Simulation of Air and Mist Drilling for Geothermal Wells
Authors: Mitchell R.F. 
Published: December 2007

An improved method for calculating downhole temperatures, pressures, fluid densities, and velocities during air drilling has been developed. Improvements on previous methods include the following.A fully transient thermal analysis of the wellbore and formation is used to determine the flowing temperatures.The effects of flow acceleration are included explicitly in the calculation.The slip velocity between the gas and the cuttings is determined by the use of a separate momentum equation for the cuttings.The possibility of critical flow in the wellbore is tested and appropriate changes in the volume flow rate and standpipe pressure are made automatically.The standpipe and flowing pressures are predicted.The analysis is conservative. The effect of the cuttings on the wellbore flow will tend to overpredict the required volume flow rates.

In this paper, the basic equations of fluid flow for a gas with cuttings and mist are presented along with a numerical method for their solution. Several applications of this calculational method are given, showing the effect of flow rate and standpipe pressure in typical air and mist drilling situations.

Introduction

Air and mist drilling have several advantages over conventional drilling fluids for geothermal drilling. The principal advantages are higher penetration rates, longer bit life, and no lost-circulation problems. The usual disadvantages, such as control of fluid influx and high pressure zones, are not normally a factor in geothermal well drilling.

To realize these advantages, it is important to maintain adequate circulation. Determining the required volume flow rate to maintain this "adequate" circulation has always been difficult. The best available technique has been the analysis developed by Angel. This analysis allows estimation of volume circulation rates for various hole sizes. drillpipe sizes, and penetration rates.

One difficulty with Angel's result is that the equation giving the volume flow rate must be solved by trial and error. This difficulty is avoided by use of the charts prepared by Angel, provided the case of interest is tabulated or can be estimated from similar cases. A second difficulty is that the drill cuttings are assumed to travel at the same velocity as the air. Angel notes that this is not a conservative assumption, and the analysis presented here demonstrates that the flow rates he predicts are 20 to 30% low. The downhole temperatures used for Angel's chart are assumed to be 80 degrees F at the surface, increasing 1 degrees F/100 ft of depth. There is no convenient way to convert to other temperatures. A final consideration is that the Angel charts do not apply to mist drilling. The addition of water to the air requires increases in both the volume flow rate and standpipe pressures to maintain the same penetration rate.

The flow model discussed here was developed for Sandia Laboratories by Eneriech Engineering and Research Co. for use in a general purpose wellbore thermal simulator called GEOTEMP. Some features of GEOTEMP have been described in previous papers; however, the major technical features are summarized as follows.

JPT

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