Analysis of factors affecting the internal seawater flow performance of pressure-retaining drilling tool for seafloor drill during drilling process

Abstract

In order to solve the low core collection rate during the natural gas hydrate pressure-retaining exploration, a pressure coring technology for seafloor drill is introduced, and the reasons for the decline in the core collection rate are analyzed. The mathematical model for predicting the water pressure change in the core barrel is established, and the design criteria of drainage path parameters of the drilling tool is formed and verified by engineering application. The results show that: when the valve spool opening x reaches 3.0 mm, the half cone angle α reaches 30°, and the equivalent annular area A1 reaches 30 mm2, the water pressure ΔP in the core barrel tends to be stable. Under the condition that the drilling speed is 20mm/s-50 mm/s and the ratio of the effective flow area of the screen is 0.2–1, the change of water pressure ΔP is little. When the valve spool opening is 4.0 mm, the velocity and pressure change of the seawater at the cone of the valve spool are stable. The engineering application in sediment formation shows that when adopting pressure coring method, the core collection rate exceeds 70% within the drilling speed of 20–30 mm/s.