Swelling/Extraction Test of a Small Sample Size for Phase Behavior Study

Swelling/extraction tests are single-contact phase-behavior experiments to measure the solubility of CO2 dissolved in crude oil and the amount of hydrocarbon that CO2 can extract or vaporize from crude oil. The tests are commonly conducted in a visual PVT cell with a large sample size (40-100cc). In this paper, an easy operated apparatus capable of determining phase behavior with a significantly smaller sample size (3 to 14 cc) is described. The apparatus consists of a high-pressure view cell, high-pressure and precision syringe pump filled with CO2, a water bath, and accessories to measure the temperature and pressure. The device is capable of determining vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibrium commonly observed in a high pressure CO2 enhanced oil recovery process. The solubility of CO2 in oil, the expansion volume of oil due to the dissolution of CO2 as well as the phase transition during the test were quantified with excellent reproducibility. The molar volume of oil saturated with CO2 correlated linearly with the mole fraction of dissolved CO2 suggesting ideal mixing in the liquid phase. The phase behavior between CO2 and crude oil samples with different composition, temperature and pressure is discussed. Introduction A swelling/extraction test is a common phase behavior experiment to determine reservoir fluid volume and composition changes due to CO2 dissolution at reservoir temperature. The test is usually conducted in a constant volume, high pressure view cell initially filled with a predetermined amount of stock-tank oil. CO2 is injected progressively with pressure increase in a number of discrete steps. The change of crude oil volume due to the swelling is measured and the amount of CO2 dissolved in the oil is calculated by assuming that vaporization of crude oil components into the equilibrium vapor phase is neglible until the MMP is approached. At a higher pressure, light hydrocarbon components are vaporized or extracted and the hydrocarbon rich oil phase shrinks with increasing pressure. Phase behavior at high pressures can be viewed. The saturation pressure, solubility of CO2 and swelling factor are commonly used for tuning the equation of state (EOS) for modeling phase behavior. Various sizes of view cells, 140 cc (Hand et al., 1990), 170 cc (Harmon et al., 1988), 190 cc (Orr et al., 1981) and 450 cc from commercial laboratory have been used to study the phase behavior. The sample size for the swelling test described by Holm and Josendal (1982) is 30% of the cell volume. Thus, the test performed in these cells would require 40cc to more than 100 cc of liquid sample. An inherent difficulty of large volume cells is the length of time required to reach equilibrium after the pressure is changed and the difficulty of mixing large volumes of oil and gas under pressure. This paper describes a small volume high pressure view cell that was developed to investigate the mass-transfer process occurring in swelling/extraction tests when CO2 dissolves in the oil phase. The total cell volume is 26 cc with the sample size adjustable for the test. This paper summarizes the design of apparatus, experimental procedure and the tests conducted on CO2 with three crude oils produced from reservoirs in Kansas. Experimental Apparatus and Procedure Apparatus. The apparatus consists of a high-pressure view cell, high-pressure and precision syringe pump filled with CO2, a water bath, and accessories to measure the temperature and pressure. Figure 1 shows a diagram of the experimental