1. Introduction
Hancheng mining area has complex geological conditions, high gas content in coal seam, high gas pressure, difficult drilling construction in coal seam 3, difficult gas drainage, low efficiency, high cost of mine gas prevention and control, and tight mining replacement. Improving the effect of gas drainage is an important way to reduce the cost and increase the efficiency. Directional long borehole hydraulic fracturing technology can increase borehole coverage and effectively improve borehole gas drainage effect.
2. Treatment solution
In Wangfeng coal mine, the roof directional long drilling of No.3 coal seam is constructed on one side of the main inclined shaft, and the main hole is drilled along the stable layer of the coal seam roof. After the main hole construction is completed, the "backward" technology is used to construct the branch hole to the coal seam, and the branch hole exposes the No.3 coal seam, and the final hole is not less than 200m long. The second opening structure is adopted for drilling, with the first opening diameter of Φ 193mm and the hole depth of 60m. After the construction is completed, the Φ 146mm casing is put in to seal all the first opening sections; the second opening diameter is 120mm.
Wangfeng mine borehole design profile
Construction of a comb shaped long hole in the roof of 3309 working face 1 return air chute, in the north belt lane of No.2 well in Sangshuping, the main hole of the design borehole is 580m long, and the construction horizon of the main hole is about 2.0-5.0m away from coal seam 3; One branch hole shall be constructed every 60m in the horizontal section of the main hole, with a total of 8 branch holes, each of which is 50m long, The drilling depth along the coal seam shall not be less than 2m after the coal is found in the borehole, and the forward branch opening process shall be adopted, The drilling azimuth is 0 ° and the dip angle is 5 °. The bedding construction is started near the hole depth of about 80m.
Drilling design section of No.2 well in Sangshuping
Technical schematic diagram of segmented hydraulic fracturing of fixed string with long roof drilling
3. Construction situation
Through the selection of drilling rig, optimization of drilling assembly, reasonable design of drilling parameters, drilling trajectory and drilling structure, the construction of two roof directional drilling was completed in Hancheng mining area. Among them, 1# drill hole is located in Wangfeng coal mine, with 1 main hole and 4 branch holes. The length of the main hole is 261m, and the total drilling footage is 344m. The distance between the actual drilling track of the main hole and coal seam 3 is about 0.7 ~ 2.0m. The hydraulic fracturing construction was carried out by using the drilling integral fracturing technology and the casing + packer hole sealing method. The 284m long roof directional long drilling integral fracturing of coal seam 3 was realized. The setting pressure of the packer was 15MPa and the time of hole sealing was 18min; the cumulative water injection volume of the drilling hole was 874.79m3, the water injection displacement was 24m3/h and the maximum pump injection pressure was 9.4MPa. 2# drill hole i is located in No.2 well of Sangshuping, with one main hole, 588m long and eight branch holes. The total drilling footage is 1188m, and the maximum distance between the main hole and No.3 coal seam is 3.28m. Four sections of fracturing were completed by using fixed string staged fracturing technology, with cumulative water injection of 2012m3 and maximum pump injection pressure of 8.74MPa.
Drilling construction plan of No.2 well in Sangshuping
4. Achieved accomplishment
During fracturing of 1# drilling, there are many obvious pressure drops, which are consistent with the typical pressure curve model of drilling and hydraulic fracturing in the roof of soft coal seam. After hydraulic fracturing, within 86 days of drilling and extraction, the extraction concentration is 27% ~ 51%, and the average is 42.11%. The net amount of drilling gas extraction is 8.25 ~ 21.41m3/min, and the average is 17.02m3/min. The net amount of 100 m drilling gas extraction is 12.48 times as much as that of interval hydraulic drilling.
In Wangfeng mine, the pumping capacity of 100m borehole is 12.48 times of interval hydraulic drilling
After fracturing of 2# drilling, the maximum influence radius of hydraulic fracturing was 39.63m. Within 93 days of extraction, the concentration of drilling gas extraction is 19.6% ~ 56.0%, 41.28% on average. The net amount of drilling gas extraction is 0.33 ~ 1.93m3/min, 1.08m3/min on average. The net amount of 100 m drilling gas extraction is 1.2 times of hydraulic cutting seam, 4.8 times of conventional drilling in this coal seam. By using the method of residual gas content in coal seam, the effective extraction radius (the pre extraction rate is more than 30%) of boreholes with 40 days and 200 days under a certain suction negative pressure is 3.7m and 9.0m respectively.
Drilling and production curve of hydraulic fracturing in Sangshuping No.2 well
5. FAQ
(1) What is the technical principle of hydraulic fracturing.
Hydraulic fracturing technology is a conventional technology for the development of surface oil and gas fields, coalbed methane and shale gas. Its essence is to inject the fluid into the formation at a rate greater than the formation loss rate. When the injection pressure of the fluid is greater than the formation fracture pressure, the formation fractures will be formed. At the same time, a secondary fracture will be formed around the main fracture to make the original fracture system communicate with each other, so as to improve the formation permeability gas property, reduce the difficulty of gas migration, and realize the purpose of gas enhanced extraction. In addition, coal seam hydraulic fracturing has multiple functions such as changing the strength of coal body, homogenizing the in-situ stress field, inhibiting gas emission, balancing the gas pressure field, etc. It is an effective technical measure for gas control in low permeability and high gas outburst coal seam.
（2）What are the main application aspects of hydraulic fracturing technology, how is its popularization and effect.
For the mining area with the development of soft and low permeability coal seam, the hydraulic fracturing technology can be used to enhance gas drainage and improve gas control efficiency; for the mining area with the development of hard roof plate, the hydraulic fracturing technology can be used to weaken in advance and improve the mining efficiency. At present, hydraulic fracturing technology has been popularized and applied in Shanxi, Shaanxi, Inner Mongolia, Anhui, Guizhou, Xinjiang, Henan and other places. The overall application effect is good, and multi-stage research and application have been carried out in many places.
(3) What is the applicability of hydraulic fracturing technology and what are the requirements for the geological conditions of the mine.
Hydraulic fracturing construction area requires simple structural conditions, no fault, collapse column and other geological structures, good surrounding rock conditions, no obvious pressure relief channel in the formation.
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