Enhanced gas extraction system and method based on dual-pulsation and acid fracturing permeability enhancement for low-permeability coal seam

An enhanced gas extraction system based on dual-pulsation and acid fracturing permeability enhancement for a low-permeability coal seam, comprising an overpressure protection mechanism (9), a fracturing liquid delivery line (7) with a liquid delivery end extending into a fracturing borehole (2), a gas extraction line (8) with an extraction end extending into an observation borehole (3), a mixing tank (4) containing fracturing liquid, a pressure control and monitoring mechanism (5) for controlling a fracturing pressure and a frequency adjustment and monitoring mechanism (6) for adjusting a pulsation frequency, wherein the pressure control and monitoring mechanism (5) comprises a first branch (55) and a first pressure transmitter (51), a pressure data acquisition unit (52) and a first solenoid valve (53) which are connected in sequence along the first branch (55);

• the frequency adjustment and monitoring mechanism (6) comprises a second branch (64) and a third pressure transmitter (61), a second solenoid valve (62) and a frequency controller (63) which are connected in sequence along the second branch (64);
• when the third pressure transmitter (61) reaches a set pressure, a signal is transmitted, by means of the second solenoid valve (62), to the frequency controller (63) for comparison with a design frequency;
• a head end of the first branch (55) and a head end of the second branch (64) are both connected to the fracturing liquid delivery line (7), and a tail end of the first branch (55) and a tail end of the second branch (64) are both connected to a pulsation fracturing pump (71);
• the fracturing liquid delivery line (7) is connected to the mixing tank (4) and the fracturing borehole (2) and sequentially provided with the pulsation fracturing pump (71), a filter (72), a pressure gauge (73), a flowmeter (74), a throttling valve (75) and a liquid-injection line valve (76) in a delivery direction of the fracturing liquid;
• the gas extraction line (8) comprises a negative-pressure extraction tube (81) connected to an output end of the fracturing liquid delivery line (7) and the observation borehole (3), a fracturing borehole extraction valve (82) is arranged at a joint between the negative-pressure extraction tube (81) and the fracturing liquid delivery line (7), and an observation borehole extraction valve (83) is arranged at a joint between the negative-pressure extraction tube (81) and the observation borehole (3);
• the overpressure protection mechanism (9) comprises a third branch (93) connected to the fracturing liquid delivery line (7) and a second pressure transmitter (91) and a third solenoid valve (92) which are connected in sequence along the third branch (93);
• and when a pressure of the fracturing liquid delivery line (7) is higher than a set pressure, the third solenoid valve (92) is powered to be turned on to release the pressure for protection.