Based on lab tests and theoretical analysis, a dual-factor method for calculating weight distribution between the pilot bit and underremer is presented.

The novel method yields much more precise results than empirical values, thus can be used to determine weight distribution factor for an underremer-bit assembly.

A weight distribution factor f was used in this paper to represent the bit-underreamer interaction with the formation and reacts to weight inputted from the surface. As to the factor f, traditional calculation method only focuses on geometrical structure of the pilot and main hole, results arising therefrom are too rough to be used. Dual-factor method avoids such drawbacks by introducing two factors. A factor k termed sharpness represents drilling tools’ ability to penetrate the formation, and the other factor σ termed compressive stress represents formation strength. The two factors are defined by lab tests. Influence on the distribution of each factor was analyzed, and results show that weight distributed on the underreamer will increase as the formation stress σ increased, and decrease as the underreamer sharpness k decreased.

Well known in the reaming while drilling process is the importance of proper matching for the bit and underreamer. Improper drilling assembly will lead to sever vibration and decreased ROP for the drilling system. An underreamer should not be designed and employed as an independent tool. Dual-factor method provides a good reference for matching underreamer-bit assembly. Also, applying this method, design process for an underreamer can be optimized, and reaming while drilling operating will become more stable.

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