INVESTIGATION OF BIT HYDRAULICS FOR GASIFIED DRILLING FLUIDS

This paper presents a mathematical model for calculating bit pressure drop for gasified fluid drilling (GFD). The proposed model is valid for both sonic and subsonic regimes. The exact and complete solution with theoretical and experimental verifications is first presented here. Unlike the approximate models, the kinetic energy term, compressibility factor of the gas phase and internal energy changes throughout the bit are considered in the proposed model. In addition, the model uses a mixture sound velocity approach for determination of the sonic velocity boundary for the fluid. The proposed model assumes that, the dispersed bubbly flow pattern and no radial velocity gradient develops through the bit nozzles which are verified experimentally. The performance of the proposed model is also tested for airB_water mixtures.

The bit pressure drop reduces with an increase in bottom hole pressure. Although liquid rate has a noticeable effect on bit pressure drop, the effect of gas flow rate is not as significant.

The internal energy effect on bit pressure drop becomes more pronounced at high gas flow rates.

When the sonic boundary is approached there is an excessive accumulation inside the bit and a sudden increase in the upstream pressure is observed. At lower bottom hole pressures, the fluid can reach the sonic limit even at lower gas flow rates. Since sonic flow is not practical for GFD, the proposed model calculates the optimum liquid and gas flow rates keeping the flow in subsonic region.

Bit pressure drop predictions of the existing models significantly deviate from those of the exact solution. Initial kinetic energy not included in the other models is primarily responsible for this deviation.

Very little is known about the bit hydraulics of GFD. The analysis presented here is the first study investigating the exact and complete analysis of the problem.