A New Production Logging Method for Fullbore Gas Holdup Measurements in Cased Wells
Authors: Waid Margaret C. , Madigan William P. , Smith Harry D. , Vasquez Rafael B. 
Published: December 2007

The measurement of the gas holdup in a flowing cased-hole environment is a fundamentally difficult problem. Gas holdup, the estimate percent of gas in a volume of wellbore, has traditionally been computed from fluid density measurements. These estimates are inadequate for determining gas holdup in deviated or horizontal wells since the fluid density was not a fullbore measurement. A new Gas Holdup Tool (GHT TM) has been developed which provides a more accurate technique for obtaining gas holdup measurements directly. This 1 11/16-inch production logging tool is used to determine the volumetric fraction of gas in horizontal, deviated, and vertical cased wells, and provides a log of the gas holdup fraction (from 0% to 100%) in all flow regimes. Examples are presented for comparing homogeneous and stratified flows.

The tool uses a low-energy Co-57 source and NaI detectors with a new backscatter technique to accurately measure density differences of the total fluid and gas in the borehole around the tool. This new measurement provides the gas fraction (holdup) in all flow regimes, and is not affected by the materials outside the casing.

Monte Carlo modeling and experimental data over a wide range of cased-hole conditions validate empirical relationships between detector count rates and gas holdup for various casing diameters. The sensitivity of the measurement to other factors, such as pressure, salinity, and fluid type, are also investigated. The new production logging method may be used in determining points of gas and oil entry into deviated or horizontal wells and for quantitative production logging in deviated or horizontal wells with variable or unknown flow regimes.

Introduction

Logging surveys in wells producing oil, gas, and water are used to determine the holdup of each phase. The holdup of a specific phase is defined to be the fraction of the cross-sectional area of the casing that is occupied by that specific phase at a given depth. Gas holdup is the same as gas cut provided that all the produced fluids flow past the cross-sectional volume element at the same velocity. The holdup estimates are used with estimates of flow velocity to determine production rates from each zone of interest. Intervals producing oil, gas, or water, or combinations of these phases, may be identified and the information used to improve oil and gas production, reduce water production, and manage production from the reservoir. Accurate fullbore measurements of gas holdup are especially needed for determining production rates in horizontal or highly deviated wells, where the gas may be stratified or separated from the liquid phases.

Previous methods for estimating gas holdup have proven to be inadequate. The production logging tools traditionally used in combination to determine holdups are the radioactive fluid-density, the differential pressure, and the capacitance or dielectric water-holdup tools. The radioactive fluid-density and capacitance water-holdup tools respond to small samples of fluid near the center of the wellbore and require that the samples be representative of fluids in the full wellbore. The conventional radioactive fluid-density tools measure the attenuation of gamma rays in a direct line between the source and detector, and thus cannot be fullbore tools. Available water-holdup tools are suited primarily to well-mixed fluids with a low water holdup not exceeding 30 to 35%. Differential pressure measurements, when used with an inclinometer, may also be used to estimate fluid density, though these tools are not applicable in horizontal or highly deviated wellbores.

The new GHT tool provides a more accurate technique of obtaining gas holdup values directly during logging. The new tool operates in horizontal, highly deviated, and vertical cased wells and generates a 0-to-100% gas holdup log in stratified or uniform flow. The gas holdup measurement may be used to identify gas productive intervals, to determine bubble point pressure/temperature, to identify crests/troughs in horizontal wells, and to monitor gas lift mandrel performance. It is a necessary component for multi-phase flow analysis.

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