Prediction of Critical Pipe Running Speed During Tripping in Drilling Operations

Abstract

Blowout and loss of circulation are two serious accidents that can happen while drilling without the control on drillstring running speed during tripping operations. Exceeding the critical running speed during tripping-out (high swabbing pressure) and lack of control of mud rheology are the main causes for blowout. Also, exceeding the critical running speed while tripping-in (high surge pressure) and lack of control of mud rheology are the main causes for loss of circulation. Many factors which affect surge and swab pressures must be precisely selected in order to control kicks or blowouts and to prevent loss of circulation. Prediction of the critical pipe running-in and -out speeds during tripping operations is therefore very important. This can be done by the evaluation of several basic fluid flow equations. A computer program has been developed to simplify these calculations. The developed program requires fairly simple input data which can be measured in laboratory in addition to hole and drillstring dimensions. The output of this program then is transformed into graphical form from which the safe running-in and -out speeds during tripping can be predicted. As an alternative and direct way to predict the critical pipe running speed during tripping operations two correlations have been developed. These correlations account for the governing factors which affect the tripping-in and -out speeds including mud properties and drillstring and hole configuration. A comparison between the critical pipe running speed computed using the two methods has been outlined and very good accuracy based on the coefficient of linear correlation (r2) and the standard error of estimate (SEE) (r2 = 0.995 ; SEE = 0.39 for running-out correlation and r2 = 0.875 ; SEE = 0.817 for running-in correlation) have been obtained. Critical tripping speeds were found to be greatly dependent on mud weight and rheology, hole diameter and drillcollars-to-drillpipe length ratio.