Injectivity Changes and CO2 Retention for EOR and Sequestration Projects

Abstract

Abstract The apparent consequences of increased greenhouse gas emissions is encouraging increased carbon dioxide (CO2) injection into oil reservoirs for enhanced oil recovery (EOR) and we are seeing the start of carbon storage in other geological formations, especially deep saline aquifers (Rogers and Grigg, 2001). The development of CO2 plumes and their subsequent transport and dissolution into formation brine are essential mechanisms in most storage scenarios. The study of these will aid in understanding long-term injectivity reduction in EOR WAG (water alternating with gas injection) projects. This paper describes laboratory tests on sandstone and carbonate core samples. Two types of displacement tests were performed at reservoir conditions: gas injection into brine saturated core until residual brine saturation is reached with respect to gas, then this is followed by brine injection into this core that had previously been flooded to a residual gas with respect to brine. In some cases the brine was injected until all the residual gas was removed by dissolution. The level of CO2 saturation in the injected brine at reservoir pressure and temperature was varied from zero to over 90% saturation. This variation in CO2 saturation in the injected brine determined the reduction rate of the CO2 saturation or plume after residual gas saturation was reached. This information can be used in CO2-EOR WAG projects and for carbon storage into geological formations. The injection of CO2 into brine-saturated sandstone and carbonate core results in brine saturation reduction to from 62 to 82% brine in the tests presented in this paper. In each test, over 90% of the reduction occurred with less than 0.5 PV of CO2 injected, with very little additional brine production after 0.5 PV of CO2 injected. During brine injection, CO2 production was equivalent to the rate expected from brine saturated with CO2 at reservoir conditions, except for the first ~0.1 PV of the Queen Sandstone CO2 production. This indicates that in each core at high end-point brine saturation at the tested flow rate (~2 m/day) the CO2 plume was reduced through by solution, not displacement. With increasing CO2 saturation in the injected brine, the brine volume required to remove (dissolve) the CO2 plume increased proportionally. Results will be used to aid in predicting injectivity in CO2-EOR-WAG operations and CO2 plume migration and CO2 dissolution in EOR and sequestration projects.