Advances in Downhole Flow Rate Measurement and Its Application during Well Testing
Authors: Manin Y. , Herve X. , Aquilina P. , Tighe M. 
Published: December 2007

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Abstract
Production testing (drillstem testing) of exploration and appraisal wells remains of fundamental importance in the development of oil and gas reserves. The trend is for the continual refinement of data acquisition and interpretation techniques, with a push for downhole measurement wherever possible. Recent advances allow non-intrusive downhole flow rate and density measurements with the resultant potential for the analysis of data from the "nonideal" well.
This paper describes the latest downhole DST flowmeter which utilises a double venturi profile. The accuracy and resolution achievable and a brief discussion of the theory behind the measurement are presented.
The potential of downhole flow rate measurement is discussed from an operators' point of view. The advantages of acquiring flow rate data from flow periods when traditional surface rate data are unavailable and application of the rate convolution principle to downhole transient pressure data are discussed.
Data from a recent North Sea oil well are presented, exemplifying the accuracy and usefulness of such measurements. Of particular interest in this example is the acquisition of data on a continual basis while surface data collection was interrupted because of foaming problems. In addition the downhole rate information was used in a process allowing identification and validation of the reservoir model to be performed on all the drawdown flow periods.

Introduction
Over the past 10 years, several authors have described the application of downhole spinner rate measurements to both buildup and drawdown transient pressure data, using convolution methods. Recently, an example was presented, in which such rate convolution techniques were applied with rates derived from a downhole venturi flowmeter located within the drillstem test (DST) string. It was shown how the downhole rate and density information can be used in detecting and accounting for variation in gas-liquid interface below the downhole valve during a buildup interpretation.
One of the main objectives of the downhole rate measurement is to help minimize shut-in periods during well test operations, by allowing transient analysis based on drawdown data. This can be achieved through rate convolution techniques, provided that the downhole rate data are of sufficiently good quality. The hardware requirements should therefore be oriented toward maximum reliability and optimal metrological performances, in order to increase the tool operating flow range, as well as the rate accuracy and resolution, while maintaining the fullbore (2.25 in) profile and mechanical toughness needed for DST operations.
The new downhole tool presented in this paper features a double-venturi profile. For a well producing liquid phases, this system yields improvements in terms of flow rate accuracy and minimum threshold. Furthermore, the tool is designed to withstand shocks from tubing-conveyed perforating (TCP).
The quality of the measurement is proven with a field example, in which the downhole rate was used in a multi-rate test to 1) describe the downhole flow without requiring empirical pressure-volume-temperature (PVT) conversion factors, 2) provide a rate measurement when the surface equipment was either not connected or not operating properly and 3) identify and validate the reservoir model on drawdown pressure transient data.
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