CN108915650A - The devices and methods therefor of difference pressure drop during a kind of simulation coal bed gas extraction - Google Patents

Abstract

The invention discloses a device for simulating the differential pressure drop in the coalbed gas extraction process, which includes an experimental device and a servo water injection pressure pump. The experimental device includes a test cavity and a cavity cover, which are arranged at the bottom of the test cavity and the top of the cavity cover The pressure gauge measures the water pressure. The test chamber is divided into several stratified intervals by equidistant scale lines set on the inner wall. Drainage pipes are arranged on each stratified interval. A pressure sensor from the scale line; also includes a method, including the following steps: firstly, the coal sample is broken, and the coal powder and gypsum powder of the required particle size are screened and mixed, and secondly, the mixed coal sample is added to the test sample in batches. The box is extruded with a molding press, then water is injected into the model, and finally the drainage valve is opened, and the pressure contour is drawn according to the pressure data to analyze the pressure distribution; The law of difference in pressure drop between layers can provide theoretical guidance for the development of coalbed methane.

Description

A device and method for simulating differential pressure drop during coalbed methane extraction

technical field

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

Background technique

Drainage and depressurization are required during the development of coalbed methane. When the pressure is lower than the critical desorption pressure, the adsorbed coalbed methane will be desorbed and then produced. Therefore, large-area drainage and depressurization are the key to the development of coalbed methane, so there are It is necessary to conduct detailed research on the change law of the formation pressure distribution of coal reservoirs during the process of drainage and depressurization.

As an unconventional reservoir, coal reservoirs have extremely low permeability. Therefore, the seepage of formation water in coal reservoirs belongs to low-speed seepage. According to the theory of low-speed seepage: the flow of formation water requires a certain starting pressure gradient to break through Due to the binding effect of the coal matrix surface on water flow, when the formation pressure gradient is lower than the threshold pressure gradient, the formation water may not flow, resulting in the failure of drainage and depressurization. Moreover, because coal reservoirs often present the phenomenon of superposition of different coal and rock types in the vertical direction, resulting in the vertical heterogeneity of permeability and the vertical heterogeneity of the start-up pressure gradient, then the drainage and depressurization During the process, the depressurization degree of coal reservoirs in various intervals may be different, and the depressurization effect may not even be achieved in a certain local area, which will have an important impact on the development effect of coalbed methane. Therefore, it is necessary to design a set of physical simulation devices to reveal the laws and provide theoretical guidance for the development of coalbed methane.

Among the existing technical solutions, such as a coalbed methane production physical simulation device and its simulation method published in the application number 201010101527.5, it optimizes the drainage work of the gas well and reduces the cost by simulating the gas charging, water injection and production states. However, in the above-mentioned technical solution, although the step-down simulation in the production state can be realized, there are still some defects. Combining the above-mentioned technical solution and the existing problems in reality, and combining the technical solutions widely used at present, there are still main problems The defects are mainly reflected in the following two aspects: first, due to the strong heterogeneity of the coal seam, a single simulation device cannot accurately simulate the pressure change law of each layer; second, due to the superposition of different coal rock types, the coal The vertical heterogeneity of the rock and the heterogeneity of the starting pressure gradient, if the above factors are not considered in the process of drainage and depressurization, the effect of depressurization will not be achieved in some areas, although there are Heterogeneity, but not completely independent, if the overall pressure-reducing effect cannot be fully considered in the simulation, due to mutual interference and influence in actual production, the final result will be very different from the simulation result, making the simulation process Lost meaning.

Contents of the invention

In order to overcome the shortcomings of the existing technical solutions, the present invention provides a device and method for simulating the differential pressure drop in the coalbed methane drainage process, which can reveal the law of the pressure drop difference of each layer in the vertical heterogeneous reservoir. , provide theoretical guidance for the development of coalbed methane, and can effectively solve the problems raised by the background technology.

The technical solution adopted by the present invention to solve its technical problems is:

A device for simulating the differential pressure drop in the coalbed methane drainage process, including an experimental device and a servo water injection pressure pump, the experimental device and the servo water injection pressure pump are connected through a water injection pipeline, and the experimental device includes a test chamber and a chamber The body cover, the upper edge of the test chamber is provided with a snap-fit protruding block, and the cavity cover is snap-connected with the snap-fit protruding block through the snap grooves arranged on its peripheral edge, the test chamber The bottom and the top of the cavity cover are respectively provided with a water injection pipe and a water outlet pipe. Both the water injection pipe and the water outlet pipe measure the water pressure through the built-in pressure gauge, and the water injection pipe and the water outlet pipe are respectively equipped with a water injection valve and a water outlet valve. The water pipe passes through the chamber cover through the conduit to communicate with the test cavity, and the connection between the water injection pipe and the test cavity and the connection between the conduit and the test cavity are all provided with barbed wire;

The inner wall of the test chamber is evenly provided with four equidistant scale lines parallel to each other from bottom to top, and the equidistant scale lines divide the test chamber into several layered intervals, and the side chamber of each layered interval There are drain pipes on each of the drain pipes, and drain valves are arranged in each of the drain pipes. There are multiple sets of pressure sensors perpendicular to the equidistant scale lines in the test chamber, and each set of pressure sensors is evenly spaced in the longitudinal direction. .

As a preferred technical solution of the present invention, it also includes a press, which includes a pressure loading platform and four pressure guide columns, and the pressure guide columns are fixedly installed at the four corners of the test cavity, and the pressure loading platform is along the With the vertical movement of the pressure guide column, the bottom of the pressure loading platform is fixedly installed with a pressure loading platen in the shape of a square prism, and a number of intersecting anti-skid positioning patterns are fixedly installed on the bottom surface of the pressure loading platen.

As a preferred technical solution of the present invention, the locking groove is L-shaped and fixedly installed on the edge of the cavity cover, and the inner side of the locking groove is fixedly installed with several snapping teeth evenly distributed at equal intervals. The inner width of the above-mentioned engaging teeth is equal to the outer width of the engaging protruding block, and the engaging protruding block is ring-shaped and fixedly installed on the upper end of the test chamber, and the outer side of the engaging protruding block is attached to the inner wall of the cavity cover For contact, an annular airtight gasket is provided at the contact point between the outer side of the locking protrusion and the inner wall of the cavity cover.

As a preferred technical solution of the present invention, the pipe wall of the drainage pipe is provided with several evenly distributed through holes, and the through holes are used to simulate the perforation on the well wall, and make the coal rock and the inside of the drainage pipe The pipe wall where the through hole is located is fixedly equipped with a funnel-shaped jet conduit, and a swim bladder cavity is provided at the connection end of the jet conduit and the drain pipe.

In addition, the present invention also provides a method for simulating differential pressure drop in the coalbed methane drainage process, comprising the following steps:

Step 100, coal sample preparation, crushing the coal samples of each coal rock type with a pulverizer, screening a sufficient amount of coal powder with the required particle size, and fully mixing with gypsum powder according to the proportion;

Step 200, forming the briquette, adding the mixed coal samples into the test piece box in batches, adding an appropriate amount of water, and using a forming press to form under a certain forming pressure;

Step 300, inject water into the model, inject water into the model through the servo water injection pressure pump until the water outlet pressure reaches the set value, stop the water injection and close the water injection valve;

Step 400, surveying and mapping the pressure distribution, opening the corresponding drainage valve according to the predetermined drainage scheme, processing the pressure data of the pressure sensor with a computer, drawing pressure contours, and analyzing the pressure distribution.

As a preferred technical solution of the present invention, the coal samples used for forming are composed of coal powder and gypsum powder of different coal rock types and particle sizes, and the particle sizes are all larger than 120 mesh and smaller than 200 mesh. Among them, the particle size ratio of bright coal It is: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder = 93:0:7; the particle size ratio of semi-bright coal is: (60-80 mesh coal powder): (120 ~200 mesh coal powder): gypsum powder = 85:8:7; the particle size ratio of semi-dark coal is: (60 ~ 80 mesh coal powder): (120 ~ 200 mesh coal powder): gypsum powder = 70:23 : 7; the particle size ratio of dull coal is: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder=50:43:7.

As a preferred technical solution of the present invention, in step 200, the specific steps of pressure forming are:

Step 201, fully mix the coal sample and an appropriate amount of water one by one to moisten

Step 202, giving forming pressure to the coal sample each time, adding the thickness of the coal seam formed by the coal sample to the equidistant scale line each time, and keeping the forming pressure stable for 1 hour after forming;

Step 203, when forming coal samples successively, implant various sensors into designated positions of the coal seam, and install drainage valves layer by layer according to the layered intervals.

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

Step 301, turn on the servo water injection pressure pump, inject into the model, and close the water outlet valve after water comes out from the water outlet;

Step 302, keep the same water injection rate and continue to inject water, and set the water injection pressure of the servo water injection pressure pump, stop water injection when the pressure at the water injection port reaches 2.1Mpa, continue to inject water when the pressure is less than 2.05Mpa, until the water outlet pressure reaches 2MPa;

Step 303, close the water injection valve, and stop the water injection.

As a preferred technical solution of the present invention, in the above steps, when the servo water injection pressure pump is used to inject water into the cavity until the water exits from the water outlet, while maintaining the same water injection rate to inject water, draw the water injection pressure of the water injection port as a function of time. And when the water injection pressure has a peak value, stop the water injection, and draw water outwards from the water injection port until the water pressure at the water injection port drops to 30-50% of the peak value, and then continue water injection with the same water injection rate again.

As a preferred technical solution of the present invention, in step 400, the predetermined drainage solution specifically includes: opening several mutually independent drainage valves or selectively opening drainage valves at target layers according to requirements.

Compared with prior art, the beneficial effect of the present invention is:

According to the design plan, the present invention can simulate the law of pressure drop transmission under the superimposed state of different permeability coal rock types, and the design of multiple stuck holes in the locking groove makes the experimental device not only suitable for the superimposition of four rock layers, but also can According to the needs, the pipe wall of the drainage valve in the rock formation is covered with small holes, simulating the effect of perforation during the drainage process, and the results transmitted by the sensor are used to draw the pressure contours with a computer, and the pressure distribution law is intuitively analyzed.

Description of drawings

Fig. 1 is the overall structure schematic diagram of the present invention;

Fig. 2 is the sectional structure schematic diagram of experimental device of the present invention;

Fig. 3 is a structural schematic diagram of the drainage pipe of the present invention;

Fig. 4 is a structural schematic diagram of a press of the present invention;

Fig. 5 is a schematic diagram of pressure distribution contours of experimental results of the present invention;

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

Labels in the figure: 1-experimental device; 2-servo water injection pressure pump; 3-water injection pipeline; 4-equidistant scale line; 5-layered interval; 6-drainage pipe; 7-pressure sensor; 8-press machine;

101-Test chamber; 102-Cavity cover; 103-Clamping raised block; 104-Catching groove; 105-Water injection pipe; 106-Water outlet pipe; 107-Pressure gauge; 108-Water injection valve; Valve; 110-barbed wire; 111-biting teeth; 112-airtight gasket;

601-drain valve; 602-through hole; 603-jet conduit; 604-maw cavity;

801-pressure loading platform; 802-pressure guiding column; 803-pressure loading platen; 804-anti-skid positioning pattern.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

As shown in Figures 1 to 4, the present invention provides a device for simulating the differential pressure drop in the coalbed methane drainage process, including an experimental device 1 and a servo water injection pressure pump 2, the experimental device 1 and a servo water injection pressure pump 2 They are connected through the water injection pipeline 3. In the above, the servo water injection pressure pump 2 specifically refers to the combination of the servo motor and the water injection pump. Compared with the simple water injection pump, its distinguishing feature is that the output of the power pump is relatively Single, and the servo motor can be precisely controlled, so in fact the output power can be changed according to the needs. In this embodiment, the pressure of the water injection port is used as the main reference factor for adjusting the power of the servo motor. Changes in pressure to change the power of the servo motor, so as to achieve the purpose of controlling the water injection pressure of the water injection pump.

The experimental device 1 comprises a test cavity 101 and a cavity cover 102, the upper edge of the test cavity 101 is provided with a locking protruding block 103, and the cavity cover 102 passes through the locking recesses arranged on its peripheral edge. The groove 104 is engaged with the engaging protrusion 103 . The airtight connection between the test cavity 101 and the cavity cover 102 is realized through the snap-in connection between the engaging protrusion 103 and the locking groove 102, and further preferably, the locking groove 104 is L-shaped and fixedly mounted on the On the edge of the cavity cover 102, a number of evenly spaced engaging teeth 111 are fixedly installed inside the locking groove 104, and the inner width of the engaging locking teeth 111 is equal to the outer width of the engaging protrusion 103. The engaging protruding block 103 is ring-shaped and fixedly installed on the upper end of the test chamber 101, and the outer side of the engaging protruding block 103 is in contact with the inner wall of the chamber cover 102, and the outer side of the engaging protruding block 103 is in contact with the inner wall of the chamber cover 102. A ring-shaped airtight gasket 112 is provided at the contact portion of the inner wall of the cavity cover 102 . In the above, since the connection position of the locking groove 104 is optional, it can be flexibly adjusted according to the fullness of the coal briquettes, so that this experimental device is not only suitable for the three-layer or four-layer coal rock formation shown in the figure. Overlapping, and the thickness of the coal rock layer and the number of overlapping layers can be designed according to the needs, so as to improve the usability in the actual simulation process.

The bottom of the test chamber 101 and the top of the chamber cover 102 are respectively provided with a water injection pipe 105 and a water outlet pipe 106. In this embodiment, the water injection pipe 105 is arranged at the lower end, and the water outlet pipe is arranged at the upper end, so as to ensure that the water injection process, the pressure is The transmission is relatively uniform in the test cavity 101 of the whole experimental device, preventing the internal pressure from being unevenly distributed due to the formation of dominant channels inside. Both the water injection pipe 105 and the water outlet pipe 106 measure the water pressure through the built-in pressure gauge 107. It should be noted here that the pressure gauge 107 is an unconventional mechanical pressure gauge instead of an electronic one, and furthermore , can transmit the measured pressure data to the computer system or the control system of the servo motor. These two systems have been widely used in the prior art, and their specific structure and working mode will not be discussed here. To repeat, and the water injection pipe 105 and the water outlet pipe 106 are respectively provided with a water injection valve 108 and a water outlet valve 109, and the water outlet pipe 106 is communicated with the test cavity 101 through a conduit through the cavity cover 102, and the water injection pipe 105 is connected with the test chamber. Barbed wire 110 is provided at the junction of the chamber 101 and the junction of the catheter and the test chamber 101 .

In the above, the set barbed wire 110 can prevent the pressure concentration of the water outlet where the briquettes are located and cause the briquettes to be broken due to stress concentration, and avoid changing the shape and particle size of the briquettes at local positions in the entire device, so that the entire device is in in a unified environment.

The inner wall of the test cavity 101 is uniformly provided with four equidistant scale lines 4 parallel to each other from bottom to top, and the equidistant scale lines 4 divide the test cavity 101 into several layered intervals 5, and in each layer Drainage pipes 6 are provided on the side chambers of section 5, and drainage valves 601 are arranged in each of the drainage pipes 6, and multiple sets of pressure sensors perpendicular to the equidistant scale line 4 are arranged in the test chamber 101 7, and each group of pressure sensors 7 is evenly distributed in the longitudinal direction.

In the above content, in addition to the uniform distribution of pressure sensors 7 in the longitudinal direction, the closer they are to the drain valve 601 in the horizontal direction, the denser their arrangement is. Collection of pressure data. Moreover, in the above, the pressure data measured by the pressure sensor 7 is used by the computer to draw the pressure contour, which can intuitively analyze the pressure distribution law.

In addition, the pipe wall of the drainage pipe 6 is provided with several evenly distributed through holes 602. On the inner pipe wall of the test cavity 101 on the drainage pipe 6, numerous through holes 602 are distributed to simulate the holes on the well wall. perforation, and then simulate the effect of perforation in coalbed methane exploitation, and the through hole 602 makes the coal rock communicate with the inside of the drainage pipe 6, and the pipe wall where the through hole 602 is located is fixedly installed with a funnel-shaped jet conduit 603. The connecting end of the jet conduit 603 and the drain pipe 6 is provided with a swim bladder cavity 604, and the set jet conduit 603 and the swim bladder cavity 604 are used to improve the absorption efficiency, and avoid gas or liquid from being discharged incorrectly due to the existence of air pressure. smooth.

As a preference of this embodiment, it also includes a press 8, the press 8 includes a pressure loading platform 801 and four pressure guide columns 802, the pressure guide columns 802 are fixedly installed at the four corners of the test cavity 101, the pressure The loading platform 801 moves vertically along the pressure guide column 802, and can apply vertical pressure smoothly, and a pressure loading platen 803 in the shape of a square prism is fixedly installed at the bottom of the pressure loading platform 801, so that the vertical pressure Balanced, further, on the bottom surface of the pressure loading platen 803, there are fixedly installed a number of intersecting anti-slip positioning lines 804, which can form a pressed anti-slip pattern with the briquettes, and used in conjunction with the pressure loading platen 803, can prevent Sideways sliding due to uneven bottom surface of pressure loaded platen 803 .

In addition, as shown in Figure 6, the present invention also provides a method for simulating the differential pressure drop in the coalbed methane drainage process, including the following steps:

Step 100, coal sample preparation, the coal samples of each coal rock type are crushed with a pulverizer, and a sufficient amount of coal powder with the required particle size is screened, and fully mixed with gypsum powder according to the proportion.

The coal samples used for molding are composed of coal powder and gypsum powder of different coal rock types and particle sizes, and the particle size is greater than 120 mesh and less than 200 mesh. Among them, the particle size ratio of bright coal is: (60-80 mesh coal powder) : (120-200 mesh coal powder): gypsum powder=93:0:7; the particle size ratio of semi-bright coal is: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder= 85:8:7; the particle size ratio of semi-dark coal is: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder = 70:23:7; the particle size ratio of dim coal For: (60 ~ 80 mesh coal powder): (120 ~ 200 mesh coal powder): gypsum powder = 50:43:7.

Step 200, molding coal, adding the mixed coal samples into the test piece box in batches, adding an appropriate amount of water, using a molding press to form according to a certain molding pressure, and removing the press, and sealing the experimental device with a cavity cover .

In this embodiment, different experimental schemes can be set for the stacking of coal samples according to requirements, for example:

From top to bottom, they are dull coal, semi-dark coal, semi-dark coal, and bright coal, or from top to bottom, they are bright coal, semi-bright coal, semi-dark coal, and dull coal. That is to say, in actual coal seams, various Overlapping types are all possible.

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

Step 201, fully mix the coal sample and an appropriate amount of water one by one to moisten

Step 202, giving forming pressure to the coal sample each time, adding the thickness of the coal seam formed by the coal sample to the equidistant scale line each time, and keeping the forming pressure stable for 1 hour after forming;

Step 203, when forming coal samples successively, implant various sensors into designated positions of the coal seam, and install drainage valves layer by layer according to the layered intervals.

Step 300 , injecting water into the model, injecting water into the model through the servo water injection pressure pump until the water outlet pressure reaches the set value, then stopping the water injection and closing the water injection valve.

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

Step 301, turn on the servo water injection pressure pump, inject into the model, and close the water outlet valve after water comes out from the water outlet;

Step 302, keep the same water injection rate and continue to inject water, and set the water injection pressure of the servo water injection pressure pump, stop water injection when the pressure at the water injection port reaches 2.1Mpa, continue to inject water when the pressure is less than 2.05Mpa, until the water outlet pressure reaches 2MPa, this can ensure the entire The formation pressure inside the cavity is between 2MPa and 2.1MPa to avoid high pressure near the water injection valve and low pressure near the water outlet valve, which will affect the simulation effect;

Step 303, close the water injection valve, and stop the water injection.

Moreover, further, in the above steps, when injecting water at the same water injection rate through the servo water injection pressure pump to inject water into the cavity until the water comes out of the water outlet, draw the water injection pressure of the water injection port as a function of time, and when the water injection pressure appears At peak time, stop water injection, and pump water outward from the water injection port until the water pressure at the water injection port drops to 30-50% of the peak value, and then continue water injection at the same water injection rate.

What needs to be added is that the advantages of using the above water injection method are: on the one hand, intermittent water injection can make the water injection efficiency higher, and the pressure distribution in each interval position is relatively uniform; on the other hand, it is because the coal The various components in the rock are relatively complex, and the internal structure is relatively chaotic. There will be some blockages during the pressing and forming of the press. Direct continuous water injection may cause blockage of the channels between the coal seams. The cycle of "water injection-water injection-water injection-water pumping" can avoid temporary blockage of pores and roars, so that the pressure of water injection can be evenly distributed throughout the model.

Step 400, surveying and mapping the pressure distribution, opening the corresponding drainage valve according to the predetermined drainage scheme, processing the pressure data of the pressure sensor with a computer, drawing pressure contours, and analyzing the pressure distribution.

In step 400, the predetermined drainage scheme specifically includes: opening several mutually independent drainage valves or selectively opening drainage valves at target layers according to requirements.

Based on the above, in the present invention, the stacking method of coal rock samples is specifically adopted from top to bottom as dull coal, semi-dark coal, semi-bright coal and bright coal. The distribution law of the pressure drop in the experiment is shown in Figure 5 for the drainage pipe in the layered interval.

As shown in Figure 5, the propagation speed of pressure drop in bright coal is the fastest, while that of dull coal is the slowest, which is consistent with the seepage theory. Moreover, coal seams are low-permeability reservoirs, especially in dull coals, where there may be a threshold pressure gradient. If the pressure drop gradient is smaller than the threshold pressure gradient, the water in them may not flow at all, so that the gas in dull coals cannot It diffuses out through decompression, desorption and absorption, thus becoming "dead gas" and making no contribution to gas production. In addition, Figure 5 is only a graph at a certain moment. In fact, the data of the pressure sensor will change continuously, and the distribution law of pressure drop formed by computer calculation is also a dynamically changing graph, so that the dynamic process of the entire pressure drop can be simulated in detail.

It should be further understood that Figure 5 in this embodiment is only a specific example of the application. It should be understood that even if it is the same design scheme, that is, the coal and rock are stacked in the same way, only the dim Coal, or adopting other drainage schemes, the pressure drop is different, or adopting different coal-rock superposition methods, the pressure drop is also different, which needs to be comprehensively determined according to the coal-rock superposition method and the specific drainage scheme .

It will be apparent to those skilled in the art that the invention is not limited to the details of the above-described exemplary embodiments, but that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Accordingly, the embodiments should be regarded in all points of view as exemplary and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and it is therefore intended that the scope of the invention be defined by the appended claims rather than by the foregoing description. All changes within the meaning and range of equivalents of the elements are embraced in the present invention. Any reference sign in a claim should not be construed as limiting the claim concerned.

Claims (10)

1. A device for simulating the differential pressure drop in the coalbed methane drainage process, comprising an experimental device (1) and a servo water injection pressure pump (2), passing between the experimental device (1) and the servo water injection pressure pump (2) The water injection pipeline (3) is connected, and it is characterized in that: the experimental device (1) includes a test chamber (101) and a chamber cover (102), and the upper edge of the test chamber (101) is provided with a snap-fitting protrusion (103), and the chamber cover (102) is engaged and connected with the engaging protrusion (103) through the locking groove (104) arranged on its peripheral edge, and the bottom of the test chamber (101) and The top of the cavity cover (102) is respectively provided with a water injection pipe (105) and a water outlet pipe (106). A water injection valve (108) and a water outlet valve (109) are respectively provided in the water pipe (105) and the water outlet pipe (106), and the water outlet pipe (106) passes through the chamber cover (102) and the test chamber (101) through a conduit. Connected, the connection between the water injection pipe (105) and the test cavity (101) and the connection between the conduit and the test cavity (101) are all provided with barbed wire (110); The inner wall of the test cavity (101) is evenly provided with four mutually parallel equidistant scale lines (4) from bottom to top, and the equidistant scale lines (4) divide the test cavity (101) into several layered intervals (5), and a drainpipe (6) is provided on the side cavity of each layered interval (5), and a drain valve (601) is arranged in each said drainpipe (6), and the test Multiple groups of pressure sensors (7) perpendicular to the equidistant scale lines (4) are arranged in the cavity (101), and each group of pressure sensors (7) is evenly distributed at equal intervals in the longitudinal direction. 2. The device for simulating differential pressure drop in the coalbed methane extraction process according to claim 1, characterized in that: it also includes a press (8), and the press (8) includes a pressure loading platform (801) and four pressure guide columns (802), the pressure guide columns (802) are fixedly installed at the four corners of the test cavity (101), and the pressure loading platform (801) moves vertically along the pressure guide columns (802), The bottom of the pressure loading platform (801) is fixedly installed with a pressure loading platen (803) in the shape of a square prism, and on the bottom surface of the pressure loading platen (803) are fixedly installed a number of intersecting anti-skid positioning lines (804 ). 3. The device for simulating the differential pressure drop in the coalbed methane extraction process according to claim 1, characterized in that: the locking groove (104) is L-shaped and fixedly installed on the cavity cover (102) On the edge, the inner side of the locking groove (104) is fixedly equipped with several evenly spaced and evenly distributed snapping teeth (111), and the inner width of the snapping teeth (111) is equal to that of the snapping protrusion (103). The outside is wide, and the said engaging protruding block (103) is ring-shaped and fixedly installed on the upper end of the test cavity (101), and the outer side of the engaging protruding block (103) is in contact with the inner wall of the cavity cover (102), A ring-shaped airtight gasket (112) is provided at the contact point between the outer side of the engaging protrusion (103) and the inner wall of the cavity cover (102). 4. A device for simulating the differential pressure drop in the coalbed methane extraction process according to claim 1, characterized in that: the pipe wall of the drainage pipe (6) is provided with several evenly distributed through holes (602 ), the through hole (602) is used to simulate the perforation on the well wall, and connects the coal rock with the drainage pipe (6), and the pipe wall where the through hole (602) is located is fixedly installed with a funnel-shaped The jet conduit (603) is provided with a swim bladder cavity (604) at the connecting end of the jet conduit (603) and the drain pipe (6). 5. A method for simulating differential pressure drop in the coalbed methane extraction process, characterized in that, comprising the steps: Step 100, coal sample preparation, crushing the coal samples of each coal rock type with a pulverizer, screening a sufficient amount of coal powder with the required particle size, and fully mixing with gypsum powder according to the proportion; Step 200, forming the briquette, adding the mixed coal samples into the test piece box in batches, adding an appropriate amount of water, and using a forming press to form under a certain forming pressure; Step 300, inject water into the model, inject water into the model through the servo water injection pressure pump until the water outlet pressure reaches the set value, stop the water injection and close the water injection valve; Step 400, surveying and mapping the pressure distribution, opening the corresponding drainage valve according to the predetermined drainage scheme, processing the pressure data of the pressure sensor with a computer, drawing pressure contours, and analyzing the pressure distribution. 6. the method for differential pressure drop in a kind of simulated coalbed methane extraction process according to claim 5, is characterized in that, the used coal sample of forming is made up of coal powder and gypsum powder of different coal rock types, grain size, grain The average diameter is greater than 120 mesh and less than 200 mesh. Among them, the particle size ratio of bright coal is: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder = 93:0:7; semi-bright coal The particle size ratio of the semi-dark coal is: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder = 85:8:7; the semi-dark coal particle size ratio is: (60-80 mesh coal powder Powder): (120-200 mesh coal powder): gypsum powder = 70:23:7; the particle size ratio of dull coal is: (60-80 mesh coal powder): (120-200 mesh coal powder): gypsum powder =50:43:7. 7. The method for simulating differential pressure drop in a coalbed methane extraction process according to claim 5, characterized in that, in step 200, the specific steps of pressure forming are: Step 201, fully mix the coal sample and an appropriate amount of water one by one to moisten Step 202, giving forming pressure to the coal sample each time, adding the thickness of the coal seam formed by the coal sample to the equidistant scale line each time, and keeping the forming pressure stable for 1 hour after forming; Step 203, when forming coal samples successively, implant various sensors into designated positions of the coal seam, and install drainage valves layer by layer according to the layered intervals. 8. A method for simulating differential pressure drop in the coalbed methane drainage process according to claim 5, characterized in that, in step 300, the specific steps of model water injection are: Step 301, turn on the servo water injection pressure pump, inject into the model, and close the water outlet valve after water comes out from the water outlet; Step 302, keep the same water injection rate and continue to inject water, and set the water injection pressure of the servo water injection pressure pump, stop water injection when the pressure at the water injection port reaches 2.1Mpa, continue to inject water when the pressure is less than 2.05Mpa, until the water outlet pressure reaches 2MPa; Step 303, close the water injection valve, and stop the water injection. 9. A method for simulating the differential pressure drop in the coalbed methane drainage process according to claim 8, characterized in that, in the above steps, when the servo water injection pressure pump is used to inject water into the cavity until the water exits the water outlet, keep When injecting water at the same water injection rate, draw the water injection pressure of the water injection port as a function of time, and when the water injection pressure peaks, stop the water injection, and pump water from the water injection port until the water pressure at the water injection port drops to 30-50% of the peak value , and then continue to inject water at the same water injection rate. 10. A method for simulating differential pressure drop in the coalbed methane drainage process according to claim 5, characterized in that, in step 400, the predetermined drainage scheme is specifically: open several mutually independent Drain valve or selectively open the drain valve of the target layer.