Research on Flow Field Optimization in Large-Slim Boreholes

Large-Slim (L-S) boreholes are characterized by having two or more annular sections with significantly different equivalent diameters within the same open hole interval. This wellbore configuration is commonly found in deep and super-deep wells. Because of the disparity in annular clearance diameters, L-S borehole structures often encounter issues such as inadequate cuttings removal in the upper portion and high frictional losses in the lower part. It is challenging to strike a balance between wellbore cleaning and the workload of surface equipment, necessitating optimization. Considering the High-Pressure High-Temperature (HPHT) performance of drilling fluid, a flow model for L-S boreholes was established, and a flow field adjustment tool was developed. The flow rate distribution and flow resistance models of this tool were also constructed, thereby forming the flow field optimization technology for L-S boreholes. Taking the PS-6 well as an instance, an optimization analysis was carried out. The results indicate that, while ensuring cuttings removal throughout the annulus, the bottom hole pressure has been decreased to an extent of 15 MPa, and the standpipe pressure has been reduced by 27 MPa. This has great importance for efficient and rapid drilling in complex wellbore structures.