Dispersion and two-phase flow in material from different carbonate pore classes

Carbonate reservoirs are currently estimated to contain more than 50 % of the world's oil reserves. The economical value of improving knowledge on fluid flow in these reservoirs is therefore vast. Carbonate reservoirs are commonly much more heterogeneous than sandstone reservoirs, due to their biological origin and local deposition. Also, diagenetic processes may cause selective dissolution which further increases heterogeneity. On average, carbonate oil reservoirs have lower recovery factors than sandstone oil reservoirs. Lønøy (2006) recently presented a pore-type classification for carbonate rocks based on sedimentological and diagenetic characteristics. The classification scheme includes 20 pore-type classes, counting interparticle, intercrystalline, intraparticle, moldic and vuggy porosity, as well as mudstone microporosity. Furthermore, the classification also comprises pore sizes and porosity distributions. In this thesis, dispersion tests in water-saturated state and at residual oil saturation, and water floods were performed on 11 carbonate core samples from three pore classes (as defined by Lønøy, 2006); intercrystalline uniform macropores, moldic macropores and tertiary chalk. The dispersion analysis involved a new approach, calculating tracer (added NaCl) concentration through measurements of conductivity and temperature, and simulating the results using Coats and Smith's (1964) differential capacitance model to estimate dispersivity and amount of dead-end pores. In addition, the effect of flow rate on dispersion was studied for three carbonate core samples. Also, the impact of salinity and ion valency (Na+ and Ca2+) on absolute permeability was investigated for two carbonate samples. The new approach to dispersion analysis generated large data sets, making it possible to learn more about the connectivity and heterogeneity of pore structures. Permeability measurements varying ion type and concentration showed that soluble minerals (such as anhydrite) may cause dissolution and lead to significant increase in permeability. Also, the results indicate that the Ca2+ content in brine affects permeability; this is consistent with previous studies on clay in sandstones. The flow rate effect study confirmed that velocity should be kept high enough to avoid diffusion-dominated flow. Heterogeneous core samples are more sensitive to velocity due to a higher amount of dead-end pores. Even though the number of core samples was low, a strong link between flowing fraction and recovery factor was found. Homogeneous samples also showed a more predictable relation between dispersion profiles at the two saturations. Analysing the extension of the pore class concept to fluid flow, there seems to be no direct correlation between the selected pore classes and dispersion characteristics. This study has shown that, apparently, water-flooding performance is more closely linked to flowing fraction, wettability and permeability than pore classes