The Influence of Drilling Conditions on Annular Pressure Losses

Abstract

SPE Members Abstract Combinations of drill string motion, annular eccentricities and fluid temperatures along the length of the annulus affect fluid flow regimes that control annular pressure losses. In a research well with conventional annular configuration, increases in annular pressure losses were observed to follow increasing drill string rotation. Drill string rotation can result in the formation of centrifugal instabilities that can increase annular pressure losses before the transition to turbulent fluid flow. These instabilities partially take the form of Taylor vortices which lead to irregular fluid flow. Rapid and cyclic changes in rotation rates can lead to synchronous changes in down hole pressures. Introduction Annular pressure losses have received considerable attention in theoretical analyses, laboratory evaluations and actual well measurements. Much of this effort has been directed toward the effects of isolated variables. This has been the case whether the variable has been drill string rotation, annular eccentricity or drilling fluid temperature and properties. With the increased attention to severe drilling conditions such as slim hole drilling and high temperature/high pressure drilling, the understanding of the actual down hole phenomena during drilling operations becomes more important. This is particularly true when small margins exist between safe drilling operations and severe well control problems. Without a doubt the evaluation of an isolated drilling variable remains an important step in understanding the effects that drilling conditions have on annular pressure losses. However, as many of the variables are dependent on others, a goal of the drilling industry should be an better understanding of the total effect of integrating and combining these variables into operational applications. In a slim hole configuration with the use of unconventional drilling machinery, an increase in annular pressure loss with drill string rotation has been reported by Bode, Noffke and Nickens. In their study it was shown that the ratio of annular pressure loss with rotation to that without rotation could be 2.2 at 600 rev/min. In addition, it was stated that in this slim hole configuration the annular pressure loss can contribute 90% of the total fluid circulation pressure loss. This is just the inverse of more standard drilling where around 10% of the total circulation pressure loss can originate from the annular pressure losses. In a comparison of well data with theoretical models some of the effects that temperature have on annular pressure losses have been presented by Minton and Bern. In that study care was taken to obtain data for steady state conditions and to compensate for small fluctuations in the circulating fluid so that the effects of temperature could be evaluated. By including factors for temperature and pressure there was a fair degree of agreement between the calculated values and the down hole data measurements. The highest down hole steady state temperature was 120 degrees C. It was stated that the accuracy of these predictive techniques require further refinements. In that study the effects of any drill string rotation was not mentioned. Additional studies on down hole measurement of annular pressure losses in conventional annular dimensions have been reported. As the primary variable in these studies was pump rate, the effects of drilling fluid temperatures or drill string rotation were not considered. The well site and laboratory studies on the effect that drill string rotation has on annular pressure losses readily suggest the existence of possible contradictory results. P. 553^