CO2 sequestration for enhanced gas recovery: New measurements of supercritical CO2–CH4 dispersion in porous media and a review of recent research

Abstract

The enhanced recovery of natural gas by the injection and sequestration of CO2 is an attractive scenario for certain prospective field developments if the risks of gas contamination or early CO2 breakthrough can be assessed reliably. Simulations of enhanced gas recovery (EGR) scenarios require accurate dispersion parameters at reservoir conditions to quantify the size of the miscible CO2–CH4 displacement front; several experimental studies using core-flooding equipment aimed at measuring such parameters have been reported over the last decade. However, such measurements are particularly challenging and the data produced are generally afflicted in their repeatability by limited experimental control and in their accuracy by systematic errors such as gravitational and core-entrance/exit effects. We review here the existing experimental data pertaining to EGR by CO2 sequestration and also report new measurements of longitudinal CO2–CH4 dispersion coefficients at temperatures of 40–80°C, pressures of 8–12MPa and interstitial velocities of 0.05–1.13mms−1 [14.2–320ftday−1] in 5–10cm long sandstone cores with permeabilities of 12 and 460mD. The core-floods were conducted in both a horizontal and vertical orientation, with significant gravitational effects observed for low velocity floods in horizontal cores with high permeabilities. We also analyzed the effects of tubing and core entrance/exit effects on the measurements and found that the latter resulted in apparent dispersion coefficients up to 63% larger than would be due to the core alone. Our results indicate that dispersivities for CO2–CH4 at these supercritical conditions are less than 0.001m, which indicates that excessive mixing will not occur in EGR scenarios in the absence of conformance effects such as heterogeneity coupled with injection well pattern. Inclusion of such conformance effects is essential for detailed reservoir simulation.