The pressure behaviour of a well can be aeasily measured and is extremely useful in analyzing and predicting reservoir performance or diagnosing the condition of a well. Various instruments to measure flowing and static pressures in oil and gas wells have been in use since the 1920’s. The recording devices that have been used include mechanical, (the Borden tube which records via a stylus mark on a blackened metal sheet), sonic (echometers which measure liquid levels) and electronic instrumentation (which measure pressure and temperature).

BY:
DOMINIQUE BOURDET AND PIERS JOHNSON

1. INTRODUCTION

The pressure behaviour of a well can be easily measured and is extremely useful in analysing and predicting reservoir performance or diagnosing the condition of a well. Various instruments to measure flowing and static pressures in oil and gas wells have been in use since the 1920’s. The recording devices that have been used include mechanical, (the Borden tube which records via a stylus mark on a blackened metal sheet), sonic (echometers which measure liquid levels) and electronic instrumentation (which measure pressure and temperature). Continuous recording instruments, such as the Amerada gauge, have been available since the early 1930’s. Today the preferred instrument is the electronic (memory) gauge.

One of the earliest applications of bottom-hole pressure measurements in wells was the determination of static or average reservoir pressures from the bottom-hole pressure measured after a well had been shut-in for between 24 and 72 hours. Although this static measurement indicated the average formation pressure in the permeable and productive reservoir it soon became apparent to engineers that static pressure measurements depended considerably on the time for which the well had been shut-in. Thus, the lower the permeability, the longer the time required for pressure stabilisation in the well. This lead to the realisation that when a well was shut-in, the rate of the pressure build-up would be a reflection of the reservoir permeability around the well.

Since a well test and subsequent pressure transient analysis is the most powerful tool available to the reservoir engineer for determining reservoir characteristics and planning production schedules, the subject of well test analysis has attracted considerable attention. Petroleum Engineering literature alone includes more than 500 published technical papers on this subject whilst the field of ground water hydrology also contains a similar number of publications on pump test analysis. A well test is the only method available to the reservoir engineer for examining the dynamic response in the reservoir and considerable information can be gained from a well test. Therefore, well testing is a subject which should be considered seriously.

After static pressure measurements, the most common methods of transient (time dependant) pressure analysis required that data points be selected such that they fell on a well-defined straight line on either semi-logarithmic or cartesian graph paper. The well test analyst must then insure that the proper straight line has been chosen if more than one line
can be drawn through the plotted data. This aspect of interpretation of well test data
requires the input of a reservoir engineer. Equally important is the design of a well test to
ensure that the duration and format of the test is such that it produces good quality data
for analysis.

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