Abstract

A novel acoustic dewpoint and bubblepoint detector may be a valuable device for natural gas processing industries. The device is free from mercury and can be applied to a broad range of phase transitions. Even in very lean gas systems or opaque heavy oils, this technique has application when high accuracy and automation are required. In the new state of the art Acoustic Resonance Technology (ART) system at Hycal Energy Research Laboratories Ltd., two acoustic transducers are used, one to stimulate and the other to detect normal mode vibrations of reservoir fluids in a small cylindrical resonator. The acoustic spectra, along with temperature, pressure and volume measurements, are recorded at close intervals throughout the phase envelope. The time domain data collected are processed to obtain the specific condition of phase transition. The high pressure ART system is capable of operation from -40 °C to 150 °C (-40 °F to 300 °F) and pressures up to a maximum of 70 MPa (10,000 psia). The system is capable of operation in an isothermal mode with variable volume or in a constant volume mode with variable temperatures.

The accurate determination of bubblepoint and dewpoint pressures at reservoir temperature are crucial for natural gas processing, transportation and metering. Results of two systems (a binary mixture and live reservoir fluid) are presented. The onset of bubblepoint and dewpoint are easily obtained from the processed data. Since the frequency response of sonic speed is influenced by density, micro- and macroscopic structural features, at the phase boundaries, the acoustic response shows a sharp abrupt change. The interpretation of results can be free from operator subjectivity. Comparison of the ART results with those obtained using visual methods and equation of state calculations show excellent agreement. These results will be presented in this paper.

Introduction

In reservoir engineering, detailed knowledge of the phase behavior of reservoir fluids is very important in planning, managing operations involving production, transporting, processing and utilization of fluids. Although visual techniques have been used commonly in the petroleum industry, when it comes to very lean gas mixtures and very dark fluids, detection of phase change can be a serious problem.

The recent AR technique is a highly accurate, independent method and can be complementary to, or a substitute for, visual method. It is fast and additional information can be obtained in one measurement. AR exploits the advancement in PC technology for data collection and control of measurements. The evolution of the resonance response of reservoir fluids as a function of changing conditions of pressure, volume and temperature is measured in AR in real time.

The results yield accurate information related to reservoir fluid phase behavior (bubblepoint, dewpoint, etc.). It has been shown in earlier work that some useful information about fluids can be inferred reliably from the acoustic signature.1,2,3

This paper illustrates the use of a versatile acoustic system in determining accurately the dewpoint or bubblepoint of gas condensates and reservoir fluids.

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