This paper concludes the analysis of the results from two sets of experimental tests performed at PETROBRAS real scale test facility aiming the evaluation of solids return times in aerated fluid drilling. It reviews important results on effects such as liquid and gas injection rates and particle diameter from air-water experiments1 and extend the discussion by adding the effect of the viscosity of liquid phase and annular back pressure on the transport capacity of solids in a vertical well with an aerated polymer-based drilling fluid. Results indicate that the gas has a major effect in accelerating the liquid phase, which would be responsible for carrying the particles to the surface. The trend was confirmed in the experiments with polymer-based mud, where lesser return times were associated with the fluids with higher liquid-phase viscosity. The concept of actual liquid velocity coupled with a procedure for particle sedimentation velocity calculation in non-Newtonian fluids adequately reproduced the experimental results.
Optimizing gas and liquid flow rates in light weight fluid drilling design is a complex task which involves knowledge on two-phase flow hydraulics. A lot of effort has been spent in the prediction of the impact of gas and liquid flow rates on bottom hole ECDs (Rommetveit et al.).
Naturally, a desired ECD in the bottom can be achieved by several combinations of liquid and gas flow rates. The decision of which values to use will depend on downhole motor requirements and hole cleaning criteria. These parameters usually define the operational window to work while drilling a certain well.
Suppliers generally specify the required minimum and maximum liquid flow rates for driving downhole motors. On the other hand, flow rate requirements for adequate hole cleaning using aerated fluids poses as a much more challenging task.
Compared to the widespread implementation of the various underbalanced drilling techniques worldwide, very little has been done in investigating hole cleaning in light weight fluid drilling. The most challenging drilling scenarios are the highly inclided and horizontal wells. Some important work can be cited, such as the attempt from Vieira et al., that working in low pressure and temperature pilot-scale surface facility, developed empirical correlations to estimate minimum liquid and gas flow rates requirements for proper hole cleaning in inclined and horizontal air-water drilling. Li and Walker applied concepts developed to estimate hole cleaning time and wiper-trips velocity for conventional overbalanced conditions to underbalanced drilling while working in very similar conditions as Vieira. More recent comprehensive modeling efforts can be found in the work of Doan et al. and Zhou et al.. This last work offers data from a well instrumented pilot scale facility, including pressures up to 500 psig and temperatures up to 80°C, but with small gas-liquid ratios.
Although academia and part of industry already turned their attention to more complex situations, the truth is that many questions regarding optimum flow rates design still exist while drilling vertical wells, which are still the typical candidate of aerated mud drilling. Under this scenario, Guo et al. proposed a simplified model for liquid gas flow rate prediction which would provide a given solids concentration in the annulus. Adewumi et al.8 performed pilot scale experimental studies for air/solids flow. A major problem is that scaling down techniques seems limited in representing adequately the phenomena involved in the three phase flow. Primary field experience indicates that fluid effective velocities of 120 and 150 ft/min would clean vertical and directional wells, respectively.
The minimum velocity requirements for hole cleaning depends on several aspects, including fluid and solids properties, wellpath, etc. Consequently, in many cases, the velocities normally used in the field may be much greater than necessary, resulting in high drilling costs.
This was the motivation for the development of an experimental program on PETROBRAS real scale test facility, aiming the determination of solids return time for different conditions.