阅读说明：本技术 一种ucg-ccs覆岩裂隙发育高度预测方法 (UCG-CCS overlying strata fracture development height prediction method ) 是由 徐友友 郭广礼 李怀展 于 2020-04-01 设计创作，主要内容包括：一种UCG-CCS覆岩裂隙发育高度预测方法。适用于煤炭气化活动后的二氧化碳燃空区封存裂隙发育高度的研究。其步骤为：1)根据无井式煤炭地下气化“双曲线”型隔离煤柱的结构特点,判断隔离煤柱形态；2)兼顾气化过程中的高温效应和煤柱的长期稳定性,构建煤柱二次剥离模型；3)通过分析耦合二氧化碳后的煤柱应力分布,进而构建二氧化碳封存后的煤柱应力分布模型；4)通过建立的数值模型,结合影响因素,利用层次分析法确定影响岩层裂隙发育高度中的权重；5)据此提出一种二氧化碳封存后的覆岩裂隙发育高度预测方法。这种方法兼顾了煤炭地下气化后的煤柱长期稳定性及不同应力分布条件下的覆岩结构,从影响因素出发,化繁为简,可以有效的达到预测二氧化碳封存后的覆岩裂隙发育高度的目的。(A method for predicting the development height of UCG-CCS overlying strata fractures. The method is suitable for the research on the development height of the sealing crack of the carbon dioxide combustion space area after coal gasification activity. The method comprises the following steps: 1) judging the form of the isolated coal pillar according to the structural characteristics of a hyperbolic-type isolated coal pillar for underground gasification of the shaft-free coal; 2) considering both the high-temperature effect in the gasification process and the long-term stability of the coal pillar, and constructing a coal pillar secondary stripping model; 3) constructing a coal pillar stress distribution model after carbon dioxide is sealed and stored by analyzing the coal pillar stress distribution after carbon dioxide coupling; 4) determining the weight in the development height of the rock stratum fracture by using an analytic hierarchy process through the established numerical model and combining the influence factors; 5) therefore, a method for predicting the development height of the overlying strata fracture after carbon dioxide sequestration is provided. The method gives consideration to the long-term stability of the coal pillar after underground coal gasification and the overlying strata structure under different stress distribution conditions, changes the process from traditional to simple from influencing factors, and can effectively achieve the purpose of predicting the overlying strata fracture development height after carbon dioxide sequestration.)

1. A prediction method for the development height of UCG-CCS overlying strata fractures is characterized by comprising the following steps:

the method comprises the steps of constructing a hyperbolic-type isolation coal pillar normally underground, constructing an isolation coal pillar secondary stripping model which takes high-temperature effect in the gasification process and long-term stability requirements of the coal pillar into consideration, analyzing underground gasification stress distribution of coal, establishing an isolation coal pillar mechanical model after carbon dioxide sequestration, determining overburden fracture development height influence factor weight by using an analytic hierarchy process, establishing a overburden fracture development height model after carbon dioxide sequestration by using the weight, and completing prediction of overburden fracture development height through model analysis.

2. The method for predicting the fracture development height of the overburden rock by coupling underground coal gasification and carbon dioxide sequestration according to claim 1, which is characterized by comprising the following specific steps of:

step 1: in order to ensure stability, the underground isolation coal pillar is set to be a hyperbolic-type coal pillar structure generated by a well-free underground coal gasification technology, and curve arcs on two sides of the underground isolation coal pillar structure are semicircular curve arcs taking the thickness of a coal seam as the diameter;

step 2, constructing a coal pillar secondary stripping model by using F L AC3D numerical simulation software under the condition of considering coal pillar stripping generated by long-term weathering of the coal pillar, wherein the construction of the coal pillar secondary stripping model needs to consider both high-temperature effect in the gasification process and long-term stability requirement of the coal pillar, and the coal pillar secondary stripping model comprises a coal pillar primary stripping model under a overburden load and a coal pillar secondary stripping model under the overburden load;

step 3, establishing a secondary stripping model of the coal pillar by using F L AC3D numerical simulation software, simulating the change of the gasified coal, specifically, applying upward stress (7.38Mpa) which meets the requirement of supercritical carbon dioxide to a top plate to simulate the stress of the carbon dioxide after sealing, extracting the stress value of the top plate of the coal seam to obtain the stress distribution of the gasified and sealed coal, and further establishing a coal pillar stress distribution model after sealing carbon dioxide by analyzing the stress change rules before and after sealing;

and 4, determining the development height of the overlying strata fracture after carbon dioxide sequestration by utilizing an analytic hierarchy process according to an F L AC3D numerical model and by combining the influence factors including the overlying strata structure, the working face inclination length, the mining depth, the mining thickness and the coal seam inclination angle to determine the weight of each factor in the development height of the overlying strata fracture and integrating the actual load, the actual sequestration stress and the distribution size data of the coal pillar stress of the overlying strata, thereby achieving the purpose of predicting the development height of the overlying strata fracture.

3. The method for predicting the overburden fracture development height after the underground coal gasification coupling carbon dioxide sequestration according to claim 2, wherein the specific steps of establishing a coal pillar secondary stripping model are as follows:

1) determining the shape of the gasified isolation coal pillar to be a hyperbolic curve type according to the characteristic of double-channel control retreating gasification of the underground gasification of the shaft-free coal;

2) according to the dead weight stress of a rock stratum and the hyperbolic characteristic of an isolation coal pillar, the hyperbolic isolation coal pillar stripping process is abstracted into a stripping process with 2 stages:

firstly, the bent part of the upper half part of the coal pillar is stripped to form a rectangular structure, the other positions are not stripped to be used as a primary stripping model,

then stripping the upper half part of the rectangular coal pillar to form a curve shape, and taking the rectangular coal pillar without stripping at other positions as a secondary stripping model;

after the coal pillar is stripped twice, the coal pillar is maintained in a fixed form and cannot be changed, and mathematical calculation is better performed, so that the boundary of the stripped coal pillar is approximately equivalent to a curve, the boundary curve can be approximately regarded as 1/4 ellipse, the oblateness of the boundary curve is approximately equal to the lower half part of the coal pillar, and the oblateness of the boundary curve is smaller, so that the coal pillar can be equivalent to an oblateness triangle for calculation;

the calculated secondary stripping model for building the coal pillar is as follows:

primary stripping of the model:

secondary stripping of the model:

in the formula (d)₀The length of the coal pillar bending part; d₁The width of the stacked body stacked near the coal pillar; m represents the coal thickness, k represents the coefficient of crushing and expansion and theta represents the angle of repose of the heap; gamma is the average volume weight of the overburden, and H is the average mining depth of the overburden; d. b represents the top width of the coal pillar and the equivalent combustion space area width after the isolated coal pillar is stripped for one time; d₁₀、b₁Representing the top width of the coal pillar after the secondary stripping of the isolated coal pillar and the equivalent width of the combustion space area, wherein L is the advancing distance of the gasification working surface;₁the roof strata collapse angle is obtained.

4. The method for predicting the overburden fracture development height after the carbon underground gasification coupling carbon dioxide sequestration according to claim 2, wherein the specific steps for constructing the coal pillar stress distribution model are as follows:

1) establishing a numerical model by using F L AC3D numerical simulation software and the existing drilling data, and simulating and excavating a plurality of working faces to simulate the stability of a hyperbolic briquette column under the condition of twice stripping;

2) through a numerical simulation means, F L AC3D codes are used for extracting the stress of a coal pillar top plate, Origin software is used for drawing, and the stress distribution of the UCG-CCS coal pillar is captured and sealed by comparing the UCG and the carbon dioxide coupled with the underground coal gasification, so that the difference of the distribution sizes of the UCG and the UCG-CCS coal pillar is analyzed;

3) according to the retreating type ignition control technology of underground coal gasification, in the whole gasification process, isolation coal pillars are reserved among gasification channels, so that the influence condition of sealing stress is increased in a stripe mining coal pillar stress distribution model, long-term underground water influence is considered, the influence of an underground water weakening function is considered on a UCG-CCS coal pillar stress distribution model, and a new coal pillar stress distribution model is finally constructed and distributed as follows:

in particular, the stress distribution function

Wherein f (x) is the overall stress distribution function of the coal pillar, D is half of the width of the coal pillar, and D₀A, b, c is the width of the coal pillar single-side stripping₁D is a model coefficient which represents the change degree of the stress of the coal pillar along with the increase of the width of the coal pillar;

when x → d₀D, the position is actually the edge of the stripped coal pillar, the stress of the position is the sealing stress and the caving gangue side stress, and the characteristic that the stress is reduced along with the increase of the injection distance is considered, so the sealing attenuation coefficient is multiplied on the basis; when x → Y-D, the pillar stress reaches the peak value; wherein K 'is a sealing pressure attenuation coefficient (0-1), K' is a side stress coefficient, sigma₁The maximum principal stress is shown as follows according to Wilson two-region constraint theory, wherein gamma is the average volume weight of the stratum, H is the mining depth, L is the gasification channel propelling distance:σ₃γ H, wherein: c is the cohesion of the coal column, Mpa,the internal friction angle, the cohesion and the internal friction angle of the coal pillar can be measured through experiments, the above formulas are combined, parameters are solved, and the result is as follows:

in addition, after the supercritical carbon dioxide is injected, because the viscosity of the supercritical carbon dioxide is smaller than that of a liquid state, the diffusion speed is higher than that of the liquid by two orders of magnitude, so that after the supercritical carbon dioxide is injected, the carbon dioxide can be quickly filled in a combustion space area and can be diffused into a coal pillar along a primary crack, certain influence can be caused on the stability of the coal pillar, and the influence function is similar to an underground water influence function, so that a water weakening function is introduced and corrected to reflect the influence of the immersion of the supercritical carbon dioxide on the coal pillar:

g_(c)＝(1-R_x)(1-w_x)²+K₁R_x

in the formula: r_xIs the pillar softening function; w is a_xAs a function of water content, R_xAnd w_xAll of which are continuously reduced along with the distance of the coal pillar, and the influence area is a yield area g_(cq)And elastic nucleus region g_(ce)；K₁Is a correction factor;

the stress distribution above the final isolated pillar should be:

namely a UCG-CCS coal column stress distribution model.

5. The method for predicting the overburden fracture development height after the carbon underground gasification coupling carbon dioxide sequestration according to claim 1, wherein the method for predicting the overburden fracture development height in the step 2 specifically comprises the following steps:

1) combining with literature analysis, summarizing and summarizing more key influence factors, and judging the weight of each factor on the fracture development height by using an analytic hierarchy process;

2) according to the weight, controllable influence factors increased due to different sequestration conditions are combined: actual load factors of the overlying strata, actual sealing stress factors and the stress distribution of the coal pillars; the height of the overburden fracture development after the carbon dioxide is sealed is predicted by using the following formula,

H＝f(C₁、C₂、C₃、C₄、C₅、C₀、σ、F_s、f(x))

wherein: f is an influence function; c₀The controllable influence factors are increased by different sealing conditions and are determined by the specific place for sealing; sigma is the actual load of the overburden; f_sThe stress is actually sealed; and f (x) shows the size and distribution of the stress distribution of the coal pillar.

Technical Field

The invention relates to a method for predicting the development height of a overburden fracture, in particular to a method for predicting the development height of the overburden fracture after underground coal gasification coupled with dioxide sequestration.

Technical Field

In recent years, evidence shows that global greenhouse effect is becoming more and more intense, which is caused by the utilization of fossil energy with overload, with the continuous development of science and technology and the adjustment of energy structures of various countries, the proportion of fossil energy, especially coal resources, is gradually decreasing, and the proportion of clean energy is gradually increasing, so that the global clean energy revolution is being developed, and the underground coal gasification technology is gradually applied to the global market as one of the main means for the "fluidized" mining of coal resources, and meanwhile, the problem of the utilization of underground space in a combustion space area caused by the underground coal gasification technology is urgently needed to be solved.

If the CO generated by underground coal gasification and other industries is used₂The CO in the atmosphere can be reduced by sealing and storing the coal in the underground gasification combustion space area₂The content of the carbon dioxide can also improve the utilization rate of the combustion space area, the greenhouse effect can be effectively relieved to a certain extent, the combination of the technology is called as underground coal gasification coupling carbon dioxide capture and sealing UCG-CCS, and the safe application of the technology is realized. This is also the core technology of the present invention.

Disclosure of Invention

Aiming at the technical problems, the method for predicting the development height of the overlying strata fracture after the underground coal gasification coupling carbon dioxide sequestration is simple and has high prediction precision.

In order to realize the technical purpose, the method for predicting the development height of the overlying strata fracture after the underground coal gasification coupling dioxide sequestration comprises the following steps: the method comprises the following steps:

the method comprises the steps of constructing a hyperbolic-type isolation coal pillar normally underground, constructing an isolation coal pillar secondary stripping model which takes high-temperature effect in the gasification process and long-term stability requirements of the coal pillar into consideration, analyzing underground gasification stress distribution of coal, establishing an isolation coal pillar mechanical model after carbon dioxide sequestration, determining overburden fracture development height influence factor weight by using an analytic hierarchy process, establishing a overburden fracture development height model after carbon dioxide sequestration by using the weight, and completing prediction of overburden fracture development height through model analysis.

The method comprises the following specific steps:

step 1: in order to ensure stability, the underground isolation coal pillar is set to be a hyperbolic-type coal pillar structure generated by a well-free underground coal gasification technology, and curve arcs on two sides of the underground isolation coal pillar structure are semicircular curve arcs taking the thickness of a coal seam as the diameter;

step 2, constructing a coal pillar secondary stripping model by using F L AC3D numerical simulation software under the condition of considering coal pillar stripping generated by long-term weathering of the coal pillar, wherein the construction of the coal pillar secondary stripping model needs to consider both high-temperature effect in the gasification process and long-term stability requirement of the coal pillar, and the coal pillar secondary stripping model comprises a coal pillar primary stripping model under a overburden load and a coal pillar secondary stripping model under the overburden load;

step 3, establishing a secondary stripping model of the coal pillar by using F L AC3D numerical simulation software, simulating the change of the gasified coal, specifically, applying upward stress (7.38Mpa) which meets the requirement of supercritical carbon dioxide to a top plate to simulate the stress of the carbon dioxide after sealing, extracting the stress value of the top plate of the coal seam to obtain the stress distribution of the gasified and sealed coal, and further establishing a coal pillar stress distribution model after sealing carbon dioxide by analyzing the stress change rules before and after sealing;

and 4, determining the development height of the overlying strata fracture after carbon dioxide sequestration by utilizing an analytic hierarchy process according to an F L AC3D numerical model and by combining the influence factors including the overlying strata structure, the working face inclination length, the mining depth, the mining thickness and the coal seam inclination angle to determine the weight of each factor in the development height of the overlying strata fracture and integrating the actual load, the actual sequestration stress and the distribution size data of the coal pillar stress of the overlying strata, thereby achieving the purpose of predicting the development height of the overlying strata fracture.

The concrete steps of establishing the coal pillar secondary stripping model are as follows:

1) determining the shape of the gasified isolation coal pillar to be a hyperbolic curve type according to the characteristic of double-channel control retreating gasification of the underground gasification of the shaft-free coal;

2) according to the dead weight stress of a rock stratum and the hyperbolic characteristic of an isolation coal pillar, the hyperbolic isolation coal pillar stripping process is abstracted into a stripping process with 2 stages:

firstly, the bent part of the upper half part of the coal pillar is stripped to form a rectangular structure, the other positions are not stripped to be used as a primary stripping model,

then stripping the upper half part of the rectangular coal pillar to form a curve shape, and taking the rectangular coal pillar without stripping at other positions as a secondary stripping model;

after the coal pillar is stripped twice, the coal pillar is maintained in a fixed form and cannot be changed, and mathematical calculation is better performed, so that the boundary of the stripped coal pillar is approximately equivalent to a curve, the boundary curve can be approximately regarded as 1/4 ellipse, the oblateness of the boundary curve is approximately equal to the lower half part of the coal pillar, and the oblateness of the boundary curve is smaller, so that the coal pillar can be equivalent to an oblateness triangle for calculation;

the calculated secondary stripping model for building the coal pillar is as follows:

primary stripping of the model:

secondary stripping of the model:

in the formula (d)₀The length of the coal pillar bending part; d₁The width of the stacked body stacked near the coal pillar; m represents the coal thickness, k represents the coefficient of crushing and expansion and theta represents the angle of repose of the heap; gamma is the average volume weight of the overburden, and H is the average mining depth of the overburden; d. b represents the top width of the coal pillar and the equivalent combustion space area width after the isolated coal pillar is stripped for one time; d₁₀、b₁Representing the top width of the coal pillar after the secondary stripping of the isolated coal pillar and the equivalent width of the combustion space area, wherein L is the advancing distance of the gasification working surface;₁the roof strata collapse angle is obtained.

The concrete steps for constructing the coal pillar stress distribution model are as follows:

1) establishing a numerical model by using F L AC3D numerical simulation software and the existing drilling data, and simulating and excavating a plurality of working faces to simulate the stability of a hyperbolic briquette column under the condition of twice stripping;

2) through a numerical simulation means, F L AC3D codes are used for extracting the stress of a coal pillar top plate, Origin software is used for drawing, and the stress distribution of the UCG-CCS coal pillar is captured and sealed by comparing the UCG and the carbon dioxide coupled with the underground coal gasification, so that the difference of the distribution sizes of the UCG and the UCG-CCS coal pillar is analyzed;

3) according to the retreating type ignition control technology of underground coal gasification, in the whole gasification process, isolation coal pillars are reserved among gasification channels, so that the influence condition of sealing stress is increased in a stripe mining coal pillar stress distribution model, long-term underground water influence is considered, the influence of an underground water weakening function is considered on a UCG-CCS coal pillar stress distribution model, and a new coal pillar stress distribution model is finally constructed and distributed as follows:

in particular, the stress distribution function

Wherein f (x) is the overall stress distribution function of the coal pillar, D is half of the width of the coal pillar, and D₀A, b, c is the width of the coal pillar single-side stripping₁And d are all modelsThe coefficient represents the change degree of the stress of the coal pillar along with the increase of the width of the coal pillar;

when x → d₀D, the position is actually the edge of the stripped coal pillar, the stress of the position is the sealing stress and the caving gangue side stress, and the characteristic that the stress is reduced along with the increase of the injection distance is considered, so the sealing attenuation coefficient is multiplied on the basis; when x → Y-D, the pillar stress reaches the peak value; wherein K 'is a sealing pressure attenuation coefficient (0-1), K' is a side stress coefficient, sigma₁The maximum principal stress is shown as follows according to Wilson two-region constraint theory, wherein gamma is the average volume weight of the stratum, H is the mining depth, L is the gasification channel propelling distance:σ₃γ H, wherein: c is the cohesion of the coal column, Mpa,the internal friction angle, the cohesion and the internal friction angle of the coal pillar can be measured through experiments, the above formulas are combined, parameters are solved, and the result is as follows:

in addition, after the supercritical carbon dioxide is injected, because the viscosity of the supercritical carbon dioxide is smaller than that of a liquid state, the diffusion speed is higher than that of the liquid by two orders of magnitude, so that after the supercritical carbon dioxide is injected, the carbon dioxide can be quickly filled in a combustion space area and can be diffused into a coal pillar along a primary crack, certain influence can be caused on the stability of the coal pillar, and the influence function is similar to an underground water influence function, so that a water weakening function is introduced and corrected to reflect the influence of the immersion of the supercritical carbon dioxide on the coal pillar:

g_(c)＝(1-R_x)(1-w_x)²+K₁R_x

in the formula: r_xIs the pillar softening function; w is a_xAs a function of water content, R_xAnd w_xAll of which are continuously reduced along with the distance of the coal pillar, and the influence area is a yield area g_(cq)And elastic nucleus region g_(ce)；K₁Is a correction factor;

the stress distribution above the final isolated pillar should be:

namely a UCG-CCS coal column stress distribution model.

The method for predicting the development height of the overlying strata fracture in the second step specifically comprises the following operation steps:

1) combining with literature analysis, summarizing and summarizing more key influence factors, and judging the weight of each factor on the fracture development height by using an analytic hierarchy process;

2) according to the weight, controllable influence factors increased due to different sequestration conditions are combined: actual load factors of the overlying strata, actual sealing stress factors and the stress distribution of the coal pillars; the height of the overburden fracture development after the carbon dioxide is sealed is predicted by using the following formula,

H＝f(C₁、C₂、C₃、C₄、C₅、C₀、σ、F_s、f(x))

wherein: f is an influence function; c₀The controllable influence factors are increased by different sealing conditions and are determined by the specific place for sealing; sigma is the actual load of the overburden; f_sThe stress is actually sealed; and f (x) shows the size and distribution of the stress distribution of the coal pillar.

Has the advantages that:

in view of the fact that at present, no research on the development height of the coal underground gasification coupling carbon dioxide sequestration overburden rock fractures exists. Therefore, the method for predicting the fracture development height of the overburden rock by the underground coal gasification coupling carbon dioxide sequestration is provided in a numerical simulation mode, and the research result provides decision basis for judging the fracture development height of the overburden rock by the underground coal gasification coupling carbon dioxide sequestration.

Drawings

FIG. 1 is a diagram of the steps of the method for predicting the development height of a overburden fracture according to the invention;

FIG. 2 is a schematic view of the structure of the "hyperbolic" type insulating coal pillar of the present invention;

FIG. 3 is a schematic view of the present invention showing the primary stripping of the coal pillar;

FIG. 4 is a schematic illustration of the single side stripping of the coal pillar of the present invention;

FIG. 5 is a schematic view of the secondary stripping of the coal pillar of the present invention;

FIG. 6 is a comparison graph of a coal pillar stress fit of the present invention;

FIG. 7 is a water-conducting fissure zone development height hierarchy model of the present invention;

Detailed Description

The invention will be further described in detail with reference to the figures and the specific implementation process:

as shown in figure 1, the method for predicting the development height of the overburden fracture is characterized by comprising the following steps:

step 1: judging the shape of underground coal gasification isolation coal pillar

At present, the most advanced underground coal gasification technology in the world is a well-free underground coal gasification technology, the core technology is a strip mining-surface mining gasifier retreating type control gas injection underground gasification technology, the gasification technology consists of a gas inlet, a gas outlet and a gasification channel, the retreating type ignition control technology is used for preventing a top plate from large-scale collapse, an isolation coal pillar is reserved between the gasification channels, and the gasification fire source is burnt to be in a radiation combustion state from the middle to the periphery, as shown in fig. 2;

step 2: establishing an isolation coal pillar twice-stripping model considering long-term property

1) One-time stripping of coal pillar

As shown in fig. 3, under the action of the stress of the overburden rock, the bent coal pillar at the top end part can peel off firstly: the top end of the coal pillar is directly stressed by load pressure to generate horizontal expansion deformation, and the coal wall of the yielding area is separated from the coal pillar under the action of horizontal thrust inside the coal pillar. The upper half part of the stripped coal pillar is rectangular; and the coal pillar bottom is because of peeling off the body and roof scattered body constraint effect, and the peeling off phenomenon is lighter, and the latter half is approximate trapezium structure:

considering the symmetry of the coal pillar, a side was taken here for the study:

according to the trigonometric function relationship and the equal area method shown in fig. 4, the following are specific:

S₁＝S₂(3)

the above formulas are combined to solve the relationship between the two formulas:

from the relationship of the trigonometric function, one can obtain:

because the boundary of the coal underground gasification isolation coal pillar is approximately in a hyperbolic curve shape and is shown in a certain radian, the bending degree is defined as the arch depth ratio, and the definition formula is as follows:

therefore, equation (5) is equivalent to:

when f is 1, the boundary of the hyperbolic-type coal pillar is a semicircle taking the coal thickness as the diameter, the stripping degree of the hyperbolic-type coal pillar is maximum, in this case, the calculated stripping body repose angle is about 39-45 degrees, the calculation result is larger than the value range of the general empirical value, and the maximum single-side stripping of the coal pillar is half of the coal thickness at this time, which is the result under the limit condition.

2) Secondary stripping of coal pillar

As shown in fig. 5, after the "hyperbolic" coal pillar is peeled once (i.e. the upper half is peeled by the bending part, the whole coal pillar is rectangular, and the lower half is approximately trapezoidal), under the premise of considering the long-term stability of the coal pillar, the upper half of the coal pillar is rectangular after the bending coal pillar at the top of the coal pillar is peeled, and this shape characteristic is similar to that of the rectangular coal pillar, so the secondary peeling of the "hyperbolic" coal pillar can refer to a rectangular coal pillar peeling model, i.e. under the action of a load, the coal pillar in a yield region will expand and deform until the coal pillar is separated from the coal pillar, and is stacked at the lower half of the coal pillar at a certain stacking angle, and the coal pillar will undergo the secondary peeling phenomenon in the process of bending-rectangular peeling,

after the coal pillar is stripped twice, the coal pillar is maintained in a fixed form and cannot be changed, and mathematical calculation is better performed, so that the boundary of the stripped coal pillar is approximately equivalent to a curve, the boundary curve can be approximately regarded as 1/4 ellipse, the oblateness of the boundary curve is approximately equal to the lower half part of the coal pillar, and the oblateness of the boundary curve is smaller, so that the coal pillar can be equivalent to an oblateness triangle for calculation.

After the hyperbolic briquette pillar is stripped for the first time, the upper half part of the hyperbolic briquette pillar is rectangular, and the actual bearing load of the isolating briquette pillar at the moment is as follows:

the actual load bearing of the isolated coal pillar after the gasification activity is shown in the formula (8), which is established based on the currently mature limit balance theory, the theory is constructed by idealizing the coal pillar shape (regarding the coal pillar as a complete rectangle), and the two-time stripping of the coal pillar in the invention is also regarded as a shape convenient for mathematical calculation, so the actual load bearing of the "hyperbolic" coal pillar after the two-time stripping can be analogized as follows:

wherein:

S₁and S₂The area of the single-side bent part of the hyperbolic briquette column is shown; d₀The length of the coal pillar bending part; d₁The width of the stacked body stacked near the coal pillar; m, k and theta denote the coal thickness, the coefficient of crushing and expansion, and the angle of repose of the heap, respectively; gamma and H respectively represent the average volume weight and the mining depth of the overburden; d. b represents the top width of the coal pillar and the equivalent combustion space area width after the isolated coal pillar is stripped for one time; d₁₀、b₁Representing the top width of the coal pillar after the secondary stripping of the isolated coal pillar and the equivalent width of the combustion space area, wherein L in the formula represents the advancing distance of the gasification working surface;₁indicating roof strata collapse angle.

The bearing capacity of the coal pillar is obtained after the coal pillar is stripped twice, and then, the difference of UCG-CCS coal pillar stress distribution is researched.

And step 3: building a coal pillar stress distribution model by analyzing UCG-CCS coal pillar stress distribution

1) UCG-CCS coal column stress distribution analysis

In order to further compare and understand the coal pillar stress distribution rule between UCG and UCG-CCS, a plurality of numerical models are respectively established to simulate 4 gasification working faces and 2 sealing working faces, in the numerical calculation process, because stress is accurately applied to each stress node, a roof rock stratum and the rock stratum above the roof rock stratum are firstly 'delete', after sealing stress is applied, the roof rock stratum and the rock stratum above the roof rock stratum are added, finally, the node stress data before sealing are read again (a stress field before sealing is added), and finally, the numerical calculation is carried out until iterative balance. The extracted data analysis results are shown in fig. 6:

the stress of the coal pillar area is distributed in a quadratic function mode, the core stress of the coal pillar is far larger than the stress of two sides of the coal pillar, the fact that the upward stress applied to the combustion space area enables the overlying rock layer to move upwards, the rock layer is partially separated from the coal pillar, the stress of two sides of the coal pillar is reduced, in the process, the stress distribution of the coal pillar is different from the stress distribution of a non-sealed coal pillar, the quadratic function coefficient of the coal pillar stress of the sealed area is a negative number, the absolute value of the quadratic coefficient of the coal pillar is basically equal, and the fact that the stress distribution form of the coal pillar without considering and considering the initial stress is the same is explained.

2) Building a coal pillar stress distribution model

According to the stress distribution diagram of the elastic core area of the coal pillar, the following assumptions can be made:

(x) is the overall stress distribution function of the coal pillar, D is half of the width of the coal pillar, D₀A, b, c is the width of the coal pillar single-side stripping₁And d are model coefficients which represent the degree of change of the stress of the coal pillar with the increase of the width of the coal pillar. The stress distribution function should be solved under the following boundary conditions:

when x → d₀D, the position is actually the edge of the stripped coal pillar, the stress of the position is the sealing stress and the caving gangue side stress, and the characteristic that the stress is reduced along with the increase of the injection distance is considered, so the sealing attenuation coefficient is multiplied on the basis; when x → Y-D, the pillar stress reaches the peak value; wherein K 'is a sealing pressure attenuation coefficient (0-1), K' is a side stress coefficient, sigma₁The maximum principal stress is defined as follows according to Wilson two-region constraint theory, wherein gamma is the average volume weight of the stratum, H is the mining depth, L is the propelling distance of a gasification channel:

σ₃＝γH (13)

wherein: c is the cohesion of the coal column, Mpa,is the inner friction of coal pillarThe friction angle, the cohesion and the internal friction angle can be measured through experiments, the parameters in the formula (10) can be solved by combining the above formulas, and the results are as follows:

in addition, after the supercritical carbon dioxide is injected, because the viscosity of the supercritical carbon dioxide is smaller than that of a liquid state, the diffusion speed is higher than that of the liquid by two orders of magnitude, so that after the supercritical carbon dioxide is injected, the carbon dioxide can be quickly filled in a combustion space area and can be diffused into a coal pillar along a primary fracture, certain influence can be caused on the stability of the coal pillar, and a water weakening function is introduced and corrected to reflect the influence of the supercritical carbon dioxide immersion on the coal pillar by analogy with an underground water influence function:

g_(c)＝(1-R_x)(1-w_x)²+K₁R_x(15)

in the formula: r_xIs the pillar softening function; w is a_xAs a function of water content, R_xAnd w_xAll of which are continuously reduced along with the coal pillar distance, and the influence area is a yield area (g)_(cq)) And elastic nucleus region (g)_(ce))；K₁Is a correction factor. The stress distribution above the coal pillar should therefore be:

the formula is the UCG-CCS coal column stress distribution model.

And 4, step 4: determining weight of factors influencing overlying strata fracture development height by using analytic hierarchy process

According to the relevant documents, the influence factors of the mining method are summarized to be mining conditions and geological conditions which are respectively mining thickness, working face inclined length, mining depth, coal seam inclination angle and overlying rock structure, and the determined hierarchical structure model is shown in FIG. 7:

and comparing the chromatographic structure models pairwise, and grading according to the importance degree of the chromatographic structure models. The table below gives the 9 importance ratings and their assignments for the element-to-element comparison results. The matrix formed by the results of the pairwise comparisons is referred to as the decision matrix. The decision matrix has the following relationship:

in the above, the following steps: a is_ijThe importance of element i is compared with the importance of element j.

TABLE 1 analytic hierarchy Process evaluation Scale

aijValue of	Definition of
		1	The i element is as important as the j element
3	The i element is slightly more important than the j element
		5	The i element is more important than the j element
7	The i element is significantly more important than the j element
		9	The i element is absolutely more important than the j element
2,4,6,8	Between the above values

1) Construct judgment matrix A

According to the 5 influence factors of fig. 7, an analytic hierarchy process judgment matrix a is constructed by combining the evaluation scales between the influence factors in the table:

2) matrix consistency check

Due to the complexity of the research problem and the diversity of subjective recognitions, the decision matrix a is not necessarily completely consistent. Therefore, in order to evaluate the reliability of the constructed judgment matrix, it is necessary to perform a consistency check on the judgment matrix. For the consistency check of the matrix, the eigenvalue and the eigenvector of the constructed judgment matrix are judged first, and the position of the maximum eigenvalue is located, so as to calculate the consistency check index CI, which is defined as:

in the above consistency check index, n is the order of the judgment matrix, λ_maxIs the maximum characteristic root of the judgment matrix. Since λ is continuously dependent, then λ_maxThe more n, the more serious the inconsistency of A, the consistency index is calculated by CI, and the smaller CI, the larger the consistency is. And using the feature vector corresponding to the maximum feature value as a weight vector of the influence degree of the compared factor on a certain factor of an upper layer, wherein the larger the inconsistency degree is, the larger the judgment error is caused. Thus can use λ_maxThe magnitude of the n value is used to measure the degree of inconsistency of A.

CI is 0, with complete consistency; CI is close to 0, and the consistency is satisfactory; the larger the CI, the more severe the inconsistency. In order to measure the CI size, a random consistency index RI is introduced, and in the calculation, the data in table 2 can be used for expression:

TABLE 2 average random consistency index RI standard value

n	1	2	3	4	5
						RI	0	0	0.58	0.90	1.12

If there is an error caused by a random reason in the consistency, there may be a deviation in the reliability of the consistency check result, so it is necessary to combine CI with the random consistency index RI to construct a consistency check coefficient CR, which is expressed as follows:

and when CR is less than 0.1, the judgment matrix A meets the consistency, otherwise, the judgment matrix A does not have the consistency, when the judgment matrix does not pass the consistency inspection, the judgment matrix is required to be reconstructed, and the iteration is sequentially circulated until the judgment matrix passes the consistency inspection.

When checking the consistency of the judgment matrix, lambda_max5.23, CI 0.0594, CR 0.053 < 0.1 satisfy the consistency check condition, from which the influence weight vector is derived as follows:

TABLE 3 influence factor weights

The above table shows that, among the several factors influencing the development height of the water-conducting fractured zone, the sensitivity degree sequentially includes the overburden structure, the working face inclination length, the mining depth, the coal seam thickness and the coal seam inclination angle, wherein the overburden structure plays an important role in the development height of the water-conducting fractured zone, the weight of the overburden structure reaches 0.5128, the overburden structure refers to the soft and hard combination degree of the top plate, whether the rock stratum playing a control role exists in the overburden and the lithology of the overburden, and for the carbon dioxide sequestration after gasification, the development height of the water-conducting fractured zone is influenced and the rock stratum closure degree under the sequestration pressure effect is also influenced.

And 5: and finally establishing a overburden fracture development height model after carbon dioxide sequestration by combining the steps.

The influence factors and the weight of the overlying strata fracture development height are determined by the steps, if different geological conditions are considered, controllable influence conditions need to be added on the basis of the influence factors, and the final overlying strata fracture development height from the main control factors is as follows:

H＝f(C₁、C₂、C₃、C₄、C₅、C₀、σ、F_s、f(x)) (20)

wherein: f is an influence function; c₀The controllable influence factors are increased by different sealing conditions and are determined by the specific place for sealing; sigma is the actual load of the overburden; f_sThe stress is actually sealed; and f (x) shows the size and distribution of the stress distribution of the coal pillar.

Due to the complexity and uncertainty of the development height of the overlying strata fracture, geological conditions and a sealing and storing process, a universal method for predicting the development height of the overlying strata fracture and corresponding parameters thereof are difficult to provide. The method can respectively construct the form and related parameters of the overburden fracture development height of each sealing area on the basis of experimental simulation, theoretical analysis, engineering examples and statistical data according to the characteristics of the sealing location, the basic form of the model is linear or approximately linear or nonlinear, and the specific form can be continuously checked and corrected in the sealing process of each area.