Precise fracturing and efficient scour prevention method for far and near field key layer of extra-thick coal seam

Abstract

The invention relates to the field of coal mining rock burst prevention and control, in particular to an efficient anti-impact method for precise fracturing of far and near field key layers of an extra-thick coal seam. According to the method, firstly, the positions of the key layers are judged, then the strength characteristics of the key layers are assumed to be weakened, the assumed and weakened strength characteristics meet the rock burst safety requirement, and then the fracture step pitches of the key layers are calculated on the basis of the assumed and weakened strength characteristics. And according to the breaking step pitch at the moment, interval manual fracturing breaking is carried out on all the key layers in the advancing direction of the working face, and therefore accurate fracturing can be carried out on all the key layers, and the rock burst safety requirement is met. According to the method, the fracture step pitch of each key layer is determined on the basis of the strength characteristics after hypothesis weakening, the fracture characteristics of the key layers are consistent with the fracture step pitch of the rock stratum which is hypothetically weakened, and the contribution of each key layer to the impact tendency is similar to that of the rock stratum which is hypothetically weakened due to the fact that rock burst mainly occurs when the key layers are fractured; and the rock burst safety requirement is met.

Description

Technical Field

[0001] This invention relates to the field of rockburst prevention in coal mining, and particularly to a method for precise fracturing of key layers in near and far fields of extra-thick coal seams to achieve efficient rockburst prevention. Background Technology

[0002] After coal seam mining, from bottom to top, caving zones, water-conducting fracture zones, and flexural subsidence zones will be formed. Among them, the rock strata in the water-conducting fracture zone are regularly blocky fractures. Generally speaking, the development height of the water-conducting fracture zone is directly proportional to the coal seam mining height. That is, the greater the coal seam mining height, the higher the height of the water-conducting fracture zone after mining. Moreover, studies have found that as the coal seam mining height increases, the ratio of the development height of the water-conducting fracture zone to the coal seam mining height also gradually increases.

[0003] After coal seam mining, strata movement and damage are transmitted from bottom to top. Among them, the rock strata that control strata movement and damage are called key strata. Key strata generally refer to rock strata with greater thickness and strength in the strata. The rock strata between key strata are generally called load strata (a combination of weak rock strata). They have less strength and do not control strata movement. They move together with the key strata below them.

[0004] When coal seams are mined at considerable heights, especially in extra-thick coal seams, water-conducting fracture zones develop to great heights, affecting numerous critical strata. In most cases, these fractures reach the surface directly, preventing the formation of tortuous subsidence zones. Due to their thickness and strength, critical strata are prone to rockbursts upon fracture, particularly under conditions of deep coal seams, numerous critical strata, and high critical strata strength.

[0005] Artificial pre-fracturing of key strata before coal seam mining is a crucial means of preventing rockbursts. For coal seams with relatively low mining heights, the number of key strata affected by mining is smaller, meaning fewer key strata are involved that could cause rockbursts. Consequently, fewer key strata require fracturing, resulting in less fracturing work. Fracturing parameters selected based on experience tend to be conservative, fracturing points are more densely spaced, and the size of the fractured key strata blocks is generally smaller to ensure effective rockburst prevention.

[0006] However, when the coal seam depth is large, especially in the mining of extra-thick coal seams, the mining involves many critical strata, meaning there are many critical strata that can cause rockbursts during coal seam mining, requiring fracturing of many critical strata. In such cases, adopting a conservative fracturing approach results in an enormous workload. However, current technology lacks a reliable method for determining the fracturing block size (fracturing location) parameter of critical strata. Especially when multiple critical strata need to be fracturing, accurately determining the fracturing block size (fracturing location) parameter for each critical strata becomes a key challenge restricting the efficient and safe mining of extra-thick coal seams. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention proposes a precise fracturing and efficient anti-shocking method for key layers in ultra-thick coal seams, comprising the following steps: Step 1: Identify the key strata in the overlying strata of the extra-thick coal seam based on the key strata theory; Step 2: Weaken the strength characteristics of each key layer by using an assumption method, determine the impact hazard of the extra-thick coal seam under the condition of no artificial fracturing intervention based on the weakened strength characteristics of each key layer, and ensure that the weakened strength characteristics of each key layer meet the impact hazard requirements of the extra-thick coal seam when mining under the condition of no artificial fracturing intervention and meet the rockburst safety requirements. Step 3: Calculate the fracture step distance of each key layer based on the weakened strength characteristics of each key layer; combined with the working face advance range, perform artificial fracturing and fracture of each key layer along the working face advance direction based on the fracture step distance of each key layer after weakening. Step 4: Conduct safe mining of the extra-thick coal seam in the working face.

[0008] Preferably, in the second step, the strength characteristics weakened by the assumption method include, but are not limited to, uniaxial compressive strength and elastic modulus.

[0009] Preferably, in the second step, multiple sets of key layer weakening schemes that meet the rockburst safety requirements are selected, and each set of schemes includes the weakening intensity characteristics of each key layer.

[0010] Preferably, in the third step, assuming that the fracturing step distance after the nth key layer is weakened from bottom to top is Ln, the artificial fracturing method is as follows: starting from the mining position of the working face, along the working face advancing direction, the nth key layer is artificially fracturing once every Ln distance.

[0011] Preferably, in the third step, for the multiple key layer weakening schemes that meet the rockburst safety requirements in the second step, the scheme with the lowest construction cost and construction time for artificial fracturing is selected.

[0012] Preferably, in the third step, the artificial fracturing method includes, but is not limited to, hydraulic fracturing, carbon dioxide or nitrogen fracturing.

[0013] Invention points and beneficial effects: 1. Addressing the problem of large workload in fracturing during the mining of extra-thick coal seams, this invention first identifies the location of critical strata, then assumes weakened strength characteristics for each critical strata while ensuring these weakened characteristics meet rockburst safety requirements. Based on these weakened strength characteristics, the fracturing step distance for each critical strata is calculated. According to this fracturing step distance, each critical strata is subjected to intermittent artificial fracturing along the working face advancement direction, thus enabling precise fracturing of each critical strata while meeting rockburst safety requirements. Furthermore, it facilitates the selection of the construction scheme with optimal cost and time.

[0014] 2. This invention determines the fracture step distance of each key layer based on the assumed weakened strength characteristics, so that its fracture characteristics are consistent with the fracture step distance of the assumed weakened rock layer. Since rockbursts mainly occur when the key layer fractures, the contribution of each key layer to the rockburst tendency can be similar to that of the assumed weakened rock layer, thus meeting the rockburst safety requirements.

[0015] 3. This invention only requires manual pre-fracturing of each key layer according to the assumed weakened strength characteristics, based on the calculation of the fracture step distance of each key layer along the working face advancement direction. It does not require fracturing of the entire area of ​​each key layer, and the fracturing position is accurate, with a small amount of fracturing work. Attached Figure Description

[0016] When considered in conjunction with the accompanying drawings, the invention will be more fully and better understood, and many of its accompanying beneficial effects will become readily apparent, through the following detailed description. However, the accompanying drawings, which are provided to further illustrate the invention and form part of this invention, and the illustrative embodiments thereof, together with their descriptions, are used to explain the invention and do not constitute an undue limitation thereof, wherein: Figure 1 —Schematic diagram of the key layer structure of the extra-thick coal seam in the near and far fields of this invention (working face advancing direction); Figure 2 —A schematic diagram of critical layer fracturing (working face advancing direction) determined based on the critical layer fracturing anti-shock parameter determination method of the present invention. Figure descriptions: 1-Extra-thick coal seam; 2-Subcritical layer one; 3-Subcritical layer two; 4-Subcritical layer three; 5-Main critical layer; 6-Loose layer; 7-Loaded layer / weak rock layer combination; 8-Site of artificial fracturing. Detailed Implementation

[0017] To better understand the technical content of this invention, specific embodiments are described below in conjunction with the accompanying drawings. Various aspects of this invention are described with reference to the accompanying drawings, which illustrate numerous illustrative embodiments. The embodiments of this invention are not limited to those shown in the drawings. It should be understood that this invention is implemented through any of the various concepts and embodiments described above, as well as the concepts and embodiments described in detail below, because the concepts and embodiments disclosed in this invention are not limited to any particular implementation. Furthermore, some aspects of this invention can be used alone or in any suitable combination with other aspects disclosed in this invention.

[0018] like Figure 1-2 As shown, this invention proposes a precise fracturing and efficient anti-shocking method for key layers in ultra-thick coal seams, including the following steps: Step 1: As Figure 1As shown, the key layers in the overlying strata of the extra-thick coal seam 1 are determined based on the key layer theory. In this embodiment, from bottom to top, the key layers are: sub-key layer 1 2, sub-key layer 2 3, sub-key layer 3 4, and main key layer 5. The strata between the extra-thick coal seam 1 and sub-key layer 1 2, between each key layer, and between the main key layer 5 and the loose layer 6 are the load layer / weak strata combination 7.

[0019] After the mining of the extra-thick coal seam 1, the water-conducting fracture zone will generally develop to the surface. The failure of all key strata will have a rockburst impact on the mining of the extra-thick coal seam 1. Based on the strength characteristics of the key strata (including but not limited to uniaxial compressive strength and elastic modulus), the rockburst risk of the extra-thick coal seam 1 under the condition of no artificial fracturing intervention is determined. That is, the rockburst risk is determined under normal mining of the extra-thick coal seam 1 without artificial fracturing intervention in the key strata. The rockburst risk at this time can be characterized by the advance support pressure, the coal pillar side support pressure, and the stope pressure (the support force required by the hydraulic support).

[0020] Step 2: Weaken the strength characteristics of each key layer by using an assumption method, including but not limited to weakening its uniaxial compressive strength and elastic modulus. That is, assume that the strength characteristics of each key layer are the weakened strength characteristics. Based on the weakened strength characteristics of each key layer, determine the impact hazard of the extra-thick coal seam 1 under the condition of no artificial fracturing intervention, until the weakened strength characteristics of each key layer meet the impact hazard requirements of the extra-thick coal seam 1 when mining under the condition of no artificial fracturing intervention and the rockburst safety requirements are met.

[0021] For example, if the main critical stratum 5 is sandstone with a uniaxial compressive strength of 70 MPa, and the subcritical stratum 2 is also sandstone with a uniaxial compressive strength of 60 MPa (other subcritical strata are not listed here), directly mining the extra-thick coal seam 1 would pose a high risk of rockburst. In this case, the strength characteristics of each critical stratum are weakened. For instance, if the main critical stratum 5 is assumed to be mudstone with a uniaxial compressive strength of 30 MPa, and the subcritical stratum 2 is assumed to be mudstone with a uniaxial compressive strength of 25 MPa (other subcritical strata are similarly weakened), the rockburst risk of the extra-thick coal seam 1 under conditions without artificial fracturing intervention is determined. If the rockburst safety requirements are met, it indicates that the weakened strength characteristics of each critical stratum meet the rockburst safety requirements. If not, one or more critical strata are further weakened until the rockburst safety requirements are met.

[0022] Preferably, multiple sets of key layer weakening schemes that meet the rockburst safety requirements are selected, and each set of schemes includes the weakening intensity characteristics of each key layer.

[0023] Step 3: As Figure 2As shown, based on the weakened strength characteristics of each key layer, the fracturing step distance of each key layer is calculated. Specifically, based on the weakened strength characteristics of sub-key layer 1-2, sub-key layer 2-3, sub-key layer 3-4, and main key layer 5, the fracturing step distances of sub-key layer 1-2, sub-key layer 2-3, sub-key layer 3-4, and main key layer 5 are calculated respectively. Based on the fracturing step distances of each key layer at this time (after weakening) and the working face advancement range, the artificial fracturing fracturing positions of each key layer are determined. Specifically, from the mining and recovery position of the working face, each key layer is artificially fracturing according to the weakened fracturing step distances of each key layer (see [link to artificial fracturing position]). Figure 2 (Artificial fracturing location 8 in the text). For example, if the working face advance length is 2000m, and the fracturing step distance of subcritical layer 2 is determined to be 20m based on the weakened strength parameters, and the fracturing step distance of main critical layer 5 is determined to be 60m based on the weakened strength parameters, then from the working face mining and recovery position (cut-out position), along the working face advance direction, artificial fracturing is performed on subcritical layer 2 every 20m, and from the working face mining and recovery position (cut-out position), along the working face advance direction, artificial fracturing is performed on main critical layer 5 every 60m. Other critical layers are also artificially fracturing in the same way (see the artificial fracturing location section). Figure 2 Artificial fracturing location 8 in the middle.

[0024] Preferably, for the multiple critical layer weakening schemes that meet the rockburst safety requirements in the second step, the scheme with the lowest construction cost and construction time for artificial fracturing is selected.

[0025] Among them, artificial fracturing methods include, but are not limited to, hydraulic fracturing, carbon dioxide or nitrogen fracturing.

[0026] Step 4: Conduct safe mining of the extra-thick coal seam 1 in the working face.

[0027] Of course, the above description is only a preferred embodiment of the present invention. The present invention is not limited to the above-described embodiments. It should be noted that any equivalent substitutions or obvious modifications made by those skilled in the art under the guidance of this specification fall within the scope of this specification and should be protected by the present invention.

Claims

1. A precise fracturing and efficient anti-scour method for key layers in ultra-thick coal seams, characterized in that: Includes the following steps: Step 1: Identify the key strata in the overlying strata of the extra-thick coal seam based on the key strata theory; Step 2: Weaken the strength characteristics of each key layer by using an assumption method, determine the impact hazard of the extra-thick coal seam under the condition of no artificial fracturing intervention based on the weakened strength characteristics of each key layer, and ensure that the weakened strength characteristics of each key layer meet the impact hazard requirements of the extra-thick coal seam when mining under the condition of no artificial fracturing intervention and meet the rockburst safety requirements. Step 3: Calculate the fracture step distance of each key layer based on the weakened strength characteristics of each key layer; combined with the working face advance range, perform artificial fracturing and fracture of each key layer along the working face advance direction based on the fracture step distance of each key layer after weakening. Step 4: Conduct safe mining of the extra-thick coal seam in the working face.

2. The method for precise fracturing and efficient anti-shocking of key layers in ultra-thick coal seams in both near and far fields according to claim 1, characterized in that, In the second step, the strength characteristics weakened by the assumption method include, but are not limited to, uniaxial compressive strength and elastic modulus.

3. The method for precise fracturing and efficient anti-scour of key layers in ultra-thick coal seams in both near and far fields according to claim 1, characterized in that, In the second step, multiple sets of key layer weakening schemes that meet the safety requirements of rockburst are selected, and each set of schemes includes the weakening intensity characteristics of each key layer.

4. The method for precise fracturing and efficient anti-shocking of key layers in ultra-thick coal seams in both near and far fields according to any one of claims 1-3, characterized in that, In the third step, assuming that the fracturing step distance after the nth key layer is weakened from bottom to top is Ln, the artificial fracturing method is as follows: starting from the starting position of the working face, along the working face advance direction, artificial fracturing is performed on the nth key layer every Ln distance.

5. The method for precise fracturing and efficient anti-scour of key layers in ultra-thick coal seams in both near and far fields according to claim 3, characterized in that, In the third step, for the multiple critical layer weakening schemes that meet the rockburst safety requirements in the second step, select the scheme with the lowest construction cost and construction time for artificial fracturing.

6. The method for precise fracturing and efficient anti-scour of key layers in ultra-thick coal seams in both near and far fields according to any one of claims 1-3, characterized in that, In the third step, artificial fracturing methods include, but are not limited to, hydraulic fracturing, carbon dioxide or nitrogen fracturing.

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