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Evaluation of Subsurface Pressures Obtained from Drill Stem Testing: Abstract
This paper dealt with the data produced by bottom hole pressure recorders in drill stem testing. Fourteen examples of very accurate subsurface pressure versus time charts and their interpretations were given. Basic points of various types of testing charts, as well as unusual cases, were discussed. The importance of accuracy of the recording instruments and in the handling of the records so produced was carefully pointed out.
A steadily increasing interest in the evaluation of drill stem testing pressure charts has become prevalent throughout the Oil Industry. Proper interpretation of this data is highly important to the proper use of drill stem testing.
It should be understood that careful consideration must be given to all factors involved in the planning of the test in order to obtain the desired results. These in part include:
1. The removal of all tight spots in the hole.
2. Removal of all drilled cuttings and/or carvings from the well bore.
3. Proper conditioning of the drilling fluid for the testing operation.
4. Careful selection of the proposed packer seat.
5. Clear cut instructions to the service company's representative in order for them to supply the correct tool assemblies and necessary instruments to obtain the most accurate record possible of the entire testing operation.
This paper dealt with the subsurface recordings obtained on a group of selected drill stem test charts which indicate significant conditions prevailing during the test. These charts were recorded by the B. T. (Bourdon Tube) Pressure Recording Device which is a large size of parallel design to the Amerada RPG-3 Instrument. The charts produced with this instrument are read with a micrometer chart reader whose error is plus or minus one thousandth of an inch. The deflection readings are then converted to pressure from individual calibration curves which are drawn on 10x10 cross section paper, the vertical scale having a value of .100" per inch and horizontal scale equal to 100 PSI per inch. With this reading technique extreme accuracy is obtained.
The accuracy of the recording gage is maintained by re-calibration on a 60-day interval using a dead weight tester with a maximum error of .1 of 1% based on a "G" factor of 980.076 CM/SEC2 at an operating temperature of 68° F ± 10°. The instruments are calibrated at three previously selected temperatures in an oil bath with temperature control of ± 2° F. This extreme care and control during calibration insures instruments of the highest rated accuracy and sensitivity to the industry for drill stem testing.
As can be seen by the relatively few charts which were discussed, it is necessary to have thorough knowledge of the basic tools and their auxiliary assemblies. Also, extreme care should be taken in preparation of the well prior to testing. The instruments used must be the best available both in accuracy and sensitivity. The chart reproductions must be an exact duplication of the original without falsification or retouching. With all the above adhered to, very intelligent study must be made of the charts, the recovery and the entire available data in order to obtain the full value of each drill stem test.
Acknowledgments and Associated Footnotes
1 Formation Testing Service Halliburton Oil Well Cementing Co.
Copyright © 2006 by the Tulsa Geological Society