Abstract

The acoustic resonance method has the potential to detect liquid-vapor and liquid-solid phase transitions over a broad range of temperatures and pressures. The versatility of the technique is an advantage for measuring reservoir fluid properties, particularly for opaque oils that cannot be analyzed with conventional visual methods.

In this work, the acoustic resonance method is used to detect liquid-vapor and liquid-solid phase transitions from reservoir fluids. The expected trend in the resonance frequency is established from acoustic and thermodynamic theory as well as experimental observations. The predicted trend is then used to screen the AR measurements for resonance frequencies that correctly identify phase transitions.

Although experiment and theory did not match well, the screening procedure correctly identified liquid-vapor phase transitions in all cases. However, a significant number of false transitions were also identified. The results for liquid-solid phase transitions were less conclusive because independent measurements were not available.

Introduction

Phase transitions play an important role in every stage of oil production and their significance for oil and gas production is well known. In a gas condensate reservoir, liquid drops out of the reservoir fluid below the dew point and is left unrecovered1. In an undersaturated black oil reservoir, the appearance of a gas phase results in low recovery1,2. Wax and asphaltene precipitation, two examples of liquid-solid transitions, can cause operational difficulties and necessitate costly treatments3,4.

The visual method is often used for the detection of liquid-vapor transitions. For the detection of liquid-solid transitions, the techniques of light transmittance5, electrical conductivity6, refractive index7, viscometry8, and flow loop test9 have been reported. Most of these techniques are designed for titration tests and are limited to low temperature and pressure conditions. A versatile technique for detecting phase transitions over a wide range of temperature and pressure conditions has yet to be found.

In this work, the acoustic resonance (AR) method is used to detect both the liquid-vapor and the liquid-solid transitions from reservoir fluids. This technique is based on the observation that the sonic speed in a fluid shows a distinct change at a phase transition. To detect a phase transition, a fluid sample is placed in a cylindrical chamber and the resonant frequencies of the fluid filled chamber are measured as the pressure or temperature of the chamber is adjusted. Since the resonance frequency is approximately proportional to the sonic speed in the fluid, phase transitions can be identified by measuring the change in resonance frequency.

The AR method has been implemented on an apparatus developed at Hycal Energy Research Laboratories Ltd. Although phase transitions have been identified before with the apparatus 10,11, an experienced operator has always identified the resonance peaks by inspection. A systematic approach and a more rigorous error analysis are required before the technique is ready for general use.

In this paper, a screening procedure is developed to identify the resonant frequencies based on their expected behavior. The expected resonance frequencies and their response to changes in temperature and pressure are found from acoustic and thermodynamic theory.

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