Hydrocarbon residue in a Danish chalk reservoir and its effects on CO2 injectivity

Abstract

Depleted oil and gas chalk reservoirs in the Danish North Sea are considered for Carbon Capture and Storage (CCS) as part of Denmark's strategy to reduce CO2 emissions. While these reservoirs are well understood after decades of hydrocarbon production, the interaction between CO2 and hydrocarbon residues (HCRs) is less understood and pose a major challenge. This study investigates the effects of supercritical CO2 (sc-CO2) injection on HCRs in a core plug from the Upper Cretaceous chalk reservoir of the Halfdan Field, Danish North Sea, after sc-CO2 flooding for nine days in near reservoir conditions (85 °C and 27.6 MPa). The HCRs from before and after the sc-CO2 flooding were analyzed using Extended Slow Heating (ESH®) pyrolysis, Thermal Desorption Pyrolysis Gas Chromatography-Mass Spectrometry (TD-Py-GC/MS), and organic petrography methods. Results indicate that sc-CO2 can effectively mobilize and put into solution all lighter hydrocarbon fractions, leaving behind heavy immiscible asphaltene-rich hydrocarbons and solid bitumen. In the post-CO2 flooding sample, the asphaltene-rich, immobile oil and solid bitumen aggregates were physically trapped at various points, though primarily at the outlet section due to a sudden change in pressure. The results indicate pressure differences cause up to 10.5% of total rock volume precipitation of pure solid bitumen, which can significantly alter the ongoing flow of sc-CO2 in the reservoir. The HCRs also show an increasing proportion the heaviest, solid bitumen fraction from the inlet of sc-CO2 injection to the outlet of the plug. The increase in solid bitumen content away from the sc-CO2 inlet point is suggested to be caused by selective solubility together with imbibition of hydrocarbons within the flowing sc-CO2 phase, thereby changing the bulk polarity and hence the solubility - here referred to as the Avalanche Effect.