Precision in Bottom-hole Pressure Measurement
- E.R. Brownscombe (The Atlantic Refining Co.) | D.R. Conlon (The Atlantic Refining Co.)
- Document ID
- Society of Petroleum Engineers
- Transactions of the AIME
- Publication Date
- December 1946
- Document Type
- Journal Paper
- 159 - 174
- 1946. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 5.7 Reserves Evaluation, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 5.5.2 Core Analysis, 4.3.4 Scale, 2 Well Completion, 5.6.2 Core Analysis, 5.6.4 Drillstem/Well Testing, 5.2 Reservoir Fluid Dynamics, 2.2.2 Perforating
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Errors in measurement of reservoir pressure include: (1) gauge errors, and(2) interpretation errors.
Gauge errors may be reduced by: (a) reading charts with a comparatormicroscope, (b) use of hard metal stylus points to give clear lines, (c)insuring temperature equilibrium, and (d) making multiple tests. Checkcalibrations are equally as important as check well tests. A statisticalanalysis of such check runs shows quantitatively the advantages of astandardized "zero pressure line" and of the use of field calibrationsmade at bottom-hole temperature on the same chart run in the well.
Interpretation errors include: (a) fluid heads, which should be corrected tothe top of the perforations on the basis of well-fluid gravity and from the topof 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 entirelyinadequate to overcome these, necessitating a reservoir study based on build-upcurve data, reservoir history, core analysis and any other availableinformation.
Interest in engineering studies of the pressure changes in oil and gasreservoirs is increasing. This type of work yields important information anddoubtless will be greatly extended in the future. One of the factors limitingthe work, however, is the accuracy with which the reservoir pressures can bedetermined; for if the inaccuracies in the pressure measurements represent anappreciable percentage of the pressure changes, the engineer is seriouslyhandicapped, and must delay the reservoir study until the reservoir pressurehas changed a relatively large amount. This delay can greatly reduce thebenefits of such studies, at least from the point of view of aiding theleasing, drilling and completion programs. Thus, from the point of view ofreservoir analysis, it is frequently important to obtain the most accuratepressure data possible early in the history of any field-particularly for thegasdistillate fields now being developed.
This paper deals with improvements in the measurement of bottom-hole pressuresthat permit pressures even as high as 4000 or 5000 lb. per sq. in. to bedetermined with gauge errors of only 1 to 2 lb. per sq. in. Since pressures outin the reservoir rather than in the bottom of the hole concern the reservoiranalyst, differences between bottom-hole pressures and actual reservoirpressures 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 recordingsystem.
c. Errors caused by failure to allow the gauge to reach temperatureequilibrium.
2. Interpretation errors:
a. Errors caused by a fluid head between the gauge position in tubing and thepressure' at the datum level in the reservoir.
b. Pressure gradients in the reservoir.
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