractures grow wider and their tips are stretched and deformed by the freezing and shrinking forces on both sides. Consequently, new fractures are formed.

Keywords

• coal seam borehole
• liquid nitrogen injection efficiency
• liquid level change
• heat and mass transfer
• coal damage