Recently high pressure water jet drilling is proposed for coalbed methane development by connecting nozzle with coiled tubing directly. However, rock breaking efficiency is still compromised because of the hydraulic energy loss when water flows through the small size coiled tubing. Cavitation is the formation of vapor in a liquid when the local pressure in the fluid is reduced to its saturated vapor pressure. The violent bubble collapse can cause severe damage to the materials caused by the high collapse pressure and temperature.

In this paper, based on bottomhole flow field analysis and nozzle survey, a convergent-divergent nozzle is optimized to trigger cavitation in water jet for multilateral drilling. A mathematical model coupling the flow field and bubble dynamics is used to predict the effective standoff distance for cavitation erosion. The classic Rayleigh-Plesset equation is solved by first order Euler finite difference at each step with a thermodynamic model. Then, laboratory experiments are conducted in a pressure vessel to validate the calculation results based on pitting analysis.

The results show that the divergent section of the nozzle is critical to generate low pressure regions inside the throat section, but not hindering bubble transportation towards the specimen outside. The optimized divergent angle is 20° and divergent length is 4 times nozzle diameter. In addition, it is demonstrated that the cavitation bubbles formed inside the nozzle can be transported to the outside flowfield against adverse pressure gradient due to its inertia. The effects of flow rate and ambient pressure on the effective standoff distance are investigated. Given certain ambient pressure, there exists a threshhold of flow rate, below which cavitation bubble can't be transported outside the nozzle. The experiment shows the maginitude of the cavitation impacts lies in the range of 800 MPa to 2700 MPa.

The present research, to our best knowledge, is the first systematic investigation on utilizing cavitation to increase erosion rate in multilateral CTD drilling. The nozzle proposed has a good compatibility with the coiled tubing. The proposed mathematical model can be used to assist engineering design and operational parameters adjustment.

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