Abstract

The paper describes a novel positive displacement flowmeter developed to measure the mass and volumetric flowrates of multiphase crude oil fluids in flowlines. After an introductory review of the technical and economic incentives for the development - particularly offshore and subsea, the paper goes on to explain the hydrodynamic problems that paper goes on to explain the hydrodynamic problems that hither to have inhibited the use of conventional metering techniques for multiphase flow measurement.

The paper then describes how these problems have been resolved by a novel combination of simple, well tried principles. The design concepts of the new meter are principles. The design concepts of the new meter are outlined, together with a brief review of its development from conception to the completion of endurance and calibration tests on two field trial meters. Special features of the positive displacement mechanism design, which enable the meter to withstand rigorous multiphase flow regimes, are explained. Field trial results confirming the meter's capability to resist such conditions are reported, as are the results of independent checks of its measurement accuracy, assessed over a wide range of flow regimes and void fractions. Comparison is made between the performance of the meter and that of a test separator system.

Introduction

Motivated by increasing oilfield development costs, oil companies have invested heavily in recent years in efforts to develop a flowmeter capable of measuring multiphase fluid flowing from a wellhead. Incentives for these developments have been the prospect of significant savings in the capital costs of reservoir exploitation and of improved efficiency in subsequent operations.

Because of the complex nature of multiphase fluid flow regimes, direct measurement of the flow using conventional metering devices has not so far been successful. Experimental flowmeters based on more sophisticated techniques have also yet to meet target performance criteria. British Petroleum have resolved the measurement problem by adapting the principles of simple positive problem by adapting the principles of simple positive displacement measurement to the particular conditions of multiphase flowlines. Short term performance targets have been based on results from present day procedures using test separators followed by conventional single phase meter measurements. The accuracy standards that will be demanded for 'fiscal' multiphase meters are not yet defined.

INCENTIVES

Measurement information on the instantaneous and cumulative flow of fluid from each well or from well clusters is required routinely for many purposes. These include:

For reservoir management - determining depletion policy.- reservoir modelling.- planning secondary/tertiary and enhanced recovery.

For well management - optimising well performance.- providing evidence of well impairment.

For 'fiscal' purposes - export/allocation data from wells or well clusters.

At present, information on individual well performance is based on periodic well test procedures using test separators. These procedures are both lengthy and manhour intensive and involve bulky, space consuming equipment, high in capital and installation costs. Furthermore, the measurement data produced by this method can be of uncertain accuracy, especially under high loading conditions with gas or liquid carryover into the separated streams. Strictly, the data also relates only to the specific period over which the test is carried out.

These limitations in the present procedures suggest the advantages that can derive from an ability to measure single well production in a direct and continuous way. Assuming that a multiphase meter is of compact design, and dependent upon whether it must be surface mounted or can be installed subsea, the consequent benefits that can accrue for an offshore development scheme may be summarised under economic and operational/technical headings. Economic savings depend upon the scale of the development and are not quantified in this paper.

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