Papers - - Production Engineering - Precision in Bottom-hole Pressure Measurement (T. P. 1942, Petr. Tech., Nov 1945, with discussion)

Errors in measurement of reservoir pressure include: (I) gauge errors, and (a) interpretation errors. Gauge errors may be reduced by: (a) reading charts with a comparator microscope, (b) use of hard metal stylus points to give clear lines, (6) insuring temperature equilibrium, and (d) making multiple tests. Check calibrations are equally as important as check well tests. A statistical analysis of such check runs shows quantitatively the advantages of a standardized "zero pressure line'' and of the use of field calibrations made at bottom-hole temperature on the same chart run in the well. Interpretation errors include: (a) fluid heads, which should be corrected to the top of the perforations on the basis of well-fluid gravity and from the top of the perforation to the datum level on the basis of reservoir-fluid gravity, and (b) gradients around the well. Conventional shut-in periods may be entirely inadequate to overcome these, necessitating a reservoir study based on build-up curve data, reservoir history, core analysis and any other available information. Interest in engineering studies of the pressure changes in oil and gas reservoirs is increasing. This type of work yields important information and doubtless will be greatly extended in the future. One of the factors limiting the work, however, is the accuracy with which the reservoir pressures can be determined; for if the inaccuracies in the pressure measurements represent an appreciable percentage of the pressure changes, the engineer is seriously handicapped, and must delay the reservoir study until the reservoir pressure has changed a relatively large amount. This delay can greatly reduce the benefits of such studies, at least from the point of view of aiding the leasing, drilling and completion programs. Thus, from the point of view of reservoir analysis, it is frequently important to obtain the most accurate pressure data possible early in the history of any field—particularly for the gas-distillate fields now being developed. This paper deals with improvements in the measurement of bottom-hole pressures that permit pressures even as high as 4000 or 5000 lb. per sq. in. to be determined with gauge errors of only+ ±I to 2 lb. per sq. in. Since pressures out in the reservoir rather than in the bottom of the hole concern the reservoir analyst, differences between bottom-hole pressures and actual reservoir pressures are also considered. In general, errors in pressure measurement may be divided into: 1. Gauge errors: a. Errors due to inaccuracies in reading the pressure charts. b. Errors due to irreproducibility of the pressure element and recording system. c. Errors caused by failure to allow the gauge to reach temperature equilibrium. 2. Internretation errors: a. Errors caused by a fluid head between the gauge position in tubing and the pressure at the datum level in the reservoir. b. Pressure gradients in the reservoir.