Improving water saturation estimation in carbonates through integrated formation factor porosity and capillary pressure analysis

Understanding the formation factor porosity relationship is critical for accurate water saturation estimation from well logs, particularly in carbonate reservoirs characterized by complex lithologies and heterogeneity. In this study, formation factor porosity relationship and capillary pressure functions were evaluated for the Defa Field in Libya to improve the reliability of key petrophysical parameters such as the cementation factor and saturation exponent, which are often affected by formation complexity and drilling induced effects.

This study utilized log data from three wells located in the Defa Field, each having different gross water zone thicknesses. Formation resistivity and porosity were interpreted from well logs, and cross plots were generated to derive the formation factor porosity relationships. The Archie parameters “a” and “m” were determined for each well individually. Additionally, capillary pressure data were evaluated using the Leverett J function method, allowing vertical water saturation profiles to be constructed based on average values for each core. Zonation analysis was also performed to distinguish porosity and permeability variations within each reservoir interval.

Results indicate minor variation in the Archie parameters across the three wells. The tortuosity factor “a” ranged from 1.1 to 1.4, with an average of 1.26, while the cementation factor “m” ranged from 2.39 to 2.6, averaging 2.49. The saturation exponent “n” varied between 1.7 and 2.4, with an average of 2.05. Cross plots between formation factor and porosity confirmed a good correlation across the wells. The Leverett J function analysis revealed smooth saturation profiles and consistent capillary pressure gradients, reinforcing the reliability of the log derived formation properties. Notably, the permeability and porosity trends were consistent with depositional facies and structural variations across the Defa Field.

This study presents a comprehensive integration of formation factor–porosity relationships and Leverett J function analysis for a carbonate reservoir. The combined use of zonation, Archie parameters, and capillary pressure data enhances confidence in log derived petrophysical properties, leading to improved water saturation modeling and more reliable reservoir performance predictions.