Recent Developments in the Interpretation and Application of DST Data
Authors: Maier L.F. 
Published: December 2007

This paper presents a summary of the latest drill-stem test interpretation techniques which may be applied by well operators to the majority of field cases. Recommendations are submitted for obtaining better DST data, and the evaluation of data received is discussed. Oil, gas and multiphase flow are considered. Methods of calculation are advanced to help determine reservoir pressure, productivity index, transmissibility, damage ratio, radius of investigation and absolute open-flow potential. Application of calculated information to well completion is discussed, and several gas case histories are tabulated which illustrate the general correlation between DST information and actual well performance.

Introduction

The information contained with a drill-stem test report may be considered to be of four general categories:factual data including statistical well information and a description of the testing tools;measured data concerning the recovered fluids and their properties, the time periods involved and general remarks based on observations during the test;recorded pressure and temperature data; andinterpretation calculations, where applicable.

Greater emphasis is now being placed on the quality and completeness of the physical measurements and observations made during the test. This has been necessary in order to complement the accuracy and dependability of the pressure-recording instruments. Interpretation is considered an important part of the drill-stem test, and for this reason the necessary calculations are now being done on the majority of tests run in Canada. This information has proven to be of considerable assistance to the well owner in well-completion. formation-evaluation and hydrodynamic studies. This paper is an up-to-date presentation of practical methods of drillstem test interpretation and suggestions for improved testing techniques generally applicable to Canadian conditions. Previous authors' interpretation methods are combined with new developments and modifications to present a comprehensive interpretation guide for the great majority of DST results encountered.

Evaluation of Basic DST Data

Before he can begin the drill-stem test interpretation, the well operator must evaluate the basic data which were measured and recorded. Examination of the pressure data and the charts will reveal whether or not the test was satisfactory from a mechanical viewpoint and will help to verify the accuracy of the gauges. A zero-pressure base line is drawn on the chart by the bourdon tube pressure-recording instrument prior to assembly in the testing string, and the recorded pressure must "zero-in" on this base line at the time of tool assembly and "zero-out" during disassembly after the test. Since stair-stepping pressure curves may be caused by gauge malfunction, they must be considered to be unrepresentative of true formation pressures. Depending upon the severity of the steps, such pressure data may be of some value but must be used with extreme caution. If all gauges appear to have recorded correctly, the proper way to check their accuracy is to compare their recorded pressures at such key points as initial closed-in pressure, final flow pressure and final closed-in pressure. If the initial and final hydrostatic pressures are clearly defined on the charts, these may be used as well. The recorded pressure difference between any two gauges at these key points is compared with the predicted difference due to the hydrostatic head between the gauges. The discrepancy between the recorded and calculated difference is divided by two, and this quantity is calculated as a percentage of the average pressure of the two gauges. This is equal to the possible per cant error of each gauge, assuming their accuracies to be equal. The volumes of liquid recoveries often are difficult to determine accurately due to intermingling of the various fluids produced and, occasionally, due to severe gas-cutting. Gas flow measurements may be hampered by liquid flow through the flare line.

JPT

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