| There are many technical problems in coal mine, such as rock burst, hanging hard roof, low extraction efficiency in coal seam with low permeability and high methane content, low cavability of hard and thick coal. All these problems become bottlenecks limiting the safety and efficiency of mining. Hydraulic fracturing technology is an effective way to change the stress field and transform the macroscopic and microscopic structure of coal and rock mass. With the development of technology and equipment, the determination of reasonable pump time is urgent to be determined. This paper adopts physical simulation, theoretical analysis and field tests to analyze and study the reasonable pump time during hydraulic fracturing.Large size true triaxial hydraulic fracturing experimental system was used to conduct physical simulation tests. By analyzing typical test results, the following conclusions can be made:(1)the process of hydraulic fracturing can be divided into three stages :water filling the pipe stage, pressure rising and initial fracturing stage, cracks extending stage(2) hydraulic main cracks is perpendicular to the minimum principle stress and the spatial morphology of hydraulic main crack is ellipsoid(3) the formula of total pump time is + +1 2 3=tT T T T, where 1T,2T,3T are the time of three stages respectively.Based on energy conservation law, Bernoulli equation, the calculation of crack extending range and crack seepage volume and the tests conclusion, the fracture initiation time and fracture propagation time are calculated. And then, the formula of total pump time of hydraulic fracturing for coal and rock mass is calculated as follow:Compare the pump time calculated in this paper and traditional pump time and verify the accuracy and reliability of the formula proposed above based on field hydraulic fracturing practices. Reasonable pump time can increase recovery ratio of top coal, decrease top coal lumpiness and reduce coal wall ribbing and roof caving. |
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