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

1 . 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.

2 . The enhanced gas extraction system based on dual-pulsation and acid fracturing permeability enhancement for a low-permeability coal seam according to claim 1 , wherein high-pressure bushings are arranged on sections, connected to the fracturing liquid delivery line ( 7 ), of the first branch ( 55 ), the second branch ( 64 ) and the third branch ( 93 ), and a high-pressure hose is arranged at a joint between the gas extraction line ( 8 ) and the fracturing liquid delivery line ( 7 ).

3 . The enhanced gas extraction system based on dual-pulsation and acid fracturing permeability enhancement for a low-permeability coal seam according to claim 1 , wherein openings of the fracturing borehole ( 2 ) and the observation borehole ( 3 ) are sealed with high-pressure capsules, and a high-pressure sealing joint ( 21 ), corresponding to the liquid delivery end of the fracturing liquid delivery line ( 7 ), is arranged at the opening of the fracturing borehole ( 2 ).

4 . An enhanced gas extraction method based on dual-pulsation and acid fracturing permeability enhancement for a low-permeability coal seam, comprising:

S1, carrying out detailed research on a target coal seam ( 1 ) to complete sampling, then performing proximate analysis, elemental analysis and mineral content determination on a coal sample, and obtaining, according to mineral-acid solution reaction kinetics, fracturing liquid most suitable for the target coal seam ( 1 );

S2, drilling a fracturing borehole ( 2 ) and an observation borehole ( 3 ) spaced apart from and parallel to the fracturing borehole ( 2 ) in the target coal seam ( 1 ), and then mounting the enhanced gas extraction system based on dual-pulsation and acid fracturing permeability enhancement for a low-permeability coal seam according to claim 1 ;

S3, starting the pulsation fracturing pump ( 71 ) to fill the fracturing borehole ( 2 ) with prepared fracturing liquid by means of the fracturing liquid delivery line ( 7 ), controlling a fracturing pressure of the fracturing liquid to 4 Mpa-6 Mpa by means of the pressure control and monitoring mechanism ( 5 ) and adjusting a pulsation frequency of the fracturing liquid to 30 Hz-40 Hz by means of the frequency adjustment and monitoring mechanism ( 6 ), performing continuous and periodic fracturing on the fracturing borehole ( 2 ), and when water flows out from the observation borehole ( 3 ), stopping delivering the fracturing liquid into the fracturing borehole ( 2 );

S4, starting again the pulsation fracturing pump ( 71 ) to fill the fracturing borehole ( 2 ) with the fracturing liquid, controlling the fracturing pressure of the fracturing liquid to 2 Mpa-3 Mpa by means of the pressure control and monitoring mechanism ( 5 ) and adjusting the pulsation frequency of the fracturing liquid to 15 Hz-20 Hz by means of the frequency adjustment and monitoring mechanism ( 6 ), performing continuous and periodical fracturing on the fracturing borehole ( 2 ), and when water flows out from the observation borehole ( 3 ), stopping delivering the fracturing liquid into the fracturing borehole ( 2 ); and

S5, repeating S3-S4 five times, turning off the liquid-injection line valve ( 76 ), turning on the fracturing borehole extraction valve ( 82 ) and the observation borehole extraction valve ( 83 ), and periodically sampling and detecting gas concentrations in the fracturing borehole ( 2 ) and the observation borehole ( 3 ) at intervals to generate data.

5 . The enhanced gas extraction method based on dual-pulsation and acid fracturing permeability enhancement for a low-permeability coal seam according to claim 4 , wherein in S1, an acid solution formula range corresponding to contents of main minerals in the coal sample under different conditions is solved by means of an interface reaction model to obtain the fracturing liquid most suitable for the target coal seam, wherein the acid solution formula range is solved according to the following reaction rate expression:

J

=

K

⁢

C

s

m

where, J is a reaction rate, mol/(cm 2 ·s); C s is an acid concentration of a rock surface at a reaction time t, mol/L; m is the number of reactions; K is a reaction rate constant, mol/[cm 2 ·s·(mol/mL) m ];

assume common logarithms are adopted at both sides of the formula, an acid-rock reaction kinetics calculation formula is determined:

l

⁢

g

⁢

J

=

l

⁢

g

⁢

K

+

m

⁢

l

⁢

g

⁢

C

lgJ and lgC are plotted, the reaction rate constant K and m are obtained by means of intercepts of straight lines, and then a reaction kinetics equation is obtained; according to the reaction kinetics equation, types and contents of fillings in the coal seam and properties of an acid solution, the acid solution formula range corresponding to the contents of the main different minerals in the coal sample is obtained;

a dissolving effect of acid fracturing liquid on the minerals in coal is based on a solid-liquid reaction, and acid solution formulas corresponding to the contents of the minerals in the coal sample under different conditions are solved by means of the following solid-liquid reaction kinetics model formula:

C

=

m

⁢

t

n

+

t

where, C is a dissolution proportion, %; t is a reaction time, s; m is maximum dissolution of the acid solution on the minerals and reflects a dissolving capacity of the acid solution to the minerals, and n is a reaction time when half of the minerals are dissolved by the acid solution and reflects a dissolution rate of the acid solution to the minerals;

the contents of the minerals in the coal sample are quantitatively measured, and according to the acid solution formula range corresponding to the contents of the main different minerals in the coal sample, the optimal fracturing liquid is selected by comparing values of m and n in the kinetics equation for dissolving the minerals by the acid solution according to a selection criterion that the value of m is as great as possible and the value of n is as small as possible.

6 . The enhanced gas extraction method based on dual-pulsation and acid fracturing permeability enhancement for a low-permeability coal seam according to claim 4 , wherein in S2 and S3, when water flows out from the observation borehole ( 3 ), the fracturing liquid is stopped from being delivered into the fracturing borehole ( 2 ) for 30-45 min.

7 . The enhanced gas extraction method based on dual-pulsation and acid fracturing permeability enhancement for a low-permeability coal seam according to claim 4 , wherein the fracturing pressure and pulsation frequency in S3 are twice the fracturing pressure and pulsation frequency in S4.