Correlation of Minimum Miscibility Pressure for Impure CO2 Streams
Authors: Sebastian H.M. , Wenger R.S. , Renner T.A. 
Published: December 2007

The displacement efficiency of oil by CO2 is highly pressure dependent, Miscible displacement efficiency i-S pressure dependent, Miscible displacement efficiency i-S achieved only at pressures greater than a certain minimum. This minimum miscibility pressure (MMP) is a function of temperature and of composition of the injection gas. This paper examines the effect on MMP of up to 55 mol% impurities in the CO 2 -rich injection gas, We developed a correlation of these data based on the mole average pseudocritical temperature of the gas and the MMP of pure CO 2 with the same oil at the same conditions. In general, we found that increasing the Tcm of the gas lowered the MMP and decreasing the Tcm increased the MMP. Injection gases with the same Tcm but with very different compositions were found to have the same MMP. Various correlating parameters were tried. but the Tcm was found to be the most suitable. This correlation is compared with others in the literature and found to be superior in the case when the drive gas contained both light and intermediate components.

The correlation indicates that miscibility in a field-wide CO2 flood may be maintained by a reinjection of impure CO2 streams if sufficient intermediate hydrocarbons are present in the produced gas to offset the effects of lighter present in the produced gas to offset the effects of lighter gases. Because CO2 cleanup is a major cost in field-wide CO2 flooding, reduction or complete elimination of produced-gas cleanup will have a positive impact on process produced-gas cleanup will have a positive impact on process economics. The operational and regulator), aspects of injecting an impure CO2 Stream, however. must be considered to optimize the recycling scheme.

Introduction

For optimal displacement efficiency CO2 flooding should be conducted at displacement pressures greater than a certain minimum defined previously as the CO2 MMP, The prediction and determination of this CO2 MMP has been the topic of several previous authors. Pure CO2 is not always available as an injection gas. Impure CO2 streams, however, are available from a variety of sources, including natural reservoirs and process plant waste streams. Typically, impure CO2 contains significant amounts of nitrogen, H2S, and hydrocarbons. Metcalfs has shown that the presence of impurities can affect the pressure required to achieve miscible displacement. pressure required to achieve miscible displacement. Another potential source of impure CO2 is the produced gas from wells in a field undergoing a CO2 flood. produced gas from wells in a field undergoing a CO2 flood. Reinjection of produced gas could reduce the Cost Of CO2 flooding because high-purity cleanup of the fluid is expensive. The CO2-rich produced gas streams contain a wide variety of components from methane and nitrogen to intermediate hydrocarbons-such as ethane, propane, butane, and H2S. A correlation of the effect of these impurities on the CO2 MMP would be useful for modeling purification and reinjection alternatives, and for screening potential reservoirs for miscible flooding by impure CO2 streams.

Several correlations are available for predicting the MMP of pure CO2 with reservoir oils. The correlations of Johnson and Pollin and Alston et al. take into account the effect of injection gas composition. Johnson and Pollin's work, however, is applicable only to binary gas mixtures of CO2 plus nitrogen or CO2 plus methane. Our study develops a correlation to predict the MMP of impure CO2 streams but does not address the prediction of the MMP of pure CO2, We use the Tcm of the drive gas as a correlating parameter. Rutherfords originally suggested that miscibility was a function of the Tcm for hydrocarbon drive gases. Jacobsens found this to be true for H2S- and CO2 -containing streams if the critical temperatures of CO2 and H2S were adjusted slightly. The new correlation was developed for CO2 -rich streams, and the dependence of the MMP on the Tcm of the drive gas was determined. Alston et al., working independently of this effort, have derived a correlation for the effect of impure CO2 streams that is based on the weight average critical temperature as the correlating parameter. Although the Alston et al. correlation is similar to ours in that it predicts a decrease in the MMP with an increase in predicts a decrease in the MMP with an increase in critical temperature, data presented in this work indicate that the Tcm is a better correlating parameter. This correlation is also applicable over a wider range of drive-gas composition.

We compared our correlation (with the Yellig and Metcalfe correlation for pure MMP) with the correlations of Alston et al. and Johnson and Pollin. For the ranges of oil compositions and temperatures considered herein, the Yellig and Metcalfs correlation of pure CO2 MMP has been adequate.

Experimental Studies

A series of experimental MMP determinations of reservoir oil displaced by impure CO2 was conducted in a slim-tube apparatus. The experimental system shown in slim-tube apparatus. The experimental system shown in Fig. 1 is essentially the same as that reported by Yellig and Metcalfe.

JPT

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