Analysis of Cuttings Backflow and Flow Path Erosion of Different Size Core Tools

In the process of rock coring drilling, poor rock-carrying ability and short bit life are the main reasons leading to low drilling efficiency, among which the cuttings deposition at the bottom of the annulus and the abrasive cuttings wear the drilling teeth are the main factors. Therefore, taking an ultra-deep well in the northwest of Sichuan Basin as the research object, a three-dimensional fluid-structure coupling numerical model of coring tool was established to analyze the flow velocity law of bottom hole and annulus under different sizes, and the influence of injection parameters on the flow channel erosion rate was investigated. The results show that: (1) There is a linear positive correlation between bottom hole flow rate and annulus flow rate with different sizes of coring tools. The attenuation rate of flow rate from bottom hole to annulus is about 90%, and both can meet the minimum rock carrying rate. (2) The inner wall of the outlet of the throat is in the area of sudden shrinkage of the fluid channel, and the erosion wear is the most serious; secondly, the fluid channel of the inlet of the throat decreases but does not decrease sharply, and the erosion phenomenon is more serious but relatively weak; (3) The comparison and analysis of Φ140, Φ203 and Φ172 core tools showed that the erosion rate in the erosion area decreased from 10-3 to 10-4 and 10-5 under the same injection displacement, showing an order of magnitude change; However, when the injection rate increases to 35 L/s, the erosion rate in the two areas is of the same order of 10-4. It is concluded that in the process of rock coring, it is necessary to select a larger size coring tool, reasonably increase the injection displacement, optimize the inlet and outlet flow channels and wall structure, avoid excessive increase in erosion rate, and reduce the impact of erosion effect on the fatigue life of drill teeth and wall. The research results can provide technical support for solving practical drilling problems on site.