Research on a Triaxial Rate of Penetration (ROP) Model Related to Unloading in Oil & Gas Drilling

In oil & gas drilling engineering, the rock breaking efficiency as well as the trajectory of the Well should be quantitatively and accurately described by an ideal triaxial Rate of Penetration (ROP) model, taking into account the various objective and subjective factors. However, with the existing triaxial ROP models, it is difficult to achieve these goals. The applications of the existing ROP models are limited, especially in under-balanced drilling (air drilling, foam drilling etc.), because the unloading effect (i.e. the effect of the bottom hole differential pressure on the formation force) has been rarely considered. On the basis of a rock/bit interaction model, a new triaxial ROP model including the unloading effect, is developed in this paper. This new model focuses on the effect of the bottom hole differential pressure on the formation-forces. Case studies in this paper also indicate that the inclination and the azimuth forces of the formation will increase with the decrease of bottom hole differential pressure. This explains the reason why the under-balanced drilling has a greater tendency to produce hole deviation, than that in mud drilling. The new model presented in this paper, is applied to predict the well trajectory, and the ROP of the well Longgang-2, in China's Sichuan oilfield. The results of this field application indicate that the new model is quite feasible.