G. Paul Willhite

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

Permeability reduction mechanisms involved in in-situ gelation of a polyacrylamide/chromium (VI)/thiourea system

In this paper an investigation of the in-situ gelation of a polyacrylamide/chromium(VI)/thiourea system under flow conditions in unconsolidated sandpacks is presented. High flow resistance was observed in a localized section of the sandpacks that was consistent with deep-bed filtration mechanisms. A method to measure the size of aggregates in gel solution is also presented. Results from this method showed that the gel aggregates grow with reaction time and distance traveled through the sandpack, supporting the conclusion that filtration is a dominant factor in the in-situ gelation process under flow conditions.

Modeling of three-phase liquid/liquid equilibria

This paper describes a model developed from thermodynamic principles for correlating phase behavior of two-and three-phase systems. The model is based on the representation of excess Gibbs energy with the UNIQUAC equation. A set of computational algorithms is described for estimating the UNIQUAC model parameters from regression of the equilibrium phase data and for predicting phase behavior throughout the entire composition range. The model has been tested for ternary systems and a pseudoternary system. Agreement between the measured and predicted data was excellent for most systems studies. The model was able to match the ternary phase behavior of two- and three-liquid-phase systems that have not been previously simulated. For ternary systems, the estimated UNIQUAC parameters from correlation of the three-phase tie-triangle data describe the phase behavior in both two- and three-phase regions.

A kinetic study of the reduction of chromium(VI) to chromium(III) by thiourea

Abstract The reduction of chromium(VI) to chromium(III) by thiourea is a key aspect of the chromium(VI)–thiourea–polyacrylamide gel polymer system used in oil recovery processes. A study was undertaken to develop a kinetic model for the reduction reaction. Reaction mixtures were prepared and chromium(VI) concentrations were determined from the mixturess visible absorbance. The reaction rate was found to depend on the concentrations of chromium(VI), thiourea, and polyacrylamide and on solution pH. A kinetic model for the reactions was developed using the experimental data and previously proposed reaction mechanisms. The model accurately predicts the reaction rate in solutions that do not contain polyacrylamide and predicts a rate that is approximately correct for solutions containing polyacrylamide.

Correlation of the Flow of Flocon 4800 Biopolymer with Polymer Concentration and Rock Properties in Berea Sandstone

An experimental program was carried out to determine the mobility of Flocon 4800 biopolymer in Berea sandstone cores. Experimental work conducted at 25C included rocks with brine permeabilities ranging from 15.5 and to 848 and for polymer concentrations of 500–1500 ppm. Frontal advance rates varied from 0.1 ft/d to 117 ft/d. Flocon 4800 was found to follow a power-law model during flow through porous rock. Correlations were developed between polymer mobility, polymer concentration, and brine permeability of the rock after contact with polymer. Using these correlations, it is possible to estimate the mobility of Flocon 4800 in chemical flooding processes conducted in Berea core material within the range of polymer concentrations studied. Mathematical models derived from capillary bundle approaches and rheological parameters derived from steady shear measurements produced poor predictions of polymer mobility in Berea sandstone cores.

Spectroscopic determination of chromium(VI) during the reduction of chromium(VI) to chromium(III)

Reduction of chromium(VI) to chromium(III) by thiourea between pH 3.0 and 5.5 is a key aspect of the chromium(VI)/thiourea/polyacrylamide gel polymer system used in enhanced oil recovery processes. A method has been developed to determine chromium(VI) concentration during the reduction of chromium(VI) to chromium(III) in this pH range. The reduction reaction is run in the presence of an acetic acid/sodium acetate buffer which reacts with the chromium(III) produced and prevents the formation of a brown precipitate which forms in the absence of the buffer. With interference from the precipitate eliminated, chromium(VI) concentration is determined from the visible absorbance of the reaction mixture and the unique molar absorptivity spectra of the five chromium species present in the reaction mixture. An average error of approximately 1% between known and measured chromium(VI) concentrations was demonstrated over a chromium(VI) concentration range from 0.0005 to 0.0025 M.

Measurement of Molecular Weight Distribution of Polyacrylamides in Core Effluents

A procedure to measure the molecular-weight(MW) and size distributions of polymers in core effluents by size-exclusion chromatography (SEC) and low-angle laser-light scattering (LALLS) was used to analyzed Berea core effluent samples containing either a hydrolyzed or unhydrolyzed polyacrylamide (PAAM). The average MWs of the PAAMs were measured with a precision better than {plus minus}5% at a 95% confidence level. For both PAAMs, the polymer in an early effluent sample after breakthrough had a narrower MW distribution and fewer low-MW molecules than the injected polymer. This led to a higher weight-averaged MW, M{sub w}, for the early effluent than for the injected polymer. The effluent M{sub w}, decreased with increasing PV of polymer solution injected and approached the injected polymers M{sub w} after a large throughput. The results of the MW distribution measurements, which suggested a relatively large inaccessible PV for larger polymer molecules, are discussed in terms of steric-exclusion effects and dependence of retention rate on polymer size.

VISCOMETRIC MEASUREMENT OF CHROMIUM(III)-POLYACRYLAMIDE GELS†

Gelled polymers are being used increasingly to modify the movement of injected fluids in secondary and enhaced oil recovery processes. A common gelation process involves the reduction of Cr(VI) to Cr(III) in the presence of polyacrylamide. The Cr(III) reacts or interacts with the polymer to form a gel network. Although correlations of gelation time with principal process variables have been obtained, viscometric data have not been reported during or after gelation. These data are needed for fluid flow calculations in surface equipment and estimation of flow behaviour in reservoir rocks. A Weissenberg Rheogoniometer, with cone and plate geometry, was used to obtain viscometric data for the gelation of polyacrylamide and chromium (III). Solutions consisting of polyacrylamide polymer, sodium dichromate-dihydrate and sodium bisulfite were gelled under a steady shear field at constant temperature. The shear stress versus time profile for the galation process was interpreted to define a gelation time and to det...

Investigation of three phase regions formed by petroleum sulfonate systems

The three-phase region of the Witco TRS 10-80 sulfonate/nonane/isopropanol (IPA)/2.7% brine system was investigated in detail. A method is described to locate phase boundaries on pseudoternary diagrams, which are slices of the tetrahedron used to display phase boundaries of the four-component system. The three-phase region is wedge-like in shape extending from near the hydrocarbon apex to a point near 20% alcohol on the brine/alcohol edge of the tetrahedron. It was found to be triangular in cross section on pseudoternary diagrams of constant brine content, with its base toward the nonane/brine/IPA face. The apex of the three-phase region is a curved line where the M, H+M, and M+W regions meet. 18 refs.

Phase Boundaries of Microemulsion Systems as a Way of Characterizing Hydrocarbon Mixtures

The research reported in this work extends the work of J.G. Dominguez et al. to mixtures of pure hydrocarbons. The locations of the lower phase boundaries for Region 4 were determined for four types of mixtures prepared with pure hydrocarbons ranging from C/sub 6/ to C/sub 18/. In all phase behavior experiments, the lower phase boundary of Region 4 was a straight line when volume of alcohol was plotted against volume of brine. Furthermore, the slope of this phase boundary was found to be a linear function of alkane carbon number (ACN) for pure hydrocarbons and equivalent alkane carbon number (EACN) for mixtures of pure hydrocarbons. The correlation of a property of the phase diagram (the slope of the lower phase boundary) with EACN suggests a new approach to characterization of hydrocarbon/surfactant systems. This work is of interest to enhanced oil-recovery processes. 13 refs.