Measurement of Phase Behavior of Hydrocarbon Mixtures Using Fiber Optical Detection Techniques
Authors: Dandekar Abhijit Y. , Stenby Erling H. 
Published: December 2007

Abstract
In this paper we present a novel technique for experimental determination of dew and bubble points of oils and gases under reservoir conditions. Extrinsic fiber optic sensors mounted on a 80 mL titanium equilibrium cell are part of the measurement set-up. Formation of a new phase in the sample confined within the cell is sharply indicated by creation of characteristic changes in the fiber optic signal. This allows for precise detection of dew and bubble points. The entire equipment is transportable and can easily be carried to field locations for quick and accurate studies of bottom hole samples. Phase behavior measurements (dew and bubble points) of three model systems and a real reservoir fluid are presented.

Introduction
The natural gas and petroleum industries increasingly demand accurate high-pressure phase equilibrium data of petroleum fluids, i.e., multicomponent hydrocarbon mixtures. The data is needed for effective reservoir management and enhanced oil recovery processes, and they comprise among others the saturation properties (dew and bubble points) of the petroleum fluids under reservoir conditions.
Saturation pressures of petroleum fluids under reservoir conditions are usually determined at a constant temperature in high-pressure stainless steel vessels (equilibrium cells) with sapphire windows, The sample volume/pressure is varied by injection or removal of mercury and the saturation point is observed visually as the formation or disappearance of a phase. Bubble points can sometimes be detected as a break point in a pressure versus volume curve. The relatively large sample volumes (200–2000 mL) used in such cells make the establishment of equilibrium time excessive after a temperature or pressure change, and the visual detection technique is in itself operator-dependent and difficult to automate. Moreover, from a practical point of view, the oil-producing industry cannot always afford to spend expensive time in connection with shipping of samples to conventional laboratories where such analysis is carried out.
The apparatus described in this paper attempts to address all the above issues. A precise determination of phase boundaries is achieved from measurements of fiber optic signals. The size of the equilibrium cell is 80 mL, and the apparatus is completely automated. The sample is kept in continuous contact with the fiber optic sensors. The characteristics of the fiber optic signal are in this way monitored while the pressure is reduced along an isotherm or the temperature is reduced along an isobar, beginning with a single-phase sample. The value of the fiber optic signal shows a slight increase, when the pressure is slowly reduced along an isotherm until a phase boundary is reached. This value shows a rapid increase (in the case of a bubble point) or decrease (in the case of a dew point) when the saturation point is reached.

Apparatus
The apparatus was designed and manufactured by the ROP company of France to measure dew and bubble points for temperatures from 20 to 150 C and for pressures up to 700 bars.
A detailed view of the apparatus is shown in Figure 1. The size of the equilibrium cell is 80 mL. The total dead volume is determined to be 17.5 mL. The internal volume of the cell is varied by a mechanically driven piston. Viton 'O' rings placed on Teflon seats mounted in the grooves of the piston permit the maintenance of high pressures in the equilibrium cell. A small magnetic stirrer is placed on the piston to speed up the achievement of homogeneous sample and a rapid equilibration between the coexisting vapor and liquid phases. P. 55^