Acoustic Determination of Liquid Levels in Gas Wells

Abstract

Abstract A new method has been developed for acoustically predicting liquid levels in natural gas wells. A sound wave generated in the wellhead is reflected by the liquid surface. Distance to the liquid surface is determined from the reflection time of the sound wave and the velocity of the sound wave in the natural gas system. Using this method, liquid levels can be predicted within 2 percent of true depth. Tests in gas pipelines show that if system conditions are accurately known, distances to reflecting surfaces can be determined to within 0.5 percent. Introduction In the natural gas industry there is a recurring need to obtain accurate reservoir pressures on gas wells. Accurate reservoir pressures are a must for making good reserve determinations and analyzing gas well performance. Bottom-hole pressure can be measured directly with a bottomhole pressure instrument, or it can be calculated from a surface shut-in pressure. Economic considerations give added impetus to the calculation technique. Calculating bottom-hole pressures from surface pressures requites that a correction be made for the presence of any liquids in the wellbore. If liquids are standing in the wellbore and are not accounted for, the calculated bottom-hole pressure will be in error. Therefore, whenever the calculation technique for determining bottom-hole shut-in pressures is used, it is necessary to know the liquid level in the wellbore. A device commonly used to measure the liquid level in the wellbore is an acoustical well sounder which offers a reliable method of determining liquid levels under most wellbore conditions. However, wellbore conditions do exist from which interpretable acoustic test data cannot be obtained. This article introduces and evaluates a concept for determining liquid levels in those wells from which usable acoustic data cannot be determined by a conventional acoustic well-sounding device. Acoustic Well-Sounding Instrument Acoustic sensing devices for locating anomalies in pipes are not new. The first such device was used over 50 years ago for locating lost mail cartridges in the pneumatic system which interconnected the postal substations in New York City. Not until over a quarter of a century later was a similar device introduced to the oil and gas industry for locating the liquid in a producing oil or gas well. Since its introduction, use of the acoustic sensing instrument - commonly called an acoustic well sounder - has become common within the industry. The acoustical method of determining the liquid level in a gas well incorporates the principle of generating a sound wave at the wellhead and recording the echoes from the tubing collars and liquid surface. Firing a blank cartridge in the wellhead is the usual means of generating the sound wave. Reflecting sound waves are converted to electrical signals and recorded on a strip chart. The recorded signals show the position of the liquid against a depth scale provided by the tubing collars. Fig. IA is a well- defined acoustical well-sounding record showing the shot deflection, tubing collar reflections and liquid reflection. To accurately determine the liquid level in a gas well with an acoustical well sounder requires a tally of the tubing in the hole and a well defined tubing collar reflection record. The tubing tally requirement normally is easily fulfilled. Obtaining a well-defined tubing collar reflection record, however, sometimes proves to be a more formidable problem. That is, in some cases the tubing collar reflections are not well defined in the surface record. It is generally believed that, when collar reflections are poorly defined, foreign material such as paraffin has collected around the collars, smoothing an otherwise irregular surface and thereby reducing the ability of a collar to reflect a pressure disturbance. Some of the newer-type tubing strings (buttress joint, streamlined, etc.) do not contain a reflecting surface at their tubing joints. Fig. 1B is an acoustical well-sounding record showing a well defined shot deflection aid liquid reflection, but uninterpretable tubing collar reflections. New Concept Calculating, as opposed to measuring, bottom-hole pressures has become common practice within the gas industry. Calculated bottom-hole pressures have proved to be quite reliable when the liquid column pressure is considered in the calculation procedure. JPT P. 601ˆ