Multiscale pore structure characterization and pore network modeling of middle east carbonates

Gündoğar, Satı Aslı
The complex interplay between structural and wettability heterogeneities is responsible for the limited pore network modeling studies on carbonates. In this study, our purpose is to develop structurally representative pore networks of Middle East carbonates and to predict their macroscopic difficult-to-measure relative permeability and capillary pressure hysteresis curves and residual oil trends under mixed-wettability conditions. In comparison with the conventional pore-scale models, particular emphasis is put on multiscale pore structure characterization of complex carbonates. Scanning electron microscopy (SEM) imaging at multiple magnifications (75X and 750X) as well as physical core measurements, namely helium pycnometry and high-pressure (33,000 psi) mercury intrusion porosimetry (MIP) are employed to determine quantitative pore network descriptors. Mosaics constructed from overlapping low magnification (75X) views reveal abundant evidence of unresolved microporosity within the carbonate samples. Multiscaled SEM pore-size distributions incorporate microporous features as small as 0.077 μm and macropores as large as 310 μm into a single pore system. A pore body-throat classification method is developed with an initial premise of strong spatial size correlation between pore bodies and their connected throats. Six pore classes are identified with unique pore- and throat-size distributions. The average coordination number ranges mostly from 3 to 5, and irregular triangle-shaped cross-sections dominate the carbonate pore geometry. The pore body to throat aspect ratio is generally large (>2.5), indicating the potential for significant residual oil saturation. A novel quasi-static two-phase flow simulator is developed for a complete primary drainage-secondary imbibition-secondary drainage cycle with arbitrary wettability. Incorporating wettability alteration and contact angle hysteresis, the critical roles of corner wetting films and intermediate layers on fluid continuity and accordingly on recovery are investigated. The model predictions display a significant dependency on the interaction of pore structure, wettability, and saturation history. During spontaneous imbibition, snap-off becomes dominant in the systems with high aspect ratio and results in large amounts of trapped oil. It is shown that residual oil saturation is lower for more strongly oil-wet systems by means of the abundant presence of oil layers, while the endpoint water relative permeability increases gradually with the oil-wetness. The continuity of a phase is deduced to be the critical factor for its relative permeability rather than its abundance in the pore system. It is concluded that our newly proposed network model succeeds to represent experimentally hard-to-measure capillary pressure and relative permeability hysteresis trends for mixed-wet systems.  
Citation Formats
S. A. Gündoğar, “Multiscale pore structure characterization and pore network modeling of middle east carbonates,” Ph.D. - Doctoral Program, Middle East Technical University, 2017.