A new low-energy gamma ray tool for fullbore measurement of gas holdup in a cased well

Document Type

Conference Proceeding

Publication Date

1-1-1996

Journal

SPWLA 37th Annual Logging Symposium

Abstract

© 1996 SPWLA. All rights reserved. The measurement of gas holdup in a cased-hole environment is a fundamentally difficult problem. Traditionally, an estimate of the percentage of gas in a cross section of a wellbore has been computed from fluid density measurements. These estimates are often inadequate for gas holdup in horizontal or highly deviated wells since the fluid density measurements are not fullbore measurements. To accommodate the need for increased fullbore accuracy over a greater range of applications, a new Gas Holdup Tool (GHTT") provides a more accurate technique of obtaining these gas holdup values directly during logging. The GHT tool is a 111/16-inch-OD through-tubing production logging device used to determine the volumetric fraction of gas over a cross-sectional volume element of the wellbore. The new tool operates in horizontal, highly deviated, and vertical cased wells and generates a O-to-100% gas holdup log in stratified or uniform flows. The tool requires a low-energy cobalt-57 source and a sodium iodide detector located a short distance from the source and separated from the source by a tungsten shield. A backscatter technique is used to accurately measure density differences between fluids and gases. The measurement consists of counting gamma rays scattered back from the production fluid to the detector, the count rate being associated with the gas volume fraction (holdup). The measurement is not affected by the composition and density of materials outside the casing. Monte Carlo modeling (using MCNP) and experimental data collected over a range of casing sizes at standard temperature and pressure conditions are used to validate the model and establish empirical relationships leading to the association of count rates with the gas holdup in different fluids and casing sizes. The sensitivity of the measurement to other parameters such as fluid type (oil, fresh water, salt water) and casing size are investigated. Correction is made for dead time, and compensation is made for downhole PVT conditions. A calibration algorithm and design of a calibration fixture and portable wellsite verifier are based on the relationships obtained from MCNP modeling and data collected. An algorithm based on these relationships results in a log that reports gas holdup and does not require the traditional post processing of fluid density logs. A log example, generated in a controlled experiment simulating dynamic conditions of horizontal stratified flow, is given to illustrate the application of the new tool as a fullbore gas holdup tool for logging horizontal wells. A log example from a vertical well logged by the initial prototype of the GHT tool is discussed.

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