SULFATE IN ROCK SAMPLES FROM CARBONATE RESERVOIRS

摘要

Sulfate has earlier been reported to alter the wettability of both outcrop and reservoir chalk rocks to more water-wet and improve the oil recovery by increasing the spontaneous imbibition of water. In the reported study,trim off rock samples from two different oil chalk reservoirs have been analysed for sulfate and elements.Water has also been injected to core plugs from the same chalk reservoirs to determine the concentration of easily accessible sulfate.The sulfate concentrations in the reservoir chalks are compared with the sulfate concentrations in outcrop chalks used as analogous rocks. Many of the trim off samples from the two chalk fields were found to contain rather high sulfate concentrations high.The concentrations of easily accessible sulfate were also determined to be rather high in reservoir core plugs from the two chalk fields.Both in trim off samples and core plugs from the two chalk reservoirs the sulfate concentrations were found to be higher than the sulfate concentrations reported to affect the wettability of chalks. Since rather high sulfate concentrations have been found in the reservoir chalk rocks,it is recommended to modify the procedures for preparation of reservoir core plugs to establish correct wettability.The core plug preparation procedures should depend on whether the core plugs are contaminated with sulfate or the sulfate is present in the original reservoir rock.