Wettability effects on the matrix–fracture fluid transfer in fractured carbonate rocks

Experimental oil–water and water-oil displacements in stacked, moderately oil-wet limestone core plugs are presented. The in-situ development of the oil saturations in the matrix and in the fracture was between the stacked core plugs obtained during each displacement using magnetic resonance imaging (MRI). The mechanisms of fluid flow into and across open fractures were studied as function of fracture surface wettability, oil saturation in the matrix and the injected fluid. Oil droplets formed on the fracture surface during oil- and water injections at moderately oil-wet conditions, whereas no droplets were observed at strongly water-wet conditions. Water needed to overcome a threshold pressure to enter the moderately oil-wet matrix block downstream of the fully water-filled fracture. Growing oil droplets contributed to the fluid transfer across the open, partially oil-filled fracture between two separated matrix blocks, and oil transport occurred without associated fracture pressure increase. The re-occurring droplet growth-detachment-growth behavior and the flow of isolated oil globules in fractures may have an effect on a commonly used transfer functions in dual-porosity/permeability formulations, which are based on the assumptions of instantly water filled fractures, uniform pressure distributions and time-independent shape factors.