CN108915650B

CN108915650B - Device and method for simulating differential pressure drop in coal bed gas drainage and mining process

Info

Publication number: CN108915650B
Application number: CN201810749038.7A
Authority: CN
Inventors: 许浩; 赵天天; 汤达祯; 候伟; 翟羽阳; 熊先钺; 王伟; 刘玉龙; 丁飞飞
Original assignee: China University of Geosciences Beijing
Current assignee: China University of Geosciences Beijing
Priority date: 2018-07-10
Filing date: 2018-07-10
Publication date: 2020-12-08
Anticipated expiration: 2038-07-10
Also published as: CN108915650A

Abstract

The invention discloses a device for simulating differential pressure drop in a coal bed gas drainage and production process, which comprises an experimental device and a servo water injection pressure pump, wherein the experimental device comprises a test cavity and a cavity cover, pressure gauges are arranged at the bottom of the test cavity and the top of the cavity cover for measuring water pressure, the test cavity is divided into a plurality of layered sections through equidistant scale marks arranged on the inner wall, a drain pipe is arranged on each layered section, and a plurality of groups of pressure sensors vertical to the equidistant scale marks are arranged in the test cavity; also included is a method comprising the steps of: firstly, crushing a coal sample, screening coal powder with required particle size to mix with gypsum powder, then adding the mixed coal sample into a test piece box in batches, carrying out extrusion molding by using a forming press, then injecting water into a model, finally opening a drainage valve, drawing a pressure contour line according to pressure data, and analyzing pressure distribution; the method can reveal the rule of the pressure drop difference of each layer of drainage and depressurization in the longitudinal heterogeneous reservoir, and provides theoretical guidance for the development of the coal bed gas.

Description

Device and method for simulating differential pressure drop in coal bed gas drainage and mining process

Technical Field

The invention relates to the technical field of coal bed gas drainage and mining development, in particular to a device and a method for simulating differential pressure drop in a coal bed gas drainage and mining process.

Background

Drainage depressurization is needed during coal bed gas development, and after the pressure is lower than the critical desorption pressure, the coal bed gas in an adsorption state can be desorbed and can be produced later, so that large-area drainage depressurization is the key for coal bed gas development, and fine research on the change rule of coal reservoir stratum pressure distribution in the drainage depressurization process is necessary.

The coal reservoir is an unconventional reservoir and has extremely low permeability, so the seepage of formation water in the coal reservoir belongs to low-speed seepage, and according to the low-speed seepage theory: the flow of the formation water needs a certain starting pressure gradient to break through the restriction of the coal matrix surface on the water flow, and when the formation pressure gradient is smaller than the starting pressure gradient, the formation water may not flow, so that the drainage and the pressure reduction cannot be carried out. Moreover, the coal reservoir layer often presents a phenomenon of overlapping of different coal rock types in the longitudinal direction, which causes heterogeneity of permeability in the longitudinal direction and heterogeneity of starting pressure gradient in the longitudinal direction, so that in the drainage depressurization process, the depressurization degree of the coal reservoir layer of each interval may be different, and even the depressurization effect may not be achieved in a certain local area, which may cause important influence on the development effect of the coal bed gas. Therefore, a set of physical simulation device is needed to be designed to reveal the rule in the coal bed methane, and theoretical guidance is provided for the development of the coal bed methane.

In the existing technical scheme, for example, the physical simulation device for coal bed methane production and the simulation method thereof disclosed by application number 201010101527.5 optimize the gas well drainage and production work and reduce the cost by simulating the inflation, water injection and production states. However, in the above technical solution, although the pressure reduction simulation in the production state can be realized, there are some defects, and the main defects existing in combination with the above technical solution and the practical problems, and the technical solution widely used at present, are mainly reflected in the following two aspects: firstly, because of strong heterogeneity of coal seams, a single simulation device cannot accurately simulate the pressure change rule of each layer; secondly, due to the superposition of different coal rock types, the heterogeneity of the coal rock in the longitudinal direction and the heterogeneity of the starting pressure gradient are caused, in the process of draining and depressurizing, if the factors are not considered, the effect of depressurizing can not be achieved in local areas, although the heterogeneity exists in each area of the coal bed, the heterogeneity is not completely independent, if the overall depressurizing effect cannot be considered comprehensively during simulation, in actual production, due to mutual interference and influence, the final result is greatly different from the simulation result, and the simulation process loses significance.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a device and a method for simulating differential pressure drop in a coal bed gas drainage and production process, which can reveal the rule of drainage and depressurization layered pressure drop difference in a longitudinal heterogeneous reservoir, provide theoretical guidance for coal bed gas development and effectively solve the problems in the background art.

The technical scheme adopted by the invention for solving the technical problems is as follows:

a device for simulating differential pressure drop in a coal bed gas drainage and production process comprises an experimental device and a servo water injection pressure pump, wherein the experimental device is communicated with the servo water injection pressure pump through a water injection pipeline, the experimental device comprises a test cavity and a cavity cover, the edge of the upper end of the test cavity is provided with a clamping protruding block, the cavity cover is connected with the clamping protruding block in a clamping manner through clamping grooves arranged on the edges of the upper end of the cavity cover, the bottom of the test cavity and the top of the cavity cover are respectively provided with a water injection pipe and a water outlet pipe, the water injection pipe and the water outlet pipe measure water pressure through a built-in pressure gauge, the water injection pipe and the water outlet pipe are respectively provided with a water injection valve and a water outlet valve, the water outlet pipe penetrates through the cavity cover to be communicated with the test cavity through a guide pipe, and the connection part of the water injection pipe;

experimental cavity inner wall is by supreme four equidistance scale marks that are parallel to each other that evenly are equipped with down, the equidistance scale mark will be experimental cavity and divide into that a plurality of layering is interval to all be equipped with the drain pipe on the side cavity of every layering interval, at every be equipped with drainage valve in the drain pipe, be provided with the pressure sensor of multiunit perpendicular to equidistance scale mark in the experimental cavity, and every pressure sensor of group is equidistant evenly distributed on vertical.

The pressure loading platform vertically moves along the pressure guide columns, the bottom of the pressure loading platform is fixedly provided with a pressure loading pressing plate in a regular quadrangular frustum pyramid shape, and the bottom surface of the pressure loading pressing plate is fixedly provided with a plurality of mutually crossed anti-skidding positioning grains.

As a preferred technical scheme of the invention, the clamping groove is fixedly arranged at the edge of the cavity cover in an L shape, a plurality of uniformly distributed occlusion latches are fixedly arranged at the inner side of the clamping groove, the inner width of each occlusion latch is equal to the outer width of the clamping convex block, the clamping convex block is fixedly arranged at the upper end of the test cavity in an annular shape, the outer side of the clamping convex block is in contact with the inner wall of the cavity cover in an attaching manner, and an annular airtight gasket is arranged at the contact position of the outer side of the clamping convex block and the inner wall of the cavity cover.

As a preferable technical scheme of the invention, the wall of the drain pipe is provided with a plurality of uniformly distributed through holes, the through holes are used for simulating perforation on the well wall and communicating coal rocks with the interior of the drain pipe, funnel-shaped jet flow guide pipes are fixedly arranged on the wall of the through holes, and a swim bladder cavity is arranged at the connecting end of the jet flow guide pipes and the drain pipe.

In addition, the invention also provides a method for simulating differential pressure drop in the process of coal bed gas drainage and mining, which comprises the following steps:

step 100, preparing coal samples, namely crushing the coal samples of various coal rock types by using a crusher, screening sufficient coal powder with required particle size, and fully mixing the coal powder with gypsum powder according to a ratio;

step 200, forming molded coal, namely adding the mixed coal samples into a test piece box in batches, adding a proper amount of clear water, and forming by using a forming press according to a certain forming pressure;

step 300, injecting water into the model, namely injecting water into the model through a servo water injection pressure pump until the pressure of a water outlet reaches a set value, stopping injecting water and closing a water injection valve;

step 400, surveying and mapping pressure distribution, opening a corresponding drainage valve according to a preset drainage scheme, processing pressure data of the pressure sensor by using a computer, drawing a pressure contour line, and analyzing the pressure distribution.

As a preferred technical scheme of the invention, the coal sample used for forming consists of coal dust and gypsum powder with different coal rock types and particle sizes, the particle sizes are all larger than 120 meshes and smaller than 200 meshes, wherein the particle size ratio of the bright coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder 93: 0: 7; the semi-bright coal has the following grain diameter ratio: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder is 85: 8: 7; the particle size ratio of the semi-dark coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): 70 parts of gypsum powder: 23: 7; the particle size ratio of the dull coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): 50 parts of gypsum powder: 43: 7.

as a preferred technical solution of the present invention, in step 200, the pressure forming comprises the following specific steps:

step 201, successively and fully mixing and wetting a coal sample and a proper amount of clear water

202, forming the coal sample by applying forming pressure each time, adding the thickness of the coal layer formed by the coal sample to the equidistant scale lines each time, and keeping the forming pressure stable for 1 hour after forming;

step 203, implanting various sensors into the designated positions of the coal seam when the coal sample is formed successively, and installing drainage valves layer by layer according to the layered intervals.

As a preferred technical solution of the present invention, in step 300, the concrete steps of model water injection are as follows:

step 301, switching on a servo water injection pressure pump, injecting water into the model, and closing a water outlet valve after water is discharged from a water outlet;

step 302, keeping the same water injection rate and continuously injecting water, setting the water injection pressure of a servo water injection pressure pump, stopping injecting water when the pressure at the water injection port reaches 2.1MPa, and continuously injecting water below 2.05MPa until the pressure at the water outlet reaches 2 MPa;

and step 303, closing the water injection valve and stopping water injection.

As a preferable technical solution of the present invention, in the above step, when water is injected into the cavity by the servo water injection pressure pump until the water outlet is drained, and the same water injection rate is maintained, a water injection pressure variation graph of the water injection port with time is drawn, and when the water injection pressure has a peak value, the water injection is stopped, and the water is pumped out from the water injection port until the water pressure of the water injection port is reduced to 30-50% of the peak value, and the water injection is continued again at the same water injection rate.

As a preferred technical solution of the present invention, in step 400, the predetermined drainage scheme specifically includes: and respectively opening a plurality of mutually independent drainage valves or selectively opening the drainage valve of the target horizon according to requirements.

Compared with the prior art, the invention has the beneficial effects that:

the invention can simulate the law of pressure drop transmission under the superposed state of coal rock types with different permeabilities according to a design scheme, and the design of a plurality of clamping holes of the clamping groove ensures that the experimental device is not only suitable for superposing four rock strata, but also can be designed automatically according to the requirement, the pipe wall of the drainage valve in the rock strata is fully distributed with small holes, the perforation effect in the drainage and production process is simulated, the result transmitted by the sensor is drawn by a computer through a pressure contour line, and the pressure distribution law is visually analyzed.

Drawings

FIG. 1 is a schematic view of the overall structure of the present invention;

FIG. 2 is a schematic cross-sectional view of an experimental apparatus according to the present invention;

FIG. 3 is a schematic view of the structure of the drainage pipe of the present invention;

FIG. 4 is a schematic view of the press of the present invention;

FIG. 5 is a schematic view of a contour line of a pressure distribution according to the experimental results of the present invention;

FIG. 6 is a schematic flow chart of the present invention;

reference numbers in the figures: 1-experimental apparatus; 2-servo water injection pressure pump; 3-water injection pipeline; 4-equidistant graduation lines; 5-a layering interval; 6, a drain pipe; 7-a pressure sensor; 8-a press machine;

101-a test chamber; 102-a cavity cover; 103-clamping the convex block; 104-a clamping groove; 105-a water injection pipe; 106-water outlet pipe; 107-pressure gauge; 108-a water injection valve; 109-water outlet valve; 110-wire netting; 111-engaging latches; 112-gas-tight gasket;

601-a drain valve; 602-a via hole; 603-a jet conduit; 604-swim bladder cavity;

801-pressure loading platform; 802-pressure guide post; 803-pressure loaded platen; 804-anti-skid location pattern.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

As shown in fig. 1 to 4, the present invention provides a device for simulating differential pressure drop in a coal bed methane extraction process, including an experimental apparatus 1 and a servo water injection pressure pump 2, where the experimental apparatus 1 and the servo water injection pressure pump 2 are communicated through a water injection pipeline 3, and in the above, the servo water injection pressure pump 2 specifically refers to a device formed by combining a servo motor and a water injection pump, and compared with a simple water injection pump, the device is characterized in that: the output of power pump is comparatively single, and servo motor can accurate control, consequently can change output's size as required in reality, and in this embodiment, then regard as the main reference factor of adjustment servo motor power through the size of water filling port pressure, change servo motor's power through the change of water filling port pressure to reach the purpose of controlling water injection pump water injection pressure.

The experimental device 1 comprises a test cavity 101 and a cavity cover 102, wherein the upper end edge of the test cavity 101 is provided with a clamping convex block 103, and the cavity cover 102 is clamped and connected with the clamping convex block 103 through clamping grooves 104 arranged on the peripheral edge of the cavity cover. Through the interlock connection of block protruding piece 103 and card fixed recess 102, realize the airtight connection of experimental cavity 101 and cavity lid 102, and further preferred, card fixed recess 104 is the edge of L type fixed mounting at cavity lid 102, the inboard fixed mounting of card fixed recess 104 has a plurality of equidistant evenly distributed's interlock latch 111, the inner width of interlock latch 111 equals the outer width of block protruding piece 103, block protruding piece 103 is cyclic annular fixed mounting in the upper end of experimental cavity 101, and the block protruding piece 103 outside contacts with the laminating of cavity lid 102 inner wall the contact department in the block protruding piece 103 outside and cavity lid 102 inner wall is equipped with and is annular gas tightness packing ring 112. In the above, since the connection position of the clamping groove 104 is selectable, flexible adjustment can be performed according to the degree of filling of the molded coal, so that the experimental device is not only suitable for stacking three or four layers of coal strata as shown in the figure, but also can design the thickness and the stacking number of the coal strata according to the needs, thereby improving the usability in the actual simulation process.

The bottom of the test cavity 101 and the top of the cavity cover 102 are respectively provided with a water injection pipe 105 and a water outlet pipe 106, in the embodiment, the water injection pipe 105 is arranged at the lower end, the water outlet pipe is arranged at the upper end, so that in the water injection process, the pressure is transmitted in the test cavity 101 of the whole experimental device uniformly, and the internal pressure is prevented from being unevenly distributed due to the fact that an advantageous channel is formed inside. The water injection pipe 105 and the water outlet pipe 106 measure water pressure through a built-in pressure gauge 107, and it is to be specifically noted herein that the pressure gauge 107 is an unconventional mechanical pressure gauge, but an electronic pressure gauge is adopted, and further, measured pressure data can be transmitted to a computer system or a control system of a servo motor, the two systems have been widely applied in the prior art, the specific structural mode and the working mode thereof are not repeated herein, a water injection valve 108 and a water outlet valve 109 are respectively arranged in the water injection pipe 105 and the water outlet pipe 106, the water outlet pipe 106 passes through the cavity cover 102 through a conduit to be communicated with the test cavity 101, and iron wires 110 are arranged at the connection part of the water injection pipe 105 and the test cavity 101 and at the connection part of the conduit and the test cavity 101.

In the above, the wire netting 110 can prevent the coal from being broken due to stress concentration caused by the pressure concentration at the water outlet where the coal is located, so as to avoid changing the shape and the granularity of the coal at a local position in the whole device, and ensure that the whole device is in a uniform environment.

Experimental cavity 101 inner wall is by supreme four equidistance scale marks 4 that are parallel to each other that evenly are equipped with down, equidistance scale mark 4 divide into experimental cavity 101 for a plurality of layering interval 5 to all be equipped with drain pipe 6 on the side cavity of every layering interval 5, at every be equipped with drain valve 601 in the drain pipe 6, be provided with the pressure sensor 7 of multiunit perpendicular to equidistance scale mark 4 in experimental cavity 101, and every pressure sensor 7 of group is on vertical equidistant evenly distributed.

In the above, the pressure sensors 7 are arranged more densely in the horizontal direction as they are closer to the drain valve 601, in addition to being uniformly distributed at equal intervals in the longitudinal direction, and thus the function of the arrangement is to adapt the stress variation of the area close to the drain valve to the collection of pressure data. In addition, in the above, the pressure data obtained by the pressure sensor 7 is used for drawing the pressure contour line by the computer, so that the pressure distribution rule can be intuitively analyzed.

In addition, a plurality of through holes 602 are uniformly distributed on the wall of the drain pipe 6, a plurality of through holes 602 are distributed on the wall of the test cavity 101 on the drain pipe 6 to simulate perforation on the wall of a well, so as to simulate the effect of perforation in coal bed gas exploitation, the coal rock is communicated with the inside of the drain pipe 6 through the through holes 602, funnel-shaped jet flow guide pipes 603 are fixedly mounted on the wall of the through holes 602, a swim bladder cavity 604 is arranged at the connecting end of the jet flow guide pipes 603 and the drain pipe 6, the jet flow guide pipes 603 and the swim bladder cavity 604 are arranged to improve the absorption efficiency, and gas or liquid is prevented from being discharged unsmoothly due to the existence of air pressure.

Preferably, the test device further comprises a press machine 8, the press machine 8 comprises a pressure loading platform 801 and four pressure guide columns 802, the pressure guide columns 802 are fixedly installed at four corners of the test cavity 101, the pressure loading platform 801 vertically moves along the pressure guide columns 802 and can stably apply vertical pressure, a pressure loading pressing plate 803 in a regular quadrangular frustum pyramid shape is fixedly installed at the bottom of the pressure loading platform 801, so that the vertical pressure is balanced, further, a plurality of mutually crossed anti-slip positioning grains 804 are fixedly installed on the bottom surface of the pressure loading pressing plate 803, a pressed anti-slip grain can be formed with the molded coal and can be used in cooperation with the pressure loading pressing plate 803, and lateral slip caused by uneven bottom surface of the pressure loading pressing plate 803 can be prevented.

In addition, as shown in fig. 6, the present invention further provides a method for simulating differential pressure drop in a coal bed methane mining process, including the following steps:

step 100, preparing coal samples, namely crushing the coal samples of various coal rock types by using a crusher, screening sufficient coal powder with required particle size, and fully mixing the coal powder with gypsum powder according to a ratio.

The coal sample used for forming is composed of coal powder and gypsum powder with different coal rock types and particle sizes, the particle sizes are larger than 120 meshes and smaller than 200 meshes, and the particle size ratio of the bright coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder 93: 0: 7; the semi-bright coal has the following grain diameter ratio: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder is 85: 8: 7; the particle size ratio of the semi-dark coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): 70 parts of gypsum powder: 23: 7; the particle size ratio of the dull coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): 50 parts of gypsum powder: 43: 7.

and 200, forming molded coal, namely adding the mixed coal samples into a test piece box in batches, adding a proper amount of clear water, forming by using a forming press according to a certain forming pressure, removing the press, and sealing the experimental device by using a cavity cover.

In this embodiment, the stacking of the coal samples can set different experimental schemes according to requirements, such as:

from top to bottom, dim coal, semi-dim coal, bright coal or from top to bottom, bright coal, semi-dim coal, that is, various types of overlap may occur in an actual coal seam.

In step 200, the specific steps of pressure forming are as follows:

And 300, injecting water into the model by a servo water injection pressure pump until the pressure of a water outlet reaches a set value, stopping injecting water and closing a water injection valve.

In step 300, the concrete steps of model water injection are as follows:

step 302, keeping the same water injection rate for continuous water injection, setting the water injection pressure of a servo water injection pressure pump, stopping water injection when the pressure at the water injection port reaches 2.1MPa, continuing water injection under the pressure of 2.05MPa until the pressure at the water outlet reaches 2MPa, thus ensuring the formation pressure in the whole cavity to be 2MPa-2.1MPa, and avoiding the conditions of high pressure near a water injection valve and low pressure near a water outlet valve to influence the simulation effect;

and step 303, closing the water injection valve and stopping water injection.

And furthermore, in the above steps, when the same water filling rate is maintained after water is filled into the cavity by the servo water filling pressure pump until the water outlet is drained, drawing a change graph of the water filling pressure of the water filling port along with the time, when the water filling pressure has a peak value, stopping water filling, pumping water from the water filling port outwards until the water pressure of the water filling port is reduced to 30-50% of the peak value, and then continuing water filling at the same water filling rate again.

It should be added that, the method of injecting water as described above has the following advantages: on one hand, intermittent water injection can make water injection efficiency higher, and pressure distribution at each interval position is more uniform, on the other hand, because various components in coal rocks are more complex and the internal structure is more disordered, in the press forming of the press, some blockage can exist, continuous water injection is directly carried out, the blockage of channels between coal seams can be caused, and temporary blockage of pores and a throat can be avoided through the circulation of water injection-water pumping-water injection-water pumping, so that the pressure of water injection can be uniformly distributed in the whole model.

In step 400, the predetermined drainage scheme is specifically: and respectively opening a plurality of mutually independent drainage valves or selectively opening the drainage valve of the target horizon according to requirements.

Based on the above, in the invention, the overlapping manner of the coal rock samples specifically adopts the mutual overlapping of the dim coal, the semi-dark coal, the semi-bright coal and the bright coal from top to bottom, the drainage scheme specifically is to open only the drainage pipe in the layering interval where the bright coal is located, and the distribution rule of the pressure drop in the experiment is shown in fig. 5.

As shown in FIG. 5, the pressure drop propagation speed is the fastest for bright coals and the pressure drop propagation speed is the slowest for dim coals, which are in accordance with the percolation theory. And the coal seam belongs to a low-permeability reservoir, particularly in dim coal, a starting pressure gradient may exist, and if the pressure drop gradient is smaller than the starting pressure gradient, water in the coal seam may not flow completely, so that gas existing in the dim coal cannot be desorbed and diffused out through pressure drop, and the gas becomes 'dead gas', and does not contribute to gas production. In addition, fig. 5 is a graph at a certain moment, actually, data of the pressure sensor can be changed continuously, and a pressure drop distribution rule formed by computer calculation is also a graph with dynamic change, so that the dynamic process of the whole pressure drop can be simulated in detail.

It should be further understood that fig. 5 in this embodiment is only a specific example of the present application, and it should be understood that even though the same design scheme, i.e. the coal-rock stacking mode is the same, only the dim coal is opened during the drainage process, or other drainage schemes are adopted, the pressure drop situation is different, or different coal-rock stacking modes are adopted, the pressure drop situation is different, and the comprehensive determination needs to be carried out according to the coal-rock stacking mode and the specific drainage scheme.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.

Claims

1. The utility model provides a device of simulation coal bed gas row adopts in-process difference pressure drop, includes experimental apparatus (1) and servo water injection force pump (2), communicate its characterized in that through water injection pipeline (3) between experimental apparatus (1) and the servo water injection force pump (2): the experimental device (1) comprises a test cavity (101) and a cavity cover (102), wherein the edge of the upper end of the test cavity (101) is provided with a clamping convex block (103), the cavity cover (102) is clamped and connected with the clamping convex blocks (103) through clamping grooves (104) arranged at the peripheral edge of the cavity cover, the bottom of the test cavity (101) and the top of the cavity cover (102) are respectively provided with a water injection pipe (105) and a water outlet pipe (106), the water injection pipe (105) and the water outlet pipe (106) measure the water pressure through a built-in pressure gauge (107), and a water injection valve (108) and a water outlet valve (109) are respectively arranged in the water injection pipe (105) and the water outlet pipe (106), the water outlet pipe (106) passes through the cavity cover (102) through a guide pipe to be communicated with the test cavity (101), the connection part of the water injection pipe (105) and the test cavity (101) and the connection part of the conduit and the test cavity (101) are provided with wire nets (110);

four equidistant scale marks (4) which are parallel to each other are uniformly arranged on the inner wall of the test cavity (101) from bottom to top, the equidistant scale marks (4) divide the test cavity (101) into a plurality of layered sections (5), the mixed coal samples are added into the test cavity (101) in batches, forming pressure forming is carried out on the coal samples each time, the thickness of a coal bed formed by adding the coal samples each time is just equal to the equidistant scale marks, a drainage pipe (6) is arranged on a side cavity of each layered section (5), a drainage valve (601) is arranged in each drainage pipe (6), a plurality of groups of pressure sensors (7) which are distributed perpendicular to the equidistant scale marks (4) are arranged in the test cavity (101), and each group of the pressure sensors (7) are uniformly distributed at equal intervals in the longitudinal direction;

still include press (8), press (8) are including pressure loading platform (801) and four pressure guide posts (802), pressure guide post (802) fixed mounting is in the four corners of experimental cavity (101), pressure loading platform (801) is along pressure guide post (802) vertical motion.

2. The device for simulating differential pressure drop in the process of discharging and mining the coal bed gas according to claim 1, characterized in that: the bottom of the pressure loading platform (801) is fixedly provided with a pressure loading pressing plate (803) in a regular quadrangular frustum pyramid shape, and the bottom surface of the pressure loading pressing plate (803) is fixedly provided with a plurality of mutually crossed anti-skidding positioning grains (804).

3. The device for simulating differential pressure drop in the process of discharging and mining the coal bed gas according to claim 1, characterized in that: card fixed recess (104) are the edge of L type fixed mounting at cavity lid (102), card fixed recess (104) inboard fixed mounting has interlock latch (111) of a plurality of equidistant evenly distributed, the inner width of interlock latch (111) equals the outer width of the protruding piece of block (103), the protruding piece of block (103) is cyclic annular fixed mounting in the upper end of experimental cavity (101), and the protruding piece of block (103) outside and cavity lid (102) inner wall laminating contact the protruding piece of block (103) outside is equipped with the contact department of cavity lid (102) inner wall and is annular gas tightness packing ring (112).

4. The device for simulating differential pressure drop in the process of discharging and mining the coal bed gas according to claim 1, characterized in that: be equipped with a plurality of evenly distributed's through-hole (602) on the pipe wall of drain pipe (6), through-hole (602) are used for simulating the perforation on the wall of a well to make coal petrography and the inside intercommunication of drain pipe (6), equal fixed mounting has jet flow pipe (603) of leaking hopper-shaped on the pipe wall at through-hole (602) place, is equipped with swim bladder cavity (604) at the link of jet flow pipe (603) and drain pipe (6).

5. A method for simulating differential pressure drop in a coal bed gas drainage and production process based on the device for simulating differential pressure drop in a coal bed gas drainage and production process of any one of claims 1 to 4, which is characterized by comprising the following steps:

step 200, forming molded coal, namely adding the mixed coal samples into a test cavity in batches, adding a proper amount of clear water, and forming by using a forming press according to a certain forming pressure;

the concrete steps of model water injection are as follows:

step 303, closing a water injection valve and stopping water injection;

6. The method for simulating differential pressure drop in the coal bed gas drainage and mining process according to claim 5, wherein the coal sample used for forming is composed of coal powder and gypsum powder with different coal rock types and particle sizes, the particle sizes are all larger than 120 meshes and smaller than 200 meshes, and the particle size ratio of bright coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): gesso = 93: 0: 7; the semi-bright coal has the following grain diameter ratio: (60-80 mesh coal powder): (120-200 mesh coal powder): gesso = 85: 8: 7; the particle size ratio of the semi-dark coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): gesso = 70: 23: 7; the particle size ratio of the dull coal is as follows: (60-80 mesh coal powder): (120-200 mesh coal powder): gesso = 50: 43: 7.

7. the method for simulating differential pressure drop in a coal bed methane drainage and production process according to claim 5, wherein in the step 200, the pressure forming comprises the following specific steps:

8. The method of claim 5, wherein in step 300, when the same water injection rate is maintained after water is injected into the chamber by the servo water injection pressure pump until the water is discharged from the water outlet, the water injection pressure at the water injection port is plotted against time, and when the water injection pressure reaches a peak value, the water injection is stopped, and the water is pumped out from the water injection port until the water pressure at the water injection port is reduced to 30-50% of the peak value, and the water injection is continued at the same water injection rate again.

9. The method for simulating differential pressure drop during drainage and mining of coal bed methane according to claim 5, wherein in step 400, the predetermined drainage scheme is specifically: and respectively opening a plurality of mutually independent drainage valves or selectively opening the drainage valve of the target horizon according to requirements.