New insights on rock alteration by oil: An in-situ investigation at the nano-scale using atomic force microscopy

From oil-recovery, through environmental remediation of oil spills, to CO₂ storage in depleted reservoirs, the alteration of rock by oil is known to impact flow dynamics in porous reservoirs and hence controls the efficiency of reservoir development and environmental remediation projects. Whereas many studies focus either on the adherence of individual oil components in model systems or on the related macro-scale multiphase flow responses, this study investigates nano-scale mechanisms of oil alteration along the internal rock surface providing insights how these length scales interconnect. The use of atomic force microscopy allows to identify the location of oil along the internal rock surface after alteration by oil and after water flooding. The results show a persistence of water films in one out of three crude oils tested. Oil components were observed to adsorb patchy to the rock surface and fluid-fluid interfaces. After the waterflood, oil remains trapped within the roughness of the grain surface. These findings illustrate that underlying assumption of a homogeneous alteration along a grain surface, used for common wettability alteration models are too simplistic and may need adjustments for specific oil-brine-rock systems.