ENHANCED EXTRACTION SYSTEM AND METHOD BASED ON ULTRASONIC EXCITATION AND ACID FRACTURING INTEGRATED PERMEABILITY ENHANCEMENT FOR COAL SEAM

Description

BACKGROUND OF THE INVENTION

1. Technical Field

The invention belongs to the technical field of permeability enhancement-based extraction of coal seams, and particularly relates to an enhanced extraction system and method based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam.

2. Description of Related Art

With the continuous extension of the coal mining depth, the characteristics of high crustal stress, high gas adsorptivity and low permeability of coal seams become more obvious. In addition, under the influence of the geologic structure and geologic metamorphism, some coal in the coal seams is soft, and pores and fractures are often filled with minerals, hindering gas diffusion and seepage and leading to a low permeability of the coal seams.

At present, the hydrofracturing technique is applied and promoted to coal seams. However, due to the continuous extension of the coal mining depth, the maximum horizontal principal stress effect becomes more remarkable, most fractures generated by hydrofracturing extend parallel to the maximum horizontal principal stress, and uniform pressure relief of the fractures in coal cannot be guaranteed, leading to the formation of more blank zones; and the increase in the mining depth results in faster closing of the fractures generated by fracturing, so an expected extraction effect cannot be fulfilled. Due to the deficiency of full-life-cycle assessment of the application range and the environmental effect and the immaturity of matching techniques in the fracturing process, the gas fracturing technique for coal seams is still in the test and research phase. Although ultrasonic excitation has been used at present to improve to some extent the original pore and fracture structure of coal seams, it cannot solve the problem of mineral filling in pores and fractures in coal seams, leading to low gas extraction efficiency.

BRIEF SUMMARY OF THE INVENTION

The invention aims to provide an enhanced extraction system and method based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam, which integrate ultrasonic excitation and acid fracturing and make full use of their advantages to improve gas extraction efficiency and solve the problems of low permeability and low gas extraction efficiency of a coal seam caused by the hindrance of some minerals filled in pores and fractures to gas diffusion and seepage in the prior art.

For this purpose, the technical solution adopted by the invention is as follows: an enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam includes an ultrasonic excitation and acid fracturing integrated drilling device located in an extraction borehole and an ultrasonic control box and a mixing tank which are located outside the extraction borehole, wherein a liquid inlet end of the ultrasonic excitation and acid fracturing integrated drilling device is connected to the mixing tank by means of a fracturing pump; the ultrasonic excitation and acid fracturing integrated drilling device includes a liquid feed rod, an ultrasonic transducer, a fracturing cavity and a conical drill bit which are sequentially arranged from left to right and a plurality of openable tool assemblies located on left and right sides of the fracturing cavity in mirror symmetry; each openable tool assembly includes a slot, a spool vertical moving rack arranged beside the slot and capable of moving upwards and downwards, a tool gear fixed to a lower left corner of the slot and a tool fixed to a rotating shaft of the tool gear, the tool gear is meshed with the spool vertical moving rack, a side, away from the rotating shaft of the tool gear, of the tool is provided with an arc-shaped edge, a spine of the tool is matched with an opening of the slot when the tool is retracted into the slot, and a reset spring connected to the edge of the tool is arranged in a middle of a horizontal surface of an inner side of the slot; when the fracturing cavity reaches a set pressure, the spool vertical moving rack moves upwards, and the tool gear rotates clockwise to drive the tool to be opened; and when the fracturing cavity does not reach the set pressure, the tool is retracted into the slot under the action of a resilience of the reset spring.

Preferably, the number of the openable tool assemblies is six, and the number and position of the openable tool assemblies that are opened are selected according to a hardness coefficient of coal or an area of a target permeability-enhancement region. The number and position of the openable tool assemblies that are opened may be selected flexibly to adapt to various coal seam environment, such that the application range is wide.

Further preferably, a high-pressure and corrosion-resistant hose led into the fracturing cavity is arranged in the liquid feed rod, a liquid inlet of the liquid feed rod, the fracturing pump and the mixing tank are connected by means of the high-pressure and corrosion-resistant hose, and a control valve is mounted between the fracturing pump and the mixing tank, such that the transportation of fracturing liquid is facilitated, and the connection is proper; and the ultrasonic control box and the ultrasonic transducer are connected by means of a cable, such that the connection is proper.

The invention further provides an enhanced extraction method based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam, including the following steps:

• • S1, acquiring a sample in a coal seam, carrying the sample back into a laboratory, and performing a gas desorption test on the sample with ultrasonic waves at different frequencies to determine an optimal frequency f₁; then, performing proximate analysis, elemental analysis and mineral content analysis on the acquired sample, and obtaining an optimal acid fracturing fluid proportion by theoretical analysis and experimental study;
  • S2, constructing four extraction boreholes in the coal seam at different intervals, disposing the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam in one of the extraction boreholes, mounting extraction sieve tubes in the other extraction boreholes, and connecting the extraction sieve tubes to negative-pressure branches by means of hoses;
  • S3, performing gas concentration detection on the extraction boreholes where the extraction sieve tubes are mounted;
  • S4, turning on the ultrasonic control box to adjust an ultrasonic frequency of the ultrasonic transducer to f₁, turning off the ultrasonic control box 20-30 min later, and then turning on the fracturing pump to inject fracturing liquid in the mixing tank into the extraction borehole to perform fracturing; when a pressure of the fracturing liquid in the extraction borehole is close to a maximum horizontal stress, turning on again the ultrasonic control box to adjust the ultrasonic frequency of the ultrasonic transducer to f₁, and turning off again the ultrasonic control box 20-30 min later;
  • S5, turning off the fracturing pump after S4 is repeated for five times; then, turning on the ultrasonic control box to adjust the ultrasonic frequency of the ultrasonic transducer to f₁, and turning off the ultrasonic control box 20-30 min later; and
  • S6, after the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam is drawn out, connecting the extraction boreholes to the negative-pressure branches by means of the hoses, and performing gas concentration detection on all the extraction boreholes.

Preferably, in S1, a theoretical analysis and experimental study process for obtaining the optimal acid fracturing fluid proportion includes: testing an original permeability of the sample as reference, then selecting an acid solution according to mineral species in coal, then determining mass fractions of acids required according to mineral contents in a test result and a chemical reaction ratio of minerals, and finally, designing a pulverized coal corrosion experiment and a permeability testing experiment according to the mineral contents of the coal and proportions of relative contents of minerals under the precondition that an actual pre-extraction and permeability enhancement time is taken into account, to obtain the optimal acid fracturing fluid proportion.

Openings of all the extraction boreholes, where the extraction sieve tubes are mounted, are sealed with capsules, such that gas overflow during gas extraction is effectively avoided, potential safety hazards are reduced, and the accuracy of gas concentration detection data is guaranteed.

The enhanced extraction method based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam further includes a step S7: replacing the fracturing liquid in the mixing tank with clear water, turning on the fracturing pump, and then cleaning a pump body and the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam, such that the situation where fracturing equipment is corroded by the acid solution left in the fracturing equipment is effectively prevented, and the service life of the equipment is guaranteed.

In S2, the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam is mounted in the second extraction borehole from left to right, and a distance from the first extraction borehole to the second extraction borehole, a distance from the third extraction borehole to the second extraction borehole and a distance from the fourth extraction borehole to the second extraction borehole are respectively 10 m, 20 m and 40 m, such that the gas concentrations of coal seams under the influence of and away from the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam by three distances are detected to obtain the gas radiation distance of the influence of ultrasonic excitation and acid fracturing, and the design is reasonable.

In S4, in a case where the hardness coefficient of coal is less than 1.5, only two openable tool assemblies work; and in a case where targeted permeability enhancement is performed on a specific region, the openable tool assemblies close to the region work, such that the design is reasonable.

The invention has the following beneficial effects:

(1) Compared with the prior art that pores and fractures are filled with some minerals that hinder gas diffusion and seepage, this scheme combines ultrasonic excitation and acid fracturing, mechanical vibrations are generated by ultrasonic excitation to disturb an original stress field, the pore pressure and structure of coal change constantly under the action of the vibrations, and micro-fractures are generated in the coal to accelerate gas desorption of the coal seam to form a large quantity of free gas; because the inner surface and fractures of the coal seam are filled with a large quantity of minerals which seriously affect gas flow, acid fracturing is adopted to continuously corrode the minerals in the pores and fractures to dredge gas flow channels to increase the permeability of the coal seam, under the action of acid fracturing, primary cracks expand to communicate with cracks generated by ultrasonic excitation to further form complex seepage network channels, the large quantity of free gas produced by ultrasonic excitation flows into the extraction boreholes via a fracture network, and under the intercoupling effect of ultrasonic excitation and acid fracturing, the pore structure of coal is greatly improved, the connectivity between pores and fractures is enhanced, gas desorption is promoted, the influence of the principal stress effect is avoided, a complex fracture network is generated, the permeability of the coal seam is increased, the gas extraction efficiency of the coal seam is improved, the design ideal is novel, and the concept is ingenious.

(2) Four extraction boreholes are constructed in the coal seam at different intervals to obtain the influence on gas concentrations in the extraction boreholes at different distances, and before and after ultrasonic excitation and acid fracturing are performed on the selected extraction borehole, the gas concentrations in the other extraction boreholes are detected to gain a full understanding of the radiation distance of the coal seam subjected to permeability enhancement; after S4 is repeated for five times, the fracturing pump is turned off, and then the ultrasonic control box is turned on to adjust the ultrasonic transducer to perform excitation separately to allow acid fracturing liquid to further react with minerals, thus increasing the permeability of the coal seam and guaranteeing the permeability enhancement effect on the coal seam; because tensile cracks, which are prone to closure, will be generated if the pressure of the fracturing liquid in the extraction borehole exceeds the maximum horizontal stress, the ultrasonic control box is turned on again when the pressure of the fracturing liquid in the extraction borehole reaches the maximum horizontal stress; and by performing fracturing when the pressure in the extraction borehole is close to the maximum horizontal stress, secondary cracks may fully communicate with primary cracks to generate shear cracks or cracks that are prone to closure.

(3) The enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam is adopted to realize ultrasonic excitation and acid fracturing; when the fracturing cavity reaches a set pressure, the spool vertical moving rack moves upwards, the tool gear rotates clockwise to drive the tool to be opened; when the fracturing cavity does not reach the set pressure, the tool is retracted into the slot under the action of the resilience of the reset spring, and in this case, the force applied for downward movement of the spool vertical moving rack is less than the resilience of the reset spring, such that the design is reasonable, and the structures are linked with each other; the side, away from the tool gear, of the tool is provided with the arc-shaped edge, such that the safety in use is guaranteed; after the tool is opened, the edge cuts the coal seam on the inner wall of the extraction borehole to further expand the fracture surface, thus ensuring that free gas eventually flows into the extraction borehole via the fracture network; the spine of the tool is matched with the opening of the slot, such that overflow of the fracturing liquid is reduced; and the flat and long tool facilitates the buildup of pressure and is unlikely to be blocked, such that the design is reasonable.

(4) The ultrasonic control box is configured to control the ultrasonic frequency of the ultrasonic transducer to provide ultrasonic excitation, and the mixing tank is configured to inject acid fracturing liquid into the extraction boreholes to corrode minerals in pores and fractures, the liquid feed rod ensures that the fracturing cavity extends deep into the extraction borehole, and the conical drill bit is configured to break up large coal briquettes in the drilling process, such that the design structure is reasonable, and the practical value is high.

To sum up, the invention has the advantages of realizing intercoupling of ultrasonic excitation and acid fracturing, improving the pore structure of coal, enhancing the connectivity between pores and fractures, promoting gas desorption and improving gas extraction efficiency of coal seams and making structures linked with each other.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic installation and use diagram according to the invention.

FIG. 2 is a schematic structural diagram of an ultrasonic excitation and acid fracturing integrated drilling device.

FIG. 3 is a schematic structural diagram of an openable tool assembly.

DETAILED DESCRIPTION OF THE INVENTION

The invention is further described below in conjunction with embodiments and accompanying drawings.

As shown in FIGS. 1-3, an enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam is formed by an ultrasonic excitation and acid fracturing integrated drilling device a located in an extraction borehole 2 and an ultrasonic control box 6 and a mixing tank 8 which are located outside the extraction borehole 2.

A liquid inlet end of the ultrasonic excitation and acid fracturing integrated drilling device a is connected to the mixing tank 8 by means of a fracturing pump 7.

The ultrasonic excitation and acid fracturing integrated drilling device a is formed by a liquid feed rod a5, an ultrasonic transducer a1, a fracturing cavity a2 and a conical drill bit a4 which are sequentially arranged from left to right and a plurality of openable tool assemblies a3 which are located on left and right sides of the fracturing cavity a2 in mirror symmetry.

A high-pressure and corrosion-resistant hose 9 led into the fracturing cavity a2 is arranged in the liquid feed rod a5.

A liquid inlet of the liquid feed rod a5, the fracturing pump 7 and the mixing tank 8 are connected by means of the high-pressure and corrosion-resistant hose 9.

A control valve 4 is mounted between the fracturing pump 7 and the mixing tank 8.

The ultrasonic control box 6 and the ultrasonic transducer a1 are connected by means of a cable.

Each openable tool assembly a3 is formed by a slot a31, a spool vertical moving rack a32 arranged beside the slot a31 and capable of moving upwards and downwards, a tool gear a33 fixed to a lower left corner of the slot a31 and a tool a34 fixed to a rotating shaft of the tool gear a33.

The tool gear a33 is meshed with the spool vertical moving rack a32.

A side, away from the tool gear a33, of the tool a34 is provided with an arc-shaped edge, and when the tool a34 is retracted into the slot a31, a spine of the tool a34 is matched with an opening of the slot a31.

A reset spring a35 connected to the edge of the tool a34 is arranged in the middle of a horizontal surface of an inner side of the slot a31.

When the fracturing cavity a2 reaches a set pressure, the spool vertical moving rack a32 moves upwards, and the tool gear a33 rotates clockwise to drive the tool a34 to be opened.

When the fracturing cavity a2 does not reach the set pressure, the tool a34 is retracted into the slot a31 under the action of a resilience of the reset spring a35.

The number of the openable tool assemblies a3 is preferably six, and the number and position of the openable tool assemblies a3 that are opened are selected according to a hardness coefficient of coal or an area of a target permeability-enhancement region.

The conical drill bit a4 is mounted at a right end of the fracturing cavity a2.

An enhanced extraction method based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam includes the following specific steps:

S1, a sample in a coal seam 1 is acquired and carried back into a laboratory, and a gas desorption test is performed on the sample with ultrasonic waves at different ultrasonic frequencies to determine an optimal frequency f₁; then, proximate analysis, ultimate analysis and mineral content analysis are performed on the acquired sample, and an optimal acid fracturing liquid proportion is obtained by theoretical analysis and experimental study.

In S1, a theoretical analysis and experimental study process for obtaining the optimal acid fracturing liquid proportion is as follows: first, the original permeability of the sample is tested as reference, then an acid solution is selected according to mineral species in coal, then mass fractions of acids required are determined according to mineral contents in a test result and a chemical reaction ratio of minerals, and finally, a pulverized coal corrosion experiment and a permeability testing experiment are designed according to the mineral contents of the coal and proportions of relative contents of minerals under the precondition that an actual pre-extraction and permeability enhancement time is taken into account, to obtain the optimal acid fracturing fluid proportion.

S2, four extraction boreholes 2 are constructed in the coal seam 1, the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam is disposed in one of the extraction boreholes 2, the ultrasonic transducer a1 is connected to the ultrasonic control box 6 located outside the extraction boreholes 2 by means of a cable, the fracturing cavity a2 is connected to the mixing tank 8 located outside the extraction boreholes 2 by means of the fracturing pump 7, and extraction sieve tubes 3 are mounted in the other extraction boreholes 2 and connected to negative-pressure branches 11 by means of hoses 10.

In S2, the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam is mounted in the second extraction borehole 2 from left to right, and a distance from the first extraction borehole 2 to the second extraction borehole 2, a distance from the third extraction borehole 2 to the second extraction borehole 2 and a distance from the fourth extraction borehole 2 to the second extraction borehole 2 are respectively 10 m, 20 m and 40 m.

S3, gas concentration detection is performed on the extraction boreholes 2, where the extraction sieve tubes 3 are mounted, are detected.

S4, the ultrasonic control box 6 is turned on to adjust the ultrasonic frequency of the ultrasonic transducer a1 to f₁, the ultrasonic control box 6 is turned off 20-30 min later, and then the fracturing pump 7 is turned on to inject fracturing liquid in the mixing tank 8 into the extraction borehole 2 to perform fracturing; when the pressure of the fracturing liquid in the extraction borehole 2 is close to a maximum horizontal stress, the ultrasonic control box 6 is turned on again to adjust the ultrasonic frequency of the ultrasonic transducer a1 to f₁, and the ultrasonic control box 6 is turned off again 20-30 min later.

In S4, in a case where the hardness coefficient of coal is less than 1.5, only two openable tool assemblies a3 work; and in a case where targeted permeability enhancement is performed on a specific region, the openable tool assemblies a3 close to the region work.

S5, the fracturing pump 7 is turned off after S4 is repeated for five times; then, the ultrasonic control box 6 is turned on to adjust the ultrasonic frequency of the ultrasonic transducer a1 to f₁, and the ultrasonic control box 6 is turned off 20-30 min later.

S6, after the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam is drawn out, the extraction boreholes 2 are connected to the negative-pressure branches 11 by means of the hoses 10, and gas concentration detection is performed on all the extraction boreholes 2.

Openings of all the extraction boreholes 2, where the extraction sieve tubes 3, are mounted are sealed with capsules 5.

S7, the fracturing liquid in the mixing tank 8 is replaced with clear water, the fracturing pump 7 is turned on, and then a pump body and the enhanced extraction system based on ultrasonic excitation and acid fracturing integrated permeability enhancement for a coal seam are cleaned.