Compressibility Factors for CO2-Methane Mixtures

Abstract

Compressibility factors from a high-ratio CO2-natural gas mixture are compared with other experimental data and values calculated using two correlations. Discussion includes the use of these factors in monitoring reservoir performance for the SACROC Unit CO2 injection project. Introduction The SACROC Unit CO2 injection project in Scurry County, Tex., required transporting 200 Mcf/D of CO2 for 200 miles. A feasibility study of the mode of transportation resulted in selection of a "supercritical-pressure-state" pipeline system. A study was made of available data on compressibility (z) factors for CO2-natural gas at supercritical pressures, as well as of two engineering calculation methods. This study indicated that the z factors should be measured rather than taken from published information. This paper compares our measured compressibility factors with calculated and published compressibility data, and describes applications of z factors to the various phases of the SACROC Unit CO2 injection project. phases of the SACROC Unit CO2 injection project. Experimental PVT Data There were two reasons for measuring z factors rather than relying on published information:Reamer et al.'s data for the CO2-methane system did not include C2+, which is contained in the pipeline gas.For engineering purposes, corresponding-state and equation-of-state calculation methods are not accurate enough near the critical point. CO2 pipeline design conditions include the critical point. Our measurements were made in a visual PVT cell using the CO2-natural gas mixture described in Table 1. Conditions ranged from 680 to 3,001 psia and 49 to 120 degrees F. Tables 2 and 3 and Fig. 1 show the experimental results along with the calculated values and Reamer et al.'s data. Experimental Method The gas used for the compressibility-factor measurements was prepared from pure compounds. It was analyzed by determining the CO2 in an Orsat apparatus (thus removing the CO2) and chromatographing the re-reminder of the gas. The liquid-vapor, two-phase boundary of the gas was estimated, and the gas pressure was kept above the cricondenbar to insure single-phase transfer from a storage vessel to the visual PVT cell. In the visual cell, the pressure was decreased in steps and the volume was measured at each step. This pressure-volume relation was determined at 49, 70, 90, and pressure-volume relation was determined at 49, 70, 90, and 120 degrees F. At 49 and 70 degrees F we observed bubble points and a dew point. Compressibility factors were points and a dew point. Compressibility factors were calculated from the pressure, volume, and temperature values (z = pV/RT). Experimental Results Table 1 shows the analysis of the gas used, along with the measured specific gravity and volumetric expansion values. Table 2 shows the measured z factors for the four temperatures. These data are plotted in Fig. 2 with the observed bubble and dew points. JPT P. 81