Effect of Oil Composition on Minimum Miscibility Pressure-Part 2: Correlation
- F.M. Orr Jr. (Stanford U.) | M.K. Silva (New Mexico Petroleum Recovery Research Center)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1987
- Document Type
- Journal Paper
- 479 - 491
- 1987. Society of Petroleum Engineers
- 5.2.2 Fluid Modeling, Equations of State, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 4.1.9 Tanks and storage systems, 4.6 Natural Gas, 5.2 Reservoir Fluid Dynamics
- 2 in the last 30 days
- 1,211 since 2007
- Show more detail
- View rights & permissions
|SPE Member Price:||USD 5.00|
|SPE Non-Member Price:||USD 35.00|
Summary. A new correlation for CO2 minimum miscibility pressure (MMP) is proposed and tested. The correlation is based on experimental evidence that extraction of hydrocarbons from a crude oil depends most strongly on the size of the hydrocarbon molecules. Thus, an oil rich in hydrocarbons that are efficiently extracted should show a lower MMP than a heavier oil. The only crude-oil data needed for use of the correlation are a carbon-number distribution, which can be obtained easily by chromatography. The density of CO2 at the MMP is correlated against a weighted-composition parameter. The MMP of any oil at a given temperature is estimated as the pressure required to yield that CO2 density. The effects of contaminants in CO2 can be estimated easily by calculating the pressure required to give a mixture density equal to that required for pure CO2. Correlation accuracy was tested by comparing predicted and measured MMP's for 35 oils. The proposed correlation produces MMP estimates accurate to within about 10% for most oils. Comparison of correlation predictions with those of other commonly used correlations indicates that the proposed correlation is significantly more accurate, particularly for recombined oils.
Development of miscibility in a CO2/crude-oil displacement is the result of extraction of some hydrocarbons from the oil by dense CO2. There is considerable evidence that extraction of hydro-carbons from a crude oil is strongly influenced by the density of the CO2. As its density increases, CO2 extracts more and heavier hydrocarbons than does lower-density CO2. Improvement of extraction with the increase in CO2 density that accompanies increasing pressure accounts for the development of miscibility. Slim-tube displacement experiments, which suppress adverse effects of viscous instabilities and hence are nearly one dimensional, are commonly used to determine an MMP for a given crude oil. While exact procedures for MMP determination vary, all yield a pressure above which little additional improvement in oil recovery occurs with further increases in pressure, and thus are a measure of the pressure (density) at which extraction is effective enough to produce high local displacement efficiency. In this paper, we examine how variations in crude oil composition influence MMP. The size and structure of hydrocarbon molecules present in a crude oil must also affect how such components partition in CO2/hydro- carbon mixtures, and hence must influence the density required for efficient extraction. Holm and Josendal developed a correlation for prediction of MMP based on the idea that the density of CO2 at the MMP could be correlated with the amount of C5-through-C30 hydrocarbons present in the oil. According to their correlation, oils containing large C5-through-C30 fractions require lower CO2 densities than heavier oils. Orr and Jensen agreed that CO2 density determines extraction efficiency, but argued that at low temperatures (below about 120F [49C]), the density of CO2 increases so rapidly with pressure near the extrapolated vapor pressure (EVP) that effects of oil composition on MMP are small. pressure (EVP) that effects of oil composition on MMP are small. Thus, they suggested that at low temperatures, the vapor pressure, extrapolated if necessary, is a good estimate of the MMP. Yellig and Metcalfe also offered an MMP correlation for west Texas oils that depended on temperature only. Johnson and Pollin suggested another empirical MMP correlation that included contributions of the oil molecular weight and aromaticity. Holm and Josendal also speculated that aromatic content might influence the MMP. Alston et al. proposed a different empirical MMP correlation that included composition contributions based on the molecular weight of the C5+ fraction and the ratio of volatile (C1 and N2) to intermediate (C2-through-C4, CO2, and H2S) mole fractions. A correlation for the effects of contaminants in the injected CO2 was reported by Sebastian et al. In a companion paper, we present results of an investigation of the effects of molecular size and structure on component partitioning in CO2/hydrocarbon mixtures. That study indicates that partitioning in CO2/hydrocarbon mixtures. That study indicates that while the chemical structure of a hydrocarbon molecule (paraffin, naphthene, or aromatic) does influence partitioning in the presence of CO2, for the distribution of molecular types and sizes present in many crude oils, the size distribution of hydrocarbons present in the oil has the greatest effect. In this paper, we apply that result to the development of a correlation for MMP that accounts for variations in crude-oil composition. The correlation proposed here parallels the one offered by Holm and Josendal in that we correlate the density of CO2 at the MMP with a parameter that reflects the composition of the oil. To construct the parameter, we use component-partitioning data for a CO2/crude-oil system of the type presented in Part 1. In the sections that follow, we describe the structure of the correlation and the assumptions behind it, present results of experiments to test the validity of the assumptions, and compare correlation predictions with measured values. We also offer a simple procedure for estimation of the impact of contaminants in the CO2 on MMP, also based on the idea that there is a characteristic density of CO2 at the MMP for a given oil.
Correlation of MMP's
Alkanes lighter than C13 are individually miscible with liquid CO2. If an oil contains sufficient hydrocarbons heavier than C13, two phases will form over a wide range of pressures. In such cases, phases will form over a wide range of pressures. In such cases, the light hydrocarbons will partition between the phases. The component-partitioning data reported in Part 1 indicate clearly that for hydrocarbons in the C5-through-C30 range, smaller molecules are extracted more efficiently by dense CO2. Large molecules are extracted less efficiently, regardless of whether they are paraffinic, naphthenic, or aromatic. Because aromatic and naphthenic components occur predominantly in heavier fractions, from which extraction will be poor regardless of molecular type, and because any aromatics and naphthenes that do occur in the C5-through-C12 range frequently have alkyl side groups, it seems likely that the cyclic character of such molecules has a relatively small effect on the solubility of those molecules extracted most effectively by CO2. It should be possible, therefore, to account for variations in oil composition by considering in more detail the effects of molecular size on partitioning. Component-partitioning data, like those shown in Figs. 10 and 12 of Part 1, can be used to assess how much more effectively small hydrocarbon molecules are extracted than large ones. We propose, therefore, a modification of Holm and Josendal's correlation based on the following assumptions.
|File Size||1 MB||Number of Pages||13|