Application of combined drilling mode of pilot hole reaming in the control of directional well trajectory

Directional wells are affected by a number of factors. Their actual drilling trajectory sometimes is not along the designed track, posing great challenge to drilling operation. This paper does not take into account the influencing factors like formation properties, drilling parameters and wellbore trajectory, but uses simple and practical formula which also satisfies the engineering requirement to calculate and analyze the theoretical geometric build-up rate by combination of single-bend, monostable screw drill assembly. The paper studies the effect of wellbore and drillstring size on the build-up rate and finds that high build-up rate can be obtained more easily in small hole than in large hole under the same conditions. Hence, it is suggested that a small hole may be drilled first where high build-up rate is required, then reaming to the designed size with the combined drilling mode. Take Well X as an example, in directional wells where borehole trajectory is hard to control, the combined drilling mode can meet the engineering requirement, reduce drilling costs and shorten drilling cycle. The research results provide theoretical guide and reference for the design of wellbore configuration and control of directional well trajectory.