Ultra-short radius radial well (URRW) drilling is one of the crucial ways to improve the recovery efficiency and rejuvenate mature oilfields. However, due to the lagging development in drilling theory and related facilities, the technology is still encountering various bottlenecks such as high pressure loss, low rock breaking efficiency and low penetration rate, etc. Supercritical carbon dioxide (SC-CO2) jet has a low specific energy and high rock breaking efficiency when is used in drilling. Besides, the hydraulic power can be lowered due to its lower threshold pressure and viscosity compared with water. Thus it can be used in URRW underbalanced drilling and is expected to solve problems above. But properties of CO2, such as density and viscosity, are very sensitive to temperature and pressure, which makes prediction of flowing temperature and pressure distribution in wellbore very complicated.

In this paper, a theoretical temperature and pressure prediction model was developed based on conservation laws of mass, momentum and energy balance. The state-of-the-art equation of state for carbon dioxide proposed by Span and Wagner (1996) was adopted to calculate the in-situ density and isobaric specific heat for a certain segment of wellbore. Besides, the model of Vesovic et al. (1990) was adopted to calculate the viscosity and thermal conductivity of SC-CO2. Temperature and pressure were coupled during calculation by using an iteration scheme.

The results indicate that supercritical state of carbon dioxide, that is SC-CO2, can be reached at a relatively shallow depth. The temperature and pressure distributions of URRW drilling are quite different from those of conventional drilling. And temperature and pressure change dramatically due to the special structure of drillstring and narrow radial laterals. Finally, parameter analysis was conducted by calculating the influences of circulation rate, inlet fluid temperature, casing pressure and circulation time, well depth and borehole geometry on temperature and pressure of drilling fluid.

The novelty of this research is that the flowing temperature and pressure distribution in wellbore during drilling a URRW is studied accurately for the first time. Besides, the feasibility of drilling a URRW with SC-CO2 is validated from the aspect of temperature and pressure. This calculation model can be used to help design wellbore structure and operation parameters.

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