Wettability Alteration During Water Flooding and Carbon Dioxide Flooding of Reservoir Chalk Rocks

Abstract

Abstract In the presented work the wettability conditions of reservoir chalk plugs have been characterised after water flooding with formation water, after spontaneous imbibition and water flooding by sea water, and after the first cycle of water alternating carbon dioxide flooding (CO2-WAG). Core plugs from a fractured chalk reservoir in the North Sea were used in the experiments carried out at reservoir conditions. Easily accessible sulphate in the core plugs was removed before the core plugs were drained to initial water saturation by using the porous disc method. Wettability acquisition was then carried out using stock tank oil. The wettability conditions of the chalk plugs were characterised by the spontaneous imbibition of water and the water-wet area determined by the sulphate wettability test. The water-wet area of reservoir chalk plugs water flooded by formation water was found to be rather small. The spontaneous imbibition of formation water was also rather low for companion core plugs. The prepared reservoir chalk plugs therefore appeared to be close to mixed-wet or preferential oil-wet. Sea water has earlier been found to improve the spontaneous imbibition of water into reservoir chalk plugs from the same field. After spontaneous imbibition and viscous flooding with sea water, the water-wet area of the core plugs was found to be on the average slightly larger than for the core plugs water flooded by the formation water. When the same core plugs were flooded with CO2, most of the oil was produced. After the following sea water injection, the water-wet area was found to be larger than before the simulated first cycle of the CO2-WAG flooding. During long term spontaneous imbibition experiments followed by viscous water flooding by sea water, slightly alteration of the wettability conditions towards more water-wet was observed. In the first cycle of a CO2-WAG process, the reservoir chalk plugs were found to become even more water-wet. Alteration of the wettability conditions to more water-wet during CO2-WAG processes can be important for the oil recovery in fractured chalk reservoirs, and should therefore be studied further.