Downward directional drilling nitrogen foam power-law multiphase flow slag unblocking technology

In order to solve the technical bottleneck problem of downward directional drilling coal cuttings blockage and gas drainage with concentration but no flow, the nitrogen foam power law multiphase flow slag carrying and unblocking technology of downward directional drilling was proposed. Theoretical analysis, numerical simulation, om-site industrial experiments and other methods were used to research the causes of coal cuttings blockage caused by downward directional drilling gas drainage in coal mines, and the theory and practical application of nitrogen foam power law multiphase flow slag carrying in downward directional drilling annular. Numerical simulation was carried out by a turbulence model containing wall function coupled with gas flow and coal deformation, and the elastic displacement of the borehole wall caused by the fluid pressure after drilling extraction was analyzed. Numerical simulation with a three-dimensional Coulomb failure criterion model solved for the pressure changes caused by extraction, as well as the stress, strain, and displacement changes induced by the pressure changes, and the collapsing risk of downward directional drilling. The stability of nitrogen foam, the fluidity of foam in drill pipe, at the drill bit, in the annulus, the suspension performance of coal powder in foam, the pressure loss of foam slagging in the annulus, and the upward slagging stress on the foam were analyzed by the power law multiphase flow theory. Based on the theoretical analysis of downward directional drilling annulus nitrogen foam power law multiphase flow slag carrying theory, the stable nitrogen foam generator, foam generating injecting system, downward directional drilling nitrogen foam drilling process, and gas drainage drilling nitrogen foam secondary unblocking process were designed, and the downward directional drilling nitrogen foam slag removal on-site industrial experiment was conducted. The results indicated that, the frictional drag force of the borehole wall and the centrifugal force at the bends could cause the aggregation of coal particles and increase the risk of borehole wall collapse. The coal powder particles carried by the gas will collide with the borehole wall to deform or detach the borehole wall, which would lead to the collapse and blockage of the extraction borehole. The theoretical system of downward directional drilling annulus nitrogen foam power law multiphase flow slag carrying and unblocking technology, which included nitrogen foam stability, fluidity, coal powder suspension performance, annular space foam pressure loss, and analysis of slag removal force, can well provide theoretical support for downward directional drilling nitrogen foam slag removal. In the on-site industrial experiments, the initial mixing and pure amount of the nitrogen foam slag removal drilling hole increased by 6.5 and 6.4 times, respectively, compared with the water drainage drilling hole, and the mixing and pure amount increased by about 10 times in 40 days, which indicated that the nitrogen foam slag carrying and unblocking technology can significantly improve the gas drainage efficiency of the downward directional drilling hole.