Wettability and capillary pressure measurements on Hillerslev outcrop chalk

Measurement of wettability and subsequent establishment of capillary pressure curves was carried out for three small Hillerslev outcrop chalk specimens. This was done by utilizing Amott cups and an automated Beckman centrifuge at Rogaland Research, Stavanger, Norway. The dimensions of the specimens were D = 38 mm and H = 50 mm. The fluids used were synthetic Valhall formation water and the laboratory oil Isopar-L. Two of the specimens tested were originally water-wet, while the wettability of a single specimen was altered to a homogenous neutral to slightly oil-wet state. However, the wettability alteration affected the chalk, and the specimen broke in one end. Consequently, a new diameter and height were obtained, i.e. D = 37.9 mm and H = 36.2 mm. The modified U. S. Bureau of Mines (USBM) method was used to obtain both the Amott-Harvey and the USBM wettability indices. The method consists of five steps: (1) initial oil drive, (2) spontaneous imbibition of water, (3) water drive, (4) spontaneous imbibition of oil, and (5) oil drive. It was concluded that Hillerslev outcrop chalk is strongly water-wet and that it can be altered towards a homogeneous neutral to slightly oil-wet state using crude oil added with 1 weight% Dodekane acid. It was found that the USBM wettability index cannot be obtained for strongly water-wet Hillerslev outcrop chalk. Water-oil capillary pressure curves were established for the specimens based on the measurements during the modified USBM method. However, due to centrifuge limitations, the capillary pressure curves were not fully completed. Further, fractures were induced during centrifuging. It is evaluated that the obtained capillary pressure curves are not fully representative for the specimens. However, a good estimate of the residual oil saturation Sorw was obtained, especially for the strongly water-wet specimens. Based on the fact that the capillary pressure curves were not fully completed due to centrifuge limitations, and that fractures were induced in the specimens even at these lower centrifuge speeds, it is evaluated that capillary pressure curves cannot be obtained in the centrifuge for Hillerslev chalk.