Investigation of Enhancing Drill cuttings Cleaning and Penetration Rate Using Cavitating Pressure Pulses

Drilling efficiency is governed by rock cuttings removal by hydraulic forces. The
\nmechanical force introduced by the drill bit removes the rock chips from the parent
\nrock. The chips will be held down until the downward forces due to overburden
\npressure are overcome. The turbulent jet that flushes away these chips consists of
\nstatic impingement and dynamic pressure fluctuations. Instead of providing high
\npressure and hence enhancing the pressure fluctuations of the turbulent jet by rig
\npumps, the existing fluid pressure can be used more effectively.
\nA fluid passing a Convergent-Divergent venturi demonstrates significant pressure
\nfluctuations due to the cavitation phenomenon. As the fluid passes the vena-contracta,
\naccording to the Bernoulli’s principle, the fluid velocity increases and hence the
\npressure decreases. If pressure drops below the fluid vapor pressure, cavitation occurs
\nand bubbles are created.
\nDifferent prototypes were designed to investigate the probability of cavitation
\noccurrence by using CFD simulations. The successful designs were venturis with
\ndiameters of 4 mm and 12 mm. Simulation software applies tetrahedral meshing to the
\nprototype geometry for robust simulation results when geometry of the tool is
\ncomplex. The results obtained confirmed the pressure pulses and occurrence of
\ncavitation.
\nAn experimental setup consisting of a 12 mm venturi, two pressure sensors at
\nupstream and downstream, and 3 load cells in a triangular combination, and a flow
\nmeter was used. The flow rate range was from 10 USGPM to 70 USGPM. The
\ncavitation started at 25 USGPM with a shear noise that is the characteristics of a 
\niii
\ncavitating flow and the sensors recorded the pressure pulses at this point. The
\nmagnitude of pressure peaks ranged from 150 psi up to 600 psi.
\nThe second stage of the experiments was to investigate the effect of venturi and axial
\ncompliance in drilling. Compliant element used in these experiments consists of two
\nplates with rubber mounts embedded between these two plates in an equilateral
\nconfiguration. The rubber mounts enable the displacement of the upper plate on the
\nbase plate. An 8 mm venturi was also mounted on the drill string behind the bit as the
\nvibration source.
\nThe experimental results show that the tool starts to cavitate and produce vibrations.
\nThe tool was tested with compliance and without compliance to seek the effects of the
\ncompliant element. Results show that when rigid (no compliance), the vibrations
\nproduced, did not have any significant effect on the rate of penetration (ROP).
\nHowever, with integration of the compliant element, the vibrations produced by the
\ntool intensified the natural vibration of the compliant element and the penetration rate
\nincreased.

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