Experimental investigation and prediction of saturation exponent in carbonate rocks: the significance of rock-fluid properties

Hydrocarbon reserves are commonly estimated from electrical logs based on the Archie’s law. Therefore, understanding the concepts and precise determination of Archie’s parameters play an essential role in reservoir studies. The electrical properties of carbonates are affected by both microstructures and wetting characteristics of these heterogeneous rocks. Understanding of these effects and prediction of Archie’s parameters are significant theoretical and experimental challenges. In this paper, these effects were analyzed separately using rocks with various lithologies including limestone, dolomitic limestone and dolostones. Samples with different sedimentary textures (grainstones and packstones), porosities and pore types were also considered under different laboratory conditions. For microstructure effects analysis, 16 heterogeneous carbonate samples having a wide range of microstructures were selected. To decrease wetting effect, the washed samples were tested in air–brine injection system at ambient conditions to measure saturation exponent. Subsequently, in order to investigate the influence of the wetting characteristics on this exponent, 25 homogeneous limestone samples were employed for water–oil injection under reservoir conditions. The porous plate was used as a standard technique to determine the saturation exponent as an electrical index. The results showed that pore connectivity and wettability are the main factors affecting the saturation exponent. Heterogeneity, including the presence of large pores and bimodal texture, is another effective factor that complicates the relationship between saturation exponent and wettability. Furthermore, results indicate variation of the saturation exponent with fluid saturation. Finally, equations were obtained to interpret and calculate the saturation exponent using capillary pressure data by mercury injection method. The derived equations clearly demonstrate the significant impact of the studied parameters on the saturation exponent in carbonate rocks.