Low Invasion Corehead Reduces Mud Invasion While Improving Performances

A corehead was designed, manufactured and tested to reduce fluid invasion of the core. This is obtained by minimizing the exposure time of the core to the drilling fluid in increasing the rate of penetration (ROP). The design incorporates a medium heavyset polycrystalline diamond compact (PDC) cutting structure developed in accordance with cutting models and balancing methods used for drill bits. The highest ROP is achieved by a particular hydraulic design: flow ports shape and positioning to clean the cutting structure enhance the drilled cuttings removal while preventing drilling fluid in the throat of the corehead. Moreover, an internal lip works with a special inner barrel shoe to effectively seal off mud flow from the throat. All the design features have been subjected to laboratory tests, including measurement of pressure drop across the corehead and flow visualization studies. Flow visualization tests include high-speed filming of the flow and paint tracing to indicate the special flow pattern. In conjunction with lab tests, a numerical simulation was performed using fluid dynamics software to optimize hydraulic parameters. The low invasion core bit has been used in numerous applications. The performance achieved was significantly better than the average achieved over a period of years using various PDC coreheads. The rate of penetration was increased by a factor of 4.8 and bit life by 2.3 (often with reusable condition).