Measurement and Interpretation of Fluid Levels Obtained by Venting Casing Gas
Authors: Nolen K.B. , Gibbs S.G. 
Published: December 2007

A new computerized fluid level measurement method is described. Instead of using the traditional gas gun and microphone system, a transient wave is created by venting a small amount of gas from the casing and the fluid level is located with help of an ordinary pressure transducer. The method measures acoustic velocity of well gas external to the well in a known length of coiled tubing. This eliminates the need to count tubing collars to determine velocity. Much of the equipment is off-the-shelf, and cost is less than with traditional systems.

Results from field measurements show that the new method provides accuracy which is comparable to traditional systems. The new technique presents an uncluttered result without electrical or digital filtering which clearly shows the fluid level in the majority of cases. The simplicity of the return echo helps differentiate other objects and conditions that might pose as fluid level such as uphole leaks, liner tops, and tubing anchors.

The paper discusses many practical applications of the technique in locating fluid levels. It also describes how CO2 movement within the reservoir can be tracked as a by-product of measuring fluid levels. The paper also illustrates how the wave equation can be used to explain various fluid level echoes encountered in the field.

Introduction

Well surveillance has always been important in oil production operations. In the modern era, many surveillance methods are employed. Periodic production tests, monitoring with POCs, dynamometer surveys. and fluid level measurements are the principal tools. Fluid levels are often the most cost effective way of providing surveillance.

Fluid level is an indirect indicator of wellbore pressure. Fluid level is important because it is involved in so many cause and effect relationships. When fluid level is high, flood response may have occurred, a tubing leak may have developed, or lift equipment may have failed. Discovery of a high fluid level should prompt the operator to discover the cause. Sophisticated diagnostic procedures can identify lift equipment problems. Simple follow-up pressure tests can identify tubing leaks. Larger equipment can be sized to handle increased production made possible by flood response. A fluid level near the reservoir suggests that maximum production is being obtained. The rate of decline of fluid level is sometimes indicative of well inflow problems. Long term buildup surveys can verify the existence of wellbore damage. Thus it is seen that measurement of fluid level is a central activity in the oil producer's effort to maintain or increase production.

At any given time, most wells are operating satisfactorily. They do not need in-depth investigation on a continual basis. All that is needed is a simple. cost effective way of Sensing a change in the well. Historically this has been provided by the fluid level instrument. When a problem is suggested, more intensive investigations can be made as appropriate.

Because of the importance of fluid level measurement as a surveillance tool, improving and extending the art and science of making fluid level measurements is a worthy field of investigation. The subject method is yet another effort in this direction.