Characterization of Carbonate Reservoirs Using the Ultracentrifuge

ABSTRACT

The heterogeneous, vuggy and fracture carbonate reservoirs in Alberta present a challenge to the researcher who tries to characterize them. Even when core is obtained, it may be so heterogeneous that any detailed screening of physical properties and (enhanced) oil recovery potential becomes uneconomical very quickly. Centrifuge technology offers an important alternative for a quick and inexpensive screening of such heterogeneous systems. The application of such a screening program for cores of the Rainbow Keg River Reefs is presented in this paper.

Approximately 60 plugs from five reefs were screened for wettability, capillary pressure characteristics, waterflood recovery and gravity assisted immiscible injection potential. The concept of homologous times was used for scaling. Berea sandstone and Baker dolomite plugs were also used for reference purposes. Ternary wettability plots, USBM characteristics, capillary pressures, waterflood and gas injection recoveries were obtained. The results were compared to reference tests using outcrop rocks with crude and refined oils.

The reefs were characterized as generally mixed-wet with the tighter matrix showing water-wet tendencies. However, very broad wettability and capillary pressure characteristics were observed, indicating the existence of multiple types of pore networks within each reef. The work also showed that there is no apparent correlation between physical properties such as porosity and permeability with wettability indicators such as USBM and Amott indices.

It was also evident that the hysteresis observed in the centrifuge experiments is different than the hysteresis observed in the traditional porous plate measurements. More specifically, the value of the “irreducible” wetting phase saturation seems to shift between the first drainage cycle and the second drainage cycle. The plugs were tested under a variety of conditions which include crude and refined oils at room and reservoir temperatures. The reservoir core was tested on both “native” and “restored” state. It was found that the widest hysteresis occurred when crude oil was used. The hysteresis was more significant in carbonates than in sandstones. This can only be partially attributed to the fact that the carbonates have significantly smaller pore volumes. However the trends on clean carbonates, are opposite to the ones of reservoir rock.

A full version of this paper is available on AAPG Datapages Online.