Study on optimal design method of efficient rock-breaking nozzles under high confining pressure in deep wells

High-pressure jet drilling technology is an efficient and clean drilling speed increase technology, which has been widely used in middle and upper strata drilling. However, with the global oil and gas exploration and development gradually into deep water and deep depth, the high confining pressure environment of deep wells causes the poor rock-breaking effect of water jets, which seriously restricts the application effect of high-pressure jet technology. Aiming at the problem of the poor impact of jet rock-breaking under high confining pressures, an optimal design method of high confining pressure efficient rock-breaking nozzle in deep wells is proposed in this paper. The method combines theoretical analysis, numerical simulation, and laboratory experiments. Taking cone-straight nozzles as an example, the jet performance of cone-straight nozzles with different specifications under normal pressure and high confining pressures is analyzed, the optimal structure of cone-straight nozzles under high confining pressures is selected, and comparison experiments under high confining pressure verify the rock-breaking performance. The results show that the optimal structure of the cone-straight nozzle is as follows: the nozzle contraction angle (α) is 50°, the length-diameter ratio (l/d) is 2, and the nozzle inlet/outlet ratio (D/d) is 8. Compared with the conventional 0# nozzle, the rock-breaking depth in the confining pressure range of 40-80 MPa is increased by about 40.24 % and the rock-breaking diameter is increased by about 18.23 %. The research is expected to provide theoretical support for improving the application effect of high-pressure injection technology.