Errors Inherent in Rig Measurements of Drillpipe and Casing Pressures

Abstract

A conunon complaint of drilling personnel is that modem well control strategies require pressure measurement accuracies not possible with typical rig equipment. This paper documents a study made (1) to identify and quantify sources of significant error inherent in rarote measurements of drill pipe and casing pressures, (2) to determine their cumulative effects on measurement accuracy and (3) to define the pressure response limits of a typical rig system. For this study, the pressure measurement system was duplicated in the laborato:ry using rig canponents supplied by several vendors.Response characteristics of each system canponent were studied separately and are presented in four graphic displays and one table. A numerical example details their use in the analysis of a complete system. Conparison of predicted and observed pressure responses for several system configurations confinned the results and method of analysis developed in this study.Gage-reading correction factors can now be calculated with certainty. Of greater significance, is the ability to predict the upper limit of reliable response for a given pressure measurement system. A detailed analysis can show how it is possible that a system equipped with a 10,000 psi gage can indicate pressure extremes no greater than, say, 5000 psig.Errors in the rarote measurement of drill pipe and casing pressures have been and continue to be a problem in drilling and well control operations. Drilling personnel attending the Louisiana State University (LSU) IADC Well Control School relate References and illustrations at end of paper. 427 experiences of pressure measurement errors that vary from 100 to sometimes several thousand psi. As a consequence, they will argue that precision pressure control, as dictated by m::>dem well control procedures, is unrealistic and therefore, of no great concem to the operator.Errors of the same magnitude have been duplicated at the LSU Research and Training Well Facility and in the laboratory, using standard oil field equipment. Reported here are the results of a study to identify and quantify the source of these errors and to develop a methodology by which corrections for similar rig systems could be determined.As used here, remote measurement refers to those systems that connect the pressure source to the indicator gage or sensor by means of a hydraulic fluid link. The gage can be either digital or analog and is located on or near the rig floor, for example on the choke control panel. Figure 1 depicts the pressure measurement system of interest. It has four basic canponents: ( 1) pressure source, (2) gage protector, (3) transfer fluid link, and (4) pressure measuring device.Gage protectors are normally installed on the standpipe, choke and pump manifolds. The pressure derived fran any one of these sources will, henceforth, be referred to as the process pressure. The gage protector serves to prevent mixing of the process fluid (drilling mud, gas, salt water) with the clean transfer fluid (instrument oil) which contacts the pressure in...