Study On Permeability-increasing Technology Combining 3D Swirling Water Jet With Hydraulic Fracturing

For the soft and low permeability coal seams, especially the single outburst coal seams, it is difficult to reach the standard of gas extraction by using the existed gas extraction technologies. To increase the coal seam permeability is the key to solve this problem. Structure reformation of coal is the focal point to increase the permeability, and with water jet and hydraulic fracturing for coal seams as the representative technology of the hydraulic technology is an effective way to structure reformation of coal. In this paper, the hydraulic technology for increasing permeability was studied by using a combined method of theoretical analysis, numerical simulation, laboratory testing and field applications.Then,field test and application of this technology is implemented. The study on permeability-increasing technology combining 3D swirling water jet with hydraulic fracturing is of important theoretical and practical significance.The main research results of this paper are as follows:Through the methord combining theoretical calculations with the numerical simulation, the mechanical mechanism of coal seam permeability-increasing by reaming penetration boreholes was studied under non-uniform stress field. The results showed that: with the increase of reaming diameter, the plastic zone area shows a rapid growth approximating to a parabolic, while the equivalent diameter of plastic zone grows substantially linear. Therefore, seam relief and permeability increase can be achieved by the water jet reaming.The characteristics of flow field of the swirling water jet nozzle under submerged condition was studied with ANSYS, and it was optimal selection that the jet impeller orientation angle of 45 ° is more suitable for site reaming. The rock breaking and reaming performance of the swirling water jet nozzles, was tested under submerged conditions with the water jet test system in the laboratory.The numerical simulation of flow field of the 3D swirling water Jet under conditions of non-submerged is done. The results show that: the jet velocity, especially the tangential velocity, generated by the rotating jet nozzle is greater than that when not rotated. Thus, it will improve the jet ability of coal breaking and reaming. On this foundation, a method of 3D Swirling Water Jet for Reaming Boreholes is proposed.With the elasticity theory, the control role of in-situ stress to the initiation pressure and initial crack propagation azimuth during penetration boreholes hydraulic fracturing is studied. According to the analysis on horizontal stress distribution characteristics in different directions around the penetration borehole after the water jet reaming, the results show that practicing hydraulic fracturing after water jet reaming, the cracks are easily expand along the maximum principal stress direction. Based on the theory of elastic mechanics and percolation mechanics, the inducing effect to the late crack development by the induced stress which was producted by the early crack is studied. Using the RFPA2D-Flow simulation software, the initiation, extension and expansion of cracks under different boreholes arrangements and different fracturing ways are simulated. The simulation verified the guidance role of the control borehole to crack propagation, and confirmed that: during the surroundingborehole synchronization fracturing, under the inducing effect to the late crack by the early crack, the late crack reinitiated propagation and then ultimately formed into the mesh structure. The mechanism of permeability-increasing technology combining 3D swirling water jet with hydraulic fracturing is revealed.The technical method of permeability-increasing technology combining 3D swirling water jet with hydraulic fracturing is proposed. With collocation of conventional boreholes and reaming boreholes, the combined layout mode of center boreholes and surrounding boreholes is formed, this method is to increase the seam permeability through the center borehole fracturing or the surrounding boreholes fracturing synchronously. A complete new equipment, mainly made up of combination of high-pressure swirling water jet head, high-pressure auxiliary slagging drill-pipes, high-pressure rotary joints, remote monitoring and controlling system, used for reaming with 3D swirling water jet in underground coal mines, is developmented. With the self-developed outfit, this permeability-increasing technology was tested and applied in four typical mines and achieved a good field effect: It could increase 0.79 ~ 3.79 times of the pure gas extraction amount of boreholes. It is of broad site application prospects.