Analyzing Well Performance
Authors: McCoy James  N. 
Published: August 2010

This paper discusses the basic theory of well production, and thedetermination and calculations of both producing and static bottom-holepressures using the fluid level instrument. Several methods of producing wellsare compared and a general completion policy and production technique isrecommended.

INTRODUCTION

ANALYZING WELL PERFORMANCE is an important step toward increasing profits byimproving production techniques. Generally, the analysis is made by fieldobservation and examination of well data. The acoustic liquid level instrumentoffers valuable supplemental information, because downhole pressures can bedetermined from the depth-to-liquid measurement.

THE ACOUSTIC LIQUID LEVEL INSTRUMENT

The fluid level instrument determines the depth to the liquid level in awell by generating an acoustic wave in the casing annulus gas and thenmeasuring and recording the echoes received from the collars and the liquidlevel as the wave travels down the annulus. No tools are lowered in the well, nor are the rods disturbed in any manner.

During actual operation, the instrument is connected to the casing annulusopening. A blank shell is discharged into the casing annulus, emitting a soundwave which travels down the annulet, gas column. Each tubing collar reflects aportion of this sound and the reflected sound energizes a microphone. Thissignal is amplified and recorded on a moving strip of paper. The liquid levelin the well reflects a very high percentage of the sound and is recorded as arelatively large pulse on the paper. The number of collar reflections to thetop of the liquid and a tubing tally or estimate of the average joint lengthindicate the depth to the liquid. A sample chart from the Echometer is shown in Figure 1.

ACCURATE LIQUID LEVEL DATA

Liquid level data are generally very accurate, and accurate downholepressures can be calculated. Errors may occur, however, and a discussion of thepossible problems is worthwhile. Generally, the depth to liquid is reasonablyaccurate the liquid can usually be determined within 30 ft, and often muchcloser.

Difficulties in obtaining accurate liquid level data and hence bottom-holepressure can be classed as follows:Gaseous columnsFoaming liquid levelInterpretation of chartsPoor well response

Gaseous Columns

The problem of a high liquid column being gaseous can occur. A gaseousliquid column exists only on wells which are venting gas from the casing.Gaseous columns are formed when the density of a liquid column is greatlyreduced due to large gas bubbles migrating up through the liquid column. Thegas bubbles can be! at least 1 cubic foot in size. The gas migration in thecasing annulus results in lighter columns in "slim" hole completions than inlarge annular completions. The gaseous liquid column can almost always beidentified during the liquid level test. It causes a noisy downhole conditionto such an extent that the sensitivity control on a liquid level instrumentcannot be set at high settings without considerable pen movement.

The weight of a gaseous column varies considerably; hence, if a well has ahigh gaseous liquid level, the well may or may not be pumped down. From theauthor's experience, if the liquid level varies considerably (over 500 ft) at aconstant producing rate, the gaseous liquid column is mostly gas, causing verylittle back pressure, and the well is pumped down. Probably the bestcharacteristic of the gaseous liquid column is the rapid movement of the liquidlevel. Repeated shots taken hours or days apart will not indicate the sameliquid level depth. The gradients of gaseous columns in a well are difficult todetermine.