Comparison Of Calculated And Observed Pressure Drops In Geothermal Wells Producing Steam-Water Mixtures

Abstract

Abstract This paper contains comparisons of calculated and observed flowing pressure profiles from geothermal wells located in the United States and the Philippines. Comparisons are included for tubular flow as well as flow through the casing-tubing annulus. Our comparison shows that for tubular flow, the Orkiszewski correlation makes the best prediction, whereas for annular flow, no clear-cut choice of a correlation can be made. Introduction The capability to accurately predict flowing pressures in a geothermal well producing steam-water mixtures under various operating conditions is of value for several reasons: general engineering essential to evaluation of the geothermal reservoir and proper reservoir management; optimization of wellbore design from well deliverability considerations; and minimization of scale deposits in the wellbore. This predictive capability is especially important because of the difficulty of running flowing pressure surveys in geothermal wells. These wells are characterized by very high fluid velocities, which sometimes make it impractical for the pressure recorder to traverse downward in the well. There have been cases of pressure recorders thrown out of the wellbore due to high fluid velocities. In calculating flowing pressure profiles for oil wells, phase transfer between oil and gas requires a rather phase transfer between oil and gas requires a rather simple treatment, and is accomplished through the use of solution gas-oil ratio relationships. In geothermal wells, however, phase transfer between water and steam attains critical importance, and calculations must incorporate the steam tables accurately. Pressure profile calculations for geothermal wells vary from those for oil wells in another important aspect in that the temperature of the fluid must be computed precisely. For calculations included in this paper, a computer program incorporating three previously published program incorporating three previously published correlations for predicting two-phase flow in vertical pipes—coupled with equations for phase transfer and pipes—coupled with equations for phase transfer and wellbore heat loss—was used to calculate pressures in a number of geothermal wells in which flowing pressure surveys had been run. The paper contains comparisons between the calculated and the observed pressure profiles. profiles. This comparison encompasses a wide range of flow rates and wellhead pressures, and includes tubular as well as annular (between casing and tubing) flow. The three correlations used are: Orkiszewski; Hagedorn and Brown; and Beggs and Brill. The phase behavior relationships used in this work are for pure water, and do not include the effects of dissolved salts. However, the waters in the wells we studied were of low salinity. Also not considered is the effect of non-condensible gases present in the fluid. We do not consider this to be a significant limitation, because in our observation wells, noncondensible gases, consisting almost entirely of carbon dioxide, constituted a small fraction of the steam phase. This comparison of computed and observed pressure drops in flowing geothermal wells can help determine the degree of confidence an engineer should have in results predicted by the three correlations evaluated. The best correlation does a satisfactory job of predicting pressure drops, and can be used in predicting pressure drops, and can be used in deliverability prediction calculations. Optimization of wellbore design of future wells in a partially developed field can be accomplished by calculating production rates for different flow string diameters at a production rates for different flow string diameters at a given wellhead pressure, and comparing the benefit of increased flow rates against the higher cost of drilling and completing larger diameter wells. Since the precipitation of calcium carbonate scales, encountered in many hot water wells, is related to the pressure and temperature conditions in the wellbore, pressure and temperature conditions in the wellbore, calculations can be made to estimate the depth at which scale precipitation would commence for various wellbore diameters and mass flow rates. This can assist the engineer in the selection of operating conditions that will tend to cause scaling at shallower depths, thus requiring easier clean-up operations.