Observations of Wetting Alteration in Sandstone and Carbonate Rocks Across Scales (10-9 - 10-5m) Using Atomic Force Microscopy and XCT Imaging

In the presence of multiple fluids wettability determines the preference of a fluid to adhere to solid surfaces over another fluid and thereby is one of the key parameters controlling multiphase flow in porous rocks. While minerals found in the subsurface are naturally water-wet, in hydrocarbon reservoirs the wetting preference may be altered by the interaction of the surface-active compounds present in the hydrocarbons and the mineral surfaces ("ageing"). This wettability alteration is influenced by rock, crude oil and brine properties, which can lead to a spatial distribution of wettability. This spatial distribution would be input for any modelling of multiphase flow. One of the key questions is whether the dominating factor for the spatial distribution of wettability is the spatial distribution of the minerals in the rock fabric, or the initial crude oil-rock contacts, i.e. the initial fluid distribution present during the ageing process. In this work, we observe the wetting alteration in a multi-mineral sandstone and a mono-mineral carbonate rock. In the sandstone, wetting alteration is controlled by the distribution of minerals, whereas in the carbonate, wetting alteration is controlled by fluid distribution and local mineral texture.

The observations involved the combined and coordinated use of Atomic Force Microscopy and X-ray μ-CT imaging. A Berea sandstone and two Estaillades limestone samples underwent wettability alteration by exposure to crude oil at high temperature and pressure for at least 28 days. For the Berea, crude oil drainage was performed by flooding. For the Estaillades, the samples were initialised using a centrifuge at two different capillary pressure values. After the wettability alteration, waterflooding was performed by injecting brine at low capillary number and the samples were imaged with X-ray μ-CT. The fluid arrangement was analysed and compared to images acquired after waterflooding for a Berea and an Estaillades sample that were not exposed to crude oil to alter the wetting state of their mineral surfaces. In the case of Estaillades, Atomic Force Microscopy was also employed to investigate surface roughness and fluid film distribution, before and after waterflooding.