Abstract

The determination of phase transitions (bubble points, dew points, hydrate points, solid precipitation and cloud points as well as critical saturations) in porous media has been an area of interest for many years. Classic techniques involve the use of expensive and in many cases, low resolution and inaccurate types of imaging equipment, such as CT scans, MRI, etc., which often have limited utility for full reservoir temperature and pressure evaluations. Acoustic technology has been med in the past to determine accurate phase transitions in bulk samples of reservoir fluid. This paper describes an extension of this technology which allows the technique to also be used to determine the point of a phase transition in actual samples of reservoir porous media at full reservoir conditions of temperature and pressure using reservoir fluids. Further extension of the work holds the promise of the ability to evaluate critical and possibly in-situ fluid saturations in an accurate, cost-effective and non-invasive fashion.

Introduction

Acoustic Resonance Technology (ART) is based on measurement of the response of a fluid or fluids contained in a medium or cavity to acoustic variable stimulation. The medium in this context is a cylindrical consolidated sandstone. The study of state and time evolution of the resonance response of fluids in a core or porous media under variable and well-controlled conditions of pressure and temperature is one of the most powerful tools of modem times. Ultrasonic pulse techniques have been used1 previously to determine the bubble nucleation rate during depletion experiments in rock samples. There is a strong need for determining the critical or phase behavior of live fluids in the core sample. The following paper discusses a novel technique based on acoustic resonance technology (ART) to determine phase transitions of pure ethane in porous media.

EXPERIMENTAL
Sample

Pure ethane (purity 99.99%) obtained from Praxair Company, Alberta was used as a fluid for phase transition measurements. A cylindrical core sample of consolidated sandstone with 22% porosity and 162 mD permeability with a length of 3.33 cm and a diameter of 3.70 cm was used for the following measurements.

Procedure

The core material was mounted in a ductile sleeve and sealed in a 316 SS core assembly capable of applying biaxial overburden pressures up to 2000 psia. The core heads were equipped with special acoustic transducers to transmit and receive acoustic data through the sample. The core holder was also equipped with a digital strain gauge to measure pressure and a platinum resistance thermometer (PRT) for temperature measurement. The entire assembly was mounted in a temperature controlled oven. The core material was evacuated at 35 °C (308 K), pressurized with pure ethane to 735 psia and stabilized for approximately one hour at 35 °C.

The AR control system, discussed by Sivaraman et Al2,3 and developed at Hycal Energy Research Laboratories, for bulk fluid (bottom hole reservoir fluid) phase behavior studies was used for the current investigation. The system was connected to the transducers which were mounted on the core heads.

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