CO2 EOR Production Properties of Chalk

Abstract The production properties of chalk from the Tor and Ekofisk Formations of a Danish North Sea oil reservoir were compared in two core flooding experiments at the conditions of the reservoir. Both experiments started with a water-flooding, and then continued with a CO2-flooding. The Tor core sample was saturated with live oil to So=85.0 %, while the Ekofisk sample was saturated to So=63.7 %, the difference reflecting a difference in capillary pressure. The remainder of the pore space contained formation water. The water-flooding resulted in Sorw=25.3 % for the Tor chalk, and Sorw=37.7 % for the Ekofisk chalk. The difference in Sorw reflects different production properties during water-flooding of the Tor chalk and the Ekofisk chalk. After changing the flooding agent to CO2, the oil production resumed in both experiments, and resulted in SorCO2=5.8 % for the Tor chalk and SorCO2=4.6 % for the Ekofisk chalk. The Tor chalk produced 70.2 %OOIP during the water-flooding and 23.0 %OOIP during the CO2-flooding. In contrast, the Ekofisk chalk produced 40.8 % OOIP during the water-flooding and 52.1 % OOIP during the CO2-flooding. The inferior production properties during water-flooding for the Ekofisk chalk did not result in similar poor production properties during the CO2-flooding. The Tor chalk and the Ekofisk chalk showed production profiles that were very similar in the later part of the CO2-flooding, but differed in the early part, where the Tor chalk produced water, while the Ekofisk chalk produced oil. It is concluded that the Tor chalk and the Ekofisk chalk had different production properties in the water-flooding, but similar production properties in the CO2-flooding. Because the water-flooding left much more oil in the Ekofisk chalk, the CO2-flooding could recover a much larger oil volume from this rock. Neither the Tor sample nor the Ekofisk sample showed evidence of dissolution after the experiments. Both samples remained within the elastic regime all through the experiments. A significant potential for CO2 EOR may be present in Ekofisk chalk that is non-economic for water-flooding.