Advanced weakening project of hydraulic fracturing with directional long boreholes and subsection in hard roof of Baode Mine
1. Introduction
In the process of coal mining, due to the thickness of hard roof, high strength of rock and strong bearing capacity, it is not easy to collapse naturally after mining in the working face, which will form a large area of suspended roof in the goaf, resulting in the ahead stress concentration of the working face and the problems such as spalling, pressing frame, floor heave, etc.; with the advancement of the working face, the area of suspended roof reaches a certain degree, and the hard roof collapses and accumulates in a large area. The rapid release of energy gathered in roof and coal seam is easy to cause dynamic disaster, equipment damage and casualties, which seriously restricts the safe and efficient production of the coal mine.
a. Large deformation of roadway b.Gangue falling between supports
Fig1 The behavior of mine pressure under the hard roof
2. Staged hydraulic fracturing solution
Based on the fracture mechanics, combined with the production and geological data of the treatment area, combined with the key layer identification technology, the target layer of the fracturing was optimized, and the multi-point dragging segmented hydraulic fracturing technology of the long directional boreholes was adopted to control the roof dynamic disaster in the working face in advance. This technology has many advantages, such as precise control of target horizon (error< 0.5m), wide range of fracturing influence (radius > 30m), weakening ahead of production (before mining) and so on. By using open hole staged hydraulic fracturing equipment, the hard roof was fractured one by one from the inside to the outside. The single hole can achieve more than 10 sections of fracturing construction, realize the effective weakening of the target layer and high-pressure stress relief and transfer, and realize the leading regional weakening of strong mine pressure disaster. Two fracturing drilling fields were arranged in the scheme. The layout form was 1 group of vertical boreholes, and the section was located in fine-grained sandstone 18m away from the roof of coal seam 8, with a total of 6 boreholes.
Fig2 Schematic diagram of multi-point drag staged fracturing operation
Fig3 Layout plan of fracturing boreholes
3. Work situation
Baode Mine took three months to complete the staged hydraulic fracturing construction of two drill fields and four boreholes in working face 81307. A total of 34 sections of hard roof staged fracturing weakening construction were carried out, with a total of 1727.37m fracturing tool string, 2269 min fracturing time and 1558.29 m3 injected water.
a. Schematic diagram of fracturing pump b. Hydraulic fracturing borehole orifice
Fig 4 Hydraulic fracturing working site
4. Achieved accomplishment
In the process of fracturing operation, the maximum pressure was 26.33MPa, the maximum fracture pressure drop was 15.55MPa, and 167 times of obvious pressure drop above 3MPa, forming an effective three-dimensional fracture, reducing the overall strength of the roof. Through the analysis and comparison of the change of the roof support resistance when the working face passed through the fractured area and the unfractured area, it can be seen that after the fracturing construction in the No.2 drilling field, the roof pressure strength was significantly reduced. During the period of periodic pressure, the support resistance was reduced from the average of 457 bar to 393.6 bar after fracturing, with a decrease of 13.9%. The pressure range was significantly reduced, and the floor and coal wall of the return air chute were not developed. The obvious deformation phenomenon verifies the effectiveness of the weakening effect of the hard roof fracturing.
Fig5 Three dimensional variation characteristics of rock pressure evaluation in the fractured area
Fig6 Plane characteristics of ground pressure evaluation in fracturing drilling field
5. FAQ
Q1.What are the advantages of segmented hydraulic fracturing in the treatment of hard roof？
A1: Segmented hydraulic fracturing can form continuous three-dimensional fractures and effectively reduce the strength of the strata by designing a reasonable interval between the fracture sections.
Q2.How to realize hydraulic fracturing construction safely and efficiently？
A2: Reasonable design of hydraulic fracturing construction scheme, strict training of operators and examination, regular maintenance of fracturing equipment and timely troubleshooting of other unsafe factors.
Q3.How to select the target layer of fracturing？
A3: In view of the middle and low key layers, through leading weakening, the caving height of roof strata under the mining effect is full of goaf, forming effective support for the overlying strata, weakening or eliminating the overlying dynamic load effect.
Q4.How to select the location of the fractured section？
A4: According to the drilling situation and the rock sample collection and analysis, the fractured section is selected in the position with stable trajectory, stable lithology and relatively complete hole wall.
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