A method for evaluating the risk of water inrush from coal seam floors based on the mechanism of hydraulic fracturing

Abstract

This invention discloses a method for evaluating the risk of water inrush in coal seams based on the hydraulic fracturing and lifting mechanism. The method involves: 1) experimentally measuring and obtaining the fracture pressure and reopening pressure patterns of the floor seam under in-situ stress in the study area; 2) experimentally measuring and verifying the fracture pressure and reopening pressure patterns of the floor seam after mining in the study area; 3) dividing the floor seam into zones using the fracture pressure and reopening pressure as indicators; and 4) performing water inrush analysis based on the zoning of the fracture pressure and reopening pressure. This method, based on the hydraulic fracturing and lifting water inrush mechanism, establishes multi-factor coupled discrimination indicators for the fracture pressure and reopening pressure of the floor seam, and proposes corresponding criteria for water inrush. These indicators can comprehensively reflect the disaster-causing nature under the combined action of stress, rock mechanical properties, and water pressure during mining, achieving a scientific evaluation centered on the water inrush mechanism.

Description

Technical Field

[0001] This invention relates to the field of water hazard prevention technology in underground mining engineering. Specifically, it is a method for assessing the risk of water inrush at the coal seam floor based on the fracturing and lifting mechanism. Background Technology

[0002] Mine water inrush is one of the most threatening disasters in coal mining. Nationwide, the problem of coal seam floor water inrush is particularly prominent in the North China coalfield. The coal seams in the North China coalfield are hosted in Carboniferous and Permian strata, with a base of hundreds of meters of Ordovician limestone (hereinafter referred to as "Ordovician limestone") strata. Due to the well-developed karst fissures in the Ordovician limestone and its replenishment by atmospheric rainfall, the Ordovician limestone aquifer is the main source of coal seam floor water inrush in North China. With the increase in mining years and mining depth, the pressure of the coal seam increases year by year, and the threat of Ordovician limestone water inrush increases accordingly. Conducting a floor water inrush risk assessment before coal seam mining is an important water control measure to ensure safe mining. Therefore, how to conduct a scientific and reasonable floor water inrush hazard assessment is particularly important.

[0003] Currently, the assessment methods for the risk of water inrush in the foundation mainly fall into three categories: assessment methods based on mathematical statistics, assessment methods based on water inrush mechanism, and assessment methods based on multi-source data fusion. Among the mathematical statistics assessment methods, the most representative is the water inrush coefficient method. This method is a statistical assessment method based on the results of early pressurized mining practices, and it has the advantages of being simple, easy to use, and highly accurate. However, as coal mining progresses to deeper levels, it has been found that the assessment results of this method tend to be conservative. Assessment methods based on water inrush mechanism generally start from the perspective of the nature of the water inrush and use mechanical mechanisms as the core for evaluation. However, due to the large number of mechanical parameters involved and the difficulty in obtaining them, this type of method has not been widely used. Among the assessment methods based on multi-source data fusion, the most representative is the vulnerability index method. This method integrates the weight coefficients of multiple main control factors of water inrush, and has the advantage of scientific rationality. However, the selection of the main control factors and the determination of the weights of each factor are greatly affected by subjective factors. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to provide a method for evaluating the risk of water inrush at the bottom of coal seams based on the mechanism of hydraulic fracturing and lifting. This evaluation method is consistent with the nature of deep water inrush, has mechanical theoretical support, is not affected by subjective factors, and has strong applicability.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] A method for assessing the risk of water inrush at the bottom of a coal seam based on the fracturing and lifting mechanism includes the following steps:

[0007] (1) The fracture pressure and reopening pressure of the bottom plate under the stress state in the study area were obtained by field measurement;

[0008] (2) The law of bottom plate fracture pressure and reopening pressure after mining in the study area was obtained by field measurement and verified;

[0009] (3) Use the bottom plate rupture pressure as an indicator to classify the bottom plate into zones;

[0010] (4) Use the pressure of the bottom plate reopening as an indicator to divide the bottom plate into zones;

[0011] (5) Perform water inrush analysis on the bottom plate based on the zoning of the bottom plate rupture pressure and reopening pressure.

[0012] The above-mentioned method for evaluating the risk of water inrush at the bottom of coal seams based on the mechanism of hydraulic fracturing and lifting, in step (1), selects a study area that is not affected by mining disturbance, conducts in-situ stress measurement, and obtains the maximum principal stress σ1 and the minimum principal stress σ3 under the in-situ stress state;

[0013] Rock strata samples were taken from the study area and mechanical strength tests were conducted to obtain the tensile strength σ of different rock strata. T The pore water pressure P0 was obtained by using hydrological observation wells in the aquifer between the coal seam and the Ordovician aquifer.

[0014] The above-mentioned method for assessing the risk of water inrush at the bottom of a coal seam based on the fracturing and lifting mechanism is based on the fracturing pressure formula:

[0015] P b =3σ3-σ1-P0+σ T (1);

[0016] Reopening pressure formula:

[0017] P r =3σ3-σ1-P0 (2);

[0018] Calculate the distribution patterns of bottom plate fracture pressure and reopening pressure in different rock strata under in-situ stress conditions.

[0019] The above-mentioned method for evaluating the risk of water inrush in the coal seam floor based on the fracturing and lifting mechanism, in step (2), actual measurements are taken at different locations of the floor of the goaf after mining in the study area to obtain the maximum principal stress σ1 and the minimum principal stress σ3 after mining.

[0020] The distribution patterns of bottom plate fracturing pressure and reopening pressure in different rock strata after mining were obtained using formulas (1) and (2).

[0021] The rupture pressure of the base plate was verified using the hydraulic fracturing method.

[0022] The above-mentioned method for assessing the risk of water inrush in the coal seam floor based on the hydraulic fracturing mechanism, in step (3), divides the floor into three zones according to the hydraulic fracturing theory:

[0023] When the rupture pressure of the base plate is less than 0, i.e., P b <0, the surrounding rock is in a tensile state, corresponding to the water-conducting fracture zone of the bottom plate, and this range is divided into the fracture free conduction zone;

[0024] When the pressure at which the base plate ruptures is between 0 and the pressure of the pressurized water P, i.e., 0 < P b <P, where the pressure of the confined water is higher than the rupture pressure of the base plate, and the confined water conducts fracturing and lifts within this range, then this range is defined as the fracturing and lift zone;

[0025] When the pressure at which the base plate ruptures is higher than the pressure of the pressurized water, i.e., P b If the pressure of the pressurized water is lower than the rupture pressure of the base plate, and the pressurized water cannot conduct fracturing within this range, then this range is designated as the water-resistant fracturing zone.

[0026] In the above-mentioned method for evaluating the risk of coal seam floor water inrush based on the fracturing and lifting mechanism, in step (4), when the integrity of the floor strata is poor, the mechanical strength of the floor strata itself is low and cannot impede the confined water. In this case, the tensile strength σ of the floor strata is ignored. T The base plate is divided into three zones using the base plate reopening pressure instead of the rupture pressure:

[0027] When the pressure of the base plate reopening is less than 0, i.e., P r If <0, then this range is classified as the reopened free conduction zone;

[0028] When the pressure of the bottom plate reopening is between 0 and the pressure of the pressurized water P, i.e., 0 < P r <P, where the pressurized water pressure is higher than the reopening pressure of the base plate, and the pressurized water rises within this range, then this range is designated as the reopening rise zone;

[0029] When the pressure of the bottom plate reopening is higher than the pressure of the pressurized water, i.e., P r If the pressure of the pressurized water is lower than the pressure of the base plate reopening, and the pressurized water cannot be guided up within this range, then this range is classified as the water-resistant reopening zone.

[0030] The above-mentioned method for assessing the risk of water inrush at the coal seam floor based on the fracturing and lifting mechanism is based on the fracturing and lifting criterion P. b <P, evaluate the three zones of the base plate fracture pressure classification:

[0031] If the fracturing guide zone touches the confined aquifer, the confined water will be fractured and guided up to the ruptured free guide zone within the intact base plate, eventually causing a water inrush.

[0032] If the aquifer is above a hydraulic fracturing zone, water inrush will not occur.

[0033] The above-mentioned method for assessing the risk of water inrush at the coal seam floor based on the fracturing and lifting mechanism is based on the fracturing and lifting criterion P. r<P, evaluate the three zones of pressure division for base plate reopening:

[0034] If the reopened free riser zone touches the confined aquifer, the confined water will be fractured and rise within the intact base plate to the reopened free riser zone, ultimately causing a water inrush.

[0035] If the aquifer is above a water-resistant reopening zone, then water inrush will not occur.

[0036] The above-mentioned method for assessing the risk of water inrush at the bottom of a coal seam based on the hydraulic fracturing and lifting mechanism classifies the integrity of rock strata according to the joints and fractures, and uses the rock index RQD classification.

[0037] When RQD > 90, or RQD = 75-90, the rock strata are of good integrity and the "fracture pressure" index should be used for evaluation.

[0038] When RQD = 50–75, RQD = 25–50, or RQD < 25, the integrity of the rock strata is poor, and the "reopening pressure" index should be used for evaluation.

[0039] The technical solution of the present invention achieves the following beneficial technical effects:

[0040] The water inrush mechanism during pressurized coal seam mining, where confined water is driven up by fracturing within the floor, is considered to be the combined effect of floor rock stress, physical and mechanical properties, and water pressure. Under the influence of mining activities, the floor stress changes, and if the floor fracturing pressure drops below the water pressure, water inrush will occur. Therefore, a coupled stress model is established.

[0041] (σ1, σ3), rock mechanical strength (σ T ), the pressure of the base plate rupture (P0) under pressurized water pressure (P) b ) and reopening pressure (P) r The evaluation indicators aim to achieve a scientific and reasonable evaluation of water inrush in the coal seam from the perspective of the mechanism of water inrush in pressurized coal seam mining, so as to guide the work of preventing and controlling water inrush in pressurized coal seam mining.

[0042] Based on the hydraulic fracturing and water inrush mechanism, this method establishes a multi-factor-coupled discrimination index for floor rupture pressure and reopening pressure, as well as corresponding floor water inrush criteria. The index can comprehensively reflect the disaster-causing nature under the combined action of stress, rock mechanical properties and water pressure during the mining process, and achieve the scientific evaluation purpose with water inrush mechanism as the core. Attached Figure Description

[0043] Figure 1 Schematic diagram of the three-zone structure of the base plate of this invention

[0044] Figure 2 Numerical computation model;

[0045] Figure 3a The pressure field is the initial reopening pressure; Figure 3b The pressure field represents the initial rupture pressure;

[0046] Figure 4a The pressure field for post-mining reopening pressure; Figure 4b The pressure field represents the post-mining fracturing pressure.

[0047] Figure 5 For measuring the pressure field of the base plate. Detailed Implementation

[0048] principle:

[0049] The mechanism of water inrush caused by hydraulic fracturing in the foundation plate, based on the theory of hydraulic fracturing, states that when the pressure of the confined water P is higher than the rupture pressure P of the foundation plate... b When the pressurized water is in a state of pressure fracturing and rises within the base plate, a water inrush will occur when the pressurized water fracturing and rises through the channel and the base plate failure zone.

[0050] Fracturing lift formula:

[0051] P b =3σ3-σ1-P0+σ T

[0052] In the formula P b σ0 is the rupture pressure, MPa; σ1 and σ3 are the maximum and minimum principal stresses, MPa, respectively; P0 is the pore water pressure, MPa; σ0 is the pore water pressure, MPa; T It represents the tensile strength of the rock strata.

[0053] Rupture pressure P b During the hydraulic fracturing process of the base plate, the pressurized water overcomes the surrounding rock stress and rock mechanical strength, causing the pressure at which the base plate cracks to become the fracturing pressure, P. b =3σ3-σ1-P0+σ T (1).

[0054] Reopening pressure P r The pressure at which pressurized water overcomes the surrounding rock stress and reopens the fractures in the base plate during hydraulic fracturing is called the reopening pressure, P. r =3σ3-σ1-P0(2).

[0055] The specific evaluation process includes the following steps:

[0056] (1) The fracture pressure and reopening pressure of the bottom plate under the stress state in the study area were obtained by field measurement;

[0057] A study area unaffected by mining disturbance was selected for in-situ stress measurement to obtain the maximum principal stress σ1 and minimum principal stress σ3 under the in-situ stress state. Rock strata samples were taken from the study area for mechanical strength testing to obtain the tensile strength σ of different rock strata. T ;

[0058] Then, based on formulas (1) and (2), the distribution patterns of bottom plate fracture pressure and reopening pressure in different rock strata under in-situ stress state before mining are calculated.

[0059] (2) The law of bottom plate fracture pressure and reopening pressure after mining in the study area was obtained by field measurement and verified;

[0060] Field measurements were conducted at different locations on the bottom plate of the goaf in the study area after mining to obtain the maximum principal stress σ1 and the minimum principal stress σ3 after mining; the pore water pressure P0 was obtained through the hydrological observation well of the aquifer between the coal seam and the Ordovician aquifer.

[0061] The distribution patterns of bottom plate fracturing pressure and reopening pressure in different rock strata after mining were obtained using formulas (1) and (2).

[0062] The rupture pressure of the base plate was verified using the hydraulic fracturing method.

[0063] (3) Use the bottom plate rupture pressure as an indicator to classify the bottom plate into zones;

[0064] When the bottom strata are relatively intact, according to the theory of hydraulic fracturing, the bottom strata are divided into three zones:

[0065] When the rupture pressure of the base plate is less than 0, i.e., P b <0, the surrounding rock is in a tensile state, corresponding to the water-conducting fracture zone of the bottom plate, and this range is divided into the fracture free conduction zone;

[0066] When the pressure at which the base plate ruptures is between 0 and the pressure of the pressurized water P, i.e., 0 < P b <P, where the pressure of the confined water is higher than the rupture pressure of the base plate, and the confined water conducts fracturing and lifts within this range, then this range is defined as the fracturing and lift zone;

[0067] When the pressure at which the base plate ruptures is higher than the pressure of the pressurized water, i.e., P b If the pressure of the pressurized water is lower than the rupture pressure of the base plate, and the pressurized water cannot conduct fracturing within this range, then this range is designated as the water-resistant fracturing zone.

[0068] (4) Use the reopening pressure as an indicator to divide the base plate into zones;

[0069] When the integrity of the bottom rock strata is poor, the mechanical strength of the bottom rock strata itself is low and cannot impede the confined water. In this case, the tensile strength σ of the bottom rock strata can be ignored. T The base plate is divided into three zones using the base plate reopening pressure instead of the rupture pressure:

[0070] When the pressure of the base plate reopening is less than 0, i.e., P r If <0, then this range is classified as the reopened free conduction zone;

[0071] When the pressure of the bottom plate reopening is between 0 and the pressure of the pressurized water P, i.e., 0 < P r <P, where the pressurized water pressure is higher than the reopening pressure of the base plate, and the pressurized water rises within this range, then this range is designated as the reopening rise zone;

[0072] When the pressure of the bottom plate reopening is higher than the pressure of the pressurized water, i.e., P r If the pressure of the pressurized water is lower than the pressure of the base plate reopening, and the pressurized water cannot be guided up within this range, then this range is classified as the water-resistant reopening zone.

[0073] (5) Perform water inrush analysis on the bottom plate based on the zoning of the bottom plate rupture pressure and reopening pressure.

[0074] According to the fracturing lift criterion P b <P, evaluate the three zones of the base plate fracture pressure classification:

[0075] If the fracturing guide zone touches the confined aquifer, the confined water will be fractured and guided up to the ruptured free guide zone within the intact base plate, eventually causing a water inrush.

[0076] If the aquifer is above a hydraulic fracturing zone, water inrush will not occur.

[0077] Alternatively, based on the fracturing lift criterion P r <P, evaluate the three zones of pressure division for base plate reopening:

[0078] If the reopening of the riser strip touches the confined aquifer, the confined water will fracture within the intact base slab and rise to the strip, ultimately causing a water inrush.

[0079] If the aquifer is located in a non-reopenable zone, then water inrush will not occur.

[0080] The integrity of rock strata is classified based on joints and fissures. It is believed that the Rock Quality Determination (RQD) classification method is sufficient.

[0081] Good (RQD > 90) and relatively good (RQD = 75-90) indicate good rock strata integrity, and "fracture pressure" should be used as an indicator for evaluation.

[0082] Poor (RQD = 50–75), poor (RQD = 25–50), and extremely poor (RQD < 25) indicate poor stratum integrity and should be evaluated using "reopening pressure" as an indicator.

[0083] The thickness of the non-fracturing or non-reopening zone within the base plate is crucial to whether a water inrush occurs. When the water-resistant fracturing or non-reopening zone touches the confined aquifer, it is assumed that the confined water will fracture and rise within the base plate, ultimately causing a water inrush.

[0084] This method, through the study of the risk assessment method for water inrush in the deep coal seam under pressure mining in the Kailuan mining area, was applied to the deep pressure mining practice in the Linxi Mine of the Kailuan mining area. This method provides strong technical support for various mines in Kailuan to solve the problem of safe mining under pressure in deep Ordovician limestone water and improve the prevention and control of hidden dangers of deep Ordovician limestone water hazards.

[0085] (1) The fracture pressure and reopening pressure of the bottom plate under the stress state in the study area were obtained by field measurement;

[0086] Since the mine lacks the conditions for actual measurement, the geostress was referenced from the measured geostress fitting law of sedimentary rocks in China; borehole sampling was carried out on the bottom plate, and the tensile strength of different lithologies was obtained by Brazilian splitting method; based on the geostress fitting law, the reopening pressure distribution law of the rock strata under geostress state was calculated.

[0087] Measured Fitting Laws of Geostress in Sedimentary Rocks in China (Jing Feng, Sheng Qian, Zhang Yonghui, et al. Statistical Analysis of Geostress Distribution Laws in Rocks of Different Geological Genesis [J]. Rock and Soil Mechanics, 2008(07):1877-1883):

[0088] σ v =0.0263H

[0089] σ H =0.0240H + 4.9125

[0090] σ h =0.0183H + 1.5673

[0091] In the formula, H is the burial depth, in meters; σ v For vertical stress, MPa; σ H The maximum horizontal principal stress is σ, MPa; h The minimum horizontal principal stress is , MPa.

[0092] The tensile strength of the rocks was determined using the Brazilian splitting method from 16 core samples from deep boreholes in the Linxi Mine.

[0093] Distribution law of reopening pressure of rock strata under in-situ stress state:

[0094] P r =3σ3-σ1=3(0.0183H+1.5673)-0.0240H-4.9125=0.0309H-0.2106

[0095] (2) The law of bottom plate fracture pressure and reopening pressure after mining in the study area was obtained by field measurement and verified;

[0096] Since there are no conditions for underground construction, the numerical simulation results will be used as the basis for assessment.

[0097] (3) Use the bottom plate rupture pressure as an indicator to classify the bottom plate into zones;

[0098] (4) Use the reopening pressure as an indicator to divide the base plate into zones;

[0099] Using this evaluation method, and taking actual working conditions and geostress patterns as a background, this study takes a working face at an elevation of approximately -1000m in the deep pressurized mining area of ​​the Linxi Mine as an example, and applies the finite difference Flac method. 3D Numerical simulation was used to obtain the variation patterns of floor stress, reopening pressure, and rupture pressure, and to analyze and evaluate the risk of water inrush at the working face floor. Since the mine currently lacks the conditions for actual in-situ stress measurement, the in-situ stress in the numerical calculations was set with reference to the measured in-situ stress patterns of sedimentary rocks in China.

[0100] The ground elevation is +29.76m, the Ordovician limestone water level is -14m, the thin limestone water level is -400m, the stratum dip angle is 16°, and the working face bottom plate elevation is -1000m; the true thickness of the working face is 5.77m, the thickness in the plumb line is 6m, the working face dip is 110m, and the working face strike is 1000m; the center of the working face inside the model is 171.6m from the top surface of the model, the center of the working face is 222.4m from the bottom surface of the model, and the total height of the model is 400m.

[0101] The initial equilibrium state of the model was obtained through numerical calculations. Based on the formulas for reopening pressure and fracturing pressure, the reopening pressure field and fracturing pressure field of the surrounding rock before coal seam mining were obtained. Analysis of the pre-mining reopening pressure field and fracturing pressure field, combined with the floor survey lines perpendicular to the strata, showed that in the initial state, the reopening pressure and fracturing pressure of the strata generally increased with increasing burial depth. The reopening pressure decreased under the influence of pore water pressure in the aquifer; the fracturing pressure fluctuated under the influence of the tensile strength of the rock strata. Figure 3a and Figure 3b As shown.

[0102] Based on actual working conditions, coal seam mining simulation calculations were performed using the initial numerical model to obtain a post-mining numerical model. According to the formulas for reopening pressure and fracturing pressure, the reopening pressure field and fracturing pressure field of the surrounding rock after mining were obtained. Analysis of the post-mining reopening pressure field and fracturing pressure field, combined with floor survey lines perpendicular to the strata, revealed a significant attenuation of the floor reopening pressure field and fracturing pressure field. The pressure fields on both sides of the working face showed concentration, but the influence on the pressure field gradually weakened with increasing distance from the mining area. In the floor reopening pressure field, reopening pressures of 0 and 11.51 (Ordovician limestone water pressure) were used as critical values ​​for floor zoning. The floor can be divided into three zones: a reopening free-rising zone (Pr < 0), a reopening rising zone (0 < Pr < 11.51 MPa), and a water-resistant reopening zone (Pr > 11.51 MPa). Similarly, using 0 and 11.51 as critical values ​​for zoning the post-mining reopening pressure survey lines of the floor, the following can also be used: The base plate is divided into three zones: the free-rising zone, the re-rising zone, and the water-resistant re-rising zone, with thicknesses of 48.64 m, 14.97 m, and 95.05 m, respectively. In the base plate fracture pressure field, the fracture pressure of 0 and 11.51 MPa are used as critical values ​​for zoning, resulting in three zones: the free-rising zone (Pr < 0), the fracture-rising zone (0 < Pr < 11.51 MPa), and the water-resistant fracture zone (Pr > 11.51 MPa). Similarly, using 0 and 11.51 as critical values ​​for zoning the post-mining re-rising pressure measurement line, it can also be divided into three zones: the free-rising zone, the fracture-rising zone, and the water-resistant fracture zone, with thicknesses of 47.72 m, 14.09 m, and 96.85 m, respectively.

[0103] (5) Perform water inrush analysis on the bottom plate based on the zoning of the bottom plate rupture pressure and reopening pressure.

[0104] Based on the actual production at Linxi Mine, Flac3D was used to numerically solve the working face at an elevation of -1000m. Combined with the measured geostress fitting law of sedimentary rocks in my country, the reopening pressure field and fracturing pressure field of the floor before and after coal seam mining were obtained. Reopening pressure and fracturing pressure were used as evaluation indicators, and three zones were divided using 0 and Ordovician limestone water pressure as critical values. The thickness of the three zones under the two indicators was obtained, and the water-resistant reopening zone (e.g.) was also determined. Figure 4a ), water-resistant fracture zone (such as Figure 4b The thickness of each layer is nearly 100 meters. It is believed that, under the condition of an intact base slab, the risk of water inrush at the working face is extremely low. Figure 5 As shown.

[0105] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method for evaluating the risk of water inrush at the bottom of a coal seam based on the fracturing and lifting mechanism, characterized in that, Includes the following steps: (1) The fracture pressure and reopening pressure of the bottom plate under the stress state in the study area were obtained by field measurement; (2) The fracture pressure and reopening pressure of the bottom plate after mining in the study area were measured and verified. (3) When the integrity of the bottom strata is relatively good, the bottom strata are divided into zones based on the bottom strata fracture pressure. (4) When the integrity of the bottom strata is poor, the bottom strata are divided into zones using the bottom strata reopening pressure as an indicator; (5) Perform water inrush analysis on the bottom plate based on the zoning of the bottom plate rupture pressure and reopening pressure; In step (1), a study area unaffected by mining disturbance is selected, and geostress measurement is carried out to obtain the maximum horizontal principal stress σ1 and the minimum horizontal principal stress σ3 under geostress conditions. Rock strata samples were taken from the study area and mechanical strength tests were conducted to obtain the tensile strength σ of different rock strata. T The pore water pressure P0 was obtained by using hydrological observation wells in the aquifer between the coal seam and the Ordovician aquifer. According to the rupture pressure formula: P b =3σ3-σ1- P 0 +σ T (1); Reopening pressure formula: P r =3σ3-σ1- P 0 (2); Calculate the distribution patterns of bottom plate fracture pressure and reopening pressure in different rock strata under in-situ stress conditions; In step (2), the floor of the goaf in the study area after mining is measured at different locations to obtain the maximum horizontal principal stress σ1 and the minimum horizontal principal stress σ3 after mining. The distribution patterns of bottom plate fracturing pressure and reopening pressure in different rock strata after mining were obtained using formulas (1) and (2). The rupture pressure of the base plate was verified using the hydraulic fracturing method; In step (3), based on the theory of hydraulic fracturing, the bottom plate is divided into three zones: When the rupture pressure of the base plate is less than 0, i.e., P b <0, the surrounding rock is in a tensile state, corresponding to the water-conducting fracture zone of the bottom plate, and this range is divided into the fracture free conduction zone; When the pressure at which the base plate ruptures is between 0 and the pressure of the pressurized water P, i.e., 0 < P b <P, where the pressure of the confined water is higher than the rupture pressure of the base plate, and the confined water conducts fracturing and lifts within this range, then this range is defined as the fracturing and lift zone; When the pressure at which the base plate ruptures is higher than the pressure of the pressurized water, i.e., P b >P, if the pressure of the confined water is lower than the rupture pressure of the base plate, and the confined water cannot conduct fracturing and lift within this range, then this range is classified as a water-resistant fracturing zone; In step (4), the mechanical strength of the bottom rock layer itself is low and it cannot impede the confined water. Therefore, the tensile strength σ of the bottom rock layer is ignored. T The base plate is divided into three zones using the base plate reopening pressure instead of the rupture pressure: When the pressure of the base plate reopening is less than 0, i.e., P r If <0, then this range is classified as the reopened free conduction zone; When the pressure of the bottom plate reopening is between 0 and the pressure of the pressurized water P, i.e., 0 < P r <P, where the pressurized water pressure is higher than the reopening pressure of the base plate, and the pressurized water rises within this range, then this range is designated as the reopening rise zone; When the pressure of the bottom plate reopening is higher than the pressure of the pressurized water, i.e., P r >P, if the pressurized water pressure is lower than the reopening pressure of the base plate, and the pressurized water cannot be guided up within this range, then this range is classified as the water-resistant reopening zone; According to the fracturing lift criterion P b <P, evaluate the three zones of the base plate fracture pressure classification: If the fracturing guide zone touches the confined aquifer, the confined water will be fractured and guided up to the ruptured free guide zone within the intact base plate, eventually causing a water inrush. If the aquifer is above a hydraulic fracturing zone, water inrush will not occur; According to the fracturing lift criterion P r <P, evaluate the three zones of pressure division for base plate reopening: If the reopened free riser zone touches the confined aquifer, the confined water will be fractured and rise within the intact base plate to the reopened free riser zone, ultimately causing a water inrush. If the aquifer is above a water-resistant reopening zone, then water inrush will not occur.

2. The method for evaluating the risk of water inrush from the coal seam floor based on the fracturing and lifting mechanism according to claim 1, characterized in that, Based on the integrity of rock strata according to joints and fissures, the rock quality index (RQD) can be used for classification. When RQD > 90, or RQD = 75~90, the rock strata are of good integrity and the "fracture pressure" index should be used for evaluation. When RQD=50~75, RQD=25~50, or RQD<25, the integrity of the rock strata is poor, and the "reopening pressure" index should be used for evaluation.

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